1973 Thunderbird stringers

I am replacing the rotted out stringers in my 1973 Thunderbird 190c and can't figure out what type of wood it was. What should I use to replace the stringers with.

Re: 1973 Thunderbird stringers

You do not need any timber, just a form (shape) to layup over. Timber was used in early boat construction because it was cheap and readily available.

They were also made with standard wax / poly resins for the most part. Problem is this is not 100% waterproof, and over time water will pass through into the timber and it rots.

When repairing these older craft you might like to consider using an epoxy resin as it bonds better to old poly resins than new poly resin. using a standard resin the mechanical bond relies on the keying the roughened surface, a single bond mechanism; an epoxy has a mechanical and a secondary chemical bond and can be 30 to 40% stronger.

Also note that an epoxy repair is lighter and stronger than standard resin pound for pound. Standard resin to Chopped Strand Mat ratio is close 3:1, that's 3 parts resin to one part CSM; the ratio for epoxy resin and cloth (Not CSM) is closer to 1:1.
Using standard CSM with an epoxy resin is a huge waste and not at all strong. The binder in standard CSM will not dissolve and the voids ratio is way too high resulting in a weaker and brittle repair / construction.

When replacing ribs we use a closed cell foam with well rounded tops and a fillet on both sides made with micro-fibres or closed cell balloons mixed with epoxy resin.

I hope this helps.

Avagoodweekend......

Re: 1973 Thunderbird stringers

I would use exterior grade Douglas Fir plywood. Not sure of the thickness, I would probably use two 3/4 inch pieces laminated together. Use what is in there as a guide. I used pressure treated for my stringers, resin will stick to it as long as it is DRY. Less than 15% moisture content. If you use pt ply you should also use stainless steel fasteners.

Hope this helps,

Have a great day

Rob.

Re: 1973 Thunderbird stringers

old salt: The tops of my stringers appear to bear the weight of the sole because the fiberglass on the tops looked all compressed. Is this normal? The other thing is is my stringers are 15" in height can I still use the closed cell form to use as my core? Also if I understand correctly I would create core out of the foam than bond it to the hull with an epoxy fillet and then encapsulate the foam core with fiberglass cloth (biaxial?). Is this right? Would I then bond my new sole to the tops of the stringers using epoxy or use fasteners like the screws that were there before?
Thanks.
Mark

Re: 1973 Thunderbird stringers

Rob: My stringers seem very thin 5/8". If I used plywood should I laminate two thin pieces 1/2 & 1/4 or would I use a single sheet of 5/8"? How do I determine the moisture content? What type joint is best to get the required 16' of length that I need?
Thanks.
Mark