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Thread: Prop Size MD21b

  1. #11
    Join Date
    Aug 2019
    Posts
    6

    Default Re: Prop Size MD21b

    Quote Originally Posted by Divemaster1 View Post
    Engines does not move the boat, the propeller does ..... so irrespective of the RPM you run at, it is the thrust that the propeller generates which moves the boat ... to generate the thrust, the propeller gets loaded and the engine governor responds to the load..

    So to move your boat at 8 knot, the propeller have a certain load (resistance to push through water), the engine governor give the fuel needed based on the load on the propeller ... changing the propeller (size or pitch) does not alter the load it will take to move the boat through the water at 8 knots ... The only thing you will achieve is spending money to run the engine at lower rev, but at same load ... ie take out same HP and thus burn same amount of diesel...

    In respect to overheat (common issue on the MD21's)... Heat main generated in the engine is a result of the combustion... heat therefore is a result of Fuel / Air volume in combustion chamber ... so by bringing down RPM, but maintaining the Fuel/Air mixture in the combustion chamber, you are reducing the volume of coolant circulating in the engine (coolant pump also run slower) ... this results in same overheating at lower rev...

    If overheating is an issue, it indicates that the cooling system need some TLC ... if you want higher speed, then resolve this first them be prepared for a higher fuel burn as you will take out more HP of the engine(s) as load increases as water resistance does...
    Great advice. Thank you for taking time to help

  2. #12
    Join Date
    Mar 2016
    Posts
    1,076

    Default Re: Prop Size MD21b

    Hi
    Something doesnít add up here. Going by your numbers somewhere in the middle. Presuming 2800 rpm, 10Ē pitch and 1.93 ratio...at 7.2 mph....youíre at 48% prop slip. This is amongst the worst Iíve ever seen. Is your coupling good ? Is the prop slipping on its shaft or key way/ taper ? Something is terribly wrong, unless Iíve missed something.

  3. #13
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,335

    Default Re: Prop Size MD21b

    Quote Originally Posted by QBhoy View Post
    Hi
    Something doesn’t add up here. Going by your numbers somewhere in the middle. Presuming 2800 rpm, 10” pitch and 1.93 ratio...at 7.2 mph....you’re at 48% prop slip. This is amongst the worst I’ve ever seen. Is your coupling good ? Is the prop slipping on its shaft or key way/ taper ? Something is terribly wrong, unless I’ve missed something.
    You've got your speedboat head on. Those are expected figures for a displacement / sailboat pushing the speeds he is climbing out the hole.

  4. #14
    Join Date
    Mar 2016
    Posts
    1,076

    Default Re: Prop Size MD21b

    Quote Originally Posted by BruceK View Post
    You've got your speedboat head on. Those are expected figures for a displacement / sailboat pushing the speeds he is climbing out the hole.
    Not sure BruceK. Physics are physics. I took the mid range figures mentioned to get what should be a fairly efficient calculation. Prop slip is prop slip. And nearly 50% is just far too much really.

  5. #15
    Join Date
    Feb 2015
    Location
    Conwy
    Posts
    4,335

    Default Re: Prop Size MD21b

    Small engine, small prop, big heavy SD boat climbing out the hole.... pushing a narrow column of water inefficiently....

    http://www.trawlerforum.com/forums/s...tor-13274.html

    "For instance, a sailboat or barge traveling at 9 mph will have a prop slip of approximately 45%, while a hydroplane traveling at over 90 mph can have a prop slip as low as 7%." : https://teaguecustommarine.com/teagu...ropeller-slip/

    etc

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