Designing a fast Raft

Discussion in 'Boat Design' started by Vinny, Apr 20, 2011.

  1. Vinny
    Joined: Apr 2011
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    Location: Dorset

    Vinny New Member

    Hi Guys,

    Im new to the forum and was just wandering if anybody was able to lend there expertise. We enter a yearly raft race for charity, last year we built a catamaran style raft and powered it with our feet using the same system as a treddle cat. (all built from scrap). However, because we did so well and there were a number of complaints and as a result the rules have been changed!

    Paddle power is now the only form of propultion and i was therefore wandering if any of you had any ideas on designs for a light, nimble and fast raft we could build. We have to negotiate a number of weirs on route.

    Thank you for your time!
  2. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    Posting the rules would be helpful, maybe a website of the race.

  3. Vinny
    Joined: Apr 2011
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    Location: Dorset

    Vinny New Member

  4. Submarine Tom

    Submarine Tom Previous Member


    I read the rules. Great fun.

    Most importantly, have your raft finished one month before the race so the crew can get in LOTS of practice time.

    12 foot pontoons with structural cross bracing joining them. Tapered fore and aft, full width (6' 6" was it?). Cram as many paddlers as you can on each pontoon and go like hell. The paddlers feet will need to go under the knees of the person infront of them to keep them out of the water.


    P.S. Oops, I missed only four team members allowed. So, the pontoons don't need to be that large in diameter. Aim to get your waterline 40% of the way up the pontoon if you're going with a circular cross-section.
  5. Submarine Tom

    Submarine Tom Previous Member

    Assuming 200 pounds per paddler (including paddle, clothes and life jacket) and 300 for the raft, you'd need just over 1 1/2 feet of pontoon diameter to achieve 60% free-board.

  6. Submarine Tom

    Submarine Tom Previous Member

    Another consideration may be to add sliding seats. You'll then need foot straps. It will add weight, cost and something else to go wrong. Cost/benefit may dictate they're not worth it. You may gain more from specialized paddles instead OR as well...

  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Ignore the 40% displacement rule. calculate the weight of your crew with gear, make as close to semicircular shaped hulls as you can and add 4" of freeboard - to get the minimum boat. make the bow and stern 3-4" deep into the water, that will make it go straight, turns to be made by paddeling the wrong direction on one side.
    I have an 11 ft cat I made to row on, with 13" width hulls and a total 5' beam. There is so much wave drag interference in the middle that I had to widen the total beam to 7'. So use the max 6'6' total beam.
    How do you think you would build the boat? Plywood?
    Does paddles mean you could use oars like Submarine Tom seems to imply?

  8. Submarine Tom

    Submarine Tom Previous Member

    I did not imply oars Jr. The rules clearly indicate paddles only.

  9. philSweet
    Joined: May 2008
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    philSweet Senior Member

    That's a mighty persnickety rules committee you have there. I think they need to lighten up. I suggest you set some concrete goal such as getting them to change the rules again next year as a result of your exploits. I don't see anything in the rules about weaponry. How far would the separation between rafts be at two minutes? Water balloon trebuchets? Maybe get the local fireworks seller to sponsor you and set off fireworks as you float down the river?

    I see no draft or air-draft restriction. What about a 16' tall raft that capsizes after the start? Advertisers would love it! It could be designed to be very fast when capsized.

    Even if you don't remember it, you know you had a good time when you're told the host burned the registration forms and is denying all knowledge of the event. Works better if the start is moved back to happy hour and the event starts and ends at a bar.

    Because it's a boat race, you really ought to have a lawyer among the crew. Just don't let him play with the cannon. Trust me on this one. DONT LET THE LAWYER PLAY WITH THE CANNON.

    "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats." »H. L. Mencken

    Best of luck, Phil
    (veteran of silly boat races in the Florida Keys)
  10. keith66
    Joined: Sep 2007
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    Location: Essex UK

    keith66 Senior Member

    There used to be a huge raft race at Southend on the Thames estuary, for over 20 years it attracted anything up to 150 rafts which covered over half a mile of beach, fun rafts could be anything at all, then there were was the speed raft class, these evolved into catamarans around 30 odd ft long with 4 paddlers a side.
    Many of these were built with an aluminium rail section like a narrow ladder on deck to take the seats with plastic or steel drums strapped to them, the drums they used are 25kg ones, those with rims that interlock or are parralel so the joins can be taped over, this coupled with a long bow nose cone made a hull that was quite light but very quick.
    Other rafts had folded up sheet grp hulls & could not be called rafts by any stretch of the imagination.
    The start was a sight to behold, as the tide came across the mud flats & lapped the beach there was no holding them & very soon the start maroon would be fired. Industrial quantities of beer & general drunkeness made the event harder to keep safe & it was axed some years ago.
    Shame cos it was Great!
  11. Vinny
    Joined: Apr 2011
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    Vinny New Member

    Thanks for all the replies.

    We were thinking of using kayak paddles this year?

    The team that won it the year before (and smashed the course record) used a pallet, filled it with plastic bottles and expanding foam. They then fibre glassed the bottom. How would you work out the ideal width and length so that it sat in the water at the correct height?

    philsweet - we have spoken to the committee a number of times about the rules, as each year there is less and less scope for ingenuity. However, they wont budge because of the risk of polluting the river. We designed a whole mechanical system last year without the use of oil! All the rafts are going to be become very samey!

    Undecided on materials as yet - last year there was a team who used paper mache on chicken wire. Looked identical to two canoes! It had hardly weighed anything at all! The overall winner was two van roofs over lapped!
  12. BATAAN
    Joined: Apr 2010
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    BATAAN Senior Member

    Here's an ancient raft design that maybe could be adapted.

    Attached Files:

  13. Pericles
    Joined: Sep 2006
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    Location: Heights of High Wycombe, not far from River Thames

    Pericles Senior Member

    Get hold of cheap thick PIR.

    Bundle it together and carve a solid waka canoe with outrigger Bind the waka hull and outrigger components securelywith string. Reinforce keel and gunnels with old garden hose or what have you, secure as necessary with duct tape and wrap end to end with thick palletising stretch film and secure again as necessary with duct tape. The PIR might crack, but, it will be held inside the stretch film.

    Amas could be constructed from the stiff cardboard tubes that are used to roll carpets around. Carpet shops have to pay to have them removed. Seal the ends and waterproof by wrapping with stretch film. I suppose if you got enough them, you build the entire raft, but it would be heavier.

    Good luck.


    Attached Files:

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