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. leaky
    Joined: Sep 2008
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    leaky Senior Member

    Hi,

    I'm having a 32 Holland (lobster downeast style, semi displacement hull) built, which I'll be finishing myself starting this spring. Being a fairly easy pushing boat, and for the economics of what I'm doing and budget, I spec'd the project out for a gas engine (versus a diesel, which is the typical power choice), but am not 100% done deciding that..

    When choosing engines, one thing I've heard, at least related to these semi-displacement hulls is "torque makes speed, not HP".. I've always taken that to mean basically you get a similar speed out of a lower HP diesel compared to a gas engine (due to the huge torque difference).

    Poking around online though, I saw the below, which is a table for a somewhat similar hull (shorter but otherwise real close ~28.5 X ~10.5, draft 2 feet 10 inches).. The chart provides examples of performance with different power packages. I don't have this sort of thing for the hull I'm building, but really what I'm interested in is the pattern, which is probably similar.

    See the below - it shows a bunch of high-rpm diesels of different sizes, compared to a couple gas engines.. What I don't understand is otherwise I'd assume the 320 HP diesel, which I'm sure has all the torque and then some of the 425 HP big block, would be sorta similar performing - but in reality the 320 HP is 5 knots slower - and even the 440 HP diesel still doesn't quite produce the same top speed (when that 440 HP is probably > 800 lbs of torque versus the gas big blocks ~500 lbs, and they are similar HP ratings)..

    Now obviously weight is a difference, maybe in why the 440 diesel is similar speeds to the 425 big block - but I don't get why the torque difference doesn't seem to show here? Can anyone take a stab?

    NGINE IDLE SPEED / CRUISE SPEED / TOP SPEED

    Marine Power 350 Chevy EFI 325hp 3.5 Kts 21.5 Kts 28.5 Kts

    Marine Power 8.1L EFI Chevy 425hp 4.0 Kts 25.5 Kts 34.0 Kts

    Yanmar 4 cly Diesel 240hp 4.5 Knts 20.0 Kts 26.5 Kts

    Yanmar 6 cyl Diesel 265hp 4.0 Kts 21.0 Kts 27.0 Kts

    Yanmar 6 cyl Diesel 320hp 4.3 Kts 23.0 Kts 29.0 Kts

    Yanmar 6 cyl Diesel 370 hp 6.0 Kts 25.5 Kts. 32.0 Kts

    Yanmar 6 cyl Diesel 440hp 6.5 Kts 27.5 Kts 33.5 Kts

    FUEL CONSUMPTION:

    (Fuel consumption is more a factor of boat speed than engine size. These are approximate figures.)

    Most diesels will give the following fuel consumtion regardless of size:

    16kts-4.5gph 20kts-6.0gph 24kts-9.0gph 28kts-15.0gph

    Gasoline engines (multi-port fuel injected):

    16kts-6.0gph 20kts-8.0gph 24kts-12.0gph 28kts-24.0gph
     
  2. philSweet
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    philSweet Senior Member

    Hi leaky,

    What matters is power. Power that is transmitted to the water. Most boats run props that are a bit smaller than what gives best efficiency. Cost is the reason - bumping the prop dia up 6 inches means bigger shafts and a heavier tranny, and the cost difference is substantial. You have a lot of tranny ratio options for these motors because they are popular sizes, so there is no trouble with the setup mechanically, its just a matter of being honest with yourself about usage and whether you want good operating costs or low initial install costs. There's a range of maybe $20k in initial costs between best and what's workable.

    Basically, for a given boat, speed, and cost objective, there is one right prop (one dia, one pitch, one rpm), and it doesn't care whether it is shafted to a gas or a diesel. You're not going to find an exact fit - ever. So you need to figure out what gets you close.

    The efficiency curve is really, really flat as far as small changes in rpm are concerned (from the optimal). You can run a shaft speed 5% faster or slower, and adjust the prop pitch, and not know the difference. This is why trannys come in the ratio step sizes they do. There is one that is close enough. These trannies and props will all cost about the same, too.

