How Torque, HP, and max RPM relate to speed/cruise? How do Gas/Diesel compare?

Discussion in 'Powerboats' started by leaky, Oct 20, 2015.

  1. powerabout
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    powerabout Senior Member

    now look at the percent difference of the diesel at idle rpm to full speed and the same with the gas engine
    thats one reason why on a planning hull the gas powered boat usually goes faster, Faster potential hull speed the bigger advantage to the wide rpm range.
    Line up diesel, gas and outboard graphs
    area under the curve as they say...
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    That is not strictly correct. A prop absorbs the power deliver to it, in the form of torque and rotational speed and converts that into thrust.

    I don't understand any of this Imperial stuff. Most of us outside the USA were taught SI units.

    Power = 2.pi.N.T

    N = rotational speed (rps)
    T = torque Nm
    Power = kW

    Well, you clearly have a totally different way of designing/selecting a prop from me; unsure why too. In 30 years I've never ever used torque as in input into designing and selecting a prop's diameter/pitch. The prop sees power as a result of the aforementioned not the other way around. The prop is designed for its one and only optimum max. efficiency from the parameters based upon the power, rpm and speed of advance into the prop. Not running at the power/rpm the prop is designed to achieve its max efficiency means it is no longer at its max efficiency...but so what? If it does what it is designed to do...that's it. Talk of torque is meaningless in this context.. :p
     
  3. leaky
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    leaky Senior Member

    Oh god, now all the engineers are going to start arguing :)..

    The main thing here that got beat into my head is that HP (kW) is really HP (kW).

    You put a 440 HP (328 kW) diesel into a certain boat and get the gearing and prop about right, it does about 34 knots (63 kph)..

    You put a 430 HP (321 kW) gasoline engine into the same boat and set it up about right, and it does about 34 knots (63 kph)..

    The fact that the diesel can make 900 lbs torque (~408 kg) at some RPM versus the max torque of the gasoline engine being 500 lbs torque (~227 kg) at some RPM is of no consequence to top speed, since that is mostly a function of HP/kW.

    :)

    This is contrary to what you hear often in the retail marine world. Folks will ask "well is that gas or diesel HP you are talking about?".. And the general consensus (albeit incorrect) is that due to the "torque" difference to get similar performance less HP (kW) is actually required in a diesel.

    I think here that part has been well flushed out anyway.

    Jon
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Hahahah...it is all about what you're used too and being consistent. I have never used Imperial units..don't understand them at all. But if using Switz cheese rather than Chaumes floats your boat..so beit :)
     
  5. leaky
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    leaky Senior Member

    Haha just noticed you do not use kg for torque (ya foot/kg's isn't used much huh?), rather Nm ;)
     
  6. powerabout
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    powerabout Senior Member

    Do you have any boats tests to back that up?
    If it was bollard pull I would agree
    There are plenty of boats powered with slide rule engineers with diesels that wont reach speed and outboards installed on boats that should of had diesels because they think with correct gearing and prop they are the same thing.
     
  7. leaky
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    leaky Senior Member

    Haha I was asking you guys so not really - however see the example on the first page. This is taken from AJ downeast boats website (Allan Johnson, builds only one hull, a 28 foot lobster boat based on the 26 duffy)..

    Was within 1/2-knot between a 440 Yanmar and 430 HP big block gas, in the example boat (which is a 28 foot AJ downeast).. Also within 1/2-knot between the 320 Yanmar and 325 small block as. I'd say the formulas work with this boat anyway.

    This particular boat is a fairly easy pushing semi-displacement, but fully keeled hull. Without a doubt they plane.

    Jon
     
  8. leaky
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    leaky Senior Member

    I do agree with you though - when they put an outboard on this style hull, they change a bunch of things on the hull...

    When you take something like a 4000 lb 25 foot center console, take the pair of outboards off, and put a single diesel in it, you do not have a better boat (although a lot of people think it makes a better boat because of their infatuation with diesel power).
     
  9. philSweet
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    philSweet Senior Member

    Regarding gas vs diesel etc.

