Over the years I have made several attempts at building model experimental hydrofoil boats. The final inspiration came from seeing the Greek, Russian built Kometa craft of the Flying Dolphin fleet (right).
Hydrofoils
'fly' through the water according to exactly the same principles as aircraft fly
though the air. The bits in the water are wings (not skis - this is quite
different), but as water is 800 times more dense than air the size of the wings
can be smaller and on an aircraft of equivalent weight. If one looks back
through the history of the hydrofoil technology one finds several different
approaches to solving various issues relating to stability, air entrapment
(ventilation) and cavitation which complicate hydrofoil boat design.
Stability is all about three things:
- Keeping the boat the right way up, since it becomes very top-heavy when supported on its foils .
- Keeping the boat straight and level without bucking around or the foils actually broaching (leaving the surface - which usually results in a 'sea-crash' as the foils fall back in the water at negative incidence to pull the whole boat into the water),
- What happens when the boat encounters a wave - does it stay right and true, or does it make some uncontrollable manoeuvre?
Air-entrapment/ventilation is about stopping air from getting dragged or sucked into the foil system as it will spoil the lift and stability completely; it tends to form a bubble around the foils which means that they are trying to fly in air rather than water.
Cavitation is similar to ventilation but it is cause by low pressure on the top surface of the foils being sufficiently to allow dissolved gasses to come out of solution and form a bubble around the foil. It can happen around the foil struts as well, and may create a pathway for ventilation. Cavitation is a real issue since it limits the 'sucking-upwards' capability of the foil to less than one atmosphere. It is an issue as real as the 'sound barrier' is to aircraft. High speed operation needs a foil design capable of coping with this. Cavitation can set in well before 70 MPH, which is why most full size hydrofoil boats don't go much over 50 knots.
As a result, four different foil systems have come to the fore:
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Some other sites of interest:
If anybody is interested in learning more about hydrofoil design or history I recommend the following website:
To the Models!
My models use a combination of surface piercing and free-surface effect foil systems.
The first quest was to find some sort easily-obtained of foil material. I found this in the form of 1-inch wide 'half-round' aluminium strip which was sold by the 2 m length by a local hardware shop for edging worktops and other things. By chance the extruded section corresponded exactly with high speed controlled-cavitation foil section.
Next was to design a foil system and model to use it. The closest full-size reference was the system used on the Russian boats, so the MK1 was design with more than a nod in the direction of Dr Alexeyev's styling.
It was not until later did I become aware of Dr Rostislav Evgenievich Alexeyev's formative work in Wing-in-ground-effect/Ekranoplan technology - my other area of passion; and so found myself following in the great man's footsteps in some small way!
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The MK1 boat (white) and Rapier (orange). Both have tandem foil arrangements of ladder/ surface piercing front foils and surface piercing rear foils. The MK1 boat tends towar54ds free-surface-effect, while Rapier is more 'V' format. If anything the MK1 format was more effective. |
