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

  1. Submarine Tom

    Submarine Tom Previous Member

    Excellent post Ian. If I may add one thing John, the larger the prop the more efficient, generally speaking.

    -Tom
     
  2. JTA
    Joined: Mar 2011
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    JTA Junior Member

    Ian

    Thanks for the very detailed explanation that even I could understand. As for making my own props, I think it is probably beyond my skills (especially if it requires welding) but I would be interested in reading/learning how it's done. If you don't mind sharing your instructions my e-mail is jennacourt@yahoo.com.

    So I did a little test on my recumbant bike in the basement for the first time this year. With no exercise in months and after a long cold winter of over eating I found I was able to keep an average/comfortable pace of 75-80 RPM; generating approximately the same Watts (75-80). When I increased the resistance and maintained the same cadence (75-80 rpm) I was generating approx. 100 Watts but I don't think I could maintain that level for a prolonged period of time (at least not until I build up some endurance).

    I spoke to a guy at Mitrpak today who actually worked with Rick and Greg who was very helpful. Although they could special orderl/manufacture a 2:1 gearbox that would accept the bike cranks directly into the box (eliminating chain) the price was nearly double that of a conventional right angle box. Being my first endeavor, I think I will go with the less expensive option. Although I did not order it yet I feel pretty sure that I will end up getting the 2:1 gearbox and a sprocket combination yielding approx. 4:1 for a total of 8:1 from prop.
    to crank.

    Now for the hull size. I know you suggested going to 18' but I was leaning towards 16' for the simple reason of one less seam (joining of plywood sheets). Any estimate on what I would be sacrificing on 2' less? Is it worth the effort/labor to glue/laminate another section to get to 18'? If you think the effort is worth it and I have to do another seam; how big should I go to maxamize speed. Do I go to 24' (3 sheets of ply joined together)? Is there a point of diminishing returns?

    If I was to stay to all of these parameters (possibly changing length based on your response to above):
    Cat hulls 16' L X 1.5' H X 1' W (4' spread between hulls)
    8:1 Ratio
    75-80 cadence @ 75 watts

    And I was unable to make my own prop; would I be able to use a 16x16 APC prop as described in earlier posts? If not any idea of what size I would need?


    BTW: I purchased the ply (enough to go bigger if I want) and ordered my epoxy/glass materials and plan on starting construction as soon as they arrive (maybe this weekend). It's still only in the 50's here during the day but I hope to have it completed by the time the warm weather gets here.

    Thanks for all your help
    John
     
  3. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    John
    Go for the 16' hulls, not a huge amount of difference. If you were racing it would be different but a cruising/exercise boat is ok. The hull sizes will give you plenty of reserve bouyancy, allow you to carry extra gear. I would make the beam 6' as this will give you better stability and less drag caused by the waves off the other hull. One thing that might help is an access hatch on the deck of each hull. They have the dual purpose of draining the hull if you get a leak and also as a storage place to keep stuff dry. Good luck with the boat building.

    Ian
     
  4. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Some builders have used an angle grinder gearbox in their boats, so that might be a cheaper alternative, especially if you can find a discarded one with a burned out motor. Probably would not work for racing, but should work for exercise purposes.

    The 16X16 should work well enough for exercise purposes, or you can order just the replacement props from some commercial versions like seacycle if you need something more rugged. You can also use some of the commercial versions as a model for gear ratios. These are rented out at many water vacation spots and you can get a feel to see if it is more or less in the range you like. Then you can tweek the ratios a bit to fit your output more exactly.

    Hope this helps.

    Porta

     
  5. JTA
    Joined: Mar 2011
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    Location: new york

    JTA Junior Member

    Ian/Porta

    Thanks for your input/suggestions. They have all been very helpfull.

    John
     
  6. upchurchmr
    Joined: Feb 2011
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    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    John,

    I haven't followed the thread well yet, but got attracted by the question about the hulls. I built a catamaran row boat that has 11' hulls which is stripped planked (~3/16" strip thickness) with 6oz cloth inside and outside. max section above water is 6" wide by 4" freeboard, below the waterline is a semicircular shape bow to stern (except for the ends which transitioned to elliptical Mostly) the hulls support my 125# wife and are extremely easy to drive, no noticable wake. This kind of construction was relatively fast and each hull weighs about 18#. I'm a little messy so you could probably do better on weight. crossarms are 2# each, total width is 5 feet. Be carefull that you don't get the hulls too close together, I did a previous boat with 12" wide hull width at 5' beam and it was a dog with noticable wave interaction. If you want a really clean path thru the water, go semicircular. The simple mold was built in 1 weekend, the hulls were stripped in about 3 days each including decks.There was a trick to making the strips. I am pretty slow with the epoxy so that took too much time. You could see the boat in the thread on woodenboat.com forum, boat designs, "skinny, light rowboat". If you are heavier the length would have to be longer assuming you want to keep ~6" width for low wave making. I could give you a length guide if I knew your weight and what you think the entire boat would weight. Good luck.

    Marc
     
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  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    John,

    After reading one of the last comments I have a suggestion. don't put in an access ports. My first boat had two for fastening the crossarms and they were the continual source of water leakage. The second boat has no openings and has never leaked (2 years). Don't get too much displacement for your weight or you will be like my first boat - totally unable to hold a course. be sure the ends are in the water, the second boat is ~1.5 inches under front and back and is a easy boat to keep on course.
     
  8. Submarine Tom

    Submarine Tom Previous Member

    John, good point on the 6 oz cloth, that was a typo, I didn't mean 16 oz.

