Wood Choice for Ribs and Stringers

[cudashark]

Dear members,
I building a Mahogany runabout using the cold-molded method. Not strip plank.

I have resigned to use mahogany for the ribs because of the issues of epoxy and white oak. I am interested in the fourm opinion for the type of wood to use for the battens and the stringers.

I also am a little confised on "hard spots" In fiberglass and polyester resin plaining hull bottoms I understand that the stringers should not be attached directly to the bottom.

On cold molded wooden boats the opposite is true?

Thanks in advance for the info....


Ray

[PAR]

I have to assume you've altered the scantlings of the structural elements (frame dimensions, spacing, stringer dimensions, spacing, etc.) to accommodate the new material properties. Given this, without knowing what you boat is I can only speculate. Douglas fur, Southern Yellow Pine and mahogany are common choices, but again each is a different weight, density and strength. A light weight runabout may use closely spaced, light materials with a light hull skin, but another boat of same size and shape, may take a completely different approach, such as bigger structural elements on wider centers with a stiff exterior skin. This isn't getting into sheathing schedules as they apply to the structure.

A "hard spot" is created anytime there is a dramatic increase in the stiffness of a piece, such as a 1/4" 'glass skin having a bulkhead directly attached to the inside. In the immediate area around this joint, there is a "stress riser" which can cause this area to fail, under certain conditions, where the surrounding area didn't see enough stress to cause serious harm to the laminate.

This is typical of stiff materials and why foam is placed between a bulkhead and the exterior skin of a 'glass hull (metal hulls can have a similar difficulty). Wood has a natural tendency (some species are much better at it then others) to resist this type of localized stress transmission. The wood cells crush, distort and bend to accept sudden increases in loading. The result may be complete recovery of the affect cells, but most often some denting, crushing and splintering can be seen, even if it is at a cellular level. Also wood can be oriented to accept dramatic increases in compression loads with little damage, because of it's physical properties, which isn't typical of metal or 'glass structures.

[cudashark]

Hello Par good to hear from you.

I find your posts enlightening.

Yes I have re-worked all the scantlings and in my mind decided on Douglas fir but I thought if I just put it out that a new idea for stringers and battens might come up.

Thanks for the info on hard spots. I suspected the same difference.

Thanks again

Ray