Modern Tug Design

(1)

Modern Tug Design with Particular

Emphasis on Propeller Design,

Maneuverability, and Endurance

BY DOROS A . ARGYRIADIS, 1 ASSOCIATE M E M B E R

T u g b o a t design, although o f utmost im- portance, has been d i s r e g a r d e d by the naval architect to a great extent and only a limited a m o u n t o f i n f o r m a t i o n is available to the d e s i g n e r in the f o r m o f technical papers. In this study, an attempt is made to c o r r e c t this lack by p r e s e n t i n g some o f the i m p o r t a n t features peculiar to t u g b o a t design. Hull f o r m and lines are treated briefly. Some design formulas are presented and a c o m p a r i s o n is made between British and A m e r i c a n designs. T h e stability of tugs is p r e s e n t e d at some length and the lines o f some m o d e r n boats as well as the particulars o f several others are given. A r r a n g e m e n t s and a c c o m m o d a t i o n s usually are based u p o n the wishes o f the tug o w n e r and are treated very briefly. T h e i m p o r t a n c e o f a g o o d preliminary

weight and trim calculation is emphasized. Several different types o f main p r o p u l s i o n machinery p o w e r plants are discussed and the merits o f each one are presented. P r o - peller design is discussed at some length. P r e l i m i n a r y desig n formulas are given for both the bollard pull and the t o w i n g thrust. C o m p a r i s o n s between the different types o f p r o p e l l e r s are made and a m e t h o d for calculating the p e r f o r m a n c e o f the propeller at any speed of the boat is presented. Maneuverability is also discussed and covers both r u d d e r design and e n g i n e controls. Formulas are given for the rudder area and the relative merits o f some o f the several types o f t u g b o a t r u d d e r s are analyzed. Finally, e n d u r a n c e and engine p e r f o r m a n c e at reduced speeds are discussed.

I N T R O D U C T I O N

T u g b o a t design is a subject t h a t has been dis- regarded b y most naval architects, with the nota- ble exceptions of Roach (24) 2 and Caldwell (8), despite the obvious usefulness of these boats. In searching the libraries for appropriate literature, the author has been amazed b y the lack of written material on the subject. This inadequacy is difficult to understand, especially if one remem- bers t h a t the design of tugboats dates back to the earliest days of steam-driven boats and the sight I N a v a l A r c h i t e c t , J o h n J. M c M u l l e n Associates, H o b o k e n , N. J. 2 N u m b e r s in p a r e n t h e s e s refer to t h e B i b l i o g r a p h y a t t h e end of t h e paper.

P r e s e n t e d a t t h e A n n u a l M e e t i n g , N e w York, N. Y., No- v e m b e r 14-15, 1957, of T I ~ SOCIETY OF NAVAL ARCHITECTS AND MARINB ENGINEERS,

of an old steam tug helping a sailing vessel dock or und0ck was fairly common in the major ports of the world around the middle of the past century. Without these h a n d y vessels in and around our harbors, a major portion of the world's shipping could not operate successfully and efficiently, and the docking, undocking, salvage and the carriage of cargoes in barges would have beeh impossible.

Tugs can be subdivided into three main cate- gories or classes; namely, (a) small harbor and utility tugs, (b) large harbor and coastwise tugs, and (c) ocean-going and salvage tugs. T h e small harbor tug represents the workhorse of the harbor, and its services would include the perform- ance of a number of rather small towing jobs and the docking of small vessels. T h e utility tug m a y 362

(2)

M O D E R N T U G D E S I G N 363 range from 40 to 65 ft in length, while the large

h a r b o r and coastwise tug usually has a length of from 70 to 120 ft. T h e services of this second class would include the docking of large vessels and the towing of barge and lighter fleets within the h a r b o r or along the coast. Finally, the salvage tug is mainly concerned with long ocean towing services and, as its name implies, salvage jobs. T h e length of this tug is usually over 125 ft ancl its freeboard is normally more t h a n the freeboard found in its smaller counterparts, in order to allow a safe and d r y ocean crossing.

T h e three t y p e s mentioned can actually be dealt with simultaneously, since, a p a r t from physical dimensions, there are few f u n d a m e n t a l differences between them. In general, the tug designer is limited in his choice of principal dimensions b y the specifications of the owner relative to power, m a x i m u m allowable draft, and free running speed, while he also has to take into account such practi- cal aspects as stability, limited length in connection with maneuverability, engine room size, propeller dimensions, hull form, and so on. Off hand, it m i g h t seem peculiar to the uninitiated t h a t length and speed are t r e a t e d so nonchalantly and assigned specific values for different classes of tugboats without a thorough investigation of the effects of length on speed and power. This subject has been treated b y L. A. Baler in references (3) and (4) in which he shows t h a t in m a n y cases it is a d v a n t a g e o u s to increase the length of the b o a t to obtain the best resistance characteristics. However, the design of a t u g b o a t does not allow the selection of the m o s t efficient length for the power available because of other, more important, considerations. A t u g b o a t is essentially a floating powerhouse and its p r i m a r y mission is to help other vessels to m a n e u v e r in restricted quarters or to tow t h e m to their destinations. AcCordingly, m o s t of its power is absorbed on the towline and only a small percentage is used for the propulsion of the b o a t itself.

With the exception of ocean-going salvage tugs and some coastwise tugs, one can safely say t h a t the resistance of the tug itself, while towing, is only a small percentage of the over-all towrope pull exerted, with the result t h a t hull-form characteristics can h a v e little influence on the towing speed. However, since the speed/length ratio of these boats will.be high in the free running condition owing to the large available power, and since normally the owner will specify some particular speed to be attained while running free, the designer should give careful consideration to the se- lected prismatic COefficient, longitudinal center of b u o y a n c y and fineness of b o d y fore and aft in order to obtain the best hull form possible. Free-

board forward, which frequently limits the free running speed, m u s t also be considered.

T h e trend of t o d a y seems to be to increase the available power over older tugs w i t h o u t a n y change in the over-all length of the boat. One reason for this is the b e t t e r and more powerful engines, such as the supercharged Diesel, available on the m a r k e t today. Again, the length of the ship-handling tug cannot be increased con- siderably o v e r 100 ft, since a n y size a b o v e t h a t length would tend .to m a k e the b o a t a w k w a r d in m a n e u v e r i n g in a n d out of tight spots. I t follows, then, t h a t the m a i n problem of the designer, after the preliminary form characteristics h a v e been established, would be to fit a propeller which would give m a x i m u m possible towrope pull at some o p t i m u m towing speed and which, a t the same time, would allow the b o a t to a t t a i n the desired free running speed.

Finally, careful consideration should be given to the main propulsion machinery control, since the m a n e u v e r a b i l i t y and hence quite a bit of the suc- cess of the tugboat, be it large or small, will de- pend largely on the response of the propulsion plant to the orders given to it from the bridge. I n addition to the pilothouse control station, an additional control station on the deck a f t of the pilothouse is r e c o m m e n d e d for h a r b o r work.

HULL-FORM CHARACTERISTICS

I t has been mentioned before t h a t the effective horsepower of a tug a t normal towing speeds will be v e r y small as compared to the total towrope pull exerted. However, the designer should give careful consideration to the hull form, so as to obtain the m a x i m u m possible t h r u s t available for towing and a t the same time m e e t the owner's re- quirements regarding free route speed. F r o m purely theoretical considerations, the prismatic coefficient should be somewhere between 0.57 and 0.67, since m o s t tugs will h a v e a free route s p e e d / length ratio of a b o u t 1.10 to 1.40 and a towing speed/length ratio of from 0.60 to 0.70. I t would appear, off hand, t h a t a greater prismatic could be used if the hull were to be designed for towing speeds, b u t an investigation of the resistance of a h e a v y displacement hull (say displacement/length ratio equal to 400) with a prismatic of 0.70 at a speed/length ratio of 1.15 shows a twofold increase in total resistance per ton of displacement over the same hull having a prismatic of 0.60, while the reduction in resistance of the lower prismatic hull over the:~0.70 prismatic hull a t a speed/length ratio around 0.60 a m o u n t s to less t h a n 10 per cent of the total resistance a t t h a t speed. On the other hand, if the speed/length ratio of the tug when running free is over 1.25, as is often the case in

(3)

