Twin flapping tail propulsion HPB

Discussion in 'Boat Design' started by jakeeeef, Dec 6, 2016.

  1. jakeeeef
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    jakeeeef Senior Member

    I'm looking for more info on types of flapping tail, for want of a better word (doesn't have to be at the tail of the boat though)- propulsion.

    Clearly single flapping tail propulsion on small craft works, but it does use a lot of energy yawing the boat from side to side, and often also rolling it to an extent.

    So I want to build a system that has two parallel symmetrical foils that are leading edge hinged and angle of attack constrained- nothing new there. But, they will move laterally inwards and outwards along the crossbeam of a catamaran- but because they will be sliding inwards and outwards on a track- actuated by lines and turning blocks they wont be swinging at the end of tiller arms.

    I am SURE that this will have been done before as it's pretty obvious but I cant find anything on it.

    I have found
    But this is not what I want to do quite as it uses tiller arms.

    The issue with this is that it uses a long rod to get the necessary range of flapping of the foil. In the context of a single flapping foil boat the yawing of the hull to an extent makes up for the pitch angle change over the sweep of the arm, so it is less damaging to overall progress than it might be. Indeed, you could argue that a canoe shaped hull with single flapping tail propulsion is acting like a fish- with the foil the tail and the hull the rest of the fish- for efficient propulsion the rest of the fish yaws (even though if like a tuna the front 2/3rds is actually rigid).

    But when there is no hull yaw with a twin setup I want to keep the angle of attack of the foils WRT the oncoming water flow consistent throughout all phases of the oscillating propulsion cycle. Using a swinging tiller flapping foil approach, in my view it is only at the exact centre of the swing (with tiller parallel to the centreline of the boat) that the constrained angle of attack is what the designer planned it to be. At the ends of its travel it's also massively influenced by the angle of the tiller itself. So setting up its constrained foil angle must be a compromise that minimises stalling, but it can only have the precisely optimum angle of attack at a couple of points in the stroke cycle. I understand that some have used bungees etc. to allow the foil to 'decide optimum angle of attack for itself' to an extent, but this sounds like an imprecise alternative to exactly constraining the angle of attack throughout the whole cycle.

    I wish to use cable adjusted moveable wedges both sides to constrain the foils' angle of attack. This will mean that the boat should have good acceleration from standstill- due to very low pitch (little angle constraint), but this pitch can be reduced (more foil angle constraint) as speed through the water increases- essentially a VPP but in flapping propulsion concept.

    As the pitch constraint will be separate for each side, I also wish to see whether this will allow turning without paying the appendage drag cost of a separate rudder. So, let all pitch constraint go on the inside blade so it's just feathering to flow in all directions and dragging that side of the boat back, but keep pitch control on the outside blade to drive the outside of the hull round.

    I'm thinking what could make my idea a non starter is the range of movement required. My method constrains the range of movement to within the practical distance between the hulls of a catamaran, whereas those flapping tiller type arrangements send the foils well outside the width of the boat- but maybe they HAVE to do that only because they only hit the foils' angle of attack sweet spot at a couple of points in the cycle whereas my system can be right on it the whole time?

    Has anybody seen twin in and out flapping foil propulsion that shuttles the foils in and out along an athwartships track? If somebody else has done it I'd like to know the design pitfalls before I start mixing up the epoxy.
    DogCavalry likes this.
  2. David Cooper
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    David Cooper Senior Member

    I've seen a much better small robotic boat on a BBC program called Techno Games (which ran for three series at the start of this century but has very little about it online). It used a single foil on the end of an arm and absolutely ripped along. Sadly I can't find any information on who made it, no picture and no stats about it.

