Small diesel working hard or Large diesel working easy?

Discussion in 'Diesel Engines' started by Don H, Nov 14, 2012.

  1. Don H
    Joined: Apr 2012
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    Location: Queensland Australia

    Don H Junior Member

    I have always had outboards so i really have no experience with the economics of a diesel.
    If we assume a theroretical boat where say 10 knots is its optimal hull speed. 50hp with its correctly matched prop achives the 10 knots but is working hard. Fuel consumption is X liters per nautical mile.
    If you then replaced the engine with 100hp and the correctly matched prop would you expect:
    1, the fuel consumption to be more? less or about the samebat 10 knots?
    2, would the engine life span be significantly longer for the higher HP engine

    My gut feeling tells me the small engine would still be better as a big outboard still loves its fuel even at low speeds but i really have no idea when it comes to diesels.

    Thanks Don
     
  2. michael pierzga
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    michael pierzga Senior Member

    Hmm..what is the correct size power plant.

    Good question...with a million answers. You should give an example of a boat and the exact way you will use it.

    Perhaps wait for a Naval Architect to give an opinion .

    I prefer more power than what is needed.

    An underpowered boat with a small prop is a dog in close quarters.

    Im presently sailing with 180hp at 50 tons. Seems pretty good..... powerful. Cruising speed at 65 percent throttle. When the going gets tough and full power is needed, I can flatten some waves.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    For best fuel efficiency pick a diesel which generates the required 50 hp in the highest part of the torque curve. The maximum hp rating will be substantially higher, like 70-100 hp, depending on the engine construction. Some engines have an almost flat curve over a wide rpm range, but near the maximum output the curve always drops.
     
  4. FAST FRED
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    FAST FRED Senior Member

    For long term service and good fuel use , the most common concept is 80% of rated HP output at 90% of rated RPM.

    So an engine rated at 65 hp would be cheapest to keep and have the longest service life on a 50HP prop requirement..

    The downside is little excess power for climbing head seas or high winds.

    A gas engine does not suffer any where near as much from being run at a light load.

    Your "400hp" car engine ay 60 mph may give 20 mpg , so burn 3GPH and be producing 30 -35HP.

    Run a 400HP diesel at 30HP for sure early death.

    Sadly with the air police limiting the number of engine MFG you may not have a great choice in engine selection.

    FF
     
  5. michael pierzga
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    michael pierzga Senior Member

  6. Don H
    Joined: Apr 2012
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    Don H Junior Member

    Thats pretty much what i expected to hear but i didnt consider a shorter lifespan of the larger engine on a low load.
    Consider a Powered Displacement Multihull with twin 250hp engines. Top speed is quoted at 23-25 knots. If we are travelling at 10 knots for the majority of the time would i be correct in saying the life span of the engine will be significantly reduced?
    From what i have read it seems that to go twice as fast approx 8 times the power is required so to travel at 10 knots 50hp should be plenty i think.
    It would seem that the small engine is the choice for fuel consumption and engine life but has a downside of not enough power for some conditions?

    Thanks Don
     
  7. Frosty

    Frosty Previous Member


    No well that subject to personal interpretation
     
  8. michael pierzga
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    michael pierzga Senior Member

    Hmm.

    Consider a 50 amp alternator on one side and a 100 amp alternator on the other side of a " small " diesel then think about lugging and service life .

    A cat is something I have no experience with. For instance , when seeking economy do you travel on one engine ?

    Also consider reality. How many engine hours and how many years will you own the vessel ?

    Are you a cheap and cheerful type ?

    Or a Continuous duty type ?
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Some forum members keep repeating that story although there is enough evidence for a question mark.

    My SUV has a 140 hp common rail diesel, but I am retired so there's no hurry and a use maybe 25% or even less. After 5 years of service it still performs like new.

    The cell phone operators in this country use solar panels for their towers where that is possible, but a few are on the north slope of a mountain. In that case they use an industrial 4 cylinder diesel running at 1500 rpm.
    Because they have little faith in a start/stop system, the diesel runs 24/7, even at night when the load is nearly zero. Service interval is 3 months (over 2000 hours), in between they fill the tank once a month.
    If there would be any truth in the lifespan story, the company would have surely installed smaller engines.
     
  10. michael pierzga
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    michael pierzga Senior Member

    Ive got 14,000 hours on my power plant. A naturally aspirated Mercedes straight six. I never operate "correctly ". The rpms are backed off with 100's of hours per year at near idle. I observe no deterioration other than premature clogging of the exhaust water injection jacket.
     
  11. Aliboy
    Joined: May 2011
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    Aliboy Junior Member

    I run my twin 330hp 5.9l diesels at 1200rpm rather than the rated 2400 - 2500rpm cruise speed for around 85% - 90% of the time. They do get a quick burst at 2300 - 2400 for 15 mins or so after a long day at 1200rpm, but so far 4000 hours without any sign of 'excessive wear'. Personally i would be looking for engines in the 100 - 150hp range if I wanted a steady 50hp. Even 200 - 250hp engines making 50hp shouldn't be an issue as long as you give them a regular run at rated cruise or higher. Other thing I would look far if after longevity is larger displacement engines rather than the latest ligh weight small displacement engines making big hp.
     
  12. Frosty

    Frosty Previous Member

    Yes its time this glazing crap was put to bed. We all do --I do every one does.

    Its the Arm chair guys again reading books.
     
  13. WestVanHan
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    WestVanHan Not a Senior Member

    Perhaps you need to look at peak vs continuous hp?

    I run my twin 640 hp 10.8 Cummins QSM11s at around 1100 rpm 95% of the time and from what I'd guesstimate that's about 100-120 hp each.
    1600 hours-perfect condition.

    I guess in the old days when the injectors dumped in one big shot of diesel it happened.
     
  14. Frosty

    Frosty Previous Member

    Perhaps we should just push that past us and get on with it.
     

  15. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

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