Bender came up with a theory last week about fin flex in chop.
Going over chop at an angle downwind can set up a rocking motion in the board, this increases the negative pressures on the fin, increasing chances of ventilation/cavitation and spin out.
The idea is that a bit of flex in the fin reduces this effect.

The cedar carbon fins I've made are very stiff, and I'm having this problem.
Introducing tip flex shouldn't be too hard, but they're weed fins, and doing a bit of mental modeling I see a problem.

Flex in a vertical fin can introduce an upward moment to the lift forces, helping planning.
But if the fin is raked back and it flexes along the chord, the horizontal component is going to be angled down, in my head this will produce negative vertical lift.
Does this sound right?????
If so is it a bad thing?
Because to make the fin flex parallel to the bottom of the board instead of along the chord will take some doing.
Perhaps it's better to make weedies low aspect ratio instead of flexi.
Any thoughts anybody???

Select to expand quote

decrepit said...

Bender came up with a theory last week about fin flex in chop.
Going over chop at an angle downwind can set up a rocking motion in the board, this increases the negative pressures on the fin, increasing chances of ventilation/cavitation and spin out.
The idea is that a bit of flex in the fin reduces this effect.

The cedar carbon fins I've made are very stiff, and I'm having this problem.
Introducing tip flex shouldn't be too hard, but they're weed fins, and doing a bit of mental modeling I see a problem.

Flex in a vertical fin can introduce an upward moment to the lift forces, helping planning.
But if the fin is raked back and it flexes along the chord, the horizontal component is going to be angled down, in my head this will produce negative vertical lift.
Does this sound right?????
If so is it a bad thing?
Because to make the fin flex parallel to the bottom of the board instead of along the chord will take some doing.
Perhaps it's better to make weedies low aspect ratio instead of flexi.
Any thoughts anybody???

Mike, ahhhhhh, ya just gave me a head ache trying to comprehend what the dynamics are your describing
So the answer from me is "wothu"

Select to expand quote

mineral1 said...



Mike, ahhhhhh, ya just gave me a head ache trying to comprehend what the dynamics are your describing
So the answer from me is "wothu"

Sorry mate, didn't mean to give you a headache, trying to cure my own.

Gee decrep the word theory might be stretching it a bit.

Now you have me totally lost

may be I have to draw it out on paper, am I confusing everybody else?

"But if the fin is raked back and it flexes along the chord, the horizontal component is going to be angled down, in my head this will produce negative vertical lift."

Decrepit, I am no fin expert but I dont think you would get negitive vertical lift, as the water is flowing parallel to the bottom of the board, and not at right angles to the chord which may create negative lift. I can understand how the foil of the fin creates lift so you sail upwind and stop the board going sideways, this is akin to a plane wing, but I dont really understand which parts of the fin and subsequent action creates the lift to raise the board out of the water, perhaps someone can explain this.

As far as flex is concerned, does it release pressure from the fin thus reducing lift and aiding control in tough conditions ?

Decrepit wouldn't building twist into the fin be easier?

Select to expand quote

choco said...

Decrepit wouldn't building twist into the fin be easier?

Good point choco, that might even work better with the control problem, keeping the angle of attack almost constant as the board "rocks".
Might not be that hard either, just keep the grain more vertical instead of running down the fin, and concentrate the carbon to the front.

More I think about it, the better the idea seems.
Big THANKS choco!!!!!

Select to expand quote

Haggar said...

Decrepit, I am no fin expert but I dont think you would get negitive vertical lift, as the water is flowing parallel to the bottom of the board, and not at right angles to the chord

It's hard to visualise, but to help take the example to the absurd limit and imagine the fin bending at a 90 deg angle if the fin is upright you've now got a horizontal wing were it bends. If now you start to rake the fin back the horizontal wing starts to angle downwards, see now how itproduce negative lift???
Obviously in the real world, the fin won't flex like this (hopefully) but the effect is still there to some extent, I've no idea how much or how detrimental it might get, just wondered if anybody else had experienced or come across this idea.

Select to expand quote

I can understand how the foil of the fin creates lift so you sail upwind and stop the board going sideways, this is akin to a plane wing, but I dont really understand which parts of the fin and subsequent action creates the lift to raise the board out of the water, perhaps someone can explain this.

As above the fin flexing creates a horizontal component to the lift, the lift then isn't purely sideways, it's also slightly upwards depending on how much it flexes.

Select to expand quote

As far as flex is concerned, does it release pressure from the fin thus reducing lift and aiding control in tough conditions ?

don't think flex releases much pressure, just changes it's direction. However as choco suggests "twist" will reduce pressure and lift and also reduce angle of attack, helping to prevent spin out.

just in case it's not clear, flex is the sideways movement of the fin away from vertical.
Twist is the rotation of the fin away from pointing straight ahead.

