Fin Loads

I have been working on some fin ideas that might benefit from knowing the difference in load from the inside fin compared to the outside fin during turns. The most obvious factor is that the outside fin is only under load for a very short time until it comes out of the water. This is a standard three fin shortboard setup.

If anyone has any opinions on this I would appreciate hearing them.




I’ve been wondering whether flex sensors could be used to measure this stuff. Check:

At .019" thick they probably won’t mess up Hydrodynamics too much - just need to be wired to something that can record changes in resistance and make the whole set up water proof. I wouldn’t be surprised if there wasn’t a Swaylocker out there who knew how to do this.

Be real interesting to stick these things all over a board, not just fins, and see what goes on.



Good to see somebody’s thinking in terms of getting some hard numbers. The trick would be in getting 'em and recording 'em, then in turn doing something with it -

How I would play with it might be in several stages:

First, instrumentation- using the flex sensors pinhead has mentioned, perhaps adding pressure sensors like , set up the fin(s) or other surfaces. Note that this need not be limited to fins. Measuring pressures and flex over the whole of a board would, I think, tell us quite a lot which we only think we know something about now.

Wiring for the sensors could be stitched into the glass - it’s real light-gauge wire that wouldn’t require a channel in the foam or anything of the sort.

Now, once you have the sensors installed, you can collect data. But how to store it, along with a timer so that you have a series of measurements?

Something like one of these: - the Basic Stamp series are microcomputers/microcontrollers which can be used for logging data ( as, for instance, resistance changes) - the model is optomised for just that.

Sensors, power source and all can be done up in a small package, something considerably smaller than a bar of wax. And they are capable of taking data from several sources simultaneously ( 16 I/O pins on these little beasties) , storing it as a time-series of measurements and outputting it ( via a serial connection) to a laptop computer. It shouldn’t be especially difficult to make a little ‘black box’ that’d be waterproof and set into a small recess in the deck of a board.

As an aside, I had a gig developing a Basic Stamp based counter and controller setup for the aquaculture industry, to size, count and package shellfish on-site in a very wet environment. When I found that it would be a whole lot easier to implement using an obsolescent ( and cheap) laptop computer I bagged the project, but size and weight of the package were not constraints as they are here. A small instrumentation package that’d easily fit on ( or in) a board could be done up cheaply and well.

Then, test and calibrate the stuff so the readings you get in actual use mean something rather than just raw numbers. Lots of ways to do that.

As there are lots of folks here who are a whole lot better at electronics than I am, I’ll leave it at that for somebody else to take further. If someone is in a marine engineering program now, this would make a very nice thesis project.

hope that’s of use


well well grasshoppah…be one with your angle of attack and set yourself free…until you better understand water flow movement under the board (a very complex phenomenon) the better you’ll understand AOA on fins and its affects…have you seen my clear test board for this?

front fins have angles set into them…toein and cant…that will affect your analysis…

Pinhead and Doc

Thanks for your responses, but I was thinking more along the lines of educated guessing. It sounds a little strange but I’m thinking that fins could be designed slightly different due to the lack of strength needed in certain directions on the fin opposed others. Like inward pressure on the outside fin compared to outward pressure on the inside fin. I’m guessing the difference is about 1 to 3 (outside to inside). I’ve broken a lot of inside fins during big bottom turns during testing (the waves were big, the fins were to weak and I’m 90kg) but not one outside fin ever breaking inward.

Maybe it could also be due to the fact that the water is hitting the outside fin at a much shallower angle because of the cant compared to the straight angle of load on the more vertical inside fin. Maybe I’m way off with this line of thinking but I’m still interested in what you guys think.

Cheers again


Thanks meerrafty

Sorry, I posted my last reply before seeing yours. I thought it would be mostly due to the angles of water flow. This all helps me.

What’s the clear test board? Sounds interesting to say the least.



try this…on a bottom turn the outside fin thats lifting out of the water is experiencing pressure from both the inside and outside…did i mentioned complex?

It can be approximated from the fin depth relative to the water surface. Of course it will vary from turn to turn. In a hard bottom turn, the bottom of the board may be angled at 20 degrees, with the inside rail fin 6 inches deeper than the outside rail fin.

The water pressure will be proportional to depth. If the outside rail fin is 2 inches under water, the water pressure will be tripled on the inside rail fin from hydropstatic pressures.

Then there is the difficult to quantify variable of water flow speed. The flow speed is fastest close to the inside rail fin, at its point of maximal curvature.