1600__

150o__

l&OO__

13oo

12oo

11oo

ioo_o

9~

700 ,-~6( ID 0 r t C~ I00 0

115~I

3

Comparison of Tugboat Particulars

B r i t i s h versus A m e r i c a n Practice

I i

i / Le igth \

1

0 / I " / / / / / / - ' J f f / _ / / /

J

,,,.. / / / / f I / / / / / / / / 200 &O0 600 800 i000 1200 Shaft H o r s e p o w e r / / / / / / / / --/

~

v I~00 ./ / / / . /

/

( / / / I /

vjU

/

/ / / - - - i c eme n t ~

_/

/

/ K E Y ... B r i t i s h Praotioe American P r a c t i c e

I

1600 1800 2000 F I o . 1 2200 0 0 O~ 0

(4)

M O D E R N T U G D E S I G N 365 a m o d e r n tugboat, the resistance of the low pris-

matic coefficient hull a t these high speed/length ratios becomes prohibitive. C o n t r a r y to the be- lief and practice of m a n y t u g b o a t designers, it would a p p e a r t h a t the m o s t suitable prismatic for a vessel of this t y p e would be the one corresponding to a speed/length ratio of a b o u t 1.10, or a prismatic of between 0.57 and 0.60. This would t e n d to reduce the a b n o r m a l l y high resistance t h a t m o s t boats of this t y p e show when running free, and at the same t i m e give a reasonably low resistance over the whole range of operation.

Residual resistance contours of t u g b o a t forms are presented in Appendix 3. These contours are derived from the same d a t a as the ones appearing in C.D. R o a c h ' s p a p e r on " M o d e r n T u g Design" (24), with the difference t h a t resistance-coefficient curves h a v e been plotted for different prismatic coefficients against the m o r e c o m m o n l y used speed/length ratio. Several curves for displace- m e n t / l e n g t h ratios of from 200 to 450 in incre- m e n t s of 50 are shown and it is hoped t h a t this t y p e of presentation m a y facilitate interpolation between different speed/length ratios, displace- m e n t / l e n g t h ratios and prismatic coefficients.

T h e block coefficient of a t u g b o a t is usually m u c h lower t h a n the prismatic coefficient, and is sometimes as low as 0.45 or 0.46. This is mainly due to the fact t h a t the bilges h a v e to be as slack as practicable to allow an easy fairing of the lines into the fine fore-and-aft body. Average values of block coefficients range f r o m 0.45 to 0.55 and corresponding values for the midship-section coefficient v a r y f r o m 0.75 to 0.85, with the m o s t com- m o n l y used value being v e r y close to 0.80.

T h e foreb0dy lines should be as fine as possible and the half-angle of entrance of the load waterline ranges from 15 to 30 deg with the median around 20 deg. T h e waterlines sometimes h a v e a slight reverse to allow the fairing of the curve into the half-beam a t or near amidships. T h e load waterline aft should be as full as possible to allow for m a x i m u m coverage and protection for the propeller. T h e a f t e r b o d y lines below the load waterline should be fair and fine in order to give the propeller the m a x i m u m possible a m o u n t of solid water, and reverse c u r v a t u r e of these lines is practically a necessity. T h e fineness of the a f t e r b o d y lines below the waterline c a n n o t be overempha- sized, since in m a n y tugboats the propeller does not seem to receive the required a m o u n t of solid water, tending to pull down air f r o m above, and in this w a y m a y cause serious and objectionable vibrations. Reference (5) gives a good analysis of the reasons of stern vibrations on single-screw vessels. Although t h a t reference deals mainly with G r e a t Lakes ore carriers, the findings can be applied to

t u g b o a t s as well. I n particular, the a u t h o r s state t h a t it seems to be a p p a r e n t t h a t wide variations in wake distribution h a v e far greater effect on hull vibrations t h a n do close clearances between the propeller and the hull. An interesting sidelight of the vibration problem of t u g b o a t s is t h a t boats fitted with K o r t nozzles show m u c h less hull vibrations t h a n boats with open p r o p e l l e r s . I t might be added here t h a t in order to avoid the sucking of air b y the propeller, a case t h a t m a y h a p p e n if the wheel does not receive sufficient solid w a t e r from ahead, some of the late river t o w b o a t s h a v e the b o t t o m shell plating extending s o m e w h a t p a s t the side-shell plating in the vicinity of the propeller, thus using in effect the same technique t h a t L. A. Baler and J. O r m o n d r o y d used to reduce the fan- tail vibrations of G r e a t Lakes ore carriers.

T h e longitudinal center of b u o y a n c y location is also quite important. T h e fine form of the after b o d y will tend to force the center of b u o y a n c y amidships or even forward of amidships. Some designers seem to be satisfied with this condition and even r e c o m m e n d such a location. However, the a u t h o r believes t h a t the best location of the longitudinal center of b u o y a n c y for the proposed design speed/length ratio is f r o m 2 to 2.5 per cent aft of amidships, a value t h a t cannot always be obtained. A compromise is here necessary, and a longitudinal center of b u o y a n c y of a p p r o x i m a t e l y 0.01 L (or 1.0 per cent) a f t of amidships seems to be the best one can hope for. I t m a y be found advisable, at times, to lengthen the vessel b y a few feet to obtain a reasonable location for the longitudinal center of buoyancy, since in m o s t designs it seems to fall forward of amidships if the design is based strictly on arrangements and accommodations.

Several authors give preliminary design formulas a n d / o r proportion figures which m a y prove helpful to the t u g b o a t designer at the preliminary stages of the work. M o s t of these are derived for British t u g b o a t s which differ from the usual Amer- ican design in t h a t t h e y h a v e shorter deckhouses and are s o m e w h a t underpowered according to modern American practice, with the result t h a t the figures presented should be used with care. T a b l e 1 gives some of the m o s t i m p o r t a n t and widely used figures in Britain in comparison with some representative values for similar United States built tugs. Some more comparisons between c o n t e m p o r a r y British a n d American designs are shown in Figs. 1 and 2.

Additional information and proportions of several types of British tugs m a y be found in W. Pollock's "Small Vessels," (22).

A. R. T a y l o r (28) gives the following formula for the preliminary estimate of the block coefficient

(5)

20 19 18 17 16 O 15 . . . .

..f

= l&' 0 m 13 .~ 11 lO @ 9 7

6 f

/

5 /

~o 50 /

J

/

60 70 C o m p a r i s o n of T ~ b o a t Particulars • '13 B r i t i s h versus A m e r i c a n Practice 12 "-~ ~--" z Ii io - / _ _ ,- 3 ./" / / 2 I K E Y I ... British Practice 0 - - A m e r i c a n Practice

I

80 90 I00" Ii0 120

Length Between Perpendiculars, Feet

130 i ~ 0 150 © ~0 OO © FIO. 2

(6)

\ M O D E R N T U G D E S I G N 367

TABLE i PROPORTIONS OF TUGBOATS--BRITISH

Class ~ 1-ocean . . . . 2-coastwise - - - . ~ 3-harbor - - - ~

Item a b c a b c a b c Vk/%/L (free) 1.04 - - 1.20 1.04 - - -1.25 1.20 - - 1.30 L/B 4.75 5.75 4.10 4.25 4.50 3.70 3.75 4.50 3.30 KG/D ~ 0.85 0.85 0.89 0.90 0.82 0.91 0.80 0.78 0.88 Cb 0.56 0.475 0.52 0.55 0.53 0.48 0.52 0. 464 0.50 C~I 0.85 - - 0.80 0.85 -- 0.80 0.85 -- 0.78 Cp 0.66 - - 0.65 0.65 - - 0.66 0.61 - - 0.64 Cwp 0.7.0 0.70 0.74 0.75 0.75 0.75 0.71 0.702 0.74 m '~ 0.09 0.089 0.091 0.09 0.091 0.091 0.09 0.091 0.092 LID 9.5 9.0 8.4 8.5 8.0 7.2 8.0 8.0 7.8 Sheer fwci (% of L) 5.75 c -- 2.5 5.75 -- 3.9 5.75 -- 3.6 Sheer aft (% of L) 1.5 - - 1.2 1.35 - - 1.2 1.25 - - 1.1 GIrl, light - - 1.0 1.70 - - 1.75 3.30 - - 1.75 2.0 B/H 2.5 2.5 2.2 2.5 2.5 2.2 2.5 2.5 2.9 A/(0.01L) 3 250 250 320 380 300 400 310 380 400 SHP (approx) 1000 - - 1500 600 - - 1200 300 - - 300 & to to to to to up 3000 1000 1800 600 900

VERSUS AMERICAN PRACTICE

4-rive~ a b 1.15 4.00 3.80 0.85 0.87 0.47 0.46 0.90 0.52 0.70 0.695 0.095 0.094 7.5 7.8 5.0 1 . 0 - - 2.0 2.5 2.5 320 310 abt 30O " BM coefficient for use with Simpson's formula for beam.

b No representative American design listed, since most of the river craft in the United States are towboats. our own small harbor tugs are similar to the river tugs listed.

