Fin Foil Pressure Calculations

General Discussion
1

So im still stuck on fin foils, i dont see the importance of foiling
the side fins of a thruster asymmetrically (flat on the inside).I have
been told on other threads i have started on the subject in sways, that
due to the pressure differential(see photo attached) that it stops spin
out and that this pressure difference performs some kind of function.
http://www2.swaylocks.com/forums/fin-foils
anyway
so i decided to do some calculations to see what the pressure
difference is, and what i came up with doesnt seem like it could
possibly be usefully at all(except maybe for those with super
heightened senses…slater perhaps?)
The pressure differential that
i come up with is -0.947kPa, now when you take into consideration that
i calculated the pressure on one fin to be approx 54.5kPa and that in a
turn you would have at least two fins in the water(one side and
trailing) that would be 109kPa, less than 1kPa differential hardly
seems like it is going to be of any functional value at all apart from
maybe marketing.
Any way i will post up all my calculations so ppl can go through them and point out to me any mistakes i have made.
Also
im no scholar so im not sure that i am applying any of this correctly
so please correct me if i am wrong, also there is the question of how a
plane can fly upside down which kind of blows the pressure differential
thing out of the water(as well as the asymmetrical fin foil having any
kind of functionallity) although any of the theory behind that is well
above me (so is this truth be told and i wouldnt be surprised if i have
it all way to simplified).

2

Ok so the aim of this was to try and calculate the pressure drop accross a standard fcs asymmetrical side fin.

some formulas that i have used.

pressure = force/area where force= newtons area= meters squared

force = mass * acceleration where mass= kilograms acceleration= meters/second squared and the acceleration of gravity= 9.81 m/s2

pythagoras theorem for a right angled triangle hypotenuse2 = o2 + a2 (osquared + asquared)

bernoulli’s equation 1/2 v1 squared + P1/rho = 1/2 v2 squared + P2/rho

where rho= density(density of water= 1000) v= velocity (m/s) P= pressure (newton/meter squared or pascals Pa)

3

first lets calculate an approximate force that could be generated on a
drop into a wave from the top, neglecting any friction of water/air etc
and just calculating a free fall drop from 0 velocity, and using my
approximate weight of 100kg(including surfboard) and remember that
acceleration due to gravity would be 9.81 m/s2.

force= mass * acceleration = 100kg * 9.81m/s2 = 981 newtons

now
we can calculate an approx pressure that could be applied to the fin
when we do our bottom turn at velocity.To keep things simple we will
calculate the fin area as a rectangle of similar dimensions as an fcs
fin of 120mm * 150mm.

fin area= 0.150*0.120= 0.018 m2 (meters squared)

pressure= force/area = 981 newtons/0.018 meters squared = 54500 pascals or 54.5 kPa

so we could assume that during a bottom turn we could be putting (in the ball park of) a pressure of 54.5 kPa on our fin.

4

now we can use the ratio of length differences to calculate the faster
water velocity over the foiled side of the fin.to simplify this we will
use triangles to replicate the fin foil as per attached picture.

section1
pythagoras hsquared= 20mm squared + 6mm squared = 20.880mm

section3
hsquared= 60mm squared + 6mm squared = 60.299mm

length of foiled side of fin = section1 h length + section 2 length + section3 h length

= 20.88 + 40 + 60.299 = 121.179mm

now
we can calculate our velocity.To make things simple lets say we are on
a wave with a 9.81m face.It would take us 1 sec to get to the bottom
neglecting friction etc (due to our 9.81 meters/second squared
acceleration) and we would have a velocity no more than our
acceleration at the bottom of the wave which would be 9.81
meters/second (which would be 35.316 km/h).

now using the ratio of lengths of our asymmetrical fin lets calculate the second velocity over the fin

distance1 * velocity2 = distance2 * velocity1

velocity2 = distance2 * velocity1/distance 1 = 121.179mm * 9.81meters per sec/120mm = 9.906m/s

5

now we can use bernouilli’s equation to calculate the difference in pressures over the fin due to difference in velocity.
P1=54.5kPa v1 = 9.81 m/s v2 = 9.906 m/s P2 = ? rho= 1000

P2 = (1/2 v1 squared * rho + P1) - (1/2 v2 squared * rho)
P2 = (1/2 9.81 squared * 1000 +54500) - (1/2 9.906 squared * 1000)
P2 = (102618.05 - 49064.418
P2 = 53553.632 Pa
P2 = 53.553 kPa

pressure difference across fin = 53.553kPa - 54.5 kPa = -0.947 kPa

6

Have you ever tried a rear in the frontside fin box? I did the other day – it’s crap. Makes a big diff in how much drive, how much you can feel the fin and how much you can push off it, which means there’s a lot of lift lost. In crap waves, mind you—I’m sure you can have a good time with double-foiled rail fins in waves with juice. They probably have to be bigger than they would if they were single-foiled though.

I’m basically certain most foil lift in surfboard fins and airplane wings is Newtonian not Bernoullian. The Coanda effect keeps the flow attached on the foiled side. The pressure diff isn’t the big thing.

You can defeat the foil on some airplanes flying upside-down, given enough horsepower and wing chord, and the reason you can do that is Newtonian too. You have to let the tail hang until the back side of the rounded (it’s flattened on aerobatic planes) foil is at a Newtonian lifting AOA.

my 2 cents.

