Kiteboard eater

We look at Paul Larsen's incredible Mark 2 version of Vestas SailRocket

Looking back through thedailysail archive the SailRocket speed sailer was first shown to us when Paul Larsen and designer Malcolm Barnsley revealed their blue model at London Boat Show all the way back in 2002 (see this here). Over subsequent years Larsen and Barnsley, with the help of their team, notably capable Swede Helena Darvelid, built their black speedster and campaigned it over the World Sailing Speed Record Council’s 500m course predominantly in Walvis Bay, Namibia.

They broke the boat on numerous occasions, often in dramatic fashion, including one backflip (see the video here), but always they have dogmatically jumped back, repairing SailRocket and got underway again. Latterly, with support from their sponsor Vestas, in 2009 they ultimately scored their best run at an average speed of 49.38 knots and a top peak speed of 52.78 knots. Unfortunately at the time the outright world speed record for the 500m course had been claimed by L’Hydroptère (51.36 knots) and has since been bettered by the pesky kiteboarders, who upped the ante when American Rob Douglas managed a 55.65 knot run at the Luderitz Speed Challenge (also in Namibia) last October. Nonetheless SailRocket’s concept had been proved and she is currently holder of the B-Class record for the 500m course.

With the secrecy of an America’s Cup campaign, over the last 16 months the Vestas SailRocket team have been beavering away at the hangar at East Cowes’ Venture Quays (appropriately the former Saunders Roe factory where they once built seaplanes and the first hovercraft) on a Mark 2 version - the orange and carbon fibre black marvel that was unveiled to the public for the first time yesterday.

The concept revisited

Before getting into the nuts and bolts of the new boat, we need to recover some ground. The SailRocket concept comes from US naval designer and rocket scientist, the late Bernard Smith, as described in his book ‘The 40 knot Sailboat’ first published back in 1963. This essentially was for what he dubbed an ‘aero-hydrofoil’. At first glance this looks vaguely like a Pacific proa, where the hull with the rig on it is down to leeward and the weather ‘hull’ flies, crew weight at the end of a lever arm counteracting the forces derived from the rig. But the fundamental concept of The 40 Knot Sailboat takes this a step further as instead of requiring weight to weather, the lift generated by the rig is counteracted by a foil in the water. So stability - one of the key components that limits the performance of every other form of sailboat - is neatly neutralised: a big feature summarised in a few words. Additional power developed by the rig simply translates into more speed.

Thus ‘the 40 knot Sailboat’ is a misnoma – at 25 knots SailRocket 2’s aft float to weather should be flying and at 50 knots the float with the rig on it to leeward is also airborne leaving just the foil and the tiny rudder and the forward float step in the water. Theoretically beyond this, the boat has no top speed, the limits being what the structure (and the skipper) can withstand. Already the concept has been proven with SailRocket Mk1, which reached in excess of 50 knots – something the team were proud to convey to Bernard Smith shortly before he died recently. Beyond this – who knows?

SailRocket Mk2 in a nutshell

At 40ft long by 40ft wide, SailRocket Mk2 is slightly larger than her predecessor, but is a significant development of the concept. She still comprises a main hull, now more of a fuselage held above the water as it sits on two ‘stepped’ floats fore and aft, a foil and rudder to weather, a giant single crossbeam and a solid wingsail canted to 30°, sitting atop a third stepped float to leeward. However there are some distinct improvements.

Most evident, and perhaps weirdest, is that the floats all appear to be constantly steering to port. In fact the floats are positioned at the angle SailRocket 2 will travel through the water, while the hull and the rig are pointing 20 degrees off to starboard into the direction of the apparent wind in order to minimise aerodynamic drag. So SailRocket 2 is not only set up to be a one tack wonder on starboard, but is also designed to operate at a specific apparent wind angle. When sailing it will appear to crab through the water and due to the orientation of the fuselage, according to Larsen, her total drag should be no more than that of a 74cm diameter sphere.

