Make a Hydrofoil Blade From Windsurfer Fins

Here's how to graft two windsurfer fins together to make the lifting part of a hydrofoil.
I lost the front hydrofoil blade from one of my Aquaskippers. This new blade works great as a replacement.
The foil section is just right for the purpose. And it's a really quick project.
The resulting blade is strong enough to use for an "air chair" or "sky ski" type hydrofoil, or any other weird hydrofoil project.
You can get windsurfer fins cheap at flea markets and yard sales.

You'll need the following:
Two windsurfer fins. Mine were each 9" long.
Epoxy. Not "5 minute epoxy". That stuff is crap.
Power Sander. I used a belt sander mostly.
Dust mask, safety glasses, clothing to keep the figerglass off your skin.

Step 1: "Design" Your Hydrofoil

Pick out a couple of fins.
Like I said, you can get windsurfer fins cheap at flea markets and yard sales.
Unless you need them. Then they're expensive. So stock up now before you actually need them.

You'll notice an odd phenomenon, which is that no matter how many of them you have, you'll never have two that are the same.
So pick a couple that are close enough and live with it. The earth isn't symmetrical either or the ocean. It's not going to make any difference.

Step 2: "Analyze" Your Hydrofoil

Every windsurfer fin in the world uses the 63a010 foil section.
That's a 63010 section modified to take the concave out of the trailing edge of the foil.
If you find a fin with a different section it won't be enough different to matter.
If you want to see what some other sections might be, browse the foil catalog at UIUC. If you want to import the foil to the XFOIL simulator and make graphs to tack to your wall, here's some more info on that collection.

Your fins won't be exactly the same thickness, so get an idea where the center lines are and maybe mark the base with that. It won't matter if the centerlines are at different levels, but you'll want both fins to be at the same angle of attack.

Step 3: Remove the Metal Bits

There might be some metal pins or fasteners on your foil.
Remove them.
The pins can be started with a hammer and finished out with a nail or punch.

Step 4: Sand a "Scarf" on the Butt of the Foil

"Scarf" is old Norse boat jargon for "taper" or "lap".
It means we're going to grind both butts off at an angle so we have more surface area between them when we glue them together.

Be very careful and don't do this when you're distracted. You have to grind opposite sides of the two foils. If you grind them the same you'll have two left feet etc.

I clamped my beltsander in a vise and used that for sanding. Don't breathe the dust or shoot it into your eyes. It'll make your skin itchy if you get it on you. If you don't care, your girlfriend/boyfriend or pet aphids might.

It's amazing how fast the fiberglass sands down with this 50 grit belt.
It's like shoveling snow.

Step 5: Alignment Pins

Eyeball it to sub-molecular accuracy and line them up.
Make sure the two foils have the same angle of attack.
Not out of plane if you know what I mean.
Drill a hole at each end for a small nail to keep them from squirting apart when you clamp them later.
Put the nail in one hole and eyeball the fins again before drilling the other alignment hole.
Just in case the molecules have shifted already.

Step 6: Glue

Mix up some of your favorite epoxy. Read the directions.
The directions will be very confusing, especially if there are metering (ketchup type) pumps involved.

Slather it all over the mating surfaces and hope for the best.

Step 7: Clamping

Mate the surfaces together, insert the alignment nails, and clamp the assembly.
Leave the heads and most of the nails protruding so you can pull them out later.

My joint was perfect so I only needed two clamps.
If yours isn't so good of if you like using lots of clamps, go ahead.
If you don't want the clamps permanently glued to the fins, you could spray something greasy on the jaws or rub them on your greasy nose.
I didn't do either because I planned to pull them off before the epoxy got hard.

Step 8: Shape the Knuckle

The next day I pulled the clamps off, pulled the nails out, and smoothed the joint area out.
I used this 50 grit sanding disk for the major butchering.
Then I switched to a sanding block with finer grits for the final smoothing.

You've made a high speed boomerang!
Sell it to the DOD. It's perfect for our national defense strategy!
Charge 2 billion dollars apiece for them. Subcontract the parts to a vendor in every congressional district. Advise those vendors to donate to their congressman. Congress will force the DOD to take it!
Name it after the price. The B2!
Life is hilarious! I'm rich! oops, we're dead!!

Step 9: Drill and Countersink the Bolt Hole

Make a hole and a divot for a 1/4" - 20tpi flat head bolt.

You may wonder if that's strong enough.
That's why you have to do it fast.
Wondering about stuff like that is what makes projects take a long time.

Step 10: Bolt It Together

Here's the finished front blade and surface feeler assembly.

I think it's called a "Shutt Strut" after the dude who invented it.
comments? corrections? Are you Sid Shutt? Steve Shutt?

I didn't trust the threads into the bottom of the vertical extrusion any more, so I drilled a pocket for a nut. When I bolted it all together it seemed very solid and secure.

Step 11: Testing!

I'd changed a whole bunch of stuff at once. Typical science.
A new wing.
A new birch surface feeler.
A new pivot joint and a massive welded repair that changed the weight distribution.

It worked great!

Some issues. The surface feeler didn't stay up on the water as well as before. I realized that this new wing's center of lift is a few inches further aft than the original wing. I could have bolted this new wing further forward.

I tried riding again with an original straight wing. I used the new surface feeler again.
It acted nearly the same. I realized the new birch surface feeler is much heavier than the original one.
That seems to matter. I guess I'll make it lighter with a grinder. Or make another out of cedar. Or use an old CD as a surface feeler instead.