Distance between bulkheads

I´m browsing some of the plywood/stringer/fiberglass/epoxy build blogs and designs and it got me thinking about structurally safe distance between the bulkheads. I really like the Easy Catamarans. Seems to be proven designs. An example of the bulkhead setup can bee seen here:

http://diy-yachts.com/forum/viewtopic.php?f=23&t=1011&start=10

Then I came across a much larger catamaran, the DH550

http://www.dixdesign.com/dh550 build1.htm

I would think both designs to be of excellent standards but the DH550 is a lot larger with many fewer bulkheads. In addition I see no mention of fiberglass over the plywood at any point.

So the question is essentially what impact it will have structural integrity to move the bulkheads from 4' to 5' or even 8' apart?

Also I was under the impression that a layer of fiberglass cloth was crucial for a plywood/epoxy construction. Would I be wrong on that point?

 

Impossible to answer. You can engineer a boat to have no bulkheads.

 

The resistance of a bottom panel depends, among other things, on the separation of transverse and longitudinal elements that limit the panel. The cross members do not need to be bulkheads, can be simple frames. So if you put appropriately separated frames, bulkheads can be located anywhere. Another very different matter is the need of watertight compartments along the length of the boat.

 

For most sheet ply designs the glass is not structural, and is fairly light, but does add a protective layer to the ply and helps stop checking etc. Having had moored ply boats with and without glass sheathing, I prefer with.

Glass is an essential element in some tortured ply designs where it adds stiffness/strength to the (necessarily thin) hull skins.

The Dix design is a pretty amazing result. There wouldn't be many boats that size built in ply these days.

 

The distance between stiffeners is dictated by the required panel stiffness for a design state which is a function of wave height, vessel speed and vessel displacement.

The stiffeners can take a form of stringers, frames, bulkheads etc whatever stiffens the hull panel to keep deflections within acceptable limits. Designers accomplish this is different ways. For catamarans tho, the interior furniture and arrangement means that all sorts of things can be utilised in order to stiffen the hull panels as everything is basically adjacent the hull panel with a central walkway, so we often see cupboards and shelves acting as stiffeners, doorways and room partitions etc... So with the interior arrangement in mind, the designer can see at what spacing the stiffeners will be located, and design the hull panel accordingly, which is mainly driven by hull panel thickness of course.

Another consideration of how it's all achieved is the ease of which its built, specially for home built designs. It's easy to cut and setup closely spaced frames and bulkheads on a strongback then apply the hull sheeting over it to get a fair hull, so it's about the build method not just the design...

 

It depends if each BHD is either a simple partition BHD or an actual WTB.

If a simple partition type BHD, then it is a matter of what that BHD is being used for other than separating 2 voids. For example, is there a cross beam being attached to it..or a chain plate or a mast etc etc. If there is no other structure that is "using" the BHDs localised stiffness/strength then you can place them where you like, its your choice.

If they are proper WTBs, then that comes down to legislation. Is your boat being built to any acceptably known standard such as ISO or other small craft code? If so, then the distance or rather minimum and hence maximum distance apart is dictated by the damage stability requirements.

Thus is it a BHD or a WTB?

Just because something is not shown, does not mean it is not there. Do you see the resin type or layup schedule on those websites...no. Thus, doesn't mean there is not one. Never assume because you can't see what it is your seeking is not shown.

 

Actually the biggest loads are usually not from waves but from fenders when side-tied with waves pushing the boat onto them. As happened to me a couple of weeks ago when a cold front went through the Greek harbour I was in

I could feel the hull side of the FP Athena 38 flexing, and no, I didn't sleep much that night

Otherwise I agree with groper. Boats are stiffened locally by the interior furniture, the boat as a whole is held together by beams and full width bulkheads. It's never a good idea to rely solely on deck/hull mouldings to do either job

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I hope to explain well and what I say does not seem nonsense: good webs are sufficient to support the hull and deck. What is important is to create a continuous ring in cross section. Transverse bulkheads, however, are needed to support the racking on the ship considered as a beam. These bulkheads, in general, need not be as strong as watertight bulkheads or tanks bulkheads. The damaged ship stability may also affect the amount of transverse bulkheads required.
It is true that some boats have to be scantlings not by the height of the waves but by the hard working conditions they endure.