# FIN BASICS- DRIVE Vs PIVOT

The word on the street is that the BASE of a fin gives it its drive and the RAKE/SPAN gives it its pivot.

[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/fin1.jpg[/IMG]

But Im thinking that its the other way around....

you could have a 2in base fin with a 11 in rake and theres no way that the 2 in base regulates the drive.

But maybe its a blend of area and where that area is distributed.

Not so much relying on finite fin dims but maybe its more about area distribution

[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/fin4.jpg[/IMG]

So a fin like above would be considered to have a small base, but it would, in reality, have lots of drive.

And this config below would be considered a set of DRIVE fins followed by a PIVOT fin  ?

[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/bonfirespitfire.jpg[/IMG]

And then, whats going on here...?

[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/BT2.jpg[/IMG]

Double the drive and double the pivot ?

( With all respect to Bill, whos assisting me with my development of a new Lazor Zap single fin.)

My point.... and I do have one....

is ...

what dimensions attribute to a fins DRIVE and PIVOT or is it a matter of area distribution ?

But…everything you think you have in a fin design can be overruled by the bottom rocker, it becomes a marraige of both parameters

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And then, whats going on here...?

[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/BT2.jpg[/IMG]

Double the drive and double the pivot ?

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Like the chewing gum ad says, ''Double your pleasure, double your fun!''      To answer your question, the angle of the leading edge of the fin,in relation to the bottom of the board, ( the angle of incidence) is the primary source of the ''pivot'' sensation, the quickness of the response to rider input. Fin position is also a major contributor to that sensation too.  Base length, AND total surface area of the fin, to a greater degree, produce the ''drive'' sensation. (when you push the board, the board pushes back)    The same fin  that feels unresponsive in small mushey waves, will feel ''drivey'' and positive in a larger faster wave.    A reaction typical of a 5in base, 8in deep, high aspect ratio glass fin.

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There are different things going on at the bottom surface of the board, than there is one inch or more away from the bottom. On rail, water is being redirected close to the board's bottom surface.

On rail, the angle of the fin relative to flow both near the bottom and away from the bottom plays a factor.

Because of this, IMO fin base length has a major affect on modern board's drive.

Its been my personal experience, experiencing lots with multi-fin systems, that fin cluster spread can make a board feel pivoty or drivey. Small spread changes can have very big effects. Havent experiment much with singles tho, no expertice there. My \$0.02

very good question....

in my personal experience, i have found that wide-based fins offered more "drive" than narrow based fins, so i am going to agree that it is based on the area of distribution that is important to produce drive OUT of a FIN.

As the other posts have mentioned, there are a number of other factors that will govern the overall performance and drive of each SURFBOARD.

the Clustering and positioning of the fins also has a huge bearing on the board's "drive" but i can explain more on that later if u want....

that's my \$0.01 worth...

regards

Deano

i think i will enjoy the info on this thread looks like it will be a good one!

Howzit Bill,Just a thought about those double fin boxes. I amsure evey body who has them has both fins inthe same position, but I wonder how the board would work if the fins were offset with one further forward than the other,might make it really have some drive or might mess up the whole thing.Aloha,Kokua

A few people have said that the base length and area control drive. Very thoughtful.

Here’s my 0.02c, from experience:

I have an old-school, round bottomed 9’8 log, on which I used to use a RFC pivot fin (9.5").  I switched it out for the Michel Junod 17 fin, which has a similar length and rake, but a greater chord all the way along the length of the fin.  Given the size of the board, I don’t notice much reduction in pivot, but the increase in drive is huge.  With the pivot fin, the board is much more passive, but with the bigger fin it projects much further up the wave out of a bottom turn.  Floaters are now much more of an option, where with the pivot fin I run out of steam by the time I get to the top of the wave (if I get there at all).

Obviously, some of this effect is simply the increase in area (big fins on big boards rule!), but I’d be interested to see how it feels with a more raked fin with similar area (therefore having a longer span, similar to my bigger fin).

Tim

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Howzit Bill,Just a thought about those double fin boxes. I amsure evey body who has them has both fins inthe same position, but I wonder how the board would work if the fins were offset with one further forward than the other,might make it really have some drive or might mess up the whole thing.Aloha,Kokua

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Good question.    I don't know the answer.    Might give the ''best of both worlds'' and then again may not.     This may stimulate someone to try it.

FWIW, this is a 9.5" fin that I use on a 9’6" Harbour Banana.  6" base, trailing edge set 7-3/4" up from the tail.

It’s a great, all-around fin for that board.  Provides drive out of turns, pivots well, holds the tail in during nose rides and doesn’t come unglued or sideslip when cutting through soup.  I was skeptical about it before using it, but experience has made me a believer.

One tangent I’ve been interested in are the fins that are cut with less fin area at the base and more out on the tips.  Not just the cutaways but some of the really aggressive shapes like show up in the Pro-Box catalogue or the cutaway keels on Tomo’s fishies or some of Halycon’s fins.  I read some comments from somewhere (prolly one of the above) that moving those wide points deeper on the fin put that drive area deeper into water that is cleaner and less turbulent than the layer that’s travelling around the base of the fin.

I’m also really intrigued by the elliptical fins and the Horan-style keels and Halycon’s “Mental” fins.   Some of those Hanalei fin models really spark my imagination.      That black metal (?) rudder+tunnel thing that Roy has going on one of his boards looks interesting, too.

Too many fins, not enough spare cash to experiment with.   I should work on my fin foiling skills.

My experience leads me to believe that a wider base adds drive. I was having a little problem with a board I have using a 9" cutaway fin, talking to a guy in the water about, he said just try a fin with more base. I’ve since used my 9" Greenough stage 4 on that board and it helps.

I have a double fin board, but it doesn’t have a leash plug, and I hate swimming, so this test could be a problem if it proves to be a mistake. I would think that whatever good things going it does when you turn one way would be negated when you turn the other way. It’s not like you can only make right turns or left turns. And I only have 2 cutaway fins that are the same size and shape.

From the comments here, all the fin area contributes to drive as well as pivot.

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