50 knot digestion...
Mon, 17 Oct 11 15:23
So now we are collating all the data to try and get a good picture of where we are at. We have built up a pretty solid VPP (Velocity Prediction Program) throughout the project and now we want to see exactly where we are in the real world relative to that.
The performance of the new foil has been pretty rapid. There are lots of variables in this game and we need to be careful before jumping to any assumptions. Whilst we are all delighted to be dealing with issues around 50 knots, we really thought that most of these flow issues would have been resolved by this speed. We believed we would be way out on the open highway by now with the foil travelling in a nice big 'super' cavity. I don't think we are that far away from it... but we may just need a few little additions to help it on its way.
It would be very nice to know exactly what is happening with the ventilated cavity that we want to cover the whole upper/suction/windward surface of the main foil. The question is how do we do it. As it is inclined at 30 degrees downwards in the water, it makes it pretty damned hard to film as the best angle is side on. We are considering such things as cameras on the flat back edge of the foil... but that only shows the bubble down the base of the foil i.e. the back, blunt edge of the 'axe-head' profile. Looking around the corner and getting any clear imagery would be a lot harder. Another thought is to have small spring loaded 'feelers' that stick upright when they are not forced back by water. The fact is that any of these systems always ends up more complicated and time consuming than you can imagine. They are projects in their own right. We will try them as a last resort. Firstly we will probably try to view the cavity externally either from another craft or merely from different camera angles on the boat. We will conduct further trials trying different sailing techniques and we will try a few simply 'stick-on' mods to the upper foil surface to see what they do.
On another note, we have begun making enquiries relating to making a ratified record attempt in a month or so. Let's see what the WSSRC make of that. I was hoping to be going faster before committing but we are simply running out of time. We may have to make a leap of faith on this one in the hope that we can continue progressing in the next few weeks before the actual attempt will start.
We will use these next couple of days before Wednesday's solid forecast to give VSR2 a good going over. We have already found a few little chafe issues. Now that we are pushing the boat we have to be extra vigilant on maintaining her. I have begun removing some of the systems that we no longer feel we will need. We had a very complicated rigging system installed that would allow us to swing the whole rig and beam fore and aft in column. We thought we would need it for the start-up phase as the boat accelerated. This hasn't been the case. It is a real joy to remove all this stuff and greatly simplify the boat.
So let's see what the brains trust back in the UK come up with. Thanks again for all your constructive input. It really is all good stuff and therefore much appreciated.
Cheers, Paul
Cavitation location?
Submitted by Arvid on Tue, 10/18/2011 - 18:41.Correct me if I'm wrong, but the foil is pulled/pushed in the direction were the low pressure is. Looking at the images further below, I can't help to think that the lowest pressure will actually be by the base of the vedge, that is behind the foil. This would effectively create a small cavity behind the foil, attempting to pull it backwards and reducing the boats speed.
Applying the same concept for the propellers of motorized boats, this is not as much of an issue because the low pressure side is infront of the propeller (the problem here is rotating it fast enough so that the water rushing towards it won't get the pressure as high as behind), and the "base cavity" will be at the back end of each propeller blade. This will act to slow the rotation, but will not (directly) reduce the speed of the boat.
Since the power of a sailing boat is limited to the wind, you'd need eighter more wind or less cavitation behind the foil to counteract it.
If my idea would be true, this wouldn't be an easy change, as you'd need to redesign the entire foil with this in mind. My guess would be that you'd then want a flat side on the high pressure side, and the low pressure side arced in the middle, and converging towards the flat side at the edges, like the back-part of a drop both infront and in the back.
Why use VSR2 to unravel foil mystery?
Submitted by Tim on Tue, 10/18/2011 - 19:05.Terrific and informative replies, Paul.
I suspect that the UK BRAIN TRUST is already on it, but at the risk of appearing to be an officious intermeddler, here's one more thought from the armchair-beside-the-wood-stove vantage point in Vermont.
