Motor Board Details

Steve Sanford

Steve Sanford

Well-known member
Good morning, All~


I just mailed off a motor board to Long Island - for a member's South Bay Duckboat. I have made a bunch of these over the years and thought I would share some of the details that go into the design and installation of this simple but critically important feature of many smaller gunning vessels.


Original motor boards on South Bays were rugged. I have seen them made from marine plywood, White Oak and Teak. Given enough seasons, though, any of these can wear out or rot. Here is one in its last throes.... As I recall, the original galvanized bolts had corroded to almost nothing within the wood.


[CLICK images to enlarge]



View attachment MB - A Motor Board 02 - BEFORE sm.JPG



Here is what I use - for numerous South Bays and a variety of Sneakboxes and Grassboats.



View attachment MB - C Motor Board 08 sm.JPG



It has standoffs for a couple of reasons. First, there is no need to mate the entire lower board to the hull. Such a large area - even if well-bedded and sealed - invites the intrusion of moisture. Next , because the standoffs hold the board about 1.5 inches off the hull, the board can now be used as a lifting handle. More than once - when the tide ran out and left me stranded - I have removed the outboard on my Sneakbox and set it on the foredeck. That allowed me to easily lift the stern and push the boat out to deeper water much more easily than drying to drag it from the bow.


So, all of the edges of the board are rounded over. This helps to maintain the epoxy seal (usually 3 coats) and allows for a comfortable hand grip. The bottom is sawn to a radius to allow access to a drain plug (not standard on South Bays but a useful addition in my opinion).


The shallow "pits" are bored to fit each engine - to prevent the outboard from twisting off the stern (I learned this lesson the hard way whilst in the 8th Grade.....)



I use four 5/16" stainless steel carriage bolts to attach the board to the hull - and so bore the holes on the drill press.



View attachment MB - B Motor Board 07 sm.JPG



I bore another hole lower down in the middle of the board. This is chamfered to take a short length of decoy line - a lanyard or "keeper" for the drain plug. As many dukboats have decked-over sterns, it is usually much easier to get to the plug from outside the boat. I typically drain the boat when it's still on the ramp after a hunt - to take advantage of gravity. And, I still carry a spare plug in my onboard tool-kit.


Here is my own Sneakbox RED~LEG early in my 2016 renovation. I had replaced the original motor board before I decided to begin my major re-build.


View attachment MB 17 Drain Plug.JPG



Here it is after the renovation with the plug ready-to-hunt (but no lanyard yet).


View attachment MB 18 Hull 12.JPG



Now back to the project at hand. As it happens, I had made up a bunch of motor boards a few years ago. I think I made 5 from the old treated 2x10 I had been using for motor boards from my days on Long Island (pre-1995). I had installed or shipped off all but one. So, I was able to prepare it for a new South Bay owner in recent weeks. He has the same engine as I have on RED~LEG - a Johnson or Evinrude (OMC) 9.9 or 15 from the late 70s or early 80s. They are light, reliable, low-profile and well-regarded by outboard mechanics.



View attachment MB 01 - 15 on transom.JPG



The first job, then, was to fit the blank motor board to the clamping pads on my outboard. I took the direct approach.



View attachment MB 02 - marking clamp pads.JPG



To make the next one easier, though, I also made up a measured template - to hang on the shop wall for future reference.


View attachment MB 03 - template for OMC.JPG



Stay tuned as I head to the drill press.....


SJS


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I use a 1-3/8" Forstner bit on the drill press.


View attachment MB 04 - Forstner.JPG



I bore each "pit" about 1/8" deep.



View attachment MB 05 - shallow boring.JPG





NOTE: For the first time this past year, a customer found that his outboard would not clamp tight. I do not recall if it was a Suzuki or Tohatsu. The clamp pads bottomed out on the motor's bracket before snugging up on the board. We fixed this with a piece of 1/2-inch plywood - custom fitted and well-sealed with epoxy - that he screwed and bedded to the aft side of the board.


I made this backer board from some 3/8-inch MDO (Medium Density Overlay) I had on hand. The Pine "bumper" keeps gas tanks or other loose under-deck gear from chafing on the bolt ends.


View attachment MB 06 - MDO Backer with Pine bumper.JPG



The backer gets a single coat of epoxy to seal the wood - but all surfaces and especially the edges get attention. I apply with a foam brush - then hang out to cure in the sun. Because epoxy usually gets a bit "wetter" during the curing process, hanging allows it to collect at a single low spot - easily shaved and sanded off after the cure.


View attachment MB 07 - Backer epoxy curing in sun.JPG



I do not hang the motor board itself. It had been sealed with 3 coats of epoxy - because it'll live on the outside of the boat - and so only the shallow pits needed resealing. I lay it flat throughout the curing so a) the epoxy would saturate the wood and b) so it would not pool in the bottom of the pit.


