Thursday, January 25, 2018

The Sportfisherman Grows a Transom

In my last post about this new powerboat project we installed the keel and the graceful S-curved laminated stem. This post mostly deals with the other end of the hull, installing the transom and the plywood gussets that will support the outboard engine bracket.

In the interim, Kevin has been busy fitting Douglas fir cleats in the corners of the internal structure of the hull. These cleats are triangular and are easier to do than epoxy fillets on these straight joints.
Douglas fir cleats fitted into the corners of the structure with temporary screws. The cleats across the top of the photo are on the edge of the girder and will be planed down to match the angle of the plywood bottom panel to increase the gluing area.
The transom was laminated previously over a curved former, from four layers of plywood, shown in a previous post. It is a strong and substantial piece of timber, so needs careful planning before starting the installation. You don't want to lift it and wriggle it into position more then absolutely necessary because it is pretty heavy and you need to show it who is boss.

This transom doesn't have the outboard motors clamped onto it, it has slots through it for a series of gussets that will be bonded onto the girders and keel inside the hull. The gussets are the internal structure of the bracket that will carry the outboard motors, cantilevered aft of the transom. The gussets must be installed in tandem with the transom, so there is time pressure to get it all done and properly secured before the epoxy goes off.

Before doing a dry-fit, test fit the various parts. The end of the keel must fit into a socket in the front of the transom. Check this with an off-cut of the transom rather than offering up the heavy transom to the keel. Check that the gussets all slide easily through their slots. There must be enough slop to leave a narrow gap (about 1mm) to fill with epoxy, to do its bonding work.

You will also need to trim the keel to its final length, which means pushing the transom in as far as it can go then measuring how far it still has to go to be in the correct position. Measure this distance onto the keel and mark the position to cut the excess off the aft end. This needs the transom to be removed then replaced again after the keel has been trimmed.

We eased this whole process by hanging the transom on the centre gusset, which was clamped very securely into its slot in the keel. This allowed us to have the keel take most of the transom weight while we fine-tuned the fit.

The centre gusset is 3 layers of 12mm plywood, fitted into a slot in the keel. Here is it clamped in place to assist by carrying the transom weight when fitting the transom.

The transom is in position and supported on adjustable stands. The centre gusset assisted by carrying the transom weight while getting it into place. The two outer gussets each side are dry-fitted to be sure that all will go together without problems.
 Next, the two legs that tie the transom to the building stocks were fitted. Each had a bevel angle cut onto one face to match the angle created by the curved shape of the transom. With the legs screwed to the stocks but not to the transom, we added a short wooden block to the side of each leg, tight up against the transom edge and projecting aft beyond the transom. They gave us additional temporary support points for the transom.

With everything fitting properly, we kept the transom in against the legs and supported on the blocks but pulled the top aft away from the girders and keel. That allowed us to spread epoxy into all contact areas. We excluded the centre gusset from this gluing process because it was dry-fitted and would need to come out again. before gluing. 
Transom supported by blocks on the support legs, with a clamp on the centre gusset preventing it from falling off the boat. Now all gluing areas can be reached to spread glue. Be sure to first brush on liberal amounts of unfilled epoxy onto all end-grain surfaces and allow it to soak into the grain before applying glue and closing the joints.
After this first phase of gluing, the second phase is gluing in the four outer gussets and setting them to the correct angles and heights. Temporary screws will hold the transom and gussets.

The third phase of gluing was for the centre gusset. Once unclamped, this came right out to give access to the bonding surfaces. Once epoxied it went back in, clamped back into the groove in the keel.
The transom has now been glued to the girders and the four outer gussets have been glued. Note the 2x4 on top of the framing, clamped to apply pressure to hold the transom against the other structure. The centre gusset has been removed
View of the inside of the transom, showing the gussets bonded to the girders, fastened with temporary screws and fender washers. The centre gusset has not yet been fitted.
All glued and waiting for the epoxy to cure.
During this process, care must be taken to get epoxy all the way through the various joints. That means working it into the joints by squeezing with a spatula or with a syringe. Clean up all superfluous epoxy while it is still wet, it will be a pain to clean once hardened.

The next phase of the project will be fitting chine stringers and planking the chine flat.

This design is not yet complete, so is not on our pricelist or website. See our full range of designs on our main website or our mobile website.