A flying cat with stabiliser foils

[David Cooper]

This is just an untested idea at the moment and the design is far from fixed, but it's time to put some kind of sketch up for people to comment on before I take it any further. I want to build a small, light, high-performance foiling cat which can both be raced round the cans and be used for long coastal journeys lasting many days. To see the diagram properly, right-click on the image below and open it in a new tab. The boat will ideally be about 12ft long (if it's practical to make it that small - I want it to be easy to handle it out of the water because there will be occasions when it has to be winched some distance up steep sloping rock on remote islands like North Rona), or up to 20ft long if you include the stabiliser foils. The width of the boat may be as much as 16ft (with the foils separated by 10ft and the masts (one on each hull) separated by 13ft). Each hull is 3ft wide (though not so wide below the waterline), providing just about enough room for two people to cram into it nearly side by side. Cabin headroom is not generous and many people will not be able to sit up fully, but it will be comfortable enough when leaning back a bit (and when lying down - it is essential that there be enough room to sleep inside).

The most obvious novelty is the addition of stabiliser foils at the bow and stern (which also serve as rudders). These are free to rise and fall according to however much lift they generate from their contact with the water, but if the boat pitch changes, whichever stabiliser foil is pushed higher relative to the hull will lock and begin to carry some of the weight of the boat (although it's free to fall again as soon as the weight comes off it). This quickly arrests any change in boat pitch where it goes bow or stern down and buys the crew time to move the weight over the main foil to bring everything back level (if the boat doesn't revert to level by itself). Note that the crew will normally move the main foil forward or aft rather than moving their own weight relative to the boat because this has a couple of advantages: it allows the cabin to be shorter and this makes the boat lighter, but it's also easier to move the foil than to move a sitting person. This means that when you gybe or tack, you should move the foil aft just before you begin the manoeuvre and allow the front stabiliser to take up some of the weight, then when you start the turn a moment later and the power from the sail is removed, the bow will come back up again. At the end of the turn when the power returns, the bow will pitch back down and you will have to move the main foil back forwards to restore the original balance. By doing this, the balance should be good through most of the turn and it will hopefully make foiling gybes and tacks a standard feature of the boat's performance whenever there's a decent breeze with reasonably flat water - this is a major goal with the design because such performance will result in better racing. Not only should the boat be stable through these unpowered turns, but it should also enter them at higher speed than normal due to the superior aerodynamics (with no one sitting out on the top) and the extra power provided by the double rig (which have a lower centre of effort and thus allow more power for the same RM). The front stabiliser foil has a fair amount of twist in it towards the tips in order to handle the change in boat pitch. Both stabiliser foils also lean back more as they rise.

The extra power from the double rig (25ft high masts combining to give 46ft of vertical mainsail) should also enable the boat to take off at lower speeds than other foiling cats, and this is even more important than top speed, particularly when the boat is being used for cruising. Note that the hulls are not designed to move quickly through the water, but are optimised for take-off speed and are also shaped to move well through the air.

The main foil is staggered, the lower part being used at high speeds and the higher for low speeds, the higher part acting not unlike Welbourn foils, only underneath the hull instead of sticking out from the side. This foil design should hopefully provide adequate heave control despite using a passive foil at a shallow angle (which may be as low as twenty degrees). The actual lengths, angles, chord, etc. of the foils will need to be experimented with to find out what works best, but the diagram gives an indication how it might end up.

There is enough power from the rig for the boat to be crewed by three people while racing, each hull having one person stay inside it throughout while the third member of the crew switches side during gybes and tacks. It may be best for this third person to make the journey from one side to the other inside the trampoline to reduce drag, but it would also guard against the risk of falling overboard and being sliced by a foil. He/she would prepare for this before the turn by getting out of the cabin and into the space between the two flexible layers of the tramp, then pull himself along a rope to get to the other side during the turn (without needing to worry about anything else during the process - he is merely a piece of movable ballast at these times, even if he's the tactician), only climbing into the other cabin once the turn is complete. This indoor pathway between the two hulls would also make this a much safer boat for a family cruise when up on the foils, removing the major risk of serious injury.

 

[David Cooper]

I started this thread after describing my design at SA (starting from the middle of this page: http://forums.sailinganarchy.com/index.php?showtopic=178470&page=2#entry5656193 ) and I was pushed into describing it after an argument about the possibility of foiling tacks on a better-stabilised system. I'll now copy across some of the details from there wherever they expand on the above.

