It does not sound reasonable to me that just because the mast was not sanded it sucks air down. I think there is something off with the design of the mast that allows it to flex under load. If the mast flexes under load, and the load is coming from the windward side, then cavitation would occur on the leeward side, which it does.

Grantmac said..
I've actually held AFS and IQ foil setups, the AFS is very flexible in comparison. Even the regular Starboard setup is significantly stiffer.
The fuselage to mast connection has absolutely zero flex. The original non-plus fuselages were not as thick as the plus and there were a few that failed but none of that has been current for at least two model years.
There have also been cracks in one part of the starboard masts but those were again not current or IQ versions.

The masts have a smooth sticker that goes along part of the leading edge which makes the surface very smooth and that can absolutely cause boundary layer separation. In my mind as someone who has a casual background in aerodynamics that is absolutely what we are seeing here and there are simple experiments to test whether a change in surface would be beneficial such as running a line of 3M Blenderm tape along the leading edge.

Grantmac what AFS foil did you have? I have the W95, so I think third generation (hollow mast) with the newest version coming out a year or two ago (solid mast). I have a big smooth AFS sticker on my foil across the top on both sides and I never drop out of the air like Nico or more relevant aeroengr who is probably going a similar speed as I am. So I am not buying it right now that the mast smoothness or sticker are causing the cavitation. Maybe there is a production issue with some masts not being built to specification? Starboard had a delamination issue in the past with boards, so why not an issue with the carbon mast layup?

Grantmac said..
I've actually held AFS and IQ foil setups, the AFS is very flexible in comparison. Even the regular Starboard setup is significantly stiffer.

Yes and not really. In torsion, the orig 95 and AFS are very close but the IQ is much stiffer. In flex (side to side), the AFS is significantly stiffer than the orig or IQ.

windfoilfan.glissattitude.com/devices/foil/chart/tors_module

windfoilfan.glissattitude.com/devices/foil/chart/flex_module

And, yes, Nico is a big boy and pushing a lot harder and faster than most anyone on this forum.

Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery.

Tell me why the air bubbles you see in Nico's video can not be due to cavitation? Cavitation does not have to lead to shock waves, only when subjected to higher pressure.

Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

In other words, ventilation is not something that normally happens on a vertical foil mast. You guys are just throwing stuff out there to cover up the fact that there is a design/construction problem with some of the iQ foils.

And, When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water. So cavitation does occur with foils, and that is exactly what you see with Nico, cavitation followed by the board crashing to the water. Ventilation would in comparison be a more gradual process on a vertical mast, and you would see the air getting sucked down the side of the mast, but you do not see that in Nico's video, instead you see an expansion of air bubbles below the water due to cavitation.

aeroegnr said..
Interesting thoughts from Nico Prien on his Defi Wind crash. I think this is the best explanation of a potential problem causing a sudden crash, and a remedy. I don't think that this is what happened to cause my crash at my first post in this thread, but I've had some crashes that seemed to act more like what he shows here.

That is very interesting. The fish foil session where i experienced at least a half a dozen non breaching violent catapults was actually the first day I used a brand new carbon foil (of course not sanded). My only explanation was hitting fish as a felt a soft "impact" with each catapult. Perhaps Nico's ventilation theory is more accurate.

thedoor said..

aeroegnr said..
Interesting thoughts from Nico Prien on his Defi Wind crash. I think this is the best explanation of a potential problem causing a sudden crash, and a remedy. I don't think that this is what happened to cause my crash at my first post in this thread, but I've had some crashes that seemed to act more like what he shows here.

That is very interesting. The fish foil session where i experienced at least a half a dozen non breaching violent catapults was actually the first day I used a brand new carbon foil (of course not sanded). My only explanation was hitting fish as a felt a soft "impact" with each catapult. Perhaps Nico's ventilation theory is more accurate.

Look no one has shown that lightly sanding the mast of an iQ foil stops it from dropping someone out of flight, that includes Nico and aeroegnr. So why doesn't someone who has been having the problem sand their mast down and find out? Better yet contact Starboard to see if they recommend doing that and if it will invalidate any warranty claim.

And again, there is no video evidence from the videos that the mast is ventilating (air snaking down the mast), which by the way does not normally occur on a vertical mast and would not drop you out of flight suddenly like cavitation does.

Paducah said..

thedoor said..
That is very interesting. The fish foil session where i experienced at least a half a dozen non breaching violent catapults was actually the first day I used a brand new carbon foil (of course not sanded). My only explanation was hitting fish as a felt a soft "impact" with each catapult. Perhaps Nico's ventilation theory is more accurate.

People who have hit fish have also mentioned that the fish oil/body parts disturb the water flow similar to dirty finger prints. I wonder if that contributed to your day after your first fish encounter?

I never saw evidence of fishyness on the foil but I don't see finger prints either

aeroengr said, "But this is like a foil-assisted nosedive.", and you are right, the foil is cavitating and so suddenly losing lift causing the board to immediately drop out of flight, that is the hallmark of cavitation, not ventilation. For what you paid for the foil I would be talking with whoever sold it to you and to SB, of course they could be the same. You have a defective foil buddy, wake up and get a replacement if it is a construction issue, and SB should know which it is.

