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Thread: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) story

  1. #1
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    Default Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) story

    I have to apologise for starting a thread about this when it may have been discussed to death on this forum (I may be going insane, but doesn't "advanced search" normally allow an "and" function - even if only by phrases in quotes - neither seem to work here).

    I simply couldn't face the 500 threads that turned up as soon as I used 2 words.

    I also gave up using t'web - even though logical operators work a bit, it still produces around 80% commerce plus about 19% click bait.

    So this is why I've written the engineering equivalent of a shaggy dog story (again).

    I did get a very interesting read by looking up Yuloh here, as several spring-loaded "flipper" designs turned up (eg. Harry Bryan's Thistle) that arguably offer a better way to finesse the ideal stiffness, and may even provide a better angle of attack through the entire sweep.

    But I wanted to see something like a large flipper or small monofin on a stick - preferably made from junk I already have or (probably better) could be sourced as boring materials for something entirely different (not "glamorous" stuff like sailing supplies, hobby stuff, or gardening, or any similar categories that always seem to cost 300% more).

    Basically I want to make something that could be used without a special rowlock or fixed hinge (so I could experiment with placement - starting with existing pegs, nooks, crannies etc.).

    And that would be shorter than a yuloh (so I can get it to the other end of a proa without braining myself - or have one at each end with one out of the water - but neatly).

    And it must be a pretty serviceable rudder when not being flippered.

    Incidentally my first try with a very basic flipper oar showed that there is probably too much yaw for any light boat without a pronounced keel - my inflatable suffered even with a weather board in the water - though a more flexible flipper could have been used over smaller angles and quicker and would probably work OK.

    It still makes me think that 2 flippers in antiphase would be better - after all I've got 2 feet that aren't doing much - has anyone tried this junkyard version of a mirage drive? Again, I'm just too lazy to read 500 threads to find out.

    Apart from these questions I feel a need to pass on some vaguely related experiences from trying to paddle an inflatable kayak/canoe in its un-"improved" state:

    However my experience of canoeing/kayaking is so limited I'm pretty sure I will sound like a nutter...

    I have a 2 man version, and with 2 people in it the waterline is somewhat longer than it is wide, so even if you are stupid enough to paddle the same side as your companion (I am) you can move forwards, and more importantly (for a lazy person) you continue to move forward-ish between strokes. It's not efficient, but it is tolerable enough to be more fun than frustration.

    But. When I am alone in it, the centre sinks, much of the bow and stern leave the water, and the waterline is almost wider than it is long, with the deepest part (the folded PVC under my backside) acting as a keel across the desired direction of travel. (NB. taming this with higher pressure is a No-No on the cheap ones).

    The result is that when I stop paddling it goes sideways - I've timed it - it starts immediately and it's turned 90 in less than a second. Slalom canoes do something similar but not quite as pronounced.

    I didn't want to put a solid floor in it and I didn't want to buy the heavy and overpriced skeg/fin upgrade, so I tied on a 30cm crosspiece with a whittled wooden plank attached - this gave me a fin with about 50 x 10cm in the water.

    The effect was startling - not only was I paddling almost a straight line, it was obvious I was travelling at least twice as far with every stroke.

    I realised that the back of the boat was still yawing a little but flexing just enough to turn the fin into a fishtail.

    I never want to paddle without one, and if I ever get a solid kayak I would want to try some flex in the fin/rudder or the mounting to have the same effect. I might even go for a near-slalom type to 'free the yaw', because I have a gut feeling that a solid keel isn't giving me that extra thrust when it blocks most of the yaw.

    Incidentally, some time later I was given the manufacturer's fin as a present - it is half the size and almost twice as heavy as the home-made version - it works OK with 2 up, but is virtually useless for a single. It is just too small - far too small to operate as a flipper and its fixing isn't stiff enough to do a good job of flippering anyway.

    Going back to the original question of flipper oars - it occurs to me that this kind of 'passive' flipper might be an answer to the excessive yaw from flippering one oar (you can see quite a lot of yaw on Harry Bryan's videos).

    An active flipper oar could be worked over and beyond a passive sprung rudder (flipper) like the one on my kayak - it might be a way to turn the residual yaw into useful thrust and reduce what's left. It could certainly be simpler than antiphase flippers.

    Sort of like a compound steam engine getting an extra push by running a low pressure cylinder off the first exhaust?

    Has anyone tried this? Any ideas on the sizes of the two fins? Is the 'passive' flipper a 'low pressure' system that needs a bigger area?

    I feel it could work as well switching active & passive - by working the tiller and using a fixed oar flipper to feed off the yaw (?). It would be a simpler system, and mounting the passive flipper to the side of the working flipper/rudder would be even simpler.

    Maybe a catamaran's twin rudders could be redesigned as flipper rudders with an option to release one to a spring as the passive flipper while working the other - or is that just too far - too crazy?

    If this turns into a five-way argument like the 'oars vs yuloh' one, then at least it'd prove I'm not alone.

    Regards.

    PS. re: the latter argument, there's a classic 'divided experts' case in "Surely You're Joking, Mr. Feynman!" - it's called the 'reverse sprinkler' (or inverse or underwater) - all the people involved were right at the top of their field, and only half of them were right about a deceptively simple device...

  2. #2
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    Default Re: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) st

    I find that folks attracted to building cats are attracted to this kind of thing.

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    Default Re: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) st

    Your double-spacing and "flipper" has stumped me. Can you tell us what a flipper rudder / flippering means? Do you mean kickup rudder and foil? Or a rudder that can be sculled back and forth for propulsion somehow? If the latter, I don't think that a yuloh blade just moves back and forth, it does a twist / scoop sorta motion that can be difficult to duplicate.


