T-16 hydrofoil racer 'Miss Calypso' competing in the 'A' Final of the 2017 edition of the QinetiQ Haslar Model Powerboat Race.Slide ShowVideoVideo
Development of the Model Hydrofoil Racer ‘Miss Calypso’
By Jerry Turner CEng FRINA
See first photo below:
A radio-controlled model hydrofoil has been developed to compete in the 2017 edition of an annual competition held at QinetiQ Haslar in the UK. The competition is open to all types of craft and is unique in that the venue is the QinetiQ ‘Ocean Basin’: a 122m x 61m x 5.5m indoor ship manoeuvring tank. It is the largest facility of its kind in Europe. The racing takes place around a complex course marked by buoys and with the computer-controlled wavemakers running. Waves are approx. 1-2m in length and vary between 150-300mm in height.
The rules permit electric power only and limit the total propulsion power available by specifying the type of battery and setting a maximum current rating of 35A on the ESC (electronic speed controller). The challenge is to produce a design that is fast around the course and can handle rough conditions while being limited in power.
A hydrofoil was chosen because it was believed that a hydrofoil had never been entered before at the QinetiQ competition and because it was felt it would make a far more challenging project than yet another fast electric monohull. Both assumptions proved to be true.
See second photo below:
LOA (Moulded) 700mm
Beam (Moulded) 150mm
Wingspan (Beam Overall) 410mm
Depth of Hull Amidships 80mm
Draught (Canoe) 37mm
Flying Height (Estimated) 40-50mm
All up weight 1.405kg (1.603kg incl carbon lid)
LCG fwd AP (Transom) 280mm
Battery Overlander NiMH Sport 6-cell 3800mAh (380g)
Motor Overlander T3536/05 v3 1500KV (97g)
ESC Overlander XP2 30A
Propeller Graupner 2-Bl Carbon Hydro ‘S’ 42mm Dia x1.6 Pitch Ratio
Max Speed 7.42 m/s recorded by GPS
‘Miss Calypso’ is a hybrid hydrofoil design having two separate surface-piercing forward foils and a fully-submerged rear foil. The forward foils are each supported by a single strut and are configured as inverted T-sections, though somewhat angled out to increase the width of the support points. This creates a tripod-like footprint and is an important feature that contributes to the remarkable roll stability, permitting very high yaw rates. The weight split is approximately 75% front / 25% rear.
The current Mk3 foil system has struts and foils made of Aluminium Alloy AISI-6061-T6 which were joined using special aluminium brazing rods. The rear foil is brazed on to the leading edge of the rudder, which is a heavily modified stock CNC aluminium rudder of wedge cross-section and is transom mounted.
Foils sections are NACA 66-012 (struts); NACA 63-412 (fwd foils); NACA 4412 (aft foil). Foils were rough milled then filed, sanded and polished by hand and checked using templates. Forward foils are constant 25mm chord with no sweepback. The wings are in two segments with spans of 100mm (lower) and 75mm (upper) and dihedral angles of 30deg (lower) and 50deg (upper). The rear foil is 80mm span and tapers from 12mm at the root to 8mm at the tips.
The front foil angles are set by rotating the adjuster brackets (marked off in degrees) and tightening the two retaining bolts with an Allen key. The rear foil angle is set by loosening the rudder clamping bolts and adjusting the rudder trim angle to achieve a specific horizontal distance of the rudder leading edge (measured at the aft foil) from the trailing edge of the fixed P-bracket ahead of it. To make in-field adjustments easier, a laminated table was produced that gives the distance to set in mm for a range of foil incidence angles.
Early developments were a (Mk1) foil which was a V-shaped 25mm chord design with single struts and two dihedral sections each side of the centerline. The foil flew immediately demonstrating good lift and early take-off but was not very stable in roll or pitch. It appeared to plunge suddenly on accelerating and could only be flown continuously on reduced throttle in order not to nosedive. The rear foil at this time was attached to the rudder tip and measured 120mm in span.
A second V-type foil (Mk2) was produced with a chord of 12.5mm which had enough lift to fly the hull but had worse roll stability than the (Mk1) as it could not pick up area quickly enough when rolled. It was modified to have a lower flying height by shortening the struts (Mk2B) but this foil continued to demonstrate an inability to fly confidently when manoeuvring or attempting higher speeds. Not as much time went into making the foil sections accurate or producing a perfectly smooth, polished surface and ventilation was frequently observed.
The current foil arrangement (Mk3) is the Mk1 system completely recycled. Mk3 shows very good stability in pitch and roll in calm conditions. Pitch stiffness and damping appears a little soft at higher speeds . In rougher water at high speed a slight pitch instability sometimes manifests as a 0.5 to 1Hz pitch/heave oscillation. This occurred when the boat was carrying extra weight in the form of +200g carbon self-righting lid and +167g Garmin GPS used to measure speed. Without the additional weight the pitch oscillations were hardly evident.
The hull is a single hard chine double-stepped mono-hull, having deep-vee sections of approx. 25deg dead-rise. The aim was to produce a hull that had reduced drag for take-off and soft impact with waves. The freeboard aft is significantly reduced to save weight. A single large access hatch is fitted in the deck. The hatch fits flush into a rebate in the deck and is sealed using waterproof adhesive tape. There is plenty of reserve buoyancy to cope with large waves.
The hull & deck construction is from laser-cut birch ply 0.9mm thick. All the internal parts were designed to interlock to make assembly easier. This made a building jig quite unnecessary. The total weight of the wooden assembly of the hull, deck and internals was 210g, before painting. The overall weight target for the boat was 1.3-1.5kg, though this was set before development of the foils was completed. An allowance for weight growth/estimate inaccuracy was factored in to the calculations.
