[sss]

As far as I know, HDPE can't be bonded together aside from using fasteners.
I need an inch of thickness to replace the rotted plywood in a 1950's Lone Star Corsair. It's a very light aluminum semi-v boat, brochure says 150#. Transom is 15" height, with the strength section being 9"x48", 1" thick.
I have some 1/2" HDPE sheet that I can double up. If I were to thru-bolt it to the aluminum sheet of the transom, would it have enough rigidity to take the force from a 20HP motor weighing around 100#?
Bonding with glue or epoxy isn't possible. But in order for 2 stacked sheets to flex, there would need to slippage between them. If there are bolts installed in precisely drilled holes(not oversized), the lateral movement between the two sheets would be eliminated unless the holes elongate or the fasteners shear.
I'm thinking 24-5/16" bolts, fender washers on each side, evenly spaced along the transom board.
Waste of time or give it a shot?

 

[DuckNut]

You can do whatever you want with your time.

That plan would never be recommended by anyone I know, but it is your boat.

Your motor clamps will not grab, so make sure it is through bolted.

 

[Capnredfish]

You asked if it would have enough rigidity. No. That material does not have rigidity. Almost anything it’s used for requires bracing under it. Even boat hatches 24 inches+/- wide have an aluminum brace under it.
Two pieces of homedepot plywood epoxied together and sealed would probably give that 1950’s craft another 30 years. HDPE has some good uses, but not as a transom material. Could you use it? Sure. Mythbusters made a boat out of duct tape.

 

[Smackdaddy53]

Wouldn’t be a light hull any more if you used that heavy stuff for the transom. I second the double 3/4” plywood transom.

 

[sss]

Thanks for the advice, I’ll scrap that idea.
Marine plywood is hard to find around here. Does it go by another name? I know not to use pressure treated due to corrosion issues.

 

[DuckNut]

I assume you are only needing to make a T or something similar that bolts to the aluminum.

If you used exterior ply and did nothing you would easily get 5+ years. For that old of boat I would probably go this route so i can inspect it every 5 years for othe issues. I think it would take longer to get the beer cold than it would to cut a couple pieces of wood and bolt them on.

 

[anytide]

any exterior plywood (CDX) will have petro based glue in it.
C side/ D side exterior grade
maybe go with BC ply for better finish.

 

[sss]

Thanks Anytide!

 

[Smackdaddy53]

A couple a sheets of regular 3/4” plywood with wood glue between and brass screws in a 6” grid then a couple coats of polyurethane will last a lifetime if you make sure to seal any holes you drill through it before running any fasteners through.

 

[sss]

Just checked out things at Lowe’s after church. They have 24x48 sheets of select plywood. They seem to be better quality than what you find for exterior use. I think I would have less chance of having a void inside. Which is more appropriate, maple, fir, or oak for the transom?

 

[anytide]

yes less voids/ checks/ knots and patches.
if your glassing it then it doesnt matter the species..
marine ply is made with better complete plies/ more bi -directional plies/ straighter grains and more adhesion quality.

if you can find a length of "microllam" LVL / para-lam or any other laminated/ engineered wood beam material your work is half done.
good luck.

 

[devrep]

you need a plywood with water resistant glue (not moisture resistant like standard exterior plywood). completely encapsulated in epoxy and bonded together with epoxy paste. most generic plywood doesn't have this. it's out there, do a little research. any holes drilled thru the wood should be over-drilled, filled with epoxy and re-drilled to size to keep moisture from seeping into the wood. whatever you do don't

use P.T. against aluminum. ask me how I know.

 

[Capnredfish]

Devrep. Little late for the sponge!

 

[DuckNut]

you need a plywood with water resistant glue (not moisture resistant like standard exterior plywood). completely encapsulated in epoxy and bonded together with epoxy paste. most generic plywood doesn't have this. it's out there, do a little research. any holes drilled thru the wood should be over-drilled, filled with epoxy and re-drilled to size to keep moisture from seeping into the wood. whatever you do don't View attachment 75772 View attachment 75774 use P.T. against aluminum. ask me how I know.

Why did you let your grandkid use the hull to sight in his .22?

 

[devrep]

Devrep. Little late for the sponge!

no, we fixed it.

 

[devrep]

 

[sss]

Anytide, thanks for the advice. I can’t find any of the engineered wood products you mentioned locally. I believe I will go with 2 layers of the 1/2” oak plywood bonded together, then skinned with a layer of .125 6061 on the inside of the transom.
In the past, I have used thinned resin mixed with extra hardener to seal plywood when I needed it to be waterproof. I’ve never used epoxy except for flies and repairing small objects so I need to do some research to understand what devrep is suggesting. Nice boat btw.
Thanks for the help.

 

[sss]

 

[Capnredfish]

Nice job!

 

[sss]

Smackdaddy53, liking your idea for the brass screws holding the sheets together with something like Titebond3 in between. I’d want to prime and paint it white before I seal it with urethane. Need to see what paints work well with urethane. Thanks for your input.