I’d guesstimate at least 3 times greater, perhaps as much as 5 times greater.

The non-surfer engineer I worked with totally underestimated the side forces on a fin. The first attempts on lightweight fins failed, they disintegrated. The design was good, the structure bad.

If we had numbers to work with I’m sure he would have designed the structure differently, and we would not have gone through that learning curve.

Still lots of variation in user pressure, but getting info like this is only a step in the right direction.

My gut feeling is that in addition to factors described by Blakestah, it has a lot to do with how they’re foiled… The flat (or in some cases concave) inside surfaces are going to generate more loading pressure than the domed outer surface at least on twins and thruster side fins.

I would think that on a traditional symmetrically foiled twin (Lis/Frye Fish) that there wouldn’t be as much difference between inside and outside surface loads of either fin.

An example of loads on an asymmetrical foil might be found on an airplane wing with the flat underside generating lift.

If you have removable side fins, you might try fitting them with flat sides outward as an experiment.

For an estimate of the maximum force on a fin (not counting striking a reef, etc. :slight_smile:

  1. Compute the dynamic pressure. Equal to 1/2 times the density of sea water (1.99 slugs/cu-ft) times the square of the speed of the flow past the fin (use speed measured in ft/sec, which is = 1.47 x speed in mph). Note that the speed may be the sum of the speed of the surfer across the water plus the speed of the foil relative to the surfer–e.g. the transient flow condition that can exist when a fin (and board) “break” free and skid (typically rotationally) through the water relative to the rider.

  2. Multipy result in step (1) by the maximum lift coefficient for the foil/fin–or the drag coefficient if the flow is perpendicular to the fin/foil. Choose the larger of the two for the calculation. A typical maximum lift coefficient will be about 1.6; some specialized foils may get up to about 2.3. A flat plate drag coefficient for a long rectangular shape is about 2.2; for a circle it is about 1.2. The planform shape for a fin will fall somewhere between those values, so a coefficient in the range of 1.6 to 2.2 should be reasonable.

  3. Result is the loading in lb/sq-ft.

  4. Multiply the result of step (2) by the planform area of the fin (in sq-ft = sq-inches/144). Gives the maximum force (in lbs) that can be exerted on/by the fin.

  5. The difference in loadings among the three fins (on a thruster) is much more likely to be due to which fins ventilate and which don’t, and/or their relative planform areas, and/or fraction of their planform area that is wetted, than from the variations in the flow speeds among each of the fins/foils.

  6. A fin/box, or glassed on fin, may also break due to the torque exerted on the base of the fin (and box, if present; or the foam/glass structure of the board itself). You can (over) estimate this (but obtain an approximate number for comparison purposes) by multiplying the force computed in step (4) by the distance from the root of the fin to its tip (in feet). A more accurate number is probably obtained using something closer to one-half the length of the fin, but if you’re doing relative values, this doesn’t matter.

  7. However, the propensity to breakage from torque also depends on a the specific design of the fin/board attachment (e.g. glassed on vs box; and if the latter, the specifics of the design of the box and how it integrates into the board structure). A potentially important factor is the torque per unit length of root attachment. To estimate this, divide the result in step (6) by the (total) length of the base of the fin that is inserted into a box, or the length of the root of the fin (in the case of a glassed on fin). Hence everything else being equal, a long, shallow keel fin will generally be much more resistant to failure than a short, tall fin of equal planform area.


… and the equal and opposite force would be the 90 kg surfer pressing down hard, maybe at 190 kg onto the top of his board. This load spread across the wetted surface of the board and fin.

Thanks Everyone. This is exactly what I was looking for.

Any way you look at it, that poor inside fin sure does get hard time. Maybe next time we might spare a thought for it and all start turning a little softer.



Sorry, no turning softer, it’s not in the rules.

Turn hard, give those fins hell.

Find the limits, then turn even harder.

Learn and enjoy.

Make better fins.

Hi Wildy

I was just saying that softer turning stuff because of the S.P.C.I.F. ( the Society for the Prevention of Cruelty to Inside Fins). They’ve been on my case ever since I started making my own. But like I always say to my fins “I bought you into this world, I can take you out!”



Good, good, I thought you were falling into that bad habit of soft turns. I went through a short stage like that and didn’t like it. I snapped out of it, in both senses of the word. Glad to hear you’re okay.

Cowboy , I’m sure being from the eastern island of australia [!] , you guys ALSO know the expression …"go hard… or go home ! " , eh ? heheh