Transom Thickness for MerCruiser Sterndrive

[TinMan007]

Hi, I have a aluminum 1979, AMF Crestline '19 Norseman with MerCruiser 140 with a Mr (pre Alpha 1) drive and need to replace the plywood transom. The existing plywood is 1" spanning the entire transom with a smaller 3/4" piece (total plywood transom thickness 1 3/4"). There is no epoxy or fiberglass adding to the thickness. My marine mechanic believes the transom should be 2" thick. Given the boat is 42 years old, it's unlikely the plywood I'm replacing is original so it's anyone's guess what was originally spec'd for the boat. Prior to my mechanic's feedback, I purchased Coosa board (instead of plywood) to match the plywood thickness. My question is, is 1 3/4" thickness of Coosa sufficient for this application? Any help is deeply appreciated.

 

[missinginaction]

So you're going to laminate that COOSA to get it to 1.75 inches? If the old 1" ply was sufficient that COOSA will be even more so. Based on my experience 1.75" of COOSA will be stronger than the aluminum hull itself.

 

[gonzo]

The range for the transom is 2" to 2.25" according to the Mercruiser manual.
Boat Construction; Transom Thickness And Surface - Mercury Alpha Installation Manual [Page 10] | ManualsLib https://www.manualslib.com/manual/797538/Mercury-Alpha.html?page=10

 

[TinMan007]

So you're going to laminate that COOSA to get it to 1.75 inches? If the old 1" ply was sufficient that COOSA will be even more so. Based on my experience 1.75" of COOSA will be stronger than the aluminum hull itself.

Thanks for the reply, yes, I'm laminating the 1" and 3/4" COOSA to get to the same thickness and set-up (a smaller 3/4" square laminated to a large 1" rectangle) as the existing plywood setup equaling 1 3/4". It sounds like you believe that combined thickness/setup of 1 3/4" COOSA is sufficient. All comments are appreciated.

 

[TinMan007]

The range for the transom is 2" to 2.25" according to the Mercruiser manual.
Boat Construction; Transom Thickness And Surface - Mercury Alpha Installation Manual [Page 10] | ManualsLib https://www.manualslib.com/manual/797538/Mercury-Alpha.html?page=10

Thanks Gonzo, I need to give that diagram some thought. I appreciate it.

 

[TinMan007]

The range for the transom is 2" to 2.25" according to the Mercruiser manual.
Boat Construction; Transom Thickness And Surface - Mercury Alpha Installation Manual [Page 10] | ManualsLib https://www.manualslib.com/manual/797538/Mercury-Alpha.html?page=10

The instructions (referencing flatness uniformity) and the diagram suggests the specification of 2" to 2.25" total thickness is for a fiberglass boat. My guess (that could be trouble) the aluminum transom plate combined with the 1" 3/4" Coosa would have a comparable strength to a 2" wood and fiberglass transom. Any thoughts?

 

[missinginaction]

I don't disagree with Gonzo. Reading the manual carefully though, Merc states that the 2-2.25" thickness applies to the area 8" either side of the transom center line. They don't call for a 2-2.25" transom thickness all the way from port to starboard.

I think you'll be fine with what you are doing. That COOSA and an aluminum plate will be here long after we're all gone as well.

MIA

 

[Howlandwoodworks]

Just beef it up with some aluminum angle stock somewhere if you can.
Crestliner 12' sportman

Elmer's

• Howlandwoodworks
• Jul 21, 2019

Elmer'e

• Howlandwoodworks
• Jul 21, 2019

Elmer's

• Howlandwoodworks
• Jul 21, 2019

 

[TinMan007]

I don't disagree with Gonzo. Reading the manual carefully though, Merc states that the 2-2.25" thickness applies to the area 8" either side of the transom center line. They don't call for a 2-2.25" transom thickness all the way from port to starboard.

I think you'll be fine with what you are doing. That COOSA and an aluminum plate will be here long after we're all gone as well.

MIA

Thanks MIA, I appreciate your input. Once I have the outdrive removed I can measure the transom aluminum plate (the hull) and add that thickness to the Coosa (1" 3/4") to determine the entire transom thickness (inside surface to outside surface). I think it's safe to assume (that could be trouble... again) the 2-2.25" spec'd in the 2001 manual is for materials going back 20 plus years (pre composite board/Coosa) and the increased strength of Coosa (compared to marine plywood) would allow for a slightly thinner transom. Any additional thoughts are appreciate.

 

[TinMan007]

Just beef it up with some aluminum angle stock somewhere if you can.
Crestliner 12' sportman

Elmer's

• Howlandwoodworks
• Jul 21, 2019

Elmer'e

• Howlandwoodworks
• Jul 21, 2019

Elmer's

• Howlandwoodworks
• Jul 21, 2019

Thanks Howlandwoodworks - funny you say.... there is a piece of aluminum angle stock running across the entire transom (inside, 8" above outdrive) starboard to port. I'm starting to feel good about what I've got going. Btw, nice 12' Sportman, such a cool boat. That must have some get-up-and-go. Any additional input is appreciated.

 

[gonzo]

The thickness requirement is for the bolts between the gimbal housing and the transom plate to work.

 

[Howlandwoodworks]

It would be best practice to find out what horsepower your boat is rated for. 140 is a large engine. Check for stress cracking.