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Let's talk about engines, gearbox, propellers

Discussion in 'General Yachting Discussion' started by Floriano, Feb 1, 2017.

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  1. Floriano, Feb 1, 2017
    #1
    Floriano

    Floriano Member

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    Hello everyone, I have a (really really dumb) question,

    For instance, when a yacht engine is running at 3000 rpm, the propellers are turning at 3000 rpm ?!!!

    Regarding the direction of rotation I know 1 thing: when you have 2 propellers, 1 is turning clockwise and the other counterclockwise BUT what about the engines ? I believe both engines turn in the same direction but only one is inverted ?!!

    Can you explain me ?!

    Thank you !
  2. Pascal, Feb 1, 2017
    #2
    Pascal

    Pascal Senior Member

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    Most pleasure craft diesels are rated around 2300 rpm (max speed) and the gears reduce shaft rpm by at least two... so typically at an 1800/1900 rpm the shaft and wheels will turn at under 1000 rpm

    Some smaller diesels may be rated to higher rpm but still have gear reduction
  3. Floriano, Feb 1, 2017
    #3
    Floriano

    Floriano Member

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    Ok thank you !

    And for the direction of rotation ? Both engines shaft are turning in the same direction but 1 of 2 is connected to a gearbox and that's why one wheel is turning in the opposite ?

    Thank you
  4. Capt Ralph, Feb 1, 2017
    #4
    Capt Ralph

    Capt Ralph Senior Member

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    Later diesels turn the same direction. It's the clutch (Transmission) that dictates the shaft direction when needed.
    Some older diesels (Detroit 6-71) you could turn the whole block around for different rotating crank shaft and propeller shaft rotations.
    Some older diesels could change the cam shaft gears and offer a counter rotating engine.
    Older gas engines were often built with a gear drive (special) cam shaft to offer counter rotation.

    Lately, all engines spin normally as they would be installed in a truck, it's the clutch that dictates the propeller shaft rotation.

    I hope this helps.

    If you have an exact model in mind, please update your thread and we can be more specific.
  5. Floriano, Feb 2, 2017
    #5
    Floriano

    Floriano Member

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    Thank you for your help !

    For example a Pershing 64' with 2x MAN 1550hp

    I know that:

    Port propeller: turning counterclockwise
    Starboard propeller: turning clockwise

    But both of the shaft engines are turning in the same direction and after ?
  6. Bamboo, Feb 2, 2017
    #6
    Bamboo

    Bamboo Senior Member

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    The engines from the aft end turn the same direction so that all engines can be installed on either side. At the gears the rotation is determined- "forward" is clockwise on the stbd side and counterclockwise on the port side- so it's just a gear selection which puts the shaft in "forward" or "reverse". Most all gears the reduction ratio is the same "forward" and "reverse" so that a gear can be installed on both sides just like the engines. BTW the reduction is usually less than 2-1 and not exactly at 2-1; the exact ratio of 2-1 can result in a harmonic vibration.
  7. Capt Ralph, Feb 2, 2017
    #7
    Capt Ralph

    Capt Ralph Senior Member

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    Yep, The port clutch (transmission) is changing the power rotation.
  8. Capt Ralph, Feb 2, 2017
    #8
    Capt Ralph

    Capt Ralph Senior Member

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    I have to share a builders setup that I have yet to understand on our boat.
    Counter rotating 12V71TI on stb. Flywheels on both turn in to each other.
    514 clutches change the rotation of both engines and spin the propellers out as normal.

    I'm sure there is a lil parasitic hp lose in there somewhere. Since 1978, still running strong.
  9. K1W1, Feb 2, 2017
    #9
    K1W1

    K1W1 Senior Member

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    Even modern engines can be ordered to turn in either direction.

    Clutches don't change rotation. Gearboxes are set up for one side to run the aft clutch when going ahead and the other one runs the fwd clutch when going ahead.

    A ZF guy once told me at 20000 hrs we could swap the gears from side to side and do another 20000 he obviously didn't know much about the 16V396's driving them
  10. Capt J, Feb 2, 2017
    #10
    Capt J

    Capt J Senior Member

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    Gear ratios can be found anywhere from 1:1-4:1 gear reduction and depends on how the manufacturer does things....I know of one 70' SF manufacturer that runs a 3:1 reduction (1000 engine rpms prop is turning 333 rpms) and another SF manufacturer 75' that runs a 1.5:1 reduction (1000 rpms, prop is turning 667 rpms). Usually in modern diesel engines, the motors turn their natural rotation and the gear changes the rotation so one runs the opposite direction. It's more efficient to have counter rotating props, boat runs straighter, and handles better (around the dock and turning at speed).
  11. Capt Ralph, Feb 2, 2017
    #11
    Capt Ralph

    Capt Ralph Senior Member

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    This is correct.
    My terminology was short and not complete.
    Usually, in the same case is the clutches and gears. Selecting the correct clutch pack, then engages the proper gear set for the desired rotation as K1W1 has corrected me.
  12. Capt Ralph, Feb 2, 2017
    #12
    Capt Ralph

    Capt Ralph Senior Member

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    I remember some boats with wheels turning inward.
    One day, I know I can try this.

    OTOH, I know if I loose a forward clutch pack and don't want to use the emergency pack lock up trick, I can always put a (counter rotating) spare prop on that shaft and use the counter clutch pack and get home.
  13. Capt J, Feb 2, 2017
    #13
    Capt J

    Capt J Senior Member

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    Raceboats experimented with inward rotation props, it added a few mph, but the boats wouldn't turn at speed and could not dock them either, so they abandoned that idea. I believe it was good ole reggie fountain who experimented with it.

    As for Kiwi's mention....it was in regards to the MTU 396's......I ran one yacht with 12v 396's, what a nightmare those motors are. Smoke like crazy at slow speeds.
  14. Floriano, Feb 6, 2017
    #14
    Floriano

    Floriano Member

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    Many thanks !
    I went on the docks, anyone know why on several yachts (starboard side close to the waterline) there is some water constantly pouring whereas the engines are shut down and even nobody on board, maybe the air conditioning ?
  15. AMG, Feb 6, 2017
    #15
    AMG

    AMG YF Moderator

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    Yes AC and also for frigos and freezers...