Based on the addition of forecastle.

NOTI~S : Figures under "a" represent values recommended by A. Caldwell, reference (8). Figures under "b" represent values recommended by A. R. Taylor, reference (28). Figures under "c" represent modern American practice.

Actually, T h i s f o r m u l a h a s b e e n t e s t e d i n a c c o r d a n c e w i t h m o d e r n A m e r i c a n p r a c t i c e a n d g a v e c o n s i s t e n t l y good r e s u l t s v, cb = 1.0s 2 v / L Cb = b l o c k coefficient

Vk = m a x i m u m s u s t a i n e d sea speed, free r u n n i n g L = l e n g t h b e t w e e n p e r p e n d i c u l a r s

A. CaldweU (8) gives t h e l i m i t for e c o n o m i c a l " speed for t u g b o a t s as e q u a l to = 1 . 5 5 ( L - V

y J,

. Vk = l i m i t of e c o n o m i c a l speed (free r u n n i n g ) L = l e n g t h b e t w e e n p e r p e n d i c u l a r s , ft V = v o l u m e of d i s p l a c e m e n t , c u f t A ~ = a r e a of m i d s h i p section, sq f t

T h i s f o r m u l a seems to give low r e s u l t s for A m e r - i c a n practice, a n d the a u t h o r wishes t o p r o p o s e t h e following m o d i f i c a t i o n of C a l d w e l l ' s f o r m u l a t o b r i n g i t i n a g r e e m e n t w i t h m o s t m o d e r n designs v = 1.85 L -- A--~] for Vk m a x i m u m v, a n d v, = 1.70 L -- A--~) for e e o n o m y or e n d u r a n c e V \1/, A m o n g t h e A m e r i c a n a u t h o r s , D. S. S i m p s o n (27) gives t h e following p r e l i m i n a r y p r o p o r t i o n s for t u g b o a t s : B / H f r o m 3.75 to 4.75 M e a n d r a f t a b o u t 10 p e r c e n t of w a t e r l i n e l e n g t h Block coefficient f r o m 0.48 to 0.55- M i d s h i p s e c t i o n coefficient a b o u t 0.75 D r a g of keel f r o m 0.04L to 0.05L M i n i m u m f r e e b o a r d a b o u t 10 p e r c e n t of m a x i m u m "beam M e t a c e n t r i c h e i g h t : a m i n i m u m o f ' a b o u t 2.5 ft i n l o a d e d c o n d i t i o n . I n a d d i t i o n to these figures, M r . S i m p s o n s t a t e s t h a t t h e deck line s h o u l d be full, especially aft, i n order t o p r o t e c t a n d p r o v i d e coverage for t h e propeller, a n d s h o u l d show c u r v a t u r e all a l o n g its l e n g t h t o f a c i l i t a t e c o n t r o l alongside o t h e r ships.

T h e i n f o r i n a t i o n a p p e a r i n g i n t h e foregoing references a n d in n u m e r o u s o t h e r articles t o be f o u n d in m a r i n e m a g a z i n e s from t i m e to t i m e h a s b e e n c o n s o l i d a t e d i n t o one plot. I t is h o p e d t h a t these sets of curves, as a p p e a r i n g here i n Fig. 3, m a y help t h e t u g designer i n t h e p r e l i m i n a r y stages of t h e design. F i n a l l y , Fig. 4 m a y a i d t h e designer in e s t a b l i s h i n g a p r e l i m i n a r y s e c t i o n a l - a r e a c u r v e for a c o n v e n t i o n a l b o a t . So far t h e d i s c u s s i o n h a s b e e n l i m i t e d t o c o n v e n - t i o n a l t y p e s of t u g b o a t s : H o w e v e r , r e c e n t l y some n o v e l t y p e s of b o a t s h a v e b e e n a p p e a r i n g i n t h e E u r o p e a n h a r b o r s a n d t h e i r designers h a v e b e e n c l a i m i n g i n v a r i a b l y t h a t t h e y are b e t t e r t h a n a n y o t h e r t u g b o a t afloat. F o r this reason, a brief dis- cussion of these b o a t s m i g h t p e r h a p s b e necessary. E. C. B. C o r l e t t (9) discusses t w o of these n e w t y p e s of t u g b o a t s ; t h e " V o i t h W a t e r T r a c t o r " a n d a t u g w i t h a " h y d r o c o n i c " t y p e hull. Figs. 5 a n d 6 show t h e profile a n d m i d s h i p section of t h e l a t t e r . T h e s e b o a t s h a v e n o t as y e t a p p e a r e d in U n i t e d S t a t e s harbors, a n d t h e i n t e r e s t e d r e a d e r is re-

(7)

9_

7_

6 3_ 2

13_

11_

i0

2_

t

0._6.. _~

3O_03 Od%~ _~

2 5 @ i0

' 0

i

~ K

_2

~0('

% 1 1 3~ 3.~ 32 @ ~2.G 1.8 16 12 1.fl

d"

~

--

Modern

American Practice

70

80

90 i00

Ii0

120

130 I~0

150 Length Between Perpendiculars, Feet

FIG. 3

©

<3

U3

(8)

0.' .' 0.65

~o.6c

8 ~o.55

P-, 0. O.&5

o.~,~

F o r e a c h , l ~ o f L e n g t h t h a t LCF i s a f t o f S u ~ c t ~ t 0 . O 2 2 t o o b t a i n F o r w a r d P r i s m a t i c & Add 0 . 0 2 2 t o o b t a i n NOTE: Prismatic

/

" /

/ / / J

Z

/ /l---

0.80 0.90 1.00 Statior 1

I /

0.I0 0.20 0.30 O.t~O 0.50 0.60 0.70

Percent of Maximum Sectional Area (Station i0)

FIG. 4 APPROXIMATE TUGBOAT SECTIONAL AREA CURVES IN PER CENT OF MAXIMUM SECTIONAL AREA (STATION I 0 )

Aft © © 00 Lg Oh ~D

(9)

f

J | . ,-. • II, ~ - - - - - t = = ~ ~ ' ' , ' "~ , - ^ ~ 0 ' ~ - I . . . . ~ ~ r - - - ~ , / , I . . . . i ' - . . . - ~ , 1 , , , , , , i ; ,

I

L ~ ?

~

' ' ' '-"~"

FIG. 5 A EUROPEAN-TYpI~ TUG WITH HYDROCONIC-TYPE HULL

- - W L

ferred to the foregoing article for f u r t h e r particulars. One obvious a d v a n t a g e of the " h y d r o c o n i c " hull m a y be mentioned here: the saving in man- hours required to build the hull b y the use of al- m o s t straight plates. This should be particularly a p p a r e n t in small boats, and Dr. C o r l e t t claims t h a t in the particular b o a t he investigated these savings a m o u n t e d to o v e r 35 per cent in m a n - hours, a figure t h a t m a y be h a r d to believe. H e also claims t h a t the bollard pull of the vessel was almost equal to the open w a t e r t h r u s t of the propeller and t h a t the t h r u s t deduction was no m o r e t h a n 2 per cent due to the solid w a t e r supplied to the propeller. Unfortunately, no direct comparisons are m a d e between a hydroconic t y p e hull and a conventional hull and thus no direct con- clusions can be drawn from Dr. Corlett's article.