When the ultimate goal of the VSR2 is to break the outright sail driven world speed records, why use the VSR2 as an experimental test platform to investigate the performance properties of the ventilating foil? Wouldn't it be a great deal more straightforward to rig various appropriately monitored test foils to a pretty fast motorized boat platform to test various foils, angles of attack/yaw angles at speed? From my viewpoint, the following advantages are immediately obtained by using a motorized boat platform:
1. You don't have to wait for the right wind at Walvis Bay;
2. Your test runs can be as long as you wish them to be, and as frequent as you wish them to be (no towing the VSR2 back to the top of the course), which should translate into LOTS OF DATA;
3. In the event that you deem a video necessary down the low pressure foil surface,it should also be easier to rig a video of the foil cavitation on a motorized test platform;
4. You don't beat up the VSR2 doing foil research, which avoids the risk of a possible VSR2 injury or catastophic failure.
After all, you seek a sailing world speed record, and unless I've missed something in the "spirit of the adventure", I would advance that it makes sense to save the boat for runs with an improved/perfected foil.
You know, in many ways when viewed from 30,000 feet, you're doing almost exactly what some famous Americans did more than a century ago...and I think you know this. History teaches that the Wright Brothers invented controlled, motorized flight. But to get to that point, they first invented the wind tunnel to test their airfoils and control surfaces, as well as inventing wind-tunnel test procedures. Then they invented an airplane propeller. Finally, they perfected a light-weight motor. The Wright Brothers were long on that which America is now so short: intelligence, driving curiousity, gumption. More the loss for us. Like the Wright Brothers, so it is with the VSR2 project, grappling with high-speed foils to advance the speed bar. Radically, we hope. I'm damned glad there are still people burning to answer questions such as, "how do we sail +60 knots across the water, and what must that be like?"
Keeping good thoughts for Wednesday!
Tim, Vermont
Foils ventilating?
Submitted by Yannig on Tue, 10/18/2011 - 10:29.I may be wrong but looking at the foil wake going up and down at short intervals, I think that may be there is ventilation happening. It doesn't seem to happen all the time, if it is possible to synchronise the GPS log to the video it would be interesting to see if there is some kind of correlation with the speed going up or down.
Are there anti ventilation fins on the foil?
Reply... foils ventilating...
Submitted by Paul on Tue, 10/18/2011 - 12:04.Hi Yannig, I'm sure there is some ventilation happening near the surface. Every now and then we see a little puff of spray but we think this is merely the inboard wake off the front planing hull temporarily hitting the foil when the forward hull clips some chop.
We have no anti ventilation fins because this foil is meant to ventilate.
I always expected a much bigger 'rooster tail' from a ventilated foil due to the deflection of the water off the working surface of the foil.
Cheers, Paul.
Hi Paul, That's great to see
Submitted by Yannig on Tue, 10/18/2011 - 14:15.Hi Paul,
That's great to see a reply!
First a disclaimer: I am no expert, my experience of ventilation is limited to loosing control on downwind legs and of cavitation to seeing outboards propellers loose their paint. I have done a little bit of reading and learnt some thermodynamic at college but that's all!
IMHO cavitation is rather different from ventilation as it involves low pressure steam whereas ventilation is just air coming down the foil. I think that you can kickstart cavitation by letting in a little bit of air but if you get along the foil a lot of it at atmospheric pressure the lift just disappears (or more precisely is consistent with what you would get from an airfoil). A cavitating foil gets its lift from the fact that steam pressure at say 20C is much lower than atmospheric pressure and thus the foil still generates a lot of lift albeit not as much as a traditional foil.
I think that if I were you, I would try to limit the quantity of air sucked in from the surface. You probably still need a little bit otherwise you would have to reach an unattainable speed for the foil to start cavitating.
That may sound patronising and it is just my understanding but if it helps you, even a little bit, I would be really chuffed!
Go, go sailrocket!
Cheers
Yannig
Main foils
Submitted by Bob Imhoff on Tue, 10/18/2011 - 05:37.Hi Paul, along with sorting out the cavitating foil, I'd suggest also running with the conventional foil. My guess is you can get to record speed with the conventional foil, the question is when does cavitation plague it.
