Fin Toe-in formula

Just having some fun here when I should be doing other things.

a = toe in distance

b = base of small triangle

c = length of base of fin

X = TE to S distance, or trailing edge of fin to stringer distance.

Y = Distance from S to projection point P

Z = Projection distance from TE to P, either forward, at, or aft of the nose.

TE = Trailing edge of base of fin.

S = Point where perpendicular line intersects stringer and TE.

P = Projection point. Where you want the fins to point to, i.e., in front of, behind, or precisely at the nose.

So you want to know how much toe-in to set your fins depending on where the fins are to project to.

For example: You have fins with a 4" base, you want the fins to project to a point 12" in front of the nose, and the trailing edge of the fin base will be 5" from the stringer. Say the board is 7’ long and the trailing edge of the fin is set 1’ from the tail.

So;

X = 5", Y = 72" + 12"= 84", and Z = (Y2 – X2)1/2 = 83.85"

Since X/Z = a/c, a = cX/Z. So a = 4"x 5"/83.85" = 0.2385 inches of toe-in.

Here’s the thing; the angles are so small that you can just approximate by using the base of the fin in place of b, which is unknown. So X/Y = a/b, or X/Y more or less = a/c, so a = cX/Y. In the same example, 5"x 4"/84"= 0.2381 inches of toe-in.

I just remember it as:

Toe-in = (TE)(Fin base) / Projection distance along stringer

Are you actually having fun making surfboards?

That’s why I stick with single fins–less math.

For tri fins, I have developed my own formula:

Angle fins=a little bit

Toe=until it looks about right

The principle has worked for me for a while, but for some reason I can only turn in one direction…

It’s all fun, but at least when I do the math I don’t get resin and fiberglass dust stuck in my eyes. As far as the eyeball approach to setting fins goes, that’s how I ended up with a board that went everywhere but down the line.

[=1]

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[ 2]For tri fins, I have developed my own formula:

Angle fins=a little bit

Toe=until it looks about right[/]

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I ussually toe in a quarter to an eigth of an inch or there abouts.

…I never use the rail line to set my toein…my trailing edge maybe???

…Using a straight center line to mark your fin placements… will give you a truer setting.

…if your fin settings looks off at the rails from using the center line,it’s because the shapes off.And remember just because the stringer looks straight,doesn’t mean it is.by a longshot…Herb

The second you change the rocker, all of the math goes out the window, more rocker, less toe in, real flat boards, lots of toe, tailwidth, surfers foot size, all the above is in the mix

A good approach on multi fin boards is to toe in fins 1|4 inch. There are so many variables with design that it’s nice to have one element that doesn’t change. You can build boards around the fin set up. That’s one thing we have total control over no matter what the rest of the board is doing.Once you have made countless boards of the same design, you can then begin to play with toe angles and cant changes.

Exceptions would be 2 plus 1 setups (1\8" toe) and double sided foiled fish fins(1\16 toe) You must mark some toe on these boards as the fin placement dots leave room for negative toe(toed out) if the fin guy isn’t careful. Marking the fins with 1\16 guarantees the fins will not have negative toe.

I wonder why noone measures the angle of toe in in DEGREES? A toe in of 1/4" or whatever changes with the size of the fin. Even pointing it at a reference to the nose seems better.

regards,

Håvard

I have been setting the toe on my boards using a reference point …12N or 12" back from the nose and somewhere between 3 and 3 1/2" off the stringer. I lay a half template on the blank and use the index side as a straight edge between my referece points. The template will flex to the rocker and is fairly accurate. Some people just use 5" up from the index dot and 1/4" in. Problem on a board over 7" it will be inside the nose. I recently made a 5’0" for one of the groms and used 3 1/2" off the stringer at 12N. Checked it @ 5" in front of index and it was just shy of 1/4". The faster a board is going to be ridden the more lift and drag is created from the “Static” toe in angle. The type of wave ridden will somewhat dictate what is optimum. (It’s all a compromise of some sort!) I personally like more tail rocker and a bit less toe-in to reduce drag but maintain lift/manuverability. (As Jim was alluding to in his post)Then when you open that baby up down the line all you feel is a sense of solid hold without excess drag. Just one more way to skin the proverbial kitty!

Krokus

Haavard,

Just use:

Tangent(angle)(projection distance along the stringer)=(fin dot to stringer distance)

and solve for the angle. The same example in the post comes out to 3.41 degrees.

I just noticed that none of the superscript or subscript notation came through on the original post. It’s just the pythagorean theorem.

Rocker would affect the calculation, but only by shortening up the projection distance, and not much at that. It’s non-Euclidean geometry, but barely.

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Rocker would affect the calculation, but only by shortening up the projection distance, and not much at that. It’s non-Euclidean geometry, but barely.

