fin flex quandry

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???

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

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

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

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.



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.



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.

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

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