No one can s a y quite enough a b o u t the stability of a tugboat. M a n y good tugs h a v e been tripped on towlines while pushing or pulling ships o u t of the docks, or driven under while running free a t full speed, when the reduction of pressure amidships uncovers a large portion of the hull and creates freak stability problems. B o t h the U. S. Coast G u a r d a n d the U. S. N a v y h a v e their own statical and dynamical stability criteria, b u t in all cases these criteria are based upon larger vessels which are not called upon to p e r f o r m the t y p e s of work a tug is. F o r this reason, these criteria should be used with g r e a t care and as a check only, b u t n e v e r as a design condition for a tugboat. Caldwell proposes the use of Admiral Simpson's formula for an approximation of the b e a m required

to give a specific metacentric height (GM). formula follows B e a m = K M =

H =

C~p =

C~=

m = This -

/Jm

height of m e t a c e n t e r a b o v e base m e a n d r a f t waterplane coefficient block coefficient

B M coefficient, roughly 0.09 for t y p e s of boats u n d e r consideration (See also T a b l e 1 and Fig. 7).

While this formula will serve well as a first approximation for the required beam, it should be used with care, keeping in mind t h a t it was developed for E u r o p e a n - t y p e tugs which h a v e shorter deckhouses and usually less power t h a n their American counterparts.

Capt. C. P. M u r p h y , U. S. Coast Guard, proposes to use the revised Coast G u a r d formula for formula G M = ( S H P X

D)'/,Sh

3 8 ~ - f S H P --- shaft horsepower

h = vertical distance f r o m the center of effort to top of the towing bitts, ft. S = effective decimal fraction of the pro-

peller slip s t r e a m deflected b y the r u d d e r

z~ = displacement in long tons, salt w a t e r

2fiB

-- least t a n g e n t of heel to deck edge ,

f = m i n i m u m freeboard, ft

the required G M for tugboats. This follows

(10)

M O D E R N T U G D E S I G N 371 EN@INF- G.ASI NG _L." ~. I E~U LWA21~- E N G t ~4E B E D ss i :

Tgm,

F I G . 6 SECTION THROUGH E N G I N E ROOM OF HYDROCONIC-TYPE T U G

B = waterline beam, ft D = propeller diameter, ft

W i t h o u t going into the particulars of this formula, it can be s t a t e d t h a t it does n o t give sufficient GIVi for m o d e r n tugboats.

T h e best formula for the required G M of a tug- b o a t is the one proposed b y C. D. R o a c h (24). 3 However, the a u t h o r believes t h a t even this formula underestimates s o m e w h a t the GN[ necessary for a tug and wishes to propose the modification of R o a c h ' s formula to r e a d as follows

= Roach's formula is:

B H P X 15 h

G M = A f i B

B H ~ = displacemetlt, lb

= brake horsepower; remaining symbols h a v e the same meaning as above. S H P X h G M -

100a -f

B S H P = shaft horsepower

h = vertical distance f r o m center of effort to t o p of towing bitts, ft

A = displacement in long tons, salt w a t e r B -- waterline beam, extreme, ft

f = m i n i m u m freeboard, ft

I t is realized t h a t this formula a t t e m p t s to measure a dynamical force b y statical methods, b u t it is believed t h a t it will give satisfactory results for t h e t y p e s of power eml~loyed today. T h e B H P of R o a c h ' s formula has been changed to

(11)

S H P to allow for differences in transmission efficieneies of the different s y s t e m s of propulsion employed. One of the best sources of information available on the differences between purchased power a n d actual power delivered to the propeller is Prof. L. A. Baier's p a p e r on propellers and propulsion (6). T h e following efficieneies are given as representative of m o d e r n trends:

BHP/IHP varying with IHP and attached auxiliaries r u n s f r o m S k i n n e r s i m p l e e n g i n e s Steeple c o m p o u n d e n g i n e s Single r e d u c t i o n g e a r s D o u b l e r e d u c t i o n g e a r s Stool b e a r i n g s S t e r n t u b e b e a r i n g s T h r u s t b e a r i n g s Electric c o u p l i n g s E l e c t r i c d r i v e S H P / B H P 75 to 85 to 9 5 % 93 to 94% decreasing 9 1 % w i t h R P M 9 8 . 5 % 9 7 % 99.5% each 98.5 to 99% 98 to 99% 98.8 to 99.4% 85 to 89% d e p e n d i n g on ac or dc a n d p o w e r r a n g e .

In addition to the foregoing figures, the following are typical values for the transmission efficiencies ( S H P / B H P ) of some of the m o s t com- m o n l y used systems of propulsion in t u g b o a t s of m o d e r n design:

(a) Diesel-electric drive. A v e r a g e efficiency a r o u n d 85 t o 87 p e r c e n t . (b) D i r e c t l y c o n n e c t e d Diesel. Efficiencies f r o m 94 to 97 p e r c e n t w i t h t h e a v e r a g e a r o u n d 95 p e r cent. (c) Diesel e n g i n e in c o n j u n c t i o n w i t h a r e v e r s e r e d u c t i o n gear. Efficiencies a r o u n d 94 p e r c e n t . (d) Diesel e n g i n e in c o n j u n c t i o n w i t h a r e v e r s e r e d u c - t i o n g e a r a n d a h y d r a u l i c or electric slip c o u p l i n g . Effieiencies a r o u n d 90 p e r c e n t . (e) Diesel e n g i n e in c o n j u n c t i o n w i t h a t o r q u e c o n v e r t e r a n d a c o n v e n t i o n a l r e d u c t i o n gear. Efficiencies a r o u n d 93 p e r c e n t for single r e d u c t i o n g e a r s a n d 91.5 p e r c e n t for d o u b l e r e d u c t i o n gears.

Before disposing of the stability problem, one more thing should be mentioned; namely, range of stability. This is largely a m a t t e r of freeboard a n d some differences in the a m o u n t of freeboard in the three classes of tugs can be expected. I n general, the freeboard of the small h a r b o r tug should be a p p r o x i m a t e l y 2 ft, while the freeboard of the large h a r b o r or coastal tug will be in the vicinity of 3 ft and the one required for the ocean- going or salvage tug ranges from 4 to 5 or even 6 It, depending largely on size. In a n y case, the freeboard should not be less t h a n 10 per cent of tile waterline beam, and the range of stability should be a t least 65 to 70 deg and in no case below 65 deg under a n y b u t the light ship conditions. I t should be mentioned here, t h a t in order to obtain this range of stability, all deck openings m u s t be watertight, doors m u s t have a r a t h e r high coaming or be of the " D u t c h " type, and there m u s t be sufficient freeing ports, strategically located in the bulwarks, to allow quick e m p t y i n g of a n y green seas on deck. Finally, all vents m u s t be carried well upwards to avoid the shipping of green seas through them.

Another helpful indication of freeboard and stability is the angle a t which the deck edge sub- merges. This angle, with the vessel fully loaded, m u s t be a b o u t 7 to 9 deg. I f it is below the criti- cal value of 7 deg, the freeboard m u s t be increased. Table 2 gives the design characteristics of some modern tugs which are believed to be typical of recent trends. Figs. 8, 9, and 10 give an idea of the lines of three representative tugs of recent years. Fig. 8 represents Model no. 4087 of the D a v i d T a y l o r Model Basin, a tug designed b y C o m m a n d e r Richards T. Miller of the United States N a v y and believed to be v e r y successful. Fig. 9 is Model no. 4093 of the D a v i d T a y l o r Model Basin, a typical E u r o p e a n - t y p e tug. Fig. 10 is the new V T B design of the United States N a v y , designed for the U. S. N a v y b y M. Rosen- b l a t t and Son of New York City.

A R R A N G E M E N T S AND ACCOMMODATIONS A tug is essentially a floating powerhouse and hence m o s t of the available space in the hull is occupied b y the main propulsion machinery and the required auxiliaries. N o t m u c h choice is left to the designer as to the location of the machinery room, and he often finds t h a t 40 or even 50 per cent of the total length of the tug is taken up b y the engine room. T h e usual practice is to provide a forecastle b u n k room for the crew members. Messing facilities, galley space, a head and wash- room and as m a n y s t a t e r o o m s as space will allow for the licensed crew are usually provided for in the deckhouse, while, if room permits, the skip- per's accommodation is to be found on a higher level, directly below and a b a f t the Wheelhouse. T h e pilothouse should be r o o m y and should afford clear visibility b o t h forward and aft. Blind spots should be eliminated and the stack should be of narrow configuration to allow clear view aft.

