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.





