A hydrofoil sailing dinghy




The boat sailing well but not foilborne

Background

About 15 years ago I was thinking about lifting hydrofoils for sailing boats and wondering what might be a quick easy way to build something to get some practical experience in this area. At the time I was thinking of a project which might take a few months of spare time, but as so often happens with such projects it is still not finalised today.

First concept

Having been a member of the Amateur Yacht Research Society for some years I was aware that the majority of hydrofoil sailing boats have three hydrofoils supporting the boat as a tripod, a logical approach. In most cases a widely spaced pair of hydrofoils is mounted forward and a single hydrofoil aft, but the reverse arrangement has also been tried. There have also been configurations with four hydrofoils either positioned as the wheels of a car, or in a diamond pattern, ie a foil at each end and two lateral foils.

Three or four widely spaced hydrofoils implies a fairly expansive structure from which to mount those hydrofoils. Typically this structure is a multi hull sailing boat, either a catamaran or a trimaran. Since I was looking for a quick lightweight boat building project I was reluctant to head off in that direction and I wondered whether something simpler and more portable could be achieved by considering just two hydrofoils, one at each end of a single hull. Compared to other sailing hydrofoils it would be the equivalent of a bicycle as opposed to a tricycle. If you had never seen a bicycle you might well think it to be a totally impractical idea but in fact bicycles work rather well, better for most purposes than the more logical tricycles. That fact gave me some hope and lead me to embark on a somewhat crazy project.

So the initial concept was a single narrow hull with hydrofoils at each end, a sail mounted above and one or more crew members slung from a racing dinghy style trapeze wire with feet resting on the hull. Steering would by a tiller acting on one of the hydrofoils. Since I was initially thinking of a 'sailing bicycle' it was natural to think of steering the forward rather than the rear hydrofoil, but I later decided to go with aft steering for mechanical simplicity. For the hull I first imagined nothing more than a length of 50mm diameter aluminium tube which would make the boat a 'sinker' in sailboard parlance. (a sinker is a sailboard having less buoyancy than the total weight of board and sailor, it can only operate by dynamically generated forces)Later I decided on a narrow box section plywood hull which would at least have buoyancy to support the weight of the sailor plus some reserve. So we have a very portable and quite easy to build hydrofoil sailing boat but would it work? Well, if you have watched an expert sail boarder water-start a 'sinker' sailboard there would seem to be a possibility that this hydrofoil boat could be sailed in the same way. You start with the wet-suited sailor lying in the water, feet in foot straps on the hull, the trapeze wire hooked to harness and the sail above him to windward. The wind catches the sail tensioning the trapeze wire to lift the sailor clear of the water and the craft moves forward, accelerating until the hydrofoils can lift the whole weight. Once on the foils the boat is steered and balanced like a bicycle - two wheels are enough for bicycle stability so two hydrofoils should be fine. Sounds easy?

First prototype

The sail I used with my first prototype was a standard sailboard sail 6.5m2 area but with the mast foot in a slot in the hull rather than being on a universal joint as for a sailboard. A forestay was used to support the flimsy sailboard mast against the tension of the trapeze wire and I also experimented with diamond stays to further stiffen the mast. Because of the mast curvature and the constraint from the forestay it was necessary to allow the mast to move a few mm fore and aft at deck level as the sail was sheeted in and out. To allow this the mast was fitted through a block which could slide in plastic slides mounted in the top of the slot in the hull. The forward hydrofoil was a surface piercing Vee shaped foil and the aft hydrofoil, which also served as a rudder, was an inverted Tee shape. Both hydrofoils were made from glass reinforced plastic using simple plywood moulds. The angle of attack of the hydrofoils could be adjusted on shore but not while sailing. The hull was made from 4mm plywood and was about 11 feet long and about 270mm square cross section with a punt shaped bow.

