The Chine Line / Scarf Joint

The line where the bottom of the boat meets the side is called the chine. For slow moving, displacement boats, that area is rounded for the most efficient travel. But, for speed boats like the one I am building, the motor is strong enough to push the boat up onto a plane.

Edges created by a sharp chine on the sides and cutoff at the transom, lets the water separate cleanly from the boat, reducing the wetted surface. This loss of frictional resistance allows planing boats to reach amazing speeds.

When the chine line is defined, it is partly visible when the boat is at rest, and more fully shown at planing speed. It becomes an important part of the visual profile.

That edge must be strong also, because the corners are where the boat may contact a dock or an object in the water. For my boat, I am using one inch thick Cherry wood to make the chine log, as it is sometimes called.

The trouble is that the boat is 20′ long and my longest cherry is 12′ long. This requires joining two pieces using a scarf joint. Below are shown cutoffs from free hand cutting the roughly one foot angled ends.

The ends are not a perfectly consistent angle, so they are sanded, using a backer support to keep them from bending.

Then, the sides were matched together and assembled with two screws to prevent sliding under glue and clamp pressure.

Below is one 22 foot chine log clamped together.

Now they are ready for glue clean up and assembly onto the boat.

The chine log dimensions were picked specifically so it could be bent on a comfortable, easy curve from front to back. This is intentionally done without steam bending which is cumbersome and can give irregular results.

When the scarf joint is well done, and the grain of the both pieces are selected for longitudinal grain, without knots, the log will bend very consistently. As it is temporarily clamped in place, it helps to show where to adjust the cross frames on the way to creating a fair and beautiful curve.


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