Back-up plate for motor mount

[mike gartland]

I've noticed a small amount of deformation of my transom due to the vertical torque applied by the weight of my Tohatsu 9.9hp 2-stroke electric start.  I am the fourth owner of this boat and although I have never trailered her with the outboard in place I suspect one of the previous owners might have.  Even the SS motor mount is slightly deformed which adds to my suspicion.  I can see no signs of deformation or structural stress cracks on the inside of the hull but am considering replacing the aging 2-stroke with a 4-stroke which will weigh maybe a couple of pounds more and will develop decidedly more propeller torque.  The bottom line is that I would like to shore up the transom with some kind of back-up plate before I change out the motor and 4-stroke motor mount.  (The existing motor mount has only bolts and standard SS washers but no additional plate to help distribute the load across the transom .)  I have contemplated using plywood (5/8 or 3/4 inch?) glassed to the inside of the hull, a plate of metal (1/4 or 3/8 in aluminum or?) or one of the high density polyethylene products (3/4 or 1 inch StarBoard?).  All have some apparent drawbacks but my main concern is getting the support I need and I would like to be able to do the work myself.  Any thoughts or suggestions on the best solution?

Thanks

Mike
WindRush 

[brackish]

Hi Mike,

I would avoid the plywood, really doesn't have great tensile or compressive strength and subject to problems with water intrusion.  If wood, balsa cut into 3/8" thick tiles from a 4 X 4 set with the end grain in compression would be stronger once fully saturated with resin and glassed in. That's what boatmakers used for decks before the poly honeycombs became more widespread.   However, I would avoid the wood altogether, and use a metal backing plate.  10 ga. stainless plate 25% larger in area than the bolt pattern  probably best or 1/4" 2024 t4 aluminum plate.  Anything more would be overkill and add more weight to an area where you don't want it.  Relatively cheap if you have a fab shop in the area.

David's suggestion to increase the number of fasteners and the larger backing fender washers would probably be all you need.  However, if it is already weak or flexing, plate may be better.

If you use a solid plate, maybe bed it in a structural epoxy paste to make sure that there are no voids and the  forces are transferred to the transom uniformly across the whole surface.

My $.02 worth, probably worth exactly that.

Frank

[Craig Weis]

Well, a little deform in the transom from the motor mounts weight holding the motor up?
As noted may have been bent by some one backed into something? Or the motor bouncing astern while motoring down the road.
So remove the motor mount.  And clean the inside of this transom area of that grey factory paint. Vacuum.
-->Make a papper pattern, top to bottom, deck to hull, and scupper to outer end of transom of the inside.
-->Cut a chunk of 10mm aluminum to the pattern and ruff up with a grinding disc. This provides a surface for epoxy and removes the smut that forms on aluminum.
-->Cut a 1/2" thick marine plywood pattern as well.
-->Mix up some epoxy resin [MarineTex would be good] and using a roller to coat the transom, aluminum, and plywood slather all that together. It is important that the aluminum go against the transom first, since it will not deform like plywood. The 'curve' of the fiberglass transom is slight but there. And the void behind the plate will be taken-up by the epoxy. The wood goes next to the aluminum.

**NOW HERE IS THE RUB. A way must be devised to squeeze all that onto the transom as a sandwitch till the [Plexus or MarineTex ] epoxy cures.
Enough pressure must be applied to push out the dent in the transom. I would make a jig and use a small bottle jack or pound a few wood wedges in a horizontial position, and let her sit over night. You'll have about 20 minutes working time so work all this out before hand.

Of course no one will do this so just add some 10 mm aluminum plates about 4" x 4" for each mounting bolt [Or one piece strips] and spread out the weight over the greatest amount of surface possible... the dent might pop out using some heat from a hair dryer or paint stripping gun or by simply letting the transom surface 'rest' a few days. I would not go with more fasteners 'cause that means more drilled holes. Hulls and holes don't go together. The new fasteners will need to be longer, 1/4 bolts with flat washers and nylocks. Use lots of 3-M 5200 slow set sealer.
skip.

[mike gartland]

Thanks to all for the well thought out and detailed suggestions.  I am leaning toward the larger surface area SS plate if I can find a source for it.  I've done some work with stainless before but that was back in my college days when i had access to a machine shop....both long gone, unfortunately.  I need to look around for a local shop that might be willing and able to provide the material and basic outline cuts.  I can take it from there.  I'll let you know how the project goes.

Thanks again.

Mike
Windrush

[mike gartland]

Since I can't seem to find a way to post pictures any longer on this CYOA website I have begun posting a few on the Sailboat Owners Association site.  I would be happy to learn if it's still possible to do so here and how it's done.  In the meanwhile you can find a few pics of WindRush sailing on her home waters of Galveston Bay (yes, she survived hurricane Ike) at

http://forums.sbo.sailboatowners.com/album.php?albumid=566

Mike
WindRush CP 23/3