pre-determined draft and displacement

[Shallowatereds]

I'm curious about a design issue.

Let's say a designer is laying out a hull, and he/she has a pretty good idea of what the hull bottom is going to look like, and where the waterline is supposed to be, and how the topsides and interior are all going to look, and mechanicals and so on...

What if his displacement doesn't support his designed water line?

In simpler terms, lets say he designs a hull to draft 4', but in the end, he's at 2'. Does the designer add ballast or redesign the hull bottom?

[Eric Sponberg]

It is time to go around the design spiral again.

A good designer will be able to estimate fairly closely what the hull shape should be to support the weights that are intended to go into the hull so that the differences on the first go-around are fairly small and manageable. What you describe is a pretty gross error. So you have to compromise somewhere. It may be a combination of both reducing interior volume and hull size, reducing weight, and adding ballast. You should try to stay within the intended design ratios in order to guarantee performance--displ/length ratio, Sail Area/displ ratio, SA/wetted surface ratio, etc. So for example, if you decide to greatly increase ballast, you are making the boat much heavier and displ/length ratio does up while SA/Displ ratio goes down. Performance suffers.

By continuing around the design spiral, making changes that gradually come into agreement, you will eventually get to the design solution and a good performing boat.

Eric

[Shallowatereds]

Great answer, Eric. Thank you very much.

And my example was just pulled out of the air, it wasn't intended to be a real situation where a designer was off by 2'.

[tom28571]

I agree with Eric, sort of. I think a design should usually start with a displacement range and a draft range. These should be fairly tight. Displacement and draft are so important that they should not be a fallout of the design process but a primary parameter.

If the desire is to find out what displacement and draft would be for a given hull shape, then you go the other way. I suspect most beginners start this way with a mental (or actual) picture of the boat in their head but is is not the best engineering practice.

Of course the design spiral will eventually take care of starting at a less optimum point but you can eliminate some bad choices and disappointment earlier the way I advocate.

[Shallowatereds]

So, you start with an idea for a hull with, say, a 4' draft and a displacement of 60,000lbs, and then design around that?

[PAR]

Most designers can visualize pretty accurately what shape and it's resulting displacement and draft will be, before committing to actual conventions. I'm a target displacement kind of guy (I have a range of displacement and draft I'm aiming for), but there are other ways too. In the same vain, most can also visualize a sail plan that fits the boat and it's related centers pretty accurately too. It's a feel thing, you get after a while.

This is why beginning designers shouldn't venture far into the unknown, rather sticking with familiar shapes and well tested concepts. The best advise a budding designer can take to heart, is to study known designs, understand their traits, why they display them, possibly how to adjust them to suit different needs, etc. The preverbal "blank sheet of paper" just doesn't exist, except in the far reaches of racing craft or specialty vessels, both of which require full understanding of what's involved.

A radically new concept, for a dramatic racer or naval warship is romantic, but way outside the realm of a novice designer. The same is true of moderate and larger size yachts. Your version of a dory (for example), after studying the type carefully, is the best test, especially if it performs as well (maybe better) then others of similar type and scale. As your confidence grows, so can your ambitions, eventually you'll get to a point where you'll see this 40'er in your head, know pretty much how much displacement it'll need, draft, Hp requirements, etc. The actual calculations will just confirm your thoughts, maybe with a surprise or two along the way (but they will not be big or unexpected surprises).

[Shallowatereds]

Thanks again. Good stuff.