Foil Cavitation at Lower Speeds Than Expected

Agreed.

Since ventilation is loss of lift owing to an air path from the lifting surface to the atmosphere..... whereas cavitation is literally the boiling of water owing to a high velocity

 

Right, if you look carefully at the video, you can see the ventilation trace starting at waterline level (outer foil) and travelling downwards slightly diagonally along the foil suction surface. A boundary layer fence about one chord above the foil apex/junction (whatever it is called in English.....) would delay the inception, if not stopping it altogether.

 

I'm quite confident that this is cavitation - not ventilation.

Any foil can cavitate at an unexpectedly low speed if the CL is larger than usual.

Since the area of surface-piercing foils gets smaller as they fly higher, they do get high CLs if they fly too high. This effect is stronger on V foils (especially with large dihedral angles) than it is on simpler slanted foils, but it can even happen on vertical rudders when their area gets too small. (It's happened to me on a Moth.)

 

Cavitation can definitely take place at 20 kt, especially for a situation where there's an acute junction between foils like the boat in the video. The velocities around the two foils basically add together at the junction, producing a local velocity that is much higher than 2D flow analysis would predict.

For example, on the AC catamarans, both the AC50s and the AC72s, cavitation always started first on the inside of the elbows of the L foils. The sections there were specifically designed to delay that onset, but it was still the most critical location.

Cavitation in the video boat could be delayed by opening up the junction, bringing the inboard foil in at a 90 deg angle to the outboard foil. Cavitation will still start at the junction first, but it might be possible to delay the onset to a speed that is higher than the boat can attain.

 

In addition to the above reasons why cavitation is possible in this case, I should add my impressions of the actual experience in the handfull of times that it happened.

The boat would be going along relatively smoothly & quietly (even when throwing significant spray). Then suddenly it would be as if I were driving on a bumpy gravel road. There was a very loud growling sound & significant vibration. The boat would slow down slightly, but would stay foiling with no particular problem. If it had been ventilation, the lift would have mostly disappeared & the boat would have crashed.

I was able to go much faster many other times (top measured speed just under 29 knots) without this effect occuring, by paying better attention to controlling the ride-height (via adjustable aft-foil incidence).

 

Thanks Doug, always interesting to hear your experiences.

That sounds more like stalling, or at least playing on the edge with the "bumpy" ride.

 

The video sounds very much like cavitation. I don't believe ventilation causes the same growling noise.

The oscillations are caused by a re-entrant jet at the downstream end of the cavitation zone. This jet of water flows back up the surface to where the cavitation is beginning and causes flow separation that detaches the vapor from the foil. This then drifts away as a bubble or cloud, cutting off the source of flow of the re-entrant jet. With the jet cut off, the flow reattaches. Then it cavitates anew, forms a new jet, and the cycle repeats itself.

 

I do not agree with the cavitation hypothesis here, particularly with the presented visual evidence of an aeration channel from the leading edge of the outer foil. Look closely from about 34 secs and forward; there are several occasions where there is a clear vortex trail reaching down to the foil junction. The rumbling noise is not typical for cavitation; the frequency is too low. It might be caused by cavitation, setting up a low frequency vibration in the foil structure, but then there would be an additional high frequency "white noise" in the kHz range, which (in my ears) is absent.

Now, with the gas cloud collecting at the junction at a depth of, say 0.3 m at slightly above 10 m/s in a water temp of 20 (?) C with a natural gas content of 2% free gas, there must be a local pressure coefficient peak of something like 1.8 to 2.0 in order to come down to the vapour pressure in the ambient water. It is possible, but first there will be a strong local pressure sink, which will trig a vortex from the foil leading edge (or any position with a strong local velocity gradient, even from far behind the foil!), down the foil suction surface. If you look again at the video, you can see the increased spray from the outside foil, coupled to a down movement from the low-pressure zone within the vee, when the speed increases, ie when the side force causes an increased aofa. That's when the aeration trail starts from that foil section. When the bubble is attached, fi within a recirculating bl zone, it can persist for very long times, causing a stable change of lift, drag or attitude.

It might seem stupid of me to argue about which gas you have in the submerged cloud; water or air, but they have different origins, and if you want to find out the next step in the development, the distinction is important.

