All Electric Jet Drive Discussion

Discussion in 'Jet Drives' started by anthonydimare, Aug 7, 2015.

  1. Yobarnacle
    Joined: Nov 2011
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    Yobarnacle Senior Member holding true course

    There are a couple parameters not indicated.
    How long a period do you want to maintain 60 knots?
    In what kind of sea conditions?
    On what body of water?
    If short duration high speed runs were all you needed, a combination of ICE generator, battery bank, and a turbo driving a water jet impeller rather than as forced air feed, might get you 60 knots for 10 minutes or so, and allow a lighter boat.
    FIVE JETS, TWO DRIVEN BY ICE, TWO ELECTRIC OFF BATTERIES, AND ONE OFF THE EXHAUST TURBINE.
    During that 10 minutes, you will travel ten nautical miles. Roughly 20 kilometers. Actually, 18.5 kilometers.
    Pretty big lake.
    Open ocean, you'll have only a few days per year calm enough to do 60 kts safely, IN AN UNDER 30 FT BOAT.
     
  2. Yobarnacle
    Joined: Nov 2011
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    Yobarnacle Senior Member holding true course

    Horsepower isn't the same as horsepower. Are we talking brake HP (BHP)? or shaft hp, (SHP) or BTU's (energy in the fuel consumed)? Or bollard pull (for tugs, 40 tons of bollard pull equates/markets as a 4000 hp tug).
    Or some other rating system?
    Different engines , different mfgs, different types of motors, frequently rate differently. And all say "horsepower."
    Minnkota advertised a 2 hp trolling motor in the 1980's.
    I called them up, and asked how many amps it drew from a 12 volt battery.
    It used only a quarter hp of electricity.
    Amazing! Energy problems solved.
    They excused their 2 hp claim, by explaining the trolling motor produced equal thrust as a 2 hp gasoline outboard.
    Horsepower doesn't drive boats. Thrust does!
    Maybe bollard pull as horsepower is the better indication!
     
  3. Joakim
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    Joakim Senior Member

    Bollard pull thrust doesn't drive the boat either. It's the thrust at speed you are travelling. That thrust can be quite equal or very far from th bollard pull thrust.

    E.g. Torqeedo Cruise 2.0 (2 kW electric power) does have equal bollard pull to 5 HP gasoline outboard (about 600 N). So they market it as "equal to 5 hp". But a typical 5 HP outboard has more than 5 HP SHP while Torqeedo only has a bit over 2 HP. Tests show that a 2.5 HP outboard has a higher top speed than "5 HP" Torqeedo in the typical 5 knots range. Although their bollard pull is typically only half (300-350 N).

    Assuming 1.8 kW at shaft for the Torqeedo and knowing it has a three blade 12" X 10" propeller you can calculate that at 5 knots the thrust will be about 400 N (propeller efficiency a bit less than 0.6).

    2.5 HP OB might have a 7 3/8" X 6" propeller, which has a clearly lower efficiency (0.44) at that point. It would need about 2.5 kW for the same 400 N thrust.

    I don't know if they have even more than 3.5 SHP or maybe Torqueedo has clearly less than 1.8 kW at the shaft.
     
  4. Frank41
    Joined: Sep 2015
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    Frank41 Junior Member

    Bollard Pull is the static force that a boat can apply to a stationary object. Typically, speed does not enter the equation.
     
  5. powerabout
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    powerabout Senior Member

    laying the 2 torque curves over the top of each other is the only comparison you can make sense of.
     
  6. kerosene
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    kerosene Senior Member

    exaclty. but if I put five reduction gear boxes in my car engine of 77kw (I know its slow) I can get static pull equivalent of massive engine. Can I know claim that the car has 500kw/700hp beacuse it has the same torque at the wheels as a comparable 700hp vehicle? Of course not.

    Thrust AT GIVEN SPEED can be related to power. Prop and shaft gearing affect are exactly like a transmission. You can gain thrust at low speed to lose it from high speed. Its not magic. Power is the best unit for measuring engines.
     
  7. powerabout
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    powerabout Senior Member

    thats why I said curves hence it includes rpm
    do it at the prop shaft.

    power is just torque x formula which is based on rpm
     
  8. whitepointer23

    whitepointer23 Previous Member

    That will be good. Don't forget the launch pics.
     
  9. kerosene
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    kerosene Senior Member

    or power x speed (rotational speed or linear speed, essentially the same thing).

    Unfortunately too many people fail to see what it means or stop to try to understand the relation.
     
    1 person likes this.
  10. Frank41
    Joined: Sep 2015
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    Location: Lake Conroe

    Frank41 Junior Member

  11. groper
    Joined: Jun 2011
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    groper Senior Member

    Your electric boat could easily be reality if your prepared to accept a few fundamental changes.

    A lightweight, minimalistic hull with no fancy luxuries, smallest possible to keep budget manageable.
    Very flat hull, low resistance , low planing speed.
    A more moderate top speed of something like 30-35mph, and 20-25mph cruise.
    An efficient prop drive for the speed regime.

    A boat like this is very feasible and could be done for a reasonable price for quiet lake recreation. A attempt at a 70mph electric jet boat of 28ft length would be financial suicide...
     
  12. Frank41
    Joined: Sep 2015
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    Location: Lake Conroe

    Frank41 Junior Member

    Groper,
    I finally agree with you. I need to knock the size way down to Jon boat size, configuration and speed or go with small tug or trawler style displacement hull, appropriate speed and expense.
    Not interested in either at this time. However, I am interested in your twin jet diesel project, which I plan to follow and add my two cents worth.
    Thanks for all of your input.
    Frank41
     

  13. Yobarnacle
    Joined: Nov 2011
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    Yobarnacle Senior Member holding true course

    I know a fellow lives in Fernandina Beach, Florida that installed a salvaged jet ski unit in a Jon boat and nearly killed himself. It was EXREMELY FAST!. And unmanageable. :D
     
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