    The efficiency curve is sensitive to diameter, and so is install cost. As a practical matter, for the type of boat you are building, you probably can't fit a prop big enough to maximize efficiency. So it comes down to comparing costs. Start with the biggest diameter wheel you can fit and work backwards to end up with a particular shaft, prop and tranny. If you don't like the price, drop down one shaft size and see what you can fit that maximizes the use of the shaft. The shaft will give you an upper limit on prop dia and power. Check costs again.
     
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  3. leaky
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    leaky Senior Member

    Thanks!

    I've heard people say before, a given boat is "not worth" putting a diesel in it because of prop size limitation.. What you said sorta contradicts that - is there any sense to the statement?

    Would it be fair to say, based on your info, that although potentially the 440 HP diesel should be able to push the example boat at a higher rate of speed, the lack of efficiency of the drive train (whatever they used in this case) is preventing it from doing so?..

    Like basically you have one engine that's 440 HP and probably 880 lbs torque.. Another that's 425 HP and 500 lbs torque.. Weight difference of maybe a few hundred lbs in an 8000 lbs boat, yet the less powerful engine is actually pushing it faster (is that the RPM range really causing that?)..

    I think most of these smaller boats come through with about the max shaft diameter anyone would use based on the keel (and sized wheel they can run).. On a Holland 32 I thought that was 22 inch and a 1 3/4 shaft but trying to look it up saw at least one guy running a 23 (may be pushing his luck)..

    There's one no-expense spared 32 holland built recently, with a 570 HP diesel (insane yes) running a 1 3/4 inch shaft, however that boat also has a blocked down keel. So I think in my case anyway that prop shaft size is basically set for all intents and purposes, builder puts a 1 3/4 in there as part of the kit price, no matter what I do it's gonna work.

    Any light to shed based on the above?

    Thanks in advance!

    Jon
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    PhilSweet has given you some excellent advice.

    Thus no truth in your statement/old wives tail at all.

    A propeller, any propeller is governed by only 3 variables:-

    1) Speed of water flow into the prop...so is there a large skeg in front that is disturbing the flow, for example...need to eb accounted for as it is not smooth open water flow.
    2) The power being delivered at the prop (less mech losses). - nothing to do with the torque at all
    3) The shaft RPM

    That is it.

    The rest is snake oil salesmanship :p
     
  5. philSweet
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    philSweet Senior Member

    440 hp @ 880 ftlb implies 2626 rpm. Run it through a 1.63 tranny and you have 1611 rpm @ 1434 ftlb (no tranny losses)

    425 hp @ 500ftlb implies 4464 rpm. Run it through a 2.9:1 tranny and you have 1540 rpm @ 1450 ftlbs.

    You need to match the prop to the speed, the power, and to some rpm that is reasonably efficient. You can choose the diameter, pitch, and tranny gearing to try to get there. You can also choose the sort of prop. That usually means you have a diameter constraint and are being forced to use more and fatter blades than are ideal. Don't get too hung up on efficiency. Play with some practical choices, and you will probably find that attractive and similarly priced options have similar efficiency. That may not be the best efficiency, but it would appear to be what works for you.
     
  6. powerabout
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    powerabout Senior Member

    make torque curves that have been modified so its prop shaft torque meaning with the rpm and engine torque using the gearbox ratio.
    Then compare them and if you could get the hull load graph you generally see the diesel can pull the same propshaft torque/at same rpm as gas engine with a differing gearbox ratio BUT it may not get the hull there without a multispeed gearbox

    Guy had a thread here wondering why is diesel outboard went so slow compared with 2 stroke outboard.
    Outboard has a torque curve to suit a planning hull, everything else is not as good ( less electric)
    As Phil says, its power, thats torque and rpm both very important when talking boats that do more then 5 kts
     
  7. NavalSArtichoke
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    NavalSArtichoke Senior Member

    P = T*N/5252

    where

    P - power output in HP
    T - torque in ft-lbs
    N - RPM

    Propellers convert the torque of the engine (as modified by any gearbox in the shaft line) into thrust.

    Engines are rated according to their power output, but selecting the right engine-propeller combination means looking at the torque of the engine and the speed at which the torque is delivered, among other factors.
     
  8. TANSL
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    TANSL Senior Member

    Excuse me, gentlemen, but I'm a little confused. One expert says that the power has nothing to do with the torque. Another expert said that the power is directly proportional to the torque.
    Where is the truth, please ?.
    I sense that I know the answer but I would like other opinions.
     