Recently, there has been a tendency to increase the over-all length of the deckhouse with the result t h a t the towing b i t t m u s t be located in a m o s t u n f o r t u n a t e position. This m a y affect consider- ably the m a n e u v e r a b i l i t y of the b o a t while towing and the towing b i t t should be located as close to the center of pivot of the b o a t as possible. E x a c t information a n d / o r tests on the best location of the towing b i t t or towing engine, unfortunately, are lacking. Several authorities r e c o m m e n d locat- ing the towing b i t t as far forward as practicable, a recommendation t h a t is all too soon forgotten b y the designer who strains for space in order to give the crew the best possible accommodations in the deckhouse. T h e result is t h a t the towing b i t t is pushed f a r t h e r aft and a larger rudder area is required to obtain the same degree of m a n e u v e r - ability while towing, until the point of diminishing

(12)

1 . 0 . 9 0 . 8 0 . 7 0 . 6 0 .5O

J

q) 0 .,-4 ¢) 0 ~.~ cO j-(

Simpso~s Formula for Beam:

BEAM - H x -

6

Owp

m

KM = Height of Metacenter above base, fe~t H = Draft in Feet ~ , Molded

l

g C ~ i Waterplane Coefficient ib Block Coefficient m x 0 b I

1. i

0 :Z

0,020 0 . 0 2 5 0.030 . . . . L . . . i _ _ L i ._ I I 0 . 0 3 5 0 . o ~ o 0 . o ~ 5 0 . 0 5 0 0.055 i Coefficient I

0.060 O. 065

FIG. 7 R E L A T I O N B E T W E E N Cwp A N D i COEFFICIENTS FOR SIMPSON'S B E A M F O R M U L A

I

o. 070

0.075 I

O. 0 8 0 O~ O~

(13)

143-ft sea • r e s c u e LOA . . . 143-0 L B P . . . 126-8 Beam (molded) . . . 33-0 D e p t h a t side ~ . . . 17-2 D r a f t forward . . . 11-6 D r a f t aft . . . 14-0 D r a f t mean . . . 12-9 Drag of keel . . . 2-6 A bare hull. L T S W . . . 755.98 a total, L T S W . . . 762.93 CB aft ~ . . . 3 . 7 8 CB below L W L . . . 4 . 7 8 Freeboard, least . . . 4-0 Block coefficient . . . 0. 515 Prismatic" coefficient . . . 0. 652 coefficient• .. . . 0. 792 W a t e r p l a n e coefficient . . . 0. 742 C F aft ~ . . . 7.12 L,'B . . . 4.08 B / H . . . 2.59 A/(0.01L) ~ . . . 324 Vk/~C/L, (free running) . . . 1.23 Tons per inch . . . 7.82 M o m e n t to trim 1 in . . . 50.20 T r a n s v e r s e B M . . . 8 . 4 7 G M , light . . . 1.67 G M , loaded . . . 3 . 0 8 W e t t e d surface (total) . . . 4910 Fuel, gal . . . 57732 Fresh water, gal . . . 1389~ Speed, full, k n o t s . . . 14.30 Endurance, full power (miles) . . . 7365 S H P . . . 1500 R P M . . . 160/200 Propeller diameter . . . 9-9 Propeller pitch . . . 7-4 R u d d e r area, sq ft . . . 77.16 Lateral plane area . . . 1520 Per cent r u d d e r to lat. plane . . . 5.08 Per cent balance of r u d d e r . . . 23.69 Design W L a b v base line . . . 14-0 Type of drive . . . D-E Bollard pull, lb . . .

T a b l e 2 C h a r a c t e r i s t i c s Ed. J . Y T B ~ Lack. Grace Helen L.

Moran design R R Moran Tracy

121-10 108-3 105-4 105-0 105-0 108-4 101-0 93-4 93-4 93-4 29-6 28-6 26-0 27-0 27-0 16-0 15-5 14-5 14-9 14-0 12-5 9-6 10-3 10-0 10-0 14-1 13-6 12-3 13-0 13-0 13-3 11-6 11-3 11-6 11-6 1-8 4-0 2-0 3-0 3-0 605.14 405 401.07 399.58 395.00 611.36 412 407.2 405.9 400.32 3 . 4 2 0.50 3.31 4 . 0 8 3 . 7 6 4 . 9 6 4 . 6 5 4.30 4.21 4.14 2-9 3-0 2-2 2-10 2-4 0. 523 0.448 0. 492 0.494 0. 488 0. 644 0. 564 0. 650 , 0. 660 0.653 0. 813 0. 795 0. 758 0. 750 0.745 0. 757 0. 701 0. 783 0. 768 0. 765 7.08 2.71 5.11 3.60 - - 5 . 2 2 3.91 3.53 3 . 8 5 3.70 3.59 2.14 2.19 2.31 2.35 2.35 398 400 403 406 400 1.30 1.27 1.30 1.35 1.22 6.25 4.80 4.72 4 . 7 4 4.78 36.85 24.80 25.25 24.11 24.31 6.67 6.90 6.24 6.87 7.02 1.10 3.20 2.40 3 . 5 0 2.42 3.05 3.60 2.86 3.75 3 . 3 0 4302 3620 3452 3387 3212 50333 15804 24900 33984 26300 6460 4035 1314 1196 2863 13.99 12.75 13 (?) 13.5 12.20 6380 2050 3250 4450 3830 1500 1500 1350 1500 1350 165/200 137/168 144/180 160/200 180/ 10-0 10-9 10-0 10-0 10-0 9-0 8-2 7-8 6-8 7-8 69.9 71.5 70.0 70.7 69.0 1330 1124 1071 1048 1048 5.24 68.0 6 . 5 5 6.74 6.66 27.2 20.0 23.0 23.71 26.05 14-0 13-0 12-3 13-0 13-0

D-E D-E D-E D-E D-E

42000 45,650 @ 1590 shp 46300 - - - - - -

All figures for this boat are estimates since it has not been built as yet.

r e t u r n s is r e a c h e d . I t is d i f f i c u l t t o s a y w h e n t o s t o p i n c r e a s i n g t h e l e n g t h o f t h e d e c k h o u s e , es- p e c i a l l y s i n c e t h e a u t h o r k n o w s o f n o c o m p r e h e n - s i v e t e s t s s h o w i n g t h e l o s s o r g a i n i n m a n e f i v e r - a b i l i t y o b t a i n e d b y m o v i n g t h e t o w i n g b i t t . I t is n o s e c r e t , h o w e v e r , t h a t E u r o p e a n t u g s w i t h s h o r t d e c k h o u s e s a n d t o w i n g h o o k s l o c a t e d v e r y n e a r t h e c e n t e r o f p i v o t o f t h e v e s s e l a r e m u c h e a s i e r t o h a n d l e w h e n t o w i n g a n d m o r e r e s p o n s i v e t o h e l m . O n e r e c o m m e n d a t i o n c o m e s f r o m L . C . N o r g a a r d (21) w h o s t a t e s t h a t t u g s w i t h t o w i n g b i t t s l o c a t e d a b o u t 6 5 p e r c e n t of t h e l e n g t h a f t o f t h e b o w , i n c o n j u n c t i o n w i t h f i n e s t e r n s a n d c u t a w a y f o r e f o o t s , h a v e b e e n p e r f o r m i n g a n d m a n e u v e r i n g v e r y w e l l . A . C a l d w e U (8) r e c o m - m e n d s t h a t t h e t o w i n g p o i n t b e l o c a t e d 5 2 p e r c e n t a f t o f t h e b o w , o r a s n e a r t o t h a t a s p o s s i b l e . I n v i e w o f t h e n e c e s s i t y o f a d e q u a t e a c c o m m o d a - t i o n s f o r t h e c r e w , i t a p p e a r s t h a t t h e b e s t t o w i n g - b i t t l o c a t i o n t h e d e s i g n e r c a n h o p e f o r is s o m e - w h e r e n e a r 6 0 p e r c e n t o f t h e l e n g t h a f t o f t h e b o w , a n d e v e n t h a t m a y n e c e s s i t a t e t h e c u t - b a c k o f t h e h o u s e a n d t h e f o r m a t i o n o f a n o v e r h u n g d e c k - h o u s e t o p . S i n c e a t u g o f t e n c o m e s i n c o n t a c t w i t h b a r g e s a n d v e s s e l s o f a l l t y p e s , a t l e a s t o n e h e a v y g u a r d s h o u l d b e c a r r i e d a l l a r o u n d t h e b o a t a n d a d d i - t i o n a l p r o t e c t i o n , i n t h e f o r m o f a n o t h e r , p a r t i a l , g u a r d s h o u l d b e p r o v i d e d f o r i n t h e f o r w a r d o n e - t h i r d l e n g t h .of t h e t u g . P r a c t i c a l l y a l l t u g b o a t s a r e e q u i p p e d w i t h a t l e a s t o n e h e a v y g u a r d , b u t r e c e n t l y o n s o m e of t h e S a n F r a n c i s c o b o a t s t h e s e s i d e f e n d e r s h a v e b e e n r e p l a c e d w i t h a h e a v y s/~ t o 1-in. s h e e r s t r a k e e x t e n d i n g t h e f u l l l e n g t h of t h e v e s s e l a n d a r o u n d t h e s t e r n . I n c o n j u n c - t i o n w i t h t h i s n o v e l c o n s t r u c t i o n , t h e c o n v e n -

