Measuring Fin Area

General Discussion
1

In this digital age, nearly all fin manufacturers publish the area of their products.  What if you have an old fin sitting around or on an old board and want to know the area of it?  You can trace the outline on a piece of graph paper and count squares, or you can get yourself a planimeter.  A planimeter is an analog computing device for measuring area.  Surveyors, civil engineers, cardiologists and scientists used them in the age of the slide rule.  They’re easily obtained on eBay.  I got my Salmoiraghi for about $30 bucks around 15 years ago.  I never knew such things existed even after graduating from engineering school, then on my first job one of the senior engineers pulled one out.  It was 20 years until I finally bought one.

Pictured are two templates that were traced with the planimeter setup, one off a twin-fin I shaped in the 90’s, and the other from a fin I found in my box likely from the late 70’s/early 80’s.  The first was 23.1 square inches area, and the second was 23.7 square inches area.

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2

I’m impressed with people who have skills, because I basically have none.   That’s why I cheat.  

 

Simple, intuitive, works great.  

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3

Wow, haven’t heard anybody mention a planimeter in-… decades. Lovely tools, back when I was studying boat design and engineering they were a must-have. You can see why:

doc…

4

Been more than decades since I’ve seen one also.  I was in the last year that slide rule was a mandatory class and even got to use a mechanical calculator (noisey, they kept it in a special room).   However like volume of a board,  fin area is about where it is placed not the totals.

5

I agree, Pete.  I think of fin placement in terms of the center of effort of the fin(s), sort of like the center of effort of sails on a boat and the relation to the keel.  I feel fin placement more than I feel fin area in my primative surfing.

all the best

6

Ah, center of effort, or perhaps center of area. There’s an old ship designer’s trick to find that, used on sailboats mostly to find the center of the immersed hull from the side, for figuring the center of lateral resistance versus, say, the center of effort of the sails. . You probably know it already, but I’ll throw it out there none the less.

You take a piece of cardboard, though you could use any thin sheet stock that’s reasonably stiff and uniform. ‘Shirt cardboard’ was what was we used back in the day.

Cut out the outline and then, trial and error, see where it balances when you stick a pin through it and give it a spin. . . Adjust, retry. 

hope that’s of use, or at least entertaining

doc…

7

That doesn’t work in fluid dynamics.  The center of pressure of flow going over a wing, propeller, fin et al is at the 25% chord.  It doesn’t matter if it is NACA010 airfoil at a Reynolds Number of 6,000,000 in water, in air, or a flat plate at a Reynolds Number of 60,000.  That’s jus tthe physics of it.  Then there is the whole elliptical spanwise lift distribution thing due to the tip vortex.  On a straight tapered lifing surface, take the root/base chord and apply it twice to the tip, then the tip chord and apply it twice to the base/root.  Draw lines from opposite endpoints.  Where they cross is the Mean Aerodynamic Chord.  Go 25% back from the leading edge of that, and that is the center of pressure.  With something curved like a surfboard fin, it is best to determiine it experimentally; though it can be approximately determined by dividing it up into several straight tapered sections and doing some mathematical gymnastics.

8

Scott, I’m sure you’re right.  But best applied to steady state laminar flow.  I think my method will serve considering hard bottom turns, pumping, etc.

all the best

9

Works for everything steady state and dynamic like hard bottom turns and pumping - which are really not all that dynamic.  A bottom turn is actually a steady state condition.  Pumping is a bit dynamic, sorta almost kinda like a bird/fish flapping its wings/fins, but it won’t change the physics of the flow across the foil and the resultant pressure distribution.  Our pumping is not dynamic like a humming bird flapping its wings.  We like to think we’re fast and agile, but the reality is we really are not at all.

10

In all honesty, that’s really neat but-

On a machining group, machine tools and the like, that I folllow, there was an ongoing post: candlesticks, but made (machined, on a ridiculously tight lathe) within 1/10,000 of an inch tolerance. Really cool… but…

Silly. 

It was a joke. Something taken to a ridiculous degree and utterly unnecessary. 

Similarly, I was helping a buddy set up a diesel driven pump o a boat, He was a tool and die man and he wanted to align a honkin’ big diesel engine with this pump within a thousandth of an inch. It was doable. But what we were using to gauge it was a casting that was barely within 1/10 of an inch. No way in hell we could measure that close with any ,meaning. 

