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 MK1 Hydrofoil

This model was featured in my "Experimental Hydrofoil" article for Marine Modelling Monthly January 1992 pages 26 - 30

All images copyright of Graham Taylor.  May not be reproduced without authors permission

 



Front view showing the ladder foil detail.  The upper foil comes out of the water and is only used for stability and take-off


Just resting


The MK1 at speed.  Going like the clappers!



The foil assemblies are fully detachable, fixed by nylon bolts which shear off on impact without damage to the hull.  Yes, this feature has been tested in an accidental full-speed run up the bank! 





After five years of trials things began to shake lose so the model was re-fitted and repainted in the theme the yellow & blue theme of the Flying Dolphins of Greece.  Sadly the refit added some weight so the model never quite regained its original performance.  I have learned to keep and sharp eye on model weight ever since.

The model did achieve some fame though - she was appear along with my MK4 Ekranoplan on the Discovery TV show 'Model Mania' (circa 2003) which was repeated forever....and ever....
 

Model Design

Hydrofoil models are very rare.  The number of different type of hydrofoil kit models available over the years could be counted on the fingers of one hand, mostly small tame electric powered offerings.

The model press has published the odd design or two for homebuilding, while some enthusiasts have built their own models from scratch, usually as a scale model of a full size craft.

The problem is that the foil system tends to be a bit fiddly and delicate, and that puts people off.

This project was about doing something different.  I wanted to build a model hydrofoil that was bigger, stronger and faster than anything than had ever gone before.  The idea was inspired by some six foot lengths of aluminium half-round strip I found in a hardware store some day.  It looked like perfect foil material - now build a model around that!

To start with the model would be powered by a .40 cu glow plug engine that turned out around 3/4 horsepower.  This should be able to give it a top speed in excess of 30 mile per hour.  So she had to be strong.  The overall length of 42 inches was set around the engine & propulsion system and the distance between the front and back foils I thought to be sensible.  The foil system has to be rugged enough to survive a full-speed collision with the bank, without damaging the hull in the event that something was to go wrong with the control system....  err.... or driver error.  With the engine at one end of the long hull and the propeller at the other the inclination of the propeller shaft could be  minimised.  Some deep thought went into the design of the hull bottom, which is stepped to take into account the forward position of the centre of gravity that was dictated by the engine position near the bow. 

Unlike most other hydrofoil models, I based the foil system on the 'free-surface-effect' configuration of Dr Rostislav Evgenievich Alexeyev and the  Central Hydrofoil Design Bureau of Russia.  The foils appear very simple but much cunning geometry went into their design to assure that the boat stayed the right way up at all times, bearing in mind that by the very nature of lifting the hull out of the water makes the thing inherently top heavy. 

The foils themselves are made from 1 inch wide 'half-round' aluminium strip that was sold in hardware stores from edging worktops. It just happened that its section was a pretty close to the ideal shape for a high-speed hydrofoil.  A double/ladder foil is used on the front, to help ensure the bow rises out of the water first and gives a lot of reserve to help prevent 'sea crash' - a classical hydrofoil problem that can occur under certain circumstances in which the bow foil catches in the water and actually pull the bow under.  At anything past medium speed the top foil runs clear of the water.  It is easy to over-estimate the foil area requirements for such a model - for sure there is a lot less hydrofoil area on my model than in almost any other model hydrofoil.

The foil strut design was equal challenging.  The requirement for ruggedness ruled out any fancy arrangements.  Also any part of the strut that ran through the water was going to causing.  Worse still it could cause 'ventilation', creating a air path that would wrap a bubble around the foils and destroy the lift. So the model has the minimum struts necessary to link the foil blades to the boat.   Even better, the front foil dihedral is arranged so that the struts meet the foil blades at a position that is just above the just above the surface of the water when running at full speed, for minimum drag.  The foil blades themselves are attached by clamps to the struts so that he angle of incidence can be adjusted by inserting packing shims.  I borrowed this idea from the full size Russian craft, some of which use the same technique.

Finally we come to the other main dimensions of the hull; its width ('beam' to proper naval architects) .  This was determined by how far apart the foil struts could be placed and still provide enough support to the foil blades.   More details of the model can be read in my article in Marine Modelling Monthly magazine  "Experimental Hydrofoil. Marine Modelling Monthly, January 1992". 

he last feature of the model is the way that  the struts are attached to the hull by nylon bolts.  Thus if the foil system does hit something solid it will shear off the hull at the nylon bolts without ripping the side out of the boat.  Yes, this has been tested .   And it works.

 

Running the MK1

I needn't have worried about the small foil area, as the model comes up on foils in about three metres from launch at half speed.  At full speed the foils run just below the water surface in true 'free-surface-effect mode, with almost no wake generated from the  eh front foil.  Wake from the rear foils is a little more evident as the rear struts duo run a little wet but most of the wake of from the boat is caused by the propeller which runs in a semi-surface-piercing mode and so throws up a fair bit of spray. 

She tends to run as if on railway lines when at high speed, due to the effect of the struts so foil borne turns are wide - just as on some of the full size Russian craft.  Fortunately manoeuvrability when off the foils is excellent and responsive; one only has to ease the throttle back to drop her off the foils in order to make sharp turns.  But the low foilborne drag means one has to allow a good five metres of gliding after cutting the throttle before she comes off the of the foils. 

Video of the model in action can be seen here:

http://www.youtube.com/watch?v=6npuFmveIZg

Many people get foil design wrong and make the foils too big - on model craft and on the full size craft also!  Hydrofoil boats s are subject to "the Cube-Square rule",  meaning that as a boat gets bigger, assuming all things are equal, the hull volume grows by the cube of its length but the foil area grows only by the square of the length.  Weight will tend to increase in proportion to the hull volume, i.e. it goes up by the cube of the length.   This means that a large craft will need much more foil area than a small craft in order to run at the same speed.  Then there is the problem of cavitation, in which the amount of low pressure in the water (e.g. on the upper surface of the foils) is limited by the point in which dissolved gasses come out of solution ("partial pressure") - this tends to wreck the lift generated on the foil - and there comes a point at around 70 knots where it is almost impossible to avoid such cavitation; a 'cavitation barrier' if you like..  The amount of low pressure needed to lift the boat depends on its weight - so the foil designer must then take care not to overload the foils.  Lastly the is the risk that for large foil areas the flow around the foil cannot be maintained without turbulence setting in, at which point cavitation is likely to follow.

Russian Cyclone hydrofoil - perhaps equivalent to the MK1 model.

All this means that a model requires proportionately much less foil area than a full size craft to run at the same speed.  It also means that a model can be run at a higher speed than would ever be achievable than the scaled full size craft.  Note one can get into a big mess here; the MK1 model is 1m long and runs at circa 30 knots. The nearest equivalent full size craft is the Russian Cyclone at 50m long, speed 50 knots.  Using a linear speed equation (scale 1:50) the 'scale speed' of the MK1 is 1,400 knots!   On the other hand, using Froude number scaling (as used by naval architects) to reduce the Cyclone to the same size as the MK1 gives a model speed of 7 knots, which would not be fast enough to raise it from the water.  Or, Froude number scaling up of the MK1 to Cyclone size gives a scale speed of 212 knots, which is still by far on the wrong side of the cavitations barrier. This all goes to show that scaling of hydrofoils is fraught with problems and it is actually easier to build an impressive working large model than it is the full size craft.   Thus MK1 was designed to be a working model but not a scale representation of anything in particular.  

 

   

See more at about Russian Hydrofoils at  http://www.mahartpassnave.hu/webset32.cgi?MAHART@@EN@@126@@946413335