I really like what I see, but my question is: Is compression of the Coosa dependent on the fiberglass outer surfaces strength?
I put some plain Coosa in a vise and it compressed about 1/3 thickness.
Would I want to do anything special to the areas that an outboard motor is bolted through the transom to keep the Coosa from compressing?
This doesn’t directly address your question, but may be helpful. I am very interested in your questions. Depending on how compressive it might be, could lead me to think about fabricating a bent metal 3 side transom saver to cover inside , top, and outside. I liked your video and had seen parts of it, but had not seen the whole thing before this one you listed. I have to look at a different video I saw quite some time ago to answer one question that arose for me when watching. I will reply again when I find that information.
Here is one of the other videos that helps explain my question regarding the video in your link. I saw where they decided to use epoxy resin instead of polyester resin because they wanted more adhesive strength. I am ok with that thinking. However, they show their layup starting with Chopped Strand Mat CSM because they wanted more tooth to bite to... likely more tooth than just a 24 to 50 grit grinding sandpaper wheel would provide on the original fiberglass skin. The problem I was remembering was that it has been shown on many videos that CSM has. Styrene binder that will Chemically dissolve when using polyester resin, but will NOT dissolve when using epoxy resin. When they then layer with 1708, it uses thread to bind the glass and is supposed to work well with epoxy resin. I had seen a different ship shape TV video that showed them only using 1708 against original transom skin and transom board putting the CSM side against transom skin using the same wet epoxy resin on skin, on board to smooth side of 1708... then thickened epoxy over wet epoxy on skin and then install transom board that already had the 1708 on it with the CSM side facing transom skin and clamp. I will try to find that video as well. I am truly not a fiberglass expert nor do I run a repair shop. I have just been trying to learn everything I can over the last couple of years so that I can do my best the first time around. I really would also like to learn from the experts and please correct me if I am wrong or have misinterpreted things. I am a high horse power guy and want the strongest transoms I can make. That big boat appeared to have twin outboards. Don’t know their size, but if it were my boat, I would be laminating another layer and adding some knees. I see big rigs taking a pounding on big water and would want piece of mind. In that first video, I wondered why they wouldn’t have wanted to use a straight 2x4 on edge on the inside top for all clamps to ensure a straight edge rather than risk a bow with the weight of the clamps hanging and trying to pull it out. I could see it on an arched G3, but wondered about a straight transom.
When I did the Lake n Sea Transom I used Seacast and was told to build a inner and outer "skin" of 1708 using 6 layers for each, 1/4" + in & 1/4"+ out then filled with composite. In video 1, Andy uses 3 layers of 1708 on each side of the 1/2" Coosa adding 1/4" to the thickness. So perhaps using Coosa for a 1-1/2" composite transom; 4 layers 1708 - 1/2" coosa - 4 layers 1708 - 1/2" coosa - 4 layers 1708 (12 layers 1708 = 1/2" + 1/2" Coosa + 1/2" Coosa) and use the epoxy. 1 outer layer to have extended 1708 to assist in tabbing in to the hull. Just a thought.....
My understanding is that the hard surface of the fiberglass helps to spread the compressive load. But I'm willing to bet that the cups on transom screws ( like a Mk 75 has ) would crush right through the stuff. You would need a transom plate of some kind if using old thumb screw mounting. And if I was using Coosa my transom bolts would have a decent sized sandwich plate behind them to keep them from pulling through. Fiberglass will deflect, crack and tear if the substrate isn't hard enough
As for using epoxy with CSM - you are correct, and I have also seen people make videos of CSM being used with Epoxy. I certainly wouldn't recommend doing that! And FWIW - I like vinylester resin. Less expensive than epoxy - but strong. Tests show it is almost as good as epoxy for bond strength. And it works with mat.
So I bit .....trying the Coosa for a 1964 Custom Craft Sunray. I spoke to my local supplier and they set me (our local ACBS Chapter) up as resto shop to allow some favorable price points and they have free delivery. Nice for the Coosa and they roll fiberglass cloth, not folded and stuffed in a box as I have received from others. (side note, I had used Seacast on the '62 Lake n' Sea but a different situation. Videos on this site and Lake n' Sea site)
There is 1.5" between the outer transom skin and the engine splash well so using 2 - 3/4" pieces would not have worked after a few layers of 1708. We decided to get 1 sheet of Bluewater 26 in 1/2" and then build out the outer sides to fit snug.
Pic 1 barn find
Pic 2 old trailer made it home
Pic 3 removing floor and wet foam
Pic 4 drying out
Pic 5 rotten transom removed
Pic 6 pounds of wet nasty floor, foam, transom and glass
Pic 7 cardboard then more rigid foam template made
Pic 8 trial fit and figuring out how to swing 2 layers up in there
Pic 9 coosa cut
Pic 10 yes, there was a thin transom plate, and some additional trim to the left and right that covered an OPEN seam. perfect area for water to roll round the fins and drop in to soak the wood! a little extra glass and gel coat would have been easy at the time?
Probably should move this over to member projects from here. More to follow. My supplier agrees with Bruce. Vinylester would be fine for the 1708 lamination but for adhering the laminated boards to an unknown boat hull composition we'll go with epoxy. Epoxy is all encompassing over poly or vinylester, I'm told you can NOT use poly or vinyl over epoxy.
A little more progress on the Coosa transom in the '64 Sunray
Pic 19 pattern trace onto 1708 and cut
Pic 20 layers of 1708 ready to laminate with vinylester on Coosa
Pic 21 outer panel had enough swing to get up behind motor well but inner layer would have to be done in two sections
Pic 22 cross section of sliced inner section showing 1/4" of 1708 on 1/2" Coosa
Pic 23 outer single piece adhered to outer transom skin
Pic 24 bolts and boards used to pull tight along with clamps on the upper section
Epoxy was used to adhere the laminated board to the outer transom skin. This will be left to cure for a few days. Bolts that were used to pull the laminated board tight with the outer skin along the bottom were wrapped in painters taped so they could be easily knocked out once cured.