    While looking at the videos you linked to though, I saw a link to this: - it seems to solve a lot of the transmission problems.
  3. jakeeeef
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    jakeeeef Senior Member

    Thanks David,
    I've seen the Treddle Cat, and it's a simple, low maintenance spin on the idea and a great way to power a two person cat with little in the way of mechanism to go wrong. However, it cants its foils (like a Hobie Mirage Drive), so is not quite what I am describing.
    Each of the Treddle Cat's paddles describes part of an arc of a screw propeller, then 'tacks' and reverses back again. If, as I suspect it does, it uses solid hinged foils- probably windsurfer fins or centreboards, then it is performing like a propeller with no twist on its blades- so a pretty inefficient one. Towards the end of its pedal cycle when mounted in a cat- so not under a hull, the upper part of the foil is likely to be trying to shift water upwards from the surface.
    The Hobie Mirage drive is a little better in some ways as it has flexible fins that do allow twist- but the precision of the degree of this twist all along its blades is somewhat imprecise due to the nature of the materials involved. It also always has a small portion of its actuating chains and its central pin in the water flow causing significant drag- and in this regard the Treddle cat is an improvement, albeit a niche one only suitable for catamaran mounting and two person drive.

    I have a Mirage drive I have been playing with and like it in many ways, but question its durability and efficiency. I'm working on a design that is:
    Simpler, more durable and cheaper to build- due to fewer moving parts
    More suitable for multihull mounting (so does not require to be fixed in a hull)
    More efficient- (none of the actuating mechanism/ hinge underwater, accurate control of the foils' angles of attack at all times along the full length of the foils).

    Of course the Mirage drive is successful and will continue to be so as it is very plug and play, has an elegant shallow water drive ability (that mine won't unless I make the foils kick up around a nose pin- which will need to be done individually), and is relatively compact and lightweight.
  4. jakeeeef
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    jakeeeef Senior Member

    The other advantage that my proposed system will have over the Mirage Drive is pitch change on the fly. The Mirage system (if you have the later 'sails') does allow the pitch to be changed for different paddlers- but this is only achievable by pulling the drive out and adjusting individual thumb screws to change the 'clew' tension, so it's a case of stop, adjust while getting blown backwards, refit and go again. What you really want is slack clews as you start off and then to tighten them up, on the fly as you accelerate. Or to be able to slacken them off (so changing down to a lower gear) if you hit a gust of headwind or become more tired.
    Ultimately it is this pitch adjustment that could enable foiling by setting them at the powerful pitch required to get up onto the foils, then tightening up once the vessel is foilbourne for the correct pitch for that speed.

    It's also possible for mine to reverse it's fulcrum from the front of the foil to the back- hence allowing reverse gear. Although It's unlikely I'll bother with that as it's further weight and complexity and a paddle works fine for reverse- especially as there would be one aboard anyway in case of mechanical failure.
  5. David Cooper
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    David Cooper Senior Member

    My worry would be about the amount of friction that might be involved in running something along a track, though that might not be a real issue and it could be the right answer. Going back to the videos you linked to though, with a long arm sweeping, you have changes of pitch at the ends of that travel, but that could be eliminated easily by using two parallel arms connected up as a parallelogram, which could also allow the arms to be shorter while still improving the performance. This is still inferior to using a track in one key way in that it will move the foil fore and aft to some degree at the ends of its travel, and that affects the effective foil pitch there as a result, but even with a track you have an issue with the effective pitch changing as you accelerate and decelerate the foil (as a necessary consequence of stopping to change its direction), so the ends of the stroke are always going to be inefficient anyway, although fortunately the power is low at those times to match and so it may not matter.

    The other problem with all this though is with the foil shape - if you have the ideal shape for moving it to the left, it's horribly wrong for moving right, and making it flexible so that the water bends it will curve it the wrong way. You really want to switch between left and right foils at the end of each stroke, and that could be done by having two foils at 90 degrees to each other such that the one that's not in the water is held horizontally (pointing sideways). When the active foil reaches the end of its stroke, it starts to be pulled back the other way but it would simply be pushed back and upwards instead, automatically rotating the other foil down into the water in its place. This switching over of the foils would take place where it's action is highly inefficient anyway, so it wouldn't be any great loss, and it would maximise the efficiency of the stroke through the crucial mid range of its sweep. With this system, there would be no low-drag rest position to leave it in when not powering it, so it would need to be lifted/rotated clear of the water for that (perhaps by unlocking it and rotating it on round until one foil is pointing directly upwards). This same system could be used with a track, so if the track friction is low enough, that might be the best direction to take this in.

