Re: Please somebody help me. Simple question.

DELFTship forum Hull modeling Please somebody help me. Simple question. Re: Please somebody help me. Simple question.

John R. Coil

Nothing wrong with your English. I just misunderstood what you meant when you mentioned control points.

As for the other I may be able to help you a tiny bit.

There are a number of different ways to calculate resistance and some seem to rely on on the assumption that your hull isn’t too different than the series of examples on which they are based. Somehow, and I’m not saying I know how, these schemes come up with a type of hull that’s “typical” and if your hull is indeed similar then they do a pretty good job.

But if your hull is not similar, as indeed my relatively high prismatic coefficient but still balanced sailboats (no “power stern” or anything like that) I’ve been puttering around with seem to be dissimilar, then the results can be squirrelly. The Delft series included with the free version of Delft Ship just doesn’t behave rationally with my models: looking more like a roller coaster than a resistance curve.

It seems likely to me that FreeShip just uses an entirely different type of resistance calculation. Either one based on a different “typical” hull or one that actually tries to calculate resistance using some of the available equations and, likely as not, a healthy dose of assuming stuff.