Short board fin position formula. Or am I Tripping!

General Discussion
1

I,ve had this running around inside my head for some time now. Surfding (on another thread) prompted me into writing it up.... and also to see if  i'm crazy or not. What ya reckon?

 

Sorry if its hard to follow, I didn't like school that much... well I didn't go much anyway.

 

A few points to paint the picture

 

1. It appears (to me) if you use a ratio of fin placement (front and back) divided by board size then multiply that  by a required new board size to obtain a new fin placement, the fin placements get rather excessive as the board grows in length.

Ratio, e.g 11.25" (side fins for 6' board) divided by 72" (6') multiplied by 96" (8') = 15"

 

2. We need a ratio formula that will provide a truer fin placement when scaling boards.

 

3. Lets say for argument sake, each customer rides the same size short board as their height (it might become clearer why later)

 

4. the surfers stance from front foot to back foot then to the groin area forms a isosceles triangle, then from the groin to head a straight line.

 

5. Lets say the groin area is approx 1/2 the height of the surfer

 

6. The distance between the surfer’s feet (stance) will increase "using ratio" by the hight to the groin area (creating the triangle), Not the overall height of the surfer/board.

 

7. How to find the new fin placements in relation to what I’ve just said.

 

8. Find the two extremes of the board sizes in the new list of fin positions you want to make, e.g 72" (6') to 96" (8')

 

The 72" board has 3.5 (back fin) 11.25 (side fins) now find, with ratio, the measurements for the fins for the 96" board. Then divide the DIFFERENCE between both the fin positions of both 72" and 96" by 2 (because stance to groin triangle is approx 1/2 height of surfer/board)

 

Ratio. 3.5 (back fin on 72") divided by 72 then multiply by 96" = 4.666666. The difference between the two back fins is 1.1666666 divide this by 2 = 0.5833333. Then add on to original 3.5 from 72" board = 4.0833333 (this becomes the new position for 96" board)

 

Ratio. 11.25 (side fins on 72") divided by 72 then multiply by 96" = 15. The difference between the two side fins are 3.75 dividing this by 2 = 1.875.  Then add on to original 11.25 from 72" board = 13.125(this becomes the new position for 96" board)

 

This is our NEW fin positions for 96" board 4.0833333" and 13.125"

 

Now to find a fin position for every board you want to increase by an inch (in between the two boards 72" to 96").

 

Get the distance in between back fin of the 72" board and the back fin of the 96" board. And also, a distance between the side fin of the 72" board and the side fin of the 96" board.

 

*B back = 0.5833333 (distance between 72" and 96")

 

*S side = 1.875 (distance between 72" and 96")

 

Now because there is 24" in between 72" and 96" divide above *B and *S by 24. That will give you the amount we have to increase both back and side fins by for every inch (in board length) we want to increase for our new fin positions.

 

*B 0.5833333 divided by 24 = 0.02430555" (every inch up add this much, back fin)

 

*S 1.875 divided by 24 = 0.078125" (every inch up add this much, side fin)

 

 

Am I tripping?

 

 

 

 

 

2

I like your idea a using percentages when scaling.  I do something similar when working with the template, length versus width.

FIns are a little different, because they react with the rider’s position on the board.  On a short board designed for quick turning, I like to keep the front fins under my back foot.  If I want more stability for a step-up tube rider, I like my fins just behind my back foot. 

Foot placement is about where the foam makes the sweet spot.  A wider or thicker tail puts me back further.  A narrower or thinner tail puts me forward.  Depending on the ride you want, the fins move accordingly.

My 16" bat tail has fins at 11".  On this board,  I often find my back foot on the leash cup, behind the front fins.  And it carves short radius turns. 

My 13 1/2" rounded pin step-up has it’s front fins at 12".  But because of the board’s narrowness in the tail, I find myself naturally standing further forward.  My back foot is in front of the front fins.  That board doesn’t turn nearly as short a radius, but is super smooth and stable in the tube.

3

Yorky I have been using this formula for years. Board building is very subjective as you know!

I haven't made that many boards (4,011) to date. So I may not be qualified to make comments. However I try to share whatever works for me and what I have actually done and tested.

Let's say I make a 6'0" Thruster High Performance Shortboard and set the fins as follows:

3 1/2" Center and 11.25 Front the 7'2 I would set at 4 1/8" Center ane 13 1/4" Front

I seen some pro boards with the center at 3 5/8" and the fronts at 11 1/4" however lately it seems that most are at 3 1/2 on shorties, (Grom Boards 3 1/4" Center and 10 5/8" Fronts)

The formula would work like this:

(Length of Board/Center Fin Standard) x New Length

(Length of Board/Front Fin Standard) x New Length

Now if I'm Doing a Gun or a Long board I use this formula only I use smaller side bites on the LB like GL's and a 6" center fin you can adjust in the box.

