Now with regards to plywood:
As I am currently also rebuilding my Starcraft transom I have gone through all of those options myself. Where I landed as it seems that you have is that good ol' plywood is going to be the most affordable, least time consuming and easiest to work with. Something to be aware of is that there are strong opinions out there on this. One fella on here even used old hockey sticks to replace the transom wood! So at the end of the day you will need to decide for yourself. All of that said I'll try to throw some information out that may help.
With regards to Plywood choice I would try shopping with a local lumber yard that specializes in LUMBER ONLY. The kind of place a contractor would order wood from if he were building a house. These will usually be found in industrial areas so you may have to drive a good distance to find a decent lumber yard. I would recommend you search them online and call them up. Once you get a salesperson on the phone you just mention that you are looking for something that will be similar to marine grade ply but not treated for exterior use. I highly recommend you contact them in some way ahead of time because they won't necessarily have everything in stock. As I was shopping for plywood I found that marine ply is actually difficult to find. Most of the lumber yards I contacted had a similar product that they kept in stock that they recommended.
The advantages I found shopping with a lumber yard:
- They offer more products than home depot does.
- I found that I could get a far better quality grade of plywood for not much more money than Home Depot.
- I have done a lot of carpentry work and I can tell you from experience that the wood from home depot isn't the best. They have affordable wood, and the most commonly needed cuts, but I tend to shop elsewhere if I need quality wood for structural or aesthetic reasons.
- A lumber yard will be more knowledgeable on the products that they carry and will be able to help you choose the best option.
To boil the opinions on marine ply down for you:
- Some say marine ply is the best, or only option. One of the arguments being that you want it to last as long as possible.
- Others say that marine ply is too expensive, and hard to find. They will also argue that a high quality ply (with fewest voids possible) with some water resistance should be sufficient, and that you should be able to rely on the epoxy to do a majority of the work. Just remember to be wary of some plywood that is treated for exterior use.
If you decide to go with anything of lesser grade than marine ply just make sure to do a very good job with your epoxy work. I'm sure that this goes without saying but make sure that you get the edges of the transom wood and inside any bolt holes that are drilled or holes for through hull fittings and such. As far as a specific plywood product I can recommend for you? I think you are better off contacting your local industrial lumber yards and seeing what they have available, and pricing.
Now, for an interesting wrinkle that Starcraft Chieftan and Islander owners will be aware of; the transom has no "skin" on the inside. As you mentioned above, there is only the aluminum sheet on the exterior of the hull and the two sheets of plywood on the inside. The reason this is significant is that any time you take on water that transom wood may get wet as there is nothing at all separating it from the bilge area. So where other boat owners will have their transom wood completely encased, ours will be exposed to any moisture in the bilge area. All the more reason to keep a good strong bilge pump and trailer it with the plug out!
I hope this helps! Don't forget to let us know what you decide!
Ten years ago I seriously messed up the transom 77 Chieftain 22’
Where I messed up a couple times.My orginale plywood transom appeared to be 2” thick
It mite have been swollen. I believe Starcraft didn’t use ¾” plywood I think it was 3 layers of 5/8” which is 1 7/8.
The flange this plywood sits in was a tad smaller than 2 “ BTW orginal transom was in pretty good shape it was painted on exposed inside.
- I actuely had to grind down the thickness around the perimeter of wood to get it to slide into the flanges.this grinded down around the perimeter never got sealed now is flaking off.
2.The lumber seller didn’t have ½” marine grade plywood. Sold me ½ regular I thought well I’m sealing it anyway wronge.
3.glued all 3 plywoods together ¾” x 2 an the ½”in middle w/ construction glue
, lumber place say it was waterproof wronge. look close you can see the 1/2" middle piece the marine is in better shape but is delaminating
- sealed w/ cheap urethane from home depot wronge.
- I had to cut the transom skin off the vessel to remove the transom. left the star side half exposed used 5200 on the top of this star side wronge.
I sealed top of port side w/ 5200 an fabbed a new aluminum transome skin this port side isn’t rotton.
The small section ontop of star side is where the ½”ply got rot. See the allen key stuck down about 4 “ so it’s not totally rotton. The exposed ¾” ply is in pretty good shape however there are spots where I can stick a screwdriver into maybe a few layers of ply
See when I left the top transom skin off this plywood was exposed to rain.
I thought so what is been covered in urathan an top was coated w/ 5200 wronge.
Again port side is unaffected by any rot. Where the outboard engine mount bolts are seam dry.
.
I need to redo the transom before next cruise. again w/ ¾” x2 marine plywood an I have a aluminum scafold
to reinforce the 1 1/2" plywood plus an overkill kneebrace.
Sealed w/ the proper sealant this time in addition to the urathan I used cheap fiberglass resin, this vessel is exposed to all weather so she's been rained on for ten years didn't help matters next home will have a carport cover.
https://www.amazon.com/.../B00.../ref=ox_sc_saved_title_4...
all this to illistrate how not to do a transom, I guess I got a little lucky she lasted 10 years instead of 30.
edit: Starcraft used stainless steel though hull screws to hold the transom wood into the flanges.
I think if I do find a decent sheet of marine plywood the 2 sheets would be stronger w/ the screws left in there being sealed.
I may have to take a road trip to the coast to get decent marine grade. the coosa board is looking to be the best choice.
Thanks for any tips