View attachment MB 08 - epoxy sealing of boring.JPG



After a careful hand sanding with 100-grit paper, I wipe everything thoroughly with acetone, then apply a prime coat of flat oil paint.


View attachment MB 09 - prime coats.JPG



I followed this the next day with the top coats. Two coats are enough for the backer. The owner will add another coat to the motor board after installation. I do not paint the mating surfaces of the standoffs - so that the 3M 5200 adhesive caulk used for bedding with have direct contact with the epoxy.



View attachment MB 10 - top coats.JPG



I was careful once again to not let paint puddle within the pits.


View attachment MB 11 - paint in boring.JPG



The holes in the backer are over-size. I bored them to 7/16" so locating the backer over the 5/16" carriage bolts during installation will not be too fussy.



View attachment MB 12 - 7-16 hole in backer.JPG





Almost done.....


SJS













 
I recommend not putting fresh paint on the mating surfaces of the vessel itself. I want the 3M 5200 to get a nice grip - so the transom area should be finish sanded (100 grit or so) then masked before installation of the motor board. The hull should be painted prior to board installation, of course.



View attachment MB 13 WSB motor board masking.JPG



I typically dry fit the board before bedding and fastening. The board has been bored for the 5/16" carriage bolts. The bolts should not be driven in to their shoulders at this stage. Instead, each bolt hole should be bored through the hull - making sure the board is centered and plumb on the transom. Once all 4 holes have been bored, then actual installation can commence.


The bedding with 5200 must be done carefully. I spread a liberal amount - at least a 1/16-inch thick - over the entire mating surface with a putty knife. I also coat the shaft of each bolt with 5200. Although I also squirt some 5200 into each bolt hole, I do not want to presume that it will distribute itself thoroughly. When I do insert a bolt, I twist it in to help spread the bedding.


The backer itself does not need the strength or permanence of 5200. A cheaper house caulk or silicone will suffice for bedding the backer. I use s/s fender washers - 5/16" x 1.5" - and Nylock nuts. When fastening, each bolt should be tapped into the wood to bury its shoulder - rather than asking the nuts and washers to draw it in.


Note the cut-out at the bottom to allow for a future drain plug.



View attachment MB 16 SSB Motor Board - backing plate.JPG



The 5200 squeeze out around the board should be smoothed to a nice radiused fillet. A finger works best for me. It takes several days to fully cure - but can be sped along with some water sprayed on during the installation process. The final topcoat of paint can now be applied.



View attachment MB 14 D Motor Board - aft.JPG



The heavy eye bolt on the stern deck is for the safety chain/cable from the outboard.



View attachment MB 15 Motor Board - fwd.JPG



UPS tells me the motor board and backer board have made their way to Long Island. I hope to see some "as built" photos - and especially some "sea trials" soon.



All the best,


SJS






















View attachment MB 14 D Motor Board - aft.JPG
 
Hi Steve, very informative post as always and excellent timing as I'm looking towards the motor board on my rebuild. Any idea what the lifespan difference is between a white oak board vs treated? Also, is there a minimum width for the standouts?

Thanks,
Bob
 
Bob~


re White Oak v. treated SYP - I have no good way to compare the lifespans. Earlier on, I always used White Oak - but was disappointed in how it would deteriorate - to the point of crumbling - over time. But, I had only treated it (motor boards and thatch rails) with Cuprinol and duckboat paints. My earliest treated 2x10s seemed to last with this approach. Nevertheless, I now coat mine with 3 coats of epoxy and 3 coats of paint. I would guess that White Oak, too, would do well with such added protection.


Regarding the standoff dimensions. As usual, I use my "seat of the pants" engineering. I want enough room between the board and the hull to easily fit a gloved hand. And, I want enough meat around each bolt. So, my standoffs are about 1.5 inches in both dimensions. The inch-and-a-half width also gives ample mating surface for the 5200, I think.



All the best,


SJS

 
I got about 15 years from my white oak board. A few friends back in the 90's rebuilt out boats about the same time 1992.
One friends board broke at the ramp about 2005. I checked mine that year and found the board was rotting on the inside so I replace it.
Another friend is still using the same board no issues.
One thing I know about treated lumber used for bench and bleacher boards is some rot other check/crack within a few years and some seem to last for ages.
 
Steve, great motor board primer. Thanks for sharing this. I wasn?t familiar with these until I started looking at the low freeboard duck boats.
Jim
 
Good morning, David~


I'm glad this was helpful.


Funny should ask about adding it to my website. I tend to neglect my website - in favor of duckboats. I sometimes post on both sites - but each takes time. There is tons of stuff here that should be on my own website - but time is always the barrier.


All the best,


SJS

 
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