Reply...Re main foils...
Submitted by Paul on Tue, 10/18/2011 - 12:08.Hi Bob, I can't wait to try the normal foils on. VSR2 should be a real weapon with them fitted as she is already hitting similar speeds to the old boat in similar winds with the ventilated foil. Right now we want to focus on the ventilated foil as that is where the real big gains will come.
I agree that many records could still fall to the other foil.
We'll see.
Cheers, Paul.
Time for the brain trust to step up
Submitted by P.Flados on Tue, 10/18/2011 - 01:40.Typical of something never done before, the devil is probably in some detail.
The real question is which detail?
Good luck, I hope the guys deep into all of the theory can at least come up with some "try this" items that will point the way to finding the culprit.
Reply... re 'The brains trust'
Submitted by Paul on Tue, 10/18/2011 - 12:51.Hiya Paul...
Yeah it's hard at this stage to know where the 'devil' is. We may be very close to some tipping point already and she just needs that extra nudge. This really makes it all fascinating as you are pushing right up against conventional understanding. This great boat is simply a tool that allows us to focus on the real issues in the real world. The boat is of course very impractical... but very practical as a tool for pushing foils to their limits. I think there are similarities to the Bell X-1 in this respect. It was a very impractical plane that needed another modified plane just to launch it... but it proved a point and was perhaps the quickest and cheapest way to reliably do it (note the colour of VSR2... sort of homage).
There are so many variables. We have to narrow them all down. Our brains trust don't like to make any assumptions until they have looked at all the evidence. The first thing they rightfully question is the quality of the evidence. We are forever chasing high quality data and it is notoriously hard to get.
I think that the 'try this' method is highly relevant as we can get a solid answer very quickly and way before we waste a lot of money on more complex single purpose measuring devices. It's often our first stop. Tomorrow we will add some simple 'strips' to the upper surface of the foil and see what happens. We will probably add it to the rear of the upper surface. If we do have fully ventilated flow then it shouldn't make much difference. It should be removable on the course so we can do a run by run comparison.
Cheers, Paul.
What about using paint or grease as a Telltale?
Submitted by Kuhilani1 on Tue, 10/18/2011 - 00:28.I've seen model tests where they use some sort of brightly colored paint or grease on the hull. The pattern left in the paint helps show areas of streamline and turbulence. Could you guys use something like that? It seem fairly simple to employ?
REPLY... grease paint telltales...
Submitted by Paul on Tue, 10/18/2011 - 12:55.Hi kuhilani...
I figure it would be a bit more difficult in our case as we are trying to record what is happening at one particular instance in the high speed flow regime. The start up and slow down attached flow patterns would probably affect the final 'image'.
having said that, I've never used the stuff and will be more than happy to be proven wrong as it sounds like the easiest option. It is noted as an option.
Cheers, Paul
Grease paint telltale on foil
Submitted by Kuhilani1 on Tue, 10/18/2011 - 18:57.Aloha Paul,
I guess with the grease paint telltale u would need to do a bit of deduction like Sherlock Holmes, but I suspect the resulting end patterns would probably show u the regions that experience more or less cavitation than others? Or venting as well?
If the foil is ventilating
Submitted by USA 3257 on Mon, 10/17/2011 - 21:08.If the foil is ventilating markedly before it cavitates this could create a large drag hump. With enough air from the surface the pressure may never get low enough to truly cavitate and you simply plow a bunch of atmosphere into the water with each pass.
Good luck and tell Alex the secret to the data loggers is to hit them on the 57.
REPLY... Is the foil ventilating
Submitted by Paul on Tue, 10/18/2011 - 13:03.Hi Karl....
The foil is meant to be ventilating before it Cavitates. We figured that it would start ventilating in the mid 30's and this would be where the drag hump would be (which you refer to).
We thought it would definitely be established by the mid 40's... but this does not explain why the boat should be up against something in the low 50's. Maybe it isn't... that's what we are looking at now.
The foils do drag big aerated ditches in the water and ventilation is used as a more stable form of cavitation.
