I’m making some koa wood fins that will be glassed onto an 8’6" solid balsa gun (thruster arrangement)
in your opinion what is the optimal thickness for those fins?
not fin positioning or cant or toe in. Just thickness.
PS For extra credit: explain your answer!
Well, if you’re one of those crazy guys that likes to use ratios to define your fin thickness …I like to stick to around 10:1. -Carl
Standard thickness is 0.25 inches.
This thickness, in glass, allows reasonable stiffness. Anything thinner and flex begins to play a substantial role. Single and thruster fins have evolved over decades to be as thin as possible while maintaining adequate stiffness.
Thinner fins are more sensitive, and stall at lower angles of attack. Fatter fins are less sensitive, and stall at higher angles of attack.
I prefer my fins as thin as possible given stiffness constraints, because I like more sensitivity in my fins, and I find stalling to be a non-problem except in some really marginal circumstances. However, even using carbon fiber, I can only go marginally thinner than glass fins because of the strong relation between fin thickess and stiffness. So it is not really worth the effort for me. So I make mine 0.25" thick.
The most common reason given for fatter fins is that using appropriate light core fin materials, buoyancy can be positively effected, and this will have a substantial impact on performance when the board is weighted lightly (like a top turn). The board will sit higher through the top turn, and be easier to torque around as a result. Whee!
Drag reduction in a thruster, when going straight, is also a bonus in thicker fins (because they are less sensitive, the thruster fins generate less lift and less drag when you go near straight and the rail fins are in “snowplow” configuration). This bonus is totally irrelevant in my surfing trucks though because they don’t do the snowplow…
hth
Keith,
Since the board is a gun, you're probably not too worried about stalling the fins due to excessive angle of attack. So, a finer foil is better. The fins we make quite often are in the .32" range at the base and taper down from there. Your core is going to end up pretty fine. But, don't skimp on the laminate schedule. You want a solid base flex.
Keith,Since the board is a gun, you're probably not too worried about stalling the fins due to excessive angle of attack. So, a finer foil is better. The fins we make quite often are in the .32" range at the base and taper down from there. Your core is going to end up pretty fine. But, don't skimp on the laminate schedule. You want a solid base flex.</blockquote></div>I calculated 5 layers of glass, on each side, should by themselves provide enough stiffness to largely match normal size glass fins.
Of course it will vary somewhat if you use different cores, with a wood like koa 3 or 4 might be enough, I would make a glass fin to compare with (or grab one).
You don’t want flexy fins in a gun!
…go with the Tomatdaum thickness (tapered) and Blakestah number of fiber layers…
thanks for your thoughts everybody. Don’t know if you’ve ever worked with koa wood, it is hard as NAILS. Can’t imagine it would flex unless you worked it down nearly paper-thin.
5 layers of glass seems like a lot to me, using that wood anyway. I’ve done walnut fins before and didn’t use anywhere near that much. Of course they were thick keels…
Carl, 10-1 ratio, measured which way? height of fin or front to back? thickest part to thinnest part? Never used that method…
10-1 ratio would be aspect ratio.
i.e. the thickness is 10% of the chord (fin length FRONT to BACK at that point).
Keeping a 10-1 ratio will naturally taper the fin, for as the chord gets smaller towards the tip, the fin gets thinner!
id probably go with a 12% chord, as this is generally considered the optimal for foils in water, though im guessing that surfboard fins have their pecularities and the optimal ratio might be different!
James
What Keeno said. I’ll be sending you something.
10-1 ratio would be aspect ratio.i.e. the thickness is 10% of the chord (fin length FRONT to BACK at that point).
Keeping a 10-1 ratio will naturally taper the fin, for as the chord gets smaller towards the tip, the fin gets thinner!
id probably go with a 12% chord, as this is generally considered the optimal for foils in water, though im guessing that surfboard fins have their pecularities and the optimal ratio might be different!
James
have you measured very many thruster fins for thickness vs chord length?
to be honest no,
the response just came off the top of my head without thinking about the real world, as per usual!
i can imagine that the chord is prob. much less than 12%, i guess about 5%ish. it seems pretty thin (in theory), but im guessing that there’s a reason that their so thin.
go on… show us your wisdom and explain why.
james
It seems that with the weight of the solid balsa, leaving a bit more thickness in the fins would allow for more overall flow. 8’6" solid balsa is a huge factor in this…
Quote:10-1 ratio would be aspect ratio.
i.e. the thickness is 10% of the chord (fin length FRONT to BACK at that point).
Keeping a 10-1 ratio will naturally taper the fin, for as the chord gets smaller towards the tip, the fin gets thinner!
id probably go with a 12% chord, as this is generally considered the optimal for foils in water, though im guessing that surfboard fins have their pecularities and the optimal ratio might be different!
James
have you measured very many thruster fins for thickness vs chord length?
Of course. They are usually somewhere around 14:1. But that doesn’t mean you have to fall in line. -Carl
We all know what Bert Burger and Greg Loehr would say. . . go thicker !
I like fins thick or thin, even thick AND thin, depending on this that and the other thing. . .
