Wooden boat floors strength

Hi,

Could you please explain me which forces are applied to the floors of the wooden boat? I don't know if it's ever possible to calculate exact numbers, but I'd like to understand the principle.

In particular, I wonder what happens when the boat heels. As I see it, it's mainly the hull form, not the floors stiffness that keeps ballast keel from swinging. But there are scantling rules for the floors height (e.g. "floors height shall not be less than N inches for this boat") - what stands behind these rules?

The reason I'm asking is that I'm building a wooden boat (George Buehler design) and used metal floors (made from L-bar, the same height as wooden ones, almost according to the scantling rules). I have noticed that frame/keel connection is not very rigid, due to the elasticity of the floors (L-bar bends and I'm able to roll the frame athwartships with my hands). This might be OK, or might be not - I have never seen the boat being built before... And I don't think that planking will help much here - the distance between the rabbet and floors is small, so planking/floor/keel will not form a rigid triangle to resist bending/rolling much.

It's a crucial isssue for me now (do I need to re-make it?), so I will very appreciate your comments.

Best regards,
Andrey

 

Is that a Hagar keel?

The floors are a very important structural element and is typical of most timber frame construction types. There are scantling rules covering this, but their application can become quite involved per boat. General sizes and dimensions can be estimated easily, for particular hull types, but loading can become laborious to calculate.

George uses very stiff floor structures and their marriage to the frames should make the structure quite stiff.

When you move the frames side to side, is the keel twisting with the movement or is it truly flex in the metal floors? You look to be using 3" angle on top of the keel with 2" flat stock, up the planking? The flat stock will have little resistance to fore and aft loading without a flange. Generally these flanges (like on the angle stock) will face the greater opening side, which means the "L" will face aft, in the forward sections and forward in the aft sections, permitting easier access then if mounted the other way.

In answer to your concerns, yes, these things need to be quite firm and a solid marriage to the keel. Anything less and the fasteners will tear loose as the boat twists, pants and bellows underway.

In George's designs, if three floors are bridged (like a mast step would) and securely fastened, this should be strong enough to lift the completed boat off her cradle (plus some). This is an idea of the loads they will be expected to carry.

 

No, this is Juno keel.

The keel is 10"x10", and it certainly does not twist when I push frames! This is a flex in the metal floors (not in the keel-floor or the floor-frame fasteners, but in the floors themself).

The floors are made from 4"x6" angle, 1/3" thick (90x140x8 mm). "Wings" are welded on top of it - 4" or 100 mm high, the same thickness. It gives the total height of the floor of about 10" (250 mm). Frames are fastened with 4 1/2" bolts per side. Each floor is bolted down with a pair of 1" keel bolts.

Yes, I can roll frames fore and aft easily, but I don't think that longitudal stifness will be such a problem...

Yes, I did it as you described!

Does it mean that one shall not be able to shift the top of the frame 1/2" sideways by applying the force of 10-20 kilograms (pushing near the sheer, which makes a 6' lever)?

 

Sorry about the mix up. I've been on a couple of Juno's.

If your welded up floor assembly is flexing (athwartship) you can put a flange on top of the "wings", maybe over the angle stock too. This would add compression strength to the assembly. You could also box it in to provide the additional stiffness necessary.

If memory serves me, the floors were spec'ed at 4" stock, approximately 10" deep (solid or laminate) which is a very stiff section of lumber. Metal floors (keels and frames also) are a great way to save weight, but the materials need to have similar stiffness (as well as other physical properties) for the substitution(s) to be effective.