Hydrofoil

Introduction

I embarked on this project primarily to teach myself about hydrofoils and the problems associated with their design. A hydrofoil is a fast boat that uses underwater wings to lift the hull out of the water to reduce drag and improve efficiency. Many have been built around the world of varying complexity and for a variety of roles. Russia has built a lot of hydrofoils for high speed river transport, with many being exported to Mediterranean countries like Greece for use on coastal routes. Italy has also produced many designs, the first hydrofoils I saw were built in Italy and ran from Southampton to the Isle of Wight in the UK. The most complicated hydrofoils have of course been built in the USA, Boeings jetfoil and the US navy's PCH being the most notable types. Using fully submerged foils these craft need an electronic control system to maintain stability (like a modern jet fighter), they also feature retractable foils to allow them to enter shallow harbours and water jet propulsion.

The initial idea was to create a boat that would be stable, simple to build and simple to operate without the need for complex control systems and mechanical linkages. The stepped planing catamaran hull was chosen to give as much lift as possible from the hull with the minimum of resistance at high speed in order to help the foils lift the boat out of the water as quickly as possible. Using a catamaran hull helped to make the control system simple, by using two shafts with a wide separation steering was achieved by independent throttle control,  with outward turning propellers and high angle of incidence of the shafts this also produced a favourable rolling moment in the direction of the turn. This approach proved very effective with no requirement for rudders or any other control surfaces, turning circle was around 4m at maximum speed.

Design

I spent a long time contemplating the foil system, in particular the stability in steady running conditions and the transition from displacement to foil borne mode. The simple way to achieve stability in a hydrofoil is to use a surface piercing, V shaped foil as shown above, this works because when the craft rolls there is a longer length of immersed foil on the lower side producing a righting moment. This also gives stability in heave as less foil is immersed as the boat rises and vice-versa when it falls. The stability of the surface piercing foil is dependant on the angle of the V. The alternative to a surface piercing foil is the fully immersed foil with some kind of control system be it electronic or surface following floats with mechanical linkage to elevons to maintain stability, this was immediately discounted for this project due to the added complexity.

So, having established that a surfacing piercing foil was the way to go the next question was: should both foils be surface piercing? With a forward surface piercing foil to provide stability in roll and heave a fully immersed aft foil is sufficient to provide pitch stability but there could be an advantage with an aft surface piercing foil. With a submerged aft foil the pitch of the boat will change with speed because the lift generated is dependant only on speed and angle of attack the only way to negate this is to put all the load on the forward foil and use the aft foil purely as a stabilizer, . With an aft surface piercing foil with the same loading as the forward foil it is possible to maintain a constant trim across a broad range of speeds and reduce trim changes across the landing and take off regimes. I decided initially to try a fully submerged aft foil because it does not need to be tuned as accurately to work, although I have left flexibility in the design to fit a surface piercing foil aft later.

Pond trials

The first couple of trips to the pond threw up one major problem. On first opening up the throttles the boat rose quickly out of the water and promptly ploughed in as soon as the bow foil pierced the surface, this was caused by ventilation of the foil. When the water pressure on the upper surface of the foil falls below atmospheric pressure, if there is a point at which the flow can separate from the foil then it will allow air to shoot along the top of the foil and destroy any lift being produced. My first approach to trying to rectify this problem was reducing the angle of attack on the bow foil so that the boat would lift off more gently and shifting the centre of gravity aft to reduce the load on the bow foil (the aft foil can't ventilate because it's fully submerged). This improved the situation but did not eliminate the problem, I got a couple of good runs across the pond but when it did plough in again it was dragging an air bubble along with the foil preventing it from taking off again without stopping first, so it was back to the work bench to try some other solutions.

The first thing I looked at was fitting fences to the top of the foil to stop the air from progressing all the way along the foil, I spent some time dragging airfoils up and down the bath with different profile fences attached to test there effectiveness before settling on the ones in the photo above. I hoped that the fences would reduce the severity of the ventilation problem but I thought they were unlikely to help shed the air bubble if a plough in did occur so I glued a couple of turbulence generators on to the outboard ends of the bow foil in the hope that this would help. These measures proved effective on the next set of trails, the boat experienced a small amount of porpoising on take off and quickly settled down to run very smoothly and proved quite manoeuvrable. The additional anhedral  foils above the main foil were fitted later to provide additional stability on take off and gave enough extra lift to give a smooth transition from hull borne to foil borne regimes.