Trials with the first prototype

This first prototype was not a success, but perhaps I just did not try hard enough for long enough. I sailed the boat four times in all and never sailed it with enough wind that the sail could support my weight on the trapeze wire, hence I never really gave it a chance. At that time I had done a little sail-boarding but a water start was beyond my ability so I always attempted to start this craft by up-hauling. I would stand on the hull with the rig lying in the water to leeward. I would then ease out the tackle on the trapeze wire allowing me to lean backwards until the rig came up from the water. A moment later I would find myself lying floating in the water with the craft more or less upright but because the trapeze tackle was let out it would be heeled to leeward by perhaps twenty degrees. If I then sheeted in the craft would start sailing but because of the leeward heel and the drag of my body in the water it would carry huge weather helm which was impossible to steer against and after sailing a few yards it would come head to wind and fall on top of me. Probably I needed to tighten in the trapeze tackle as the craft came upright, or at least before starting to sheet in, but I never managed to adjust the trapeze tackle quickly enough while still keeping hold of both tiller extension and mainsheet. With a lot of practice and some improvement in the details of the design perhaps it could be done. But after a particularly frustrating swimming session in a muddy creek sometime in late autumn I gave up and dumped the boat at the end of my garden wrapped up in a polythene sheet.

Starting again

A few years ago I renewed my interest in sailing hydrofoils and took a look at my old project which had been abandoned for twelve years. Damp had found its way into the polythene cover and the plywood hull was soggy with some of the joints coming unglued. I spent a few weekends attempting to refurbish it using epoxy resin and filler to strengthen and seal all the joints but when I immersed it in the local canal and found that it still leaked like a colander I realised that I was wasting my time with a refurbishment. I then decided to build a new version to a higher standard. Remembering my difficulties with up-haul starts I decided that this time I would fit two small floats each side to give just enough stability for it to float upright at rest. By doing this I was moving away from the original concept of a really portable device you could carry down the beach tucked under an arm, instead it was becoming a small lightweight sailing dinghy. I suppose that I was also making it into a trimaran, but because the floats are tiny it is really closer in character to a 'winged' monohull dinghy such as the international moth, albeit with a little buoyancy in the extremity of the wings.

My intention was still to use a trapeze wire to provide lateral righting moment and if I could manage to trapeze with feet on the outriggers rather than the main hull I could gain considerable extra righting moment and hence sail power. Indeed, trapezing properly in this manner the available righting moment should be something like double that possible with a sailboard and with the hoped for low drag lift from the hydrofoils there seemed to be a possibility of really high speed if the ergonomic and control problems could ever be properly sorted out.

The boat carried on roof rack

Hull structure

The general arrangement of the hull is shown by the picture of the boat on a car roof rack. The hull and outriggers were built from 4mm plywood mainly joined using fillets of epoxy mixed with microballoon filler. Unless you are a real expert with foam or honeycomb cored composite construction I think you would find it difficult to improve on plywood and epoxy for a small lightweight boat building project. Certainly it is well nigh impossible to achieve adequate panel stiffness and puncture resistance with a comparable weight single skin grp panel. When I started building this boat I thought I was cutting it fine using just 4mm ply but in fact the resulting hull is more than robust enough. Because of the small dimensions there are no large unsupported areas of ply and I could have used thinner ply for many parts. The parts where thicker ply are justified are areas which come into contact with the crew's boots rather than with just water. The whole of the front half of this boat is lightly loaded and could perhaps be made of ply as thin as 1.5mm which would save a few kilos. The floats might also be built a bit lighter but they do become immersed to something like 800mm depth when the boat is being righted from capsize and are thus subject to a hydrostatic pressure of about 8000Pa (about 1.2lbs/sq inch). When you work that out as a force on a hull panel it is more than you might have expected and this may govern the minimum thickness of ply in those areas which can be deeply submerged.

Rig

The rig has the general appearance of a sailboard rig but I resisted the temptation to use a standard sailboard rig, even although that would have been a cheap option since I already had a sailboard and a couple of rigs for it. Sailboard rigs are highly developed to be just strong enough for their intended application. I felt that a boat which was intended to be sailed with a trapeze needed a stronger rig, both to take the localised loading from the trapeze wire and to cope with the heavier aerodynamic loading possible due to the crews weight being applied at greater leverage than for a sailboard.

I also felt that to utilise the greater righting moment due to the trapeze I should have less twist than a modern sailboard rig. In recent years sailboard rigs have adopted a large amount of built in sail twist. This has allowed sailboards to use larger sails than in the past since overpowering can be avoided by feathering off the upper part of the sail while maintaining drive from the lower part. If you watch these modern rigs in action you see that the sail is only filled along its whole height when off the wind or when getting onto the plane. Once the board is planing and the apparent wind strengthens and moves ahead the whole upper part of the sail is feathered off or even becomes loose and floppy which greatly lowers the centre of effort so that the board remains manageable with the limited righting moment available. Since I envisaged that my boat would have more righting moment than a sailboard I felt, rightly or wrongly, that I should have less sail twist than most sailboard sails.

