Another wooden fin build thread

Here is a series of pictures that show the steps that I went through to make the fin that is going on my latest board project – a 10’ old school balsa gun.

I started out by gluing together some balsa and mahogany.  I used the same mahogany that I used on the tail block of the board, and the same balsa that I used for the board.  You can see at the far end of the table is a piece of 1/16" thick G10, and a couple of pieces of 4 oz fiberglass.

I made a sandwich of wood + fiberglass + G10 + fiberglass + wood.  I used RR epoxy to glue it all together.  I had to put the sheet of G10 at an angle in order to make it fit my template.  The top was brushed with epoxy, so that I could easily attach a template to it with Super 77 spray glue.

After the epoxy hardened I cut out the outline with a bandsaw.

 

Below is a masonite template that I stuck to the blank using Super 77.  Note:  I covered the bottom of the template with blue tape before spraying, so that my template wouldn’t get gooey. A flush trim bit on my router table allowed me to easily clean up the outline.

 

Here’s the side view.

 

Before foiling I drew some guide lines.

 

Here it is all foiled.  I used 220 grit on a 6" PowerPad.  In the past I used rougher grits, and found that I was removing too much material too quickly.  The 220 allowed me to go just slow enough to have a sense of control.

Here’s another angle.

 

Next step: I’m going to apply a cheater coat of epoxy to the fin, and then glass it to the board.

This is the third board where I have made fins for it.  I continue to improve my build process, and am getting faster.  I am always open to tips and suggestions.

 

Nice work Swied. Thanks for posting.  I love seeing the step by step stuff.

 

Just one observation about the foil-  you’re maximum thickness looks like it’s about 5% back from the leading edge.  NACA foils are about 30% back from the leading edge.  I think they’ve found that is the ideal position to maximize lift.  My guess is that having the max thickness so far forward may lead to cavitation.  I could be totally wrong.  I’m not a engineer.  The engineers or fin guys here can correct me or elaborate.

 

I agree Jeff, that fin may stall out (slide out) easily, you want the max thickness somewhere between 20-30% of the base length, i think… def not that close to the leading edge.

 

Very cool process Swied.  I like your use of G-10.

I did not know that.  I appreciate the feedback.  One of the main reasons why I posted so much detail, was so that people could point out the flaws in my process, and help me improve.

I was also wondering about my reference lines.  I followed the curve of the front of the fin.  Is that standard practice?

 

When I have foiled fins I have followed the trailing edge curve when grinding behind the max thickness line, and followed the leading edge curve when foiling in front.  Maybe (ProBox) Larry can jump on here and tell you how a pro does it.  That man is a machine.

I’ve always drawn a line at max thickness from base to tip…at about 30% of the chord (the fore aft distance).