How you will justify this body plan?

Discussion in 'Props' started by manon, Jul 2, 2013.

  1. manon
    Joined: Dec 2007
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    manon Junior Member

    Dear members,
    Please see the attached body plan.
    L wl X B X T = 47.37X10X2 (Extreme B/T is due because of the draft limitation).
    Displacement = 757 tonne (Block coefficient 0.8)
    Light weight =
    Main Engine = 350 BHPX1
    Propeller dia = 1200 mm
    Gear Ratio = 1:2.54
    Pitch = unknown (yet, since it is not docked I cant measure pitch, and owner cant say the pitch value)

    Expected light weight speed = 10 knots
    Measure Speed = 4.5 knots

    Propeller immersion = 60 mm above the water surface (this was the light ship trial condition).

    Where is the real problem about the speed? Other than the pitch of the propeller I have all data. Pitch is off course important and should be match properly but will that make so big difference in speed? Is there any factor about the loss in speed? I have suggested to make trial with proper ballast and immerse the propeller. But will the immersion of propeller gain the desired speed?

    For your convenience, I have uploaded the body plan.

    Waiting for your valuable comments. Please let me know if you need any other data.
     

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  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Two questions:

    What is the engine rpms?
    How deep was the water during the trial runs?
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    With the propeller out of the water it is very possible that cavitation finished with it in less time than sea trials time. Not only does the propeller performance is very low but can lose some of the blades.
    On the other hand, it makes no sense to check the speed of the light ship since this condition can not be achieved ever in normal working conditions. What is important is to know the speed at normal load conditions.
    Regarding the body plan, the sleeve is very large relative to strut. Sure the initial stability is so great that the boat does not fulfill the criterion of maximum GZ over 25º.
    In addition, you must put many longitudinal bulkheads to avoid the effect of free surfaces in tanks.
     
  4. manon
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    manon Junior Member

    Engine rpm= 1000
    water depth around 4 meter.
     
  5. manon
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    manon Junior Member

    Thanks for your comment.
    As I have told the owner to trial taking more ballast so that the propeller get enough water. Trim looks suitable at the light condition.

    About the lines, please explain 'sleeve is very large relative to strut' more. Will this body plan effecting the speed? Off course a fuller ship like this is not meant for better performance in terms of speed, but should I suggest to change the aft body now to achieve higher speed? Will that improve the speed?

    Light ship is not a trial condition, I already have told the owner. But with properly submerged propeller, how much speed can be gained?
     
  6. TANSL
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    TANSL Senior Member

    The issue is not how fast you will win with the propeller in the water. The real problem is that the propeller may end up broken if we run out of water ..
    Shapes of aft body, of course, are crucial to get a good inflow to the propeller. But I fear that without some tests in a towing tank, it is very risky to say anything valid.
    When the B / D is large, the initial value of GMt is very large. The curve representing the values ​​of GZ relative to heel, has a tangent at the origin whose value matches initial GMt. Thus there is a curve as indicated in Figure 2: the maximum GZ value is reached for a value relatively small of heel and, therefore, is not satisfied the stability criterion which requires that the maximum value of GZ occurs at more than 25° (30º in some ships) heel.
     

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  7. TANSL
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    TANSL Senior Member

    Apparently you need a very shallow draft boat.
    I'm not sure what I will say is totally correct. Other members of this forum to express their comments.
    Could the solution using two propellers, of smaller diameter, more effective, for example, providing them with nozzles?
     
  8. manon
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    manon Junior Member

    TANSL
    Thanks for your valuable comments.
    Large angle stability of the hull seems okay of this hull, maximum GZ occurs at 30 degree. Actually I am more concern about the aft section design and the propeller inflow. As you have said it is very difficult to say without towing test which is right, but here the owner does not want to pay for any test! So, I have to rely on good aft and for section design.

    The test trial will take place again with more ballast so that the propeller is submerged properly. Let see how it goes.
     
  9. TANSL
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    TANSL Senior Member

    Good luck and let us know sea trial results.
     
  10. manon
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    manon Junior Member

    Off course I will let you know the result. For the time being I am uploading some pictures if it helps to understand the shape.
     

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  11. TANSL
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    TANSL Senior Member

    The boat seems quite correct. The only reason to take the propeller out of the water is probably because, in light condition, has a very large negative trim. Is there a possibility of placing a liquid ballast tank aft?. Or you may want to take all weights that are hosted in the forecastle and place them in a store (new?) aft.
     
  12. MikeJohns
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    MikeJohns Senior Member

    You really need that prop pitch info first. Everything else is speculation until you know what speed you are actually trying to drive it. But what RPM did you get wide open throttle. Prop looks to be on the small side.
     
  13. manon
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    manon Junior Member

    TANSL,
    Sorry for the late reply.
    The vessel was trialed again with ballast water to submerged the full propeller. (Note that, at the light condition the about 50 mm was out of surface and the vessel was well trimmed by aft).

    With submerged propeller, with the current speed was little less than 7 knot. That means the speed has increased a bit, but still not up the satisfaction.

    Shipyard has got some suggestion from somewhere which they want me to verify. the suggestion was as following:

    ME 350BHP, GB ratio 2.54:1

    Propeller: Dia 1500mm , pitch H:1050mm, 4 blades, Type: MAU4-54,

    calculated speed: 10.1 Knot.

    ......
    So it seems that they are suggesting to increase the prop dia if possible and expecting the speed upto 10.1 knot!! At this point what i understand that if the yard modifies the aft end and manage to accommodate 1500mm prop, it is not possible to get 10.1 knot at full load displacement (around 830 tonne).

    My calculation gives 7.5 knots with BAR=0.530, P/D =0.642, D=1500mm. I assume that 10.1 knot is the ballast speed.

    Do you think my calculation is okay?
     
  14. johneck
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    johneck Senior Member

    What is the current pitch and diameter? Is the engine reaching rated RPM?
     

  15. manon
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    manon Junior Member

    Current Pitch is 1100 mm and dia 1300 mm and Engine reaches its rated rpm
     
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