And in surfing, we know essentially the cube root of nada* about such things as how a board travels across the surface of the water. Lots of handwaving, lots of ‘duuude, it does this’ but the cube root of nada again as regards actual measurements. Actual science or engineering? Nothing. 

So yeah, it’s lovely to do the calculations with Reynolds numbers and all. Times like this, I very much miss my friend and mentor Terry Hendricks, who did this sort of thing for a living among many other things. He could do this…and dismiss it as meaningless. 

But the thing is, is it meaningful? Having precise fluid dynamics for a fin, on a board that is travelling across a medium in a way that we can only wave our hands and say ‘cool, dude’? We’re not talking about , say, an F-35 that has been tested to a fare thee well in wind tunnels, finite elkement analysis and many, many more ways. Same same America;s Cup hulls, or these days foils- and yes, Terry was heavily into those.

It’s a freakin’ surfboard, where we don’t even know what way it really points on it’s journey along a wave. Let alone, say, the angle of attack as said fin goes down the line. We can be real sure it isn’t right down the longitudinal axis of the board, but beyond that? Good luck. Let me know when you’ve rented the wave pool. 

I don;t think so. Not yet. Long way to go before that. In the meantime? Spinning some cardboard around a pin is probably close enough. 

 

doc 

*‘the cube root of fuc# all’ is how I would put it, among other boat design types. We’re a coarse lot…

 

11

I hear ya Doc. Feeling a little coarse myself lately. Not that I’m not interested  in modeling. It just doesn’t seem worthwhile here. 

 

all the best to you and Scott. 

12

From alchemy to super-computers, one variable at a time.

13

Scott Jarratt is spot on guys.

And let’s not overthink it.

The physics help in correlating a design parameter too a feeling while surfing, but in the end it is still just trail and error.

Fin area will help your fin engage at lower speeds, increasing area when a fin is fully engaged makes not much difference but you need to make sure to get it just high enough for your conditions, weight, style, fin placement, …

While fin placement will make a difference in any situation.

14

Well, its entertaining.  A few things come to mind.  One, the thread awhile back on what makes a surfboard go.  Some said gravity, pure and simple, sliding downhill down the face of the wave.  Others (including me) said No, its not just gravity, its the moving force of the wave energy traveling through the water that makes the surfboard go (in addition to gravity).  There was no agreement, and it went on for pages.  We can’t even decide for sure what makes a surfboard go.

Another thing that comes to mind is something I read in the book The Wave by Susan Casey.  Author attended a scientific symposium on wave energy and measurement, top scientists the world over convened to discuss the matter.  In the end, she said there was no agreement on anything, not even on what a wave really is.  Waves, it turns out, are so complex that they defy comprehensive analysis.  Not only more complex than we imagined, but more complex than we can imagine.  Or words to that effect.  Which is why for example oil platforms that use computer wave modeling to withstand anything nature can throw at them routinely fail.  Nature, it seems, will outsmart the best wave modeling computer programs that man has devised.

I have to say I am impressed with the old school engineering method for finding area, the planimeter.  I had never encountered that particular analog device before.

15

Since Huck brought up the what makes surfboards go thread, I have to comment.

Maximum “sustainable speed” is determined by gravity and wave height (physics).  [In general, wave height is determined by wind/storm and/or seismic energy.]

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16

I would do it thiis way too. finFoil is very easy to use. If you have messed around with shaping software, the control point editing will seem natural.

  1. Measure depth of fin (vertical distance base to tip, perpendicular to base)

  2. Take picture of fin dead flat to one side, leading edge on left hand side

  3. Open desktop version of finFoil

  4. Set depth of fin in your favorite units

  5. Drag in picture from desktop or folder into a side view of finFoil and adjust position and size to match depth

  6. Adjust fin contours to match background image

  7. Read off area from information panel

If you have other CAD type programs that can accept a raster image, the same can be done but you would have to do more drawing and scaling manually, then ask the system for area. Sometimes I will place a ruler next to the fin or whatever object I am taking a picture of, to use for tracing, so as to have a dimension for verifying scale.

17

So true.      So very true.

18

Seems like a fin (skeg) would affect the center of lateral resistance.

Lateral resistance is affected by surface area?

Rail and fin interaction?