    Semi-circular (cross-section) hulls for sure.

    -Tom
     
  9. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    Hull Shape

    I agree that the semi-circular section for the hull, gives you the smallest amount of wetted surface area. John mentioned he was building his boat out of sheet ply, difficult to make a half round section. A long thin hull 16' long x
    1' beam can be easily built by having a rectangular section, the difference in performance would be minimal. The wave interaction between two cat hulls could be minimised by having two assymetrical hulls, the inside edge straight and the curve on the outside. Waves generated between the hulls would be small, might even make construction easier.
    The access hatches I mentioned are about 150-200mm in dia. I have had a leak and this is the only way to get the water out short of cutting a hole. Its like getting a flat tyre, hardly ever happens but when it does thats when you need it.

    Ian
     

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  10. upchurchmr
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    upchurchmr Senior Member

    Ian,

    If you look at drag for airfoils it might suggest why you should not make an asymmetric set of hulls. A symmetric foil will a certail CD at zero angle of attack, straight ahead. when you add camber, or bend the airfoil, the graph of lift to drag moves over to the left, which means that at zero angle of attack, the CD has gone up. Since you have two hulls you get twice the penalty.

    Being fixated on semicircular hulls, I can't quite understand how to build the hulls you drew. Of course, a square section flat bottom would be just as easy symmetric of asymmetric.

    If you really want a square section, have you ever seen how Jim Brown built his searunner series of Trimarans? Basically he built the shape he wanted out of single curvature plywood, but did not connect the adjoining edges. then he rounded the mating edges as much as possible, leaving just the inner edges touching each other. Fiberglass the outside, fill the inside with epoxy and microballoons (for lightness) and fiberglass the inside. The result is a nicely rounded hull from plywood without compound curveature. His boats go around the world.

    Marc

     
  11. upchurchmr
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    upchurchmr Senior Member

    Paddle wheel fans,

    About 10 to 15 years ago a group of bicyclist decided to win some local boat races for HPV. The boat ended up about 25', 8 feet wide (to fit on a trailer), 4 road race bikes welded together on top of a frame which supported twin inflatable huls. The paddle wheel was ~6' dia and ~7.5' wide. I never got to see it in action, but it was reported to throw a rooster tail about 30', went over 20 mph, and was really hard to turn. It was ridden by some dedicated road riders. Needless to say it demolished the competition and was retired after several years, because no one would enter the races.

    One of the riders told me that each blade hitting the water sounded like a small shotgun, in rapid succession.

    Brute force but I would have like to have seen a race.

    Marc
     
  12. I57
    Joined: Feb 2008
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    Location: Melbourne, Australia

    I57 Senior Member

    Cd?

    Marc
    Hate to sound dumb but what is the CD and what sort of forces does it apply to the hulls. The boat in question is for recreational purposes in sheltered waterways. A boat that is long and thin with a square section performs well and is easy to build. For this type of hull and where its used there is not a lot of advantage in a semi-circular hull. What I had in mind with the assymetric hulls was to reduce the waves between the hulls and keep the person dry.

    Ian
     
  13. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Ian,

    Sorry about the CD bs.

    Cd is coefficient of drag. One of the parts of a drag equation would be a number that shows the relative effect of the shape you are using. The lower the Cd the less drag for a equal size foil section. I guess you would have to trust me (Im not sure I would if I was you - except this time) but all that means that the curved shape develops more drag than a symmetric one. I guess as usual the only way to really prove it would be to make a tow test. Not very likely I suppose. I can assure you with a good shape 6" wide with 4" separation, you will not see any appreciable extra drag between the hulls, even with a symetrical shape.

    I am sure the square section hulls work, I would just rather go faster for my exercise effort. Different strokes and all that. My wife kept saying she wanted the exercise and was fine the my rather pitifull first attempt. Now with a better boat she says the same thing as she is leaving me in the dust, spray, whatever. Sounds like you had the same goal as I did. One of my design constraints was to take the boat to the lake in my pickup truck, and have my wife be able to launch it. 11' long works fine for her but I sink it almost to the decks. did you take a look at the boat? It is on the forum at woodenboat.com unders design/plans, "Skinny, light rowboat".

    Good luck, Marc

     
  14. xj35s
    Joined: Nov 2009
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    xj35s Junior Member

    I am going to derail this thread a little. I have a different issue. Big boat like a tug boat. I found these extremely cheap fan blades and was thinking of using two mounted solid with a rudder behind each. I wonder how these would run in water and if I could actually turn two of them by pedal power. I have the drive line consisting of a large Craftsman angle grinder gear box. I would link the two props together by bike chain inside the hull.

    The price is almost free. I'm thinking maybe running as surface piercing so maybe 3/4 the prop is in the water. Think I could turn one around for counter clockwise props ?

    I know this is Goofy and kind of far fetched But seriously, what do you think ? I could consider a deep cycle and electric motor to power these too. Will they push my flat bottom 250# chugger ?

    I found the blades here. http://www.sciplus.com/category.cfm/subsection/18/start/34/maxrows/11/srch.fp/1
    [​IMG]

    4'x4'x12' basic pocket cruiser.

    [​IMG]
     

  15. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    These fan blades are very inefficient in water compared to model prop blades which are available for $2 to $15 depending on size and pitch. I did try something like this blades and there is just to much resistance and they must be geared down quite a bit. You hull is also looking to be high resistance with the boxy shape.

    Hope this helps.

    Porta

     
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