# Water lines and levels in a boat

Discussion in 'Boat Design' started by timwithoutaboat, Aug 30, 2008.

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### timwithoutaboatNew Member

How does a boat designer know how a boat will sit in the water? ie where the water line will be and level surfaces inside the boat? Is it always parallell with the bottom of the keel?

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### the1muchhippie dreams

its ALOT of math,, weight, displacement,loa, beam,, i think they have like a 2 year class on it at alot of colleges,,hehe
i hope you lke #'s,,, cause them "smat" people will answer soon,,,,, and they'll give you all kinds of #'s to play with,,hehe

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### timwithoutaboatNew Member

I'm not afraid of #'s and as it's now 2am here I think i'll go and count some sheep. see ya.

4. ### Guest625101138Previous Member

In a static condition the line between the centre of gravity and centre of buoyancy will be vertical. Also the weight of the boat when in air will be equivalent to the weigh of water displaced to support the hull.

The density of saltwater is 1025kg/Cu.m. So a boat weighing a tonne will displace a bit less than 1Cu.m.

Not many boats have a dead straight keel so the notion of the waterline being parallel to the keel does not mean much. For sure a larger ship will have a straight keel line and it will normally have static trim with keel level.

So the first thing is to estimate the boat/ship weight. You then consider a hull shape for the intended use and the dimensions are adjusted until the desired displacement volume is achieved. This can be an iterative process because the boat weight will depend to some degree on the size of the hull. After you have settled on the hull you move the weight around until the centre of gravity is directly above or below the centre of buoyancy when correctly trimmed to waterline. If you get the calculations wrong then the boat will not sit with incorrect trim. It will roll and pitch to shift the centre of buoyancy until it aligns with the centre of gravity. This is why you store cargo in set locations such that trim is correct.

There is readily available computer software to assist with the above. It still requires a good deal of effort to get it right but the software eliminates the number crunching. Google Delftship, register and download the free version if you want to try something.

It does pay to work through the above in some simple way like just a box shape so you understand what the software is doing. It is easy to determine the volume of a box.

All the above relates to the static conditions. It will apply to a hull sitting or moving slowly. As speed increases there are dynamic forces that can alter trim. A planing hull is at the extreme end of the scale from static. Its attitude is controlled primarily by dynamic forces. This is a whole other area of study.

Rick W.

5. ### lazeyjackGuest

Thank you R W

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### the1muchhippie dreams

ya same here,,,,Thanks Rick

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### Wynand NRetired Steelboatbuilder

and this also explains why some keels are more forward or aft than others relative to the LWL of hull. It is the heaviest and easiest thing to move around to get LCB and LCG aligned vertically.
BTW, the hull "pivot" point longitudinally is not at the LCB/LCG, but at the LCF (longitudinal centre of flotation) which is the centre of the waterline area, and is usually a little bit aft of the LCB.

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