The next most evident change is that the cockpit has been moved to the front of the boat and there are now two of them. The reason for this is primarily one of usability – with the cockpit on Mk1, Larsen says that the force of water was such that it felt like his face was being shot blasted. Even located at the front behind a small windscreen (not fitted yet) he will wear a full face helmet and a racing car-style harness system.

In the front cockpit where there is a small steering wheel operating the rudder that sits beneath under the front float (ie the rudder is at the front of the boat). There are two controls – one for the mainsheet, the latter to articulate the angle of the beam (for reasons we’ll explain later). The pilot hauls in the sheets manually and these are then held in Spinlock clutches, released via two foot controls – to bail-out you stamp on both pedals. These controls can both be operated from either cockpit, so the crew, who is facing aft in the rear one, can either come along as a passenger to enjoy the filling-loosening ride or can take part in sailing.

The solid wingsail configuration is the improvement that will most aid the handle-ability of SailRocket 2 as unlike the old rig this self-feathers. On Mk1 the rig had to be lowered as the boat was taken back up the course whereas on the new boat it will not. This will enable the team to have a faster turn-around time between runs and will speed up the development process.

The wing is bigger with its working area increased from 16 to 18sqm (although its total area including the wingtip is 22sqm), but the biggest visual difference between the Mk1 and Mk2 rigs is that there is a substantial hook off to leeward – the wingsail forming a V-section when viewed fore and aft. As mentioned, at record breaking speeds the leeward float is designed to take off. On Mk1 this was achieved from lift developed by a flap on the trailing edge of the cross beam. On Mk2 there is short secondary wing off to leeward, a distinct improvement, as Larsen explains in the video accompanying this, because it forms an endplate to the main wing, is a better lever arm to get the leeward float airborne and no longer affects the flow over the lower part of the main wing. The wing feels like ithas a much higher aspect ratio.

Back up to weather and the foil set-up will be what allows Vestas SailRocket 2 to pass the speed record and beyond. On Mk1 the foil was located beneath ‘the hull’ but on the new boat it hangs out to weather and there is a neat arrangement where the inboard end of the foil plugs into a C-shaped strut attached to the fuselage, to ease launching.

However it is the section of the foil that is most significant. At present Vestas SailRocket 2 has two options – a ‘standard’ foil to operate in sub-cavitation speeds, estimated to be around 52-55 knots, and an entirely different wedged-shaped ventilating foil (above) that should allow her to go beyond. Why not just use the latter? Because it is very draggy at low speeds and at this stage the team can only theorise as to whether it will enable Vestas SailRocket 2 to reach the speeds where it will become effective. Ideally you would use the first foil to get up to 50 knots and then somehow swap in the ventilated foil to take you beyond this. But the fact that the designers have moved this foil from beneath the hull on Mk1 to outboard of the fuselage on Mk2, where there is no potential to achieve the added efficiency of an endplate effect on the ‘conventional foil’, indicates they believe their future to be with the ventilated option.

One idea would be to tow SailRocket up to 40 knots and then unleash her with the ventilated foil, but this isn’t allowed under the World Sailing Speed Record Council rules for speed records.

Aside from the choice of foil the biggest unknown facing the team is, erm, how to get SailRocket 2 started. As Larsen succinctly puts it: “We know how to run. We’ve just got to learn how to walk.” For with the beam at its high speed angle of 90 degrees to fuselage, the relative position of the centre of effort of the wing and the foil meant they were unable to get the SailRocket 2 model to go forwards. To solve this potential problem they have a number of weapons in their armoury, the most significant from a design and engineering point of viewing being their ability to articulate the cross beam so that the leeward float (and rig) moves forward by up to 3m. So again, as Larsen guides us through in the video, the plan at this stage is to start on a reach with the beam forwards (rather like getting going on a windsurfer), sheet on, let the boat pick up speed, ease the beam back and bear away...record.

