thought this was an interesting way to approach fin selection.

www.bluefinz.com/technique/finselection/finselection.asp

anyone already do this...?

cheers
Gestalt.

38 is probably a bit too big for a 5.7, but it depends on the style of fin. In wind where the 5.7 is starting to become good the 32 should be perfect, although the bigger fin will resist spin out and give you instant pointing etc UNLESS it becomes overpowered. Fin size is largely rig size dependant also, you have to have the rig power to counter the force from the fin.

I think the article is very general and it depends alot on the style of board- hypers get fin overpowered very easy and I would think 1 foot off would be a pretty big fin. I would probably look at is as (without measuring yet) 1 foot off plus 2cm as your maximum fin and go down from there... I have 2 fins for each sail, about 4cm increments.

Gestalt, thanks, didn’t have any idea on how to select a fin to suit a board and conditions.
The topic may generalise but for a uninformed new person at this sport, it’s been an eye opener and informative. Even if its not 100% regulation law, it gives me a start point.
Once again thanks.

Mineral

Hey folks, Think I'm in Mk7's school of thought on this one.

I use as follows:


5.7 - 30/31 cm (18 - 20knots), down to a 24 in 30 knots

6.0 - 33 (15 knots) - 28 (25-28 knots)

7.0 (35 - 31cm) (a 38 would really help me to plane earlier)


(ballistic conditions (4.7 with a 22 or 24)

Agreed also that bigger fins really help in lighter conditions.

What does not seem to be mentioned is 'technique' smaller fin really requires greater skill to get on the plane and you usually sacrifice some pointing ability.

Best small fins for pointing and going quick are the Torquay Brand - but no longer made !!

- J

yeah vando, but what do you think of the article....?

slalom board comes in the post tomorrow...

but i got some tastey large fins to try out in it. one being a custom torquay world cup slalom....

upwind is fun too....

got said board today....

thanks windsurfing perth.... nice one.

you will like the fin MK, it is the one that comes with the board and it is smallish........

i think i am in love. will i now change brands....


p.s. got a 7.4 for the job, if not will stick an 8.5 on it. will still try my 45cm torquay....

hey mk.. check out the superx on high.tv

huge fins and sails. these guys, jump, loop duck gybe, freestyle etc and the fins are massive.

Depth and width of fin equales lift ok
What about the actual foil shape ?
whats the fastest, having a thick or thin foil set, what ,just in front of the centre point, middle or behind the centre point
what about the leading and trailing egde

Thats how i understood it to .Then you got the stiffness to consider on those large fins. Seen some great pics of them bending up to about 1 foot when crew over power them.
Suprised a wing keel system hasnt got a niche market maybe it has

hi nebbian,

speed fins have the thickest part of the foil towards the centre of the fin. the whole purpose is to reduce drag and increase speed.

look towards c3 fins for this concept.

another thing is that mfc fins are starting to move the max thickness around on there fins. using 50% in the tip half and 30% at the base. have a look at the goya 4s.

these ideas about thickness have been around for ages. i am not to good with the aeronautical maths but i know that when making fins i was given some basics.

thick foil near the front for lift and control. keep it upright or rake it forward and you go to windward better.

thin foil max thickness around the middle for speed. rake it back (straight leading edge) and you get some control.

curved leading edge for control.

edit......
i was always told and i always did sand the trailing edge of my race fins blunt. reduces spinout. i am wondering, if the leading edge was a mirror of the trailing edge with a speed fin to allow it to slice the water more efficiently, should the leading edge also be sand blunt or more like a knife?

maybe you guys are right???

check out this link... look into NACA 66-010 foils..

http://www.swaylocks.com/forum/gforum.cgi?do=post_view_printable;post=167328;guest=2123878

maybe time for some sandpaper and angle grinder on one of your new fins mk....

what are you sandy point guys doing with your fins?

just as a theory,,,, which may be flawed...

if the fins max thickness is only say 5% of the chord does that mean that the point of max thickness can be moved further aft. the resultant flow will stay laminar due to the small max thickness ratio.

end result being less drag with enough grip?

Gestalt,

The theory that that guy posted isn't the accepted one in the aeronautics world. That theory went out with the dinosaurs, unfortunately it's still around in one form or another and while it's good enough for total newbies, it really doesn't cut the mustard when you start actually measuring what's going on, changing values and measuring what happens. He's also talking about having two or more fins on a board, which isn't what most windsurfers use...

For an asymmetrical foil, think of it this way. Firstly figure out where the flow separates as it goes around the fin. This depends on angle of attack, and how blunt the leading edge is.
Now draw a line equidistant from the top and bottom of the foil, along the middle of the foil, starting at the flow separation point on the leading edge and continuing along to the trailing edge. You now have a curved line. You can think of the water as following this curved line.
Now here's where the magic happens:
You can make the fin as thick or as thin as you like, so long as that curved line is in the centre of the foil, and it will have pretty much the same lift!!
If you make it thinner, then there's less drag, but it's harder to physically build a strong airfoil. If you make it thicker, then you get more drag, but it's easier to make it strong.