"I had intended to have the centre foils go down from the outside side of the hulls to maximise RM, but I now realise that I can get more RM by putting them on the inside side of the hulls and moving the hulls further apart (without changing the separation between the foils), thereby eliminating the need for racks and improving the aerodynamics for higher speed. The main foils can be removed from the hull sideways from the case so there's no need to drag them up through a slot (other than for raising them a couple of feet), and this also allows them to break out if they hit debris, so this gives me the freedom to use a more complex arrangement than is normal and to reduce the loading on the vertical section where the lifting sections branch off from it - they have less leverage in the forces applied there than you get from a much longer lifting section turning off to one side only."

[Since then, I've made it possible to move the main foil some way forward and aft, and this makes it easy for the foil to break out without damaging the hull if it hits debris, so I no longer need a door in the side of the boat for the foil to break out through, and it will still be easy to remove the foil without needing to reach under the hull.]

The following is an older idea for the stabiliser foils, but it's worth posting here as it may be that the two systems should be combined into a hybrid system:-

"The stabiliser foils are essentially just long "poles" of oval section (the cross section shaped like a greatly elongated running track as used in athletics) with a V or diamond lifting section near the bottom and a short rudder section below that. These poles rise and fall freely (vertically) through simple, robust cases (which pivot for steering in the usual way of rudder cases) with the rarely-immersed pole part running against wheels/rollers in the case. A cable may be used to help raise these foils using bungee power (with a spiral pulley wheel to change the leverage and thereby provide constant force from the bungee, or perhaps a stronger force when they're higher up) - this would reduce the drag that the stabiliser foils generate while they follow the water surface. The rudder section below remains immersed almost all the time. The downside of the bungee is that it will slow the descent of the foils when they're free to fall, but there should be no great rush to get them down - that's something that will need testing on the prototype to find the right level of force to use. I haven't settled on a locking mechanism (which needs to arrest upward movement of the foil while still allowing movement the other way), but I have worked out a simple way of coupling two of these foils so that the higher one always locks (though I won't attempt to describe it here)."

"I have my doubts as to whether either system will actually prove to be viable, but a third option is to use a hybrid of the two systems with the height of the stabiliser foils being set manually in part with automatic height variation taking place over a more restricted range. A key issue is how much weight a stabiliser foil needs to be able to hold up in order to buy enough time for the crew to shift the their weight over the main foil to restore correct balance."

"The boat is equivalent to a much bigger one than the hull length suggests. You have to consider the amount of sail it carries, and notice that when the stabiliser foils are locked and actively lifting (rather than merely holding up their own weight), they extend the effective length of the boat towards 20ft. (When sailing upwind, my design has more power available than an F20 FCS and my main foil is generating lift in a better direction.) Conventional boats only generate lift from the bow in displacement mode, and they don't need that lift so far forward in light winds, so what actual use is it on a foiler? It certainly allows faster acceleration in strong winds from a stationary start where my design would have to have the power added more gently to avoid being blown over forwards while the front foils are generating no lift, but is that such an important ability once the aim is to stay up on the foils all the time in those conditions? My hulls may well be a bit too short and fat, but the dimensions are not set in stone yet: the shape will ultimately be dictated primarily by what's optimal for take-off speed, although if there's any room to make the cabin wider by a few inches without compromising that performance significantly, I'll take it if it leads to a critical gain in comfort. It's possible, of course, that two different designs will be required to satisfy the two different applications (racing vs. cruising), but if the performance of the cruising version is anywhere close to that of the racing one, few people will want to spend money on the substantially less versatile racing boat in return for a fraction of extra speed or very slightly lower take-off speed, and it would also be placing it into arms-race territory where the kind of people sailing it are going to keep switching to new designs anyway for small performance gains. I want something that could be a popular class for decades, so it needs to tick as many boxes as possible, and the most important thing it needs in addition to being good to race is that it should be a proper freedom machine, opening up the way to adventure. By luck, the indoor comfort that can be added to improve the boat's potential for cruising can also make it a better racer by improving both the aerodynamics and safety. I've also eliminated the stays and mainsheets to reduce drag (the former allowing all the power to be dumped in an instant) and made it practical for pedal power to be used to pull the sails in, this making it easier to handle the extra power."