The mast is a custom made carbon fibre spar which is larger diameter than a sailboard mast and I think has about four times the stiffness of a sailboard mast. The mast was made with an in-built curve so that despite the stiffness it can still be inserted into the luff pocket of the curved luff sail. The boom is a lightweight half wishbone since it is not required to hold on to it and this half wishbone is joined to the mast with a specially made hinge fitting which allows the boom to pivot in the plane of the sail but not to rotate around the mast. This means that the boom is in the same plane as that in which the mast is pre-curved and with a stiff mast I believe that this is one measure which reduces sail twist since the top of the mast does not bend off to leeward. The other measure to reduce sail twist is simply to cut of the sail with a tighter leech than a modern sailboard sail.

There are some other advantages in fixing the orientation of the mast relative to the sail. One is that it is possible to make the mast with slightly greater wall thickness on the compression side than on the tension side. This is desirable since a thin walled carbon fibre structure is inherently much weaker in compression than tension. I also made the middle section of the mast oval to increase for and aft stiffness. Perhaps an even better section would be an egg shaped oval to reduce the nose radius a bit, Tom Speer and AYRS member who is a professional aerodynamics expert reckons that could be an advantage.

The sail is fully battened and there are camber inducers on four of the seven carbon fiber battens. The foot of the mast fits into a carbon fiber socket which is fastened into the hull structure. The mast rotates in machined bearings at top and bottom of this socket. Each bearing consists of a thin stainless steel sleeve bonded onto the mast and running in a liner of ultra high molecular weight polyethylene (UHMWP) which is moulded into the socket. The lower of the two bearings also includes a UHMWP thrust washer to carry down thrust. UHMWP seemed to be an appropriate material since it is almost universally used for artificial hip joints which are similarly subject to wet salty conditions and oscillating movement under load. The foot of the sail is cut to be at just a few mm above deck level and yet the socket which carries the mast extends above deck level to give extra separation between the mast support bearings. This is made possible by a carbon fibre bracket which is bonded to the mast just above the top of the mast socket and extends down to deck level to carry the lower fixing of the sail luff. Up towards the top of the mast there is a fixing for a forestay and trapeze wires. When the sail is set and the mast fully curved under the normal luff tension the forestay attachment point has to line up with the axis of the two mast rotation bearings so that the forestay does not slacken as the mast rotates. This is achieved by having a significant kink in the mast just above the top of the mast mounting socket. A further advantage of this is that it places the rotation axis for the whole rig closer to the centre of effort of the rig to reduce sheet load.

The mast, mast socket and boom were my first attempt at carbon fibre laminating and it was a bit of a learning curve. I also made the mast rotation bearings and the various fittings in aluminium and titanium but since machining bits of metal was already one of my pastimes this part of the work was easier for me than the laminating. All the carbon fibre was vacuum bagged and since the components were all long and thin I was able to use lay flat tubing (available from a packaging consumables supplier) for the vacuum bags. But what a lot of different materials and bits and pieces are needed for this kind of job! There is carbon, both cloth and unidirectional, resin with various speed hardeners, peel ply, breather cloth (I used old curtains , maybe a false economy), cleaning cloths, parcel tape (some brands are ok with epoxy, others are useless and I don't know how to tell which is which), vacuum bags, release agent, rubber gloves (essential), various solvents, numerous brushes, mixing pots etc. The list seems endless and for a small one off job it is hard to avoid wasted left overs so it is not a cheap hobby. I am pleased with the result although another time, with the benefit of this experience, I believe that I could improve the laminating technique to make the rig one or two kilos lighter which would definitely improve the boat. I think it does take practice to achieve anything like the published optimum resin to carbon ratios whilst ensuring that the carbon is properly wetted out everywhere. One problem is that because of the black color of carbon, areas of resin shortage do not show up as they do with resin and glass laminating. Another time I would certainly investigate the possibility of carbon prepreg for more consistent properties and cleaner working.

