fin cant method...

I’ve seen some references to fin cant (tilt) expressed as “X” degrees… hard to measure with concave, vee, etc. Read somewhere that another way is measure the distance between the trailing edge of the bases and also the distance between the tips.

Anybody have a measuring tape and care to post their favorite board specs at those points?


Read somewhere that another way is measure the distance between the trailing edge of the bases and also the distance between the tips.

sounds like more trouble than it’s worth…especially when trying to make those measurements accurately while setting your fins. a more simple means might be to lay a straight edge over the board from rail-to-rail behind the fins…eliminates any confusion caused by vee / concaves.

I know I’ll get flack, but the best way to set fins is visual, relative to the bottom shape.

If the fins look too splayed out, then they probably are.

If they look too vertical, then they probably are.

So many variables in between, especially when they will look different on a concave compared to a vee bottom.

Of all the fins I set, and with lots of experimentation, the ones that looked right went unreal. Take into account crooked stringers, slightly asymetrical shaping, etc, and you will have nightmares trying to get measurements to match.

Measurements are a good guide, but what it looks like is what counts. Measure once, look twenty times.

I like your style, Wildy.

I can’t say I’d be able to do it myself…but I sure can tell from looking at someone else’s board, if I think the fin angles look right or not.

a more simple means might be to lay a straight edge over the board from rail-to-rail behind the fins…

Yeah but… the straight edge tilts when on a vee. If somebody will take the measurements, I’m interested in a reference range. Maybe 4 cm? 1 1/2"??

Two theories…

Theory One says that the angle of the fin should be measured perpendicular from the rail-to-rail plane

first…make sure your board is sitting level on the rack (use a level) and stabilize it with rags or foam.

second…plumb the fin so you know it’s vertical

third…measure and cant the fin to the desired angle

Theory Two says cant is measured relative to the bottom of the board, from the plane of the vee or concave where it intersects with the plane of the fin, since that’s the dynamic that matters - ie. where the water actually flows.

So…4 degrees to you might not feel the same as 4 degrees to me, depending on how we measure. The simple answer is…measure the angle of a few boards you like and do it the same way when you glass on a fin.

Have Fun!

Sheesh - You guys are killing me! I’m just curious how the trend is gonna work out using those positions as measuring points, not debating the pros and cons of every method out there… anybody?

That’s funny John. I just finished a thruster. It’s got a bit of vee. I measured the distance between the tips of the rail fins to the tip of the rear fin. Looked good. While laminating it on I knocked it over. God damm. So I ‘eyeballed it.’ Looked good. Finished the board yesterday. I thought. Took it out and was looking at it in the afternoon light. The dang thing is just barely tilted to port. Guess I’m not finished. mike

the angle on most boards is measured from the shaped bottom - bottom contours included. Period.

I am with wildy and benny on this one…like my dad always says, “trust your eyes.” I am a nuerotic measurement-taker, fin postion marker, and cant angle calculcator, but when it comes down to it, something is always off by a 1/16th here or a degree or 2 there. I use measurements to get as close as possible, then I basically eyeball it. If it looks wrong, it always is! If it looks right, it is probably still wrong…but as long as I think it is right the board will go!

which brings me to rule number 2. Once the damn fins are in, the metrology stops! I figure I am way more of a perfectionist than whoever the board is for, so if it satifised my eyeballing, it will satisfy theirs…

just as a side note, rule 3 is to lie thru my teeth when someone asks, “did you use gorilla glue to reattach a tiny piece of the nose here?” in reference to the nose of every blank I shape after I inevitably slam it into the wall trying to carry it in off the porch/shaping bay. haha

You’ll need a scientific calculator, but here’s a formula that should work for that.

Average cant = arcsin ((distance from fin tip to tip - distance from base to base)/2*fin length from base to tip)

Make sure calculator is set to degrees, not radians, and also note that this is the average cant of each fin, so you have to be sure to set the fins to exactly the same angle.


Something about rocket science comes to mind here. I see your point. Vee, concave, double vee, whatever, the point is the bottom isn’t flat between the fins.

Here’s a fairly simple solution. Use a strip of wood that’s about 1/4" think and an inch or so wide, cut it to length about 1/4" short of the width between your fins. Mark the center of the length of the wood strip. Glue or staple rubber strips at the end of the wood strip. Use something thicker than the height of the ridge of the vee. These act as little support feet spanning the ridge of the vee. Set it up with the centerline of the wood over the stringer and now you have a flat reference that is perpendicular to the stringer. It should work for any bottom contour as long as it symetrical on both sides of the stringer. The just set your protractor, pre-cut angle guage, or whatever on the strip when you align the fins.