(14)

M O D E R N T U G D E S I G N 375 o f M o d e r n T u g b o a t s

Nancy 100 ft 100 ft Paterson Win. and T

Moran Tams Drake tug Moran

105-0 101-6 100-6 100-10 94-41/~ 93-4 90-0 89-10 88-4 84-2 26-0 25-11/~ 24-0 24-1 25-0 13-6 12-10 12-7~ 13-5 12-1 9-7 8-6 9-0 9-0 7-8 12-0 11-0 9-0 11-0 9-6 10-91/~ 9-9 9-0 10-0 8-7 2-5 2-6 0 2-0 1-10 365.17 295.80 269.6 304.7 263.5 370.67 300.31 274.0 309.4 266.7 3.66 3.08 0.80 2.88 3.08 3.75 3.57 3.47 3.70 3.43 2-5 2-8a/~ 3-51/~ 3-1 3 - 7 ~ 0. 506 0. 485 0. 490 0. 515 0. 527 0. 658 0. 654 0. 650 0. 664 0. 602 0. 771 0. 742 0. 747 0. 775 0,862 0. 803 0. 748 0. 742 0. 777 0. 708 4.64 4.75 2.4 4.84 3.00 1 3.84 3.83 3.99 3.95 3.61 t 2.40 2.58 2.67 2.41 2.89 371 328 314 362 364 1.28 1.32 1.34 1.28 1.32 4.54 4.14 3.81 4.11 3.50 23.30 20.00 19.34 20.36 14.70 6.68 6.71 6.21 6.20 6.16 2.90 2.52 2.78 2.32 2.73 3.46 3.19 3.04 2.58 4.52 3138 2891 2615 2947 2612 25820 22808 24000 13920 25880 3122 2555 2167 600 2400 12.76 12.75 13.10 12.50 12.50 4568 4030 4360 2350 5035 1000 1000 1200 1000 1000 160/200 200 120/150 185/225 8-10 8-10 8-10 9-4 8-6 7-0 7-0 7-0 9-4 6-4 62.7 52.4 55.5 56.6 65.28 968.4 853 798 866 704.6 6.38 6.14 6.96 6.53 9.28 24.64 22.70 23.89 23.60 22.50 12-0 11-0 9-0 11-0 9-6

D-E D-E D D-E D-E

. . . . 32800

M i k e 85-ft J. W. 56- f l

Moran tug Coppage tug

93-0 85-0 64-10 56-0 82-6 74-2 56-8 48-9 23-0 23-0 19-0 16-6 11.1 10-61/~ 8-4 7-4 ~/~ 8-1 7-6 5-9 4-2 9-0 8-6 6-9 6-0 8-51~ 8-0 6-3 5-1 0-11 1-0 1-0 1-10 249.03 203.3 90.99 58.2 252.6 205.9 92.05 59.04 2.84 2.62 2.01 1.88 3.42 3.10 2.33 1.96 2-11/~ 2 - 5 ~ 2-0 1 - 8 ~ 0. 527 0. 538 0. 496 0. 478 0. 660 0. 649 0. 639 0. 617 0. 799 0. 830 0. 777 0. 774 0. 782 0. 747 0. 738 0. 737 3.14 2.88 2.33 3.58 3.82 3.48 3.25 3.21 2.70 2.88 3.04 3.55 371 400 392 396 1.43 3.52 3.15 1.89 1.49 15.1 12.91 5.72 3.57 5.91 6.30 5.79 5.14 2.40 2.28 2.07 2.20 2.95 2.89 2.21 2.44 2508 2135 1267 1028 16581 10965 4768 3340 2139 1000 1374 1242 11.25 1610 750 800 500 330 160/200 321 370 8-3 5-7 5-0 6-4 3-4 3-4 46.1 42.24 22.4 16.4 717 553.4 340 263.4 6.43 7.64 6.59 6.24 24.55 21.60 22.50 22.20 9-6 8-6 6-9 6-0 D-E D D D t i o n a l g u n w a l e b a r h a s b e e n r e p l a c e d w i t h a h e a v y h a l f - p i p e s e c t i o n w e l d e d all a r o u n d t o t h e s h e e r s t r a k e a n d t h e d e c k a t side. T h e o b v i o u s a d - v a n t a g e of t h i s c o m b i n a t i o n of h e a v y s h e e r s t r a k e a n d p i p e is t h a t i t p r e s e n t s a s m o o t h s u r f a c e a n d p r e v e n t s a n y " h a n g i n g - u p " of t h e t o w a n d / o r o t h e r o b s t r u c t i o n s . A t t h e s a m e t i m e , s u c h a c o m b i n a t i o n m a k e s i t m u c h e a s i e r on h a w s e r a n d t o w l i n e w e a r f r o m c h a f i n g on t h e side. F u r t h e r - more, L. C. N o r g a a r d (21) in discussing t h i s t y p e of c o n s t r u c t i o n , s t a t e s t h a t i t h a s l o w e r e d r e p a i r costs c o n s i d e r a b l y b y t h e e l i m i n a t i o n of f e n d e r s a n d g u a r d s .

A g a i n , since t h e t u g i n v a r i a b l y will c o m e in con- t a c t w i t h o t h e r vessels, all d e c k h o u s e s s h o u l d be p l a c e d well i n b o a r d a n d t h e h u l l s h o u l d b e g i v e n s o m e t u m b l e h o m e ~ a t l e a s t a f t of t h e f o r w a r d one- t h i r d l e n g t h . F o l l o w i n g t h e s a m e r e a s o n i n g , t h e b u l w a r k s s h o u l d b e r a k e d i n b o a r d , o r s t e p p e d s o m e w h a t i n b o a r d of t h e shell. T h e s c a n t l i n g s a n d s t r u c t u r e of all t u g b o a t s s h o u l d b e h e a v i e r t h a n t h e A m e r i c a n B u r e a u of S h i p p i n g R u l e s r e q u i r e , k e e p i n g in m i n d t h a t a t u g will be ex- p e c t e d t o r e c e i v e a n d w i t h s t a n d c o n s i d e r a b l e p u n - i s h m e n t d u r i n g its useful life. A b a r k e e l s h o u l d be i n s t a l l e d w h e n e v e r possible, t o m i n i m i z e t h e d a n - g e r t o t h e h u l l f r o m a c c i d e n t a l g r o u n d i n g .

As t o t h e q u a n t i t a t i v e v a l u e s of s c a n t l i n g s , s e v e r a l a u t h o r s , a n d in p a r t i c u l a r A. C a l d w e l l (8) r e c o m m e n d t h e following sizes for t u g s of m o d - e r a t e size: F r a m e s s h o u l d be a b o u t 1/~ in. d e e p e r t h a n r u l e siz'es. T h e f r a m e s p a c i n g s h o u l d b e r e d u c e d b y a p p r o x i m a t e l y 3 in. f r o m t h e f o r w a r d m a c h i n e r y b u l k h e a d t o t h e s t e m . I t is r e c o m m e n d e d t h a t a n i n t e r c o s t a l side s t r i n g e r b e l o c a t e d f r o m t h e f o r w a r d m a c h i n e r y b u l k h e a d t o t h e bow. D e c k b e a m s c a n be of r u l e size. B e a m b r a c k e t s t o f r a m e s s h o u l d be a b o u t 3

(15)

l e

--.___

/

Fro. 8 M O D E R N T U G L I N E s - - D T M B M O D A L No, 4087 j; ! ' \ o ~ 6 • . , 74.-~- a,'~

- - - 7

_/

/

> f

/

FIG. 9 A EUROPEAN-TYPE T u G - - D T M B MODAL NO. 4093

Y,./

/ / ~LWL

S

2~ r.¢3 ¢)

(16)

M O D E R N T U G D E S I G N 377 i " ' \ 0

\

\ \\ ", ~/

\

\ i

/

/=---.. ~-\ \ \

o i "n~t = 0 i

G

~ , ~ 0 o f f l e ~ Z o

~2

times the d e p t h of the b e a m a n d deep knees should be fitted on every third f r a m e forward a n d a f t of the m a c h i n e r y space.

