fin design and sailboat keels?

So I was reading the Nov/Dec 1996 longboard magazine devoted to longboard design last night and I happened upon an article about fin design and function. They traced the evolution of fin design from the 1930's through oddities such as the Velzy V-fin, Dave Sweet Bat fin (had to throw that in), the white Owl flow-through, to the modern O'Fish'L type of fin. It was interesting to note the design differences between the older models and the more modern version of the fin and the parallel to sailboat keels. More specifically the how the surfboard fin has evolved from a greater rake (aspect ratio?) to something with less rake (or angle from the deck to the leading edge of the fin). I draw my parallel from sailboat keels from the early (sloop) America's cup boats from 1900-1980's, http://www.acmuseum.com/poster.html where the keels were severely raked. While sailboats of the 70's (San Juan's I think) the keel looks more like the modern fins found on surfboards today. So why are there no fins with the decreased rake like the Melges 24 http://www.melges.com/24melges.html (look at the keel in the lower right side of the page). I was thinking (could be a major problem here) what are the basic differences between the form and function of the modern sailboat keels in relation to surfboard fins?

The function of a keel on the sailboat is to offset the sideways force of the wind on the sails and convert this into foreword momentum while decreasing the drag as much as possible, and to keep them upright (i.e. lead), which is similar to the surfboard (except for the lead).

1. You want decreased surface area = drag, while decreasing side slippage. 2. You want to be able to power out of a turn and maintain speed.

So why are surfboard fins not shaped like the modern rockets like the Melges? Is it due to the problem of depth versus surface area (less rake thinner blade = increase in surface area as a function of the length to a lesser degree than one with a greater rake)?

Or is it a lack of tank testing and money for R and D in the surfboard industry, whereas the racing sailboat industry lives and dies by tank testing.

Or is it some other factor that I cannot seen to grasp?

I did find one fin that was similar to a modern keel, the Ben Lexcen Wing, which mirrors the Americas cup boats of the 80's.

Oh yeah it's flat here,

<snip>

I had one (Ben Lexcen keel). I used it on my semi-racy 6'8' winged swallowtail, and it felt both stiff and it wanted to force the board into pearling. I got the idea after a go-out or two that it wasn't meant fot that kind of board. I think I also tried it on my 9' log. It worked better on that, but I stayed with more traditional fin shapes. The guy who helped develop it for surfboards, Cheyne Horan, rode very wide-tailed boards and it appeared in movies and vids like he had to push his boards a lot harder to get them to turn.

I had a Butterfly Fin, which is a shallower version of the Velzy V-fin. It allowed (gave) me a quick sideslip in a hollow section on a small wave on my log. It didn't provide enough 'grip' in the wave the day I got back to FL during the Tropical Storm Dennis swell of '81. It worked well enough until it allowed the board to spin out, with me landing on the side of my head. It felt like a blast of air blew though my head

[snipped]

The main difference I see (besides the necessity to carry the ballast as low as possible on a sailboat) is that a surfboard fin operates in a very different , disturbed, surface zone of the water, with much cavitation and entrained air often effecting the flow around the foil. Fin keels on sailboats operate much deeper in a relatively undisturbed flow.

On a keel-type sailboat, the speed ranges are usually very different than with a surfboard as well. Until a sailboat begins to actually plane, it would never attain the speed through the water of a surfboard on a good drop.

Having said that, though, I have seen some higher aspect-ratio, less raked-back 'hatchet'-style fins that do mimic the fin keel of some sailboats. It seems to me, though that fin shape development hasn't really gone a long way since Greenough developed the flexible, high aspect-ratio glass fin for his kneeboards.

Just for a little FYI - aspect ratio is generally the ratio between length and width of the wing/keel/blade/sail/fin/whatever.....

Fer instance, a Greenough Stage IV; a long thin fin, has a pretty high aspect ratio. The fin on a Velzy popout (which is just about square ) has a low aspect ratio....

But I digress.....

Yah..it's pickin' up a little here, but still not enough to push my fat old ass, so I guess this is gonna get my usual flat spell beat it to death approach.....

Okay... thing is that the parallell between sailboat keels and surfboard fins is severely flawed:

1) the keel serves as both moment arm for ballast and as resistance to lateral movement. The surboard fin is more like the feathers (fletching, for you trivia buffs) on an arrow or those (flights ) on a dart: it pretty much has a basic purpose of keeping the tail of the board at the back.

2) Boats operate as both planing hulls and displacement hulls pretty much on a flat surface, leaned away from the direction of turn. The rails of a board make it happen, and on non-flat surfaces, and always as a planing hull.

3) The function of the fins on a tri, quad or twin fin ...is not really understood. It works and that's really all we know.

See http://groups.google.com/groups?q=group:alt.surfing+authordbchguy&s... &hl=en&safe=off&rnum=35&ic=1&selm=330e8c8c.36df5d46% 40usw-ex0107-055.remarq. com for a very good analysis.

Multiple keels on a boat are either useful for beaching the thing and keeping it upright (see a bunch of boats on the English East Coast including a fairly famous design by Laurent Giles , the name of which escapes me just at this moment ) or to allow a little more lateral resistance in an otherwise not too resistant hull that has to operate in shallow water and they don't wanna use a centerboard.

As for the lexcen wing fin...and the velzy fin and all the others...

The surf industry is all about hype and selling crap, not about functional improvement.

Cavitation, the formation of bubbles from water vapor or dissolved gases due to extremely low pressures, in essence boiling the water, does not happen around surfboard fins. There's not a low enough pressure. It can happen in pumps and at the leading edges of ship propellors.

What does happen is flow separation around the fins and the formation of a turbulent wake, with lots of drag resulting. Separation is the point at which the flow past the fin reverses direction. It leads to stall of airplanes and drag on golf balls (and moving the separation point back on the golf ball by adding dimples leads to less of that drag). The shedding of eddies within the wake can lead to the 'singing' of telephone wires in the wind, or surfboard fins on a hard bottom turn, and it did lead to the resonant self- destruction of the Tacoma Narrows bridge.

Ordinarily I wouldn't nitpick about this but it's a technical thread and the word 'cavitation' got used a number of times.

Thanks for the correction, no sense in me going around babbling more than I already do.

Have you read 'to engineer is human' by Henry Petroski fairly good book for the non-engineer (me).

Just air entrained earlier where the board is slicing through the water, that then gets caught in the fin wake.

Cavitation is a major problem with turbomachinery because of the wear it causes, when the gas bubbles come back into contact with the pump impellor or boat prop, they collapse (remember the water wouldn't ordinarily be boiling) back into a liquid. This collapse pits the hell out of the toughest metals, and would eat a surfboard fin like Foondoggy nibbling daintily on a Krispy Kreme.

A M24 has a rudder to turn so you dont care how forgiving the keel is. On a surfboard you have to introduce rake and toe to take care of turning, a stright high aspect foil would work like crap with this much rake and toe so you make it shorter and cant it back.

M24's along with any high performance skiff (18 footer, 49er) were designed to go straight fast and handle like crap especially at low speeds. Surfboards need to go fast but a bigger priority is that they handle well.