overpowering your vessel

Discussion in 'Diesel Engines' started by scott2640, Feb 23, 2017.

  1. scott2640
    Joined: Dec 2016
    Posts: 5
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Rochester, NY

    scott2640 Junior Member

    I have heard various things about using a larger than necessary diesel engine to power a vessel. specifically i'm referring to a full displacement trawler (21 ton) operating on a single screw at low RPM utilizing a less than a 100 HP engine (as the designer recommends).

    setting aside the extra cost of a larger engine than needed and the lost fuel economy, is it bad for a modern diesel engine to be run under loaded?

    i have heard it both ways. yes- the engine needs to be properly loaded to run efficiently and maintain its well being and avoid internal build up.

    I have heard no- modern engines, particularly turbos, suffer no ill effects being run continually at low RPM.

    As I shop for used engines, i find very few for sale in the 100hp range. however, there seem to be many 6bt's and comparable lehmans in the 200+ range at good prices.

    what say you?
     
  2. Scot McPherson
    Joined: Jan 2017
    Posts: 103
    Likes: 4, Points: 0, Legacy Rep: 23
    Location: New England

    Scot McPherson Senior Member

    For best results in fuel efficiency, and improved life cycle management of your engine, you want the engine to drive the hull at cruising speed at the manufacturers recommended RPM.

    You can get away with a lot, but don't expect your engine to run as efficiently, cleanly, or last as long as it could. It depends on when you want to spend your money...Now for better, or later for good enough today that needs fixing replacing sooner rather than later.
     
  3. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    As above, diesel engines have an rpm that will give best efficiency. Using a larger engine at some other rpm is inefficient.
     
  4. JSL
    Joined: Nov 2012
    Posts: 721
    Likes: 35, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    use it or lose it!
    always check with the experts but here's my 2 bits worth.
    diesel engines like to work so long hours under light load is not good... at least for older engines, they soot up and run rough. New, computer controlled tier 3 and 4 might be okay?
    If you are running bio-fuel, 'babying' the engine might be worse still, especially if you are near the fuel's expiry life.... about 90 days.
     
  5. Barry
    Joined: Mar 2002
    Posts: 1,084
    Likes: 62, Points: 48, Legacy Rep: 158

    Barry Senior Member

    This is what was explained to me with information supplied by one of the largest Volvo distributors in the pacific northwest
    The old diesel engines had quite low fuel pressure going into the injectors, maybe around 2000 psi and only one pulse of fuel with a larger orifice and poor atomization. These engines at low rpm could load up with unburned or poorly burned combustion residue
    The new electronic engines, run common rail pressure in the neighborhood of 25,000 psi, use smaller orifice injectors for better atomization, and often have a pre detonation pulse of fuel, a detonation pulse of fuel and a post detonation pulse of fuel, meaning better combustion and less residue.

    As I wanted to troll at 4 knots with a 350 hp diesel, ie low rpm, I was worried about the naysayers and contacted the Volvo OEM sales manager and he said much the same thing but recommended a short higher rpm run before shutting down for the night.

    Regarding efficiency or poorer fuel burn at say half of max rated horsepower. If the target speed of the hull is 9 knots and it takes 80 hp to push it at that speed. If the transmission and prop is properly (sic) matched, the fuel consumption should be very close to the same if you use a 100 hp diesel or a 150 hp diesel. ie so many pounds of fuel per hour to produce a certain hp per hour.
     
  6. Sparky568
    Joined: Jan 2017
    Posts: 84
    Likes: 4, Points: 8, Legacy Rep: 10
    Location: Northeast USA

    Sparky568 Junior Member

    Newer modern electronic diesels you always want to to see 100 percent load at governed speed and max load fuel, water, gear, folks, beer what have you to make sure it is propped right. Max efficient speed will be determined by hull form. My experience is with downeast (lobster) style boats and there are some hulls that operate quite well with 450 to 500 Hp. Throw anything bigger 600+ and you gain maybe a few knots while burning a lot more fuel.
     
  7. Barry
    Joined: Mar 2002
    Posts: 1,084
    Likes: 62, Points: 48, Legacy Rep: 158

    Barry Senior Member

    So are you suggesting that all diesels run at 100 % of rated horsepower when cruising?
     
  8. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    Only if the rpm with best efficiency is the same for both engines.
     
  9. Barry
    Joined: Mar 2002
    Posts: 1,084
    Likes: 62, Points: 48, Legacy Rep: 158

    Barry Senior Member

    What efficiency are you referring to? If you are instead saying that if they take the same amount of fuel burn to move this boat at 9 knots, then your comment is the correct. Which is what I was saying. Sometimes, larger engines will use less fuel at certain hp outputs than smaller engines.
     
