Getting the fins in the right place

This is my first post on Swaylock’s but I have been sucking up all the high quality knowledge of the members for quite a while. I have made the jump to making my first surfboard. I have it shaped, it’s a 6’ 4" fish, 22" wide, 2 3/4" thick, 18" nose & 16" tail. I’ve been over and over the forums for fin placement & I think I have it down pretty close but I want to run the numbers past the experts here. The fins are ones I made from plywood from a template I downloaded from Sway’s & they are keels with a 7 1/4" base and 4 5/8" high with one side foiled. From what I gleaned from the forum the back edge of the fin should be 7" off the trailing edge, 1 1/4" off the rail, 1/4" toe in & 4 degree cant. I am trying to add pics, because everybody likes pics…




The classic keel placement is: the backs of the fins should line up with the butt crack and the “pins” of the swallows.  No cant, no toe.

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Nice looking board and fins.

LeeV: Is that pretty much THE accepted way to place keels on a fish?  Would toe-ing the fins in and canting them give a more modern ride or take away from a proven design? There are so many different ways discussed here that it is very confusing for a newbie board builder. Being my first board, I just want to get it as close to a rideable board as possible, learn from it and apply it to my next board. Thanks again for your reply, and thanks benr for the compliment. I tried to pinline it but the paint bled under the tape and spread a bit, so maybe I can get a do over after the hot coat & cover it up. I’m going to laminate it next week using Solarez so I’ll have time to get it right without the worry of the resin going off in the middle of the lam. I’m making this board as a celebration of my 50th year of surfing.

I have to agree with LeeV on the classic keel placement . The keel is a low aspect fin thus to angle (toe in) the fin would just cause drag and possible cavitation at speed. On the other hand looking at the shape of your board I am not so sure this classic keel set up is the best for your board. I think that High aspect fins with some flex and angle (toe In) would work better.

After looking at your numbers again , I would have to change my opinion and say the low aspect keels would work fine for a fast down the line board but the high aspect fins would work good if you  slash the face

 

 of the wave. I guess what I am trying to say is the fin set up is going to depend on your style of surfing.

OK, excuse my ignorance, but what is a high aspect fin? Something with less base and more upright? I do surf off my back foot and in my mind I slash like Mick Fanning, but reality is, I’m 61 and probably just doin’ the ol’ wiggle butt. Thanks for your input.

I believe that there was something to the various developments in twin fin design way back when guys like Mark Richards were experimenting...  I.E. - traditional keels fell out of favor, the super wide, nearly parallel tails got pulled in and fin settings allowed more maneuverability in 'normal' waves.  Keep in mind that the original fish design (Steve Lis) was meant to be ridden as a kneeboard at 'slab' type breaks like Big Rock in San Diego.  I've personally seen countless 'classic' fish riders gag out on the shoulder as their boards refused to come around during an attempted cutback.  They simply weren't made for much besides riding the tube.

Mark Richards has been at it (perfecting twin fins) for over 30 years and is worthwhile referencing on this topic.  Current fish designs, even the 'classic' fish designs, have been tweaked considerably from the originals.  Steve Lis' current boards are looking more like MR designs than the original fish.

 http://www.surfscience.com/topics/surfing-lifestyle/shaper-interviews/mark-richards-twin-fins-and-super-twins

SS: Looking at toe-in and cant, is there an ideal placement?

MR: Fins have to be definitely angled in and leaning out significantly. Fins placed parallel to the stringer and with no lean out do not work...they track.

Great stuff in the surfscience interview. MR knows his twinnies. I’m glad I posted up the question this morning, I’ve leaned a lot today. Thank you, John for the the link…

Interesting. I had a very similar sized twin keel. Rear edge of the fins ( 7'' x 5'' glass on fibreglass ) was 7'' from the tips ( though it was a shallow swallow, not a buttcrack ). They were toed in and canted a little bit. Fuggen awesome board, fast n loose. Also, i recently made a small wide eggy twin. I thought i'd try no toe on it. Was fun, but just today i took it for its first surf after routing out the finboxes and adding 1/4'' toe in. IMHO it works much better with toe in.

 

 

 

I’m both a front footed and rear footed surfer with a non-crouching style , but I surf just like Larry Bertleman.

~ Mick is a front footed surfer.

