Calculating Propeller / Reduction: HP or Torque?

Discussion in 'Boat Design' started by Emmett Smith, Apr 10, 2014.

  1. Emmett Smith
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    Emmett Smith Junior Member

    Hey folks, signing in to ask for guidance from the experts...

    So, I am repowering a displacement cruiser from 1910. I have some options for power, but also need to choose the proper reduction gear and propeller. I have been using some calculators to help, one at vicprop.com and another excel calculator given to me by a friend. They seem great and they help, but I have two questions:

    These calculators ask for max rated hp and rpm of the engine. But, rather than using the rpm/hp at peak hp, isn't it better to use the rpm/hp at peak torque? On both my potential engines (Gray Marine 6-109 and Mercruiser 6-165) the peak torque is at much lower rpm than peak hp. The calculations are quite different when I use peak torque instead of peak hp. Which is a better indication of what the engine will like, and where it will be efficient?

    Second question: the calculators are great at telling me which prop to use with what reduction and visa-versa, but they don't give any guidance about what is the most efficient combination of the two. Any suggestions on how to determine that?

    Displacement cruiser 35' X 8'6", 2' molded draft, ~10k lbs, desired cruising speed 11 kts. Gray marine is max 109 hp @ 3400 rpm, peak torque is 196 ft lbs at 82 hp @ 2200 rpm. Mercruiser is max 165 hp @ 4200, peak torque is 242 ft lbs, at 138 hp @ 3000. I do have a prop, 20" X 16" LH, but may switch it out depending.

    The original 1910 motor, if you're curious, was a 4-cylinder 5" X 5" 2-stroke, rated 28hp. Don't know rpm but would have turned less than 1000. Original prop was 24 X 30 reversible.

    Thanks!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt you could leave out the resistance Vs speed characteristics of your hull and still get a precise answer, but others will know better.
     
  3. Emmett Smith
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    Emmett Smith Junior Member

    I would be glad to work out and provide that information if someone could tell me how.

    But really the question is about theory, I am not expecting a specific recommendation for my boat. What is the primary factor for propeller calculations, torque or hp?

    The calculators ask for hp/rpm at peak rated hp, and then they use those numbers to calculate torque. But by the time peak hp is reached the torque has fallen off, and I wonder if it might be better to give the calculator the peak torque instead. It is often hard to determine peak torque, where max hp/rpm is easy to find for any engine; this may be why the calculators ask for those specs instead of torque.

    But what do I know?

    Thanks for reading.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    HP

    Its called trail and error by simple iterations when doing by hand calculations, or you can buy software to do it for you. There is no one formula that fits all.
     
  5. Emmett Smith
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    Emmett Smith Junior Member

    I guess part of my hesitation is I am not going for speed, but efficiency. This hull doesn't need 109 hp to reach is optimal speed, and I don't intend to wind the engine up. So why does peak hp/rpm matter? If I do the calculations at the rpm I would like to operate at, I end up at higher torque. Is it bad to size a prop based on that?
     
  6. jonr
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    jonr Senior Member

    Max rated HP and rpm are irrelevant if you don't ever want to operate there for efficiency or wear reasons. Refer to a BSFC map if you want the peak efficiency operating point. A bigger, slower turning prop will generally be more efficient.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    A prop is designed to work at one pint only and thus to maximise its efficiency. The inputs are power, rpm and speed of water flow into the prop.

    So, you can elect to:

    1) Design the prop to absorb all the power you have, and then when cruising about, you have lower efficiency as your rpm shall be lower too. The amount is to be determined. (In real terms does it make a difference??)
    or
    2) Design the prop at the speed you want to cruise about at, but you will not be able to go much faster, despite having reserves of power, as the prop shall cavitate.

    Alternatively, use a CPP.
     
  8. jehardiman
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    jehardiman Senior Member

    As far as efficiency of the prop, swing the largest, slowest turning one you can find.

    As far as where on the Hp-rpm curve you should place the maximum torque is more about engine rating usage and engine/gearbox longevity than what the prop needs.

    SO.....For a given speed of advance, behind a given hull, a propeller operating at a given RPM will absorb a specific amount of torque and produce a specific amount of thrust. For this to be a viable "operating point", the thrust needs to be exactly equal to the drag of the hull and the torque produced at the shaft needs to be greater than that absorbed when the engine is at a throttle position necessary to produce the required RPM. This is why marine engines (internal combustion or steam) generally have more torque (i.e. effective piston stroke) at lower rpm than other types of engines.