Thanks mate, let us know how you go

Mike, I'm going to venture into this topic, but only gently as I dont want to come across as an expert on all aspects of fin design. I know a bit about speed, but slower sailing through chop has many other facets that I am not an expert in.

Having said that I think some things will translate well from my speed sailing experience to sailing in chop. Firstly, a stiffer fin (with less twist) should be more resistant to spinout. This is an easy one to explain. The more fin that is being used to create lift (all other things being equal) the straighter the fin is with the flow, so the better the flow will remain attached to the suction side. Conversely if you have a lot of twist, then only a small portion of the fin at the base will be doing all the work creating a higher suction peak and higher angle to the flow at the base of the fin, and so more inclined to get flow separation and ventilate close to the hull.

Sideways flex is another matter. If the fin is very upright and only flexes laterally then at slower speeds it can create lift from the vertical component of force and this lift should help the board glide across chop by slowing the descent between the peaks of the chop. The board is more likely to skip gently across the tops of the chops reducing wetted surface area and drag. This is assuming that the fin is controllable and the twist is minimal.

At high speed a flexible fin is unlikely to be stable enough unless you put a lot of rake in it. But highly rakes fins can flex in two dimensions. Lateral flex (left and right under the board) is OK, but flex in a rotational sense where the tip is being pushed horizontally around the base of the fin will unload the tip and behave the same as having a lot of twist, which is not so good. This effect is unlikely to cause a downward force unless the fin curls back up towards the board. And even then the forces will be minimal because the tip is effectively unloaded.

If a fully raked fin is constructed properly these attributes can be controlled to reduce the flex effect. But I suspect that raked fins will always be a compromise because efficiency and spinout resistance are not their best attributes.

My guess as to why your stiff fin is spinning out is probably more to do with the foil section, or possibly still suffering from some twist due to the rake. If its the foil section, check that the leading edge is not too blunt or that there is not too much curvature in the foil close to the leading edge. This will create a high suction peak that can be the cause of your problems. Did you base your shape on a particular design that you can share with us? Would you describe the foil section as being very fair and of high precision? Imprecision in the foil shape could also cause trouble.

Another possibility is that the fin is too small. Raked fins produce less lift and if too small can be easily overpowered. There is a trend for many sailors to use small fins in speed sailing, but the best speeds are regularly done on bigger fins around 24-26cm unless you are on a very broad flat course or using asymmetric fins. Check GPSSS to see what fins achieve the top speeds. And raked fins need to be even bigger, perhaps 24-26cm in overall depth, making them much longer along the span.

Mal

Thanks Mal for sharing your experience and wisdom.
Looks like I'll forget about twist and flex and concentrate on accurate foils.

I wouldn't describe the fin as precise, but I have been working on it and last time out on it didn't have a problem when a bit underpowered, I hit the 32kn point where it had previously played up.
It hasn't spun out fully loaded up going upwind, only when going fast (for me) over the back of chop at an angle, that's when the board starts this rocking motion.

I'm in the middle of putting some cut outs in the tail of the board, I have a feeling there's too much area there, that's contributing to the problem.

The foil chord thickness ratio is about 9% and max thickness about 40% from the front, entry is fairly fine somewhere around .3mm leading edge radius, and fairly constant curve back to about half way, then gradually straightening out to the 2/3 point and dead flat from there to the back.
Yoyo sent me a foil section based on the eppler 836 foil, after I'd made this one, and it's quite similar apart from the eppler foils hollow rear section.
I must admit I was more worried about the entry being too fine, causing it to stall at higher angles of attack, than being to thick causing a bigger suction peak.

It's all a very precise juggle isn't it, and my wave board/fins experience isn't a lot of help!!!!
Guess I'm going to have to get a lot more precise about measuring foils, stiffness, twist and flex. Then I can document what's happening. See now why everybody is going the CNC route, not really an option for me I'm afraid.

Hi Decrepit,

Don't know if this is going to be of any help, but here's my 2c:

Why not try laying up your fin so that the majority of glass is running parallel with the bottom of the board? That way the fin will prefer to flex on the same plane as a vertical fin.......

bit like if you cut a fin shape out of corrugated iron, as long as you have the corrugations running horizontally the shape will only flex horizontally! (not sure that's made it any clearer...)

S

yeah that'd probably work, but timemachine has talked me out of the idea, think my problem is more to do with inaccurate/bad foil section and maybe tail area too great. I'm concentrating on those at the moment and will see what happens next time I get enough wind to break 32kn