Im no expert but I reckon it would the fin not seated properly in the box and causing air/turbulence and hence the spin out??

Don't think it's turbulence due to lose fin, I'd bet on first theory, air getting sucked down through screw holes. I've noticed the same thing. If I hit the bottom with the fin, this can angle the fin back a bit more and loosen the rear screw, this will inevitably lead to an increased trail of bubbles in my wake, and a struggle to keep going straight.

I also had a split rubber washer, that also increased spin outs.

Air being sucked down via venturi can cause enough cavitation to spin out, I once had to sail back with my foot over the front screw hole on a mates board, as speed built up you could hear it sucking louder and louder until bam it would let go like the fin snapped!

Thanks for feedback.

Mark _australia said..

Don't think a fin that is not down flush with the board would have a marked effect, US box is gappy as hell and with a screw head poking out.

Agreed that US box is gappy, but none of the gaps join up with air on the surface/deck of the board as can be the case with tuttle/power type boxes.

Imax1 said..

Ive always thought those rubber washers were for pooofs and never used them.

I thought the same, but in a slightly more politically correct manner . I did not appreciate the role they would have in sealing the air connection between deck and fin.

decrepit said..
Don't think it's turbulence due to lose fin, I'd bet on first theory, air getting sucked down through screw holes. I've noticed the same thing. If I hit the bottom with the fin, this can angle the fin back a bit more and loosen the rear screw, this will inevitably lead to an increased trail of bubbles in my wake, and a struggle to keep going straight.

I also had a split rubber washer, that also increased spin outs.

The "loose rear screw" describes my situation pretty accurately. "struggle to keep going straight" means spinning out (as opposed to any reference to persons mentioned by Imax1) I assume?

Von said..
Air being sucked down via venturi can cause enough cavitation to spin out, I once had to sail back with my foot over the front screw hole on a mates board, as speed built up you could hear it sucking louder and louder until bam it would let go like the fin snapped!

Interesting observation. Maybe you should post more often . I would think there only needs to be a narrow pocket of air along the base of the fin, and once this pocket is long enough it will reach past the tail of the board and join up with surface air, which can then be drawn in by the negative pressure on windward side of fin very easily. Maybe the "bam" is when enough air has been let in to allow this air pocket to extend past the tail and join with air surface.

Clarence

^

Got it Mark. Thanks.

Heres something to think about.
You know when your pushing hard on the fin over rough chop , sometimes it gets loose and feels a little faster ( not spinning out as such , but mabee just a tiny bit , and u still have to go down wind to clear it ), makes me think,
Does a EVER SO SLIGHTLY cavitating fin go faster ? ( like having a smaller fin ) ?
Adjustable fin size with cavitation , air release hole mabee half way down the fin ??????

Sometimes its just easier to spinout when your not powered up and fully planning.

Imax1 said..

Imax1 said..
Heres something to think about.
You know when your pushing hard on the fin over rough chop , sometimes it gets loose and feels a little faster ( not spinning out as such , but mabee just a tiny bit , and u still have to go down wind to clear it ), makes me think,
Does a EVER SO SLIGHTLY cavitating fin go faster ? ( like having a smaller fin ) ?
Adjustable fin size with cavitation , air release hole mabee half way down the fin ??????

Von said..
Have a look at some of the fin designs from the late 80 early 90s (my era) lots of holes and slot's everywhere!

I dont mean holes or slots like in the old fins , but purposely feed air from the top of the board ( via a tube inside the fin )halfway down the fin . The faster you go the more air it would suck down.
Probably a crap idea

There is lots written on spraying air over propellers to reduce cavitation which seems a little counter intuitive. Physics is weird.

en.wikipedia.org/wiki/Solutions_for_cavitation_in_marine_propellers

Piv said..
It's definitely from leaky screw holes. Chalk an cheese with rubber washers.

I like a definite answer, esp if it agrees with original theory.

Piv said..
Just being a bit pedantic but this is ventilation not cavitation.

I think you are correct that it is technically ventilation- not cavitation.

Cavitation is where there is vapourisation of water due to speed of it past the solid surface (which creates excessively low pressures)- creating loss of liquid water contact against the foil. Occurs where surface flow is over 40 knots (approx) in near surface situations. Does not require entry of air from atmosphere (for example).

Ventilation is where air is sucked in from surface on low pressure side of foil.

Clarence

Another theory ..
Is having the fin loose ..the fin is not in the right position that it was designed for and may move from side to side slightly .
i have power box fins ,the water doesn't come out of the top of the fin box if the fin is tight in the box.Thats with a urethane washer .the rubber ones
do work too .
If the fins got water coming out the top ,you really should tighten it .so it rides to its true angle .
this may be causing the spin out .
The water is being pushed up in there a 50 Kms per hour or more .

i reckon yes it does make a difference .

AUS729 said..
This topic reminds me of an ex girlfriend.

Power Box?
Or screw loose?

I think that's really 3 categories, you can divide the last one in 2 depending on word sequence, loose screw or screw loose.

Never tried tap washers on the ex girlfriend but I reckon it could've only improved things !!

Mastbender said..
From what I've been led to understand, which could still be in question, is that the vid above about cavitation, can't really apply to windsurfing because we don't produce the kind of speed to make steam out of the water.
But we can produce enough speed to make the water move away from one side of the fin to produce a bubble, filled not with air or steam, but of vacuum.
Which in itself would also call into question of air coming down a bolt hole to create cavitation, when bolt holes aren't required to make fin spin out.
Maybe the bubble could be filled with air or vacuum producing the same effect?
That's beyond my pay grade.

Hmm, well I've been told by one who knows, that cavitation is becoming a problem for the fastest guys, ie over 50kts, and foils need to be carefully designed to minimise the effect. It doesn't necessarily lead to spin out but it does reduce lift and increases drag.

And air getting sucked onto the fin isn't cavitation it's ventilation. And as demonstrated, even a semi vacuum in water will produce steam.

Certainly sealing the bolt hole reduces/eliminates the trailing bubbles and spin out.
Although if you're spinning out that doesn't have to mean there's a bad seal on your bolt holes, there's many other causes, but it's probably the first thing to check.

clarence said..

Spin out as most folks know it is ventilation, not cavitation.

IMHO, the real reason for spin out, is when a greater angle of attack, or drift angle no longer creates a correspondingly larger force, at that point you spin out. i.e. it stalls,

ventilation, flow separation, and cavitation could all cause this. These dutch guys think cavitation can happen at lower speed that 50knots at high angles of attack...

www.marin.nl/upload_mm/8/3/e/1806737901_1999999096_120_e_8.pdf

Though they seem to be saying that cavitation does not necessarily have an adverse effect on the stall characteristics but certainly increase drag and reduce lift...