The new SailRocket is designed to break records in 20-30 knots of wind, however the wind speed it requires to go beyond 55.65 knots is something of an unknown. According to designer Malcolm Barnsley in a worse case scenario she should achieve 60 knots in 26 knots of wind. In practice it could be a lot less. They’ll have to wait and see.

As Larsen puts it: “Assume a flat plate in the water – what would it take to skull drag that thing at 60 knots through the water? We built that boat. So if we get performance that is slightly above the most horrendous plate [which they certainly will] then we get a lot more efficiency and a lot more performance. That can mean you can go faster until the boat just rips itself apart because the structure is limited or it means that you come down the wind speed range and start breaking records in 22 knots. That, to me, is a nicer thing.”

Paul Larsen and designer Malcolm Barnsley comment:

Malcolm Barnsley, designer of the first SailRocket and part of the five man design team on Mk2, says he first came across the 40 Knot Sailboat book when he was experimenting with model yachts in Gosport, aged 16. At the time he couldn’t believe the numbers quotes by author Bernard Smith - achieving 14 knots with a 2ft long model - were true until he built his own. Since then he has become the aero-hydrofoil's staunchest advocate.

“There aren’t any other configurations that I’ve come across that have no limit. You could say that if you use symmetrical T-foils and you pull down equally and push up equally on either side [ie in a trimaran-style format, a la Blue Arrow] in theory that could go on forever. But if you look at the actual physics and maths what you find is that the total lift you are generating in the water is getting bigger in faster relationship than it is with ours. With ours the forces are in simple relation and the foil force is basically equal to the sail force.”

Barnsley describes the stability equilibrium between foil and wingsail: “If the foil is aligned well enough with the centre of pressure of the wing, then those two items as a pair are in a neutral relationship, they don’t know how to correct themselves, so you do rely on the rest of the boat to steady itself and that’s why there is a platform. But the difference is that steadying job doesn’t actually get any bigger as the wind speed and boat speed go up – it is just a little correction, it is a steadying force, it is not a grouching, levering type force and that is really what is unique about it. The big forces have this neutral relationship and the little forces don’t get any bigger and in all other arrangements they do.”

With the ‘hull’ now constantly out of the water its streamline shape is much more aircraft-like. However according to Barnsley the net gain from this isn’t massive – it is probably worth a knot or two. However the more slippery aero profile may prove vital to get them up to the speed where the ventilated foil starts to operate efficiently.

Barnsley believes developments they have made with the wing to improve its efficiency are more significant and hence why they have only needed to raise its working area by 2 sqm, while driving force has increased by “a substantial percentage”.

As mentioned, the wing feathers on the new boat and Barnsley cites how vitally important this is, not just for being able to get the boat back up the race track between runs, but for launching and general usability: “Before we had this crazy system where you didn’t know what to do with the wing and if you let the sheet go, you still had full power on. If that isn’t a stupid design feature...We somehow agreed on that at an early stage and it was a hell of a mistake and made the boat virtually unmanageable. It was a circus-act of the utmost amazement that managed to keep that boat operating in strong winds. I know because I stood there holding the boat in 25 knots of wind and you just know that a moment’s lapse of concentration and you’ve wiped the rig out - and that is just standing on the beach! So Paul learned to do a very difficult juggling act. By most people’s standards that boat was unsailable. But on this one it should be a lot easier to handle.”

Larsen agrees that the new wing should be a lot more manageable: “There are so many things when we built this boat that we build because of the absolute fear we lived in when we were handling Mk1 and all its bad habits. You couldn’t let the main off on that boat, you couldn’t turn the power off, whereas if this wing works, if anything goes wrong, we are not so far down a scenario that we can’t get out of it. We can just turn it off, like a normal boat – let the sail go, stop, round up and sit there in the water and have no fear of it backwinding and crashing and stuff. But we have to make sure that all those systems work. In theory the boat should nearly be able to go back to unpowered like a kite - it will float to the window where it wants to go.”

Part two of our SailRocket epic comes shortly