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Thread: Hydraulic Propulsion

  1. #1
    Join Date
    Dec 2008
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    Now cruising in the SUN! In Malta.
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    Default Hydraulic Propulsion

    My cat has one Lombardini 34hp diesel engine with hydraulic transmission to the two propellers. This has worked very well since buying the boat in 2008. I am now wanting to change the two-blades folding propellers for 3-bladed folding ones and in the course of preparation I had the shaft revolutions measured with a strobe pistol. To my surprise it came out at 1:1. engine revs : shaft revs. I know of the Vetus hydraulic system which has an approx. 1.7:1 ratio.

    Has anyone here come across such a 1:1 ratio?
    Roger

    ------------------
    I look to the future, because that's where I am going to spend the rest of my life - George Burns

  2. #2

    Default Re: Hydraulic Propulsion

    If the 2 blades have been working very well, why do you want to change 3 blades?

    2 blades are usually more efficient than 3 although the greater blade area of 3 will provided more thrust but will result in less speed through the water.

    Just curious

  3. #3
    Join Date
    Dec 2008
    Location
    Now cruising in the SUN! In Malta.
    Posts
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    Default Re: Hydraulic Propulsion

    Hi Peter,

    My understanding is that two-bladed propellers are the most in-efficient propellers around. I want to change to three-bladed ones to give me more power and speed when motoring into wind and waves as this is where I currently fall off the vmg.
    Roger

    ------------------
    I look to the future, because that's where I am going to spend the rest of my life - George Burns

  4. #4

    Default Re: Hydraulic Propulsion

    In reality a single bladed prop is the most efficient (and the least practical for obvious reasons).

    By adding more blades you increase the rotational drag that will put more load on the engine. I would be very wary in your case given that a total of 34 hp is not that much and hydraulic drives are mechanically quite in-efficient.

    You would probably do better to play around with the gearing. Find the speed you want to go at in certain conditions then adjust the gearing to match the performance curve of the engine. Large power boats with variable pitch propellers will often bring the boat up to cruising speed then adjust the pitch until the engine is at its optimum revolutions.

    Also the success of choosing the right propeller is always hit and miss. Propellers are produced in fixed bands diameter and pitch. If your demands happen to match that of a particular propeller you will be very happy. If your demand falls in the middle of a band for a particular manufacturer you will likely be disappointed with the result.

    Peter

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