Helena Darvelid / www.sailrocket.com

60 knots in sight

Paul Larsen updates us about his Vestas SailRocket 2 campaign

Wednesday October 26th 2011, Author: James Boyd, Location: Namibia

Fingers crossed, touch wood – at some point over November-December this year we could see the world sailing speed record down a 500m course taken over 60 knots and it could be a ‘boat’ that does it rather one of the pesky kiteboards or sailboards.

Having launched Vestas SailRocket 2 this spring in Cowes (to find more about this see the video here), so this extraordinary yacht, designed to the concept laid down by Bernard Smith, is now down in Walvis Bay, Namibia and according to her pilot, Paul Larsen, prospects are good. Already Vestas SailRocket 2 has hit 50 knots and that was with a passenger, a journalist from Wired magazine, in the back seat.

“It was a little bit unexpected,” says Larsen of that run. “It was quite gusty that day. Here, if it is a day that is going to be quite windy, often it is only when the day really starts to max out that it locks into the correct true wind angle that we need to do a good run. So it hadn’t quite settled and we were getting gusts of 21-31 knots, shifting through 15-20 degrees but I thought ‘let’s see how the boat goes with someone else in the back of it’, because we haven’t done that before.”

Initially there was a problem to get started because of the extra weight in the bow. As Larsen describes it: “I need to spend a lot of time drifting sideways with everything fully stalled – the sail and the foils, etc. It is like backwinding a jib through a tack – it is a matter of balancing everything so the boat rotates, and it wouldn’t do that, so we didn’t think we would get started.”

Eventually they did get going and with a big gust towards the end the recorded average speed was 50.05 knots. Larsen admits he was worried about losing it at the end of the run as Vestas SailRocket 2 accelerated as she headed up before eventually she did slow. Unfortunately the run being a low tide, in the process they ended up with the foil stuck in mud. The foil, which has a razor-sharp leading edge, was ground away a little by this contact and as a result they were unable to carry out another run that day which could have been a record breaker.

Crucial to the current set-up is that Vestas SailRocket 2 has been sailing with a non-cavitating lifting foil (ie the foil works only on one surface, rather than on both as most foils are designed to).

While Larsen says he and the team still have a lot to learn about this foil, his initial impressions are extremely positive. “With this foil, I am totally confident it doesn’t have any stability issues like a traditional sub-cavitating foil will when you start getting to those [high] speeds. You should just be able to push this foil more and more.”

A particular worry when Vestas SailRocket 2 was launched was how this non-cavitating foil would perform at low speeds. However this has proved not to be a problem. “We thought we were going to have problems between 30 and 40 knots,” says Larsen. “But in two runs it just powered through that and then in another three runs it was over 50 knots. I am pretty sure in another couple of runs it will be up to 60 knots. I am not worried about the potential of that foil to hit 60 knots at this stage – the boat is so stable.”

The key is that Vestas SailRocket 2 has sufficient power to haul foils that are inefficient and draggy at low speed down the course. As Larsen puts it: “We made the decision to simply make a boat that can drag the worst possible foil as fast as any boat potentially can – to give the boat the efficiency, the robustness, the strength and the ability to sail in lumpy conditions that you would associate with 30 knots of wind and to give it the aerodynamic efficiency and clearance above the waves, etc. And everything is paying off, because we assumed a worst case scenario for the foil of a lift over drag ratio of somewhere around 4.5-5:1. We even suspect now that we might even be struggling to achieve that. So the way to get the performance out of the boat is simply to sail it at this stage in a little bit more breeze.”

And there remains much development to be done with the foil. At present they have done little to tweak it. Larsen says it is quite a basic section, because they figured that they could always add bolt-ons, such as adding steps to it to trip the flow over it or help stabilise the cavity on the foil’s upper surface.

Back in Cowes when the boat was launched, Larsen was concerned about the ability to set off down runs, thus Vestas SailRocket 2 has an articulating beam that allows the centre of effort of the rig to be moved forwards. In reality, Larsen says that they haven't needed this. “It is a real relief, because the boat is getting a lot simpler, and it is nice to shed those systems.” And removing systems, alleviates not just weight and complexity but also the possibility of breaking something, which on a finely tuned instrument such as Vestas SailRocket 2, could result in catastrophy.

Since arriving in Namibia they have modified the floats by adding second steps to their underside and enlarging the planing surface at the front to encourage it to lift off at start up, as well as giving it the ability to sail two-up.

One aspect of the boat that hasn’t worked out is the self-feathering rig that was supposed to enable them to haul Vestas SailRocket 2 back up the course without having to drop the rig. “We have found that it is a lot more complicated than we expected, just because when you are towing the boat with the rig up backwards and the boat is trying to sit sideways to the wind and there is not enough hull in the water to make the hulls track in a straight line at low speed, because the foils all so small,” says Larsen. So they are back to dropping the rig at the end of the run.

What has pleasingly proved to be the case is how quickly Vestas SailRocket 2 has got up to speed - much faster way than Larsen anticipated. “The progress has been quicker than we can almost come up with theories. We get a whole group of data from this boat and while we analyse that we go and do another run and all of a sudden we are going 5 knots quicker. We have got a very well developed VPP for this [non-cavitating] foil, but we are not going along in steady states on this boat. Even in that 50 knot run it is a series of surges down the course, and I’m sure that anyone from the kitesurfers to the windsurfers to even the Macquarie Innovations boys, would tell you that it is always the case down the course. It is not like you are slowly rumbling between 48 and 54 knots.”