Eppler did a heck of a lot of maths and experimentation to prove this, and it's pretty much accepted that this is what happens. He designed a whole series of foils with the same 'centerline curve', with different thicknesses, and surprise surprise they behave pretty much the same when the flow is laminar. Of course, a thicker foil will stall quicker, and a thinner one is harder to build strong, but the theory stands.

He also did a lot of maths to figure out where to put the thickest point of the airfoil, and it works out to be (from memory) something like between 30% and 40% of the way back from the front.

Now for symmetrical foils:
The reason you get lift is mainly due to the blunt leading edge. Again, do the same thing you did for the curved foil, firstly figure out the angle of attack, draw your centre line and you see that you actually get a slightly curved 'center' of the fin. The thicker the leading edge, the more lift you get, because you get more curve in that centre line.

All of this depends on a knife-sharp trailing edge, if you round off the back of your fins then the theory says that you're increasing drag, reducing lift and actually making it more prone to spinout. There's no way I'd round off the back of my fin!!

Having a sharp leading edge means that you get no curve in the 'center line'. You may as well use a flat plate for a fin, you'd get about the same amount of lift. It won't behave nearly as well as a rounded leading edge and you get a lot less lift.

That's the theory anyway

Harrow,

A flat plate stalls very easily for a low angle of attack, and low lift values. Not recommended. You're better off with a proper airfoil, even if you could make everything as stiff as possible.

I was thinking, how about making a fin that changes shape depending on which tack you're on? If you make the front and back of the fin stiff, with a floppy centre part, then it would behave like a sail and you'd get a curved center section in the right direction no matter which tack you were on! Hmmmm.... think I've got an old fin lying around somewhere... the middle bit would have to be made of unobtanium though

Slightly off topic, but I reckon the simplest way to bust 50 knots is to address the huge drag that must be occuring ABOVE the water. I'm talking about the huge un-aerodynamic blob called the SAILOR!

These guys are getting close to 90km/hr. Stick your arm out the window at that speed in a car and feel the drag. I know from my ironman triathlon days that I got an extra 2km/hr on the cycle simply by placing my arms close together on a set of aerobars, and that is at a speed of only 30-40 km/hr.

It would be a heap of hassle, but could some sort of aerodynamic 'trailing edge' be attached to the sailors legs? Or even his torso or head? I guess it would be a bit like some of the large luff pockets on race sails that prevent the turbulence occuring behind the mast.

You'd really look like the blue streak in the 'aero suit' that I'm imagining, but I reckon that elusive '50' would be cracked in a flash! Am I nuts?

Gestalt,

Was the boat heading upwind?

nebbian, there are a couple of extra things to consider beyond section efficiency and stall characteristics. Firstly (for speed sailing at least), there is cavitation avoidance. Secondly is how the moment affects the geometric twist of the foil.

So, there are some fins with maximum thickness further aft of the conventional 35% to 37%. The main justification is to deepen the drag bucket by going for extended laminar flow at the low angles of attack seen on a speed course. Long favourable or neutral pressure gradients terminating in a questionably abrupt recovery. Similar reasoning to eppler's- both for delaying cavitation onset and to achieve some laminar flow. The predictions are for lower drag assuming that early transition does not occur.

But... using conventional materials poses a challenge. Shifting the maximum thickness aft also shifts the geometric twist aft, but the moment centre does not move back so far. The end result is a "negative" twist starting at certain load points, which increases the effective AOA toward the tip (opposite to what a sail does). The result is that the spanwise lift distribution changes dramatically, the tip cavitates before the rest of the foil (since it is taking a higher portion of the load), and the result is a slow powerful fin. Adding some reflex increases the moment with speed. OK if its negative to start with...

The geometric twist can be biased by raking the fin, which also scales down the velocity (and therefore increases the cavitation margin) but this adds extra effects which can be detrimental to extended laminar flow. C3 uses this to make some exceptionally fast and versatile fins. They are far from a machined chunk of G10, and the layup is absolutely critical to achieving the optimum twist. Its also a very thin foil so is quite sensitive to surface finish.

Going thiner does not necessarily decrease the drag. Thinning the foil decreases the lift coefficient. So if your drag does not scale down with the lift (eg constant skin friction component) thinner foils can often have a higher drag in the operating region than a thicker design. Thinner foils do have lower peak velocities so therefore are better at avoiding cavitation.

Sorry guys if this went a bit OT...

if you want to go fast......practice.
and study what world cup sailors use in their fin quivers.