T h e shell plating of a tug of m o d e r a t e size from

the after m a c h i n e r y bulkhead to the stem should be a t least 1~ in. thick. T h e sheer strake should be a t least ~/~ in. thick. T h e r e m a i n d e r of the plating can be of rule size.

T h e gunwale angle should be a t least a 31/~ in. )4 31~ in. X ~ in. angle. Preferably it should be fitted below the deck to allow the stepping-in of the bulwarks.

Deck plating. T h e stringer plate should be a t least 0.35 in. thick all the w a y . T h e r e m a i n d e r of the plating c a n b e of rule size.

Bulwarks should be a p p r o x i m a t e l y 1/~ in. thick and should be fitted a b o u t 6 in. inboard of the shell whenever possible. Their height should be a b o u t 36 in. for larger tugs, b u t as low as s a f e t y permits for small h a r b o r tugs. T h e sheer of the bulwarks forward should be a b o u t 6 to 12 in. more t h a n t h e corresponding deck sheer, while it can be a b o u t 3 in. less t h a n the corresponding deck sheer aft.

T h e floors in the engine r o o m should be a t least 1~ in. thick.

T h e guards should consist of a t least a 4 in. half:round section for the main guard and propor- tionately less for the secondary guards. This requirement depends mainly upon the t y p e of serv- i c e the tug will be required to perform and can

v a r y widely. "

T h e installation of bilge keels is r e c o m m e n d e d for a t least the midship one-third length of the boat.

M o r e and more tugs t o d a y are equipped with formidable fire-fighting equipment:. T h e usual location of the fire monitors is on the top of the pilothouse and either directly forward or a b a f t the pilothouse.

One last word a b o u t accommodations a n d arrangements. I t is obviously i m p o r t a n t to keep the tug t r i m m e d under all loading conditions in such a w a y as to h a v e the propeller submerged a t all times. T h i s means t h a t the designer m u s t give careful consideration to the location of fuel t a n k s and ballast tanks and m u s t m a k e good prelimi- n a r y trim calculations to ensure t h a t the propeller does not come out of the water under a n y loading or.trimconditions. A trim calculation cannot be made, of course, without a good weight estimate. I n the preliminary stages of the design, the figures given below m a y prove of use to the designer.

D. S. Simpson (27) gives the following average weights for preliminary c_alculations for m o d e r a t e sized tugs:

(a) Steel hull weight . . . L B D X 0.003 (b) Deckhouse weight . . . lbd X 0.001 (c) G e a r a n d e q u i p m e n t weight . . . L )< 0.35 (d) Joiner and carpenter work . . . . L B D X 0.001

(17)

©

(18)

M O D E R N T U G D E S I G N 379 m |

f

&N

°!

0

2

r~ O O Z Z

T h e foregoing figures are i n l o n g t o n s a n d t h e s y m b o l s h a v e the' following m e a n i n g : B = b e a m inside g u a r d s , m a x i m u m , ft L = l e n g t h on t h e w a t e r l i n e , f t D = d e p t h a m i d s h i p s (or a t m a x i m u m b e a m section), f t l l e n g t h of d e c k h o u s e , f t b = a v e r a g e b r e a d t h of d e c k h o u s e , f t d = a v e r a g e d e p t h of deckhouse, f t / M r . S i m p s o n a l s o s t a t e s t h a t t h e r e m a i n d e r of the weights d e p e n d s t o a g r e a t e x t e n t u p o n t h e o w n e r ' s r e q u i r e m e n t s a n d c a n n o t b e e s t i m a t e d w i t h o u t some k n o w l e d g e of w h a t t h e m a n n i n g a n d a c c o m m o d a t i o n s are to be, how m u c h a n d w h a t t y p e of p o w e r is to be installed, a n d w h a t t h e r a n g e of t h e b o a t is expected to be.

I n c o n j u n c t i o n w i t h weight, space a n d r a n g e re- q u i r e m e n t s , t h e d a t a g i v e n i n T a b l e 3, e x t r a c t e d from t h e U n i t e d S t a t e s N a v y r e q u i r e m e n t s for t u g b o a t s , m i g h t p r o v e useful in t h e p r e l i m i n a r y stages of t h e design. T A B L E 3 S T O R E S FOR T U G B O A T i t e m L b / m a n / d a y C u f t / t o n Dry provisions . . . 3.25 77 Refrigerated provisions: Freeze . . . 1.16 107 Chill.. . . 2.37 92 Dairy . . . 0.26 120 Total refrigerated . . . 3.79 98.5 Clothing and small stores . . . 0. 146 267 Ship's store and ship's serv-

ice store . . . : . . . 0. 965 169 Special clothing . . . 38.0" 3.25 b Potable water . . . 25.0 c a P o u n d s p e r m a n . b C u b i c f e e t p e r - m a n . e G a l l o n s p e r m a n p e r d a y for t o t a l t i m e f o r r a n g e .

F i n a l l y , A. Caldwell (8) gives some r e l a t i v e w e i g h t d a t a for h u l l o u t f i t t i n g , etc., for t u g b o a t s . T h e s e d a t a are s h o w n i n T a b l e 4. A n y o n e u s i n g this t a b l e s h o u l d r e m e m b e r t h a t t h e y refer to B r i t i s h designs w h i c h differ f r o m c o n t e m p o r a r y A m e r i c a n designs. Therefore, t h e c o m m e n t s re- g a r d i n g T a b l e 1 a p p l y t o t h i s t a b l e as well.

Fig. 11 shows t h e g e n e r a l a r r a n g e m e n t a n d o u t - b o a r d profile of a m o d e r n tug. W h i l e this t y p e of a r r a n g e m e n t is c o m m o n , i t is b y n o m e a n s t y p i c a l a n d m a n y v a r i a t i o n s of t h e same t h e m e c a n b e o b t a i n e d . T A B L E 4 I t e m ~ Classes 1-Ocean . . . 2-Coastwise.. 3-Harbor . . . . 4-River . . .

APPROXIMATE HULL AND OUTFIT WEIGHTS IN PER CENT

Castings Equipment

and and

Steel Wood forgings outfit

70 10 . 10 10

70 10 10 10

75 8 10 7

(19)

MAIN PROPULSION MACHINERY

In years gone by, m a n y successful tugs h a v e had steam reciprocating machinery as their main power of propulsion. T h e main a d v a n t a g e s of such an installation are, of course, rather obvious. With the slow turning s t e a m engine a propeller with a large diameter can be used, the pitch-to- diameter ratio can be close to u n i t y and the fuel burned is generally much cheaper t h a n Diesel oil. One also can control the propeller revolutions from practically zero to full power revolutions and in this way obtain a great degree of maneuverability. However, m a n y disadvantages h a v e forced the steam engine out of the picture and m o s t boats of t o d a y are equipped with Diesel engines. Several authors h a v e c o m m e n t e d on the relative a d v a n - tages of steam and Diesel drives, such as E. F. Moran, Jr. (18), C. D. Roach (24), and P. G. T o m a l i n (30). All of t h e m come out in favor of the Diesel engine and the trend of t o d a y justifies t h e m completely. Some of the disadvantages of the s t e a m engine as c o m p a r e d to the modern Diesel drive are: T h e space t a k e n up b y the boilers; the large crew required to operate the s t e a m power plant; the cost of s t a n d - b y operation; the t i m e necessary to bring up the s t e a m to the prescribed t e m p e r a t u r e and pressure; and last, b u t not least, the high specific fuel consump- tion of the s t e a m power plant.

Diesel engines, with some m e t h o d of connection between the engine and the propeller, are almost universally employed as main propulsion units in American tugboats. F o u r different ways of con- necting the engine to the propeller .~e listed here as representative of modern trends :

(a) Directly connected, reversible Diesel. (b) Nonreversible Diesel with reverse reduction gear drive or torque converter.

(c) Diesel-electric drive (nonreversible Diesel). (d) Nonreversible Diesel with conventional reduction gear and controllable-pitch propeller.

