The Transom Challenge (and keel)

As mentioned in the previous post, the cold weather was limiting me on what I could do in my shop in terms of using epoxy on the build. However, I don't think it much mattered that I didn't have a warmer shop because I took up the majority of the cold weather season just to build the transom. No doubt about it, I was probably over-thinking the situation.

Crude sketch

Before I get into my transom build experience, I should explain what the transom is/does. The transom is the surface that forms the stern (rear) of the vessel. In small craft, it is usually the member on which the outboard boat motor is mounted.The transom member can be straight up/down or it can be tilted either forward or aft. The tilt is referred to as "rake". In the attached crude sketch, the transom is noted and it can be seen that the transom is raked aft. The amount of rake from vertical is denoted as the transom angle.

The transom of the Malahini consists of a plywood outer panel that is strengthened by attaching a mahogany frame to the inboard bottom and sides of the plywood. A U-shaped frame is epoxied and screwed to the inside of the plywood transom. As shown in the crude sketch, the bottom of the transom plywood AND the bottom frame member have to be cut at an angle to achieve the proper rake which is given as 12 degrees in the plans. The patterns provided give the final dimensions of the transom assembly on the outboard (ply) side which meant that I would have to ensure that the plywood and frame lumber was initially cut oversize relative to the final dimensions in order to account for the rake angle. I chose to use a rake of 13 degrees based on discussion with other builders.

Meranti plywood is expensive and I knew I had to be certain before I took a saw and cut the beveled bottom. I looked at what other builders had done to cut the angle cut accurately and of course, there were a multitude of ways to do it and opinions. I chose the method I was going to use and decided to make a dummy transom out of cheaper wood first to get the process down. I layed out the pattern of the transom on BOTH sides of the plywood, made a circular saw jig, and set the angle on the saw blade. I cut a bottom frame member out of 1x pine and tacked it to the inside of the transom. I cut the bevel on the bottom of the dummy transom and was very happy with the way it came out. I set the dummy partial transom on the building form and felt good about everything and told myself that I was cleared to build the real transom.

transom frame temporarily fastened

I carefully transferred the pattern lines to both sides of the plywood. After pre-cutting all the frame members exactly to the pattern lines earlier in the build process, I had to make another bottom frame member due to the reason noted above. I temporarily fastened the frame components to the inside of the plywood and set up the work piece to safely make the cuts. The first thing I noticed was that the saw didn't want to rip through the material with ease like it did on the dummy transom. I cut 1/2 of the bottom edge and did not like the result.....rough edge and inconsistent angle along the edge!

 Before cutting the other half of the bottom frame, I would need to replace blade and figure out how to fix the messed up side. I was flustered and did what many other builders have done....go to the Glen-L builder forum and ask for advice. I got the answer that I had to re-cut the bad edge, but one builder did the trigonometry and figured that my transom height relative to the plan dimensions would only be 3/32" short. I did re-cut the bad half and cut the other half correctly the first time. He was right. The height is about 3/32" shorter than plan.

angled backer board

angled notch setup

With the intimidating bevel cut completed, I removed the frame components in order to assemble. Before putting the frame together permanently, I needed to cut out the batten and chine notches on the bottom member. Unlike the other frames, these notches have to be cut ahead of time because once mounted to the plywood, access would make cutting extremely difficult. Since the edge is beveled, the notches would have to be cut at the same angle. I cut an angled backer board on my table saw and attached it to my miter fence using double faced tape. The bottom frame member was positioned and clamped to the backer and saw cuts made inside the area where the notches needed to be. Once several cuts were made, I cleaned out the waste with a chisel. Multiple iterations of repositioning and clamping were necessary, but the result was as hoped for. The transom frame components were brought to the layout table and assembled with gussets just like the other frames and once dry, this frame was epoxied and screwed to the plywood.

transom temp height blocks

The transom has to be built with at least a double layer of plywood in the area where the motor is to be mounted. This posed a problem for me in that I wasn't sure how to "hang" the transom assembly on the building form. I used the frame alone and hung it on the form and attached temporary mounting blocks to the frame and the building form. The assembled transom would initially be set at the correct height using just these blocks. I knew that I would have to do more than this later, but I felt this was sufficient until permanent mounting to the keel happened.

The transom assembly was put together with the frame and additional layer of plywood called the motor board. The top of the transom has a cutout for the motor and on either side of this cutout it has an arc profile. I used the full size pattern of the transom and made a poster board template to lay out the arc. I cut the excess plywood close to the layout line and I will fine tune this shape once the hull is uprighted and are working on the decking.