    Happily for the OP, his lobster boat is right in the center of the big-picture powering chart. There are dozens and dozens of options available to him. Not so for all of us, but for him, it is a smorgasbord, or smordieselbord, if he prefers.

    No one is saying there aren't differences, or reasons one might prefer one and somebody else the other. But they are both going to cruise at 28 knots if both motors are rated 280hp at cruise and are geared and propped correctly.
     
  10. leaky
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    leaky Senior Member

    Ya - I'd say with this one though the cruise is really 20 knots with the diesel, somewhere around 16 with the gas, (might be 18 but we have no stats on that).. That's the most efficient speed, which is real typical.

    The example boat, and what I'm building which is real similar, are designed to cruise in the 15-20 knot range when powered to reach such speeds, while also being able to "go" if someone wishes to do so at the expense of fuel.

    Actually with larger models in the 32+ range, it's pretty common to have the best miles per gallon down closer to idle/displacement speeds - those making 100+ mile trips out to the fishing grounds will sometimes run them at 8-10 knots for maximum efficiency. That does not seem to be feature of this smaller hull though.

    Jon
     
  11. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Pot-ay-toe, Po-tah-toe

    Torque and power are two sides of the same coin: what the engine does while it's working.

    I'm bi-lingual, so I have to understand it all:
    Imperial and 47 flavors of metric/SI

    Same formula basically for converting torque in ft-lbs to HP
    N - RPM
    1 HP = 33,000 ft-lbs/min

    HP = Torque * 2* pi rad/rev * N /33,000 or
    HP = Torque * RPM / (33,000 / 2 pi) = Torque * RPM / 5252


    Torque is meaningless, until your propeller isn't pitched properly to absorb the torque of the engine at the vessel's designed speed and engine RPM.

    Sure, there are going to be times when the vessel will be operating off its design point, but that's no excuse for not doing the job properly when the vessel is operating at its designed speed and engine output.

    Apparently you don't design using modern propeller performance charts, like the Wageningen B-Series data, which plot Kt and Kq against J.

    [​IMG]

    J = Va / n D

    Kt = T / p n^2 D^4
    Kq = Q / p n^2 D^5

    eta 0 = (J / (2*pi)) * (Kt / Kq)

    n - Propeller revs per second.
    p - density of the water
    Va - speed of advance
    D - propeller diameter
    eta0 - propeller open water efficiency
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Nope, i use standard Bp-delta charts. Like below:, simple.

    Stnd G-B series props.jpg

    Not sure what you're suggesting here? Care to clarify?
     
  13. powerabout
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    powerabout Senior Member

    with those stats from the builder I would go diesel
    No free ride there for the gas engine
    there is planing and there is planing
     
  14. leaky
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    leaky Senior Member

    Yes I'd say it's more on the diesel side of the curve, but not by a whole lot.

    The cummins 5.9 engines were probably perfect in something this size, for someone in the US anyway, good HP at low weight, low cost to maintain. Unfortunately with the environmental regs, they shelved this engine. CAT also made a decision not to make the smaller engines initially when coming up to Tier III emissions code. JD's are just too heavy and heavy duty, they don't really make high performance stuff in this size anyway. There is the 6.7 cummins now, just it's adding 300+ lbs to the weight, I'd rather not do that.

    The newer Yanmar light duty twin turbo V8's look pretty good for this boat, but they have a reputation for being outrageously expensive to maintain (even on the simple things like proprietary filters at 4X the cost of what you'd buy for another diesel). So even after the added 20K initial cost the odds are you are incurring more expense, and at least my impression is the twin turbo versions are not going to be so long lived and low maintenance - ie maybe 2X gasoline engine life, but not 4X.

    Will checkout Iveco/FPT too, not sure what they have in this ballpark yet - may still go the diesel route, it is just a lot less attractive without a US engine and given the fuel savings is kinda minimal - especially at my 300 hours/season.

    $20,000 US buys a lot of fuel in this sort of boat, and a gas engine is quieter and cleaner burning, less to insulate, less vibration to overcome.

    Jon
     

  15. powerabout
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    powerabout Senior Member

    I agree all up costs, gas beats diesel in many pleasure boats
     
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