Some questions about dual pedal prop drive on a cat

Discussion in 'Boat Design' started by Deadweasel, Apr 13, 2015.

  1. Deadweasel
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    Deadweasel Junior Member

    I have an idea for a dual prop drive on a catamaran design (props on angled shafts about amidships, hulls 18' long with about a 6' beam overall).

    Now, I know I'm breaking almost every design rule in the book by jumping right to modeling the thing, but I'm trying to refine what it is we actually want, and imagery does a better job at sparking imagination and generating further ideas.

    The questions that have arisen while toying with design ideas are:
    • Since I'm not obsessed with speed or efficiency -I just want something faster and bigger than a crappy rental paddle wheeler- is there any advantage to using two three blade props driven by chain-to-shaft-to-twin-gearbox system? (pics attached below).
    • Is there a game-breaking issue with trying to use the multiple gear system from a bike to get higher prop rpm once started off? I'm thinking of a full mtb crankset with three chain rings and derailleur assembly. I've only seen one example of this and the video doesn't show the boat in actual operation and doesn't comment on performance.

    Our idea so far isn't intended to be a racer, but could be a nice fishing boat as long as it moves at least as well as a Hobie Mirage cat (I'm not a fan of the sit-on-top kayak design), so I don't mind sacrificing some efficiency and speed, so long as I don't get reduced to the velocity of an unladen sea cucumber. :p

    General overview of the concept
    [​IMG]

    Close-up of the drive and prop retract mechanicals
    [​IMG]

    Overview of deck spars and pilot seating
    [​IMG]
     
    Last edited: Apr 13, 2015
  2. Deadweasel
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    Deadweasel Junior Member

    Another question for the folks who are familiar with the mathematics surrounding such things:

    Because there are apparently so few sources for props at the scale a pedal powered boat lives at, is it feasible to take an existing design and scale it up or down to suit a particular design?
    For example, can I use a three blade design intended for a full size boat with a design similar to mine, scale it down to a size suitable for a testing model, scale it back up from the mockup to my full intended size, and expect performance figures to scale along with it all?

    I'm quite inexperienced in this field, but my gut says there might be unexpected effects due to the changing size of the active prop surface in relation to water density, which of course doesn't change appreciably by itself in freshwater.

    Can anybody confirm (or bust) my gut? Be gentle please, I just had breakfast. :p
     
  3. daiquiri
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    daiquiri Engineering and Design

    Nice model. :)

    If you can get 150 W continuous output from your heart, lungs and muscles (run to a gym and measure your state of fitness ;) ), then you will probably be able to glide over the water surface with ease at 5 kts on that boat, which is twice the average walking speed. The maximum speed could be something like 6-6.5 kts, depending on your maximum power output.

    You will need a good propeller, designed specifically for low power and high advance ratio. Some 80% propeller efficiency is the target here. I am not aware of any commercially available prop in this power range, so you might need to make one (or a pair) ad-hoc for this project. The propellers for RC airplane models will IMO not do the job.
    In the power range which you are considering (2x75W or 1x150W for a leisurely cruise) it won't make too much difference whether you will opt for one or two props. However, one bigger prop will be more efficient than 2 smaller ones. The transmission drive for a single prop will be simpler to manufacture too, and more efficient. So, if it was my project, I would opt for a single prop.
    You can expect some 98% bike chain and 95% gear transmission efficiency, for an overall combined 70%-75% propulsive efficiency here.
     
  4. Deadweasel
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    Deadweasel Junior Member

    daiquiri: thanks so much!

    I would much rather only have to deal with one prop if possible, so I'm more than happy to read that a second prop won't make much difference (even if I don't completely understand the why of it... yet!).

    I'm also pretty sure those hulls are a bit fatter than they need to be to stay shallow on the draft, but I'll know more about what direction I should be heading with them once I can get settled enough on the design to start putting together some floating models for rudimentary testing.

    As for the prop itself, yes I'm aware I'll probably end up having to fabricate my own, and that's a whole set of disciplines unto themselves, but one that I'm quickly feeling more confident about delving into after having read through other such forays here on the forums.

