My 1975 SeaCraft 25' Seafari rebuild thread

[flyingfrizzle]

Originally Posted by Blue_Heron

Looking good! How thick are you going to make the inner laminate skin?
Dave

Thanks, The original inner skin looks like it only had one layer of a thicker woven roven with csm under that against the wood. It was barely an eighth inch thick if that thick, probably less. I was surprised just how thin it was for the large transoms is in theses 25's. The outer hull skin is a very thick 3/8" plus but that was the only thick part. The two wood layers were tacked together with nails every 6" or so with no glass between them at all. I will be going back a lot thicker. I know the outboard bracket will need more support than the outdrives did. I may build up about a 1/4" thick of glass on the inside once it is in place. I will start out with a layer of 1708 with 4" ears to lap onto the hull sides and bottom for tabbing. Then two layers of 1700 for thickness on just the core then a second 1708 with 6" ears for tabs then 2 more 1700 layers then a final 1708 with 8" ears for tabbing. That should give me the thickness I need and three over lapping tabs to the hull. I will also come back and add some additional tabbing strips around the edges for additional support. I will be pulling some nice flits in the corners as well to help the tabbing radius as it rolls off of the core. All said and done it will be a lot more than the thin original factory inside layer plus have the 1/8"- 3/16" of glass between the plys as well. I may be over doing it a bit but that is one large 7'x4' span of transom especially with the bracket and outboards hanging on it. I will also add some knees to the transom down to the inner stringers plus when I build the bracket I will try to align the stringers in it to match up with the ones in the hull to the knees so that it all supports off one another.

[Bushwacker]

Originally Posted by flyingfrizzle

. . . I know the outboard bracket will need more support than the outdrives did. I may build up about a 1/4" thick of glass on the inside once it is in place. . . . I will also add some knees to the transom down to the inner stringers plus when I build the bracket I will try to align the stringers in it to match up with the ones in the hull to the knees so that it all supports off one another.

A bracket will evenly distribute motor loads across the entire transom, so you'll actually have lower concentrated loads and stresses on your rig than you'd have on a conventional outboard or outdrive transom mount!

I think the knees and 1/4" inside thickness are overkill, but I agree that lapping the cloth around into the hull bottom and sides is a good idea. That glass between the plies may add a bit of compressive strength but it adds NO bending strength to the transom because it's at the neutral axis. (When you bend an I-beam, the compressive and tensile stresses are directly proportional to the distance from the neutral axis, which is at the center of a symmetrical part, so there is NO bending stress at the neutral axis. A cored panel is similar to an I-beam, so the max bending stresses are at the outer surface. This is why cored panels can be so stiff and strong with relatively weak and lightweight core materials like foam. As long as you use epoxy or vinylester resin for good bond strength, that transom will be stiff as a brick!

FYI, Don Herman uses lots of Coosa in his brackets, so if you want some for a future project, it might be worth sending him a PM. He might be able to sell you just one piece for less than the retail you'd probably pay at at most places.