Getting the custom sail made was difficult. The sail maker I first contacted was one with special experience of making sails for international moth sailing dinghies, which seemed to me a fairly similar type of boat. Unfortunately he failed to deliver the goods and this delayed the project for about a year. After about 6 months he told me for the first time that he was unable to proceed with the job because he could not get camber inducers large enough for my oversized mast section. Perhaps he hoped that would be the last he would see of me but that weekend I took a camber inducer out of my old windsurfing sail and moulded some scaled up copies with glass and epoxy. He did then promise to get on and finish the job in a month. A few months later he still had not started and I then went to Reflex sails who made me a superb custom sail from start to finish in a fortnight. Most of the sail is clear Mylar, the edges are reinforced with multilayered mylar. Reflex sails ignored my hand crafted camber inducers telling me that I had copied them from a somewhat outdated design!

Making this one off custom rig was quite a lot of work, several times more work than building the plywood hull. It also accounts for nearly all the cost of the project to date. But I feel that it is a pretty good looking rig and although I don't have a wind tunnel or other means to prove it (who does?) I would be surprised if it is not more efficient than most dinghy rigs. The area is about 8.7m2 which is similar to many of the large sailboard rigs but with the reduced sail twist I would expect it to be more powerful albeit probably less controllable. The mylar sail sets without a wrinkle and with the wide luff pocket and camber inducers the sail is very smoothly faired on the lee side. In the photo of the boat at the top of this page you can see some slight wrinkles diagonally across the mylar. These are due to downwards tension from the mainsheet, with a modified mainsheet arrangement I have been able to eliminate them.

Hydrofoils

The hydrofoils are the same ones I made many years ago for the first prototype of this concept as mentioned above. They are moulded from glass reinforced polyester resin with a good proportion of unidirectional glass for bending strength. The forward Vee foil is solid, the strut of the aft tee foil is hollow but the base of the tee is solid. The angles of the foils can be adjusted and set while the boat is on shore but not while sailing. The aft foil is a little bit flexible considering that it carries most of the weight of the boat. When the boat has sailed fully foil borne the steering has felt a bit wobbly and this may be due to the foil strut flexing but I have been a bit too preoccupied at these times to look closely to study what is happening. If I make a new stern foil it would certainly be stiffer and carbon fiber would help here.

The angle of incidence of the foils depends on the pitch attitude of the whole boat as well as the angular setting of the foils relative to the hull. Since this boat has a dead flat deck I have used the plane of the deck as a datum to set the angles of the foils. The photo of the boat at the top of this page shows the boat quite close to take off sailing in a force 2 I think. It might only need two or three more knots to get fully foil borne. Judging from this photograph the deck is pitched approximately 2.5 degrees from horizontal which would make the angle of incidence of the forward foil about 5.5 degrees and the stern foil about 3.5 degrees. Based on these foil angles the calculated take off speed is about 17 knots. I quickly learnt that it is easy to set too high an angle of attack, especially on the forward foil.

Accepted wisdom is that fully immersed Tee shaped hydrofoils offer a better lift to drag ratio than surface piercing Vee shaped foils. This was the main reason that I chose to place the crew weight well aft on this boat, to keep as much of the weight as possible on the supposedly more efficient aft Tee foil. Perhaps I have overestimated the benefit of this and I would do better to have crew weight a bit further forward with a correspondingly larger forward Vee foil. Certainly this would make trapezing easier since there would be less tendancy for the wire to pull the crew forward. It would also avoid the bow up trim at displacement speed which could help a lot with the low speed maneuverability. If I were to make such a change the benefit of having a fully immersed foil aft would be reduced and perhaps the boat would actually be better with surface piercing foils both ends. Apart from anything else that would allow a shallower draft reducing the risk of writing off the foils and hulls by inadvertently sailing into shallow water. If I continue with this project the foils could be the next area for experiment.

