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  1. #1
    Join Date
    Aug 2001
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    Default Interesting Technical Question

    I am trying to find out how a twin shaft propped boat performs running on one engine. If you want to really extend range to a thousand or two miles on a trawler style boat there may be a higher mpg if you run on one engine.

    However one engine running without locking the other shaft means that the non-working prop is actually having to be turned and is turning the gearbox etc. hence the dragging prop is a hindrance. There is also a slight rudder offset which increases the drag of the rudder.

    I know of one boat that has measured fuel consumption and found that running on one engine used more fuel to achieve the same speed than running on two engines at lower speed - no doubt due to the above to factors. However, I cannot find any source that has actually determined if there is a gain in mpg by running on one engine with the idle shaft locked. In this situation there may be an overall mpg gain- as the only drawback is the rudder offset alone.

    Does anyone have any experience or figures on this subject?
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  2. #2
    Join Date
    Nov 2002
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    5,880

    Default Re: Interesting Technical Question

    Isn't this just a different take on the perennial raggie problem - when sailing do you allow the prop to spin or not? I think the conclusion is 'it depends on the boat, prop and gearbox'. Get a floscan, do some trials and work it out.

    Another advantage of a single engined boat [img]/forums/images/graemlins/smile.gif[/img]

    Rick

  3. #3
    Join Date
    Nov 2001
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    1,429

    Default Re: Interesting Technical Question

    I always understood it was best to let the prop revolve.

  4. #4
    Join Date
    Sep 2004
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    Default Re: Interesting Technical Question

    I have tiddly engines on my boat, manoeuvring and drive for windless days. Running on one engine I can not get up to the same speed as two, 4.5kts instead of 6.

    The single engine must also be raggin its balls off to get 4.5. To get say 4.5 with both engines I can do this running at 1800 - 2000 rpm as opposed to 3000 odd. This makes a massive difference to fuel use when you look at the consumption curves, although my consumption is probably akin to your boat heater!

    Always lock props when not in use, especially when sailing, though mine are 43% blade ratio so drag is much less than a mobo prop.

    I agree, that running both is more fuel efficient for me. Though with twin sailing rudders I do not have the other problems you mention.

  5. #5
    Join Date
    Aug 2001
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    Default Re: Interesting Technical Question

    Not exactly the same as the raggie problem.

    The only test on a twin screw I have seen to date shows that the mpg is worse with just the one engine - so the best guess for a single engined boat is try to stop the prop rotating or let it rotae without it rotating the gearbox (Freewheel).

    Its simple to measure one engine versus 2 at the same speed - that is not the issue, I am fairly happy with the conclusion that two engines are better). It is difficult to measure one engine only with the other shaft locked - that is the issue. There are different shaft locking systems but its too late when you have had the expense of installing one.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  6. #6
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    Default Re: Interesting Technical Question

    How do you lock the shafts?
    I think this is difficult with twin 700hp engines.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  7. #7
    Join Date
    Sep 2004
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    Default Re: Interesting Technical Question

    For me it is just a case of putting them into reverse, they have a clutch arrangement (Yanmar) that means they will not lock in forward.

    Many yachts achieve locking by adding a disk brake to the drive shaft with the pads locked to the hull, pull a push pull cable like a hand brake and the shaft stops. Though you would seriously have to remember to unlock the brake with 700hp. Ah the smell of burning in the morning!

  8. #8
    tcm is online now Registered User
    Location : Caribbean at the moment
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    Default Re: Interesting Technical Question

    http://www.bruntons-propellers.com/p...able/autolock/

    This confirms what i understood - that one must not freewheel for long distances. I ran the (slightly busted, but not terminally) engine on tickover for 20 miles or so.

    I'm afraid i can't report details of fuel economy, in the same way that i haven't a clue how much i have spent on lots of other things!

  9. #9
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    Aug 2001
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    Default Re: Interesting Technical Question

    My gearbox maker states its Ok to free run the box for up to 8 hours - however there is no point in doing this as the combination of prop drag and rudder drag makes it pointless.

    What I am after is trying to find if someone has measured the difference between having a shaft lock and not.
    Paul
    St Francis 50 Cat
    http://www.multihulls4us.com

  10. #10
    jfm's Avatar
    jfm is offline Registered User
    Location : London/Antibes
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    Default Re: Interesting Technical Question

    We have instantaneous fuel flow rate metering, which adds the fuel burn rate data for both engines, and also reads SOG from the GPS so can give a readout of instantaneous MPG. Unquestionably running one engine in gear at idle rpm gives a better mpg figure than running 2 engines at idle revs in gear, despite the rudder offset losses and the fact one prop is being dragged. Indeed, running one engine at idle rpm in gear gives the highest mpg figure ever achievable on the boat. This is all as you'd expect.

    But turning to your question about locked vs unlocked shaft, on ours the unpowered prop spins freely. I have never done it with the unpowered shaft locked. I dunno for sure but gut feeling says that locking the prop would increase drag, compared with just letting the prop spin. Surely the components are following the path of least resistance, so allowing the prop to spin as it wants to causes less drag than locking it?

    The gearbox manuals however say do not let it spin for long. Summink to do with the gearbox oil circulation pump not powered, so the internal lubrication in the gearbox isn't functioning in "free wheel" mode.

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