I decided to fit this board with a large tunnel fin.
Here is a shot of the mold that I will use to form the fin resting on the board for perspective.
You can also see the materials that will go into the fin. A sheet of brown paper and slats all cut to size.
I start by wetting out the paper on both sides with epoxy:
Next I thicken the epoxy with some wood flour (saw dust) and apply a layer of that to the paper:
Then I lay out my slats making sure to press each one firmly into the paste so that it oozes up through the space between each slat:
Now I carefully lift that whole mess, lay it onto the mold, and apply another layer of thickened epoxy to make sure I have all of the gaps between slats filled:
Finally I wrap the whole mess in stretch wrap to keep things in place while the epoxy sets up. When the epoxy is cured the stretch wrap will peel right off.
Normally I would use two layers of slats staggered to cover the seems and a layer of paper in the center. This creates a very strong fin. In this case, the slats I’m using are on the thick side so I’m only using one layer. This means that I will only be able to properly foil the outside of the fin and I will leave the center flat or un-foiled. This is a bit expirimental. I’m not exactly sure how the fin will perform this way. I reason that it will be OK as follows:
Normally I would say that an unfoiled fin will cause drag and that properly foiling it will serve to reduce this drag. In the case of a tunnel fin, foiling the interior edge creates a bottleneck for water flow as the inside diameter becomes smaller at the apex of the foil. That, in theory, will create its own form of drag. Forgoing the foil eliminates the bottleneck and so relieves this element of drag. However, now we have an unfoiled surface and so introduce surface drag in the form of friction. The question is; which element of drag is more signifigant? The answer is; I have no idea. And so I’m going to experiment and see if I can tell the difference. My hypothesis is that I will not be able to.