Performance to date

The boat has so far sailed for distances of up to perhaps 50 to 100 m with the hull fully clear of the water and supported on both hydrofoils. This has been achieved in about force 3 wind strength. Once the hull is completely clear of the water there is very little spray or wake. If you are wondering why there is no photograph to show this it is because there has been no camera in the right place at the right time. Photographs are hard to come by when testing a new boat out in the middle of a big lonely lake with no support craft. I have not yet measured the speed when foil borne but I can say that on one occasion when the boat was fully foil borne it easily overtook a Laser 4000 dinghy, but I realise that is not a very scientific way to assess the performance. I doubt that my boat would be as fast as a sailboard under any conditions but a direct comparison has not been possible since the weather conditions when I have sailed my boat have been too calm for board sailors to bother to unpack their equipment!

I have recently purchased a hand held GPS receiver and I have also been given an old trailing log. I tried out the GPS by taking it with me when I went cross country running the other day. I found that it gives excellent readings when measuring a steady speed but can be misleading when speed is fluctuating. My GPS also has a resettable peak speed reading. This is definitely unreliable, if I could believe the maximum speed reading from running with my GPS I should win the London marathon! I am thinking of attaching the GPS to the craft and just leaving it storing position readings throughout an hour or so of sailing trials. I have seen freeware software which should be capable of downloading this data from the GPS and plotting the entire track of the boat on a screen and calculating speeds all along the track. I have not tried this yet but if it works it would be much easier to study the data back at home than to try to read the GPS display while concentrating on sailing the boat.

I did take the boat to the Weymouth speed trials this autumn and I made two timed runs of the 500m course at about 9 knots averaged over the 500m. The wind at the time was too strong for my present skill level and so the boat was never foil borne during these runs, indeed at times it was almost stationary as I struggled to recover from near capsizes. I am sure the boat has briefly sailed faster than 9 knots, perhaps quite a lot faster. Even 9 knots is quite speedy compared with most 12 foot sailing dinghies. However, if you make the comparison with a sailboard rather than with other sailing dinghies then there is a very long way to go, the sailboard is a hard act to follow.

The boat has only briefly sailed fully foil borne but it often sails with a large proportion of the total weight carried by the hydrofoils and some weight still on the aft part of the central hull. The photograph at the top of this page shows the boat sailing in this manner. Judging by the proportion of the hull immersed compared with the proportion immersed when the boat is stationary I would guess that something like 75% of total weight was being carried on the hydrofoils at the moment this photo was taken. Most of that weight would be on the stern hydrofoil. It seems plausible that in this situation the hydrofoils are making a useful reduction in overall drag even although the boat is not fully foil borne. It is notable that in this picture there is no visible spray from the forward hydrofoil but there is a great amount of spray and wash from the aft section of the main hull which is planing but probably planing rather inefficiently. I suspect the hull is too narrow for efficient planing and any rebuild might feature a wider planing surface towards the stern.

The boat was fitted with trapeze gear when first launched but this equipment has never been used and has now been removed from the boat, for the time being at least. My skill level at the present time is simply not up to sailing this cantankerous craft using a trapeze wire. To get the full potential from the boat I assume that I will at some stage need to put the trapeze gear back and find out how to use it or alternatively rebuild the boat with some other method of increasing righting moment.

Capsize

The reader will not be surprised that this boat is really easy to capsize. Fortunately it is also an easy boat to right from a capsize. When the boat is capsized with the crew not on board it floats on one of the small side floats together with the forward part of the central hull with the buoyancy of the mast preventing complete inversion. The first action of the crew is to pull the main hull down to the water immersing the lower float. It is then possible to place a foot on the bow of the immersed float to help climb up over the underside of the main hull to scramble aboard just forward of the forward float beam. Sometimes the boat comes upright during this process, if not I just sit on the main hull and lean back holding onto the upper float to right the boat.

One difficulty is that if the crew becomes separated from the boat during capsize the boat can drift away from the crew faster than it is possible to swim. A heavier sailing dinghy probably drifts more slowly when capsized and a sailboard certainly drifts more slowly since it has very little air drag once the sail is lying flat. I have mostly sailed the boat in winds of force 2 to 3 and under those conditions I can catch up with the drifting boat swimming with the hindrance of a wet suit and life jacket. But on one occasion I sailed the boat in a stronger wind, probably a force 4 and found that I was unable to get back to the boat after capsising. I was eventually rescued by a sailing club safety boat but it was about the most scary incident I have ever had when sailing and I have been thinking about a safety line attached in some way which would not impede sailing the boat.

