I’ve got two possible fin templates to use as twin on fish I am soon to make. Anybody know any simple way for a non-calculus type person to figure out the surface area of these non-rectangular, irregular shapes so as to make each one the same “size” by area? (Did that sentence make sense?)
Draw them both on graph paper and count the squares as best you can.
Sure it’s crude, but good enough to give you an indication of comparable areas.
Of course that doesn’t take foil into account, so the actual surface area would be much harder to calculate.
When I was a kid I ran string around the perimeter of fins until the ends of string met and were fastened together. The loop of string was removed and formed into a rectangle. Measure sides and multiply length by width = total area.
I’d also play with making new fin designs using that same loop/area of string. Quick and easy!
if u go to a drafting supplier (of know a draftsman) you may be able to get a cheap planimeter - calculates area in any scale
I thought about the string perimeter thing too and it will make a good estimation but with some ‘radical’ shapes such as the narrow neck on the stage 6 it may be inaccurate, for example 12 x1 and 8 x 5 rectangles have equal perimeters but 12 vs 40 square units
I have surfed a 6-0 x 19 3/4 x 2 3/4 bonzer bottomed single back to back (on a true ames greenough 4c 9.5" and then a stage 6 9.0" that I thinned the blade 2/3rds from its original thickness and rounded the blade outline a bit as well) in head high good points waves on the same day and found that on mushier or weaker waves (even all the way up in the box ) (in general I like bigger fins further up when surfing single fins) the 9.5" 4c was simply too much fin while the stage 6 loosened it up quite noticably while sacrificing some drive, both at the same location (all the way up) in the fin box. The 4c with its overall more traditional flex pattern and base length seems to hold in and drive better on late drops and harder bottom turns where the board in general starts to max out (head high very hollow powerful) or a few feet overhead semi hollow, where I would much rather be on a multifinned board with more tail rocker.
I have experimented with using paddle fins as rail fins on quad setups originally starting with a traditional 4.5" (i think) deep “common” template (they were actually red/x glass front fins from a romonosky quad) then shifting the area around to a paddle fin approach. I have made two standup fish (both quads) and two spoons (quads as well) and found that you can go bigger/deeper in general with the paddle fins than traditional templates, and still not get too stiff or tracky, and they hold in fine. And use less toe though I have not experimented with cant. Right now the latest template for the front I am using has 1/2" up size all around than the original I spoke of and 5.5" depth, 3 3/4" base; trailers are traditional templates about 4" deep. overall a LOT of fin, on the standup board with too little tail rocker (my fault) too stiff but still manageable with pilot adjustments to excel in surf from 1-5 feet. On the spoon (which is destroyed at the moment) it seems like I could put more fin still if I wanted to.
So if you want to try the stage 6 paddles as twins (cooll idea i’ve been meaning to try myself) then I would reccommend getting your basic outline and rake from the original keels, up sizing 1/2" more all around the outline perimeter, and maybe another 1/4 inch depth.
One day I surfed someone elses quad fish with the curved turbos right after mine and it went way better standup, but was still more akin to a thruster in terms of reliable feel and desire to be pumped, the paddles (when set straight no cant) in the quad setup seem a compromise more towards allowing the glide of a single fin/ keel hauling. but still break up the outline to allow an acceptable amount of looseness.
Bottom line is it all works, you as the pilot can always make the final adjustments, one good sized day i tore the trailer on my 6-1 x 18 5/8 x 2 1/4 thruster on rock dropping into a set and surfed it on the same size waves another 30 minutes as a twin fin (knowingly) (futures medium flex black and yellow carbon rtms), still made some all the way through, still held in some (not really critical but still a little hollow) tube sections, just use more rail. Or look at liddles forward fin placement, still holds and drives. I guess its just how much down the line rail you wanna feel and how much tail pressure vert you wanna go, and all the infinite compromisal levels in between.
Good luck with your ideas, I hope some of this rambling helps.
edit- oh yeah there is a allaboutsurf.com hydrodynamica photo of daniel thompson holding up a fish which appears twin keel, but the fins do have a little
greenough look to them, i wonder how the board went.
also a big advantage of your twin paddle idea would be the minimum base contact area with the board which would nurture flex (??) but I imagine the same could be acheived by as dale has said just glassing the front 1/3 or have of a traditional keel, also I remember mister Lee V. has surfed paddles and traditional fins and has mentioned something along the lines of very directional but not as drivey as a traditional template) (??)
by the way wildy I vagabond through once in a while and have always enjoyed your posts and approach to surfing, now that i think of it your fondness for the inline single fin makes sense as my thoughts for the quad is the front fin for direction (paddle outline) and rear for drive/stability (traditional outline). Just some rambled thoughts, hell hopefully only one pair at a time is in the water at a time anyway haha take care.
…love the google ad down the bottom of THIS thread , too !! [HOW do they work these out !!!]
Good Dr., I think I’d go with the keels on the fish , personally , or a nice 5x6" or 5 1/2 x6" or 6x5" or 6x5 1/2 " [whatever !!] twinny. [“twin fin templates” thread has a few , if you’re stuck ?]
