Transom replacement or design flaw?

[bikerider]

I have a 12 ft Starcraft, model sl-12 aluminum boat. It has a 5hp four stroke outboard on it. I notice that the transom on it flexes when I get past about 1/2 throttle. Enough that I don't go much more than that. I thought the transom wood was shot. But, as I've started removing screws, and what not, the transom wood seems surprisingly solid still. I'm wondering if the transom flexing under power is a design flaw. The transom wood in the center is only about 4 1/2" top to bottom, and about 5 1/2" top to bottom on the outside ends. There is no place (that I can find) where a brace would have been on the inside of the boat, and the very back of the boat has no brace either. It is just the aluminum skin. This thin transom wood seems like a poor design that would flex with any outboard except an electric trolling motor. What do you all think?

 

[Scott Danforth]

The wood looks like it dried out and delaminated.

I would get some 3/4" exterior grade plywood, laminate two pieces together to make a new transom and be back on the water by the weekend

 

[cyclops222]

Cut 2 FULL width Ply wood pieces to go SIDE TO SIDE. Should be thick enough to just easily slide under the inside Aluminum skin. It should go from side to side. Trim as needed. If you can slide in a 12" HIGH piece ? Do it. You might need to remove some bolts. Use S S hardware.

 

[racerone]

They forgot ( saved money ) by not putting a brace ( knee ) between transom and bottom of the hull.-----Easy fix for me here.

 

[airshot]

Owned many small 12' and under aluminum boats over my 60 plus years of boating, not unusual to see some flexing at the transom. How much is ok is the question !! These small boats are made to be light in weight so they can be cartopped. The makers use minimum strength materials to save weight. Just the way it is...when you get into larger, higher hp rated boats you will see far less transom flexing. If it bothers you, do as mentioned and replace that old wood with plywood, not that difficult and much stronger, even add an extra knee or other support....won't hurt a thing and might help you feel safer.

 

[cyclops222]

On my Lowe 16' With the 30 hp. I bought a piece of Aluminum heavy gauge, 2" X 2 " angle. Cut it to fit completely from side to side at the top of the transom. Some notching for motor mounting bolts. SOLID
Did add a few bolts, nuts and washers close to the hull sides. The engine clamps are on the angle.

 

[Grub54891]

That bottom piece looks compromised to me. At this point I would replace that, make the plywood go further down and it’ll be much stronger. Adding a knee brace would be appropriate also.

 

[Chris1956]

The transom cap is cracked thru. Likely from the plywood flexing. That does not help with lateral strength. See if you can find a way to repair/replace it.

 

[cyclops222]

If you have a LONG shaft motor on a SHORT shaft transom ? It will have a lot of extra pushing TORQUE on the transom. Oh well.

 

[bikerider]

The wood looks like it dried out and delaminated.

I would get some 3/4" exterior grade plywood, laminate two pieces together to make a new transom and be back on the water by the weekend

I purchased the plywood already. I was planning on doing just a transom wood replacement, but don't want to end up in the same boat if this is a design flaw as well/only.

 

[bikerider]

Cut 2 FULL width Ply wood pieces to go SIDE TO SIDE. Should be thick enough to just easily slide under the inside Aluminum skin. It should go from side to side. Trim as needed. If you can slide in a 12" HIGH piece ? Do it. You might need to remove some bolts. Use S S hardware.

I'd consider that. The tray that the current transom wood rests in is riveted to the back aluminum skin of the boat. It seems dicey cutting that tray out without developing leaks.

 

[bikerider]

Owned many small 12' and under aluminum boats over my 60 plus years of boating, not unusual to see some flexing at the transom. How much is ok is the question !! These small boats are made to be light in weight so they can be cartopped. The makers use minimum strength materials to save weight. Just the way it is...when you get into larger, higher hp rated boats you will see far less transom flexing. If it bothers you, do as mentioned and replace that old wood with plywood, not that difficult and much stronger, even add an extra knee or other support....won't hurt a thing and might help you feel safer.

The flexing made me think I might develop cracks or a break if I went much beyond half throttle. I already have the plywood for new transom wood. I still need to research more about adding a knee, but I'm assuming it would have to be welded in. I also saw an aluminum plate that goes between the transom and outboard. I have linked it below.

Transom Engine Mounting Bracket - Standard 1.75" Thickness - Tiny Boat Nation

Custom Outboard Mounting Bracket - Transom Wrap 1/8" Thick Aluminum Transom Bracket to securely mount your outboard and reinforce the critical part of your transom. Designed for the common jon boat and V-Hull transom at 1.5 to 1.75" Thickness. Extra Wide 2" available for thicker transoms. 11"...

tbnation.net

 

[bikerider]

On my Lowe 16' With the 30 hp. I bought a piece of Aluminum heavy gauge, 2" X 2 " angle. Cut it to fit completely from side to side at the top of the transom. Some notching for motor mounting bolts. SOLID
Did add a few bolts, nuts and washers close to the hull sides. The engine clamps are on the angle.

Something like this?

Transom Engine Mounting Bracket - Standard 1.75" Thickness - Tiny Boat Nation

Custom Outboard Mounting Bracket - Transom Wrap 1/8" Thick Aluminum Transom Bracket to securely mount your outboard and reinforce the critical part of your transom. Designed for the common jon boat and V-Hull transom at 1.5 to 1.75" Thickness. Extra Wide 2" available for thicker transoms. 11"...

tbnation.net

 

[bikerider]

That bottom piece looks compromised to me. At this point I would replace that, make the plywood go further down and it’ll be much stronger. Adding a knee brace would be appropriate also.

That bottom piece is riveted to the back skin of the boat. Is there a way to get rid of it without having to worry about holes/leaks? Taller plywood seems like a better setup. I have never seen another aluminum boat with such thin transom wood like mine.

 

[bikerider]

The transom cap is cracked thru. Likely from the plywood flexing. That does not help with lateral strength. See if you can find a way to repair/replace it.

That actually happened once when I lifted the outboard off the boat. One of the clamps got caught between the top cap and wood and it cracked it when it was lifted. The top cap does need to be replaced.

 

[bikerider]

If you have a LONG shaft motor on a SHORT shaft transom ? It will have a lot of extra pushing TORQUE on the transom. Oh well.

It is a short shaft motor. I made sure of that when I bought the outboard. It seems like a poor design because of the leverage. The boat is rated for a 10 hp max outboard. I can't imagine putting that much power to that transom.

 

[bikerider]

They forgot ( saved money ) by not putting a brace ( knee ) between transom and bottom of the hull.-----Easy fix for me here.

I'm really wondering about that. I assume the knee has to be welded in. Is that correct?

 

[Chris1956]

I would expect the top piece adds a lot of strength to the transom.

 

[racerone]

Put a knee in with rivers or screws.

 

[Scott Danforth]

I purchased the plywood already. I was planning on doing just a transom wood replacement, but don't want to end up in the same boat if this is a design flaw as well/only.

If the original wood is delaminated, it won't act as a solid piece of wood, it would bend like a phone book