how far apart should my bulk heads be

[mattoc]

hi im currentylr designing a 33ftt foot catamaran and have looked all over the net for infomation regardeing how far apart my bulkheads should be, but i keep getting diffirnt awnsers on similar size cats.

my cat hulls will be fibre-glassed, with 9mm ply and four 2"/4" beams, two of which will reach accross the whole width of the cat (bow and stern) the other two will reach the inside of each hull. my bulk heads will reach across the inside width of the cat (one hull to the other) these are 1"/4" planks

I will have four stringers on each outer parts of the hulls, my choice of timber is dougless fir everything wood will be resined and screwed,

I have talked to designers and 50% say the ply hulls are of the form and the fibre galss is for the strenth so it makes very little differance bettween 900mm apart and 1400mm apart. the other 50% say buy my sample plans and go from there.

any help will be alot of help.
thanks

[Eric Sponberg]

Mattoc,

This is really a matter of engineering the geometry of the structure for the materials and the design at hand. This is what naval architects do. You can have whatever bulkhead spacing you want--you just have to engineer the size of the stringers, and their spacing, along with the thickness of the hull to achieve that bulkhead placement. Keep going around, refining the results, until you get a size of structure, thickness of hull, and size and spacing of stiffeners that is an acceptable weight, not too intrusive on the interior, and is easy to build. It is a complicated process, but it can be figured out. I know this does not give you a precise answer, and that is because there isn't one. For as many different designs as there are, there are just as many correct answers.

You should look into some engineering books for more guidance. The book "Fiberglass Boat Design and Construction" by Robert Scott, and available through the Society of Naval Architects and Marine Engineers in the US, is a classic textbook on fiberglass boat structures.

Eric

[FAST FRED]

If there to assist in keeping a holed boat afloat ,

there are specific rulles for spacing water tight bulkheads that consider the trim when flooded.

FAST FRED

[rxcomposite]

There should at least be a steering bulkhead, engine room bulkhead, and a collision bulkhead. If the boat is large, there is an additional bulkhead in the middle for damage stability.

[safewalrus]

Never mind all the technical stuff; what do you want to put into the hulls? At the size your building as long as you put a couple in to hold the shape (a watertight one close to the front as a collision bulkhead is good) you'll get away with that!

However if you want to be all technical the guys above have given a reasonable answer but basically it's up to you - do your thing man it's your boat!

[waikikin]

Foolish answer mr Walrus, when it comes to connective stuctures on multis get some pro advice.Rgards from Jeff.

[safewalrus]

More minimal structures eh? thet will get you killed for sure!

Actually I have a better system, I leave well alone, multis are for mad young things with no sense or money! Still if thats your poison go for it! Pray answer me one question as your so clever? What sort of computers did the polynesians use when they built their multi hulls, after all it would be impossible to build a proa just by lashing two sticks at right angles to the log hull then lashing another log to that? must have been some tremendous strains in those lashings - what was the mathamatical equation used to work it all out? And pray how did they work out how to get the mast to just the right height? not to tall that it fell over or too small that it couldn't carry the exact sized sail made out of perfect coconut fibre matting woven by highly trained seamstress who under stood ever nuance of the thread and could make a mat that would just take the mathamatically formulated exact wind that neither gusted nor blew to hard or to soft!

Please tell us we're all dying to know

[longliner45]

what kind of wood are you going to use for construction?I have a plywood sailboat the glass is a barriar,the strenght comes fromribs every 3ft and 1by4stringers every 5 inches a backbone made of alaskan yellow ceder 10 by 10 inches a keelson made of white oak and a white oak keel. also 3300lb of lead held on with 10 1 inch ss bolts.structural wood is very important.

[mattoc]

thank you all so very much for your information I have taken all your responses into consideration , thank you once again and happy SAFE sailing