Probably a dumb question, but since this one occurred it is a little confusing.
How important is it to have a good seal at the top of the screw holes and the fin box in tuttle box (or powerbox, or anything where the attachment comes through the deck)?
Recently went for a sail with fin not really tightly screwed in (only a quickie in 12-14 knots) and board spun out several times with no excessive back foot load or speed.
I am assuming air getting in through the poorly sealed holes in top of fin box allowed air to get in and ventilate fin, leading to spin out.
Any thoughts?
Clarence
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??
Never considered it - could be air.
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.
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.
WOW !!!!
That makes sense , mabee ive noticed that kind of thing.
Ive always thought those rubber washers were for pooofs and never used them.
That's it , im trying rubber washers 
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!
Select to expand quoteVon 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!
Again WOW !
If u can hear air ( over all the other noise ) sucking down a screw hole , it's got to be doing no good !
Thanks for feedback.
Select to expand quoteMark _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.
Select to expand quoteImax1 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.
Select to expand quotedecrepit 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?
Select to expand quoteVon 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
Select to expand quoteclarence said..
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.
That's my point,in relation to ka43's idea that it may just be ill fitted fin. I don;t think it is. I am supporting your idea that air may be drawn down thru screw holes
Venturi effect may be stronger than you think. Would be interesting to hear from someone who could work it out.
Quick fix I use is a tap washer. What you also may not realise is that it is not just the seal between the washer under the screw and the surface of the board. You also have to seal the gap around the thread on the screw itself. The washer must be a reasonably tight fit around the screw.
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 ??????
Have a look at some of the fin designs from the late 80 early 90s (my era) lots of holes and slot's everywhere!
Sometimes its just easier to spinout when your not powered up and fully planning.
Select to expand quoteImax1 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 ??????Select to expand quoteVon 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 
Select to expand quoteImax1 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
Just being a bit pedantic but this is ventilation not cavitation. It's definitely from leaky screw holes. Chalk an cheese with rubber washers. Generally ventilation reduces Cl max ( lift ) and increases drag on foils. But there are always exceptions.
Select to expand quotePiv 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.
Select to expand quotePiv 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
For the tech-heads/nerds out there.
Cavitation explained.
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 .
Select to expand quoteAUS729 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 !!
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.
Select to expand quoteMastbender 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.
Select to expand quotedecrepit said..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.
Ventilation and cavitation are different things.
Cavitation can happen deep in the ocean a long way from any atmosphere....as seen in the submarine example.
Ventilation is where air from the surface gets onto the low pressure (upwind) side of the fin- it needs a source of air (like the atmospehere) to make it happen.
Spin out as most folks know it is ventilation, not cavitation.
What decrepit said sounds correct (cavitation at 50 knots)... the general rule of thumb I heard many years ago was that cavitation occurs at speeds of 40 knots and over.
As far as I know cavitation itself may not cause spin out, but the problem would be the trail of gaseous bubbles (water vapour) reaching past the tail of the board which can then allow atmospheric air to join into the stream of bubbles..then possibly ventilation and spin out.
The screw hole question was about trying to understand how big a role the seal around them would play in ventilation of the fin (and hence low speed spin out).
Clarence
Select to expand quoteracerX said..
>>>>>>>
These dutch guys think cavitation can happen at lower speed that 50knots at high angles of attack...
>>>>>
..
Yep, that makes sense, it's about how great the negative pressure is. I think my source is more concerned with heading downwind at low angles of attack.
Temperature is obviously going to be in the mix as well, but I've no idea how much.
