CHAPTER V - CONTROLLING DRAFTS
CONTROLLING DRAFTS
When you start making your calculations for a particular voyage it is most important to know where to start i.e. where your controlling draft is.
Sometimes this is quite easy.
1. Say, you are loading at Tubarao for Rotterdam when it is summer in the Northern Hemisphere. Draft available at Tubarao is 22m, at Rotterdam 23m, your summer draft is 17.52 meters - simple you can load in Tubarao up to the summer mark assuming Tubarao is in the Summer Zone - if it is in the Tropical Zone you will be able to load up to Summer Loadline plus the consumption in the Tropical zone till you reached the Summer Zone.
2. Say, you are in Tubarao, loading for Rotterdam, when it is winter in the Northern Hemisphere. - you can load such that when you enter the Winter Zone off
Cabo Finistere you are at the Winter Mark of 17.16 metres.
Your controlling draft would therefore be 17.16 metres when entering the Winter Zone
& you would have to start your calculations from that point. It would then work out that your sailing draft from Tubarao would be Winter Draft + consumption from Tubarao to Finistere. When calculating I would start at Finistere and then work backwards to Tubarao
3. Say, you are loading in Tubarao, then discharging at a port where the allowed draft is 16.00 metres - simple - your controlling draft would be at the disport - you would base your cargo to arrive even keel at the disport with 16.00 metres draft after calculating what your fuel etc. ROB would be at the disport.
4. You are loading at Newcastle, Australia, where max. draft is 15.50m, your summer draft is 17.52 metres, for Rotterdam where max draft is 23.0m. Simple your controlling draft is Newcastle & you would start your calculations to sail out of Newcastle even
keel, then work for arrival Rotterdam, taking ballast to avoid arriving down by the head.
5. You are on a Panamax size where you are required to load as follows -
Disport 2 is Eregli, Turkey where max draft is 11.5 metres. Your summer draft is 13.25 metres. Loading at Saldanha Bay, South Africa. Disport 1 is Constanza, Romania. You have to load for Constanza all you can, that is possible, assuming you are loading maximum for Eregli. Max draft in Constanza 17m. All ports & the entire route in the summer zone. Max draft in Saldanha Bay 20m.
In this case the initial controlling draft is Eregli. You would have to start your calculations based on first arriving at Eregli with an even keel draft of 11.50 metres after calculating what your fuel etc. ROB would be at Eregli..
Then work your fuel consumption backwards to sail out of Saldanha Bay, at the summer loadline - you would then work out the cargo you can load for Constanza, & calculate the trim you would have to sail out of Saldanha Bay with, such that after fuel consumption en route, & discharging in Constanza you would arrive at Eregli with the even keel draft of 11.50 metres - your arrival draft & trim at Constanza in this case is not really a problem.
6. You are loading on the Cape-size, summer draft = 17.52m, at Tubarao, where max.draft is 23m, for Pohang where max. draft is 17.40metres. Consumption of fuel en route causes a rise of 15cm. Both ports & route in the summer zone.
Here even tho the draft available in Tubarao is much higher than in Pohang, the departure draft in Tubarao is the controlling draft. You are permitted to load up to your summer mark of 17.52 metres & no more - when you do that in Tubarao - your consumption en route to Pohang cause a rise of 15 cms - thus your draft arrival Pohang would be 17.52 - 0.15 = 17.37m. Of course you would have to adjust your trim departure Tubarao, to arrive, after the fuel consumption, even keel at Pohang.
7. Cape-size situation - summer draft = 17.52 metres - all ports & route in summer zone.
1st loading port - Sept Isles, Canada - iron ore lumps. Max draft at Seven Islands less than summer draft.
2nd loading port Saldanha Bay, South Africa. - iron ore pellets. max draft 20m.
1st discharge port Kimitsu, Japan - discharge the Saldanha Bay cargo. Max draft 20m 2nd discharge port Nagoya, Japan - limiting draft, reqd to disch the Sept Isles cargo.
Required to load max cargo for Nagoya - no bunkering en route.
This was a situation on SILC (actually the cargo was iron ore concentrates - 2 types).
Now, in this calculation, you would have to start with Nagoya, after calculating what your arrival Nagoya bunkers etc.would be then work the fuel etc. consumption backwards to S.Isles, via Kimitsu & Saldanha Bay, & then see whether with the max.
Nagoya cargo on board, you were within the limiting draft in S.Isles -it was - but if it were not then the limiting draft would be at S.Isles - in this case it was at Nagoya - then after calculating your weights at S.Isles, work your fuel consumption to Saldanha Bay - on basis of that work your figures to sail out of Saldanha Bay at your summer draft, then discharge the Saldanha Bay cargo at Kimitsu, then after consumption of fuel Kimitsu, to arrive at the max. draft at Nagoya.
Please remember there will be probably be density changes which you have to take into account.
Also vessel must be within allowable stress limits at all times.
This page intentionally left blank.