  10. Sparky568
    Joined: Jan 2017
    Posts: 84
    Likes: 4, Points: 8, Legacy Rep: 10
    Location: Northeast USA

    Sparky568 Junior Member


    No. Not at all. Remember electronic engines have computers with preset limits based on engine speed and fuel consumption parameters. The resultant information equates to load. Once an engine reaches 100% load even if not close to max rated speed, under hard acceleration, once it reaches 100% engine load it will not increase RPM or fuel consumption i.e. power. This is to save itself from an abusive operator. However, if 100% of available HP (under propped) is not achieved at governed speed you will not get all the available power at cruise rpm typically 80 to 85% of rated RPM.
     
  11. Barry
    Joined: Mar 2002
    Posts: 1,084
    Likes: 62, Points: 48, Legacy Rep: 158

    Barry Senior Member

    What you are describing is getting the maximum of horsepower out of an engine at various rpm. This is a horsepower curve and probably the poorest fuel consumption for that particular horsepower at that RPM that the engine can produce.

    Ignoring acceleration, the only time a marine engine sees the maximum horsepower that it can produce at any rpm is when the horsepower requirements of the propeller meet the horsepower capability of the engine.

    At all other times the horsepower production of the engine matches the horsepower requirements of the prop and is below the max horsepower engine curve.

    There have been quite a few threads suggesting that a marine diesel HAS to run at close to max horsepower or the engines life will be impacted.

    I don't accept this as there are millions of light truck diesels in the world that have gone 6000 hours plus, under lightly loaded and low rpm conditions.

    For example my 6.7 Powerstroke CAN produce about 140 hp at my cruise rpm of 1200 prox but the fuel consumption, give me an indication that it is only producing about 65-70 hp.
     
  12. Sparky568
    Joined: Jan 2017
    Posts: 84
    Likes: 4, Points: 8, Legacy Rep: 10
    Location: Northeast USA

    Sparky568 Junior Member

  13. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    That is only possible when the larger engine is operating at a higher efficiency than the smaller engine.

    By efficiency, I mean power out over power in (hp produced for fuel burnt, if you prefer to think of it that way).
     
  14. Scot McPherson
    Joined: Jan 2017
    Posts: 103
    Likes: 4, Points: 0, Legacy Rep: 23
    Location: New England

    Scot McPherson Senior Member

    I think this thread is getting a little off course. It's not about maximum horsepower, but rather which engine is best. Sure you can put in a bigger engine, but you'll still want to run it at full operating speed which means pushing the boat a touch faster, but it also means more fuel consumption. Running the engine below operating speed is bad for Diesel engines. It's better if you can get an engine and prop that is designed to push the boat at cruising speed when the engine is running at optimal RPMs for that engine. That's the ideal. If you want to push your boat faster, then you want a bigger engine, but you'll also want to run the engine at operating speed the majority of the time, and not at the cruising speed of the hull because you are running it at a lower RPM and at a lower load than designed. It's just not a great idea.
     

  15. Barry
    Joined: Mar 2002
    Posts: 1,084
    Likes: 62, Points: 48, Legacy Rep: 158

    Barry Senior Member

    This is a good graph and explains my point. The max rated, governed rpm is for one point only. Not all the way along the rpm axis. The graph clearly shows that engine might only be producing 50 to 60 percent of maximum available horsepower at specific rpm, especially in the lower rpms, with no detriment to the engines longevity.

    So as to the OPs query. Can I install a higher horsepower engine as they are cheaper and easier to buy and then the come back that you should not do this as you will not be running at close to 100 percent of max load, does not hold water.

    Would I install a 600 hp engine if I only need to take 100 hp off it. Probably not. Would I install a 200 hp engine and then take 100 hp off, sure. If the engine is a new electronic engine, high injector pressure, multiple pulse, there should not be any issues.

    The question is this.
    Would a 100 hp rated engine at max load, max target rpm use more fuel than a 150 hp rated engine running at 100 hp, or 75% of rated load?
    In spite of a larger displacement

    We used to build jet boats in years past. I don't have the tables in my hands at this moment but will give you a run down. Our hulls were the same. So the horsepower requirements at 32 miles per hour was the same

    Our first engine, carbed, put out 260 hp at max rpm. We used an A cut impeller. At 32 miles per hour, at 3800 rpm, got about 3 mpg.

    When we switched to 454's, carbed which could produce 330 hp, I think, we switched to an A3 Impeller. The engine ran near 2900 rpm, cruised 32 mph and gave us 4 mpg

    So larger displacement, better fuel economy. Of course the impeller loaded up the larger engine more effectively

    I have not lost sight of the fact that the OPs question was for a displacement hull and his designer suggested a 100 hp engine.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.