Pedal Boat Design

Discussion in 'Boat Design' started by BG_Geno, May 28, 2006.

  1. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

  2. John Hazel
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    John Hazel Junior Member

    Yes that seems better. You can go to much smaller tubing in such an arrangement. One guide to use is to ask if the structure would stay in place if there were swivel joints at all connections. If the answer is yes then you have axial (compression or tension) loading on the parts instead of bending loads. It takes a much heavier part to resist a bending load than and axial one.
     
  3. BG_Geno
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    BG_Geno Senior Member

    WOW...I never thought of it like that, as in your test. It seems that if both "A" frame members were cantered inwards (the front one tilted aft, the back one tilted forward) a little, say 15 degrees or so it would add some stiffness as well.

    I will have to rethink the structure I guess. Thanks for the input.

    Geno
     
  4. Willallison
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    Willallison Senior Member

    Also better for the braces to meet at the corners (intersections) rather than part way up the cross-beams as you have them now.

    I would 2nd Raggi_thors' suggestion of a little more volume in the ends - particularly the bow (I assume it's the bow) which you show as being very pointy: though for a different reason.
    For a displacement hull (as this will be unless you can paddle REALLY fast!:D ) longer is always faster - so you should aim to build the hulls as long and narrow as you can, whilst still maintaining their strength. It's waterline length that's important. Any overhangs are simply added weight for no real benefit.
    So I would suggest that you make both ends more like the stern - almost vertical.

    I didn't look at Tim's hydrostatics, but he's right when he says that you need to know the where the LCB is. You want to make the boat so that the LCB is located in the same place fore and aft as your passenger(s) going to sit. A boat will naturally trim so that the balance of the weights (the LCG - longitudinal centre of gravity) and the LCB (the longitudunal centre of buoyancy) are in line vertically. If they're not, the boat will trim until they are.
     
  5. BG_Geno
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    BG_Geno Senior Member

    The opperatiing enviroment for this boat will be the bay between Port Isabell and South Padre Island in south Texas. 2 feet is deep for the bay (sans the shipping chanel of course) and the water is seldom flat. Are you sure a little bit of bow "strake" (likely the wrong term, sorry) wouldn't help with smaller waves? The pointed bow/stern of your typical canoe does an OK job of splitting the water in the direction of travel, but it is also rather high--mine are not.

    As for your comments on LCB and LCG...I have searched these forums and there seems to be several definitions of what these terms mean. Could you please explain your point in novice/laymans terms--or point me to a resource that might clear it up for me? I of course want to build the best boat I can...

    As always, thanks for all the great input Guys.
     
  6. Poida
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    Poida Senior Member

    BG, after paddling an inflatable dive kayak back to shore in a head wind may I suggest that you contemplate paddling your craft in a reclined position with a nose cone up front to break the wind. When you are paddling or peddling you can't stop to take a break in a head wind as you could lose more ground than you have made.
    Poida
     
  7. BG_Geno
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    BG_Geno Senior Member

    It will be a recumbant seating arrangement as wind is common in our bay. That is another reason I had originally wanted to keep the seating/deck as low as I could. I have also contemplated a nose cone as well, for the stated reasons. The gains on earth based recumbent cycles are quite obvios when riding--as I am sure they would be on the water.

    Thanks for the input =)
     
  8. Willallison
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    Willallison Senior Member

    LCG - longitudinal centre of gravity: is the sum of all the weights - including the hull, seating, supports, etc, etc - in other words, everything. In your case it should also include people, as this will be the greatest single weight on board (no offense intended!:D )
    Your boat is like a sea-saw - nothing more than a plank of wood with various weights placed along its length. The LCG is the point of balance along that plank. If you could pick the boat up and balance it on your finger, then the LCG would be at the point where it doesn't tip down towards either end.

    LCB - longitudinal centre of buoyancy - is much the same thing except it's the centre of the submerged volume, rather than the weight. So if you had a rectangular box, the LCB would be 1/2 way front to back. Similarly if your boat's ends are ther same shape (so you have the same taper from the middle to each end) then the LCB will once again be at amidships.

    There are also VCB / VCG and TCB / TCG which are vertival and transverse centres as well, but you probably needn't worry about them...