    Oh, and if you put four foils on the thing, you could have your reverse gear foils too with the same high performance, but you'd then need to lift the whole thing up when resting. That would add more aero drag though, so it probably isn't a great idea if you're hoping to get up on hydrofoils.
  6. Skyak
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    Skyak Senior Member

    "Brilliant minds think alike" is that the quote or is it "Wizeasses pass off bragging as a complement"?

    Anyway I had the same idea. I planned to try it on my Hobbie 3.5 but now I have a different plan -"full fish". My point is that there is little reason for both foils to move and I am better off paying more attention to getting the right angle of attack on the tail foil and optimizing the dorsal foil for size profile and AOA. There is potentially an added benefit to the tail foil benefiting from the vortex off the dorsal.

    There is a Youtube video of a little RC model that did a great mechanism like you are suggesting. It even had great steering by altering the sweeps. I will look for it.
  7. jakeeeef
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    jakeeeef Senior Member

    That's a clever idea David. I guess the only possible issue with it from a practical point of view in a cat would be the further increase in the beam of at least the foil depth (perhaps more as you wouldn't want the foil tips to collide with each other when switching on the inside either), if the device was to be operated along a cross beam and the foils aren't staggered. The lazy foil will always be out horizontal when the active foil is driving- but with 'handed' foils they will be smaller than symmetrical ones. There will be a certain area at the ends of the stroke where the pedal stroke will have a lot of travel and go light as the foils need a bit of motion to fully switch. As you say though this light on the legs part of the stroke does occur fortuitously pretty much where you would want it to.
    No reason it wouldn't work well in a tri though! The outrigger stabilizer hulls could be short and fitted on a separate crossbeam fore or aft of this drive crossbeam. I'm working on a Mirage drive tri at the moment based on a single scull main hull so have most of the component parts for a prototype already.
    I ultimately want to try to power a vessel by a standing 'cross trainer' type motion that even if converted to a seated version involves some considerable rolling impulse that a trimaran might not enjoy. The reason I want to go for a standing cross trainer type approach is not for efficiency reasons (on aerodynamic grounds alone it's a fairly poor use of energy), but it is a very popular and understandable range of exercise motion for a lot of people who have been to gyms. Hobie have done a Mirage drive standing board, but it doesn't employ arm power. I've not yet seen a 'cross trainer on water' that allows this exercise to be conducted while also moving reasonably efficiently.
    I guess if you carry on this path (with the foils) too far you run into the danger of inventing first the Voith Schneider propulsion system and ultimately the screw propeller if you're not careful. But the benefit for human power is in having larger, slower moving more optimally shaped foils (than can be practically built and operated as a screw propeller). There is also none of that hydrodynamically messy stuff going on around the shaft, any support brackets and the early stub parts of the blades that in most screw propellers have to be fatter (for strength- to support the outer parts of the blades) than you would optimally want them to be.

    I think getting the track friction very low is not an insurmountable problem. I'm thinking aluminium box section crossbeams fore and aft of the foils- with the foils mounted to cars that have sets of low friction wheels (I'm thinking rowing boat seat wheels- but only because that's what I'm familiar with- much smaller wheels could be better) encapsulating the box section all sides.

    I did also think of round carbon crossbeams with teflon tape over them- and the foil cars with teflon sleeve bearings to run up and down- all greased with teflon grease.

    In terms of the problem that stopping pedalling would leave you with two sets of foils fighting each other, I think the mechanics and weight of removing them from the water would be best avoided. I would imagine that it would be possible to reduce pedalling effort enormously (without stopping pedalling) and the foils would continue to go through the motions.

    Any wave action on the lazy foil- which will necessarily be horizontal and very close to the surface is a concern. I wonder if moving their relation to each other to more than 90 degrees might help? Of course, the further away from 90 degrees and the nearer to 180 degrees you make them the longer and tardier the foil switch will be.
  8. David Cooper
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    David Cooper Senior Member

    Unless you're trying to make an exercise machine, I'd steer clear of arm power and leave it all to the legs. Many experiments have been done with bicycles where arm power is added, but they all fail, perhaps because leg power is inherently more efficient - I think it's better just to work on making your leg muscles stronger rather than building up arm muscle. Using the recumbent cycle position also appears to be the best option for maximising power, quite apart form the aerodynamic and stability benefits.

