A new Sailrocket????

This thing would look incredible if it wasn’t just a prettier version of the Vestas Sailrocket. The derivative-looking concept drawing is actually Alain Thebault’s idea of a 100-knot boat, and if there’s anyone obsessive enough to spend the years it could take to refine Paul Larsen’s and the SR team’s design to a 100-knot craft, it’s Thebault. We wish he’d stick to developing his RTW mega-foiler, but sometimes following is a lot easier than leading.



About the SR2 that set a 59 knot speed record. Not your average boat....

VSR2 has to be dynamically stable in a number of conditions including a total main foil failure at 60 knots. She must remain stable when encountering either cavitation or ventilation of either foil.

VSR2 was designed to be able to handle sailing loads over 60 knots including a 1G turn with a realistic safety margin.

VSR2 has to be able to operate over 50 knots in winds from 20-30 knots and in much rougher water than the first boat.

Our boats are based around a concept where there are no overturning forces. The opposing forces of wind and water are in alignment. It’s like someone trying to push you over by pushing at the soles of your shoes. They could push you sideways, but not over. This allows us to handle a lot more power without tipping over like 99% of other sailing craft.

We are able to generate a huge amount of power by using the speed of the boat itself to make more wind. We start off stationary with a nice stiff wind at 90 degrees to our course and as we accelerate, we start adding the wind generated by the boat speed to that natural ‘true’ wind. To the boat, the wind feels like it is not only moving forward of 90 degrees but is also increasing in speed. When the boat is travelling at 50 knots, we will have turned a 20 knot breeze into a 50 knot gale. This is what the wing-sail feels. The faster it goes, the more power it gets... and the faster it goes. The only way for it to escape is to go faster until all the drag equals all the power.

Sail-powered boats rely on fins or ‘foils’ to counteract the side-force of the wind, and to stop the boat slipping sideways. When travelling through the water at speeds around 60 knots, virtually all foils in the real world experience a phenomenon called ‘cavitation’. Cavitation will happen in a fluid when you reduce the pressure so much that it effectively boils at room temperature. The liquid turns to vapour. Cavitation happens everywhere where liquids are subject to very low pressure. It can happen on boat propellers, hydraulic pumps or on the fins of high speed fish like Tuna. It is very damaging and when the bubbles collapse, because they can do so with tiny pin pricks of huge force that will erode away the hardest materials.

The foils we have all been using so far have relied on the water passing over both of their sides just like air on the wing of an aircraft. On one side there is high pressure creating lift and on the other side there is low pressure which is actually sucking the wing up (or foil sideways). Underwater, it is typically when this suction becomes too great that we experience cavitation. The foil loses grip on the water. The pressure side of the foil now has to do all the work so the foil skids sideways until it is at double the angle (to do double the work). The trouble is that this now also doubles the drag. Imagine going for a speed record in your car and right when you are approaching maximum speed, you open all the doors wide open and pop the bonnet! You now need double the power to go any faster. This can also lead to a big loss of control.

If you design foils specifically for confronting cavitation, you step away from traditional tear drop profiles, as one whole side of the foil is now travelling in a vapour ‘bubble’. The shapes are more like sharp wedges where you are only using one surface at a greater angle. There are many high speed power boats that use propellers with this profile... but it has never been done effectively on a sailing boat.

With VSR2 we have made a package that has a huge amount of power on a very efficient, low drag platform. It is designed to be always seeking stability like a well-built model airplane. At high speed the pilot should be able to take his hands off the controls. This was possible on the first boat. VSR2 is designed to have enough power and efficiency to be able to drag a truly horrible plough-like cavitating foil through the water at over 60 knots. Anything better than ‘truly horrible’ will result in either higher speeds or greater efficiency in lighter winds.

VSR1 was there to show us that the concept had the power and efficiency we thought it should. VSR2 is now built to exploit these characteristics to allow us to confront cavitation head on. Just like the sound barrier, once you are through... you are through. The equation for doing 100 knots or greater will have been written and validated for the next generation. If we can make it to the end of this process, we will happily leave that to others. We will have proven a point. That is the goal of this project now. If we prove that point then we believe the outright record will simply come with the territory.

Righto, let’s try and explain a little bit more about this wing so those of you who are interested know what I am referring to.

The wing was designed by the Sailrocket team but was largely the responsibility of Chris Hornzee Jones and Wag Feng at AEROTROPE. Chris designed our first wing. It is a much more powerful, efficient and complex beast that the first wing. We also expect it to be much more mildly mannered. Whilst the overall area is 22 sq/meters, the actual driving area is only 18sq/m i.e. only 2 sq/m larger than our first wing. It does however have a number of features which make it much more efficient and more stable. It is thinner than our first wing and has a ‘reflexed’ trailing edge which basically means the trailing edge has a slight return ‘ramp’ right at the back which stops the wing from going into a negative lift mode when it is sheeted out.

