50 knot bound

Alexander Sahlin of the Swedish Speed Sailing Challenge describes their craft and their progress

During the last years some very serious projects have made large efforts to develop boats to beat the world speed record over 500m and to pass the elusive 50 knot 'barrier'. Aside from Paul Larsen's SailRocket, there is competition from the Swedish Speed-Sailing Challenge with their Paravane Speedsailer.

The three key members of the team are Alexander Sahlin, Jens Österlund and supremo Tornado builder Göran Marström.

While Marström is a well known figure in sailing circles, Sahlin is a a fluid-dynamicist at the Department of Mechanics, Royal Institute of Technology, Stockholm 1985 to 1993. He carried out the aerodynamic design of the MTL wind-tunnel built there in 1990. Personally he has won 14 Swedish championships in skate-sailing and has designed a human powered hydrofoil, called Trampofoil®, with flapping-wing propulsion actuated by the pilot jumping on the foot-plate.

Jens Österlund is an aeronautical engineer with a PhD in fluid-dynamics and currently at the Aeronautics department of The Swedish Defence Research Agency, FOI. A sailor since the age of eight he is a world-class International Canoe sailor.

Alexander Sahlin describes their Paravane Speedsailer and the challenges they face trying to break 50 knots.


General Configuration

High speed can be achieved in different ways. One obvious way is to build a very large conventional multihull. This method was successfully employed by Tim Colman's Crossbows from 1972 to 1986. Built at the size neccesary for beating the present sailing speed record of 46.52 knots such a boat has to be so large that it would be far too expensive.

Another way is that you develop something you can handle in very strong wind and sail it on flat water. Windsurfers have done this with very impressive results. However, such conditions are quite rare, and competing with the present windsurfing speed record holder Finian Maynard would require many years training and development.

The third way is to build a sailing craft that can have more aerodynamic force in the sail than the sailing weight of the boat. Such boats sail at twice the windspeed or even better at high speed. The present record holder Yellow Pages Endevour has achieved this with a boat that is about as wide as the sail-height.

Even larger sail-forces can be obtained if the heeling moment is balanced by hydrodynamic forces on hydrofoils. On sailing hydrofoils like the Windrider Rave or Hobie Trifoiler, the upward- and downward forces on the windward and leeward hydrofoils create a moment that counteracts the heeling moment on the sail.

However, the vertical acting parts of the hydrofoils has a lot of drag, so at high speed, when the aerodynamic force in the sail can be several times more than the weight of the boat, it will be more efficient to put the sail a long distance to leeward of the lateral plane so that the forces in the sail and lateral plane can act on the same line, if the sail and lateral plane are canted to windward.

There are at least three projects active at present where the heeling moment of the sail has been cancelled out in this way. These are Monofoil, SailRocket and The Paravane Speedsailer.


Paravane Stabilisation

The idea of using a paravane to windward has been suggested long ago (see AYRS 114 - Ultimate Sailing). Lots of inventors have tried model paravanes designed for sailing, but as far as we know, no one sailed a full scale paravane before us. We think our paravane is an improvement over the present state of the art, both in performance and reliability.

Unfortunately we can not discuss the details of our paravane at present. The unique feature of the Paravane Speedsailer is that the paravane is self-stabilising and only needs to be connected to the sail by two thin lines, that carries the pulling force between the sail and the paravane. Hence, we get something that works as a very wide boat without any heavy beams between the hull and the paravane. The paravane is also very light, so it can follow the water’s wavy surface at high speed with reasonable hydrodynamic control-forces.


Hull of hydrofoil?

One of the most common pieces of advice we get is that we should use a hydrofoil to lift the hull out of the water. A well designed hydrofoil may certainly have a better lift/drag ration than our planing hull and we would have used it if the hull-drag was a larger portion of the total drag. On our boat we estimate that the sail force will be about four times the hydrodynamic lift of the hull and the hull drag just 1/20 of the sail-force or 1/5 of the total drag. However, we have tried to minimise hull-drag as far as possible for our planing hull. Ocke Mannerfelt Design have helped us with that.

However lifting hydrofoils, a closed and streamlined cockpit and streamlining the paravane lines may be considered for future improvement of the boat if neccesary.


Solid wing or softsail?

The stresses in a traditional sail set on a mast would be enormous on a boat sailing at 50 knots, both because we have to sail close-hauled at an apparent wind angle of 15 to 20 degrees and because the sail-force amounts to around 6000N. To find something similar you have to look at a big ice-boat, and the rig on a such needs to be very strong, and therefore also quite heavy if you don’t use lots of stays and shrouds with lots of drag. That kind of rig would also need a lot of tuning of the mast and the sail to get the the right shape and dynamics for our case.