    I'm not sure that you're picturing my foil idea correctly as you talk about the tips colliding. Imagine that they are arranged like the letter L, but rotate that 180 degrees so that the arms are pointing with one down and the other to the left. Imagine that the back of the boat is where your face is and the front end is beyond the screen. That is the orientation of the foils when they move to the right. On reaching the end of their travel to the right, the direction is reversed, at which point the downward pointing one drags and rotates our upside down L anticlockwise, and this action brings the other arm (which was pointing to the left) down into the water where it is pushed vertical (and pointing downwards) by the water pressing against it. I don't know how long (deep) the foils would be, so if they're a foot long it will reduce the effective distance covered by each sweep significantly, but that might have less of a negative impact on an arc sweep using a foil at the end of parallel arms rather than a straight sweep along a track.

    The foils would be free to swing back if water pushes against them from the wrong side, so the drag wouldn't be enormous if you stop pedalling for a moment, and waves washing over the horizontal foil would also swing it to an angle where it causes little drag.

    I still wonder about the drag involved in a track. The force from the foil will try to twist the fitting, pressing any pulleys hard against the track, and each pulley is going to be rotating fast with this pressure on it with two surfaces dragging against each other at high speed where the rotating part rubs against the axle. I'm not an expert on how much friction is generated by advanced materials though, so it may be that it's low enough to work fine. However, I can also imagine a trackless system with two wires at different heights going round pulleys at each side of the boat which might be able to do the job without the same issues, though the foil would likely flex about a lot. My money's still on the arc-sweep solution for best efficiency.
  9. rower mike
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    rower mike Junior Member

    the above is totally true in my opinion... likely because the human lungs and cardio system can only supply a set amount of support long duration (ie > 5 minute) physical activities and often has to direct it to where its needed most.

    everything about us is (or should i say was) adapted to meet our survival needs... our ancestors needed to do a lot of walking. Even today people who aren't in good shape still walk somewhat and have the muscle mass and endurance to walk continuously for 45 minutes or more if they really had to.... even though its a full body weight exercise.

    With training, they can easily build that up to hiking up and then down a mtn in say 5 hours. We also really couldn't do much with our hands and arms when we're walking, hiking or running around... so our cardio capacity is mostly based on what our legs needed to be capable of to survive. As is where most of our muscle mass is located.

    its all a spinoff off of "walking upright"
  10. tspeer
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    tspeer Senior Member

    I'm surprised nobody has mentioned the Hobie Mirage Drive. It's a successful flapping foil propulsion unit that can be used in a variety of craft.
  11. David Cooper
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    David Cooper Senior Member

    It was mentioned in posts #3 and #4, but that's the best picture I've seen of one, so it was well worth posting it.
  12. tom28571
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    tom28571 Senior Member

    Both flippers and propellers work in the same way in that they both accelerate a mass in one directions and generate thrust in the opposite direction. At low speed, flippers can probably be more efficient because a flipper can be very large compared to a propeller with the necessity of having identical opposing blades halving the diameter. Compared to propellers, fish move their flippers, and bodies, slowly and speed is limited by the necessity of moving all that body mass at all speeds with limited power.

    Flippers can be flexible which makes them more efficient than fixed blades and none of the existing flipper powered boats I've seen other than divers flippers properly utilize that property. The flexibility greatly increases efficiency for a diver at the low speeds they travel and they can come close to a fish in that regard.

    I designed a yuloh with a flexible blade that I think recovers some of that lost thrust, especially near the beginning and end of each stroke. Not built or tested yet though.
  13. Victor R
    Joined: Sep 2019
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    Victor R Junior Member

    Are you still interested in this topic?
    Please look Flapping Foil
  14. Horton HCCI
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    Horton HCCI Junior Member

    Feel free to take a look and tell me what you think.

    Attached Files:

  15. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Instead of fishtail type action - same leading edge, naybe falling leaf action might be more efficient.
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