Learning to fly

The emergence of the Foiler Moth has been one of sailing’s most pyrotechnic developments. The Moth is a development class which by definition, means that it’s capable of true innovation, such as hydrofoiling at speeds up to 27.9 knots. It was only recently that Foiler Moths—the Bladerider, the Prowler, the Mistress, and the Hungry Tiger—became commercially
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The emergence of the Foiler Moth has been one of sailing’s most pyrotechnic developments. The Moth is a development class which by definition, means that it’s capable of true innovation, such as hydrofoiling at speeds up to 27.9 knots. It was only recently that Foiler Moths—the Bladerider, the Prowler, the Mistress, and the Hungry Tiger—became commercially available.

Foiler Moths have several unique characteristics, including an extremely narrow hull, large mesh-enclosed “hiking wings,” a rotating mast, and a sensor wand that hangs from the bow into the water. Pull the boat out of the water and things get even more sci-fi: The main foil and the rudder both have asymmetric wing foils mounted on their bottoms, making these appendages look like a pair of inverted Ts. The main foil measures 33.5 inches long and is set to roughly zero degrees to the hull; the rudder foil is 25.6 inches long and is set to a corresponding angle. On the trailing edge of main foil and some rudder foils are flaps, like those on an airplane wing. These are designed to yield maximum lift with minimum drag, enabling the boat to hydrofoil in less than 7 knots of true wind. This is achieved through input from both the sensor wand and the tiller extension.

The sensor wand is one of the keys to the Moth’s explosive performance. The wand is attached to the bow with a hinge and is held forward by shockcord or spring return device. A control rod attaches to this hinge and runs from the bow to the top of the main foil, where it connects to a bell- crank lever. The lever is attached to an internal pushrod that runs down the vertical length of the blade and controls the main foil flap.

moth

As the boat begins to sail, the wand is pulled aft by water drag, which adjusts the flap, and the boat lifts and accelerates. As it reaches cruising speed, only the wand’s tip is submerged. Reduced drag and resistance allow the shockcord to pull the wand forward, moving the trailing flap to a more neutral position. This system dramatically reduces the boat’s wetted surface area and allows it to sail on its foils, which stay submerged.

The innovations don’t stop with the bow wand. On some Foiler Moths the tiller extension attaches to a worm screw at the extension/tiller joint; a control rod joins this worm screw and runs the length of the tiller, connecting to a vertically running pushrod within the rudder. This pushrod controls the movement of the rudder foil’s flap. Twist the tiller extension and the worm screw translates rotational movement into linear movement, thus controlling the rudder flap. On the Bladerider the rudder foil is fixed. Rotating the tiller extension changes the rudder foil’s angle of attack. On all Foiler Moths, the foil systems can be seen as macro/micro tuning, with the macro (and constant) adjustment coming from the wand/centerboard and the micro tuning coming from the tiller/rudder system. “You have to sail a Foiler Moth in three dimensions. It’s similar to flying a plane in that you have to control your pitch, yaw, and height,” says 2005 Moth World Champion Rohan Veal. “Once you’re foiling, there’s little need to make much adjustment on the rudder trim. When the seas get up, though, you need to use it a lot more to keep the boat from launching into orbit.” Giddyup, sailor.

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