This formula is just to get your head around the fin placement for various lengths.

You being the Artist have the freedom to do what you want. Sometimes I will move the center fin forward and the front fins back. It's nowing when to do so requires intution.

Surfing Sunset and seeing guys on guns with the front fins real far forward would trip me out. When I did the math it all made sense.

Let's take a 9'0" Gun for paddling in to serious surf where you might surf this board with our body more centered than say a shortboard where you may be more on your back foot.

9'0 = Center Fin 5 1/4 Front Fins 16 7/8 your back foot will be futher forward than it would be for a 6'0 where your front fins will be set at 11 - 11 1/4. You would surf a 9'0 different from a 6'0 or at least one would hope so?

 

Just like the McKee Formula it's only a starting point and very useful. You can put your own twist to fin placement. Cant, Toe, Foil. Then there is formulating were you put all the elements together. Let's say a bottom has a heavy single barrel concave what degree cant do you use?

Your making a semi gun to surf closed out slabs down in Mexico or Chile how much tow do you use? {Fin Placement; Toe; Cant; Foil, Fin Shape, Size, Rake}  Understanding the function of each one and blending them together for a desired effect. The fin placement is a cheat sheet to put you in the ball park. There is so much more to the whole issue of fins and their placement. However it's pretty cool as we are finding it can be thougth provoking!


https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/fin_placement.xls

4

Hello Guy,

Cheers surfding  for you fin placement that sparked this thread, its opened my eyes to riding boards with the fins a bit further up. I’ll be doing some experimenting on this for sure… you know what I just starting writing and that’s what I came up with kind of on the spot, although i been thinking about ratios and stuff on scaling boards for a while . 

I like to have answers for things and and reasons why. As most of the guys on this site they are here because they are fascinated in surfboards and why they do what they do and how we can nail some solid info down.

There are soo many variables and this formula I just came up with on the spot and after a few to many beers very late last night. I haven’t actually  tried it in practice at all, It was just an idea.

So true, there are many variables to match the fin position to the boards characteristics, Not every surfer is scaled in height the same, and not every surfer wants the same board to ride like the next surfer. 

I think I was tripping.

see ya

5

We all have our own formulas. The Beauty is they all work. Differently and some better than others.

Most of them are just starting points anyway!

Your not tripping just thinking outside the box which is a good thing.

 

It's all Good!

SD

6

Kind of related, but i have a 7'10'' nugget/eggy thing. Started with the 2+1 setup, slowly increased the side fin while reducing the centre until i ended putting occy sides in, measuring 7 1/2'' inches back, and putting in an old 80's thruster centre fin ( fits in my LB box ). Goes insane, turns as tight as im able to turn it, and i can come all the way around through a roundhouse. THe back of the centre fin is 8'' from the tail, which to me seems heaps, but hey, it works so good im not going back to the 2+1, even in the junk.

My hack .02c.

7

Beerfan, excuse my ignorance, but how did you move the side fins up?..oh… or did you increase the size of the fins?

8

Hey SD - Thanks so much… Blows my mind.

I had done scaling like you, but I think the “magic” board I was working from had fins further back than may have been best for me to scale up from… I think my biggest guns would benefit from trying the fin set ups you provided ( I didn’t even think to look at my McKee charts, as “they were for quads,” forgetting he was working from a tri front placement), and the trip Robin Mair has me on - considering  fin placement from the front of the fin…

Well - one thing I’ll add:  I like my back foot to be able to get between the front and rear fins for turning, regardless of were I put my feet for down the line/tube riding… Nice to be able to come out of the pocket, and cut back to set up the next one.

If I’d tried the set up in your placement sheet, my back foot coulda’ been @ 3" further forward while still stepping on the turn spot, and the overall “sweet” spot woulda been that much bigger.  Glad I looked at this one.

This whole thing speaks to the beauty of movable fin systems.

9

Sorry mate, the side fins are fixed boxes, they dont move, but i started with small lb side fins, and kept using bigger and bigger fins. Then, the centre fin is an old thruster sized centre fin with the tab, and the screw, which fits in the centre fin box.

 

 

EDIT i put some pics in.

10

TaylorO, If you don’t mind,

What are the dimentions of your magic board that you were scaleing from, and where the fins were placed?

