Time to think and tinker...
Mon, 5 Nov 12 12:53
The Walvis Bay wind machine remains on safari... although we might get a chance to get out over the next couple of days. It has now been 16 days since we last had enough wind to get going. It's going from 'unusual' to ridiculous. The walls of the container begin to close in on us.
The WSSRC are now on-site so any runs we do from here will be officially ratified ones. Our record attempt period has started. The big TRIMBLE 5700 GPS is mounted on the boat. The gig is on!
The standby time has allowed us to think long and hard about our current performance predicament. We had our design meeting where Chris, Malcolm and myself had a long Skype discussion about likely scenarios. Basically we tried to reverse engineer the problem and our train of thought was as follows...
-The most likely candidate that would be giving us a sudden, large loss of performance regardless of power input is cavitation.
-The rudder is not loaded highly enough nor does it have the base area to give us such a sudden drop in performance... although we will continue to put sensors on it to make sure it isn't contributing.
-So... the most likely candidate for caviataion is the suction surface of the main-foil.
-The main foil shouldn't begin to cavitate around 52 knots unless it is 40% overloaded.
- How could the main foil be 40% overloaded at such a relatively low speed?
- If the upper portion of the foil, the part that enters the water, was ventilating (highly likely as it is at the surface), then what effect would that have on the boat?
- On checking the numbers, AEROTROPE deduced that if the transition (curved part of foil) was ventilating down its suction side... then we would lose about 30% of our lateral loading area. At 52 knots... this would lead to the lower section of the foil being overloaded by... wait for it... 43%!
-It would cavitate
- AND... the back of the boat would ride very low... as it has been doing...
- AND... pitching the foil up would most likely have little effect and may actually make it worse by leading to more upper surface ventilation (as we have often seen)
So, this all seems to fit together very nicely. This in itself kind of makes me suspicious. Nonetheless it is a great starting point. Some parts are kind of obvious i.e. that we are getting ventilation near the surface of a shallowly (?) inclined surface piercing foil but others aren't. The foil was already twisted so that it would be lightly loaded near the surface to prevent ventilation. The fact is that what happens at the surface is very hard to predict, especially in chop. We have added substantial 'fences' to the foil to try to prevent the curved part of the foil from ventilating. We have started big as it is easier to chop them down than build them up. We have fences on both sides.
Looking at VESTAS Sailrocket 2 sail past, it is easy to see the energy lost in the spray that is thrown into the air by the high pressure side of the foil... but nearly impossible to see the energy that is lost by air being sucked into the water on the opposite suction side of the foil. Both need to be considered.
It is only our 6th run with this foil. In this respect I think we are developing it in the right manner.
I think our logic is sound and obviously I look forward to seeing what happens next. We may not get the fences right first time. We have other options for modifying the foil if this doesn't work. We'll see. One at a time.
We have begun to make a few changes to the foil as we always expected we would. Subtle changes at the speeds we are already travelling can make a big difference. I'll hang onto a some of the pictures, data and details for now. If there is one thing that the past year has taught me... there is very little real-world info about the hydrodynamic arena we are about to enter into. There are lots of theories but getting the hard data from the real world to verify them is the hard bit. Some of those theories strongly oppose the path we are taking. The data we have received already validates some of our decisions. It has been expensive to come by.
Ben has also made a new asymetric rear skeg to help VSR2 get started.
So far we have just been using the old original rudder off Sailrocket 1. It has worked OK but quite often its effect was marginal. The new foil is nearly twice the size. The purpose of this foil was to help our small high speed foils give traction at low speed. VSR2 starts in a highly stalled state and makes a lot of leeway. As it is skidding sideways, the weight of me in the front makes the front float sit low and this drags the nose of the boat into the wind. She won't bear away and I just sit there. If we add more lateral resistance at the back of the boat i.e. the new big skeg... then there will be more drag at the back and if I fully over-sheet and stall the wing, VSR2 will bear away to an angle where I can sheet out the wing and start sailing. Hopefully I can get up enough speed for the small front rudder to get a grip before VSR2 turns head to wind again. You have to remember that the boat is set up to be perfectly balanced at 60 knots. This particular boat and concept is more sensitive to its static set up than most other boats as the sail/wing has a large offset from the opposing foil.
The skeg will kick up as VSR2 accelerates. If not then I will pull it up manually as soon as VSR2 begins to accelerate.
For now, we sit on standby. VSR2 sits outside fully tooled up and ready for action. The wind is blowing nicely already but we have lost confidence in its ability to build as it normally would. The team is now milling about as we wait to see if it will build enough to go sailing. I want to get out today... even if it is just to see how well the new skeg works at low speed. Tomorrow is forecast to be stronger... but then the wind drops off again until the weekend. A big day is predicted next Sunday but I don't put too much faith in forecasts that far away.
Come on Walvis... reward our patience. We're waiting.