Distance from transom to propellor?

Discussion in 'Surface Drives' started by Northwester, Nov 10, 2010.

  1. Northwester
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    Northwester Junior Member

    From a review of available surface drive designs for planing hull vessels, I've noticed that the propellor is usually placed a distance of at least 1 meter (and usually further) from the transom.

    I'm wondering what the reasons for this are.

    Can anyone provide some background on this?
  2. gonzo
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    gonzo Senior Member

    It needs to be ventilated. If it was under the hull it wouldn't work.
  3. daiquiri
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    daiquiri Engineering and Design

    I think Baeckmo is the right person to give you a correct answer, but here comes my two cents worth.

    Arneson and Arneson-alikes have a movable shaft aft of the transom, which gives it a steering capability (the prop acts as a rudder, which is absent). The length of the movable leg increases the leverage of the steering force and also provides the space necessary for the hydraulic steering actuators. Try to draw two boats - one with side-tilted prop right behind the transom and one with an equally side-tilted prop 1 meter further aft. You will see that the lever of the side force component (and the relative steering moment) is bigger in the latter case.

    Generally speaking, the more a steering device (be it prop, rudder or whatever) is distant from the boat's CG, the more efficent it is.

    There are other manufacturers who don't use steerable-shaft surface drives like, for example, Seafury and Q-SPD. They use traditional rudders instead, so their prop shafts are much shorter. But, on the other side, their rudders are placed further aft ( for the same reason seen before), so the overall length of the unit (prop + rudder) is more or less the same as Arneson's.

  4. Northwester
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    Northwester Junior Member

    I'm specifically interested in the fixed shaft design. I would like to know what would happen if a surface piercing propellor is located closer to the transom than most of the currently available designs.

    Instead of it being located more than a meter from the stern, could it be located within 10-30 cm and still be an efficient design for a deep-v planing hull vessel at speeds of 20 knots or so?

    I imagine that reversing performance could be a problem. Perhaps the water flow is too turbulent at a closer distance to the transom.
  5. War Whoop
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    War Whoop Senior Member

    You may wind up with something that goes foreword in reverse
  6. rxcomposite
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    rxcomposite Senior Member

    Arneson type needs to be away from the transom because that is where it pivots, actuated by three hydrailic arm on the sides.

    There are racing class that uses outboard motors and its propeller is located higher up in the water for ventilation. But these are no ordinary outboard motors.
    Last edited: Nov 13, 2010
  7. Jim_Hbar
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    Jim_Hbar Junior Member


    The prop can be located in front of the transom - check out Powervent.
    There just needs to be a source of air so that the surface props can ventilate.

    Hope that helps.

    Where on the Island are you located?
  8. Northwester
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    Northwester Junior Member

    Someone asked where I was located on Vancouver Island. I'm located in Nanaimo.

    The Powervent design and the somewhat similar ZF Trimax design has a spacing of about 1 meter between the propeller and the location where the propellor shaft emerges from the transom. I think this is done to deal with the problems associated with poor reversing thrust. A short sloped surface @ 45 degrees will likely mitigate some of that problem. The distance may also be set this way to ensure clean undisturbed water to give the propeller a good bite.

    I would like to try and adapt the Powervent approach and use it on my boat without modifying the existing hull. This would require constructing an outboard strut and rudder assembly and attaching that to the existing transom and incorporate within that assembly the self regulating venting system needed get the boat through the transition zone and up onto plane. I would like to avoid a system that hangs quite a bit more than a meter from the transom like the Simplicity drive does.

    My boat is a 25' Bertram deep v planing hull with 3 lifting strakes, a 32 degree deadrise at the bow and 24 degree deadrise at the transom. Fully loaded with provisions and fuel, I estimate the vessel weighs 3 metric tons.
  9. CDK
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    CDK retired engineer

    Do you mean really, really close like this?

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  10. Northwester
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    Northwester Junior Member

    Yes, but only if it suits the intended application.
  11. yipster
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    yipster designer

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  12. baeckmo
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    baeckmo Hydrodynamics

    The problem with semisubmerged props is that their (thrust)/(advance ratio) is not rising with increased load (~reduced advance ratio) for a given submergence, instead they show a serious dip in midspeed thrust. With normal hull shapes, this results in a lack of thrust, particularly around hump speeds.

    To compensate for this, the effective swept working propeller area has to be increased in this speed range. By placing the propeller disc at a certain distance from the flow separating edge, the upsweep of the flow from the bottom will cover more of the propeller disc; ie higher mass flow rates are involved in the work, which increases the developed thrust. At higher speeds, the flow straightens so that the propeller is working ~50 % submerged. With the Power Vent variety, the function is exactly the same; what counts is the distance from separating edge to propeller disc, all provided the correct amount of ventilating gas can be supplied. With Arneson drives, the shaft elevation is adjusted to achieve optimum submergence.

  13. Northwester
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    Northwester Junior Member

    The explanation makes perfect sense. Thank you. I now understand why most surface designs are as they are.

    Is it possible to accurately predict the profile of the water surface as it leaves the flow separating edge at various boat speeds through water?

    It would seem that all of the factors such as shaft angle, propellor distance from flow separating edge, propellor diameter and the volume of ventilation air have to be just so in order to achieve the desired results. I imagine that the design is also very sensitive to changes in boat trim.
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