And there still remains a lot left in the tank. Key to Vestas SailRocket 2’s performance is her non-cavitating foil, this is relatively undeveloped with a simple section, but gleaning vital data to learn about how this works is proving hard. “We have a debate going on whether it is fully ventilated or not,” says Larsen. “We know the square back of the foil is fully ventilated, but whether the full upper surface is fully ventilated of if that bubble only goes forward 25 or 50% from the leading edge, we don’t know. You can’t film this foil and, because we have got an inclined foil, and the surface you are trying to look at is pointing 30degrees downward so when you’re underway to put a camera on it at that speed is really hard. You can put a sensor on it in one spot, but that doesn’t what is happening further down or further forward on the foil. To be able to look at it from the side would be absolute gold for us. We have got a lot of theories on how we could do that, because just a simple picture would tell you everything. Unless someone has a 50 knot micro-sub, we are in trouble!”

And the faster the boat goes so more aspects of her design come into play. One of the main oddities of Vestas SailRocket 2 is that her fuselage is orientated off-axis, so that at speed it should line up with the direction of the apparent wind, thereby minimising drag. “We are only just starting to drop into that apparent wind angle where that should start to become beneficial, so there are all these other aspects of the boat that we haven’t tapped into yet.” So to hit 50 knots with a passenger on board, the pod flying far higher than it should and the wingsail not fully sheeted, bodes well for the future. In fact having a passenger on board Larsen reckons only costs about two knots of boat speed.

Having done the 50 knot run followed by the grounding, so they are currently working on the non-cavitating foil and have discovered a couple of small cracks in it. So these will be repaired and the foil bench tested for the loads they anticipate it will see at 65 knots. “If you don’t have peace of mind on this boat on a windy day, it is a very hard call to go up there and risk everything - the boat included and your own neck - on something that is not thoroughly understood. So I want to be totally sure of that,” says Larsen.

But another spanner in the works will be if the foil doesn’t pass the bench team. Then it will take time for them to make another. “There is a lot more load on the ventilated foil than there is on the sub-ventilating foil,” says Larsen. “Because it is draggier it needs a lot more power to drag it along, so there is a greater tug of war going on between the water and the air with the ventilated foil. For the same speed, it needs about 25% more strength in it to stay as stiff. So we have to see how it unfolds. If this foil doesn’t pass the next load test for some reason, the project will be set back quite some time, because they are not lollipop sticks and you can’t just go and get another one. Just to get your hands on the fibre, you have to order it and then they make it. Then we would have to start looking at plan Bs and what if scenarios to see how quickly we can turn anything around in the next month or so if we have to.”

In the meantime they have fitted the original cavitating foil back on the boat as how this will perform remain an unknown. As the foil is more efficient and less draggy, it should require less wind to get up to 50+ knot speeds than the non-cavitating foil which Larsen says requires around 27-30 knots to reach the 60 knot average speed down the 500m course.

Larsen anticipates the more traditional sub-cavitating foil will be enough to get them past L’Hydrotere’s record of 51.36 knots, but possibly not past the existing outright 500m course record of 55.65 knots set last year by American kiteboarder Rob Douglas. “I am quite curious to see how that foil performs and where it maxs out at." Once again the ultra-efficient sailing machine that Vestas SailRocket is means that even if the foil is ventilating/cavitating, it will still get dragged down the course. As Larsen explains: “When a normal boat would hit a limit with a bit of cavitation at say 56-57 knots, we might be able to hit a peak with this one still around 59 knots or something - it wouldn’t be pretty, but if it gives you a big number at the end then it might be.”

Again development of the cavitating foil has some way to go. When they first arrived in Namibia they fitted it with fences at 7° but due to the inclination of the foil, these proved to be at the wrong angle and were stalling out.

Unfortunately yesterday, they broke Vestas SailRocket 2's main cross beam. Although this sounds like a major issue, it in fact represents about one week’s work to repair. Once this is fixed and they are back up and running again, then they are anticipating starting a 28 day record period (ie when Mike Ellison, the representative of the World Sailing Speed Record Council is in Walvis Bay) on aroud 17 November. Ideally Larsen says they want to get to the stage where they have broken the record unofficially so that when Ellison turns up they merely have to wait for the right conditions, although on Walvis Bay 30 knot days are much rarer than 25 knot ones.

“It is just a matter of whether we can hold it together on this record attempt, or if we have got to do another one,” admits Larsen. “The progress has been really good, but with speed sailing you are always waiting to be dealt a cruel blow. It will be in something to do with the details. It is certainly a quick boat. It will be a beautiful beautiful day when we do get that outright record, but I really want to see this boat doing reliable runs of 60+ knots down the 500m course. If we did a solid 60+ knot run, then I’d be happy to bring it back and let everyone digest that for a while, knowing that if we did come back down here in a year or two and that structurally there is a fair bit more left in the tank. We haven’t done it yet, but I do believe we are on the way there. It is something that we can’t be that scared of, because that is the creature we built and it is time to unleash it."


Latest Comments

  • David Bains 27/10/2011 - 11:08

    It is fascinating to see Bernard Smith's "40kt sailboat" heading for 60kts!! I am unclear whether the "lifting" foil is actually lifting the hull or in fact pulling it down to resist the "lift" from the wingsail. Also I am concerned that should the forward planing surface detach and the hull nosedive, then Larsen's head (and neck!) would receive damaging impact. As has happened to some open top powerboat drivers!

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