    As far as your hullshape goes, it's not so much about rough water ability but waterline length that I suggest changing the bow shape. As I said, the longer the WL length, the faster the boat will go. The needle-like bows that you show will provide some protection against very little waves (though the will tend to slam a bit, which will slow you down also) but they will act more like wave-piercers in anything bigger than a ripple, so the boat will have a habit of nose-diving into waves. Better to have a more vertical bow (soemthing like you would se on a rowing skiff) and then incorporate some flare into the hull (so it gets wider towards the top) that way as the bow goes into a wave, the extra buoyancy will help lift it up.
     
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  9. BG_Geno
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    BG_Geno Senior Member

    See...those are laymans terms even I can understand. Thank you very much.

    As for the pointy bows, they of course would never be so pointy on the actual boat of course. I modeled them after the world record holder for pedal boats (almost 20 knots) and was approaching the problem from the wrong angle. I was seeing forward motion pushing the curvature along the bottom of the "bow" upwards over the waves. I see now that you can part waves like a canoe and use the extra displacement of the increased bow area to float over.

    Thank you very much. I will revise the hull design and repost.

    Geno
     
  10. BG_Geno
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    BG_Geno Senior Member

    OK. Moving on to proplusion.

    First issue. Prop size and number.

    There is a wonderful monohul design offered commercially called the Cadence that uses a single 3 blade 11 in dia prop. Cruise speed is reported at about 6 mph and the top end is 12 mph for athletic sprints.

    As I have a cat design, and the pedal drive train works better if the drive line splits pedal power between two shafts (one in each hull as you would imagine) I am wondering about the relative merrits of two props over a single prop with one "engine" (rider) and also with two. All my research so far shows that a human pedaling at the prefered 90 RPM can generate between .5 and 1 horse power for 3 to 4 hours without too much dificulty. How this translates to shaft horse power I do not know, but imagine it would depend on gearing, and I do have some controle over the gear/sprocket sizes.

    Second issue. Prop depth.

    At my relativly slow speeds and prop RPM, how "deep" does the prop arc need to be below the water line? I would like to keep the draft overall as shallow as I can. Would 2" between the waters surface and the top of the prop arc be enough?

    As always, thank you all for your input.

    Geno
     
  11. Poida
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    Poida Senior Member

    I would have thought pedal to paddle. Same axis. Stainless steel chain going straight down to the paddles from the pedals.

    To change axis of rotation requires gears that means loss of power due to friction
     
  12. BG_Geno
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    BG_Geno Senior Member

    Paddles like the old steam ships are simply not as efficient as a prop. The helical bevel gears in a modern gearbox are very efficient, quiet etc. Also, the weight savings would be offset by the weight/complexity of a paddle wheel and its shroud. Here is a LINK to the type of gear box I am thinking of using.

    Geno
     
  13. Poida
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    Poida Senior Member

    BG

    I usually make things by gut feeling, probably why nothing ever works.

    I would have thought that the reason for paddle steamers becoming inneficient was due to the introduction of the modern engine, ie straight line drive out of the back, making a change in direction to drive paddles, due to the loss in friction.

    Also, I don't know but just a feeling that a paddle would be more efficient at low speeds, suitable for pedals and a prop would have to run at the correct optimum speed, including correct pitch.

    I'm interested to know where you are going to mount your gearbox.

    The oputput shaft surely would have to be in the centre of the boat, which means the drive from the pedal would have to be offset?? Where's the pedals.

    Tell ya wot!! 2 peepl sitting sideways and opposite. V4 straight line drive to the prop. Now we're talking. Each foot pounding 8,250. 4 feet = 33,000 foot pounds. I'm sure at school they said that was 1 HP.

    Anyone for waterskiing?

    Gosh I'm an idiot.
     
  14. SolomonGrundy
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    SolomonGrundy I'm not crazy...

    Pedal boat

    BG,
    I saw someyhing like the boat you have pictured on the Human Powered Vessel thread. It is a long one but I too am designing & building a pedalboat, so I know a few of the challenges you face. Good Luck & let me know if I can be of any help, my HPV thread has some usefull info, & lots of extraneous gobbeldygook so finding info can be a task.
    I'm working on the driveline on mine & when I have the mock-up finished I will post it on a new thread as the old one has become cumbersome. Are you using a 90 degree bevel gear on yours?

    SG
     

  15. SolomonGrundy
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    SolomonGrundy I'm not crazy...

    gearbox

    Here's my gearbox. Precision CNC machined hardened stainless steel in an anodized aluminum casing prelubricated for life, I'm very happy with it.
     

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