Coordinate systems for sailboats

[ClarkT]

For your cad models of sailboat hulls, where do you place your origin, and why?

For V hulled boats you have the transom/keel intersection, or pick a chine at the transom. But the transom on a sailboat just seems wrong to me. Anybody have a clever scheme? How does it save you work?

regards...

[Eric Sponberg]

ClarkT,

In the olden days before computers, the fore/aft origin was at the front end of the design waterline. The back end was at the rudder post to hull intersection, usually. This length on the waterline was divided into 10 or 20 stations for easy Simpson's Rule calculations of volume, center of buoyancy, and other curves of form.

With the advent of computers, we don't have to bother with Simpson's Rule because the computer can calculate volumes and centers, etc. arbitrarily, to any draft, heel, and trim. I have found, therefore, that the most convenient zero reference is the upper forward peak of the bow.

Sometimes, on powerboat designs, I will use the upper aft peak of the transom as the zero reference, because many design factors are figured ahead of the transom.

In the end, it really does not make any difference, just so long as you are consistent within the design. Pick a reference and stick to it.

Note also that in the US, the convention is to make the profile of your hull face to the right, whereas in Europe very often the hull and outboard profile are drawn facing to the left.

A good way to judge the fairness and attractivenes of your hull lines is to reverse the profile view, and see how the hull looks pointing in the opposite direction. If it looks good, then it is good, and you have an attractive design.

Eric