Installing the Sportfisherman Backbone

In my previous post Kevin laminated the keel and stem, the two components that make up the backbone of the new boat. Now we must shape the scarph joint that will combine them into one piece and then install them.

That introduces a tricky problem that may confound some builders who may not have done this before. The problem is that the keel and the stem have to both occupy the same space before the joint that bonds them together is shaped. This joint must be very carefully shaped to create an effective scarph joint with the two parts correctly aligned. Trying to do this by measurement has a pretty big chance of an error causing a mismatch, with a bad joint the result and possibly having to make a new stem or splice in an extension on the keel.

This photo shows the keel being laminated in position, extending to Frame 1. The frame that is in front of it is Frame 0, at the forward end of the design waterline (DWL). The stem has been made to extend from the tip of the bow, over Frames 0 and 1, ending at Frame 2. The overlap between the two allows us to blend them at the joint.

A template is needed to get the cut angle correct, with the template used first on the keel, then on the stem. But if we do that off the boat the chances are that we will get it slightly out of position or at slightly the wrong angle, yielding a bad scarph. A small difference in thickness between the keel and stem will throw out the angle substantially. To remove this considerably risk, the template needs to be on the hull framing and it must be done in a way that it can be used to mark the angle on the two components individually.

To accomplish this we needed three battens. These we set up with the keel in position and the ends of the desired scarph marked onto the top and bottom of the keel. Two of the battens are clamped against the frames, one on the aft face of Frame 1 and the other on the front face of Frame 2. Both are vertical and away from the side of the keel by the thickness of a batten. The third batten is clamped to these first two against the side of the keel, in a position that the lower edge crosses the top and bottom faces of the keel at the previously marked lines showing the ends of the scarph. Clamp them all tight so that they cannot move but the keel must be free to move. The template is now complete. Draw a line onto the side of the keel along the lower edge of the third batten, then slide the keel aft out of the way.

Here is the template set up against the keel. A line is then drawn onto the keel along the bottom edge of the sloping batten, then the keel is slid aft out of the way.

The template still in position with the keel out of the way.

Now place the stem into its correct position. To do this the bow end of the stem must be trimmed to the correct deck angle and height above the building stocks, which comes from the drawings or the stem template. The stem must also be set on a block to get it to the correct height. Also, draw the waterline onto the side of the stem from the pattern, to assist in ensuring that the stem is correctly set up relative to the frames, by using the laser level. With the stem in place, draw the cutting line onto the side of it along the template exactly as was done for the keel.

The template is till in place and the stem has been slid in to replace the keel. The cut line is drawn onto the side of the stem in the same way as was done for the keel.

The keel and stem can now be removed from the framing for the scarph to be cut. Before doing this, draw the ends of the scarph line across top and bottom of both the keel and stem, then draw the scarph onto the opposite face as well. Cut the scarph with a skillsaw or hand saw. Don't cut right on the line, cut about 3mm (1/8") away from it. This will allow you to finish it more accurately with a sharp hand plane to a smooth and straight surface.

Scarph cut onto front end of the keel, then finished with a hand plane.

Now the keel and stem can be dry-fitted in place on the hull framing to check for accuracy, before being permanently glued into the structure.

Keel and stem dry-fitted and clamped to show a near-perfect fit and a nice fair curve.

At this stage the girders haven't yet been glued in, so we elected to glue the girders to the frames, to get the structure solid before gluing in the backbone. After that we glued in the keel, followed by the stem. The scarph was glued while gluing in the stem.

The new scarph joint glued, clamped and excess glue removed. A nice clean joint.

The next step in the assembly will be setting up the transom. I will blog about that next week.

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.