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seamoosecap 11-16-2010 08:54 AM

outboard motor on o'day 27' - transom strength?
i recently mounted an outboard motor bracket to my 77' o'day 27' and i have an 80 lb 15 hp outboard that i am getting ready to put on it. i was kind of surprised at how thin and flimsey the transom of the boat is. i have a nice 12" x 12" wood plate on the inside of the transom where the motor mount is mounted. the mount is rated to handle at least that big of a motor.

so my question is, has anyone had any experience mounting an outboard in a similar situation? is it normal to see visible flex in the transom fiberglass when the motor is mounted? is there any risk of damaging the fiberglass of the transom? i have not heard of anything like that happening before, but the flexure of the transom is a lot more than i expected.

any insight would be greatly appreciated.



deniseO30 11-16-2010 11:05 AM

it's a pretty common situation on most outboard sailboats. The larger the "plate" the less flex you will get. We used 1/4" alum plate on the hunter 23 I had, it was overkill even though the 10 hp merc was heavy and more then the boat needed.

tommays 11-16-2010 11:25 AM

Its going to stress and fatigue the glass if you can see it moving

I have seen many boats with custom wood (or what ever) part made on the outside to help spread the load over a larger area

seamoosecap 11-16-2010 02:23 PM

do you think epoxying some marine plywood to the inside over a large area (3'x3') would do it?

deniseO30 11-16-2010 02:28 PM

3/4 or double would work. the hard part is getting it pressed up tight. you could use 1/4" and press it into the back of the transom then and another.. then another, that way you could meet the curvature of the transom.

seamoosecap 11-16-2010 05:01 PM


how big (area) was the 1/4" aluminum plate on your hunter 23?


deniseO30 11-16-2010 05:14 PM

I forget, but it was pretty large . Plywood make more sense though.

billdre 11-16-2010 05:27 PM

Another Oday w/outboard
I have a '79 O'Day 28. The saildrive was pulled by a prior owner who installed an outboard hydraulic assist bracket rated for 15hp. It holds a 9.9hp Mariner that weights 86lb. I've had it on and off several times. This transom doesn't flex at all. This mount is backed by a couple circular plywood plates that are very thick. I've never measured them but it wouldn't surprise me if they we 3/4." The transom is also a reverse angle. Flexing would bother me. Good luck.

COOL 11-16-2010 05:33 PM


Originally Posted by seamoosecap (Post 667382)
do you think epoxying some marine plywood to the inside over a large area (3'x3') would do it?

That should help. It would be best if your plywood extended
the entire height of the transom, and was tabbed with fiberglass
at top and bottom. That will transfer some of the loading
to the hull and deck structure.
That is quite a large motor for a ODay 27,
and I think you want to eliminate any flex that
might eventually fatigue the laminate.

ccollins0601 11-17-2010 11:29 PM

The flex you are seeing is surprising. I have an O'Day 27 (1973) with a 12hp outboard. I have never seen any flex or weakness in the transom, and even use it as a step to get up on the boat sometimes when it's on the hard. I don't specifically remember what (if any) size of backing plate is used, the transom is so sturdy that it has been a long time since I looked.

One caveat: my boat was built as an outboard model (never had an inboard) so it may have different construction than the models with inboards.

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