Fastest Sailing Boat:
How Does Sailrocket Work?
A truly inspiring story…
Occasionally a news story really grabs you
The Vestas Sailrocket Team‘s attempt to break the sailing speed record is a little known, tiny team, who for 11 years has invented and engineered their way through problem after problem, on a budget that would fund just the coffee for an F1 pit crew. Determined to come back after the Sailrocket 1 crash in 2009, they used the experience to develop Vestas Sailrocket 2 (VSR2). And after 2 years of hard graft, the radical-looking VSR2 was launched to much fanfare last year. And then devastatingly failed to live up to their predictions.
What do you do when after 11 years trying, your “this is it this time” version makes all your best thinking appear flawed? Well, the Sail Rocket team kept cool and carried on believing, spending this last year re-evaluating and preparing for another attempt. And this final push brought them the new insights that changed everything.
In October our friend Dan Emuss at Independent Composites told me he’d just finished building one of the key pieces of this puzzle – a new smaller carbon fibre main foil – and that they were ready to try again. Which is when I started paying attention.
And lucky for us, this time the guys made a fantastic effort to share the record attempt experience almost live on the web. This has transported us to the apparent desolation of a windswept Namibian sandbar, right there alongside the team. And it’s allowed us to witness them achieve something remarkable.
On Sat 24th November 2012 Vestas Sailrocket 2 (VSR2) sailed a mile long stretch of Walvis Bay in Namibia, sustaining an average speed of 65.45kt over 500m, with a peak speed of 68.01kt.
Bear in mind 68kts converts to nearly 80Mph or 126 Kph. The next time you’re on the motorway – imagine hammering along over a lumpy sea in something which might break apart at any moment. I think Paul communicating with Jesus during warp speed pretty much sums it up…
Okay, so it looks impressive. And they’re obviously quite chuffed about it. But is this really such a big deal? Surely to go faster you just add a bit more sail and go out in bigger winds, don’t you?
Well no, sailing doesn’t really work like that. You can’t just stick a bigger engine in the thing. Having to generate all your power from the wind and convert it to forward motion through choppy water (under control) is a highly complex balance of forces that is really pushing our understanding of physics to the limit. And sometimes beyond:
Sailrocket 1 midway through her almost terminal crash in 2010 – proving that using a forward cross-beam as a way to lift the leeward float out of the water was a bad idea!
Firstly this difficulty is clearly demonstrated over the last 40 years, by how small a margin each record has been broken. From 26.3kts set by the British boat Crossbow in 1972, up to the last record of 55.65kts set by a kitesurfer in 2010 – the largest single gain was a 4kt increase… set by Crossbow in 1973!
Secondly in speed sailing circles “boats” are not supposed to be any good! After 1986 the speed records have all been set by windsurfers, and then from 2008 onwards, the kitesurfers have been totally dominant, apart from when l’Hydroptère (which is a huge trimaran flying along on hydrofoils) managed 51.36kts in 2009.
It is completely unprecedented for “a boat” (under control) to break the record by such an astonishing margin. 10kts beyond what many people thought was near theoretical maximum of speed sailing really is like aircraft breaking the sound barrier for the first time.
So… How did they do it?
The key which unlocked this boat is the foil which works in a new way. But it’s worth explaining a few concepts first as there’s a lot of theory to this boat.
Firstly a yacht’s rig (mast and sails) works in the same way as an aeroplane wing to generate driving force by bending the wind around the curved shape. Any aerofoil or hydrofoil works in the same way – this “classic” video explains it very well:
This is all fairly straightforward for an aircraft which is driving itself through the air to stay aloft. But a sailing boat is a bit more complicated… because it’s getting drive from the wind, it somehow has to hold the sail up against the wind and convert its force into a forward motion. This is a trick which confuses many non-sailors.
Conventional yachts do this by having a heavy keel underneath. Firstly this is another form of wing, which “flies” through the water, providing sideways “grip” to make the boat go forwards rather than just being blown sideways off downwind. Secondly, it’s a big weight that, once the boat starts to lean over, provides the righting stability to push back against the lift produced by the wind.
In speed terms, this is not great. Firstly you’re having to drag a heavy boat through the water. Secondly leaning over is not efficient – most of the force is wasted in the wrong direction, and the more you lean over the less well the sails and foils work. So once a conventional yacht has reached top speed (perhaps 10 -12 knots at the right wind angle), any further wind increases are wasted in further heeling over, rather than turning into more speed.
After World War 2 a step change occurred at the hands of Uffa Fox, a highly imaginative and intelligent boat designer with an adrenaline addiction. By all accounts he was somewhat of a “bon viveur” – and incidentally became very good sailing mates with Prince Philip, who you may not know is also a great sailor.
Anyway, Uffa understood that weight was bad for speed and began developing lightweight “planing” dinghies. By doing away with the keel, building a lightweight hull and using the crew weight and skill to keep the boat upright – you could skid over the water like a motorboat, rather than wading through it like an old barge.
