Hull speed MPG - Monohull vs pontoon

Discussion in 'Hydrodynamics and Aerodynamics' started by parkland, Feb 3, 2013.

  1. parkland
    Joined: Jul 2012
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    parkland Senior Member

    I keep reading conflicting and incomlete information.

    For a houseboat sized boat, for low speed travel, is a monohull or pontoons going to provide better economy?
    Are pontoons generally effecient at low speeds?

    There are plenty of large (40 ft) trawler style boats that can operate at 6 liters per hour or so and still get 6-7 knots,

    What could be expected from a 40 ft pontoon houseboat with a diesel as well at the same speed? (8 ft wide, 40 ft long, 36" inch diameter pontoons)

    I am hoping the economy would be even better, but a few comments I ran accross lead me to believe otherwise.
     
  2. Fanie
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    Fanie Fanie

    It depends what you mean by "pontoons".

    If they are well shaped displacement hulls then they will be hard to beat by any other hull shape. The advantage of a catamaran setup is you are not bound so much by width, iow you can easy go 6m wide hence win the space. They handle poor water conditions very well, very stable and is fuel efficient right up to hull speed, which allows smaller motors or motors not having to run full throttle all the time.

    A mono hull will make more drag if it goes wider and if narrow will roll more and have less space.

    Of course if you plan to use drums to form a "pontoon" even a mono will outperform that.


    I need to add - I mentioned smaller motors but beware that if you have a windy craft and the wind is up it's going to push you a LOT. You need enough power to turn into the wind and still make way. Invest in a decent anchor. My friend has a 12M x 6m party boat and he has 2 x 160hp motors on it - if the wind is up those motors grunt to overcome the wind ok, otherwise in tame conditions they merely idle...
     
  3. parkland
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    parkland Senior Member

    I was reffering to aluminum "off the shelf" pontoons.

    I have been scouring for information. and it seems that monohull trawlers can easily manage 5-7 kt speeds even into the 40+ ft range, using 1-1.5 gal/ hr. Or SOME can, anyways.

    My exact question, is let's pretend that we find a trawler that is 44 ft long, 14 ft beam, and can push 7 kt's at 1.5 gal hour with a cummins 6 cylinder with 24" prop.

    Now if we built a pontoon deck boat, 44 ft long, 14 ft wide, 36" diameter pontoons, and gave it the same cummins engine and 24" propellor, how much fuel would it use at 7 knots ? (Assuming it is loaded with 15,000 lbs of cabin and equipment, the 50% submersed mark...)



    Assuming everything else is equal... no waves, etc.
    So basically, 2 identical boats, just one is built on a hull, and the other is built on a pontoon deck arrangement.
     
  4. Fanie
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    Fanie Fanie

    Well shaped hulls vs poorly blunt shaped hulls that has to push part of the water in front of it to displace it I'd say less fuel at almost double the speed.
     
  5. yipster
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    yipster designer

    "depends" a pro would reply :D i'm no NA pro but let me theorise; 2 hulls have less frictional drag than 3 but more than a single, aint that simple tho. Drag is made up under speed, so what speed, and consist of frictional, wave, form, residual, and windage, depht, and more. And not all drag builds up equal, slow its mainly friction but the wavedrag a monohull encouters at "hullspeed" takes a hump of power to have the boat step the wave and start planning. Cats (and tri's) with hulls with a high LB ratio say 10:1 LB (placed wide to avoid crossing wavedrag under the bridge) cut the bow wave much better with sharp bows and in displacement mode easely pass their suposed hullspeed in displacement mode and with less power. Betcha i missed things and said something wrong like over at the "any hope" thread but by keeping our loud mouths shut we wouldnt have a discussion. Offcourse the black fat line humpzone for a 24ft boat isnt 18knts, thats a bigger boat! its not even 18 km. was trying to show how things hang together and even started wondering if that power curve should not be higher at the hump. that power calculatero foind at wooden boat forem were some people try'd and liked, I still have to play and like to check MM sum. Late here and seeing 49rs fumble made me take a sleeping pill, bloody ravens! :mad:
     
  6. parkland
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    parkland Senior Member

    But I meant low speed, like 7 knots for example that many monohull trawlers exibit ecellent economy; how would a pontoon variant compare?
     
  7. yipster
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    yipster designer

    49rs vs ravens 29 to 31 1/4 to go keeps me awake. Should ha e mentioned displacement for a mono is lowest, surface should be to and drag up to 7knt for a 40ft boat, what hulls, appendages, you really have to test with forexample mitchlet, my guess all things beeing even that might make a tight comparison, mono lighter, cat slender, dont know, i put a buck on the cat. going faster the answer is clearer. Try to catch a canoe with a rowboat :p
    one more thing: 7knts IS slow if you cant go faster
     

  8. haribo
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    haribo Junior Member

    at this relativ-low-speed (fn~0,3) is a monohull better than a catamaran, the bow wave plays not the first role at this speed and the underwatersurface is smaler...... less drag for the mono

    you need abaut 20 m of the 50%submersed pontons d=0,9m for your load , so two hulls with ~10m and space for a good formed bow and stern...... 14m long sounds good, 50%submersed is the best for a slow hull

    for a perfect catamaran with 7to and 14m long you will need 25hp for 7 kn, for the not so perfekt ponton katamaran maybe 50hp

    but with this length and wight even a perfect trawler need a bit less than 25 hp

    is a cummin 6 cyl. with more than 230 hp (?) perfekt in this case?

    so if you seek the efficentest solution for 7kn look for a smaler engine and the perfekt prop..... and ask for the best hullform for fn~0,3

    most trawlers are optimised for a higher fn
     
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