Outboard mounting plate

[marsters]

I have a 14-foot Sebago Lake boat that was restored two years ago. Part of the restoration involved replacing the transom, which had been on the boat since it was built in 1936. The new stern is oak, about an inch thick. While fishing a couple of weeks ago, we noticed a crack about two inches below the outboard mounts (5-horse Honda) running most of he way across the transom but not all the way through. Accelerating the outboard opened up the crack to maybe one-sixteenth of an inch. Clearly it was the torque of the engine that caused the transom to split. To see what I mean, check this video (http://www.youtube.com/watch?v=y3uGoWPSxn8).

My thought is that the crack needs to be glued and then some sort of mounting plate needs to be put on that will spread the load of the outboard. Such a thing used to be made, but despite a valiant effort Hamilton Marine has been unable to find one, and lots of Googling has failed to turn anything up. There is such a thing made for inflatable dinghies out of aluminum, but I'd rather have something beefier and more traditional in keeping with the style of the boat.

I would appreciate any thoughts and ideas.

[Tylerdurden]

You could try some sheet stock in bronze plate but that would be pricey. Any hardwood suitable would work and look better I would think. Just epoxy it and it will take the stress. Also if you go with metal epoxy that too as it will pass the loads better than fasteners.

[Thorne]

Think of an outboard as a long lever being pulled aft -- now attach that 14' lever to the top part of the top board in the transom and jerk it around with your full body weight -- for hours. That is a good analogy of the stress / force that you are dealing with.

First off I'd say that a 5hp outboard is way too much for that boat - as evidenced by the split in the transom. I used a 6hp to power my 2200lb San Juan 21, moving it at hull speed most of the time. A 2hp should push the Sebago at hull speed in nearly all conditions.

Second, the knee / internal brace doesn't go up high enough. The outboard is attached to the top 4" or so of the transom, but the knee only goes a little ways up the top plank -- it needs to go nearly all the way to the top and fit that notch in the interior plate (if you keep the 5hp).

Third, any sort of fastener hole greatly weakens solid wood planks -- those bolts for the handles are an open invitation to splitting the oak. If you must have them attached, consider metal backing plates to spread that particular load also. If you replace the top plank of the transom, why not leave the handles off until you can confirm that the outboard won't split the plank -- then add 'em if you are feeling strong...
;0 )

If you MUST have both the handles and use an outboard, why not use some thinner oak planks for the transom top (i.e. more planks from top to bottom) and have the bolts go through the glue lines? That way there is already a split there along the line of force, and it is held together with epoxy and stronger than the surrounding wood.

As for a mounting plate, try making something up from stainless steel, and only using it when the outboard is mounted. Cost will be a tiny fraction of what plate naval brass or silicon bronze costs. Make it formed to fit over the top of the transom, with most support plate running down on the outside/aft side -- remember the force from the outboard is applied from that side, so anything mounted on the interior of the transom just gets pulled forward.

Best of luck, and you've done a lovely job of restoration!

[rbgarr]

Oak cleats glued/fastened almost from the floor to top of the transom on both sides of the motor?

[Kevin G]

Originally there may have been some diagonal cross bracing on the inside of the transom. My Sebago has it, and it seems that this would take some of the torque generated by the motor. I will send you some pics of my boat, and maybe that will help. I get the feeling that my Sebago was built about 1950.

KG

ps. I will send you the pics becauswe I can't seem to ge the hang of postin pics here.

[marsters]

Thanks. I'd love to see the pix. You don't see many of these boats. My boat has been in the family since 1936. Now that you mention it, I seem to remember some kind of cross-bracing.

[marsters]

Thank you. Ouch! Your description of the forces involved makes me feel badly for any transom with an outboard. There has been a motor on this boat ever since it came into the family in 1936. I remember an old Martin, maybe a 6, then for years there was a 10-horse Johnson. The only damage during all that time was from a tree that fell on the boat during a hurricane and holed it. Good point about the bolts from the handles.

[Kevin G]

Did you get the pics??

KG