Motorboard installed on SBDB

Dave Diefenderfer

Dave Diefenderfer

Well-known member
I need to replace the motorboard on my SBDB. I have a piece of Ash that is 2x12x36 that was part of a stash of materials and tools I bought years ago from a guy giving up on his strip canoe project. Would ash be an acceptable wood for the motorboard, supporting my 15hp?
 
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Good morning, Dave~


I'm sure Ash would be strong enough, but..... As you know, I have replaced a bunch of South Bay motor boards. They were originally made of Mahogany or marine plywood - even Teak - over the years. But 40 or 50 years take their toll.


I am currently working on a vessel I built with a friend ~ 1990. The motor board is sound but for one corner that's a bit punky. Not sure whether I will replace or repair. We made it from treated lumber (SYP).


I remain a fan of treated lumber (usually 2 x 10) for motor boards. (I priced Coosa Board and was scared away by the price....). I pick through the lumber and look for pieces that have something close to quarter-sawn grain - to avoid cupping. And, I make sure I can cut around any larger knots (> dime diameter) and get several boards out of the piece.



Two keys are:



1) very careful sealing with 2 or 3 coats of epoxy, and



2) using stand-offs against hull to minimize the surface area that beds against the transom.



Of course, the board should be bedded in 3M 5200 - and the shafts of the bolts thoroughly coated/slathered just as they are being installed.


sm 2 Motor board - fwd side.JPG



The other change I have made recently is to no longer bore the shallow (1/8-inch) sockets I have used to receive the motor clamping pads - to keep it from twisting. Now I add a 'glass plate., 1/8" or 3/16" thick. I set it in a milder (door and window) caulk and fasten with 6 s/s panheads. This approach both gives 2 fewer spots for moisture to enter the wood (even though the sockets were sealed) and also allows for easier custom-fitting when a different outboard comes along.


Tuffin - Motor plate bored for Mariner.JPG



This photo shows everything installed. You can see how the standoffs keep most of the board off the transom. The eye bolt is backed up below decks - and is there for the safety chain/cable from the OB.



Tuffin South Bay - motor mount pad.JPG



So, that piece of Ash will undoubtedly work. You are a meticulous craftsman and I know any new board with get thorough epoxy-sealing.


Hope this helps!


SJS


 
Well, with a little research (should have done before I asked) I find....

White ash is not known for being very pest or moisture-resistant at all and is not ideal for outdoor use.
 
Dave~


True enough. Ash is traditionally limited to trim pieces on boats - most famously on for rubrails, inwales, thwarts, yokes on canoes. As such, the lumber is largely open to the air - avoiding the trapping of moisture - and receiving regular maintenance/varnish. It is strong but lighter than the Oaks - and certainly much lighter than Locust. It's tough to beat, though, for oars and paddles.


I tend to use treated SYP and Mahogany for wooden parts when I need both strength and some level of rot resistance.


Happy hunting!


SJS

 
When I restored the Sam Hunt BBSB back in 2009, I bartered with a co-worker for a big slab of mahogany. I very carefully used that board and saved and used every piece I could. I still have some cut-offs worth saving, but nothing too large. I reached out to him again, as he is a wood hoarder. He agreed to sell me another piece big enough for this motor board. 14x38x8/4. I will follow Steve's design with the stand-offs. My Yamaha has a 17" height, and that extra 2 inches should put the pin just above the rear deck so I might be able to tune the motor position without removing the motor each time. I will just dress the stand-offs in the planer, but will bring the motor board down to 6/4 dressed. I think that offset will help some on the clearance when the motor is in shallow drive, but will likely shave the bottom corners of the sponsons if required.



343801107_3562392877417334_4128520046558729214_n.jpg
 
Steve

Is the motor board mounted well above the bottom of the transom. Reason I ask is it seems the standoffs would create a channel for water to rush up and splash everywhere.

Eric
 
Eric~


I've never had that problem on my own boats - and no complaints from customers. Especially with the cutout for the transom drain, there is little or no board per se below sea level.


Better lucky than good!


SJS


 
Steve, do you have a picture installed from the transom? To Eric's question, does the stand-off install flush with the lower edge of the hull, or does it sit up a little? I made mine today, have painted on straight epoxy, then added Cab-o-Sil and glued the stand-offs to the motorboard. I have not cut the final length, but have routed all but the top edge where I will adjust the height.

On the fiberglass plate, is there a layer that the clamps tighten on so the fiberglass is a stack up?
 
Good morning, Dave~


Sorry I missed your call.


Here's one installed on a South Bay - the 2 lobes are just above the round-over on the bottom. They could certainly be cut a bit higher if desired.



Transom Drain 05 - Motor Board bedded and fastened.JPG



The plates I use to hold the motor pads are laid up: 1708 with mat - 2 mm Coremat - 1708 with mat. The mat sides of the 1708 are both facing in toward the Coremat. I use polyester resin since this is all plastic. The finished thickness is about an eight-inch.


I bore right through the 'glass and the plates bear on the motor board itself. With a thicker plates - or 2 plates glued together after boring, the board itself would get more protection - but I'm not sure it'd be worth the extra work. I think sealing the end-grain of the board is the most critical job.



Tuffin - Motor plate bored for Mariner.JPG



Hope this helps!


SJS


 
Stev, yes, that does help. I will make sure I am above and not even. Just a thought, the mahogany was a full 2" thick. On the spacers, I just planed them to make a finished surface, so they are 1/2" thicker than a 2by is. Does that mean I should cheat up a little more? I did plane the motorboard to 1.5" so that the clamps will fit well. On the BBSB, I left it 8/4 and it was a tight fit with one of the motors. Especially when I added the SS wrap. I did not rely on the clamps, and through bolted the motor for security. I will probably do the same on the SBDB as it is a quick 2 bolts and I won't remove the motor often. Might slow a thief a few more minutes too.
 
Dave~


I'm not sure how the spacer/standoff thickness matters. The ones I just removed from the JAMES CAIRD were just an inch thick. On the new board I'm making for that same vessel, they are 1.5 inches thick - but also beveled to mate with the radiused transom.


I think this is a build and see situation....part of R & D......



SJS






 
Dave, nice work as usual. Great site, learn a lot of great ideas. I locked the motor clamp screws with a short chain to a s/s thru both for security.
 
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