"A crew of three would be used when racing in strong winds, but they needn't all be as hefty as F20 sailors even though the power is similar - the important thing is to be able to put enough mass in the windward hull without having to move the helmsman/helmsmen, so two lightweight adults plus one hefty one would be ideal, the hefty one being the one who keeps switching sides. Two hefty people could sail the boat instead, but they'd have a harder time coming out of a tack or gybe, not being able to apply full power until the second person has crossed the boat after handing over the steering to the first one to cross, although they could make up for it by being able to handle more power afterwards. When cruising, you'd carry extra gear, but the foiling performance needn't drop much - a bit, or a lot, of extra weight stored in the hulls is useful as ballast, allowing one person to travel alone in the boat with perhaps eighty kilograms of gear in the other hull so that he doesn't need to change sides on tacking or gybing - he would likely want to carry more gear rather than less so that he can raise the sails higher. A family using it for a long journey would avoid the higher speeds by reefing, but in light wind conditions they could use the full power to fly the boat, and they needn't weigh it down much more than a typical racing crew. The essential gear to carry shouldn't be any more than they'd need if they were travelling in kayaks."

"The length of the higher lifting section of the main foil might be as much as three feet long (though I've shown it at about 2ft in the sketch), and the lower section (which will probably slope at a shallower angle than the higher section) would likely be another two feet, so there's no shortage of lifting surface. The mainfoil on the windward side would be raised by about a foot when going upwind, but left fully down when going deep downwind. The stabiliser foils may not need to be as big as shown, and I have decided to change the way they're attached to the boat to reduce the amount of weight that they have to lift as they hold themselves up, but the drag through the water from these four foils (which also serve as rudders) isn't going to add up to all that much, being less than on the C Fly where the boat actively leans on its front foils. The connecting parts can also be sprung to take up some of the weight of the stabiliser foils so that they only have to lift part of their own weight, the only cost of this being that they will fall back down more slowly, but it shouldn't be important for them to descend particularly fast, so this may well prove to be viable."

"When it comes to whether the boat will fly or not, that's all down to physics. If you have good aerodynamics, plenty of power and the right amount of foil in the water in the right place and set at the right angle, you'd have to stuff things up badly to prevent it from doing what existing foilers already do with inferior aerodynamics, less power, and foil systems that generate less of their lift in the ideal direction. The worst aspect of my design is the drag (both hydro and aero) of the stabiliser foils (which is necessarily a bit higher than a boat that uses long rudder foils with winglets instead), but the speed of the C Fly suggests that this is not going to cause a major performance hit, and the stability gains should pay dividends. Even if I was to build the hulls using Soric in place of Nomex (which might even be worth considering to make it more bulletproof), it would still fly well."

 

[David Cooper]

The big engineering issue is how best to attach the stabiliser foils. The original sketch shows long arms connecting them to the hull, but how heavy would those arms need to be to carry the required forces, and how much extra work would the foils have to do to rise and fall with that weight resting on them? It looks like an unrealistic approach. If they are partly held up by some kind of spring force, that will take weight off the foils, but lead to them descending much more slowly.

The hybrid approach that I mentioned in the previous post would split the arms into two parts to reduce the amount of weight resting on the foils, and to illustrate that idea I've reworked the sketch a little, though it's still only aimed at showing a possible approach (primarily to prevent any troll getting a patent on it and blocking me from using my own idea). The heavier xy section would be set manually for the expected flight height (based on the wind strength and direction of travel relative to it) and locked in position at x. The relatively light yz section would be free to move up and down with the automatic locking taking place at y. Maintaining the angle of the foil while these arms move is easy enough, and it would also be easy to make the foil tilt back or forwards a little as xy moves up or down.

 

[David Cooper]

The simplest option is usually the best one to go for, and I think that's the system described in post #2 rather than the ones shown in previous sketches. My original idea was to have the stabiliser foils run up and down on "poles" held just beyond the bow and stern with room to raise most of the lifting sections out of the water when the boat's in displacement mode, but that makes the poles horribly long, putting severe loadings on them and adding a lot of weight for the foils to carry when they're unlocked. A simpler alternative would be to have the "poles" run through the hull, with the downside that the lifting sections will be completely underwater when fully raised in displacement mode. I've worked out a way of removing these foils forwards or aft along with a removable part of the hull, this also allowing the foils to break out on contact with debris and for the little chunk of hull they take with them to keep them afloat so that they can be recovered. The hull in the first of the new sketches is lengthened to something in the region of 16ft and needs a lot more structural strength in the lengthened bow. However, it may turn out that the stabiliser foils don't need to be so far away from the main foils, so it might not be necessary for that extra length - the purpose of the stabilisers is merely to buy enough time for the crew to move the main foil to correct the balance, and if on rare occasions there isn't time for that, they can dump the entire power of the rig in an instant instead.

