More fin theory

In discussing how fins work in a multi fin pod, there are two types of flow, laminar and turbulent, I do believe.

No doubt most everyone has seen the eddies and rolling off the side of your boat as you are wakesurfing. It can also bee seen off the back when your boat is unladen and you're blasting somewhere. That is a layer of turbulent, rolling, swirling water thats releasing off the hull of the boat.

That layer of turbulence is being caused by viscous drag or surface drag.

On your board you go from laminar flow at low speeds (paddling speed in the ocean) to turbulent flow at high speeds. Turbulent flow is not a bad thing because that layer of turbulence insulates the bottom of your board from the main body of still water and acts almost like ball bearings under your board. It helps speed.

While turbulent flow is great for the bottom of your board, its a nightmare for your fins.

If a plane flys through turbulence it can lose altitude rapidly as the wings can't function as effectively in turbulence. Similarly if your fins have to deal with turbulence they don't function as well, lose hold, encounter more drag, especially at angles of attack (like turns up and into the wake).

A cut-away in the trailing edge lets the turbulent water pass under the fin, so less of the fin has turbulent water pushed up against it in turns. The part of a normal fin (without a cut-away) in contact with the board won't act efficiently, have more drag, less hold. The cut away allows the fin to operate in a zone away from the board in cleaner (less turbulent) water, allowing it to perform more efficiently.

This theory is seen frequently in nature on the fins of fish and on the wings of some birds.

This cut-away can be duplicated in a fin for a board, that uses a multiple fin pod, I doubt it's overly effective with a single fin. An example:

To give a better illustration of the operation of the cut-away, I've labeled this picture. Don't you just love my artwork?

I had two sets of custom fins foiled by Halycon Custom fins. The blue'ish fins are polyester and fiberglass. The black fins are epoxy and carbon and 'glass.

Close up of the two rail fins

The blue poly fins are of a relatively high aspect template. They will release very easily at slow speeds but because of the very aggressive foil they will progressively engage more positively as speed increases. In my experimentation, these are great for sliding around up towards the spine and then when you can drive the board from the back up towards the lip, without a "too fast" boat speed.

The horizontal chord toward the tip of the fins is moderate so they can be pivoted easily even at maximum speed. They give lots of drive
once the board gets going but are washy if the board is going slow (stationary on the wake). The canards in that set are foiled exactly the same as the main fins. These fins were done with very high grade poly resin and glass cloth and "should" have a very snappy recoil speed for a poly fin (as opposed to epoxy).

The Carbon fins have a cosmetic weave that is built into the cloth that shows vertical in one fin and horizontal in the other. Actually the weave is the same in both fins and runs traverse from leading base/leading edge to tip/trailing edge. I originally thought the weave would have a "best" orientation, but I have found no difference in how this cosmentic line affects fin flex or recoil, nor was I able to find any research that indicated there was a "best" orientation. Those carbon fins are cut from a single panel and because of the way the template must be laid on it to cut the fin plug
out for a proper undercamber the weave pattern falls as it does.

The design of the carbon main fins are of an extremely powerful configuration. These fins "should" have lots of low speed drive and "should" allow James to fade well back behind the boat and swim the board up to develope lots of speed. They "should" also come off the top much harder than the blue poly templates. In my testing and "mad scientist" experiments carbon epoxy fins have extremely fast elastoviscosity. In my discussion with fin experts they indicate that two layers of carbon placed just right in the lamination stack give an excellent result. Three layers give a very very stiff fin that is hard to load unless it is foiled way down. This is fine in very big waves for super speed but is just too rigid for our little ones.

We are running a C-5/Twinzer configuration and so both fins described above have the small leading canards. The gray epoxy canards in the carbon epoxy set have a more neutral foil than the blue poly set. The undercamber is very open to match the main fins and the leading edge is softened with the double foil well into the tip area to give a very low drag effect and facilitate direction change. At least, in theory.

Pictures with the fins installed:

thanx for nice info Jeff

Hey Jeff,
The blue fins look a lot like future rasta keel fins. They are supposed to be super fast. Let us know how they work.

Hey Corey,

It was good to see you...sheesh was it last weekend? That travel is all a blur at this point. You're right they do resemble the rasta keels a little less rake. We need the skull artwork! Rich placed a very fine pattern under the gloss coat, but nothing beats a good skull.

I'll keep 'ya posted.

Thanks Petr!

We haven't had a chance to ride in almost 3 weeks, so it was good to get out today. The jury is still out on the blue poly's but the carbon ones develop a ton of drive. The best way I know to test the drive of a set of fins is to fade as far back in the pocket as possible and then see you can drive back out.

James was able to bury his rear foot and almost all the way up to his front foot and drive back out.

Pictures of James buried in the curl.

Just so there isn't any confusion, the first two pictures I didn't get a sequence shot, the last four are the sequence.

Don't know if this is the right thread to post this question but I'll give it a shot anyway since it relates to fin placement. I have heard a couple of different techniques on finding the right tow-in angle of the fins toward the nose of the board. Any insight on what has been working for everyone?

If I had it to do again, I'd start with one of the fin systems with adjustable toe. My procedure where we established the toe that James loves, was to lay the board down and measure a point 6' from the tail. I then pointed two straight edges so that they intersected at that point.

That was the start It gave me a toe just about 1/8" on a side - maybe a little more. I incremented that 1/16" at a time until I got a LOG. The usable range seemed to stay around 3/16" on a side to 5/16" on a side. More or less than that didn't work for us.

James currently rides at 1/4" on a side with the Scim's.

Thanks for the input. That is pretty much the range that I have been using and it seems to work pretty well. Thanks again