Most efficient place for propellers in this particular situation?

Discussion in 'Boat Design' started by Lemans, Mar 22, 2012.

  1. Lemans
    Joined: Nov 2011
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    Location: Belgium

    Lemans Lemans

    I expect this a very easy question as I never have seen a boat with propellers up front but here's the question anyway.

    The drawing shows a hybrid propulsion unit. The electric engine can be connected without reduction drive. In witch way is propulsion most efficient?
    Propellers in pusher configuration or in tractor configuration?
    Speed would be less than 5 knots as the battery pack stores energy for 5HP for 3 to 4 hours.
    Estimation on range?
     

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  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Nice drawing. Why the bicycles?

    There have been boats with tractor drives. Steering becomes problematic, The props are more nearly subject to fouling, and there are hydrodynamic implications that may lead to reduced efficiency. The argument can be made that the props operate in clean water but the prop wash could have some negative effects on the hull.

    Some smarter guys will jump in here, I expect.
     
  3. WestVanHan
    Joined: Aug 2009
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    Location: Vancouver

    WestVanHan Not a Senior Member

    Hi Lemans,interesting boat.

    Is this going to be used on the canals?
    I've been on the Belgian canals and was just wonderful, especially around Bruges.

    Curious if you've done your research on the batteries.
     
  4. Submarine Tom

    Submarine Tom Previous Member

    Pulling is usually more efficient than pushing but a lot can depend on details which are lacking here.

    Range: 3 knots at 3 hrs is 9 Nm

    So, about 10 Nautical Miles I'd say.

    -Tom
     
  5. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    most light aircraft operate with tractor props to good effect, the principle is the same. It can be done if you need to, but the problems of a prop in the front of the hull can be large. There are advantages and disavantages to either one.

    It seems to me, unlike an aircraft, a tractor prop would be subject to damage, grounding (getting you stuck, unlike pusher prop that can get you off a sand bar) and would cause turbulence over most of the hull. These are likely the reason the traditional location for boats is in the rear. Also, rudder effectiveness is much better with it next to the prop. Consider how much larger aircraft rudders are compared to ship rudders, operating in all that turbulent air at the back of the aircraft. The prop energize the fluid stream and makes the rudder far more effective, keeping the flow attached to it. A front mounted prop is also directionally destabilizing, requiring a larger rudder and perhaps a different hull shape (or larger fin or skag).

    Do you really want to build a tractor prop on a vessel?
     
  6. markdrela
    Joined: Jun 2004
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    Location: MIT Aero & Astro

    markdrela Senior Member

    Theoretically, to minimize drive power for a given boat speed, it's better to have the prop ingest the slower-moving boundary layer/wake fluid. This not only helps the prop itself, but also avoids having the higher-speed propulsive jet scrub on the hull.

    The only downside of a rear prop is that the nonuniform inflow into the prop produces local unsteady Cl fluctuations on the blade airfoils, which will likely increase the average local profile Cd values and thus reduce the prop's profile efficiency somewhat. But if this added profile loss is kept reasonably small via proper design blade Cl choices, then the net benefit of rear drive will be positive.

    Airplane props generally have smaller nondimensional disk loadings (and hence smaller propulsive jet velocities), so the jet-scrub cost is smaller. Also, a rear prop causes serious balance and tail ground-clearance problems which are not present on a boat. So a front prop on an airplane is usually more attractive.
     
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  7. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    For the efficiency I guess it doesn't matter much whether the prop pulls or pushes, the shaft in both cases disturbs the flow. From an engineering viewpoint pulling is always better than pushing, even more so when a long shaft is used.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    All in all, put it where you like it. :p
     
  9. Frosty

    Frosty Previous Member

    Ive just got off a roll 0n roll off ferry with both push and pull so it can go back and forth with no bow bow stern. In fact I not of any roll on roll that does not have a rotation drive fore and aft,---which ever way you look at it.
     
  10. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Frosty; does the roro use both forward and aft props at the same time or just one at a time? If one at a time, which one?
     
  11. Frosty

    Frosty Previous Member

    The one in Pinang to Butterworth uses both, they never stop even when vehicles are embarking. The boat is not tied up and is held by both props into the dock.

    When its time to leave( 10 minutes) the drives rotate and the ship moves side ways and out.

    The Hong kong to Tsim tsa tui Star ferries are the same except they tie up and stop engines these days.

    When asked to go into the engine room it was a single engine running set RPM with a shaft coming out both ends running both drives at bow and stern that can rotate 360 degrees.
     
  12. Submarine Tom

    Submarine Tom Previous Member

    So two horizontal propshafts (I guess I better call them drive shafts to avoid confusion) running azipods?

    -Tom
     
  13. Frosty

    Frosty Previous Member

    I have to alter my previous post about the Star ferries in Hong Kong --I remember now.

    They were single engine with dog clutches front and rear. You could easliy see them working if you looked down the engine room steps They were propelled from the rear pushing prop and the boats were dual direction.

    So in one direction one was push and the bow was brake.

    They were very low RPM,--you could see it was slow or the dogs would not last long.
     
  14. Lemans
    Joined: Nov 2011
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    Location: Belgium

    Lemans Lemans

    @messabout
    It would be better for electric losses in wires to put the BLDC engines as close as possible to the generator/batteries so tractor configuration looks attractive. On the other hand, if I need to use power to help the rudders for adequate steering I lose even more than I can win.
    And the bikes.... I just forgot to unload them.

    @WestVanHan
    Yes, for the canals and the small rivers in Belgium.
    I know bruges well. Wonderfull 9the century city.
    I did a bit of research on the batteries and as I have the space for 16 60/80Ah batteries I can store
    13,5 kW/h of energy. This makes 4,5 HP for 4 hours.

    @Submarine Tom
    Would it be wise to prepare the boat for both solutions?
    10Nm, nearly 20 km is an acceptable range.

    @Petros
    I consider your remark of grounding very important. You did noticed the bicycles on the boat...landing and unloading the bikes has to be done with the props in enough water.
    I don't want to build a tractor prop if there is noting to win with such a setup.
    Your input and explanation was very helpful. Thanks

    @markdrela
    “Theoretically, to minimize drive power for a given boat speed, it's better to have the prop ingest the
    slower-moving boundary layer/wake fluid. This not only helps the prop itself, but also avoids having the higher-speed propulsive jet scrub on the hull.”


    I try to understand this and have read it a few times..;-)
    Layer speeds go from vessel speed (hull contact) to zero (water speed) so, two small props closer to the hull are better than one witch is acting in layers closer to zero speed... I be good with my design using two props?

    “The only downside of a rear prop is that the nonuniform inflow into the prop produces local unsteady Cl fluctuations on the blade airfoils, which will likely increase the average local profile Cd values and thus reduce the prop's profile efficiency somewhat. But if this added profile loss is kept reasonably small via proper design blade Cl choices, then the net benefit of rear drive will be positive.”
    Using a BSDC engine gives a total freedom in rpm choice as torque is available right from the start. The 2 HP for each prop gives a few hundreds rpm's. Do I have to worry a lot about prop design at that speed?
     

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  15. Petros
    Joined: Oct 2007
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    Petros Senior Member

    prop design is very important if you want to get the most out of that very heavy battery pack. You need a prop designed to be optimized for the speed, rpm and power input you intend to use it to get max range out of your batteries.

    An improper or primitive prop design will still work, get you around, but it will not get you as far. So either you have to design one for your needs and make it, or find one that is optimized for your needs (or close enough) and buy it.
     
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