This is a free app for designing ship hulls, and also allows for some basic testing:

www.delftship.net/

They also have a Pro version that is only €178.50 and has many more testing options...I think it even has a wave tank option!

You can also download tons of pre-built models from their website, including barges, heavy-lifters, and some other styles that might be of interest to seasteaders.  There is also a Yahoo group groups.yahoo.com/group/Freeship_HTandT_Group/ that has a bunch of models and tutorials.

I haven't played with it yet, but it's free so it can't hurt.  The only problem is the free version only allows the creation of symetric hulls so I don't think spars are possible unless you get the Pro version.

Happy modeling!