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Cavitation

Forum for hydrofoil builders.


airsail
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Cavitation

Postby airsail » Wed Mar 28, 2018 9:16 pm

I have two foils, all rigid carbon except for the rear stab. One is on a 70cm mast, the other a 90. Both front wings are similar, the 90 having a better finish but both sanded with 600. But I have cavitation issues with the 90, not the 70. The 70 has downward winglets, so tried it on the 90, still cavitates at about 26kmh.
Does anyone have a fix that worked for them? I

grigorib
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Re: Cavitation

Postby grigorib » Wed Mar 28, 2018 9:59 pm

Do you really get cavitation problem when surface of the wing gets damages due to air micorbubbles exploding on the wing surface?

BWD
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Re: Cavitation

Postby BWD » Thu Mar 29, 2018 2:43 am

I’m going out in a limb and saying you’re talking ventilation not cavitation (which happens at 55 knots plus typically). The foil is crashing because of ventilation, air getting tracked down along the mast from the surface. Either pre-existing mast surface condition or bad foil shape could be to blame.

opie
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Re: Cavitation

Postby opie » Thu Mar 29, 2018 4:03 pm

phpBB [video]


I am no expert but you need to make sure there are no small nicks anywhere. Some foils just don't ride well with even small imperfections. When mine starts to act funny I always use glazing compound like in the video. Good luck.

plummet
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Re: Cavitation

Postby plummet » Thu Mar 29, 2018 7:13 pm

BWD wrote:
Thu Mar 29, 2018 2:43 am
I’m going out in a limb and saying you’re talking ventilation not cavitation (which happens at 55 knots plus typically). The foil is crashing because of ventilation, air getting tracked down along the mast from the surface. Either pre-existing mast surface condition or bad foil shape could be to blame.
Can you start a cavitation process at a lot lower speeds by breaching the wing? Ventilating the wing tip that creates a lower-pressure zone that propagates cavitation once the initial ventilation has ceased?

Watch this vid from 17-21 seconds. The wing breaches and ventilates. Then it submerges again but continues to ventilate or cavitate well after the initial ventilation has stopped.

phpBB [video]

BWD
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Re: Cavitation

Postby BWD » Fri Mar 30, 2018 12:37 am

plummet wrote:
Thu Mar 29, 2018 7:13 pm
BWD wrote:
Thu Mar 29, 2018 2:43 am
I’m going out in a limb and saying you’re talking ventilation not cavitation (which happens at 55 knots plus typically). The foil is crashing because of ventilation, air getting tracked down along the mast from the surface. Either pre-existing mast surface condition or bad foil shape could be to blame.
Can you start a cavitation process at a lot lower speeds by breaching the wing? Ventilating the wing tip that creates a lower-pressure zone that propagates cavitation once the initial ventilation has ceased?

Watch this vid from 17-21 seconds. The wing breaches and ventilates. Then it submerges again but continues to ventilate or cavitate well after the initial ventilation has stopped.

phpBB [video]
Unless the hydrodynamic conditions depress the pressure on the wing surface to the point that the water is above its boiling point for that pressure, it is not cavitation. Cavitation is steam bubbles with no surface air in them. Surfaces are damaged when the bubbles collapse on them because there is a shockwave generated.
The same phenomenon, low pressure, is also responsible for air pockets/bubbles from the surface sticking to the wing tips, top, or trailing edge. Since the pressure is low, the bubbles can appear to grow and stretch. Loss of lift and crashing can happen but not extreme shockwaves and erosion of the surface.

airsail
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Re: Cavitation

Postby airsail » Wed Apr 04, 2018 2:07 pm

Ok, thanks, we will call it ventilation, air passing along the mast and onto the wing. Will try some crude fixes to start with, slap a vortex generator on either side, this will tap any air off and send it to the trailing edge and not down the mast. Failing that a wing fence, yep, crude but effective. At least it gives a starting point to nail down the problem.

opie
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Re: Cavitation

Postby opie » Wed Apr 04, 2018 8:40 pm

Please let us know what you try and the results. There's definitely something to be learned here.

fluidity
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Re: Cavitation

Postby fluidity » Mon Apr 23, 2018 10:09 am

airsail wrote:
Wed Apr 04, 2018 2:07 pm
Ok, thanks, we will call it ventilation, air passing along the mast and onto the wing. Will try some crude fixes to start with, slap a vortex generator on either side, this will tap any air off and send it to the trailing edge and not down the mast. Failing that a wing fence, yep, crude but effective. At least it gives a starting point to nail down the problem.
I'm not a foiler yet but I was thinking the mast as a common element must be passing air down to the foil region. Something to remember when I start my own foil adventures!

Mossy 757
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Re: Cavitation

Postby Mossy 757 » Mon Apr 23, 2018 4:26 pm

George's latest tech video from Delta Hydrofoil recommends any surface finish be in the 1000 grit+ range for exactly this reason.


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