 

Interesting. I had a very similar sized twin keel. Rear edge of the fins ( 7'' x 5'' glass on fibreglass ) was 7'' from the tips ( though it was a shallow swallow, not a buttcrack ). They were toed in and canted a little bit. Fuggen awesome board, fast n loose. Also, i recently made a small wide eggy twin. I thought i'd try no toe on it. Was fun, but just today i took it for its first surf after routing out the finboxes and adding 1/4'' toe in. IMHO it works much better with toe in.

Your board looks good!  - but those fins don't look foiled enough, to me.  The entire fin should be foiled, not just rounded in the front and back, which is what your pic looks like.

I'm not a fin-atic, just a backyarder...but I would expect that top layer of ply to look something like the top layer on these...

I just think, for all the work you've put into this first effort, its a great looking board, I'd spend a bit more time on foiling the fins, I think you'll be much happier with the board's performance, and look.

Oh great.... now that they're glassed you decide to tell him?  Of course you're right - not exactly the latest NACA design but they'll probably go OK. 

The foils on some of my old longboard fins are terrible but it doesn't seem to make that much difference.

Here is a link showing how a thicker leading edge allows a greater angle of attack on sailboat keels.  Not sure if the concept applies to multi-finned surfboards?

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Oh great.... now that they're glassed you decide to tell him?  [/quote]

No hay problema - fins made of fiberglass are shaped after they're glassed =).  Anyway, its a good looking board, I hope he gets the fins just right, and comes back with a great ride report...and some beach pics of the board!

BTW, I didn't fully understand the implications of the info you linked to, but I bet Thrailkill does!  Fin thickness...hmmm, something else to consider.

Hi Huckleberry -

As I understand, it simply means that as the keel (fin) is rotated through wider angles of attack the water will flow more efficiently around the blunt forward edge and thicker foil.  The full implication as applicable to surfboards is likely not fully understood by me either but I have some hunches based on my own experience with high ratio aspect fat foiled fins on my own boards.  The easiest way to understand is often by simply making them and surfing them.  Too much theory and math principles just add to my confusion.  I can usually understand how something under my feet feels and that's what I really want to know.

On a recent 'fish' design I made for a benefit auction, I placed single foiled keels (slightly concaved inner surface) lined up in the basic position LeeV suggested as "Classic" (in line with butt crack and tips) but with slight toe-in and slight cant.

A friend won the auction, his sons have been surfing the board and so far the feedback has been good.

 

disclaimer : I’m no expert.

I was in your position a few years ago - my first build , a fish ( 6’8 x 22 ish) - agonising over fin placement .

I read all the applicable threads and looked at pics of fish by the masters ( Steve Brom springs to mind).

In the end , bearing all that info in mind , it came down to holding the keels  on the board and moving them around til they looked good.

7 inches up , 1 3/8 off the rail and toed 1/4" over the length of the keel ( 7 1/2 ").Cant standard , 4 deg . I think.

The board went / goes really well within the limitations imposed by it’s size.

There’s a long thread about it titled 'the fish that won’t get away ’ or something.

I’ve also measured the placement on a 5’10 Aviso Van Straalen rasta fish - placement almost identical to what I just mentioned.

I used the same placement on my more recent twin keel too but with boxes for adjustment , and that works fine.

 I posted the surfermag article about ’ rocket fish’ ( which boards like ours are more properly called) years ago which you might enjoy reading - search ’ rocket fish article ’ and it’ll come up.

 

Sweet advice Lee. I’ll give it my best shot at getting them in the right spot, but if it ain’t right, time for a Probox set up. I did make some way better fins that are foiled more like they should be. But that brings up another question, I got a piece of Alder from lowe’s that is 1/4" thick x 6" x 2’ that has a nice grain to it  & I am wondering if that would work for fins or is plywood a better choice? I’m guessing the plywood is the stronger of the two.  Thanks…

Thank you all for your input. I appreciate all the info I can get. I did take a good look at the foil of the fins and they do look like the “not this” pic in huckleberry’s post, so I’ll be having a second go at foiling the fins so they look more like the way they should. This time I’ll leave a tab on the base to be able to anchor them in my vice for foiling. From all the great info here I’m definately going to cant & toe them in. I’m picking up the resin and glass tomorrow and hopefully we’ll have some dry warm weather and I’ll get this sucker put together. I’ll probably have questions about the next phase too, Thank god for Swaylock’s…