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Spitfire's Features:

Retractable Hydrofoils

Starboard Front Foil

Three hydrofoils, or underwater 'wings' lift the boat out of the water once the take-off speed has been exceeded (about 12 knots).

The use of hydrofoils is the key to Spitfire's performance. Once out of the water, the drag, or resistance to movement, is drastically reduced - allowing the boat to accelerate to much higher speeds than would be possible in the same conditions without foils.

Spitfire has three hydrofoils: the two main foils (one of which is pictured above) are 'surface piercing' and the third is a fully submerged 'T' foil, attached to the rudder's tip. The foils are placed in such a way that the two main foils support most of the weight of the boat, and the rear foil is there mostly for balance and trim adjustment.

Rudder Foil
Foil Retracted

Using surface piercing foils has many advantages. Firstly, as the boat's speed increases, less 'wing' area is required to support the boat's weight. So, the foils 'lift' themselves, and the boat further out of the water as boat speed increases. This leaves less drag-producing surface area in the water, which is perfect for good performance. Secondly, the foils are inherently stable. If the boat rides too high, there will be less foil area in the water and the boat's weight will force them back down. Similarly, if the boat is too low, the extra submerged area will lift it out again.

The aerofoil cross-sections used on Spitfire were developed at BDG Marine. The latest in aerodynamic modelling software was used to design and analyse the critically-important aerofoil shapes. The aerofoils were optimised for minimimum drag, with good resistance to ventilation and cavitation. No fences were used on the hydrofoils, and to date there has been no ventilation or cavitation observed.

Spitfire's foils are able to be retracted, to enable shallow water operation, and beaching.

'Soft Wing' Sails

BDG Marine, in conjunction with Windrush have developed a double sided sail for Spitfire. These unique sails have been provisionally patented. The design aim was to provide maximum forward force with the sail area available, and minimise the heeling moment to prevent the boat tipping over.

Two sails & masts were used, one on each hull, in order to give Spitfire more sail area, while keeping the sail's centre of pressure as low as possible. This keeps the sideways force from the sail low, so the heeling moment is minimised and the boat has a higher maximum speed before tipping.

The system comprises a rotating mast, and flexible battens. The sails rotate, pushing camber (or curvature) into the sails. This gives the sails a more aerodynamic shape, which increases their efficiency (that is, more propulsive force is created from the available sail area).

Sail Cross-Section

Twin Free Standing Masts

Computer rendering of sails

Cylindrical bodies, such as cables and ropes can produce up to ten times the drag of streamlined bodies like aerofoils. Hence, the masts were designed with no cable supports. Although the masts themselves have a circular cross-section, the sails form around both sides to create an aerofoil shape, with a round leading edge.

Two sails were used in order to keep the centre of sail area as close to the hull deck as possible, in order to minimise the heeling moment.

Raised Cockpit & Storage

In order to meet Australian Yachting Federation (AYF) regulations, the cockpit floor had to be at least 240mm above the waterline (2% LOA). So, the cockpit was positioned well above the waterline, in a central 'pod'.

This pod also has covered storage space, aft of the cockpit. Also, being high above the water (the floor can be up to 2.5m above water level when foiling), with no forward obstructions such as masts, the pilot has a great view of the water in front, and stays relatively dry. In fact, during sea-trials, this has become the most popular area for the crew to sit.

Front and Top View Drawings

Wide Beam for High Speed

The centrelines of Spitfire's hulls are located 8 metres apart, giving the boat excellent heeling stability both on and off foils.

This is important because, in high wind, high speed conditions the sails create a much greater (but unwanted) side force than forward propelling force. This is an unavoidable problem with sail boats of all types, as it can push the boat over on its side. The faster the boat gets, the worse this problem. So, the maximum speed of any sailing boat is limited by its ability to 'right' itself, or stay upright against this large side force.

With 500kg ballast + crew, BDG's mathematical modelling predicted a maximum speed greater than 45 knots (on foils) before Spitfire heels over.

Aerodynamic Design

Spitfire has not only been designed for low hydrodynamic drag below the waterline, attention has been paid to minimising the aerodynamic drag above the waterline. Hence, the beams are aerofoil shaped, the number of cables and ropes in the airstream has been minimised (eg. the masts are unstayed) and the hulls have been kept sleek and streamlined wherever possible. There is an added benefit to giving the beams an aerofoil shape; the front beam actually produces a some lift, which lowers the load on the foils, and hence drag, and therefore provides a little extra speed.

Computer Rendering


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