Inflatables only: Catamaran vs Monohull

Discussion in 'Boat Design' started by edsteveed, Feb 6, 2015.

  1. edsteveed
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    edsteveed Junior Member

    Inflatables only: Catamaran or Monohull for sail and solar motor?

    Dear forum,

    I know about the advantages and disadvantages in general. And i am kind of new to this but i think it is a bit different when it comes to <5m inflatable sport boats?
    Inflatable catamarans <5m dont have more space than a standard inflatable for example cz they are not wider. But my highest priority is to get as less "water resistance" as possible.


    [​IMG]

    VS

    [​IMG]


    If i put lets say a 5HP engine on both boats with the same size. A catamaran version and mono (with keel) version. How big is the difference of the speed? Is it just 3% more or 30% or none? Is there a big difference in stability?

    My plan is to power the boat with a electric engine (battery charged by solarpanel which is mounted at the front), and a small sail. I run the boat on a 100km long lake.

    Before i buy the cat-version i prefer to ask people who know about it :)

    Thx
    ED
     
  2. FAST FRED
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    FAST FRED Senior Member

    The narrower conventional boat will row easier when the batt dies.
     
  3. edsteveed
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    edsteveed Junior Member

    I will repeat the question again:

    How big is the difference of the speed? Is it just 3% more or 30% or none? Is there a big difference in stability?

    @FAST FRED
    If no wind no sailing.... no gas no engine... no batt no electr. motor.... That's why i choose sail AND solar.... so whats ur point with ur answer?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    With the minimal power at your disposal, the difference in speed will be, well, minimal. Neither boat is ideal for creeping along at low speed, and sailing could be a challenge, unless you have the wind well behind the beam.
     
  5. edsteveed
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    edsteveed Junior Member

    Thx for ur answer Mr Efficiency

    Stability:
    Sailing is a challenge because of drifting to easyor different reason? Still a challenge even with V-shaped hulls? A Leeboard would solve that problem i think.

    Speed:
    If none of these boat design are made for slow speed. Which one would u recommend?
    I read that a 50KG sailing cat (without V shaped hulls) gets up to 25km/h with 5HP. Thats more than enough for my purpose.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Leeboard will be needed, yes. But the stern of those inflatables is not giving a smooth exit for the water flow, so creates drag there. What is the desired cruise speed, that you will be content with ? 25km'hr is hardly a practical target with low power.
     
  7. edsteveed
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    edsteveed Junior Member

    Thx again.

    Yea. The stern problem. Can not do much against that i guess....
    I am not 100% sure what kind of sail i am installing. But sure the desired speed depends a lot of the sail. I dont have a number in mind. I just try to get the most speed out of the natural power i get on this lake here for less than 2000 Euro but i still want to have a comfy boat for 4 people. So it will be around 4,30m (14') long and 2m (6,7') wide with ca. 80Kg.
    Is there a better "shape" for my purpose (must be inflatable). Or which design would u choose?

    Further info:
    Thats how the bottom shape looks like:

    Cat:
    [​IMG]

    Mono:
    [​IMG]

    P.S.: Any sail suggestions which is easy to built? (hard to get parts here in sumatra)
     
  8. Kailani
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    Kailani Senior Member

    Inflatibles are not easy to do yourself.
     
  9. portacruise
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    portacruise Senior Member

    FWIW as I don't know how things would scale up for your larger boats.

    PC
     
  10. edsteveed
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    edsteveed Junior Member

    Thx portacrouse

    U gave me the answers i expected to get. So the best choice is the cat version as i thought.
    I want to bring it to the ocean some day so it must be inflatable which is also more light. Is it? I also want to do some trips for one or more weeks and sleep in it. So i thought a solid aluminum floor is useful and also makes the boat more stable.
    I might just use one 1000W motor which brings me up to 1,5HP. One car battery should be able to give me juice for around 45 minutes and weights around 20Kg. A 250W solar panel (20Kg) will recharge that battery within 6 to 12 hours (i am very close to the equator). Plus i also have a sail and no rush to be somewhere ;)

    Quick weight calculation:
    Battery: 20Kg (44lbs)
    Solar panel: 20Kg (44lbs)
    Motor (inkl. charger, cable, etc...): 20kg (44lbs)
    Inflatable boat: 90Kg (200lbs)
    Sail on a 5-6m aluminum mast: ???
    ________________
    Complete: < 200Kg (440lbs)

    I am a bit concerned about the stern. Is it really really bad compared to a smooth stern which u see with most sailing cats?

    Thx for ur help so far.
    Ed

    P.S.: If i buy two pontoons and make my own cat. Its just a lot of work i think. Plus the "floor" of the boat will be higher which makes it less comfortable and also less stable. Is that correct?
     
  11. FAST FRED
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    FAST FRED Senior Member

    I might just use one 1000W motor which brings me up to 1,5HP. One car battery should be able to give me juice for around 45 minutes and weights around 20Kg.

    Do the math!1.5 HP for an hour !

    Probably a 100A draw , maybe 5-10 min from a car start battery
     
  12. edsteveed
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    edsteveed Junior Member

    I thought i did my math:

    1000W = around 1,5HP
    A car battery with 100Ah is easy to get.
    Wh = Ah x Volt
    100Ah x 12V = 1200Wh = 1,2Kwh

    I thought with 1,2KWh i can run a 1,2Kw machine for one hour. Theoretically. Or am i wrong?
    But yes. 100A is hard to handle.... Didnt thought about that.
     
  13. Jim Caldwell
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    Jim Caldwell Senior Member

    You can only use 50% of a lead acid battery or you will kill it very quickly.
     
  14. portacruise
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    portacruise Senior Member

    Hope this helps.

    PC
     

  15. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Battery's are rated 2 different ways, CCA is for start batts ability to start an engine deep cycling destroys them.

    Deep cycle batts are rated at what is called the 20 hour rate.

    It is done with a load that kills the batt dead after 20 hours.

    A batt should not be drawn down over 50% for best service life.So you get to use only half the 20hr number.

    Sadly a battery with a 100AH 20 hr rate will be far lower in capacity if the current is a more rapid discharge
    The 10 hour rate would be less than half the 20 hour rate and a 5 hour rate even less again.

    Discharging at a 100AH rate (about a 3/4 hp motor draw will kill most batts rapidly.

    The cure is to discharge at about 1/10 the batts 20 hour rating , so to you would need at least 1000AH of batts , 800-1000 lbs of lead to use for power.

    This all assumes Lead Acid batts , the far more expensive lithium can be discharged more rapidly , but may need forced cooling to do it.

    Just ask Boeing!
     
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