7

In one second you could freefall 4.9m (1/2at^2), but the rest makes sense for a 4.9 meter wave.

But, even for a rough approximation I think we need the force from redirection of the water stream. Maybe assume the water approaches at a certain angle to the (toed) fin and exits parallel to your rear triangle’s hypotenuse, then an estimate of the dimensions of the jet of water redirected by the fin. Then the rate of change of momentum in each direction is the force this effect allows the fin to give the surfer for drive.

8

Try two similar size rears in the rail fin boxes.

If you want to know why performance rail fins are flat on one side.

If you have a lot of time you could make a styro glider and buff out some double foil wings too and see how it works.

Then you can ride some single foil rail fins and make some single-foil wings for your new glider.

9

Thanks for the reply janklow, i dont know much about newtonian lift but i found this on the interweb.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

does it apply different for fluids?

10

Hi D-rock, i think i get what your saying about redirection of water stream, same thing janklow is saying about newtonian?If this is true does that mean that double foiled single fins have cr@p performance as well, due to the symmetrical foil?can you come up with any figures via calculations to give us some rough ideas?

Also how much difference can it actually make considering the foil is carved into a 6mm thick sheet(ie we are hardly talking big foils)?is this enough for your average surfer to notice?

Are these foils a bit of a marketing thing?kind of like with computers where a new bit of tech is hyped up as new and improved performance when in reality the actual performance increase is minimal(although definately a performance increase all be it a small increase).

ie does joe blow who has been surfing a minimal for a couple of years really notice the difference?

11

would this experiment be somewhat misleading though given the area of the wings compared to the mass of the object?i would guess that all this would do is make any increase in the styrofoam gliders performance much more obvious.

Im not saying that there is no performance increase in the asymmetrical fin, just that the performance incraese appears to be very small according to what i ahve calculated.

As for trying the rear fins on the side i am currently doing that, so far no real noticed difference.But i wouldnt say that i have done a thorough trial and that i am not satisfied yet.

on a side note i hardly notice the difference between a set of glass and plastic fins, although i tell my missus that i do (and i suppose myself) because they cost alot more.They definately look much better than plastic though.

Mind you i am only an average surfer at best, infact kook is probably a good description.

12

Jeez, I think the picture you posted explains the whole thing. If you foil only one side you get more power from the inside fin. Less drag too. Symetrical foil on a fin that is only engaged on one side? Why?

13

thats the whole point, using some numbers to calculate what the picture is saying makes the simple picture not explain anything at all.

14

apparently this is wrong

15

and apparently this is wrong as well (newtonian as talked about earlier)

16

Interesting. I can tell you from experience that nothing changes a fins performance more than changing the angle. Changing the angle of incidence through use of foiling the fin differently will certainly change performance.

17

Ok now possibly we could be getting somewhere, so maybe its possible to change the angle somewhat by changing a asymmetrical fin with a symmetrical foil.maybe this is the major contributor to any change in performance noticed by some ppl given that fin placement can be a very personal thing?.

So really it is incorrect to say that a symmetrical fin is of lesser performance than an asymmetrical fin?, because it is the placement of the fins angle that would be the major contributor to the perfomance, which is possibly the change happening to some ppl that have tried symmetrical foils as the rail fins?and so far all the theorys behind why we are using asymmetrical fins seem to be based on incorrect theory?.

So given that fin placement is a very personal thing, no doubt fin angle fits into that catagory and can be different as well.So to say that the perfomance of a symmetrical fin is no good as opposed to an asymmetrical fin is like saying that a board with a different fin angle to what you prefer is no good, in reality it is a matter of personal taste?.

18

On your last question, definitely.

I wouldn’t want to put a centre fin in on an angle, nor would I want to put an asymetrical fin as a centre fin.

Unless, of course, like an asymetrical board, people start to tune fins for a particular wave or direction. That’s when we head into a new realm. It will come, eventually.

19

Incorrect theory nr1 is incorrect because it has nothing to do with the length of the path.

What is important is the distance between the streamlines and the formation of the wake. Outside the boundary layer the flow can be assumend inviscid and incompressible so bernouilli can be applied. Because the mass flow between two streamlines is constant, you can calculate the velocity and the pressure with bernouilli.

To get the correct lift and drag vectors you have to know the pressure distrubution over the whole surface and you have to know where the stream detaches from the surface (begin of the wake), it’s in the wake that bernouilli isn’t true anymore.

Something new and interesting in the windsurfindusty: http://www.smartfins.nl/technology.html

20

I wouldn’t want to put a centre fin in on an angle, nor would I want to put an asymetrical fin as a centre fin.

what about if the fin placement were set up to take into account the change in angle due to a symmetrical side fin?and if not why not?

So what could some possible reasons for staying with asymmetrical fins on the rails be without trying anything else??

-maybe thats just how it has always been done?

-I
guess foiling the inside if you are foiling them for a living is extra
work that you dont really need to do, which is less productive?.

-from
a business prospective maybe it doesn’t make sense to have all three
fins identical because if you break a fin it will be easily replaced
which may result in less fin set sales?.

-The general surfing
public think that the side fins work due to the bernouilli position on
lift and therfore wont buy anything else?.