T h e direct-connected, reversible Diesel is clearly an a t t e m p t to incorporate all the a d v a n t a g e s of the steam reciprocating engine into a power plant t h a t would not h a v e the inherent disadvantages of steam. These are slow, h e a v y - d u t y engines and starting is normally done b y air. In a harbor tug power plant, where m a n e u v e r i n g is a prime requirement, a large capacity of air m u s t be pres- ent a t all times, resulting in large and cumbersome air tanks and compressors. An accidental loss of air will m e a n loss of m a n e u v e r a b i l i t y and m i g h t prove disastrous. Accordingly, the United States Coast G u a r d regulations state t h a t sufficient air m u s t be available for twelve starts, with the com- pressor capable of recharging the air t a n k s in 60

rain. This is hardly sufficient today, when m o s t controls of a t u g b o a t are on the bridge and the master, under adverse circumstances, m a y use up all, or very nearly all of the air before realizing t h a t the engines have reached sufficient revolutions for starting. T o overcome this difficulty, m a n y tug owners specify larger air tanks t h a n this with a capacity of as m a n y as 40 starts.

Another serious disadvantage of the direct-connected Diesel engine is its weight, since this slow- turning engine weighs much more t h a n the conventional m o d e r a t e to high-speed Diesel. Finally, some of the directly connected Diesels have a high enough starting r p m so as to s t a r t the b o a t with a jerk, an action which often results in broken lines and hawsers.

T o avoid all the disadvantages of the slow-running Diesel, clutch-operated, nonreversible Diesel drives have been developed. These systems in- elude, in addition to the regular Diesel engine, some kind of mechanical clutch and reversing mechanism plus a conventional reduction gear, or a clutch w i t h o u t a reversing mechanism and a reduction gear incorporating a reversing feature. Several types of clutches with or w i t h o u t reversing features have been developed, the m o s t i m p o r t a n t of which are the Falk Airflex type, the M a y b a e h , the American Blower, the Westinghouse, and the Elliot clutches. T h e last two are electric, the American Blower is hydraulic, the M a y b a c h is mechanical, and the F a l k is operated b y air. Some companies also have developed reverse-reduction gears which, as the n a m e implies, incorporate a reversing feature. D e - L a v a l S t e a m T u r b i n e C o m p a n y has developed the so-called H i n d m a r c h - D e L a v a l reverse reduction gear and Western G e a r Works has developed a similar unit. Several other companies h a v e p r o t o t y p e reverse reduction gears in the making, b u t only the D e L a v a l and Western G e a r units have been actually installed on boats. One of the m o s t serious dis- a d v a n t a g e s of this t y p e of propulsion with a n y of the systems mentioned is t h a t the propeller is not able to absorb all the power developed b y the engine at all times and t h a t a corresponding reduction in engine revolutions would be necessary as the ship's speed is changed from the design speed to a n y other lower speed. Again, if the design speed is some towing speed, the revolutions will have to be k e p t constant f r o m there to free route speed to avoid overspeeding the engine, with a re- sultant serious loss in total t h r u s t available.

T o overcome this serious disadvantage, N a - tional Supply C o m p a n y has developed a torque converter, similar to its industrial t y p e A 342-100 converter, b u t with the addition of a second sta- tor, pump, and turbine unit to incorporate a re-

(20)

M O D E R N T U G D E S I G N 381 versing feature. While this unit is still in the ex-

perimental and testing stages, it promises to incorporate all the a d v a n t a g e s of the Diesel-electric drive without the h e a v y transmissioli-losses asso- ciated with it.

T h e Diesel-electric drive uses the Diesel engine as a generator to produce electric power and a propulsion m o t o r to convert the electric power thus generated b a c k into mechanical power. I t is cus- t o m a r y to use a conventional reduction gear in conjunction with this t y p e of drive, so as to avoid an unusually large and b u l k y electric motor. T h e propulsion m o t o r can be either of the single or d o u b l e - a r m a t u r e t y p e and b y weakening or strengthening the field of the motor, full power ab- sorption at all speeds of the vessel can be realized. This advantage, plus the fact t h a t m a n e u v e r - ability and control of the s y s t e m is excellent under all conditions, h a v e m a d e the Diesel-electric drive one of the m o s t popular ones in modern tugboats~ Reversing is obtained b y reversing the field of the motor, and reversing times of from 2 to 3 sec h a v e been quoted.

Several other advantages of the Diesel-electric drive over the conventional direct-connected Die- sel are listed, see also references (10, 18, 24 and 3 0 ) ; the abilil~y of the power p l a n t t o obtain a b o u t 80 per cent of the Free or towing speeds with only one of two prime movers; the free selection of propeller a n d engine speeds, thus avoiding as m u c h as possible " c o m p r o m i s e " designs; the constant rotation of the engine in one direction; the ability ,of the power plant to furnish large quantities of electricity to other boats and shore installations; the use of electric auxiliaries to s t a r t the engine, to operate the steering engine and 'the possibility of having an a u t o m a t i c towing machine. T h e a d v a n t a g e s of the latter are dis~ cussed at some length in reference (18) b y E. F. Moran, Jr. T h e a u t h o r states t h a t in long-distance towing the a u t o m a t i c electric towing m a - chine has been found to p l a y a m o s t i m p o r t a n t p a r t through its inherent ability to select and m a i n t a i n line tensions and keep the towline length a t the desired scope without constant supervision. These p r i m a r y advantages, the writer continues, h a v e come to attention m a n y times, particularly under adverse weather conditions. T h r o u g h the ability of the machine to p a y out in time of excessive pull and to reeve in when too little pull pro- duces an excessive bite, the wear and tear of the towing cable has been reduced substantially and the life of the cable has been prolonged.

T h e m a i n disadvantages of the Diesel-electric drive is the transmission loss of the system. F o r example, in a conventional installation with the main engine developing, say, 1500 hp, the effi-

ciency of the generator and m o t o r would be ap- proximately 92.5 per cent each so t h a t the total transmission efficiency would be a b o u t 83.7 per cent allowing 2.2 per cent for Voltage drops, in- cidental electrical losses, and so forth. T h u s the shaft horsepower developed would be 1256, a loss of 244 horsepower f r o m B H P to S H P . On the other hand, if a torque converter were used in conjunction with a conventional reduction gear, the slippage losses in the torque converter would a m o u n t to 3 or 4 per cent and the t o t a l transmission efficiency would be a p p r o x i m a t e l y 95 per cent, giving an S H P of 1425 for the same 1500-bhp engine, a net gain of 169 hp, or 11.3 per cent of the total power developed.

A system employing a controllable-pitch propeller consists of a nonreversible high or m e d i u m - speed Diesel engine, a conventional reduction gear and, of course, a controllable-pitch propeller. i Several such propellers are in the m a r k e t today, such as the K A - M E - W A , the Liaaen-Wegner and the Baldwin-Lima-Hamilton, to mention just a few of a still expanding field. T h e a d v a n t a g e s of the controllable-pitch propeller are fairly obvious and would include the significant reduction in weight over the Diesel-electric or reverse-reduction-gear drives and the ability to m a t c h the wheel, to some extent at least, to the main engines at all speeds. However, this, like every other system, has some disadvantages t h a t m u s t be considered and evaluated before a decision as to the method of propulsion for a particular t u g b o a t can be reached. Some of the disadvantages are strictly h y d r o d y n a m i c in nature and will be discussed later on, b u t one serious disadvantage should be mentioned here; namely, complication of control and increased maintenance expenses. I t is true t h a t controllable-pitch propellers h a v e not been on the m a r k e t long enough to say one w a y or another if their complicated control system will re- quire more extensive and expensive m a i n t e n a n c e work than, say, the Diesel-electric system, b u t all indications so f a r - p o i n t towards t h a t conclusion. Be this as it m a y , the a d v a n t a g e s of the controllable-pitch propeller are such t h a t it merits v e r y serious consideration in every design. One also should keep in mind t h a t several controllable- pitch propellers are available t o d a y and t h a t one particular wheel m a y be far superior from b o t h the mechanical and h y d r o d y n a m i c points of view to a n o t h e r propeller, similar in all appear- ances. I n short, it is the b e l i e f o f the author t h a t the whole field of controllable-pitch propellers should be investigated carefully before such a m e t h o d of propulsion is discarded in favor of the more conventional fixed-pitch wheel.