    Last edited by Thorne; 11-28-2018 at 10:46 PM.
    "The enemies of reason have a certain blind look."
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  4. #4
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    Default Re: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) st

    Quote Originally Posted by Thorne View Post
    Your double-spacing and "flipper" has stumped me. Can you tell us what a flipper rudder / flippering means? Do you mean kickup rudder and foil? Or a rudder that can be sculled back and forth for propulsion somehow? If the latter, I don't think that a yuloh blade just moves back and forth, it does a twist / scoop sorta motion that can be difficult to duplicate.
    I definitely don't want a kickup rudder - though I've seen people kick them up a little so the rudder is out flat under the surface, and then work the tiller to make way when there's no wind - it looks like it works better nearly up - probably the play in the hinges improves the angle of attack.

    I often rely on simple 10 flippers for propulsion in the water - I always keep a pair in the kayak - along with the sea anchor (a rice bag with strings). Together they are the safety system and also the system of last resort - I often get caught by an offshore squall that sometimes last an hour (there are a lot of persistent katabatics where I go).

    After paddling like crazy I can rest my arms by hanging my legs with flippers over the back (easy and comfortable on an inflatable) and flip away like crazy (with less wind drag too), and then back to the arms. If I get totally knackered I can deploy the sea anchor for a rest (but I've never had to). The absolute last resort is to abandon the boat - I'm usually in the Med so the currents are a very minor problem - with flippers (and no wind friction) I can cover a kilometre in 15 minutes without even getting out of breath.

    wizbang 13 is right - if you have a small sailboat then you might not have an outboard motor, but there's still a lot to lose if you have no auxiliary power at all - and rowing or paddling can be a bitch on a multihull (mine's a 3m proa - see inflatable proa).

    Adding a Hobie Mirage drive would be adding good sense - but also a lot of money - more than 5x what the rest of the boat cost.

    Harry Bryan's Thistle drive could be made for what? 50? and a couple of weeks of hard but rewarding graft.

    It uses what I'd call a flipper on the back - it would be a rudder/flipper (fludder?) if he used a tiller to work it but obviously pedals have advantages (if you don't have to move about trimming a sail and especially if you aren't moving end to end on a proa).

    Thistle is here www.youtube.com/watch?v=3PdOIXvfB3g with a similar (but yellow) boat here https://www.offcenterharbor.com/vide...ered-fin-boat/

    There are three reasons I don't follow too close behind what Harry built - that's apart from the desire to be awkward (Harry and others like him have already produced the necessary kudos by outflanking most of the problem - I hate it when problems are bludgeoned head-on):

    1) I only want occasional 'auxiliary power' from a usable rudder so I would prefer a simpler device - a hinged tiller (or just a tethered handle like a rear scull) but attached to a big swimming flipper - like a small "monofin" - both use graduated flex to get the tail of the flipper to behave like a fishtail, whereas Harry used hinges and springs to manage the flex.

    2) I want something quick I can try alongside a 'passive' flipper - means I'd be making two - so the simpler the better.

    3) I still prefer to stick to my original challenge brief - it either has to go with me on the plane without extra or excess bags - or I make it when I get there. 2 rubber, HDPE or fibreglass blades wouldn't break my baggage allowance, and there's enough scrap wood and cane lying about to mount them. Basic stuff like hinges can be cheaply bought from the local hardware store.

    I'd be really interested if anyone has tried the passive flipper? It definitely works with paddling but would it work on Harry's thistle?

    Regards.

  5. #5
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    Default Re: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) st

    several spring-loaded "flipper" designs turned up (eg. Harry Bryan's Thistle) that arguably offer a better way to finesse the ideal stiffness, and may even provide a better angle of attack through the entire sweep.

    But I wanted to see something like a large flipper or small monofin on a stick - preferably made from junk I already have or (probably better) could be sourced as boring materials for something entirely different (not "glamorous" stuff like sailing supplies, hobby stuff, or gardening, or any similar categories that always seem to cost 300% more).

    Basically I want to make something that could be used without a special rowlock or fixed hinge (so I could experiment with placement - starting with existing pegs, nooks, crannies etc.).

    And that would be shorter than a yuloh (so I can get it to the other end of a proa without braining myself - or have one at each end with one out of the water - but neatly).

    And it must be a pretty serviceable rudder when not being flippered.

    ......

    It still makes me think that 2 flippers in antiphase would be better - after all I've got 2 feet that aren't doing much - has anyone tried this junkyard version of a mirage drive? Again, I'm just too lazy to read 500 threads to find out.
    I've seen the two flippers in anti-phase on a small scale model - it worked well.
    Creationists aren't mad - they're possessed of demons.

  6. #6
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    Default Re: Flipper fin/rudder(s) for multihulls - with inflatables a special (shaggy dog) st

    Quote Originally Posted by P.I. Stazzer-Newt View Post
    I've seen the two flippers in anti-phase on a small scale model - it worked well.
    Thanks P.I. that's good to know, and the mention of a model is timely - I've forgotten about testing with models before and had to be reminded - but that was sailing models where the square/cube curse can play merry hell with trim - so putting a new jury-rig on an existing boat is a way to get more reliable data without the work of duplicating the whole shebang in miniature.

    However the active-passive flipper idea is a perfect match to a model - I have a kid's lifebuoy that will be the perfect keel-less hull and I can use an old battery toothbrush to drive the flipper. I can try giving the passive flipper more area, and I can fit & test a keel as the control and see if it is better or worse...

    The only problem now is finding the courage to be the "sad guy at the boating lake" one more time.

    Thanks again.

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