11

Mc Kee’s formulas,

Hope he doesn’t mind me doing this.

12

It’s (Was - as it’s no longer my favorite, been replaced by way better magic, with adjustable fins.) an 8’er round pin 18.5" wide, 2.5" thick, 11.5" tail - fins at 4"/12" - I think, maybe even 3.5"/11.5."

I’ll check tonight -

13

Those Occy front fins work really well on Mini Guns. I like the TC (Tommy Carrolls) as well. Powerful Lefties!

14

TaylorO, I step back my friend, thats a serious surfboard.

I have a stable shaping diet of 5’6 to 6’4" shortboards, mainly for Japanese surfers… of course I do bigger boards but not so ofter these days… I love shaping small boards, less walking around the bay Ha Ha.

Thanks

15

**less walking around the bay **

Classic - Are you kidding?!  My shed is 12 feet long, minus @ 7" for wall, so when I’m working on the 10’+ gun, I’m walking and ducking around “the bay” - Or in my case the one shed fits most, shape, clean, glass, clean, sand, clean, glass, clean, shape, clean… repeat as often as I can afford it…  Ha!

16

TaylorO,

LOL, thats funny… sorry but I had to have a laugh.

I had to shape a surf ski for my neighbour 12 months ago (his own escentric design) in my small bay and I was ducking and weaveing all over the place for hours and hours. Not much fun. I was totally stuffed after it.

I’m getting too lazy I think.

I watch Chris Brock shape his longer boards, he cracks me up, he’s so fussy with minor adjustments. total craftsman with so much pride for someone who has been at it for so long. He does some serious miles, in bare feet too (i’m pretty sure).

And when you start glassing and sanding in you bay things start to get congested pretty quick.

17

FWIW -

It's probably not a good idea to copy fin placement from one type of board to another anyway.  Too many other variables throw it all off. 

That said, I have placed side fins at 17"-17 1/2" up on a couple of 11 footers.  A 12' Mickey Munoz Surftech has the side fins at 17 1/2" coupled with 5" tail rocker.  My personal experience with longboards and side fins is too far forward might get a little sticky.  On Brewer designed guns and longboards with smaller side fins, Surfding's formula might be right on.  Fin size being the variable here.

It has been pointed out here that direct proportioning over great board size changes might be tricky when utilizing the direct formula on outlines.  To mimic a specific curve, it's the way to go unless you're scaling huge differences.  Otherwise the curves start getting straightened.

18

It looks like John gets it!

Fin size and configeration toe, cant. foil all come in to play that's why I share as much as I do. How you put it all together is the key anyway. That's what makes building building boards so fun. Never a dull l moment! You can share every single secrete to building a board. The cool thing is no one listens. Itsn't that great?

Life is good!

19

You can share every single secrete to building a board. The cool thing is no one listens. Itsn’t that great?

Classic - But, I like to keep in mind what’s been said around here lately, personal preference and what ever works for you…

I’ve been measuring the heck out of my most magic board - One thing that I already knew, but, in light of recent threads, have to really consider as a personal thing -

The 8’ x 20.5" x 11.5" tail, has vee starting @ 2’ from tail, (3 degree rocker kick here - and I “discovered” a secondary slight kick @ 17" just in front of the side fins… Stoked on that bit-o-knowledge!) going from 1/16" to 3/16" @ 1’/tail end of side fins - with 1/16" concaves between fin(s) and stringer… Then fading to 1/16" as concaves resolve, and near flat behind rear fin.

Makes me wonder about getting rid of most of the vee to follow thoughts on other more flat/concaved bottom types… I love the way my boards ride… What can I say.

The earlier magic board I wrote about 8’ x 18.5" has an 11.5" tail and the fins are at 4" & 12".

20

I guess the things what is Magic for one surfer may not be for another?

The 8' x 20.5" x 11.5" tail, has vee starting @ 2' from tail, (3 degree rocker kick here - and I "discovered" a secondary slight kick @ 17" just in front of the side fins... Stoked on that bit-o-knowledge!) going from 1/16" to 3/16" @ 1'/tail end of side fins - with 1/16" concaves between fin(s) and stringer.. Then fading to 1/16" as concaves resolve, and near flat behind rear fin.

Makes me wonder about getting rid of most of the vee to follow thoughts on other more flat/concaved bottom types... I love the way my boards ride... What can I say.

The earlier magic board I wrote about 8' x 18.5" has an 11.5" tail and the fins are at 4" & 12".

There are a lot of factors: Where is the board going top be surfed? Like I said the table I use is only a reference and will vary depending apond the situation and the surfer the board is intented for!