Redoing a Starcraft Islander transom using Coosa board.

rolmops · Saturday at 5:30 PM

This summer I had my boat in a slip instead of always towing it and I love it. At the same time it made me anxious about humidity and wood rotting. When I installed the current transom it was in the middle of covid and I was taking care of my wife ( she passed away). The work was not properly done and now I want to redo it the right way so I can feel safe.
The material I want to use is a 1.5 inch 4 by 8 foot sheet of coosa board.
I have never used it before but everybody seems to love it.
I need to know what to expect as in how easy is it to cut, does it need to be painted, will it hold screws? Does it go together with aluminum? Will the fiberglass chew up my saws and drills? Will I need special personal protection gear because of fiber dust?
Redoing a transom is basicly a simple but time consuming project and it does not scare me.
Please give me your opinion and tips so a simple transom change does not become a terrible job because coosa is not the right stuff.

MikeSchinlaub · Saturday at 6:50 PM

3/4 is something like $400 per sheet, I can't imagine a 1.5in sheet. Or do you mean total laminate thickness? Anyway, some answers.

rolmops said:
The material I want to use is a 1.5 inch 4 by 8 foot sheet of coosa board.
I have never used it before but everybody seems to love it.

I think it's over priced and over rated.

rolmops said:
I need to know what to expect as in how easy is it to cut,

It's very easy to cut, a jigsaw will fly through it.

rolmops said:
does it need to be painted

It needs to be glassed between each piece, and on both sides of the finished transom. You can gel the engine bay to finish like factory.

rolmops said:
, will it hold screws?

I think it was @Redneck joeiwho found a post on another forum where a guy did some tests. His tests showed good holding strength, but having worked with it myself several times, I have doubts. I think any holding power will come more from the glass layers than the coosa.

rolmops said:
Does it go together with aluminum?

I'm not sure what you mean by this. It will have no issues with aluminum, but your starcraft is fiberglass right?

rolmops said:
Will the fiberglass chew up my saws and drills?

Not really.

rolmops said:
Will I need special personal protection gear because of fiber dust?

Same as you would use for working with fiberglass. Gloves, glasses, dust mask, tyvek suit, and respirator. I really recommend a full face respirator too, both for grinding and glassing with the vapor cartridges.

jbcurt00 · Saturday at 8:24 PM

MikeSchinlaub said:
It needs to be glassed between each piece, and on both sides of the finished transom. You can gel the engine bay to finish like factory......

...... It will have no issues with aluminum, but your starcraft is fiberglass right?.

nope its an aluminum boat,

Scott Danforth · Sunday at 5:09 AM

I personally wouldn't worry about it in an aluminum boat

MikeSchinlaub · Sunday at 6:03 AM

jbcurt00 said:
nope its an aluminum boat,

Ok, you kept mentioning fiberglass so that's what I thought.

Everything else still applies though. Coosa is like a stiff foam board with some sparse fiberglass strands in it. I don't trust it because I can snap it pretty easily with my hands. If you decide to use it, I would recommend laminating more layers of thinner board with csm.

rolmops · Sunday at 7:55 AM

Thank you for your answer, you certainly gave me food for thought.
My Starcraft Islander is aluminum.
A 1.5 inch 4by8 26 grade is $582 before shipping , it weighs 120 pounds, but according to AI gluing 2 3/4 inch pieces together makes a stronger board. However, my boat has an I/O so transom strength is not as crucial as with an outboard.

MikeSchinlaub · Sunday at 11:39 AM

rolmops said:
However, my boat has an I/O so transom strength is not as crucial as with an outboard.

It still takes a lot of torque from turning. How wide is the transom on that, full width or just in the center? I ask because around the time I joined I was working a Cobalt with a small center coosa transom.

airshot · Sunday at 1:09 PM

That I/O transom needs lots of strength !! I would stick with plywood if it were me and forget the foods board or you might be replacing it again. Not sure how they justify the price on that flimsy material. If you do the plywood correctly, you will never have to do it again in your life time and probably not in your children's lifetime... My Islanders transom was all original from 1983, and just replaced two years back by the new owner. Just think how long a properly sealed plywood transom will last ?

Scott Danforth · Sunday at 2:35 PM

Coosa is a core material, not a structural material

It requires fiberglass on both sides

rolmops · Sunday at 4:02 PM

The Islander is 8 feet wide on the top and it is (if I remember correctly) a bit over 3 feet deep at its center.
The marine 3/4 inch plywood is 10 or 12 ply so the 1.5 inch will give at least 20 ply. I guess I'll go that way. I was under the impression that Coosa does not need any treatment thereby justifying the high price.
I also wonder about how to drill the holes in the transom where the bolts going to the outdrive will be attached. I always just drilled at 90 degrees into the transom, but now I wonder whether to do that or drill them perfectly level. Any advice there?

MikeSchinlaub · Sunday at 4:22 PM

There is a plate that is used to drill the holes for the outer unit and the pilot holes to hole saw the steering cut outs. Really important to get all of those lined up and drilled straight.

Pmt133 · Sunday at 5:35 PM

I made a jig from a wood block to drill the mounting holes on mine. Ran it on the drill press with the right sized bit then drilled.

airshot · 2025-11-24T10:23:37-0700

Scott Danforth said:
Coosa is a core material, not a structural material

It requires fiberglass on both sides

Maby just me, but why would anyone want to put fiberglass in an aluminum boat ?

MikeSchinlaub · 2025-11-24T10:57:50-0700

airshot said:
Maby just me, but why would anyone want to put fiberglass in an aluminum boat ?

The glass goes on the coosa before install. It needs it for strength. As a core material, it doesn't have much strength on its own, it all comes from the glass. That's why you want multiple layers of thinner coosa laminated with glass. The real advantage is that it won't rot.

Of course, when we get factory warranty work, the factory usually send something like you described, a single thick sheet with a layer of glass on each side. With the engines they put on those things though, I don't think it's enough.

Plywood on the other hand, is a structural material. It provides the strength, amd the glass is mostly for waterproofing, and in a fiberglass boat it also ties the transom to the rest of the structure.

Redoing a Starcraft Islander transom using Coosa board.

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