Adding extra chines to existing design

[unseen wombat]

Hi guys. I'm thinking of building the Payson/Bolger instant catboat, but I wonder if I can add some extra chines to it to give it a more round-bottom look. I made this hull file in the Carlson Chine Hull Designer, (file here) and tried to keep the length and beam widths the same, but added 3 chines to it.

Can any designer tell me if this is a bad thing to do? Am I causing myself problems that I don't know about? The plywood was twisted already in the original design, so I think making the strips narrower should make it easier to spring them to the bulkheads, right?

Also, I was thinking of removing the side decks and putting in benches to make it more comfortable for passengers. I expect to have two other adults and my little daughter ride with me. Is this boat too small for that you think? Will removing the side decks make it more likely to take on water when heeling?

If I did want to make the boat a little bigger, (like about 2 ft) would I have to change the size of the sail?

[alan white]

In answer to your first question, I would absolutely not change the hull sections---- so many dynamics change that you cannot (without the help of a good designer) predict the outcome. It's better to simply buy a set of multi-chine plans to begin with.
Side decks: A cat boat has side decks for both structure and safety and any catboat is going to have plenty of room anyway because they are so wide.
Yes, you can lengthen the boat. It's commonly done and no, the sail needn't be enlarged. You do have to respace stations and also redraw (stretch) stem and transom. The mast will also need to be moved if the sail isn't lengthened along with the hull. The position would be determined by the CE of the sail, which remains in the same location.
Sounds like you bought the wrong plans. Thjere are so many plans out there. Are you sure you don't want to simply look around a little more?

[alan white]

I checked out the boat---- the design derived from the Beetle cat. It is intended to be a trainer and safe (if wet) little boat for kids, and while a couple of adults can sail the boat, adding more people isn't the best use of the boat. You are much better off with a 15 ft or thereabouts design.
The Beetle cat, and all catboats for that matter, are abnormally wide relative to length for the reason that their sailing grounds are the area south of Cape Cod around Martha's vinyard and Nantucket where shifting sands create soft shoals in shallow water, so it's impossible to map depths within a few feet. Boats historically went aground all the time and a boat was needed that got its stability through great beam and not a deep ballast keel.
My feeling is that due to the quirky sailing characteristics of Cape Cod style catboats (strong weather helm, over-sized sail, weight of a mast in the eyes of the boat, and generally tepid performance compared to more narrow designs), the type is best suited for the exact kind of area that it was developed for. I see real Beetle cats up here in Maine, but really, it is an affectation of Summer rich people from the Massachusetts area who are trying to appear correct for their own type, having forgotton the more practical reasons for owning a wide and shoal cat.

[PAR]

It appears you've moved displacement around sufficiently enough to render all of Bolger's calculations useless. There are plenty of multi chine cat boats to choose from. A lapstrake version could be converted easily enough.

[unseen wombat]

Haha, oh well. I figured it might mess up the dynamics and stuff, but I thought that maybe it could be done. At least I didn't waste too much time on it.

I haven't actually bought the plans. I borrowed the book from the library and took the beam widths and the LOA from that and tried to just make something similar.

My main problem now is that the boat is too small. I really want to be able to have a couple people in there and my daughter for fun day trips. I think maybe the Glen L 15, but it's traditional construction and I had really warmed up to the idea of stitch & glue because of its lightness.

I also like the super skipjack, a whole lot, but it's some weird construction style and I'm not even sure I can figure out. But lots of people have built weekenders, so it must not be too hard.

And PAR recommended the Corsair 13, which is a good one, maybe still a couple feet too small though. The windsprint looks pretty good, and easy too.

I still don't know.

One question, when you're changing the sail rig on a boat, is lining up the CE above the CLR the only thing you have to do? Or are there other things that need to be taken into consideration? I read something once about the mast and spars having to flex.

[alan white]

There is no end to the variables that make for a good sailboat design. The relationship between CE and CLR is just one of many considerations. Mast and spar flex is not a critical variable in most designs. If they didn't flex at all, most designs would only improve.
Get yourself a copy of Dave Gerr's "Nature of Boats" and find out about what makes for a successful design. There's more usable information packed ibnto that book than any five other books on the subject.