Good to see you guys still making progress. It’s pretty hard getting motivated to grind Fibreglass everyday! 37mm Thermilite/ coosa would definitely be better then wood especially with all the holes for the engine bracket but wood will last a long long time … I might’ve doubled up the sheets of 37mm coosa.?? Can’t remember but I know I made mine too thick for the stern drive to fit thru. Have to rout a recess around it to fit it in but you won’t have that problem with the outboard bracket you got
Wow! I had no idea that Coosa board had gone up that much in price. I would do two things in that case. One, I would engineer the transom so that it would be easier to access twenty five years from now for repairs. Then I would be very very thorough about encapsulating the plywood. Isolating the two panels with several layers of glass in between so that even if one rotted a bit it would not go through to the other preserving structural integrity. Two, I would be extremely careful to not drill into that transom whatsoever. This can be accomplished by carefully laying out where needed holes are to be then drilling holes three or four (more wouldn't hurt) times the size needed. Fill these with epoxy bonding it well to the surrounding area, make it solid. Then when the final holes are drilled there is no wood in contact with the fasteners to wick moisture and cause rot. Be very anal about penetrations even the smallest ones can cause problems so make sure they all are "potted". If done meticulously your new transom should be good for several decades. This way would be orders of magnitude less expensive than the Coosa but much more work of course. In any case guarding against penetrations wicking water is crucial. Even after the areas have been 'potted' use good quality marine sealants in every hole. Gougin Brothers West System Epoxy has quite a lot of good literature in pamphlets showing their experimentation over the years in dealing with these kind of situations. Another option is to use the polyester resin as you are but for all the transom work to switch to epoxy resin and appropriate cloth. This will be more expensive but less than using Coosa. When doing your plywood spend extra time to thin (as recommended) and coat with resin (epoxy hopefully but polyester will benefit as well) soaking in as much as it will take. This is after it is cut to size. Then lay cloth on edges (difficult but doable) and sides before applying to the transom and glassing further then. This is to encapsulate the wood preventing any pin holes or 'holidays' from allowing water to attack the wood. Keep in mind a heavy lay up schedule of glass will stiffen the transom keeping any minute movement to a minimum and is that flexing which can cause hairline failures and water intrusion. So the use of knees and stiffeners as much as possible is invaluable as well. Boats were being built long before Coosa was around. You could also layup heavy glass panels in the place of Coosa which is easy to do when horizontal then cut and install just like any panel. Another option is to lay up a thick transom and then use stiffeners glassed to the inner surface much like the stringers to the hull. The deck intersection with the transom is another area that provides a great deal of stiffening and strength. Look around your area and find wholesalers for glass supplies. Buying in bulk can save quite a bit compared to buying one gallon amounts. A large roll is cheaper than several small purchases. Good luck in your project.
Thanks for the great info! Yeah Coosa is very expensive in our local market. Not being in the USA cost here goes way up due to import taxes. I will look online to see if I can import some myself.
For the transom, you could lay up a solid glass transom, obviously over many days so the curing heat doesn't get out of control. There is a boatbuilder in Australia that does solid glass transom. I believe there company is called Whitepointer. Use alternating layers of 1708 and roving woven cloth. Should build bulk quickly.
Good to see you back looking forward to the next episode she is going to be a solid boat. if you use plywood in the transim and seal it well it can last a very long time sometimes the cheaper option is a better option 👍
PS: As an alternative to Coosa check out Dyna Core products. They make cored fiberglass paneling in many configurations for many different uses. Don't know their prices but they may be much more reasonable.