 

I'm not actively developing the boat, so it's probably a moot point, but I'm still interested in whatever I can learn about it. I certainly don't consider it stupid of you to disagree.

If the video were of higher quality, we might be able to see more clearly. I don't know how much better it might have been before adding the speed data, making the enlargement, and submitting to YouTube. I'm not a video guru, but I'll try to see if it can be improved.

One thing that occurs to me is that we might be seeing both cavitation and ventilation. It's well-known that cavitation can sometimes trigger ventilation. It looks to me like the bubbles are deeper in the earlier frames, and then spread upwards. What do you think of that possibility? Here's a frame from 31 seconds:

 

Although it is a serious blow to my ego, I must admit that this image has the appearance of a stationary cavitation blister, somewhat analog to the "forehead blister" of the ITTC cavitator when run in some tanks. But we cannot see the suction surface of the inner foil, so there is room for alternatives. Anyway, I went back through the thread and noticed that there is a marked increase in transverse area close to the foil junction, which will reduce the local pressure and increase the probability for cavitation as well as down suction. You also mentioned that the phenomenon was not always occurring, even if there seems to be a coupling to foil loading and aofa. Another influence, that may have a random character, is water quality, or more specifically, its salinity and air content.

I have thought your trips were made in sea-water "of sorts". Cavitation in salt water differs from fresh "tap" water in that it normally contains far less cavitation nuclei than tap water and the vapour pressure is lower due to the influence of Na and Cl molecules, meaning that values for cavitation coefficients obtained in fresh water tests are not generally applicable. This can be very frustrating when operating in an outflow region where a river meets salt water.

Many years ago, I did some testing with supercavitating/ventilating props and the results pointed in all directions. We soon found that when crossing the Gota river outflow, we lost propulsion almost completely in some occasions. One weekend was particularly bad; the explanation came when the local news described the amazing views seen when the overflow spill dams in an upstream powerstation had been opened "for shows". The river water was oversaturated with air, and when it met the salt sea, which could hold even less gas, things went crazy. There were later reports from similar events elsewere, that fish died from "divers disease" (free gas microbubbles blocking capillary blood vessels and swelling skeletal joints) and hyperoxygenation. My point here is that ambient conditons can vary much more than one usually would realize.

 

Im not suggesting it can't. It is the speed at the localised area in question (the foil), not the boat speed per se.

Did these have fences?

I concur too.

A back of a fag packet calculation suggests this, a local pressure peak, as the "visual" numbers, such as depth 0.3m, velocity (20knots) ambient sea temps etc, appear insufficient on their own .

It would be interesting to see if running in the same conditions, but with fences makes any difference.
As that appears to be the most glaring aspect, the obvious looking air path down the foils from the free surface is unhindered.

 

The problem with this suggestion is that the conditions are rarely the same. Even ignoring the variations in water properties that baeckmo points out in post #145, differing wind strengths & points of sail result in large variations in the best ride-heights to use. A fence that would help in one condition would be a liability in others. Picking a single condition to try to optimize wouldn't be easy & could possibly be counterproductive overall.

Plus, for an amateur builder like myself, fences are a royal pain-in-the-a**!

In the early days of using this boat, I did have fences on the main foils. There were 4 of them on each 1/2 of each foil - 16 total. One day, I was sailing along in semi-foiling conditions with one of the fences just below the surface, and I could actually see flow separation occuring in the corner between the fence and the foil. Since I really disliked them anyway, that was all I motivation I needed to remove them completely. The scars in the finish on the starboard foil show where they used to be.

 

IMHO, fences are provided to avoid ventilation, not cavitation !

 

You're correct, but some (Baekmo and Ad Hoc) don't agree that cavitation is the problem.

I'm coming around to the idea that ventilation is shown at some stages, but that it is being triggered by cavitation deeper down on the foil.

 

This is fascinating to me!

I've only sailed this foiler in fresh water, and my Moth in salt water. But some of the guys in the Moth fleet who have competed in one of the high-altitude, cold water lakes in the Sierra-Nevada Mountains report that ventilation of the vertical foils (aka daggerboard & rudder) is much more common there.