The crossbeams primary function is to provide a base for mounting the rig in the required place 7.5m to leeward of the main hull. It is located on vertical pin at the main hull to allow rotation in the horizontal plane so that the fore and aft position of the rig base can be altered by adjustment of the lower (horizontal) stay lengths.
Rake of the rig can also be adjusted in the same way with the upper stays. It also acts as a support for the rig weight to lean on (or when backwinded).
The flying height of the float is stabilized passively by the aerodynamic ‘ground effect’ which acts like weak spring connecting the beam to the water surface.
By keeping the main structural part of the beam symmetrical we are able to use it on either tack* by simply reversing the flap angle.
The crossbeam is 8.3m long and weighs 35kg.
The main foil
The hydrofoil's function is to balance the lateral force generated by the sail while causing as little drag as possible.
As the sail force can approach 1 tonne at full speed it needs to be very strong and stiff enough to avoid significant bending under full load.
It is normally inclined at about 30 degrees to the vertical (parallel to the sail in front view) but we have the facility to set it more upright if desired. The Mk I design currently under test was built by DesignCraft and has the following details:
|Construction||Infused carbon shell on foam core|
Cavitation is the main problem faced in hydrofoil design. This is the phenomenon in which bubbles of water vapour spontaneously form on the foil surface due to the pressure reduction. Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation, although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots.
The main hull
The main hull of Vestas SailRocket is 9m long by 700mm wide and adopts a unique two stage form which blends the requirements for minimum ‘hump’ drag at low speeds with the requirement to ride on two well spaced but small planing areas at high speed. The third function of the hull is to act as a strong beam to react the fore and aft staying loads needed to support the rig. Low wind resistance was also a factor in design.
The forward part houses the main hydrofoil while the cockpit is located in the aft section.
The hull is built from carbon/epoxy prepreg (supplied by SP Systems) on an 8mm Nomex core and weighs 55kg without fittings. It has a removable nose cone for access to forestay fittings and in case of impact damage.
The function of this float is to support the weight of the rig and crossbeam at low speeds and at rest (by buoyancy) and at medium and high speeds by planing lift. The trim can be adjusted to control planing attitude.
At top speed the float no longer contacts the water as the aerodynamic lift of the crossbeam takes over.
It is vital that the float generates little or no sideways ‘grip’ on the water as this could upset the roll stability and trip the boat. For this reason it is shaped asymmetrically to skid easily to leeward.
The float is of carbon/Nomex panel construction and weighs 6 kg.
This acts as a compression strut to push the rig up with and to support the rig weight. In normal sailing conditions it acts like a lower tension stay. This is also a filament wound carbon/epoxy tube by CompoTech. It is 3.0m long and weighs 2.0kg.
- Fixed skeg- Hard mounted to the boat, only the trailing edge flap is adjustable. The large fixed forward section makes Vestas SailRocket track straight and dampens down any instabilities. The addition of this skeg made the craft 'sailable' over 35 knots. Without it a human couldn't steer this particular boat at high speed! It has a profile which is optimised for high speed work.
- Adjustable trailing edge skeg flap - Locked in position until we are going fast, this flap is used to steer Vestas SailRocket at high speed. It gives much more accurate control via the hand steering in the cockpit than the large and brutal low speed rudder.
- Skeg flap release trigger - Releases the skeg flap to allow hand steering and is activated by twisting the handle of the tiller grip.
- Hand steering linkages - Connects the tiller in the cockpit to the skeg flap (it works... But can be improved).
- B and G rudder angle sensor - Super reliable... and sensitive. Essential information.
- Large low-speed rudder - Vestas SailRocket is very 'unbalanced' at low speed and needs a large rudder to make it steer both onto and off the course for each run. This rudder accounts for a large part of the 'wetted area' of the whole boat when it is down. It really should be raised over 30 knots. I was actually still steering with this when we hit 44 knots. As predicted, it was not nice!!! Raising it should give us another 5 knots of boat speed at the top-end. The amount of spray it makes when down would vindicate this.
- Rear planing surface - The rear 'shoe' of vestas sailrocket. When at high speed the boat rides on the last couple of inches of this. It has a wooden core and strong carbon laminate to deal with the punishing chop. We broke one before and it nearly took us out. There are some areas of this boat where saving weight is just not a priority!
- Foot steering control lines and large rudder pull-down line.