Dr…,
Wildy has provided the simple, practical, how to method. It is also the method I use. Did I forget to mention easy?
MMMMM. The Stage VI makes me drool…I absolutely love that fin on my single, it goes like a dream.
Yo,
When you consider the area formula for a right triangle is
1/2 BxH Solomonson has the fin area problem nailed! all you need is a piece of string long enough to reach around the perimeter of the fin and a tape measure to measure the base and height of the right triangle you form out of it.
Mahalo, Rich
Halcyon,
Now that is an elegant solution. I like that. FINAL ANSWER.
With all due respect the basic geometry mentioned here is circumspect. Perimeter does not equal area. Measure the perimeter of a cut away fin and you can have a higher number than a fin of equal base and height but more actual area. The perimeter of a “C” shape is far greater than an “O” of equal radius. The perimeter continues inside the “C”. Same goes for the trailing edge of the cutaway.
Furthermore you can have equal perimeter dimensions of a square and a circle and the circle will have the greater area. That simple fact is why pipes are round and not square.
This can and has all be proven experimentally. But you can also take my word for it and you will be okay.
For simplicity counting squares on graph paper is probably the easiest most practical and accurate for now. The smaller the squares the more accurate the measurement.
No shit! perimeter doesn’t equal area? come on! You must be kidding!
Of course the total area changes with the foil ~~ I mean is that a no brainer or what?
No Worries, Rich
I’m not talking about foil.
I draw mine on graph paper.
At each 1/2 inch of depth, measure the chord length in inches.
Add them all up, divide by two. The answer is an approximation on the two dimensional area of the fin.
But this method is very useful for design purposes, too. The chord length at different depths has different purposes. On a rail fin the base chord length has the most impact on turning, and the tip chord length has the most impact on induced drag. For a center fin the chord length about 2 inches up from the base has the biggest impact on turning. And the base and tip chord lengths both have substantial impacts on induced drag. Fin tips are rounded either to convey some advantage by rotation in toe in at the tip, or to keep them from poking your eye out, or both.
Working in Euclidean Geometry in two dimensions.
The problem is with taking a curved line and inferring that straightening it will give a meaningful basis for area calculations.
Here is an example of comparing two objects one with curved line perimeter surface and one with straight lines.
A square with a perimeter equal to the circumference of a circle.
Finding area of a circle from circumference.
Area of a circle from known circumference.
First solve for diameter (D).
Diameter is Circumference divided by pi.
D= C/pi
Radius is half the diameter.
R=D/2
Area is pi times Radius squared.
A= pi x R2
Area of a square is base times height. (Also side squared.)
A=BxH
A square with a perimeter of 8 compared with a circle with a circumference of 8.
Area of the Square
2x2=4
Area of the Circle
D= 8/3.14
D= 2.54
r= 1.27
A= 3.14 x (1.27 x 1.27)
A= 3.14 x 1.6129
A= 5.064506
5.064506 > 4
With equal outside perimeter measurements as could be done with a string, the circle clearly contains 25% more area than the square.
The problem with taking a curved line and straightening it and calculating a right triangle area from it will not result in an acurate
estimation of area. Euclidean Geometry is a physical science. Fin templates are very very complicated shapes.
There is nothing wrong with counting squares on graph paper to estimate the area of a fin template.
When doing hand work the old ways still work just fine.
Thanks to all. That sound you heard was the forehead slap about drawing it on graph paper and count the squares.
Long as I"ve got your attention, this is going to be a small flex tail fish for a small person (5’3" X 130 pounds); if I decided to go w/ twin keels and am aiming for drivey, traditional down the line and big round type surfing, keel dimension suggestion would be appreciated. I’d likely go 5 X 9 on a 5’4" but how about on a 4’ 10" X 20 3/4???
haha. Me too. Pounding my head on the desk. I had to dig my HS math out of who knows where for that. But you know what? There are no simple answers anymore. We’ve heard all of them here for years. So now I think we need to really complicate things. This Clark issue has made me think we can dispense with the simple answers and really get to work and dig for some new answers. First concept toward understanding the peculiarities of curves and flats is that this understanding is fundamental to understanding surfboard and fin designs and the inter-relationship between the two. (Bert is still a little mystical on this, but he has done his homework.) So give me a little time to think this through. I’ve written one reply, but I want to edit it first. Yeah you got my attention:-)
But first, answer us this. Why Keels on a flex tail? See any potential conflicts or inherent problems down the road with that combo? Think about structural conflicts and how they relate back to design goals.
Tail approx 17 1/2 inches points about 13+ apart. Keels only glassed on for front 3 inches or so w/ a bit of relief (maybe 1/8-1/4 inch) cut from under base behind attachment to allow for tail to flex. Double foiled mounted parallel and vert. As tail flexes it will adjust ant and angle of keels…that’s the idea anyhoo…
Why not???
Next question. Style of surfing. I was thinking Tom Carroll from your description. Is that about right?
i’m thinking gg/lis/curren hybridization.