hull length and spacing with constrained beam optimization

[srimes]

Hello y'all,

I've been reading about catamarans for some time now and I understand the basic ideas of how hull l/b affects wavemaking drag and sponsoon length and spacing can affect drag. But I'm wondering how they interact with a specified overall beam and displacement.

Say I want to build a fast non-planing catamaran with a maximum beam of 9.5 feet and a displacement of 5000 lb. What lwl should I be looking for? Shorter (say 20 ft) will have less drag due to interferance, but longer will reduce wavemaking in the first place.

Thanks for the insight.

Sean

[Rick Willoughby]

Use Michlet/Codzilla to optimise the hull for the specified constraints and you can do around 10,000 shape variations per second until it finds the optimum. Saves a lot of guessing and weighing opinions.
http://www.cyberiad.net/michlet.htm

The boat you get will be optimised for calm water so you still have the challenge to determine what needs to be done to get the best result in real world conditions. However you have a sound starting point based on data rather than opinions.

If you are designing a sailing boat then you have a more complex problem of determining the displacement of each hull under different points of sailing. Upwind and reaching the limit condition is a monohull with all displacement carried on one hull.

Rick W

[srimes]

thanks. I downloaded it and now I just need to learn how to use it. Does it work for multihulls? I'm looking at a powerboat so the complications of sailing aren't an issue.

[Rick Willoughby]

It is good for power boats as you can assume level trim and equally loaded hulls. It works for at least 5 hulls. You need to choose one of the cat hull in.mlt files and adjust parameters to suit your boat constraints.

Post #25 on this thread will help you get started:
http://www.boatdesign.net/forums/showthread.php?t=19110&page=2&highlight=godzilla

The program is very powerful mathematically but it is intended for knowledgeable users so the front end simply consists of setting values in the in.mlt file. Very fast and easy once you know how to do it and it does not have the gigabytes of rubbish most other software bloats out with.

Rick