Uffa Fox (note the permanent “blown back hair” look) – and one of his designs: The International Canoe
However, whilst great fun, this approach is too unstable for very speed sailing. As many a dinghy sailor will testify, when a strong gust comes along, so the “tipping over force” can quickly increase beyond the righting moment of your weight over the side…
Many a dinghy Sailor has been known to shout “Uffa Fox SAKE!”
Therefore to go really fast you need a more stable system to convert any extra thrust from gusts directly into extra speed. This is where we get to the fundamental basis of Sailrocket. Its layout is designed to line up the force of the sail with the force of the foil resisting it in the water. This means that all the lift from the sail goes into speed, rather than trying to tip the boat over. Extra lift = extra speed. It’s a simple idea first suggested in the 1950s that’s taken till now to perfect:
Of course, the assumption is that your foil in the water is capable of taking all this sideways force and converting it into a forward motion… which is where it got really tricky for the Sailrocket team – mainly due to another physical phenomenon, known as “cavitation”.
2. Cavitation – Working With it
So we already saw that on the “upper side” of a foil is the low-pressure area. If you follow mountain climbing at all – you’ll know that as the pressure reduces, so does the temperature at which water boils. This is why (British) climbers whinge on about rubbish tea at Everest Basecamp, as their water is boiling at 68 deg C.
The point is, if you lower the pressure of seawater enough, it will boil at sea temperature. Yikes. This is a nightmare for the foil designers. You have to have a small foil to keep the drag low enough to go fast – but this increases the pressure differences even more – and you quickly get to a speed where the water is “boiling” into bubbles on the back side of the foil. This is bad because it causes a lot of extra turbulence and DRAG (which is the enemy of speed). This problem is why all their previous attempts resulted in hitting a ceiling at around 50 knots where cavitation became a major issue.
So, Sailrocket’s second genius innovation is to design a whole new kind of “cavitating” foil. Usually, a foil looks like the picture below… a round leading edge to encourage the water to flow around the back and a nice sharp trailing edge to reduce the turbulence where the water joins up again:
The new kind of foil is almost the opposite – with a very sharp leading edge and a wedge shape ending in a sharp cut off back. This wedge foil is designed to be a kind of low drag plough, getting all its lift from the high pressure side. As far as I can gather, it seems in this latest version, they are allowing air to be introduced into water behind the flat rear edge to create a nice clean run off each side of the foil followed by a stable air bubble. Also, I believe that over 60 knots, they’re expecting the increased sideload to create full “clean” cavitation, where rather than creating low pressure (and lots of chaotic bubbles), the water just cleanly breaks away from the sharp leading edge. This kind of idea has been used for years on racing powerboat propellers, but it’s never before been successfully used on a sailing boat.
3. Want a fast boat? – Get Out of the Water!
The third innovation of this boat is how it very stably lifts itself almost out of the water as it goes beyond 50 knots. Firstly, the top part of the L shaped foil lifts the back float clear of the water. Secondly, the horizontal wing sail extension lifts the downwind (wing side) float about 1/2 meter off the water. This wing extension is very efficient as it’s working in ground effect (riding on a cushion of air trapped between it and the sea).
Plus, putting it outside the wing creates an efficient wing-tip “fence” (like the winglets on the end of jumbo wings). This reduces the drag from vortex turbulence which would be created by the high pressure air rushing around the bottom of the sail to the low pressure side.
By the way, this is also why they’ve put the cross-beam at the back now. Unlike on VSR1 (which turned into a bad aeroplane), having the cross beam at the back means if it generates too much lift, it will raise the back of the boat, increasing the downward angle, meaning the lift reduces again. In other words, it’s now inherently stable… which is really good for going faster.
And Other Stuff Besides
So there you have it. The three main components that made this boat fly. There are actually quite a few other details that make all these bits work together. Many of these innovations make the boat more controllable and easier to sail and make sure it holds together as it pounds along the runway.
If you’re intrigued by what they’ve done, you can read more on the Vestas Sailrocket website and Paul Larsen’s excellent blog of their experience.
So now I’d just like to say a big congratulations to Paul and the whole Sail Rocket team. And thank you for including us all in your adventure. Hats off to you guys. I hope my brief explanation of your technology is mostly right!
Also worth a mention is the engineers at Aeroptrope, who designed the wingsail and the “supercavitating” foil. They’ve really got behind this project and have been a massive ongoing support, by all accounts. Plus of course Dan Emuss at Independent Composites who built the carbon fibre foil, using his innovative heated vacuum infusion table. This is another amazing new piece of kit invented by a tiny dedicated team!
Finally it’d be churlish not point out that Vestas Sailrocket 2 was designed and built in Cowes on the Isle of Wight. The very home town of Uffa Fox. I’d like to think he’s proud.
PHOTO CREDITS: All photos of Sailrocket have been borrowed from the www.Sailrocket.com website (thank you) some of which are courtesy of Helena Darvelid who seems as nifty with a camera as with the helm. Loads more amazing pics on her site.