Aluminum Thwarts

I obtained a 70 vintage 18.5 race boat. Moore viper. I replced the rotted wooden gunnels with aluminum, aluminum twharts and plastic endcaps. The question is, I dotn htink I attached the thwarts in the best manner to maximized hull stiffness. the boat still twist. It has a single lenghtwise center rib. The guy at a local canoe show told me to set the thwarts up without postive or negative pressure, and use one rivet to the underside of the gunnel. I had considered two but was talked out of it. I noticed one new boat at the shop that has the ends of the aluminum thwarts bent downwards 90 degrees and riveted to the to the inside of the rail, and through the hull and out thorugh the outer rail instead of to the underside of the rail as I did. Im considering that method…or using aluminum riveted L brackets holding bolted wooden thwarts (kits ar sold, but I cna make my own). How do manufacturers mount aluminum thwarts? is slight outward pressure used?

We have a '73 Moore Voyageur, 18.5’
but I’m sure the layup is much heavier than what they would have used for a Viper. Our boat started with the unusual split tube aluminum over pegs system that Moore used for boats headed for the general public. That system was strong enough in itself, but was not that stiff, so the relative stiffness of the Voyageur hull must have been due to the layup.

I’m facing a gunwale job because the wood pegs, over which the split aluminum tubes were forced, rotted, and the gunwales popped off. I will probably have to use wood gunwales because I don’t have the ability to bend aluminum gunwales to accomodate the Voyageur’s upturned ends.

Hope you find solutions for your Viper. There are very few Moores still around.

interesting. The viper weighs 62 pounds. I never considered that I might need to make a allowance for the layup being relatively light, for its day. Im thinking of dismissing my previous attempt to use one rivet to the underside of the gunnel and go with a 90 degree downward bend at the end and two rivet shot outward. I now am thinking that was what I saw on a skin coat kevlar boat recently. What say you, slight outward pressure? say moving the hull outward less than 1/2 inch on each side?

When I took out the aluminum thwarts,
I believe I recall their putting a slight outward pressure on the gunwales. The ends of the thwarts were crushed flat and bent down about 45 degrees, and then they were riveted to the inner/underside of the split T6061 tubing used for the gunwales.