    For now, it's (quite literally) back to the drawing board to simplify and slimify the design a bit, then circle back to have a bit of a stare-down with JavaProp. :p

    Thanks again, and I look forward to sharing the learning experience, even if I don't wind up with what I intended.
     
  5. Gib Etheridge
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    Gib Etheridge Junior Member

    Sea Cycle will sell you a prop. You can get 12", 14" or 16". They are designed to be used as 1 prop per person at 1:6 ratio. I would start there with a 16" and if you find that it is a bit too much reduce the radius, which is what they do to produce the smaller diameter props.

    Your design looks good to me, similar to the Sea Cycle, which reaches over 13 MPH in a sprint, has made the Seattle to Alaska run and has done the English Channel. You should check it out, and if you actually build this rig I hope you will post pictures.

    If you post over on the WoodenBoat Forum you'll get a LOT more replies to your questions. It's about 40 times as active as this one, with lots of very enthusiastic members. It should be a wooden boat though for that.
     
  6. daiquiri
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    daiquiri Engineering and Design

    16" and 1:6 ratio sounds perfect for a good efficiency at 60-80 RPM cycling - which makes a very comfortable, non-tiring and long-range ride.
     
  7. Deadweasel
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    Deadweasel Junior Member

    Heh, it may actually be a cedar strip construction for the hulls, so it should qualify!
    Thanks a bunch for your input, it's good to have some confirmation that I'm at least moving in the right direction! I've lurked about the WoodenBoat forums too, an they are in fact the site I've been reading most about potential construction methods for the final project. I will absolutely post pics when it gets to that point.

    GE and daiquiri, thanks very much for offering some hard numbers I can start playing with. Also, I didn't realize Sea Cycle is actually based very near to where I expect to be in Michigan for the build phase (I'm in Las Vegas now). How cool is that??

    Okay, I'm off to refine the design some more. I'll start up a web album to post progress pics as I go, rather than flood the forums with attachments (I know some admins get a little vein standing out on their head when somebody creates a thread filled with massive images). :D
     
  8. portacruise
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    portacruise Senior Member

    Model RC props work well, especially if you are just trying to beat the common paddle wheel design. There have been several successful designs like: http://microship.com/bobstuart/spinfin.html There is a considerable loss of efficiency when cutting the tips of RC props from my measurements, better to go to a smaller size and/or pitch. The APC 16 X 16 or others up to 20" have been used successfully. Coach Dave on pedal powered boats forum is a good source for efficient non racing designs, and Rick Willoughby's posts and separate website are good if you plan to race and need super high efficiency. A way to quickly/easily clear weeds from the prop blades should be considered as even the best folding prop designs can get clogged to a stop.

    FWIW.

    Just my humble experience, may not be true for everyone's designs/goals.

    PC
     
  9. daiquiri
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    daiquiri Engineering and Design

    RC props are being used by waterbikers because they appear as the most-immediately available and relatively inexpensive choice. But they are not the optimum props because when working in water they are subject to higher stress than what they've been designed for. They will bend, sometimes visibly, so much that the angles of attack of the blade sections will be different than nominal ones - hence the geometry of a spinning propeller is different from what it is when it is laying on the shop table.

    And I have seen reports of permanently bent or broken RC plane props after folks have pushed the pedals with too much force.

    It happens because water density is so much higher than the density of air. For the same advance ratio of the prop the aerodynamic coefficients are the same, hence in theory a well-matched RC propeller should work at it's optimum point in water too. But though the coefficients are similar (not the same due to blade bending), the torque and the root bending moment are not. The root bending moment, in particular, is almost 4 times bigger in water than in air, for the same advance ratio. A 4 times higher root bending moment means that the propeller is working at 4 times higher maximum mechanical stress. If one is pushing the pedals too hard, the thrust force can permanently bend (or break, depending on the material) the blades at any time.

    So, if you opt to go for RC plane props, I would suggest to choose a propeller model with a nice fat structure near the root.

