Load vs economy

Discussion in 'Boat Design' started by Magnus W, Jan 12, 2019.

  1. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Almost all marine diesel engines will furnish engine performance data like Engine Hp/torque curve and power demand/fuel consumption. I threw away my sets of data (from 100 Hp to 1,000Hp) as it was getting outdated. Besides, everything can be downloaded from the net these days. These are manufacturers data and as reliable as the ones from aircraft engines. The trend now is for engine to have black boxes. The manufacturers representative can come and download the history of the engine use. Say when did you last load it, what RPM, what day/time, ect.

    What you are doing is correct, plotting the fuel consumption over different load conditions and speed. But from what AH is saying, from a designer's perspective, there is more to that. I will try to find if I still have the graphs that was posted here a long time ago. I am sure it was discussed heavily.

    What you need is an accurate flow meter and a GPS in order for your data to be valid.

    And before I forgot, forget worrying about the stress on the hull. It is probably designed to operates in seas beyond what you are operating at present. Very roughly, this is equivalent to the performance envelope they use on planes. Like how many g's or negative g's the airframe can handle.
     
    Last edited: Jan 16, 2019
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Red herring. It plays not part in your attempt to do a cost analysis. The hull will always outlast the engines.

    You're not fitting a GT to your boat.
    Again, another red herring!

    Define hard data...?
     
  3. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    Not familiar with the term red herring but I sense it's some sort of critique towards my reasoning.

    1. And for this type of boat the engine can easily be replaced. The hull is in this case the single largest cost and when it dies, so does the boat. An engine is just en engine.
    (I know from experience that boats, especially commercially operated aluminum boats, does have an end of life. Don't make the mistake and compare commercial use with pleasure as they often very much differ. I know many ex commercial boats that live happily as pleasure boats long after they've become to old and weak for the harder commercial duty.)

    2. And a moose is not a giraffe. But despite that fact, both will be able to run farther if they take it a bit slower than giving everything they have. Same with engines I presume.

    3. As in stating the expected tbo in relation to how the engine is operated, taking into account for instance maximus load, average load, load distribution, cycles etc.
     
  4. rxcomposite
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    rxcomposite Senior Member

    At first I thought you are evaluating a boat or establishing a baseline using empirical data from a test boat or hull.
    You said " In my business I mostly get paid by the hour when transporting goods and/or passengers a certain distance"
    And "This has been done and as per the data there are no fuel gains to be had between 28 and 18 knots – the fuel per distance curve is flat in this region."

    Now it appears you are designing a boat. Right?

    I suggest you consult a naval architect/marine engineer to guide you or stick to the topic of designing the boat because we would not reach anything in this discussion. Designing a boat start first by defining the SOR, the way YOU intend to use the boat. Define the parameters and the design process of optimizing the boat performance will fall into place.

    I see many things wrong. First I steered you to get a manufacturer's performance curve. You said there is none.

    Second, you are now talking about the strength/longevity of the hull. And then the engine to be used.

    The strength of the hull is dependent on the condition you will operate on, say near shore, offshore, open water, ect. If it is a workboat, a certain increase in strength is added to the scantlings, say 10%.

    If it is a workboat, then the type of engine is chosen. Pleasure duty, intermittent, continous duty are the primary categories. It depends on how many hours per day/year you intend to use it. It is in the datasheet.

    There is also mention of waterjets. Jets are more efficient at high speed operations. You also need high speed diesels to match. Jets are used in high speed ferries, not workboats.
     
  5. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    I already have a few boats in operation so that's how I know how to balance the speed/time equation.

    "What has been done" is that the jet manufacturer(s) have done a performance analysis based on the hull data (the hull to be, according to the SOR) for a few of different engine choices. From this we have come to the conclusion that there aren't any gains in fuel to be had by altering speed in the region of 18-27 knots. This regardless of engine choice. Some other gains were found, like running the jet sans reduction to get rid of transmission losses, which took some doing on the part of the jet manufacturer in order to find the best engine for the jet (and not vice versa which seem to be the more common route).

    Naturally we have engine performance data from the manufacturers, impossible to calculate jet and consequently boat performance otherwise.

    I'm not sure if what I do would qualify as designing. I'd like to say I'm making an informed decision on how to best spend my earnings and investing them in what will hopefully be the best boat that money can buy. For me. As per the SOR. And I have several naval architects involved.

    I mentioned hull only as a way of illustrating what I originally asked. And still do:
    Are there any accepted methods of determining the benefits of using "whatever" at a lower load than what the rating or design approves as max? Or a rule of thumb as the one I referred to in my aircraft engine example?

    Yes, jet is the choice. Horses for courses, in my case there are many distinct benefits in running a jet which are greater than the drawbacks.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    You have not presented any evidence to support this claim.

    Nonsense. It is very easy.

    Well, either they do not have the data, because money has not been spent calculating and obtaining said data, or they do not know how to do this simple calculation.

    Yes...as noted before.
    But it is very clear from your replies, you either do not understand the reply or you do not have the data required to inform of the conclusion, yet maintain that you do? o_O

    A vessel I previously designed showing the resistance overlaid onto the jet thrust:-
    upload_2019-1-17_9-57-9.png

    At 30 knots, 700kW is required and this is at 2100rpm:
    upload_2019-1-17_9-57-59.png

    The fuel consumption is 190 l/h
    upload_2019-1-17_9-58-32.png

    So if the ruote is say 20nm, this equates to burning 127 litres on that 20nm run.

    If i reduce speed from 30 knots to 20 knots, power required is 280kW. It takes longer to traverse 20nm, it takes 1 hour, whereas before is was just 40mins. Yet at 20knots it burns 70 litres over that 1 hour trip.

    Takes 20mins longer but you burn 57 litres less fuel. In any way you wish to look at this that is a large saving in fuel.

    As noted before, you DESIGN the boat and then you have the data....clearly YOU do not have said data.

    Nonsense.
     

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  7. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    Last reply here, sorry to have the thread derail like this.

    But again, I haven’t asked about fuel. Why do you insist on answering a question never asked? It was slways and ONLY about load vs economy with regards to wear which I have stated repeatedly.
     
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Because you simply haven't a clue what you're taking about.

    And that's the point. You haven't a clue, because fuel cost IS NOT IRRELEVANT, as noted above by the very simple calculation noted above, based upon actual data.

    Geeesssss....this thread is just wasting bandwidth.

    Outta here...enjoy your self flagellating soliloquy.
     

  9. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    Can’t help myself...

    And please, no need to be rude.

    Please tell me where I said I don’t care about fuel consumption?

    I said that IN THIS CASE I’m interested ONLY in how load affects longevity of the powertrain and other components of the boat.

    Did I ever say that it’s the ONLY thing to consider?

    And a comment about jet performance. If I interpret you correctly you state that it’s easy to calculate performance without having data from the engine and jet manufacturer. That you mean that as long as you have the thrust needed for a certain speed, generic jet and diesel perfomance will give good enough accuracy?
     
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