The extra drag in displacement mode would raise the take-off speed, and although these foils also add lift, the extra drag will not make that helpful as it's better to take all the lift from the main foils. However, there may be another solution that keeps the poles short and also allows the lifting sections to be held above the water in displacement mode, and the second sketch shows this. In light winds when the boat's either in displacement mode or flying low, the case is held higher, while in stronger winds where take off is guaranteed, the case is held level with the hull. This means the take-off speed is not harmed by extra drag other than a bit of extra windage, so I think this last sketch shows the right system to build.

 

[David Cooper]

Locking and control mechanisms for the stabiliser foils:-

I've looked for ratchets that can be switched in and out under load, but that doesn't look promising, and wear would likely be high. Fortunately though, there's a ready-made solution available which is found on bicycles. A hub with freewheel sprocket and disc brake might have been designed for the task, and all the heavy wear would be on a part designed to be able to handle it. The sprocket part would be connected by chain to the rollers in the case, while the axle would be locked into the case just as it would normally be locked to the bike frame. When locking a foil, the brake would simply be applied to the disc, stopping the rotation of the hub. The freewheel block/sprocket part will remain free to rotate in one direction, allowing the foil to fall whenever it is unloaded, but it will hold up its end of the boat whenever it is generating lift. There is no difficulty in releasing the brake to allow the foil to rise higher even if it is already under high load, and the lock can also be applied while the foil is rising fast without breaking anything, so this doesn't suffer from the same problems as a switchable ratchet. More force is required to apply the brake than to switch a ratchet, but the upward force of the foil can be harnessed to apply the brake, while releasing it can still be achieved under that loading by releasing the other part of the lever.

The stabiliser foils on the windward side can be raised to reduce drag, leaving the two stabiliser foils on the leeward side to handle the steering and boat pitch on their own. When tacking and gybing, all four stabiliser foils are deployed. The crew also have the option of manually driving a foil lower for a moment to initiate a change in boat pitch, for example, pushing the front foil down a bit as a wave approaches in order to lift the bow (instead of easing the sails). A locked foil that is too high can also be driven lower manually, and the ratchet will keep it locked at the new altitude until boat pitch recovers.

There are three systems available for controlling the stabiliser foils (all three should be available in case one or two of them fail due to a breakage), so I'll describe each in turn:-

(1) Manual controlled locking

The pilot uses brake levers directly to lock the stabiliser foils, locking the front ones if the bow goes too low and the rear ones if it goes too high. Each lever would brake two foils, both fronts or both rears, and it doesn't matter whether the windward ones are deployed or not at the time. This will be the system used when racing if the more automated control methods are deemed to be against racing rules. The other methods could be used when racing otherwise, but they will certainly be used when cruising.

(2) Pendulum controlled locking

A pendulum with a heavy weight, such as a lithium battery (which will be wanted on the boat anyway to run a computer and charge phones and cameras), will move forward if the bow pitches down, and this can be used to trigger the locking of the front foils (or the rear foils if it moves aft). This will work fine on flat water, but in waves it will attempt to keep boat pitch level at all times, leading to extensive use of the stabiliser foils to generate high levels of lift whenever the water slopes steeply, thereby increasing drag. The pilot can manually adjust things to avoid that though, moving a lever back when climbing waves and forward when descending, thereby changing the angle at which the foils lock.

(3) Foil-height-comparison controlled locking

This would be fully automatic, handling waves competently without needing any manual override, although manually blipping a foil down on occasions will improve performance over waves. The higher foil of a pair (on the same hull) will automatically lock, releasing again as soon as the other foil is back level with it. There are so many possible ways of coordinating this locking that it isn't worth patenting any, but the best ones all involve tapping into the cables used to help pull the foils up (normally outgunned by the mass of the foils, but bungees attached to these cables enable the foils to run higher in the water to lessen drag). It is sufficient to change the location of a brake lever for the rest of the locking to be applied by the foil lifting a bit further and slamming on the brake.

The manual adjustments are available in all three modes. The pilot uses a steering wheel (which may be more rectangular than round) with a telescopic shaft (to allow the him/her to move further forward or back within the cabin). There are two brake levers on the wheel which are used for manual foil-locking. The pilot can drive a front foil down (whether it's locked or not) by pushing a lever forward (repeatedly if necessary), or drive a rear foil down by pulling it back (with the lever duplicated at the other side of the cabin so (s)he can continue to hold a brake on while using this lever). The main foils are moved forward or aft by pedalling, while pedalling is also used to control the double rig, so it must be switchable between those two tasks. The pedals move horizontally rather than round in circles, so there is no way to pedal backwards - there are three switch positions to select whether to move the main foils forwards, to move them aft, or to pull the sails in. If there's a pilot in each hull, they can share these tasks between them, one working the sails while the other adjusts the main foils, although the biggest adjustments of main foil position are made during tacks and gybes when the sail doesn't need any adjustment, so it's not beyond the ability of a single pilot to handle the whole task.

