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Fitting and using Hydrofoils on an International
Moth The hydrofoils that I am currently using are manufactured by John Ilett at Fastacraft in Perth, Australia. They consist of a fully submerged t-foil centerboard and a fully submerged t-foil rudder, made from a mould using pre-preg carbon fibre. The centerboard is actually a very thin straight section blade (120mm x 14.5mm) and is angled forward from the case towards the bow to stop air traveling down the blade.
On the bottom of the centerboard, there is an 800mm NACA 63412 hydrofoil wing (120mm x 14mm) that is at zero degrees to the bottom of the hull. The rudder is also a symmetrical straight blade which sits on the back of a large moulded carbon fibre outrigger, to increase the distance between the two foils underwater and improve ride stability.
The two foils each have an adjustable flap at the trailing edge. The centerboard is automatically adjusted by a ride height sensor arm mounted at the bow on a moulded carbon fibre bracket. The ride height sensor arm is actually a tapered fiberglass batten that is inserted into a slightly bent alloy tube. This tube then attaches to a hinge on the bow bracket and allows the arm to swing up and down so that it just touches the waters surface.
The constant touching of the water is maintained by some 2mm spectra passing through a micro pulley on the bow and attached to shock chord that leads back into the boat for adjustment. An adjustable cable then attaches to the top end of the alloy section on the control arm, and leads back into the cockpit where it mounts onto a block just in front of the centerboard case. The end of this cable then attaches to a pivoting arm poking through the top of the centerboard. Also attached to this arm, is a thin fiberglass rod that travels the down to the bottom of the centerboard, where it attaches to the adjustable flap.
Therefore as the hull rests in the water when traveling at low speeds, the control arm is pushed back and up which in turn adjusts the trailing edge flap so that it induces maximum lift on the centerboard hydrofoil to make the boat fly. Once airborne, the control arm drops down to the water's surface by means of the shockcord pulling it in that direction and therefore adjusting the trailing edge flap and reducing the amount of lift created. If it did not do this, then the boat would simply want to keep on lifting until the foil looses pressure as breaks the water's surface. This is then followed by a spectacular crash landing of the boat back down to the water’s surface.
If the foil does break the water’s surface, the moment is usually followed by a rather loud underwater explosion like a bomb going off underneath the boat. The effect is only increased when traveling at high speeds due to the increase in pressure around the foil. From my experience, hydrofoil cavitation on the Moth is only caused when sitting too far back on the wing bar or when sailing into a big wave. However this is easily counteracted by doing one or all of the following. Firstly more tension can be applied to the shock cord which forces the end of control arm to dig deeper in the water and hence reduce the lift force earlier.Secondly, move body weight forward once the boat is airborne to reduce the angle of attack and hence reduce lift, on both hydrofoil sections. Or finally, the rudder also has an adjustable trailing edge flap that can be manually controlled by means of twisting the tiller extension in either direction to induce lift or reduce lift. A similar experience of too much lift is also experienced when sailing into a lull in the wind. as the pressure in the rig drops, so to does the downward force in the boat. This causes the boat to lift up even higher, as it is like taking 30kgs off the boat. At the same time, there is less pressure in the rig forcing it to leeward. This means that the boat will try and come back on top of you also. The effect is only increased the higher the boat is out of the water as there is a greater fulcrum to pivot on. So in order to combat this, it is important to know how the boat should feel when you are sailing into a lull. It also helps to be more proactive than reactive, otherwise you will not be able to move quick enough to prevent a capsize to windward. If you feel it coming, trim the rudder, sheet in and move body weight inwards and forward immediatley. Surprisingly it is also quite easy and fast to sail upwind in waves. Instead of punching through waves, the boat simply sails completely over the top of them making it a lot more comfortable ride for the skipper. To do this, it involves sitting a long way down the back of the boat in order to maintain high angle of attack on the foils and hence high lift, at moderate speeds. It is an unusual feeling at first, but with an extremely efficient sail and light weight boat, it powers up immediately allowing the skipper to heal the boat to windward whilst completely airborne to carve a path upwind just as high as a conventional Moth configuration would sail, but at a lot faster speeds.
Tacking involves slowing the boat down to a comfortable speed, by slowly pointing up into the wind. As pressure decreases in the rig, the boat will try to lift slightly, however this is controlled by moving body weight forward and making small adjustments in the rudder flaps to keep the bow down. The tacking itself is not that much different from a conventional Moth, only it is a little faster, as the boat doesn’t pivot on such a long waterline anymore. With practice, hydrofoiling Moth sailors could tack quite comfortably and no different to they did beforehand, and be foiling on the new tack seconds afterwards. Downwind is also a treat as it is now possible to tack down a square run at speed instead of trying to balance on the boat like an acrobat. Reaching is also the best part of Moth sailing, but it also adds that extra fun, speed and excitement, not to mention the tactical advantage in picking the favoured side of the run that has more pressure or sailing faster with more apparent wind. Gybing is by far the most challenging part of sailing a hydrofoiled Moth, as it involves a lot of controlled speed. If you go to fast, you might go too high when you gybe. If you go too slow, you’ll probably drop the new leeward wing in the water and tip over (and that’s not all that uncommon in slow conventional Moth gybes anyway). A perfect foil-to-foil gybe involves a manageable speed and leaning into the boat so that the leeward wing is close to the water’s surface, while at the same time, making rudder flap adjustments so that the bow does not lift up. The boat will almost turn itself slowly, but one very small constant rudder trim will be enough to get you on the new gybe with the boat now healed to windward and the helmsman on the new windward wing. The mainsheet should never need to be sheeted out much anyway, so the boom should come over fairly easily, but a quick flick of the main on the way through the gybe, will see a fully battened main flip over. From here, the boat should still have enough speed to be still airborne, however if the boat touches the water, it will be back up in no time with a fully powered mainsail. Using a GPS, it is possible to get up to speeds over 18 knots in about 15 knots of wind using this configuration, however I think it is possible to go even faster, the only problem is when going that fast in such a small boat, it is a little scary at time, especially when riding on a thin blade of carbon fibre, a meter or so above the water. For someone starting out on the foils, I highly recommend finding some flat water in about 10-12 knots. Once you get the feel for it, try some more wind, but don't tackle any waves until you know how to control the boat's ride height. Other than that, just get out there and give it a go! |
