propeller design

Discussion in 'Props' started by NavArch007, Nov 3, 2009.

  1. NavArch007
    Joined: Nov 2009
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    NavArch007 Junior Member

    Hi everyone,

    Ive been working on this project for 40m crewboat vessel. The situation is as follows:
    Vessel spec:
    L=40m, B=7.2m, T=1.75m
    Design point: 21knots at 180tonn displacement.

    The vessel has a restricted propeller diameter of 1.1m due to hull clearance. So to produced the required speed, 3 propeller were necessary. Unfortunately, the gearbox ratio chosen cannot be altered due to space constrictions and a gearbox ratio of 2.95 was already chosen. Also, the vessel will have 3 engines producing 1400bhp at 2300rpm.

    To make the propulsion unit more efficient, the shaft angle was increase from 8 to 10 degrees in order to accomodate a larger propeller; and now it is possible to fit a 1.35 m diameter propeller.

    I would be interested to hear any comment and suggestions and also if possible, some calculation on expected operating rpm, BAR and pitch ratio to compare with my results. I have found that for 4 bladed 1.35m diameter:

    BAR=0.85, P/D=1.005, at an rpm of 530.

    I have been using Autopwr program. Any suggestions regarding a good propeller design program for personal use at a reasonable price.

  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    First of all, how did you arrive at 530 shaft rpm? In my book 2300/2.95=779.7!

    That is the primary dimensioning rpm. Your Froude nr (by volume) is 1.4 and she has a slenderness ratio of ~6.5 depending on WL length. This means that there should be no hump to worry about, the top speed is actually head-on the little hump there is. All provided there are no special "tricks" in hull design. In this case propellers must primarily be selected to satisfy either of two basic conditions:

    A/ Max power-max speed,
    B/ Cruising condition, added resistance due to weather conditions, allowable continuous power output.

    I guess your speed estimate of 21 knots is referring to condition "A"? What's cond "B" like?
    Last edited: Nov 3, 2009
  3. NavArch007
    Joined: Nov 2009
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    NavArch007 Junior Member

    thanks for your reply baeckmo.

    The design is basically a conversion of a previous vessel. The previous designer had opted for a gearbox ratio before specifying the propeller rpm!

    When i looked at the design, i thought that to get the best out of our propeller, we would need a bigger gearbox ratio; however such a procedure would not be possible due to space restriction on the vessel which would not enable it to accomodate a bigger gearbox.

    Condition B is 130ton displacement at 25knot.
  4. pamarine
    Joined: Sep 2009
    Posts: 144
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    Location: Norfolk, VA

    pamarine Marine Electrician

    Wouldnt "B" be below "A" in terms of speed and heavier in terms of displacement?

    Just reading the post here I would guess the speed for "B" to be in the 18 knot area but not sure what the displacement would be (add passengers, fuel, stores, etc)
  5. apex1

    apex1 Guest

    You misunderstood baeckmo.
    Condition B is at the same displacement, but continous (always lower) speed.

    And gearbox dimension is not necessarily related to ratio! The ratio can be choosen within a certain range in the same housing!

  6. Çemberci
    Joined: Apr 2007
    Posts: 127
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    Location: Istanbul/Turkey

    Çemberci Senior Member


    Here is our results
    * 4 Bladed Wageningen B
    Diameter 49.00 inç
    Pitch 48.00 inç
    Dar = % 80.74
    Speed = 26.00 knot
    Price MnAlBrz 7450 Euro
    * 4 Bladed Master
    Diameter 45.81 inç
    Pitch 47.00 inç
    Dar = % 95.07
    Speed = 26.00 knot
    Price MnAlBrz 7580 Euro

    Oktay Çemberci

  7. conceptia
    Joined: Nov 2009
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    Location: Houston

    conceptia Naval Architect

    hey navarch, just download and try the latest version of freeship @
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