More fin theory

[Surfdad]

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 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.

[Surfdad]

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:

[Surfdad]

To give a better illustration of the operation of the cuta-away, I've labeled this picture (I think that I have a repressed Kindergarten art psyche )

[Surfdad]

I had two sets of custom fins foiled by Halycon Custom fins. I'll post pictures this evening, if I don't forget! One is polyester over fiberglass and the other is epoxy over carbon and fiberglass.

The blue'ish fins are polyester and fiberglass. The black fins are epoxy and carbon and 'glass. 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 will give lots of drive
once the board gets going will but be 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. Or at least, in theory.

[LovinPowell]

Surfdad:
Your technical expertise is amazing to me. Are you as detailed in tax research?

I would think that as a fish's fin moves up and down, or essentially pivots, that a pivoting fin (to some degree) might be an interesting R & D project.

[Surfdad]

Thanks LP,

There is currently just such a product as you describe, it's Blakestah's Surftrux That link takes you to his page and a description of how it works.

I tried installing it several years ago and the way the system works is that it has springs and soft rubber "nubs" that allow the fins to swivel. An ocean wave just has a ton more power than out little wakes. So I couldn't get the rubber nubs of a sufficently low durometer reading to do any good. With NO rubber nubs, the board lost all drive, so you'd have to go much larger for most folks.

Interesting concept.

There are "flextail" boards like the old George Greenough type flexspoons and more recently Proctor's flextail

There is also something that I think is very intriquing and I'd love to try it as an experiment - it's the Tinkler Tail. It's hinged and has springs that sort of drive like you might think a porpoise's tail would work. There is an interesting video somewhere on that site that shows the drive they generate on a smallish wave, here

[Surfdad]

Pictures of the fins and installed.

[LovinPowell]

I guess the proof in those concepts is when/if the professionals on the ASP tour start using those type of fins. Without their endorsement, I don't think that they will take off as well.

[Moki]

I think you should offer a college course on surfing. I feel like I earned at least 2 credits just reading this thread

[LovinPowell]

I've flunked.

[xpjim1]

I had to drop the class. I could not find the classroom

[Surfdad]

Too funny - where is that classroom now? I post this stuff up for folks to generate interest. I get tons of email from folks that are engineers or composite specialsts and they have these incredible concepts. Showing something as simple as a custom fin gets their creative juices flowing, my knowledge is cursory compared to some of the folks I hear from. Thanks for the kudos though!

[LovinPowell]

Thanks for the post, you're awesome, keep it up. I enjoy reading them.

[xpjim1]

Too funny - where is that classroom now? I post this stuff up for folks to generate interest. I get tons of email from folks that are engineers or composite specialsts and they have these incredible concepts. Showing something as simple as a custom fin gets their creative juices flowing, my knowledge is cursory compared to some of the folks I hear from. Thanks for the kudos though!

I agree, Thanks for the post. They are definately interesting reading.

[sagmanaur]

Jeff,

I think you are an engineer in a CPA's body. Your knowledge of all these various physic's principles are amazing.

Thanks for keeping it simplistic for an electrical engineer....

Steve