Original transom material

[afterhours]

Quick question. What is the original transome core material on a 70's era potter hull 20sf? Marine plywood? Balsa?

I've seen many threads and photos about redoing the transom, but I've never run across a mention of the original core material. The photos of the deteriorated wood suggest some type of chipboard. The reason I ask, is I'm going to redo the transom, raise it 5" and hang a 2001 Honda 90hp 4s on the back. I'm leaning toward using marine ply because of my limited budget, but I'm a little concerned about weight. Considering spending a little more money on material if the final weight with motor will get me closer to the original motor package and thus similar CG and weight distribution. Seeing as I'm only putting 90hp on there, I'd like to try to get as close to designed CG as possible to improve performance.

Also, switching out the Honda for a lighter motor is a non-starter. The Honda and a doner are free from my brother. I bought this hull to fit the motor and get me offshore 15-20 miles in the Gulf and back again to see my family with some fish for dinner.

Any input is appreciated. One I begin the rebuild I'll start a thread and post photos of the progress.

[caper]

Plywood

[linesider36]

Definitely plywood.

[Eric B]

I recently had a transom re-build on my 23 Sceptre at a professional shop. They used Coosa Board instead of plywood. Very strong and light and non permeable. turned out great!

Eric B

[afterhours]

So I looked up some prices for marine ply versus coosa and the divinycell, etc. Looks like the budget dictates I'm going back with marine ply. I guess I'll just have to start exercising to make up for the heavier motor.

[Bushwacker]

Check out Okume plywood . . . more/thinner plies, but much stronger and lighter than conventional plywood! Not sure about cost but Skip's using it on his 21 and I was impressed with how light it is. If well sealed with epoxy, it would be a big improvement over conventional plywood!

[flyingfrizzle]

Most people just use Douglas Fir marine grade plywood for transoms and bulk heads. A lot of people use the Fir in transoms due to lower cost and it is stiffer. Douglas fir is stiffer than meranti and meranti is stiffer than okoume. (modulus of elasticity: fir = 1,560,000 to 1,950,000, meranti = 1,500,000 to 1,630,000, okoume = 1,140,000) Okoume and Meranti can be a better choice for marine applications but at a higher cost. Each has its place. On the down side Okoume is less resistant to rot than fir or meranti. Meranti is less resistant to rot than fir so the Fir would be the best far as rot resistance. Okoume and Meranti have a much better finish that wont check with less flaws. Doing a transom the ply wont be seen so it's not a big deal but places that are seen Okoume or Meranti is much easier to do a finish quality surface on. My Master Angler hull has a Okoume transom in it (3 layers of aquateck) and it was not sealed up well at the top. Two years in the weather it was rotted away near completely. Seems like the Fir transom weather the water much better but if it correctly sealed up with epoxy it dosent matter and you should be fine no matter what you use. I have done transoms with ac grade exterior pine and had them last but they were sealed 100%.

[shine]

On my 23 I used fir for the transom and OKoume on some bulk heads, and meranti for the sole. At the time I had access to a warehouse full of marine plywood and I chose what I like best for each application. Okoume and Meranti finish very well with just a light cloth, fir needs more glass to keep it from printing through (or checking). I prefer fir for a transom, it has great properties, it get covered with plenty of glass (no concern with printing), and it happens to be about half the cost of Okoume.

[afterhours]

Thanks Shine and Frizzle for the input. I think I'll go with Fir ply when I get around to redoing the transom this winter.

[FAS]

shine is spot on,go marine ply for strength,figure how much glass to use to reinforce the transom to be tough as you need it to be.Frizzle has the correct spects.
make it at least 2.25" thick,you will be happy....