Monday, January 15, 2018

Backbone of the Sportfisherman

The backbone of a boat is its keel and stem. For our new 26ft sportfisherman those members are made from Douglas fir, aka Oregon pine, which is laminated to the shapes needed. Laminating is the process of cutting the timber into strips, then gluing them back together again while bending them to the required shape. The thickness of strip needed depends on how tight the curve is, the tighter the curve the thinner the strips needed.

Newcomers to woodworking are sometimes petrified, or at least anxious, about tackling a laminating job. But this process of taking straight timber and changing it to a convoluted shape is strangely satisfying. The first time that you do it, you look at the result and think to yourself "did I really do that?".

It is really a fairly simple procedure and, with careful planning and common sense, has small chance of going wrong.

The hull frames form a handy and accurate mould over which to laminate the keel. The strips have the glue applied then are laid into the sockets in the frames and clamped in place until the glue has cured. It should not be glued to the frames, so the sockets are protected by tape, waxed paper or other bond breaking material. This allows the member to be removed after the glue has cured, for easy cleaning with a belt sander. The sharp corners that will be inside the hull should also be rounded off with a router while it is out of the boat.
The keel has little curvature, so can be laminated from a small number of relatively thick strips. Kevin used 4 layers of 20mm (3/4").
The stem has tighter curvature and the double curve of an S-shape, so it is laminated from thinner strips. The curve also needs more definition than the hull frames can provide, so it is laminated on a table or workbench with a former that defines the shape. In this case, the former is made from a series of timber clamping blocks that are screwed to the table. The shape that Kevin Agee followed was shown on a full-size pattern that we provided.
Shaped blocks screwed to the workbench over the template. Blue painters' tape on the blocks is to prevent the product from bonding to the blocks.
The stem glued and clamped to the blocks. This glue-up is being done in two stages because there are too many joints to safely glue in one step.
There should always be a dry run of clamping the strips onto the formers before doing the actual glue-up. This will allow you to find out before-hand whether or not there will be any problems or any tricks that will be needed to get it done with the glue starting to go off. Don't be over-optimistic and try to glue too many strips in one go. The more strips that there are the longer it will take and the more that they will slide around when you are trying to clamp them accurately. Play safe and do it in two stages to minimise the chance of failure.

Leave laminations plenty of time to cure before releasing them because releasing the clamps before epoxy has fully cured can result in some straightening of the piece. If that happens, you will not be able to pull it back into shape, possibly even by trying to clamp it back onto the former. A simple laminate like the keel with little curve is not likely to give this problem but the tight curves of the stem can do that if released too soon. In winter this could mean leaving the piece clamped to the form for a week.

Another reason for laminating from a large number of thin strips is to minimise spring-back. This is the tendency of the laminated piece to straighten out slightly from forces in the wood, even if the glue has fully cured. The more dense the wood that you are working with, the greater the chance of spring-back, so the thinner the strips that will  be required. Reducing the strip thickness and increasing the number of strips, in inverse proportion, will give better results.
The completed stem lamination, being cleaned up with a belt sander prior to trial fitting.
The completed keel lamination. The groove routed into the aft end of the upper surface is for housing the central gusset of the stern bracket that carries the outboard motors.
To speed up the lamination process and give yourself more time to work with the timber and get it right, you need an efficient way to apply the glue. Kevin used rollers to apply it quickly.
A roller applying glue to multiple strips at the same time.
The next post will cover installing the keel and stem on the frames, including how to cut the scarph to join the two together accurately.

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.

No comments:

Post a Comment