The behaviour of a high aspect ratio stiff wingsail on the other hand, can be predicted quite easily from wing-design calculations and wind-tunnel measurements. After researching this we found that just 12.5sqm was enough for our boat, which meant that a one piece, all carbon fibre laminate wing could be made small enough for practical handling and not too heavy (37kg).

Although the soft-sail alternative was estimated to be some 10 to 15 kg lighter, we found that the stiff wing boat will require 2 knots less wind to do 50 knots, so we chose that option for our boat. The only drawback is that our stiff wing is asymmetric and as a result our boat became a pure one-tack design.


Design detail

The large aerodynamic force in the sail that enables the forward thrust at the centre of effort of the sail can be very large and also creates a diving moment. Therefore it is necessary that the boat is supported on two well separated planing surfaces. One is below the centre of gravity and the other small one is on a bowsprit 7m in front of the centre of gravity. The bowsprit can be adjusted so the hull gets the optimum angle of attack for high speed planing.

As the paravane is self-stabilising in angle of attack, there is no use for a rudder on the main hull to steer the boat at speed. Instead there is a fin at the stern just for directional stability. You steer the boat by controlling the aerodynamic force in the sail - more force in the sail, and hence in the paravane, makes the paravane go faster, so the boat turns to leeward, and the opposite if you turn to windward.

In order to get stability before the paravane is launched we have a small outrigger on the port side. As soon as the paravane is launched and pulls with more than 200N the boat heels to windward so the outrigger lifts 1m above the water.


Practicality

A speedsailer will experience non-design conditions, such as sailing or being towed back to the starting point, laying at the starting point waiting for favourable wind, and starting from rest.

These problems can be solved in different ways. The Yellow Pages Endevour was operated near land, and used a tractor on land to tow the boat back to the start. If you have a very wide boat, like SailRocket, it is practical to fold down the rig when towing back the boat.

In our case we found it most practical to have the wingsail pivoted on an axis well in front of it’s centre of effort, so the sail is completely passive once the sheet is released. We found it most convenient and safe to tow the boat backwards back to the starting point, so the wind always comes from the starboard side and the pilot can stay in the cockpit.

Another quite common difficulty for speed-sailers designed for sailing very fast, at three or four times wind speed, is to accelerate through the speed range below the design-speed. If for example we had made our sail a few sqm smaller we should have needed more wind to pass the 15 to 20 knot range than for sailing at 50 knots.

The Paravane Speedsailer experiences some off-design conditions at low speed before the outrigger lifts from the water and before the sail is sheeted in for a small apparent wind-angle. At such low speed a rudder is used to correct weather helm. At some 20 knots the rudder is removed from the water.


Construction

We have built all components of the boat at the facilities of Marström AB (the world’s leading manufacturer of the Tornado) using their production techniques with pre-preg carbon fibre cured at 120 degrees under vacuum-bag in an autoclave. For the wingsail as well as the hull we have used honeycomb cores. Thanks to this we were able to build the wingsail in carbon-fibre panels without any sailcloth the weight coming in at only 37kg. The all-up weight of the boat ended up at 101kg.


Test sailing

Late 2003 we tested the Paravane Speedsailer on the water. Our objective with these preliminary tests was not to determine the top-speed but rather find out if the boat was practical and able to start from rest and accelerate to highspeed planing in the way it was designed to. After some some initial tests and adjustments we had the boat planing at some 20 knots+ on one occasion.

During spring this year we installed the present removeable rudder, Silva instruments with big displays in the cockpit and a bailer at the planing step.

On 3 and 9 July 3 we made several runs at 30 to 35 knots in some 14 knots of wind. The waves were quite high which certainly decreased the speed and caused a lot of spray, resulting in poor visibility, when the boat hit the waves. On such water where speed records use to be set waves are much smaller than we experienced, so spray will not disturb the pilot at speed. When the wind increases from 10 to 14 knots the boat speed goes from below 10 knots to some 30+ knots. During many of the runs I had the feeling that the boat was accelerating as long as the wind was on, but the wind decreased before top speed was reached.

Finally, the boat works fine now, and we are now looking for a title sponsor to support practicing at places with conditions for record speed and to make record attempts.

For more information and offers of sponsorship contact Alexander Sahlin at alexander@trampofoil.com or see their website.