I've hit fish (alligator gar) and ducks. Ducks are a soft bump, while the gar was a hard "clack" like hitting a concrete curb. No crash with the duck, total face plant with the gar. Only time I crash in loss of laminar flow is motor boat prop wash.

Sandman1221 said..
Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

In other words, ventilation is not something that normally happens on a vertical foil mast. You guys are just throwing stuff out there to cover up the fact that there is a design/construction problem with some of the iQ foils.

And, When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water. So cavitation does occur with foils, and that is exactly what you see with Nico, cavitation followed by the board crashing to the water. Ventilation would in comparison be a more gradual process on a vertical mast, and you would see the air getting sucked down the side of the mast, but you do not see that in Nico's video, instead you see an expansion of air bubbles below the water due to cavitation.

Wrong

gorgesailor said..

Sandman1221 said..
Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

In other words, ventilation is not something that normally happens on a vertical foil mast. You guys are just throwing stuff out there to cover up the fact that there is a design/construction problem with some of the iQ foils.

And, When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water. So cavitation does occur with foils, and that is exactly what you see with Nico, cavitation followed by the board crashing to the water. Ventilation would in comparison be a more gradual process on a vertical mast, and you would see the air getting sucked down the side of the mast, but you do not see that in Nico's video, instead you see an expansion of air bubbles below the water due to cavitation.

Wrong

And your source of information?, I just copied that from a hydrofoil website, and they should know!

Sandman1221 said..

gorgesailor said..

Sandman1221 said..
Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

In other words, ventilation is not something that normally happens on a vertical foil mast. You guys are just throwing stuff out there to cover up the fact that there is a design/construction problem with some of the iQ foils.

And, When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water. So cavitation does occur with foils, and that is exactly what you see with Nico, cavitation followed by the board crashing to the water. Ventilation would in comparison be a more gradual process on a vertical mast, and you would see the air getting sucked down the side of the mast, but you do not see that in Nico's video, instead you see an expansion of air bubbles below the water due to cavitation.

Wrong

And your source of information?, I just copied that from a hydrofoil website, and they should know!

From another Hydrofoil website ... though your source also mentions the speed at which cavitation occurs: "Cavitation and ventilation both appear as bubbles attached to the surface of the operating hydrofoil. This phenomenon particularly occurs over the section back (suction side) of the hydrofoil with the bubbles varying both as to size and extent. The formation of vapor bubbles will occur within a liquid in a region where the static pressure of the liquid's flow field is equal to, or less than, the saturation (vapor) pressure of the liquid. The resulting low pressure is a consequence of the local acceleration of the liquid to a relatively high velocity over the hydrofoil surface. In order for cavitation to develop, the surface pressures on the suction side of the hydrofoil must be lower than water vapor pressure. Ventilation will develop when surface pressures exist which are lower than the ambient pressure of an externally available gas supply. The gas supplied is usually air from an atmospheric source. Cavitation invariably occurs in conventional hydrofoil craft above a certain operating speed (typically over 50-60 knots). Ventilation can occur at much lower speeds."

So it is much more likely ventilation is causing the issues than cavitation considering the speeds Windfoil are currently capable of...

gorgesailor said..
Cavitation invariably occurs in conventional hydrofoil craft above a certain operating speed (typically over 50-60 knots). Ventilation can occur at much lower speeds."

The "invariably" is important - there's simply no way around it once you go fast enough, so you need a different design, like the Sailrocket 2 used.

That does not mean cavitation can't occur earlier - it certainly will occur earlier in sub-optimal conditions.

gorgesailor said..
In order for cavitation to develop, the surface pressures on the suction side of the hydrofoil must be lower than water vapor pressure.

Ventilation will develop when surface pressures exist which are lower than the ambient pressure of an externally available gas supply. The gas supplied is usually air from an atmospheric source.

Water vapor pressure is around 5 hPa at room temp, while air pressure is about 1000 hPa. That would mean that ventilation would occur a lot earlier.

Do I assume correctly that ventilation requires some kind of "connection" to the "externally available gas supply"? A foil mast obviously always has such a connection.

What is the speed with which ventilation can spread? In windsurf fin design, especially for weedies and deltas, advanced designs have some characteristics that aim to minimize the spread of ventilation to the lower part of the fin.

What about cutouts? Probably ventilated, not "cavitated"? If cavitation damages propellers, then it certainly would damage glass fiber hulls?

Sandman1221 said..

gorgesailor said..

Sandman1221 said..
Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

In other words, ventilation is not something that normally happens on a vertical foil mast. You guys are just throwing stuff out there to cover up the fact that there is a design/construction problem with some of the iQ foils.

And, When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water. So cavitation does occur with foils, and that is exactly what you see with Nico, cavitation followed by the board crashing to the water. Ventilation would in comparison be a more gradual process on a vertical mast, and you would see the air getting sucked down the side of the mast, but you do not see that in Nico's video, instead you see an expansion of air bubbles below the water due to cavitation.

Wrong

And your source of information?, I just copied that from a hydrofoil website, and they should know!

That hydrofoil information came from the Massachusetts Institute of Technology (MIT) website.