Motor mounting method

[keen2buildagain]

Hi All,

I suppose this question should be targeted towards Graham, but I value any suggestions from the forum. At present I am building the OB-20 and  in looking ahead, was wondering what is the preferred method of doing the holes thru the transom? Do I sleeve them with a s/s tube or PVC? At present the transom total thickness is approx. 2". Also I was thinking about a s/s plate for the outside as well so as not to crush the paint and glass upon tightening of the mounting bolts. Any ideas??

Trev

 

[PAR]

I dislike metal plates on transoms, just causes issues under them eventually.

 

The through bolt holes should always be "bonded". I treat them slightly different than a conventional bond, which is little more than an epoxy filled hole about 30% larger than the fastener shank. I countersink both sides of the oversize hole, before filling with a thickened structural epoxy mix. This makes a flange at each end of the bond hole to bear the weight of the washer, under the nut or the bearing pad on the outboard bracket. I seemingly minor adjustment, but it does make a significant difference to surrounding laminate or core material load paths.

 

Many outboard brackets have teeth on them, particularly the portables (under 30 HP). These teeth are intended to crush into a transom, providing some additional grip. There's two approaches, the first is to simply grind them flat with the surrounding bracket, the second is to cast them (as female receptacles) into the transom, again in some thickened epoxy.

[Ken_Potts]

There's another answer to the teeth on the outboard bracket.  On our dinghy there is a sacrificial pad of plywood that the teeth can bite into.  Every 10 years or so I'll have to replace the pad.

[keen2buildagain]

Thanks Par and Ken for your advice. The epoxy filled holes seem like a good idea and make sense to me now. I am putting a 50 or 60 Hp on the back, so I don't want to have to be taking it off to replace any pads etc. On most alloy boats, I have noticed they opt for a white "cutting board" type of material.......wonder how this would go between the motor and the transom pad?

Trev

[PAR]

HDPE pads can absorb some crush and the teeth, it's also inert, likely why it's used, but it's a poor product choice, simply because it's so slippery, which seems self defeating in this application. I grind the teeth off my outboards, the bolts will do, if dogged down good. On portable, with the upper clamps, I can see why they put the teeth in the bracket, even small outboards can be drilled for a more secure mount. If the motor has to come off and on a lot, maybe a good idea to leave them, but if it's a long term mount, I'd just blot them down and walk away. People that steal these smaller outboards are looking to simply undo the clamps and lift. Clamp locks help, but a set of bolts with nylock nuts will deter all but the most determined. Larger outboards are just bolted and bonded fastener holes, insure moisture can't get into the core to cause rot.

[Dnjost]

On my CS 17, I drilled the drainholes for the small motor well over sized, then filled with epoxy goo, and and then drilled to 1/2" diameter for drainage.  The only issue I have with the arrangement is the mizzen sheet will hang on the motor shaft if I am not diligent with tending the sheet with a racing jibe (grab with my hands and through the rig across).  

 

 

the board on the transom is bedded in polysulfide so when it get chews up, it is simple to remove and replace.  I do like the motor head right at my hand when under way.  Photo was taken prior to finishing.  I have some glimmers of how to prevent the sheet from tangling on the motor.  Check out the Flying Scot forums for some ideas on stern sheeting.  

[keen2buildagain]

Thanks guys, your advice is very much appreciated and noted.