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  1. #11
    Wiggo's Avatar
    Wiggo is online now Registered User
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    Default Re: Interesting Technical Question

    Am I missing summat here, or just being too simplistic? What happens to the prop/shaft/gearbox if you put it in gear with the offending engine shut down? Water tries to turn prop, prop tries to turn shaft, shaft tries to turn box, box tries to turn over bloody great high compression diesel lump. Like trying to push start a jumbo jet, innit?
    The nicest thing about not planning is that failure comes as a complete surprise, and is not preceded by a period of worry and doubt.
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  2. #12
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    Default Re: Interesting Technical Question

    Is it possible to do that with all singing &amp; dancing leccy controls?

    Rick

  3. #13
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    mikef is offline Registered User
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    Default Re: Interesting Technical Question

    To my way of thinking, a boat travelling at a certain speed needs a certain amount of power to drive it so the crux of the question is whether it is more eficient to provide this power through 1 engine or 2. I would bet that the mechanical/heat efficiency loss is greater with a 2 engine system than 1 engine even though that one engine is working at higher rpm to deliver the same power. I agree with jfm in that locking the dead propshaft may well cause greater drag than allowing it to spin freely but not so much as to make it less efficient than a 2 engine system
    The only circumstance where a single engine might use a lot more fuel is where the helmsman tries to get on the plane with a single engine but on most boats you can't get on the plane with just one engine so you're just wasting fuel and stressing the engine
    I believe that delivery skippers do helm boats on one engine or the other to eke out the fuel supply on long trips so I guess it must be accepted wisdom that one engine uses less fuel than two, at least at displacement speed

  4. #14
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    Default Re: Interesting Technical Question

    In fact measured mpg on a 42 foor twin screw power (twin 120 hp) boat doing 7 knots gave a 5% higher fuel consumption than using both engines!

    The rudder offset plus the fact that the free prop offers much more drag than a fixed prop (this is an accepted fact) led to the higher fuel consumption.

    The case may be different at tickover - accepted but it is not the case at higher speeds.

    If the shaft is locked there is some losss due to the water resistance of the fixed prop but this is not as great as that of a prop slowly turning a gear box.

    The two solutions are to break the shaft and insert something like a disc brake - the other is to hydraulically freeze the gear box.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  5. #15
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    Default Re: Interesting Technical Question

    I found this on the web

    "I installed Floscan fuel consumption meters recently, and there are first sea trial results!

    My boat is GB42 Classic woody, with twin Lehman 120, approx. 5200 hours. My test ride has been carried out in very unstable wind &amp; sea conditions, so that result may be accordingly inaccurate. I can provide complete table if someone interested, but real reason for this post is folowing simple experiment:
    I choose speed 7 KN, and adjusted 1) only port engine, stbd freewheeling, 2) only starboard engine, port freewheeling, 3) both engines, to reach this same speed.

    Speed RPM LPH (port) LPH (stbd) L/NM (MPG)
    7 1750 10 0 1.43/2.65
    7 1700 0 10 1.43/2.65
    7 1400 4.5 5 1.36/2.79

    Connclusion: on my boat, fuel efficiency is better when I am running both engines, than any one alone. It makes sense: when I shifted any side to neutral, autopilot needed to correct ruder for 3-5 deg to compensate it, creating non-optimal motion through the water. Plus drag of passive proppeler.

    Of course, it doesn't mean that twin engine boat is better efficient then single; it only means, that twin with both active is more efficient than twin with single engine active. "
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  6. #16
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    Default Re: Interesting Technical Question

    "I agree with jfm in that locking the dead propshaft may well cause greater drag than allowing it to spin freely but not so much as to make it less efficient than a 2 engine system"

    That is wrong - locking decreases drag.

    "I believe that delivery skippers do helm boats on one engine or the other to eke out the fuel supply on long trips so I guess it must be accepted wisdom that one engine uses less fuel than two, at least at displacement speed "

    That is true. Although they may be wrong to do it unless they lock the free shaft.

    There is a difference between locking and unlocking the shaft - its always better to lock the shaft. I am just trrying to find out by how much.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  7. #17
    mikef's Avatar
    mikef is offline Registered User
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    Default Re: Interesting Technical Question

    Typical Gludy thread then. Just posted to reinforce what you think you already know
    You're right, delivery skippers are wrong

  8. #18
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    Default Re: Interesting Technical Question

    Yup - I was wondering why Gludy bothers asking a question and then a few posts later says "accepted fact" and everyone else is wrong. Hey ho.

    Rick

  9. #19
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    Default Re: Interesting Technical Question

    Typical stupid reaction.
    I had hoped you had grown up a bit but no - the same old personal attack based on your inability to glean the facts and read the clear points..

    I am trying to find out by how much the fixed shaft improves the efficiency of the system - I do not know the answer.

    It is an accepted fact that a fixed shaft is more efficient - its also an accepted fact that many delivery skippers run on one engine then the other - so it seems that these facts clash- given that clash I wrote that:-

    "That is true. Although they may be wrong to do it unless they lock the free shaft."

    I did not know so I wrote MAY BE WRONG..

    It really is about time you grew up and got a life- the ability to read and actually understand a subject without resorting to personal attack would be a very helpful attribute.

    Now if you cannot do this I suggest you simply stay away from the thread.

    RikUp

    "Yup - I was wondering why Gludy bothers asking a question and then a few posts later says "accepted fact" and everyone else is wrong. Hey ho."

    I did not ask that question - I have stated that a fixed shaft is more efficient - if you deny that it is you that is going against what is accepted throughout the marine industry not me. All I have stated is that it is not correct to assume that a rotating prop offers less drag - the opposite is true.

    So how come you attack me personally - the opposite is true, I am siding with the industry on this efficiency question - I am also pointing out that letting one haft spin may lead to a situation where one engine running offers poorer fuel efficiency - I actually gave such an example that was measured and then pointed out that delivery skippers may not be getting the efficiency they thought.

    if you cannot follow that simple logic without resorting to personal attack then just stay away from my threads or grow up and try reading what is being said.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  10. #20
    Talbot's Avatar
    Talbot is offline Registered User
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    Default Re: Interesting Technical Question

    Only have one prop, and get over the drag problem by lifting it out of the water when not in use!

    speed is dependent on state of the bottom and amount of wind opposing passage - 7.5 kts on a good day with no wind.
    "Be wary of strong drink. It can make you shoot at tax collectors - and miss."
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