Again I'm posting these details to prevent patent trolls blocking me from using these systems which I've designed. I'm posting the details elsewhere too where they get a date and time stamp and can't later be edited. Ideally, I would have patented them myself, but I'm not currently in a position to be able to do so, and I know that other people will come up with these ideas anyway over time, so I had no option other than to reveal them. If there's a company out there that would be interested in producing boats of this kind, please get in touch so that we can team up and work on it together - that will make it possible to patent the rest of the ideas (some of the most crucial of which I haven't revealed yet) instead of sitting back and watching everything being handed away for free to all your rivals. I need a team of engineers to put this together and sort out a host of standard design details, but I've shown you enough to know that you should be jumping at this opportunity. Look at some of the key innovations I've revealed already: a competent stabilisation system with three viable modes of operation which is clearly possible to build with simple parts (and which has already been tested in one major aspect by C Fly where they've shown that front foils of this kind can keep the front end up with ease); moving the main foils forward and aft instead of moving the crew so that a long cabin isn't required, while at the same time making it easy to keep the boat pitch level through unpowered turns (which will likely put foiling tacks and gybes within reach); and the idea of moving crew between the cabins on opposite hulls inside the trampoline to reduce drag, but more importantly to improve safety (which may not be 100% elegant, but should be effective - there is no doubt that people are going to be killed by contact with foils if we don't do something like this to keep them safe). And here's another thing - if you've seen the solar powered cars that race across Australia, the similarity with a carbon-fibre foiler with cabins should give you ideas about the versatility and potential mass-appeal of a vehicle like the one I've designed.

[By the way, the boat I actually want to build is not on show here, but is merely hinted at - it's a lot narrower, has more RM without adding ballast or using downforce from foils, and it might not even be a cat. It would therefore be a mistake to rip off my ideas to build what I've described here instead of teaming up with me to build something substantially better. What I've described above is already a next-generation foiler which takes things on to a new level, but with a lot of compromises which will still limit its appeal. The real boat (that some of you might be in a position to help me build) jumps straight over that though to a next-generation foiler beyond the next generation of foilers, and its design neatly solves a number of problems with the one I've already revealed (eliminating the business of moving through the tramp which isn't greatly appealing), producing something much more practical and fun while further improving the performance, but I will reveal nothing about it until that crucial patent is in place.]

 

[Doug Lord]

You won't get much help while constantly implying that anyone who helps you is probably trying to steal your ideas!

 

[David Cooper]

You won't get much help while constantly implying that anyone who helps you is probably trying to steal your ideas!

The people who pinch ideas aren't the kind who offer help - they see an idea and get it into their heads that they can manage without the person who came up with it, so they build something using that idea without ever communicating with the originator at all. What I set out to do here is show that I don't just have one idea, but a package of ideas which are clearly capable of moving things onwards, and I have been careful not to reveal the best ones. If a company does team up with me, I won't make any announcement about that here or anywhere else until the boat is spotted on the water, so anyone who sets out to build a boat using my ideas without my involvement will soon find out that they've poured their money down the drain as they'll be up against competition from a rival company which has had full access to all the other key ideas. All I'm looking for in return is a fair share of the profits from each sale in line with what the other members of the design team are getting. We will sign a contract first, then I'll reveal all the details, and if at that point they decide not to build the boat because they think it's a dud, I will get nothing. I don't think that will happen though, because once they've seen it, they'll cancel other projects to build it.

More thoughts on the stabiliser mechanism:-

The hub with brake disc is overkill for a small version of the boat and would work best for manual locking, but I've managed to design a ratchet that can unlock easily while under high load, and this simplifies the task a lot for the automatic control modes. A differential gear allows the height difference between the stabiliser foils to be compared and converted into a single output which can switch the lock on and off at each foil. A small force is needed to hold the lock on, but the output from the differential gear (the part which would be connected to the drive shaft on a car) has enough grunt to do that (with the help of a spring which can hold it locked as the gear moves beyond the position where it applies the lock - this stops the differential gear locking up and allows the other foil to continue moving downwards freely). A metal differential gear from an RC car should be strong enough to do the job, and they cost under a tenner, but several parts of the ratchet will likely need to be machined for the purpose (unless I can modify the design enough to use off-the-shelf components).