More fin theory and design

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.

The "cut-away", if you will, on the fins of a sail fish:

[attachment=1553:100_0010.JPG]

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:

I've read enough of Surfdad posts, I can only imagine that any manufacturer that sees their caller id when you call must think

"(*#)!(!, Oh, $#(*!(#!"

LOL!

If I ever get a phone call through, I'll ask those folks. I'm not sure why, but they always seem to be busy, or lose their cell phones, or...

To give a better illustration of the operation of the cuta-away, I've labeled this picture.

I had two sets of custom fins foiled by Halycon Custom fins. I'll post picyures 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 expeiramentation, 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 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 (does that phrase resonate with you Loc?).

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

Then again...they could be REALLY expensive POS.

Pictures

[attachment=1554:101_0056.jpg][attachment=1555:101_0057.jpg][attachment=1556:101
_0058.jpg][attachment=1557:100_0059.jpg][attachment=1558:100_0060.jpg]