Time to think and tinker...

 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.

Cheers, Paul

 

Comments

thanks

Thanks for the thoughtful reply Paul. I realize you guys have spent more time thinking about this stuff than most people, which is why I put the question about weight distribution to you.

Looking at Greenbird's land yacht, it would seem there is room for an aero surface to stabilize the pitch actively. Yes you have already flipped one boat I realize.

Nice to see reality conforming to experiment for a change. Pretty massive spray; more than before?

Good luck and safe chasing 55+,

Karl

Kiteboard

I had similar thoughts to someone in comments to the previous post re: downforce requiring extra lift (downforce) from the foil.

1) the kiteboards have skegs which probably operate not unlike your foil to keep things planted, so not entirely dissimilar

2) they go faster than you (so far) with less power, so their foils create less drag, despite the fact that kites also need to counteract the vertical lift vector from the sail

3) they use helm weight more efficiently to create downforce, by putting it all right on top of the sideforce-generating surface, thereby reducing the amount of downforce required from the board/skeg

4) the board itself functions like the ventilated side of your foil

All of which is just a longwinded way of saying you should move the helm weight back, and/or perhaps put a downward lifting air wing over the front pod to keep it planted. You will probably crush the kites at some point but why nit make better use of the helm weight anyway? You represent 80 kg of "free" downforce. This will reduce front pod drag a bit also.

REPLY...Re: Kiteboard

 HI USA...

firstly, sailing downwind at 140-45 TWA in 45-50 knots is a big part of what gives kiteboards their performance. Their efficiency isn't actually that impressive.

Our weight is exactly where it needs to be.

VSR2 is designed to be both a good aeroplane and a good boat i.e. if the hydrodynamics fail (total foil failure at +60 knots) then the aerodynamics need to kick in to stabilise it all. What you are proposing is a canard aeroplane... with all the weight at the back. You did see what happened to the Mk1 boat I assume? The boat does begin to unload at the front as we accelerate anyway.

Don't underestimate the thought and designwork that has gone into this... our second boat. We have looked at every option. We did not start with a boat that looked anything like this. It kind of designed itself as we started to resolve all the key stability issues. I am yet to hear a suggestion that we haven't considered. We live and breathe this stuff on a daily basis. Malcolm and especially Chris at AEROTROPE have done a lot of very good work with this boat. We spent a lot of time and effort doing a feasibility study well before we committed to a build. To describe every detail and compromise we made would be a book in itself. Obviously, many of you will have lots of questions as to why we have done things the way we have which is what I try and answer here. If there is a potentially faster way to set up the boat... then we may have steered clear of it for the other risks it brings with it.

Right now, everyone is looking at the kites because they rule the roost. I'm a big fan of their efforts... but if we do reach our potential... which I believe we will, then it will be their turn to make big changes. I'm sure they will. VSR2 was designed with a very high speed potential exactly because of the potential of the kites. The focus speed was set at a level that they are unlikely to reach as is. It would be a waste of time to go to this much effort just to try and match them. Right now it is all about the hydrodynamics. the boat itself is great and there is little I would change. Life will begin at 60 for VSR2. It won't stop there.

Cheers, Paul

fortiter ac fideliter

My old school motto. Translated it means "stronger by greater faith".

Go VSR2.

Test of hypothesis

The latest analysis sounds good.

Hope you are on to something with the ventilation at top / overload at the bottom hypothesis.

Given the large margins of power (as noted by the rapid acceleration up to your ceiling speed, a test for this concept would be to have a foil with little or no AOA near the surface and then use only the lower portions to provide the lateral force needed. With very low AOA, fences should be more capable of limiting how far down the ventilation extends.

I am sure you have considered this along with probably a number of other choices.

It is a shame that your sitting around without enough wind to try stuff. Hopefully the wait will be over soon and what ever solution you try will make headway.

Go SR2 Go!

REPLY... Re: Test of Hypothesis

 Hi Paul,

The foil is twisted off to unload the surface piercing section. The fences seemed to work beautifully yesterday. Along with the new skeg the boats lower wind strength was transformed. The back was riding high and we hit over 47 knots with a peak wind gust of 22 knots. Things are looking good... but the proof will come with a little more breeze. It was a good shake-down run to rid us of cabin fever.

Cheers, Paul

Ventilation

Ha-Told you so on the ventilation thing. Ok im done being smug now. Good luck guys.

Get the Flux Capcitor Ready

I see 1.21 Gigawatts of wind on the horizon.
Get the Flux Capacitor Ready!

Foil Analysis

The foil analysis is fascinating. I take it that you find it as surprisingly counter-intuitive as I did/do. Does any of the current thinking process explain why your fastest run last year was when loaded with two people?

I can only imagine how much you look forward to the wind returning to check out your theories and knock down the dead-ends, one by one. I've stopped looking at the Walvis Bay wind forecast for fear that this - like checking for a boil on a covered pot - has chased away the wind.

Cheers! Tim, VT

REPLY... Re: Foil analysis

 Hi Tim,

I can't think why two people would have made any difference. Obviously it has been considered... and re-visited on a number of occasions. I think that was simply because the foil had failed in the middle and was flexing. I'm starting to think that all our foils had small issues which were different than the other... but that all foils are very sensitive to small issues at the speeds we are going.

The wind is still away... But Sunday is shaping up to be epic. It looks like a case of all-or-nothing.

Cheers, Paul.

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