The final weight of 1.4kg resulted from additional painting (+50g), the addition of navigation lights due to a late rule change (+40g). The addition of a magnetically attached self-righting lid built out of lightweight EPS foam and encapsulated in carbon/epoxy added +200g to the overall weight (total now 1.6kg) but was felt to be necessary owing to the perceived need to provide self-righting capability (racing in big waves!), even if the extra weight proved to be a slight handicap. It probably could have been made lighter in retrospect but the polished, clear-coated carbon proved to be quite a head-turner and the self-righting capability proved to be invaluable in the racing. Without the self-righting lid the boat has a positive range of stability of approx. 120degrees.
The rules stipulate that only electric power may be used to propel the boat and that a specific battery type must be used (batteries are supplied on the day by the organisers):
• Battery: Overlander Sport 6-cell NimH 7.2V 3800mAh
• Electronic Speed Controllers (ESC) are limited to a maximum of 35A total
• Multiple motors were allowed this year but the total ESC rating must not exceed 35A
‘Miss Calypso’ is fitted with a single 3-phase brushless motor (see Spec) and controlled via a 30A ESC connected to the radio receiver and battery. The motor is coupled to a 4mm diameter polished 316 stainless steel propeller shaft using a compact aluminium flex coupling.
The shaft is inclined at 15 degrees to the horizontal and runs through a rubber stern seal fitted in the shaft log and is supported by a bronze bearing in the shaft tube and a PTFE bearing in the custom Aluminium P-bracket. The alignment was very precise and the shaft runs smoothly at high speed, having only a slight transient vibration at low to mid revs.
Metal propellers are not allowed for the race and so a range of Graupner Carbon/Nylon Hydro propellers were selected for testing and down-selection. The highest speed recorded to date (by GPS) was 7.42m/s, using a 42mm diameter 2-blade ‘S’ series propeller of 1.6 pitch ratio.
TESTING & DEVELOPMENT
It is not considered feasible to design and build something novel like a hydrofoil without getting it on to the water for testing and developing. The process of trials always throws up problems and shortcomings that must be addressed if the design is to work reliably. Theory is not enough.
In fact, the three phases of design, build and test are all as important as each other. This was very apparent when looking at other team entries on the race day. Some of the boats appeared to be completely untested; consequently their teams had to spend most of the day diagnosing and fixing faults (leaks and electrical problems were very common) or simply persuading the boat to work properly.
‘Miss Calypso’ was on the water three months before the race (which took place on 24th February 2017). This gave merely adequate time to test and develop three separate foil systems. It would be true to say that development is incomplete in one sense, as there are always more ideas that could be tried out. Now the race is over ‘Miss Calypso’ may provide a useful ‘test-bed’ for further experimentation.
A great deal was learnt from the whole exercise, from developing hydrofoil design codes to Aluminium brazing; from lightweight wooden construction to project scheduling; from modern radio control electronics to learning the piloting skills to test fly a novel design of craft. Most important were the lessons of what worked and what didn’t.
On the day, the racing proved to be very challenging – the boat was extremely reliable, but as a hydrofoil with a significant wingspan it quickly became clear that speed & manoeuvrability needed to be allied with general robustness. Catching or hitting buoys, or being hit by other boats (which all happened more than once) did not do the foils any favours. Bent struts and misaligned wings do not fly very well or hold a straight course.
Performance in waves was most interesting – head seas did not seem to be much of a problem and the boat flew very well into them, until the waves got big enough to break over the boat. By contrast some of the planing boats were thrown around by the head seas. In following waves the hydrofoil seemed to slow down and could not always find the speed to lift well, sometimes rolling from side to side or ploughing into the wave troughs. The self-righting foam/carbon float added a great deal of buoyancy and this helped offset this tendency. At other times she would pick herself up and fly down a wave front as though it were no problem at all.
Despite the hazards of buoys, collisions and following seas, ‘Miss Calypso’ managed to win three heats out of five and finished in the top five out of 13 teams, making it through to the A Final. In the final race it was the boats that could charge along at top speed, make accurate turns and if necessary ricochet off buoys with impunity that did well. ‘Miss Calypso’ had to take care not to hit the buoys and gates so was frequently operating well below her top speed potential.
On the second lap of the final race ‘Miss Calypso’ struck a buoy and bent the starboard strut. This made it hard to hold a straight course after that and she did well to finish 4th overall. Clearly some work needs to go into making a more robust foil system if a hydrofoil is to challenge the narrow mono-hulls in future.
A novel hydrofoil model has been designed, built and tested. It has been developed to a point where it flew fast and manoeuvred well, so that it was believed it had a reasonable chance of success against proven and far simpler technology in the form of lightweight planing craft.
In practice, although it flew and handled well, in the Final race it could not compete on equal terms with the best planing boats due to the problems posed by flying in close proximity to collision hazards such as buoys.
On a personal note it was very satisfying to make it to the Final with a hydrofoil, (as this has not been done before) and particularly so to be awarded the prize for the Judges’ Favourite.
To develop ‘Miss Calypso’ has required a considerable input of time and effort. Early concept design work began in January of 2016; construction started in June 2016; testing and foil development started in October 2016 and ran through to February 2017. It is estimated at least 750 man-hours went into creating the model.
Further development is always a consideration and the design permits new foils to be bolted on quite easily, although future entries in the QinetiQ race would be required to be a completely new build.
For now Miss Calypso will be taking a rest until the next bright idea comes along. Perhaps it will be another hydrofoil…
Jerry Turner CEng FRINA
First runs with the Mk3 foils: https://youtu.be/evwzEriTOzg
Speed runs with the Mk3 foils: https://youtu.be/CuwFNMl0wC8
Build sequence slide show: https://youtu.be/9vXqdTBDW0g
Pictures on the International Hydrofoil Society site: http://ihs.phanfare.com/