    Cheers
     
  10. WindRaf
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    WindRaf Senior Member

    hulls are interesting, but the efficiency no: any deviation of mechanical movement absorbs energy, including the change that I think you do not need
     
  11. Tiny Turnip
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    Tiny Turnip Senior Member

    Here's a link to Rick Willoughby's page on propellors:

    http://www.rickwill.bigpondhosting.com/Propeller.htm

    Note - it appears he will make you one of his folding props at a price.

    Here is his detailed information on fabricating your own props:

    http://www.recumbents.com/wisil/hpb/Prop_Fabrication/Propfab.htm

    My cat pedal boat has second hand reconditioned twin seacycle drives. I damaged one of the abs like seacycle props hitting an obstruction on the bottom of the lake on my first test - and now run one unit with a 12" prop and one with a 16" prop.

    The only difference in use is that at comfortable cruising speed, the 12" prop will pedal faster than the 16" prop. This works quite well for me, because my usual pedal boat crew is a club cyclist, and serious cyclists find a higher cadence more comfortable than the less fit among us, <cough> so, counter intuitively, I usually pedal the 16" prop, and Paul the 12" prop.

    There is no point in having changeable gears, because the prop is in a fluid, it allows 'slip' at the 'drive interface', unlike a wheel on the road. I'm not going to attempt a more scientific explanation than that, but perhaps someone with more expertise in hydrodynamics might?

    From a users point of view, you just pedal at your maximum comfort (for all day cruise, or sprint) and the boat accelerates until the various drags and resistances stop it going any faster! Hope that makes some sort of sense.

    I would comment that the angle of your prop shafts look a bit steep; again, something for someone with more expertise than me. Other pedal boaters who have fabricated their own drives favour right angle gear boxes (Mitrepak, IIRC? I see you have noted Mitrpak in your model - the gears out of angle grinders are apparently not robust enough) and/or Ricks curved spring steel shaft approach, which seems particularly magical when it is unstayed. I understand the shaft will have a limited design life because of the cyclical flexing of the shaft as it rotates.

    Oh, and I use Trice recumbent bike seats I bought from I.C.E.

    Great model, btw! :)
     
  12. Deadweasel
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    Deadweasel Junior Member

    Perfect! It's a veritable steal at about 77USD.

    (Again relying on my gut instinct here) it sounds like what you're talking about is an effect similar to "overdriving" one of those paddle wheelers, where the prop's thrust tops out relative to the combined drag, and spinning faster won't really result in more speed unless you can spin it fast enough to get the hulls to plane up. From what I understand of it, that won't be possible without a more aggressive and robust prop design, but that design would also be extremely difficult to operate via pedal drive for very long, so it's best to focus on maximizing efficiency with a prop pitch and gear ratio that's most appropriate for comfortable cruising endurance.
    Sound about right, or did I wander into a nest of water moccasins there?


    I've been chewing over that angle too, but I'm still toying with ways to level out the shaft without dropping the draggy gearbox below the waterline too, and without adding another drive train component (which would further increase drive friction). It's the downside to working with a high-riding cat design, I suppose.

    I'm not truly sold on the twisted chain system, though it seems like that would result in a better profile from a drag standpoint, if I have to dunk the point of force direction change under the surface to level out the drive shaft more.

    Then again, don't most ski boats use an angled prop? I suppose they don't have to worry so much about efficiency losses with those big gas guzzling engines, but either way, I agree my designed angle still seems a bit steep.
    Most of the issue is sourced at the point where I decided to try building in a prop retraction system for deck level access and driveshaft protection. Another arena of tradeoffs, I guess.

    Thanks for that call out, saves me from trying to fab my own from scratch! :p.

    I greatly appreciate your comments and links (and of course everybody else's so far too!) You've helped me to solidify some thoughts and ideas in a more useful direction.
     
  13. portacruise
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    portacruise Senior Member

    Re: Ricks props- designed for speed at 1:6 and easy driven hull.
    Rick will answer questions from builders on his website email address. If you send him your sketches he can tell you if the racing prop will work in a cruising endurance or whatever situation you might want. Notice that the blades resemble model RC props a lot more than seacycle props, but I don't know if that means anything.