The main spar is a tapered, filament wound spar supplied by COMPOTECH. The ribs are carbon on 38kg Styrofoam (basically standard under floor insulation... buy it cheap by the pallet). The leading edges are 80gm glass on 5mm foam core or 200gm SP GURIT carbon at ±45 degrees on a 5mm foam core depending on what section they are. The all up weight is around 65-70kg. The wing is inclined at 30 degrees to match the inclination of the foil it opposes on the other side of the boat.

As we only need to sail in one direction, the wing is asymmetrical. It is set up for a starboard tack to suit Walvis Bay.

Some of the basic criteria for the wing are that it obviously needs to be powerful and efficient enough to drag our boat down the course at speeds over 60 knots. It also needs to be able to fit inside a 40’ shipping container. It needs to be practical to use and handle in a number of situations relating to on the course and simply getting back to the top of the course.

We endeavoured to make a wing that will allow us to tow the whole boat back to the top of the course without having to stop and lower it. This is in an effort to be able to get more runs in quickly and remove a process which is often risky in its own right.

Without going into a full thesis here, I will simply explain the wing elements and what their functions are.

WING FILLET... This is the 'elbow' in the wing where the wing effectively is joined to the end of the beam via a large, high-tensile ball joint. It's also the structural junction for all the other bits. There is a 600mm stub that sticks out the top with a steel ball on it. The main mast sleeves over this and has a cup in it. This is where the component on which the main wing rotates. The wing fillet also has a female sleeve which the horizontal WING EXTENSION fits in. The wing fillet is fixed off to the beam at a pre-set angle of 10 degrees to the wind. It is not sheeted whilst sailing but rather used as a sheeting point to control the angle of the LOWER WING which is immediately above it.

LOWER WING... This is the 7 meters of area immediately above the wing fillet. It is limited in its ability to rotate by the strut which holds the wing up in compression. It can rotate around ±45 degrees which is plenty. It can be manually linked to the MIDDLE WING for raising/lowering/transporting/storage purposes. It can also be sheeted independently from the cockpit via a small mainsheet which is linked to the WING FILLET. The LOWER WING isn't connected to the main carbon COMPOTECH spar. The spar passes through all the ribs so that they are free to float around it. This way it can spin independent of the upper wing elements and can be removed to fit in the container.

As the Wing Fillet is fixed in relation to the boat, the COSWORTH wing angle sensor uses a laser to measure the difference in angle between the bottom rib of the lower wing and the top rib of the wing fillet.

MIDDLE WING... This is the largest component of the wing. All these ribs are bonded onto the COMPOTECH spar so when the mast rotates, this whole section of wing rotates. This section of wing can rotate through 360 degrees as it is not interfered by any shrouds or supporting struts. When we get towed back up the course it can therefore fully backwind/depower. It is sheeted via a bridle which also pulls in the LOWER WING at the same rate i.e. they come in together when the mainsheet is pulled. For towing, the whole wing can be locked together using sliding pins between the sections.

TRIM FLAP... This is the small flap on the TE (Trailing Edge) of the MIDDLE WING. When removed it reduces the chord of the wing so that it fits in the 40' container. It is also adjustable to alter the 'feathering'/de-powering properties of this key section of wing. It is a slightly lighter construction.

UPPER WING/WINGTIP... This small tip section of the wing cannot rotate through 360 degrees due to its proximity to the shrouds which attach to the top of the COMPOTECH spar. For this reason it is separate. It is sheeted via a bungee that will stretch if the WINGTIP does interfere with the shrouds... but have enough strength to sheet in the small section otherwise.

WING EXTENSION... This is an interesting part of the wing that adds greatly to its overall efficiency. Previously the outboard end of the boat was flown by generating lift from the beam. This was pretty inefficient as the pressure areas from the beam and the wing were cancelling each other out. Chris and Wang at AEROTROPE who did all the structural design work and performance analysis for the wing which shaped the overall package came up with this solution. I didn't welcome the complexity it added but the performance benefits were undeniable so we went with it. Basically it does the following...

• creates a more effective lifting lever for the outboard end of the boat. This is what makes the outboard end of the boat fly.
• compliments the pressure distribution along the wing which makes the wing feel much higher aspect ratio than it is whilst maintaining a low centre of effort.
• Works in 'ground effect' due to its proximity to the water. This reduces the amount of spillage of pressure from one side of the wing to the other (induced drag)

WING LIFT FLAP... This is an active component that adds or reduces the lift of the WING EXTENSION. It will be used to actively control the flying height of the outboard/leeward end of the boat. It will most likely become fully effective at around 50 knots and be controlled by as simple a system as possible. It can't be fixed as the faster we go, the higher the beam will fly and we will lose a lot of performance. It only needs to 'just' fly the float. I don't want to be controlling it during a run. We have a number of viable options for controlling it from a surface sensing wand as seen in hydrofoiling moths to a simple means of mass balancing it so that it only generates just enough lift to fly the pod in ground effect as per VSR1. We will start with this option.

As mentioned, the wing extension combined with full wing lift flap deployment can only lift the weight of the wing and beam at around 50 knots. If we see the leeward pod flying before this... as we have at around 38 knots in initial trials... it means that the whole wing is over inclined and needs to be stood more upright.