Bestboat

When I started the transom back in December, I wasnt planing on the weather turning cold for so long. Winters in central Texas have been fairly mild most years since I moved here. Fortunately, it isnt a solid mass of cold days and we get periodic warm spells of 60 degrees or more. In fact, yesterday, it reached 81 degrees! These warm interludes allowed me to make progress, albeit, with waiting spells in between.

The last time I posted a build article, I had completed adding the first motor board. Because I want to make the transom strong enough to handle a larger outboard, I elected to add a second 3/4" motor board. There was some agonizing over the details of this. I have no previous experience with boats or outboard motors. I wasnt sure  just how thick I could go. I originally considered using 1/2" for the second board, but that would have required me to buy another sheet of plywood. Getting plywood is always a problem because none of the local lumber yards provide the product that I am looking for. The nearest supplier is in Houston which is a four hour drive. Therefore getting the wood here involves additional expense in addition to the considerable cost of the wood.

I had 3/4" plywood on hand and was hoping to use it. A visit to a local boatyard and discussions with others gave me confidence to move forward with the thicker wood. Gluing this second piece down required additional considerations. First there is the matter of the knee which braces the transom to the keel. Its attached using epoxy resin and 6" long 3/8" silicon bronze bolts. If I had simply placed the knee on top of the second motor board, the bolts would not have been long enough. I conferred with some other builders on this and decided to add a slot in the board .



Another consideration was the eventual bolt pattern and location for the outboard mount. I had trouble finding these at first, but eventually I learned that most outboards use a standard bolt pattern. Once I learned what this was, I was able to proceed.

A third consideration were the screws that I used to hold the board down (in addition to the epoxy resin) . They needed to be slightly offset from the screws used to hold the first board down. And finally, I needed to insure that all the screws and eventual structure would work with whatever cut out I needed for the outboard motor. The cut out has to be a certain minimum width to allow clearance for the motor to turn. It also needs to be a certain vertical distance from the bottom of the transom so that the motor eventually lines up correctly when installed.

Fortunately, the choices here are also standardized to 15, 20, and 25 inch motor shaft lengths. But which motor to plan on? I have somewhat solidified plans on the eventual motor I plan on using. A short shaft motor (15") is nice because it allows the motor to be low enough to be partially hidden by the boat structure. But it also requires a fairly deep cut out which I dont like.

A 25" motor (extra long shaft) provides a much shallower cut out, which is good. But the motor sticks way up in the air, is heavier, and are not as commonly available. I suspect they are more expensive as well.

Ultimately I decided on the 20" shaft length as a good compromise. Ill cover the cut out more in a bit, but I had to allow for this when laying out the screws for the second board. One additional detail was that I wanted to do the cut out after gluing all the boards down. This way I could avoid having to try to keep all the parts aligned when gluing. With all these questions answered, I proceeded to add the second board.


The additional thickness of the second board and the cut out I would eventually add meant that my building form was going to need modification to allow clearance and to provide support to hold the transom. Basically I needed to cut a section out of each building form leg  at the back end and replace the small supporting blocks of wood with a longer piece of 2 x 4 lumber.



 The cut out for the motor , as mentioned earlier, needed to be a certain size. I had worked out the width back in January so that the eventual motorwell sides would line up with other structure in the hull. It slightly exceeded the minimum width needed for the engines I had in mind. The depth of the cut is determined by measuring the motor shaft length (20") down from the keel point on the transom. It needs to be measured perpendicular to the keel rather than along the transom surface since the motor is mounted perpendicular as well. Remember that the transom is mounted at a 14 degree angle from vertical. Therefore, the cut out depth was slightly longer than 20". I also wanted to provide room for an eventual solid wood cap on the inner edge of the cutout to hide and protect the plywood edges. So this additional thickness was added to the depth of the cut out.

Then came the hard part, making the cut. I always get nervous doing this because a screw up at this point would have set me back considerably in money and time. So I spent a bunch of time rechecking all my measurements and assumptions. The cut out was laid out on the transom skin and nice rounded corners were added. Then I broke out the jig saw and proceeded to cut.

Well, as careful as I was, I still managed to slightly exceed the cut lines in a small area. Fortunately, the repair for this was easy and would eventually be covered by the edge cap. I first had to sand the cut out to its final lines. Then using some aluminum tape and two steel rulers for support, I added thickened epoxy to fill in the damaged area. Later, this was sanded to blend in with the surrounding wood and the repair worked out just fine.



The transom was temporarily re-mounted to the building form to make sure my modifications were correct and to finalize the position of the knee. I determined that the angle of the knee outer edges needed to be slightly reduced. They were at 16 degrees but needed to be at 14 degrees. The bench sander remedied this problem.





Before encapsulating the inner surface of the transom, I wanted to add the knee This required drilling holes for the 3/8" bolts. This is another of those steps you have to do that make you nervous. When drilling through thicker materials, it is is imperative that the holes be perpendicular to the surfaces or the bolts will not install correctly. I have quite a bit of experience with this from my previous aircraft maintenance days so I utilized a method I was comfortable with to accomplish this. Basically you use two vertical references to align the drill, and then start with smaller drills and step up to the final size. Then just work slowly and try to keep the drill perpendicular to the two vertical references. I used a combination square and the other leg of the knee for this purpose.


Then the holes needed to be transferred to the transom. I wanted to insure that the plywood would not tear out when the drill broke through the opposite side so I used a piece of wood clamped in place on the backside. Again I worked slowly and didnt try to force the drill through too fast.

The knee is attached using thickened epoxy and the two 6" carriage bolts. The heads of the bolts are on the outside skin of the transom. I wanted to maintain as much strength as possible here so I elected not to counter bore under the bolt heads. I plan on adding mahogany planking on  the exterior surface of the skin so I can cover these bolt heads by simply making small clearance areas in the inner surface of the planking before I epoxy them into position.

The knee was epoxied into position and the bolts inserted and assembled. The knees outer edges had to be slightly notched where it matched up to the transom frame because that wood is slightly thicker than the plywood motor board. The edge that matches up to the keel needed to be aligned with the notch in the transom frame.




And here is an older photo that shows how the knee will eventually look when the transom is attached to the boat. The motor boards are not in this older photo.


So all that remains on the transom is to do a bit of glue clean up and encapsulate the inner surface with three coats of epoxy. The outer surface, as previously mentioned, will be planked, so no need to coat that surface right now. Once this process is completed, the transom will be re-mounted to the building form and I will begin the process of installing the keel.

So until next time.......................