Fast raft design

Discussion in 'Boat Design' started by KiwiAndy, Jan 1, 2011.

  1. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    I don't know what the proper distance is for inline props (might try posting over in the 'propulsion' forum), but consider the Yamaha TRP design (twin-rotating props, on a very few select outboards). The props are immediately adjacent and inline to each other, and are COUNTER-ROTATING. Anecdotal information indicates that this setup produces uncanny holeshot (thrust) by virtue of grabbing the water better. Many shallow-water flats anglers run them on their boats because they have the ability to get the boat on plane very quickly in very shallow water - they call it "4wd for a boat". Anyway, with the limited HP of your craft, this 'hi-thrust' at low-speed may be advantageous, so you might consider making your props counter-rotating, and see if you can't build the setup so that you can adjust the inter-prop distance in the field for testing purposes, and then please let us know how it works out!

    I wouldn't bother with protecting the chains. Modern chains are quite durable. If you are using long-term in a salty environment, be sure to rinse them down with freshwater after each use. Also, use a good chain lube (check boutique bicycle shops or motorcycle shops for proper chain lube) - your basic silicone spray or graphite spray at the hardware store won't quite cut it for performance use.
  2. Submarine Tom

    Submarine Tom Previous Member

    Contra-rotating props of this nature are best spaced about half a diameter apart with the following prop about 10% smaller in diameter and of the same or slightly higher pitch, depending.

  3. Submarine Tom

    Submarine Tom Previous Member

    Turn one bike around for greatly improved visibilty, safety and balance.

  4. Submarine Tom

    Submarine Tom Previous Member

    You'd be much better off with one bigger prop than two.

    You'd be even better off with three times the length and five times the manpower, paddling.

  5. viking north
    Joined: Dec 2010
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    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    Kiwi Andy, Just completed one for my dock float/mini work barge. I bought a used 30ft. section of catwalk that siding installers used and cut it into 15ft sections. 20gal plastic soap drums from the car wash fit nicely into the cat walk rails when they (the rails) are inverted, making two pontoons. I then strapped them in with stainless 3/4 strapping using a banding machine to pull it tight and crimped it in place with stainless crimps. Rope or tie down straps could also serve the same purpose.I then fastened a few used truckers cargo jacks (square stock alum) as beams across the two pontoons. Decked it over with re cycled patio decking leaving 1/2 seam spaces to act as shock absorbers(water squirts up thru lessening the impact stress) To improve the bows i am now making up two fiberglass bows that will slide on over the ends of the front drums . If you use 45gal drums as several guys here tried to use for their floating docks they are too boyant and as a result very lively. The whole thing is made from recycled materials not to be green but it was all so economical. Hope this will give you some ideas, Geo.
  6. DaddyDog
    Joined: Aug 2010
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    Location: Windsor, Connecticut

    DaddyDog Human Power Enthusiast

    Here is a very simple exercise bike powered propeller.

    Here is a very simple exercise bike powered propeller that I built. I welded a section of black pipe to the bike's crank housing. The end of the pipe that dips into the water has the typical male tapered thread on its end; which I cut eight slits in with a hacksaw. I then used a female pipe coupler with a typical internal tapered thread. As the female coupler is tightened on the end of the male pipe it collapses the pipe and acts like a collet which in turn holds the exercise bike down tube securely. In the attached picture you will see a chromed down tube pipe sticking out of the end of the black pipe. This down tube is actually the metal tube from an exercise bike that goes from the seat to the crank housing. All the rest of the bike was cut away from this frame tube. This allows for extending and retracting the down tube pipe so as to tighten and/or loosen the chain slack between the pedal bike sprocket and the lower end sprocket that is attached to a driveshaft & propeller. It also allows for alignment adjustments too. I got lucky as the two separate pipes fitted together snugly. Once I got the assembly tight and aligned I cross drilled thru both pipes and added a bolt and nut to secure everything so it would not shift. I also added a seat clamp for some redundancy to hold things secure. The smaller lower sprocket is off a snow-blower shaft chain drive and has been cut thin enough to allow a bike chain to run on it. I used two cheapie hardware store bearings in the chromed crank housing. These cost about $4 dollars each. I then used some shafting to mount the smaller sprocket and go thru the bearings and then mounted the propeller. I have used tow props. One was home made out of a pulley and some thick metal. It worked but wasn’t too great. The next prop was from a Sea Cycle drive and cost $38 dollars. This worked much better. I added some cutting board (nylon) chain guides to keep the chain from derailing. One was mounted up high by the pedal cranks and the other down low by the prop sprocket. Without these the chain would not stay on the sprockets. I also added some wood to the bike legs to aid in mounting this contraption on my raft's deck. Using cheapie ball bearings is a must as they are loose enough to allow the water to enter and lubricate them. After each usage they have to be removed and oiled and then dried out before reassembling them or they’ll rust up solid. See additional pictures of pedal station mounted to the raft deck.

    Attached Files:

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

    Tiny Turnip Senior Member

    six drums as an outriggered monohull will be faster than 2x3 as a cat.

    six drums as a monohull with reasonably made outriggers, trimmed so the outriggers are just above the water when loaded, should still be faster than 2 x 6 as a cat.


    The pedal powered cat on the left of the picture uses professionally built Dart 18 hulls (about 17ft waterline), recumbent seats, and commercial seacycle prop drive units, one with a 12x12 prop, and one with a 16x16 prop. (more on this later) and was an unhurried workshop build.

    The outrigger canoe on the right was built with hand tools, while camping, in 2 1/2 days.
    It is 15and1/2 feet long.

    For very short sprints, the pedal boat maxes at about 5.2 knots, and the canoe at 3.9 knots.

    The pedal boat does 'all day cruise' at about 4 knots, and the canoe at about 3.

    So, although the pedal cat is faster, all the bits in the water are professional/commercial; the Dart is obviously a very fast boat in its original form, and the hulls are much more slender and slippery than achievable with ply by hand, or plastic drums.

    The greater waterline length will also give the pedal cat an advantage.

    My co pilot on the pedal boat is a serious club cyclist. I am an unfit biffer.

    We ended up (counter intuitively) preferring Paul (club cyclist) on the 12x12 prop, giving him a natural cadence of around 90rpm, and me on the 16x16, getting a comfortable cadence of around 60.(both 'all day cruise' mode)

    The seacycle units are geared at 6:1.

    My money, for winning the race, would be a paddled outriggered monohull, as long as possible, (up to about 20-25 feet max) (as long as the course is not too wiggly) and put your energy into making good, efficient pointy bows and sterns, without the complexity of home making pedal drives, with more to break/go wrong under stress, and set up efficiently (chain tension, prop bearing losses, drive leg drag...)

    But... I reckon there's more fun to be had from the pedal powered cat, both in the build and in the operation! :D

  8. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    While this is true, it has been my experience attending cardboard boat races, etc. that stability is the main issue in winning races, not fine increments in speed. With an outriggered monohull, a well-polished team of paddlers with a LOT of experience paddling together can easily pull off a win; however, chances are the team on this raft will have done little paddling together - and that would manifest into a poorly-performing vessel with outriggers constantly dipping to and fro.

    Now, if the OP and his team-mate have been paddling canoes together for a few years, then by all means go with a monohull! Otherwise, go with 5-6 drums for each hull of the cat. :p
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