Just use a Mercator projection… I believe Euler had some simple Differential equations to switch between coordinate systems… those crazy blind mathematicians, what can’t they do?

You could write the calculations out on the board in large typeface, it’s sure to sell better than those Lost boards, you know how the kids love math these days.

Or… you could print out a ruler on a transparancy and drill some sets of holes, with different amounts of toe in. Now you can repeatily, and easily, measure your fins from the stringer (or the tucked edge). Pick a median toe - maybe 3/16"… see how it works, don’t like it, do tight? toe it out a bit more next time. One could even call it “a learning process”. If you still have an urge to do math I have some books you can borrow.

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Just use a Mercator projection… I believe Euler had some simple Differential equations to switch between coordinate systems… those crazy blind mathematicians, what can’t they do?

You could write the calculations out on the board in large typeface, it’s sure to sell better than those Lost boards, you know how the kids love math these days.

Or… you could print out a ruler on a transparancy and drill some sets of holes, with different amounts of toe in. Now you can repeatily, and easily, measure your fins from the stringer (or the tucked edge). Pick a median toe - maybe 3/16"… see how it works, don’t like it, do tight? toe it out a bit more next time. One could even call it “a learning process”. If you still have an urge to do math I have some books you can borrow.

Do you have any books by Thomas Carlyle?

Anyway, Haavard had it right about the length of the fin base changing the angle of the fin if you use one measurement all the time. It’s hardly worth the sarcasm though. Like I said, it was just for fun.

Good stuff Pete. I don’t have the experience for the judgement calls other can make.

Nope… no Carlyle… I always prefered math to history or politics.

I mainly do FCS, so My holes are drilled at a fixed point - the centers of the two plugs. So no, it doesn’t affect the angle based on fin base length. It wouldn’t matter where you drill the holes, so long as their constant, the angle won’t change, nor will the point where the lines meet at the nose… only the toe-in would change for a different size fin base. That’s my whole point. Whose to tell me that 1/4" is the perfect toe-in for my boards? Nobody. The only way is for me to make one, try it, and adjust.

A longer fin base will increase my toe-in. EXACTLY. Isn’t that the point of the removable fin systems… Don’t like how the board feels - try a different template. I just changed my toe-in.

RKelly

A longer fin base would only produce more toe-in if, at the time of installation, you used the same toe in distance, i.e., 1/4" or whatever. When you use a longer fin base with a different set of fins you are adding frontal area to the fins, but the toe-in angle is the same. I guess it’s more or less the same effect, since more toe-in equals more frontal area, but I’m sure there’s an engineer around somwhere who could explain the difference. Maybe more cavitation with extreme toe vs. just having a longer fin base with the same angle.

This is what interests me about the whole question. I’ve been toying with Speeedfins and they go in a little differently–you can change toe-in and fin cant right up until the resin kicks. I gather that’s not the case with FCS. There’s got to be some validity to the concept, since there’s lots of interest in variable angle fins these days.

Thanks.

More advancement in fully adjustable fin boxes would be good.

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A longer fin base would only produce more toe-in if, at the time of installation, you used the same toe in distance, i.e., 1/4" or whatever. When you use a longer fin base with a different set of fins you are adding frontal area to the fins, but the toe-in angle is the same. I guess it’s more or less the same effect, since more toe-in equals more frontal area, but I’m sure there’s an engineer around somwhere who could explain the difference. Maybe more cavitation with extreme toe vs. just having a longer fin base with the same angle.

I am an engineer… I know that the angle stays the same, but the “toe in” doesn’t. I know its small, but the added length, combined with the angle of the fins, means that the tip of the fin will be a little closer to the stringer and the tail of the fin will be a little farther away. So if toe in = the difference in these two measurements, you just increased your toe in.

I have a fairly sound basis of fluid mechanics, but honestly I just haven’t spent the time studing fin mechanics that many around here have… I’ll state my belief anyway: I think toe-in’s major influence is the “footprint” if you will - the area that one would see if you’re standing directly in front of the fin looking at it. More toe is more footprint. This is certainly the factor that becomes dominant when you have too much toe… a lot of drag. Obviously there’s a lot more going on than that, but I don’t know that that needs to be opened up.

yeah… FCS aren’t adjustable. It was just the first fin system I got into and they’re super easy to install. But I have a bunch of different templates and sanded down a few to alter toe and cant. Works great for me, who happens to be my main customer :wink:

cheers,

I agree with you about the footprint & resultant drag – but I also think some amount of toe-in is critical to allowing the rider to pump for speed effectively. Pumping a fin with no toe-in is just pushing water sideways. People don’t pump-drive single fins (or keel fin fish)because it doesn’t work well…