  9. leaky
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    leaky Senior Member

    Thanks again guys!..

    OK so what I'm starting to think I understand (and some of you have basically written just this) is really the whole idea of an engine having a separate "torque" and a "HP" rating is kinda a farce. HP is a function of torque at the RPM where it is created and we can only really measure torque, then HP is calculated from it.

    Basically an engine has some max HP, someplace in it's range, which is based on the torque at a given RPM. That's really a true rating of an engine when you are talking how fast it could push a boat on the top end?

    So with a diesel, when they say they make more torque - what really is significant, and what someone really means, is not the actual torque figure, rather the fact that a given engine is generating a good amount of HP at a low RPM. Which potentially can give a boat a good cruise at a low RPM, and tends to be more efficient?

    But if you put a 440 HP diesel up against a 440 HP gas engine, and run them both at whatever RPM generates their max HP, negating losses of different gearing and weight differences, theoretically you can be running the exact same prop and will end up at about the same speed.

    Is that all correct?

    Thanks,

    Jon
     
  10. philSweet
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    philSweet Senior Member

    Yes. That's it.
     
  11. leaky
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    leaky Senior Member

    Along those same lines I did some quick math to understand what you see comparing the 440 Yanmar example to a 430 HP gas..

    Yanmar ~ 900 lbs torque @ 1500 RPM => 1500 * 900 / 5252 = 256 HP

    Gas ~450 lbs torque @ 1500 RPM => 1500 * 450 / 5252 = 128 HP

    Obviously - if geared right, then the Yanmar is going to push the boat a whole lot faster at that RPM..

    But at some point, way up on the top end, where the Yanmar is at ~3000 RPM, and the gas engine is at ~4600 RPM, the same calculation is going to yield a similar HP.

    And - as far as fuel usage goes, the Yanmar will be burning like 24 GPH and the gas engine 32 GPH at that point.

    Jon
     
  12. leaky
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    leaky Senior Member

    Thank you so much guys! Really learned something!
     
  13. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The point is, for the same installed power, you could have two different torque outputs, depending on which engine is selected.

    You have to choose the prop based on how much torque you want it to absorb.
     
  14. leaky
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    leaky Senior Member

    In that situation, one could change the gearing so that the outputs are the same though, correct?...

    Like in this example that philSweet gave?

    440 hp @ 880 ftlb implies 2626 rpm. Run it through a 1.63 tranny and you have 1611 rpm @ 1434 ftlb (no tranny losses)

    425 hp @ 500ftlb implies 4464 rpm. Run it through a 2.9:1 tranny and you have 1540 rpm @ 1450 ftlbs.


    And if that's correct, in the process of trying to get things right, since really it's the prop that you are sorta stuck with (ie no matter what a 22 inch prop is as big as I can go), you probably want to select a prop then try to match the gearing to the prop, then make small changes to the prop to fine tune it?

    Thanks,

    Jon
     

  15. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The use of a gearbox doesn't change the power output, except by a few percent for friction losses in the gearbox itself.

    Essentially, power in = power out - say 2%-3% loss

    What the gearbox does is change the RPM of the engine to something slower, which the propeller is better able to handle, to avoid cavitation, etc.

    Since the power into the gearbox is roughly equal to the power out, but the speed of the shaft into the gearbox is reduced by the factor of the gear ratio, this means that the torque out of the gearbox is the torque in multiplied by the gear ratio.

    For example:

    Engine produces 100 ft-lbs torque @ 2000 RPM
    Engine power: 100 * 2000 / 5252 = 38 HP
    Gear box with ratio of 10:1

    Propeller turns at 2000 RPM / 10 = 200 RPM
    Torque on propeller is 100 ft-lbs * 10 = 1000 ft-lbs
    Power out: 200 * 1000 / 5252 = 38 HP (less losses)

    The propeller is selected to absorb 1000 ft-lbs torque and the propeller shaft must also be sized to handle 1000 ft-lbs of torque, all operating at 200 RPM.

    The above is just an example; it is not intended to reflect any actual operating equipment.
     
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