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Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    hi Burnley
    Building Crusher for the raft race was a total blast - has got me hooked on boat building. I'd do it just as Rick says- butt the ply and use an internal splice (we lapped the ply and needed a fair amount of puraflex bodging to get the lap joints not to leak.) we Kerfed the chines and gunwales at the bending points which was worth the extra half an hour of carefulish sawing.
    On the length thing- have you considered a single long hull built in a front and back half that bolt together, ( Close each half with a watertight bulKhead and bolt them together.) and break down for transport and storage? I'm not going to bang on, but if it werent for navigation problems and some rule bending, 4 very little boys in a 15foot outrigger mono hull would have totally trounced 4 serious Karate heads in an 8 foot cat.
    Where's the raft race? We might have to popover and have a tilt :)
     
  2. Guest625101138

    Guest625101138 Previous Member

    Your prop would be OK for a test. Give you some idea of potential and can be compared with an optimised design on paper to see if it is worth the trouble of making another. The hull length is the man limitation on performance.

    One thing you could do is have the two engines do some performance testing. Get to a gym on a recumbent machine and see what power level you can sustain for 3 minutes. Also find the preferred cadence. Most machines can be set in power mode and the load can be adjusted to get the best feel. I was assuming 300W each at a cadence of 80 based on what you said.

    The universals will be OK if strong enough. The should be rotationally offset by 90 degrees to get smooth even torque. Also the cranks should be offset by approximately 90 degrees so the dead spots do not coincide.

    Rick W
     
  3. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Tiny, good points on the length issue. The concept could be extended to make a versatile combination of boats. If the two pieces that make a long 30' hull could be built to nestle inside one another, you could get a lot of possibilitiies.
    1.1 Very long narrow hull racing boat when bolted together lengthwise.
    2. 2 individual separate narrow hull runabouts.
    3. A high stability catamaran with two narrow pontoons when bolted sideways.
    4. Inner and outer hulls nestled together could be easily transported by auto or over water.
    5. There may be some others, if fitted in a clamshell fashion?

    Porta
     
  4. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    On your spring steel question, we have 4' lengths of what is called piano wire or music wire in very large diameters available at hobby shops here. It is very springy, but I don't know if the specks measure out to what Rick uses. The upper section of the shaft can be regular high strength tube and does not need to flex as referenced in a previous post by Rick.

    Hope this helps.

    Porta


     
  5. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

  6. burnleyfc
    Joined: Aug 2009
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    Location: Ireland

    burnleyfc Junior Member

    Good idea.

    I still don't think that I'll go with longer than 16'. Anything longer than that will look silly compared to the other rafts. You have given me an idea that I could make the 16' hulls in two 8' sections. That way I could easily add more length later by adding another section between the two. I'm not sure how I could bolt them together though; since the hulls are sealed it will be hard to get inside for access to the bolts.

    The race is in the west of Ireland so it would be a bit of a trek for you. It's not too far from the sea so if you're feeling brave you could take to the irish sea in Crusher.
     
    Last edited: Sep 4, 2009
    1 person likes this.
  7. Submarine Tom

    Submarine Tom Previous Member

    burnleyfc,

    Wouldn't look silly at all if it was crossing the finish line first now would it?

    Go longer and bolt it together.

    Only my opinion, but you do want to win don't you?

    Good luck, Tom
     
  8. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

  9. burnleyfc
    Joined: Aug 2009
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    Location: Ireland

    burnleyfc Junior Member

    Rick, when you say "under one of the hulls", how important is it that the rudder is actually UNDER the hulls? Is it ok for it to be off the back but deep enough so that the rudder is only acting under the hull? I hope this makes sense.
     
  10. Guest625101138

    Guest625101138 Previous Member

    You need a larger immersed blade if it is hung off the back of the hull. Say 300mm deep and 120mm long will work OK.

    Rick
     
  11. Skipjackrick
    Joined: Sep 2009
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    Location: San Antonio, TX

    Skipjackrick New Member

    Trimmer Drive Cables

    Has anybody ever thought about using the Drive cable from a Gas Weed Trimmer to drive the prop? A flexible Drive shaft..

    We could eliminate the need for a gear box if we went this route correct?

    However, would it hold up to the stress?

    I found a source here.

    https://sdp-si.com/eStore/Direct.asp?GroupID=454
    https://sdp-si.com/eStore/Direct.asp?GroupID=422

    Oh, and this is my first post. I have enjoyed reading all of the comments. I am just now getting into HPB's.
     
  12. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Drive cable does not eliminate need for a gear box if using efficient low speed propellers. Even when used with a gear box, there will be frictional losses when using a drive cable, plus corrosion and muck seeping into the sheath area, and the need for structural support so the bends don't pinch too tight...

    Hope this helps.

    Porta

     
  13. burnleyfc
    Joined: Aug 2009
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    Location: Ireland

    burnleyfc Junior Member

    Thanks Rick.

    When hung off the back, should the top of the blade be level (or lower) than the bottom of the hull so that the blade operates in clear water? How would a rudder in between the hulls work?

    If the rudder were placed under the hull, where would be best place to put it?
     
  14. Dennis A
    Joined: Aug 2009
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    Location: Amersham bucks uk

    Dennis A Junior Member

    Burnleyfc

    On my 10 ft pedal cat I have mounted the rudder between the hulls and the stearing is very sharp. The size of the rudder is what makes the differance, to large generates drag, to small and you can not turn tightly

    Dennis A
     

  15. Guest625101138

    Guest625101138 Previous Member

    Yes has been considered.

    With speed step up around 4 you get a nice size prop to run around 300rpm. Working a prop at cadence, say 75rpm, would require a large pitch and quite large diameter.

    The torque on a crank is quite high, typically around 20Nm and will peak 4 to 5 times this. So the cables are on the limit but workable for a while.

    The biggest problem will be the feel and power delivery. Any torsional compliance results in high RMS power level compared with average power level. It can take an eternity to get the pedals through the flat spot in the crank as the shaft unwinds. It is like pedalling in mush.

    It could be worth a try in conjunction with a speed step to avoid a right angle drive.

    Rick W
     
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