I'm going to give a brief demo in lap strake construction. If you have ever looked at a lap strake boat you will notice that the ends of the planks disrepair into each other. This gaining fit is accomplished at the stem using a rabbit. This allows the plank going onto the boat to be thicker at the edge. This thickness helps prevent splitting when fastening.
I was taught to make the joint 9" long. The first three inches begins the roll the next three continues the roll and begins the rabbit the final three complete the roll and the rabbit. ** The last three inches need to be flat for and aft. The joint must also be flat athwartships.
Below i have the two pieces stacked onto each other. The top joint continues to be nine inches long and the final three inches are also flat but their is no rabbit or a very small one. Also note the outboard edge is the thickness of the rabbit on the other joint.
When you put the two together they look something like this. My thumb is on what would be the inside of the boat this allows the outer plank to be thicker (aids in preventing splitting).
With the two pieces clamped together the forward edge disappears into the other.
The other end of the tool has a 3/4" x 3/8" notch cut into it 3/4" back from the front edge of the tool. To use this tool the planks need to be laid out on the molds. (You can see these layout lines on the mold below. The lower line is the outer edge of the plank and the upper line is the next lap.) Place the notch over the lap edge then bring the lower edge of the tool in line with plank edge line. Now mark the place on the edge of the existing plank where the corner of the notch touches.
Carefully connect these marks created with the tool. This line marks the depth of material to be taken off
This edge you have now created is the landing for the plank.
Here are some pics of the rabbit cut on the boat.
The stern is a non rabbited gaining fit.