How you will justify this body plan?

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

  1. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    OK, that is not a very good design point. The required blade area is quite high, about 1.0, and the tip speed is very low, so if your prop is probably cavitating heavily.

    Optimal diameter for the speed, power and RPM is probably about 1400mm. That will help a lot, but I doubt that you can get from 7 up to 10 kts. Pitch for 9 kts is about 1100mm with a required DAR of 0.65.

    Good Luck
     
  2. manon
    Joined: Dec 2007
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    Location: dhaka

    manon Junior Member

    Yes, you are right.
    BAR should be around 1.
    Now I want to have the following configuration:
    D=1320mm
    Pitch= 1169mm
    BAR=1.0
    GB=2.54

    With this data I cant get a speed more than 7.2 knot at full load (full load displacement 830 tonne), can I?
     
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Manon, I think your main problem here is the shallow-water blockage effect. I have seen it at work, where it caused 30% loss of maximum deep-water speed of a trawler boat.

    I have made few calcs, and imo with a 350 HP engine you can expect around 9.5 kts maximum speed in deep water, assuming 70% prop efficiency.

    However, in 5 meters deep water, by using Schlichting graphs from Principles of Naval Architecture, the approximate speed reduction should be around 35% of the maximum speed. It means that in a shallow waters you can expect your ship to travel at around 0.65*9.5 kts = 6.2 kts. That's an approximate value coming form the studies on ship models by Schlichting. The actual speed reduction will depend on your ship's particular geometry, but you can see that it is not too dissimilar from what you have measured in your sea trials.
    So, I think you should investigate into that direction too, and not look just at the propeller side (which remains the other issue, of course).

    Question:
    in the post #4 you have mentioned that the water depth is around 4 meters. It is a pretty shallow water for a ship with 2 meters draft. Are the 4 meters referred to the depth under the keel or is it the depth measured from the water surface?

    Cheers
     
  4. manon
    Joined: Dec 2007
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    Location: dhaka

    manon Junior Member

    daiquiri,
    I think you hit the main point of speed loss. The water draft from the surface varies from 4-5 meter actually.

    any way, I am going to suggest the yard as following now:

    BAR=0.7, Dia=1320mm, Pitch= 1065 mm, Maximum loaded speed = 6.5 knots. Since the yard will not change the engine now, the only change I can suggest is the prop dia, pitch etc. (I cant accommodate at prop dia higher than 1320 mm).

    Waiting for your valuable comments
     
  5. chandru
    Joined: Jan 2015
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    Location: Germany

    chandru New Member

    I can help you with NAPA. Contact me gcm4design@gmail.com
     

  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Is the RPM reaching the upper limit of the governor, or is it loaded such that it won't go over the maximum RPM?
     
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