Fin Thickness

[spaldi01]

I was recently discussing fin thickness and one of the opinions was that once you get below 9% the reduction in drag is so minimal that the disadvantages don’t make it an attractive proposition. The main disadvantages being; the loss in fin stiffness, and the reduced lift that the fin produces reducing the boats ability to point. Does anyone agree with this?

[EKEvens]

I was recently discussing fin thickness and one of the opinions was that once you get below 9% the reduction in drag is so minimal that the disadvantages don’t make it an attractive proposition. The main disadvantages being; the loss in fin stiffness, and the reduced lift that the fin produces reducing the boats ability to point. Does anyone agree with this?

I hesitate to speak for him, but I'm pretty sure Graham Bantock would disagree. His current fast fin/rudder mouldings are 6.5%

[Steve Landeau]

I, amongst others here in CA are having no problems whatsoever making a 6% fin more than stiff enough. There are some occasions where a thicker fin would be helpful, but as you probably know already, the IOM rule forces lots of compromise. The key is to get your design to work well for you. A great fin on one boat may not be the right fin for another.

[Chairman]

...the reduced lift that the fin produces reducing the boats ability to point

Hi Chris

By and large, my understanding is that a thinner foil doesn't produce less lift as such, and a thicker foil doesn't produce more lift. Both produce about the same amount of lift for a given angle of attack, and both produce about the same amount of lift as a flat plate where the angle of attack is modest. The angle of attack for a fin is the same thing as the leeway of the boat, which is a different quantity from its pointing angle.

In general, the point of having a thicker foil is to be able to maintain lift at high angles of attack (high leeway) without stalling. (The other point of a thick foil, not really relevant to a fin, is to be able to operate continuously at relatively higher lift coefficients.) This is closely connected to the nose radius which seems to be the major factor in the stall characteristic of a fin. Thicker fins allow a larger, more rounded nose radius, and so handle higher leeway angles before stalling. But because the realistic leeway of a boat while beating is maybe 3 or 5 degrees, a more rounded nose (a thicker fin) doesn't make any real difference, since the fin is nowhere near stall. This is one of the reasons that a flat plate fin can work remarkably well.

The thicker foil does have a down side at low angles of attack, and that is that it has generally a higher drag. And *that* is where the real diference comes in. The ability of a boat to point is a function of its "drag angle", which is the ratio of the lift it produces to the drag it creates. Thin fins, thick fins, flat plates, all produce roughtly the same lift. But there are big differences in drag. A thin fin (operating well below stall) produces less drag, other things being equal, than a thick foil. And, at low leeway, even a flat plate is better than a thick foil. Yes, a flat plate can be better than a thick fin...

A thinner fin in general gives a better drag angle, and hence better pointing, provided you are nowhere near stall. If you get near stall, well, the thin fin loses it easily and quickly, which is why you need to nurse the boat during pre-start manoeuvers, and during mark roundings in light airs... So the trick, in designing your thin fin, is to find and use a foil section with a wide drag bucket, a region around zero angle of attack where the foil drag is exceptionally low. A drag bucket only extends to low angles of attack, but this is exactly the region of "normal" leeway, so there is plenty of opportunity to have a rather specialist fin section, and this is where the research is centred.

[spaldi01]

You surprise me about how well a flat plate works. A flat plate fin of 6mm would be much stiffer than a foil section fin of the same width. Therefore would it be possible to make a flat plate fin of say 4mm thickness as strong as a foil section of 6mm. Could it be possible that that the thin flat plate fin would produce a similar amount of drag to thicker foil section fin?

[Chairman]

Could it be possible that that the thin flat plate fin would produce a similar amount of drag to thicker foil section fin?

Hi Chris

The hydrodynamic performance of the flat plate I've been talking about is a "theoretical" plate of "infinitesimal" thickness -- let's say 0.5 mm for argument's sake. So it isn't going to hold a 2.35 kg IOM bulb on a lever arm of around 420 mm long. As a theoretical plate, yes, at low angles of attack, it is surprisingly good. At zero attack, a flat plate has the lowest drag of anything thicker.

When you give a plate the thickness, perhaps 6mm, necessary to support the bulb, however, some "maybe"s enter the picture. If you leave the leading and trailing edges square, then drag will be pretty awful. So, you'll want to round the LE, and do something with the TE. Whatever you do to the LE will have a profound effect of drag. My guess is that a really nice, parabolic section LE nose radius and any kind of taper at the TE will give a flat plate fin (6 mm thick, 100 mm chord, t/c = 6%) that will be the equal of a 10% t/c "proper" foil section fin (to, say, NACA 0010) in terms of drag. Low angle of attack/leeway, remember! No contest outside of this limited regime, the aerofoil section wins every other time more or less regardless of t/c.

[Herb 3]

N A C A 65-07;
Eclipse 40%;
Deflection 19 m/m;
147 grams;
Chord top : 7%
" mid : 6%
" bottom : 5%.

Go to : www.gbmy.com

[edmorales]

i was browsing inside a hobby shop and i saw this section of rc helicopter spare parts. they had these carbon fiber main rotor blades on stock, symetrical foil, 60 mm wide and 660 mm long. i was wondering if it would work as a keel fin. any homebuilder out there tried this?
ed

[Muzza]

Ed,

Wouldn't the 60mm chord be too narrow for an IOM fin? Most parallel fins are about 85mm in chord aren't they?

[Andrew Baak]

Has Anyone ever tried using a windsurfing fin. Stiff G10 many different lengths to chose from?

[edmorales]

murray
i guess your right. just a thought anyway
ed