Low speed manoeuvring

The boat can be difficult to steer at low speed and tends to get caught in irons when tacking which can be most frustrating. The rudder surface, which is the strut of the aft hydrofoil, is of small area to minimise drag and is only marginally large enough when sailing slowly. Since the sail comes right down to deck level the crew has to move right aft to get past the sail when tacking and that deeply immerses the stern. This has the advantage that it forces the floats into the water giving much needed lateral stability but the deeply immersed stern also resists being steered sideways by the small rudder surface. Also the rig may be mounted too far aft for easy bearing away after a tack. I positioned the rig well aft primarily to avoid the trapeze wire pulling the crew forward when trapezing near the stern. The forward hydrofoil as originally constructed was a Vee shape with a small vertical fin protruding from the bottom of the Vee. This fin has now been cut off and the bottom of the vee has also been flattened off. This reduces the depth of the forward hydrofoil which does help bearing away after tacking.

I suspect that front steering, that is steering acting on the forward rather than aft hydrofoil, would improve low speed maneuverability. One reason is that the bows will push sideways more easily than the deeply immersed stern of the boat. The other reason is that when the boat is in irons, steering a forward rudder surface to line up with the local water flow would move the centre of lateral resistance right aft which would help bearing away. With a stern rudder this trick will move the centre of lateral resistance right forward but not right aft. However, a forward steering hydrofoil is considerably more complicated mechanically so I hope to avoid it if possible.

The boat as first constructed had inadequate working deck space for the helmsman to be able to move around easily. The picture of the boat on a roof rack is of the boat as first built with the spaces between the central hull and the floats completely open apart from the couple of wooden beams which were a lash up to provide an intermediate foot position for trapezing. About half of this open space has now been filled with ridged decking. This makes the boat more comfortable, or at least less uncomfortable, and it is easier to move around for balance.

The boat is rather slow to windward, in its present state of development it must be considered as primarily a boat for reaching. The same could be said of modern sailboards. When close hauled the speed drops to an extent that the hydrofoils do not seem to have adequate lateral area resulting in considerable leeway especially if pinched too close. With the crew forced to sit well aft the transom is deeply immersed at displacement speed making an astonishing amount of wash for such a small boat. However the boat has always been able to make progress against the wind when this has been necessary. The boat actually has a dagger board case built into the hull, although the slot is sealed. Thus there is provision to add a dagger board to improve windward sailing should I wish to do so.

Launching and Handling on shore

One of the nice things about this boat is that it is much easier to handle on shore and to launch than most sailing dinghies, let alone most hydrofoil sailing boats which tend to be particularly awkward in this respect. One person can lift the boat on/off a roof rack and carry it down to the beach, although it is easier with two persons. After fitting the rig to the hull the boat can be launched and floated in shallow water in the capsized position with the bouyant mast on the surface and the hydrofoils projecting horizontally rather than downward. The draft with the boat floating like this is only about six inches. The crew will naturally be wearing a wetsuit so it is no hardship to walk the capsized boat out from the shore until the water is about waist deep whereupon the boat can be righted and sailed away without risk of the hydrofoils grounding.

Further development

It is a bit early to say whether or not this concept is worth further development. I need to do more sailing with it to assess the performance in the current configuration and then I probably need to revisit the dreaded trapeze wire. If I find the trapeze wire to be completely impractical then maybe I could think about other means to get crew weight out from the centreline. Simply rebuilding the boat with overall beam increased by say 800mm would make a considerable difference although I think that would also make it harder to right from capsize. I have also wondered about a sliding seat such as used on the international canoe. Any such experiment would need a complete rebuild of the hull structure but that is not such a difficulty, as discussed above the hull construction was a fairly small part of the time and cost of this project. Another possible development would be to build a version of the boat for two crew members. The main incentive to do that would simply be to make more hands available for operating control lines. With the mainsheet and tiller as two controls which require constant attention a single crew member has no spare hands for such a simple task as clipping on or adjusting a trapeze wire, two extra hands on board could help a lot. Two crew members might also roughly double the righting moment with less than a doubling in water and air drag.

The boat sailing well but not foil borne

I think this picture suggests how the boat is handicapped by not using the trapeze - there should be a lot more potential if the crew weight were say twice as far off the centreline

Simple comparative analysis - hydrofoil dinghy and sailboard

A big question has to be whether this type of craft can ever exceed the performance of a sailboard. A simple and probably naive comparative analysis as discussed below suggests that this could be possible, but I am not that optimistic at the present time.