    Re: RC Prop bending distortion/breaking- Yes bending quite possible under heavy loads and acceleration. Regardless, the 1:10 spinfin link previously posted indicates efficient performance at load from the numbers posted in the text section. The RC props can be tweaked/replaced cheaply and easily with the great variety of sizes available.

    Re: Shaft leveling/profile/angles
    If an unsupported/free flexible shaft is used the prop angle will always be horizontal under load, and water disturbance to the prop will be less than when used with a rear support bearing at the prop area. There are also other benefits as illustrated in vid post #53 & 54 below:

    http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-4.html


    Here's more on building power train short cuts:

    http://www.boatdesign.net/forums/boat-design/pedal-powered-boats-23345-87.html

    Hope this helps.

    PC
     
    Last edited: Apr 15, 2015
  14. philSweet
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    philSweet Senior Member

    One other problem that ocurs in pedal boats and not so much on road bikes is the awkward feel of the pedal going round a full rotation. In the case of a bike going along at a good clip, the bike's large momentum means the speed, and hence cadence, changes very little during a pedal circuit. On a mountain bike going up a hill, you get that surge and struggle effect as you pedal. Pedal boats suffer from that effect, not so much from the boat speed change as from the change in prop slip as the torque changes. The efficiency of the propeller and the comfort of pedaling are both compromised by the pulsing. I would design a provision for incorporating some kind of flywheel to egg-gear, or cam and spring loaded cam follower, device to the driveline which will absorb torque on the power stroke and return it on the weak point of the stroke. It can be added later if needed. It might also dramatically increase the life of a marginal gearbox by removing the backlash and lowering peak torque.

    The other thing I'd do is buy a whopping great chain ring, get all your gearing on the chain, say 81:14, and use a 1:1 gearbox. That ups your input shaft speed and gets your gearbox down a size. Your 2:1 box(es) are sized by input torque and power. The static numbers for the smaller box, say an R-101, should be good enough. It's about a wash cost-wise for a single gearbox, because the chain ring isn't cheap. But the efficiency should be a bit better.

    <<edit>> though I'd add this. You can get up to an equivalent of an 85 tooth chain ring with Speed Drive hub gearing. http://www.velocityvelos.com/products/high-speed-drive
     

  15. Tiny Turnip
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    Tiny Turnip Senior Member

    Eeee! very queasy here - wading into an area where I don't know what I'm talking about, under the gaze of plenty of people who do! I'd describe it more that as you apply more force to the pedals, the prop can translate only so much of that into accelerating the boat - which it does by carving/lifting forward through the water. There comes a point where it is easier for some of the water to squish out of the way of the prop blade, i.e. the prop 'slips' and wastes the excess force creating turbulent flow, rather than cutting forward even faster and driving the boat forward. I'd guess there is a maximum possible acceleration for a particular prop and hull combination.

    I think we're probably talking about the same thing from different angles...

    The twisted chain drives generally are, I believe, a couple of percent less efficient than the right angle gearbox. If you are going for 'fastest boat possible' this would be important, as would every other odd percent here or there, and hence the rationale behind unstayed prop shafts, dipping rudders etc.

    The seacycle units chain is finer than bicycle chain, to allow it to twist more easily, and seems to be of very good quality. I have had two difficulties with the seacycle units.

    1) It is easy to over tension the chain. This makes it bind; if you pedal hard with too much chain tension, the pedals simply stop dead; very disconcerting. The 'correct' chain tension is surprisingly slack and it took me a couple of seasons to get used to this. Its partly the fear of skipping the chain off the bottom cog and then having to fish about wildly down the inside of the drive unit leg with a bit of coat hanger to get it back on.

    2) The adjustment of the chain tension is a bit fiddly/techniquey, and the screws holding the top of the plastic case on are a bit rubbish - they simply tap into the plastic. When tightening the crank nuts to hold the cranks in the correct position against the chain tension, its easy to over tighten them, as you are pushing against the side of the plastic casing, and it distorts. This can have the effect of dislocating the crank bearing from the plastic case and you get the crank at a funny angle. It looks easy on the seacycle video though!
     
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