Cheers, Paul.
Thanks for the explanation
Submitted by USA 3257 on Wed, 10/19/2011 - 08:40.Thanks for the explanation Paul. Look forward to seeing some more hammer-down runs and reading about your progress.
Foil
Submitted by Tim on Mon, 10/17/2011 - 18:23.Well, I learned something new. I had assumed that the VSR2 was so well balanced that the flow was intended to remain attached to both the high and low pressure sides of the foil, with only the blunt trailing edge (the ax-head) to be ventilated. Indeed, I look forward to what the UK brain trust resolves from the data. I'm chewing on this anew...
Paul, from your perspective, what did you observe of the boat performance on honking Saturday? Was there a steady accelleration up to the "glass ceiling" where the drag suddenly shot up and it felt like a water-brake was suddenly deployed? Or was the accelleration more like a steady but decreasing rate of accelleration until you were at speed and she wouldn't go any faster? Can you recall the sounds coming from the foil at this point? (And is there a possibility of a sound spectrum analysis to help understand what is happening on the low pressure surface?)
As to getting the WSSRC spun up and on site.... interesting considerations. I understand the risks and benefits behind the discussion. More than anything, I get the sense that there is also a deep pool of talent and brains on-site at Walvis Bay. I trust that team-VSR2 will fully argue both sides and come to a reasoned and informed decision.
As always, best of luck in decisions and outcomes!
Tim, Vermont
Reply... re 'Foil'
Submitted by Paul on Tue, 10/18/2011 - 13:28.Hi Tim,
The plan was to have the foil fully ventilated when at speed. This would involve having both the base and the upper surface of the foil ventilated. Ventilation should happen before Cavitation with our foil application. Both require a substantial drop in pressure in the water over the upper surface. The deeper the foil is submerged, the higher the pressure and hence the more resistant the foil is to either.
The interesting part is how and when the foil will transition from one flow to the other.
On Saturday, VSR2 accelerated very swiftly up to speed, especially on the big run where we had the mainsheet issues. It didn't bang into any 'walls' but the acceleration definitely slowed. VSR2 surged back and forth around the 49 knot mark. I have a very clear view of the gusts on the water ahead so try and judge what's coming. All the time I am waiting for that next 'big' lunge that will signify a big jump in speed. VSR2 should accelerate hard up to 60+ knots. I've never been there and obviously I'm a human who really wants to know what it's like. I believe we hit gusts around 26-8 knots that should have given us substantial speed gains.
The sound is amazing... but it may be due to a frequency emitted of the Trailing edges of the foil as per a normal rudder. Hmmm interesting though as it would change as the upper trailing edge moves out of the liquid and into the air cavity. Good point. I'll keep that one in mind.
Cheers, Paul.
Hi Paul Can you give us a
Submitted by markoblob on Mon, 10/17/2011 - 17:10.Hi Paul
Can you give us a simple cross section of the foil so that we know what it looks like please?
Reply... re foil pics...
Submitted by Paul on Tue, 10/18/2011 - 14:37.Hiya Marko... you ask, you get;)
cavitation
Submitted by Mike Ewart on Mon, 10/17/2011 - 16:18.Hi Paul, probably in the gandmother and egg sucking area but introducing a small diameter probably a capilliary tube down the foil and seeing if a vacuum is being generated might tell you the extent of the cavitation bubble.
REPLY... Re: cavitation
Submitted by Paul on Tue, 10/18/2011 - 14:43.Hi Mike, we are looking for cavitation along the upper surface of the foil... see pics above... and not just the thick, flat trailing edge. We are confident that this part of the foil is fully ventilated. It is hard to know how this ventilation is moving forward along the foil if at all. Capillary tubes would be a bit of a mission to fit and install. Like everything... we will keep all options in mind.
Cheers, Paul
Cavity - force feeding?
Submitted by Tim on Mon, 10/17/2011 - 18:25.And if it's determined that the foil is not super cavitating, what about the possibility of force-feeding ventilation into the low pressure area?