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Originally posted by shi thouse

landing in a bad way from a jump (havent been able to quite work this one out);

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making sure you're pointing downwind with no load on the back foot, seems to work.

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a low pressure bubble forms on the leeward side of the fin and this air pressure magically removes all performance of the fin what so ever. Does this mean the bubble is larger than the fin, or does the turblulance destroy the hydrodynamics of the planning surface??

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due to the aysmetric flow over the fins surfaces when it's under load, there's a higher pressure on leeward side than the windward, this is fin lift, it counteracts the sideways component of the sail force.

cavitation happens when the fin stalls, because the angle of incidence has become to great to maintain laminar flow on the windward side. Don't think it makes a huge difference if there's air in the gap or not, the fin's lost most of it's lift. But I think if there's air there it's much harder to recover, After all a vacuum doesn't really like being here.

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Are there other sports that are affected by this problem, in particular water sports? And what attempts have they made to try and correct this problem?

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Well I guess surfers have been "spinning out" for decades, and inpart that's driven the evolution of boards and fins, but cavitation is a major problem in a lot more than sports, after all there's a lot of money tied up in boat/ship props, when they cavitate it's a big negative to performance.

Cavitation can also come from excessive back hand pressure, try moving just the rear part of your harness line back a little bit, it may help

Alby
(Resident Elmo)

Its not cavitation that causes spinout. Its ventilation.

If the windward rail lifts on some chop, air gets sucked down the windward side of the fin (a big bubble). This kills the lift so the fin starts to go sideways. The low pressure on that side of the fin allows the bubble to expand until it pops out the back of the board. When the bubble connects to the atmosphere the air just feeds in and you have spinout. Thats why flappers were used on late 90's course boards. They allowed the bubble to dissipate and wash off before creating an opening to atmosphere which starts a full-on spinout.

Same process when landing a jump. If you land with the board pointing downwind, you are loading the windward side with high pressure (due to the direction the water is hitting the fin) until the board is in contact with the water and you put the load back on. If you land with the nose upwind, the fin will immediately be creating low pressure on the windward side and suck the air right down for a nice immediate spinout.

Separation (laminar or from stall) can start this process too- thats the spontaneous spinout we get using a crappy or too small fin. For attached flow, the lower pressure region of flow does not extend far behind the fin trailing edge, so by the time its out the back of the board, the pressure is back to atmospheric. On the other hand, the highly turbulent low pressure wake of separated flow on the windward side extends behind the board, and allows the air to be sucked in, starting ventilation => spinout.

If you look behind when you are spinning out, you will always see theres an opening in the water surface feeding air right down the windward side of the fin.

Cavitation does not cause a sudden loss of lift. Its a progressive effect. To support cavitation you need a very low pressure, which implies effective lift. The bubble expands until the pressure is too high to support the cavitation. Drag of course increases due to the change in flow characteristics. But it does not cause spinout.

Fins are complex beasties...

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Originally posted by slowboat

Its not cavitation that causes spinout. Its ventilation.

If the windward rail lifts on some chop, air gets sucked down the windward side of the fin (a big bubble). This kills the lift so the fin starts to go sideways. The low pressure on that side of the fin allows the bubble to expand until it pops out the back of the board. When the bubble connects to the atmosphere the air just feeds in and you have spinout. Thats why flappers were used on late 90's course boards. They allowed the bubble to dissipate and wash off before creating an opening to atmosphere which starts a full-on spinout.

Same process when landing a jump. If you land with the board pointing downwind, you are loading the windward side with high pressure (due to the direction the water is hitting the fin) until the board is in contact with the water and you put the load back on. If you land with the nose upwind, the fin will immediately be creating low pressure on the windward side and suck the air right down for a nice immediate spinout.

Laminar separation can start this process too- thats the spontaneous spinout we get using a crappy or too small fin. For attached flow, the lower pressure region of flow does not extend far behind the fin trailing edge, so by the time its out the back of the board, the pressure is back to atmospheric. On the other hand, the highly turbulent low pressure wake of separated flow on the windward side extends behind the board, and allows the air to be sucked in, starting ventilation => spinout.

If you look behind when you are spinning out, you will always see theres an opening in the water surface feeding air right down the windward side of the fin.

Cavitation does not cause a sudden loss of lift. Its a progressive effect. To support cavitation you need a very low pressure, which implies effective lift. The bubble expands until the pressure is too high to support the cavitation. Drag of course increases due to the change in flow characteristics. But it does not cause spinout.

Fins are complex beasties...

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Budgy Hell Chris

This explains a few of my woes

thanks

Alby

And dont land it into the wind!!

It is very unlikely a fin or hydrofoil would cavitate below 40 knots. However at 50-60 knots
the lowest pressure on the fin would be approaching water vapour pressure.

I don't know if this is a factor in breaking 50 knots but it's interesting that current speed records are around the so called cavitation limit.

mal

The "do we get cavitation?" question has been around for ages, and there are usually two answers floating around.

The "I dont think it happens" crew normally base their opinions on some observations and assumptions, such as:

1) We dont generate anywhere near as much power as a propellor on a power boat.
2) There are no signs of pitting on my fins- isn't cavitation meant to be highly violent and errosive? If it can pit metal, then soft fiberglass would surely be destroyed more readily.
3) If we were cavitating then we would get spin out at speed, which doesn't seem to be the case.

The "it can happen and probably does a lot more than we think" crew base their opinion on:

1) Tank-test proven science, some of which we rely on every time we step into an aeroplane. But its not easy to use or understand, nor completely deterministic.

The number crunching shows most good slalom fins are susceptible to cavitation in the high 30 knot range, to low 40s.

Those arguments saying its not cavitation are all speculative at best.

1) You dont need much power to create a pressure low enough to vapourise water.
2) If the bubbles are small, there may be no violent implosion.
3) To maintain cavitation (say, enough to spin out), you need low pressure. Low pressure implies that the flow is attached to the windward side, meaning its still generating lift and not spinning out.

Cant see how you could generate sustained cavitation on a fin during a spinout. Spinout is a loss of lift. This implies flow separation.
This implies ventilation or stall (which usually leads to ventilation in our case).

For sustained cavitation to be audible, there would have to be some consistent cyclic behaviour in the bubble formation and collapse in an audible frequency range. Cavitation exists in many forms. Typically starts as small bubbles, then turns into a sheet, then turns into a large thick cavity (noisy and destructive) as you go faster and/or increase the load. I dont think we could get to the latter phase. The drag introduced by the first stages would be significant enough to stop us from pushing into the next speed zone to cause the more aggressive cavitation forms.

IMHO...

The pitting in your finworks fin is probably due to the gelcoat having soft spots which get easily washed or popped out as the fin flexes and ages. Thos fins have a soft gelcoat. But it could be cavitation