As I think about it, you may want the feet a little more inboard to avoid getting resin on them when setting the fins.

I attached a quick and dirty drawing incase my description wasn’t clear. Of course the scale is out of whack. The feet will probably be more like 1/8" to clear the ridge.

OK OK - I give up. I simply thought it might be easy to have a base/tip distance reference of say 1 1/4" - 1 1/2" to aid in the setting of fins but since you guys don’t want to play, just forget it. (sitting in corner pouting)

Howzit Ryan, I just use a adjustable bevel square so I can change the cant to what ever degree I want and it reaches from the stringer to the fin and seems to work just fine.Aloha,Kokua

C’mon now John. No more pouting.

Actually I’m no good at setting fins. I’m a rookie. I’m decent with geometry and jigs though due to my machining and wood working experince.

Take a look at the jig. It’s really crude, simple, cheap, easy to make, and effective. It’s just a little strip you set betweent the fins and set your angle guide on. It levels out all the bottom contour between the fins. Say you take a strip of 3/16" or 1/4" plywood. Glue a little extra piece at each end and it levels the span between fins. The you just set your angle guide or bevel square or protractor or whatever on the end. Snug your fin face to that and let it set. Flip it around and set the other side. A little tape to hold it all. Done.

Okay. I’m a rookie so I’m probably missing some point and that’s why this jig seems simple and effective.

I remember watching John Carper’s Glassing 101 and how the fin guy had to use his eye to adjust for the bottom controur. He had and angle guide to get him close but made the fine tune by eye. That kind of bugged me. He put down a bead of hot Q-cell to set the fin in. He set the angle guide to the inside of the fin to get it close, eyeballed to fine tune it, and held it for a bit then put on masking tape to hold it. Now, I can imagine slight varaitions in the bottom but most guys can get that area pretty consistent. And I can appreciate the skill of a craftsman who can eyeball the alignment of a fin. But I thought there should be a simple tool to level out the irregularities and span the vee if it exists. That’s where the idea of this basic jig came from. To me it seems that your toe and your front to back placement are easily laid out once you know them. The tough thing is getting the angle right if you working directly form the bottom of the board.

Kokua solving the contour problemy by using a bevel square that is long enough to span most of the area betweent the fin and the stringer. That seems like a good idea too. I guess the question is what is the fin angle referenced from? If the fin angle is from the board bottom on that side, or from a line perpendicular to the stringer.

Argh…I quit… I’m thinking too hard…now I want to sit in the corner and pout.

OK…so what about double foiled fins? no standard jig will fit all fins because each has a different convex curve. For that, you gotta use the tip-to-base trick, OR… make a jig that has a concave to it, where the top of the jig hits the tip and the bottom hits the base… but then different fin depths mess with that plan…

too much hastle…

measure the angle from the shaped board bottom, because that’s the only measurement that matters… because that’s where the water flows through… which is what fins do. They change the flow of water under the board in order to change its velocity in order to create force.

Have Fun!

I dunno about the double foiled fins. Hey man, I’m just doin’ what I can. Stop putting so much pressure on me :-[

Just kidding.

So when you’re using the tip to base distance thing, how to you get the vertical line to measure from? I suppose a square and a ruler.

how do you get the vertical line to measure from?

You don’t really need one.

So what’s the difference in trailing edges measured base to base vs tip to tip? say 1 1/4" - 1 1/2" ?

Once you have those dimensions and have the bases lined up with desired toe in, a simple trapezoidal template of masonite or acrylic on a base as you describe could allow nearly instant cant positioning when glassing in fins.

That was kind of the whole idea… a set of conventional parameters for fin cant template designs. You all can keep measuring each fin set up with straight edges, protractors and variable bevel devices though.

that method will certainly work like a charm, as long as the fin depth is consistent.

I simply set the on the line I drew on the bottom of the board, toed in as I like it, then measure the cant angle with a angle finder or jig at the trailing edge of the fin, from base to tip. This method can be used with single or double foiled fins, as well as fins of any depth, since the tip and trailing edge base are always along the fin’s plane of symmetry. (The inside foil can add as much as an extra degree. If you don’t believe me, put an angle finder up to the side of any center fin and measure. It ain’t gonna be exactly 90 degrees, because the fin should become thinner toward the tip.) You can also measure the angle near the leading edge, but you might not have enough fin in that location to ensure an accurate measurement.

Have Fun!

John, I like the trapezoid idea. You’d probably need two, with slots cut in them perpendicular to the fins, and wingnuts for setting…because otherwise, every tail width would need a different width trapezoid, unless it was adjustable.