The transom assembly is essentially the fifth frame member of this boat (frame '0' on the plans) and is now ready to start being tied in with the other frames. There are multiple longitudinal wood members that will connect to all the frames, but the most important one is the keel. It is the heavy member that is on the bottom of the boat that will run from the transom forward to the stem. It gets screwed and epoxied into the notches cut into the frames and gets epoxied and lag bolted to the stem. So I set the transom aside while I installed the keel. I ensured that the centerlines of all the frames were lined up, screwed the frames to the building form so that they wouldn't move, and cut bilge drains (limbers) on both sides of the keel notches. The keel was then installed.

knee to transom

The transom is attached to the keel via a triple thick plywood transom knee. This component is a strength member and is cut so that the upright part is one degree greater than the bevel angle of the transom. I cut the transom bevel at 13 degrees and so the upright part of the knee has a 14 degree angle. The reason for the difference is that the keel rises 1 degree from transom to stem. When it is "all said and done", the building form and transom knee will stabilize the transom at an angle. I didn't want to try to install the knee to both the transom and the keel at the same time while working in an awkward position on the form. I decided to mount the knee on the transom first while it was off of the form. I temporarily mounted the transom to the form, positioned the knee where it needed to go relative to the transom and the keel, and then put temporary locating blocks on the transom so that I could accurately relocate the knee to the transom while it was off of the form.The knee was epoxied and lag screwed to the transom. I did have a little scare.....About 30 minutes after epoxying the knee to the transom, I realized I had mounted the knee upside down! I removed it, applied a little more epoxy and positioned it correctly.Whew!

I brought the completed transom with knee and located it on the building form. I did some minor shimming to get everything lined up correctly and attached the transom assembly to the keel via the knee. I did add some temporary support to the transom positioning by having a piece of 2 x 6 in the motor cutout area.

forward view

a view aft

Building Form

So I have four of the five frames built, and the next logical step is to build the transom, which includes a mahogany frame as part of the finished assembly (labeled frame '0' on the plans). The transom is the very back (aft) of the boat on which the motor is mounted. The transom will be made out of a 3/4" sheet of (expensive) meranti plywood. I started reading various sources on how to build the transom as I simultaneously looked at the plans. The transom is required to "lean back" 12 degrees (per the plan) from vertical in order for most motors to hang properly and this is obtained by cutting a 13 degree angle on the bottom edge of the transom assembly. It seemed that the more I thought about the transom, the more difficult it appeared to me. I know that I was over-thinking the situation, but I decided to put the transom on hold and to work on setting up the building form.

 The building form is a structure typically made up of construction lumber on which the hull of the boat is built upon. The structure cannot just be thrown together in a willy-nilly fashion and expect the shape of the hull to come out as the designer intended. The form is the reference structure on which the hull frames and stem are accurately placed. It should be noted that the hull is built UPSIDE DOWN on this form. This form must be fastened to an immoveable structure such as the concrete floor of my shop and must be immoveable as I push, pull, tug, hammer, etc as I put the hull together.

The fore/aft reference point used to set up the form is the very front tip of the boat - the breasthook point. The vertical reference point is the blocking on which the breasthook sits. Before getting started on referencing, I first secured a base for my form to the concrete shop floor using anchor bolts. The base is 16' long and is comprised of two 2 x 6s butted together. Upon this base, I attached 2x blocking on which the breasthook will sit. Under normal circumstances, this should be very easy and straightforward. However, the concrete finish work done on my shop floor was horrible and my floor has dips, high spot, and is not level. It took me some time to get the base boards somewhat true and to get the breasthook blocking trued up. Shims became a necessary material for me.

With the base and blocking in place, I needed to come up with a way to easily transfer measurements from the breasthook references as noted on the plans to setting up the two main stringers that make up the building form. These stringers are 12' 2 x 6s and must be parallel and level to each other. In order to achieve this, I made a reference frame out of scrap wood that was the exact height as shown on the plans. for a fore/aft reference, I just attached a board to the front of the breasthook blocking and I used the face of the board as a 0 reference. The assumption of course was that when I set the stem in place, the breasthook point would just touch the face.

Reference framework and vertical board

Stringer set up

With the reference framework in place, I then use a 6' long level and place it on top of the framework and have the other end of the level sit on top of a stringer. It was a bit of a challenge getting the stringers set up due to them being heavy and cumbersome. Once one stringer was set, I then had to ensure that the 2nd stringer was not only at the proper reference height, but also stayed level and parallel with the first stringer.  Again, this appears relatively easy on the surface, but I admittedly struggled with getting the form set up where I felt comfortable with its accuracy. I believe a lot of this has to do with my shop floor inaccuracies, but I eventually got it.

Once the basic form was set up, I then set the frames on it and added braces which will also double as mounting blocks to hold frames securely to form once I am absolutely sure all frames in proper position. At this point, no frames are locked down.

Frames loosely set on building form

This all was accomplished by November, 2013. Due to a really cold winter, work has come to a standstill in my shop for the last several months. Next up.....my transom challenges.