    Take a look at my discussion in this thread about how a propeller is calculated.

    EDIT Cross-post with Ad Hoc...
     
  9. pdwiley
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    pdwiley Senior Member

    Yeah. Good luck finding one of those, new, for a small (sub 100HP) engine.....

    PDW
     
  10. Emmett Smith
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    Emmett Smith Junior Member

    Thanks for the replies! It will take me some time to absorb the links but lets see if I understand a few things.

    First, the most important factor for prop calculations is HP, not torque. Gotcha.

    Ok, so that is as I suspected, and these calculators are designing a prop to be most efficient at the maximum hp of my engine. Since I am more concerned with efficiency than speed, and have no need to go fast except in an emergency, it seems like that is not what I want to do. It would just mean that 99% of the time I would be operating at lower efficiency than possible.

    I looked up "BSFC map" and I would love to size my prop based on that, if I can find one. But I doubt I will. Are there any rules of thumb for determining peak efficiency? Any relationship to torque, which is max force per stroke?

    So lets assume the peak efficiency is at some lower rpm than max hp. What are the dangers of sizing a prop to something lower than max hp/rpm? I do understand cavitation, and the other theoretical danger of over-revving the engine. But actually, I don't understand why that would happen. As I go back down in RPM I am climbing back up the torque curve, and the recommended prop sizes get bigger in diameter and pitch. How could using a bigger prop than recommended for max hp/rpm produce cavitation? Or put another way, am I wrong in thinking that if I size a prop for lower in the range, that that is where the engine will top out?

    For example, two results from the calculator. If I plug in max hp/rpm (109@3400) with 2 to 1 reduction, I get a recommended 3-blade prop size of 19 X 13. The same calculator at peak torque (82@2200) with the same reduction recommends a 23 X 21. That is an enormous difference!

    I have heard that but I need some limits... Are there any limits to the recommended prop size other than hull clearance?

    My other question was about the best relationship to reduction and prop size... if what you said is literally true, should I get a 5 to 1 and an 32" prop? ;)

    I actually work for a boat museum, and we have a great big reversible about the same vintage of my boat... darn ethics... :)
     
  11. jonr
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    jonr Senior Member

    If you have to guess at peak engine efficiency, 2200 rpm at 80% load is typical. But don't waste HP trying to drive your displacement hull too fast.
     
  12. Emmett Smith
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    Emmett Smith Junior Member

    Load... now that is another question. What is load, exactly?

    And if I did select a prop that made my engine top out at 2200 rpm, am I "overloading" it? Is that bad?

    The last thing I want to do is waste hp trying to drive the hull to fast, that is what I am trying to avoid.
     
  13. gonzo
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    gonzo Senior Member

    If it did OK in 1910 with 28 HP, there is no reason to repower with something that is 6 times more powerful. There are lots of small diesels in that range. The reduction gear will depend on the RPM of the engine and the target speed of the boat with the installed power.
     
  14. jonr
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    jonr Senior Member

    > if I did select a prop that made my engine top out at 2200 rpm, am I "overloading" it? Is that bad?

    Note that this would imply 100% load at 2200 rpm. So a better example would be a setup that tops out around 2500 rpm but you operate it at 2200 rpm.

    Some people call this "over proped" or "lugging an engine". These terms are misleading because they imply that it is harmful. Yet truckers operate their engines at such points for thousands of hours (and save a lot of fuel and engine wear while doing so).
     

  15. Emmett Smith
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    Emmett Smith Junior Member

    Ok, this is becoming more clear, but it sort of means the graphs and calculators I have don't really help...

    So torque peaks at 196 ft/lbs at 2200 rpm, but that is WOT full load. What I want to do is put-put along at some low rpm and low engine output and partial throttle.

    So I don't want to size a prop that will cause the engine to top out WOT @ 2200. I don't want to top the engine out at all. So maybe what I want to do is size a prop to run at partial throttle at the most efficient speed of the engine for that particular load.

    The throttle is a variable that is not reflected in my calculators or torque/hp graphs. How do I size a prop for partial throttle?
     
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