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Bakewell-White Yacht Design

World Speed Record contender Sailrocket 2's speed transitions

by Paul Larsen on 3 May 2011
SailRocket 2 sea trials Day 1 Vestas Sailrocket - copyright http://www.sailrocket.com
In 2008, Paul Larson and his Vestas Sailrocket was on the way to an outright speed sailing record when his craft crashed in a spectacular fashion, damaging it badly. It has been a long road back, but Vestas Sailrocket 2 has arrived in Namibia for their next challenge. The aim of the team is to beat the outright world sailing speed record.

The team currently holds the B-class (150-235 square feet of sail)speed sailing record with a speed over a 500m distance of 47.36 knots (87.71 km/h), but in 2008, before crashing, Sailrocket had reached a reported unofficial speed of 52.22 knots. (See the video the end of this article and relive the exultation with their speed achievement, watch the crash and then sympathise with Larson's devastation at his dream's destruction.)

The revised craft was launched March 8 2011 at an empty weight of only 275 kg/605 lb. Fabricated with materials from SP-High Modulus, the main structure is an autoclave-cured sandwich construction, comprising carbon fiber/epoxy prepreg skins over an aramid honeycomb core.

Prepregs included Gurit’s Ampreg 22, SE84LV and SE70 and some dry reinforcements. Its wing-like sail is built around a CompoTech carbon tube that acts as a spar. The wingskins are a polyester heat shrink film. According to Larsen, the entire boat, including rigging, has the equivalent aerodynamic drag of a 74 cm/30-inch diameter sphere, and its revised design enables the pilot to maneuver the craft in much rougher water than the first version could handle.

Latest from Paul Larsen on progress.

Yesterday evening we had our third session of tow testing of the bare platform. It was a beautiful evening as we towed the wing-less platform up and down speed spot with the whole team taking turns in the boat and in the RIB in order to observe all aspects first hand.

We saw on our second session the day before that the addition of the first trial 'step' on the front float made a big difference to it's ability to release from the water and step up into planing mode.

The new second 'step' on the rear underside of the hull quickly ventilated from behind at speeds as low as three-four knots. This enabled the up forces on the front of the float to win the battle and start lifting the hull upwards as speed increased. This in turn allows the first original step to 'get access' to the air and also ventilate. Voila... the hull keeps lifting and we are planing.

We wanted to see this work first hand before we went and added them to all the three floats. The back float was hence still suffering and struggling to break free from the water. We tried a few other trials with foils up and down and the beam swung fore and aft before deciding to head back in.

Yesterday, we went back out with steps on all three hulls. The leeward hull is so lightly loaded without the wing on that the step really isn't necessary. I was hoping for a bigger change on the rear float. Whilst the step was an improvement, it seems that we still have a low speed where the aft float gets caught in the wave pattern generated from the front float. There is every chance that this might just be a phenomenon caused by tow testing and that the addition of the side force of the wing could break the whole 'loop' so we shouldn't get too concerned until we conduct some sailing trials.



It was interesting yesterday to watch the foil behaviour. We were typically towing at speeds around 15-17 knots. The fact is that the foil was virtually doing nothing and you could hear it rattling around in its locking mechanism rather than being forced against either the upward or downward limiters. When the foil is fully submerged when the boat is stationary, there is as much lifting area as there is downward pulling area. The effect is that when it has an angle of attack created by any side force i.e. a sail or a tow-line, the overall force is neither up or down.

You have to remember that the foils on our boats are actually pulling the boat down into the water. On most hydrofoilers the foils are there to lift the boat up and out of the water. On the Sailrocket designs, the angled/inclined wing does all the lifting and it is the foil that keeps us down. So... when we start off and the full foil is submerged, there is no 'net' down force from the foil but the wing is pulling up. As we accelerate and the wing powers up, it gradually begins to lift the boat up with it. This means that the foil rises and the top section comes out of the water only leaving the angled lower part in the water and this is the bit that does all the 'down' work. When the boat is hauling along at full steam, the back of the boat will naturally ride at the height where the up and down forces balance. If it gets 'bumped' too high then the balance will try and pull it down and visa versa.

This means we can only do so much towing the boat around without the addition of the actual sailing loads.

It sure was a beautiful evening. It was a perfect opportunity to let every other team member go for a ride. I really enjoyed seein Ben in the cockpit. It always surprised me to see boat builders spend so much time building boats only to see them go out the shed and the next set of plans come in. Surely the joy has to be in actually seeing what you have spent so long building in its element. This way you can truly appreciate what it is you are creating. It has to make the whole experience richer.

Helena was all concentration. She knows that she will have the option to pilot this machine for a record attempt at some stage... and she knows that that day may well be approaching. It's not talk anymore. What she does and how hard she pushes it is soley up to her. There will be no restrictions and if she wants to go for an outright attempt... then that will be her call. Yesterday she also got to drive it for the first time..



The only one missing was Hiskia... but he'll probably get his turn today.

The warm East wind was blowing lightly this morning which would indicate a nice but relatively windless day ahead. We will do one more session of tow testing where we will actually measure some of these loads... and then we will roll straight into the wing trials. We might be able to do both of these today if this wind settles a little so as not to corrupt the numbers.

I must admit that I am still a little concerned about the 'hump' drag. In respect to our situation, this 'hump' is the amount of drag that builds up before the boat makes the transition into 'planing' mode. You often see it in fast powerboats where they need to use a lot of power to get started but once they are up and planing on the surface of the water, they don't need so much power. This is because they have overcome this transition from low speed displacement mode to high speed planing mode. Some shapes do it better than others. We are discovering how big our 'hump' is right now and will try and use a few tricks to bend it to our will.





Vestas Sailrocket 2 website

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