The hydrofoil dinghy as described above has two possible performance advantages over a sailboard:

Discussed above are two reasons why a hydrofoil sailing dinghy might be faster than a sailboard. On the other side of the balance the sailing hydrofoil inevitably has more air drag than a sailboard but one would expect this to be of slightly lesser importance. The sailboard hull is down at the bottom of the boundary layer between air and water, the hydrofoil hull is more exposed to the wind and probably cannot be as small and streamlined as a sailboard, although I could improve my current design in that respect. But for both types of boat the air drag of the exposed sailor dominates that of the hull. Considering the wet suited human body positioned roughly side on to the apparent wind I think we have a cross sectional area of about 0.4 m2 at a drag coefficient of about 1.0. That means a drag of say 100N at 40knots apparent wind or 25N at 20knots. If the apparent wind were aft of the beam this drag would actually be a propulsive force but sailboards reaching at speed pull the apparent wind forward of the beam. If we consider a 40 knot apparent wind 45 degrees off the direction of travel we have the sailor's body producing a rearwards acting drag component of about 70N. For comparison, if we make a guess at 7:1 for the lift to drag ratio of the planing board then the water drag on the board (neglecting lift from the rig) is 240N. So the air drag of the sailor would be worth reducing if one could do it (a streamlined helmet and even streamlined 'body armour' has been talked about) but it is not the main drag source.

Note that in the above paragraph I mentioned in brackets that I was neglecting the lift from the sailboard rig. I have asked several people why sailboards are so much faster than almost any other kind of sailing boat (the extreme record breaking boat Yellow Pages Endeavour' is the one and only exception) Often the reply includes the suggestion that the inclined rig lifts the weight of the board and sailor right off the water. I think this suggestion needs some examination. The maximum moment which a sailboard sailor can apply to the rig using his weight is something like 600Nm (75kg body weight, centre of gravity of body offset say 800mm from center or pressure of feet on board) Say the centre of pressure of the sail is 1.8m from the bottom of the mast, measured up the sloping rig (I am allowing here for the top part of the sail being feathered by sail twist). Also take the inclination of the rig to the vertical to be 25 degrees. You do see more steeply inclined rigs but I think the good speed sailors try to keep their rigs as vertical as possible. From this the horizontal component of sail force is 300N (which incidentally squares reasonably well with the drag guesses above) and the vertical component is 150N. A higher center of pressure for the sail would reduce this a bit, a higher rig inclination would increase it a bit. So yes, the inclined rig does indeed impart some useful lift to the sailboard but this lift is not a major part of the total weight. Also this is not 'free' lift, it is lift gained at the expense of some extra aerodynamic drag compared with a non-inclined rig producing the same forward drive force.

On balance this comparison so far would seem to favour a sailing hydrofoil rather than the sailboard for maximum top speed, but I am well aware that it does not work out like that on the water - at least not yet!! Why not? - well suggestions would be welcome.

Certainly the superb controllability of an expertly sailed sailboard is likely to be big factor. The board sailor can feel the effect of wind on the rig and make quick compensations for wind fluctuation in a more direct way than is possible with any other type of boat, even my one which has a more direct main sheet system than most. Also the legs and feet of the board sailor act as an active suspension system, guiding the board over each wave and keeping the planing surface at an optimum angle of attack. Perhaps this advantage outweighs the theoretical advantage of hydrofoils. Another point is that a modern sailboard has only one vertical surface underwater, the skeg. This means that if the skeg ventilates the board skids sideways. But if you have more than one underwater appendage, say a rudder and keel, or a set of hydrofoils, what happens if just one of them ventilates? The effect is probably worse than with a single skeg since you don't just hop sideways a bit, you spin round and crash. There has been speculation that this is why the Yellow Pages team never improved on their world sailing speed record which has stood for eight years. They tried but their latest boat, which was expected to be faster, spun out of control and smashed up.

So nothing beats a sailboard. Even so I intend to give this project a few more test sails and perhaps another round or two of boat mods before trying something else - or maybe booking a course of sailboarding lessons!



John Perry - November 2001



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