Power to Weight Ratio

Discussion in 'Propulsion' started by DogCavalry, Apr 16, 2022.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    I am fascinated by the disparity between the ratios of outboards, I/O's, and inboards.

    I am, as is any engineer, familiar with the different usage ratings of say, a marine diesel where the same engine is progressively derated under heavier and heavier duty cycles. Based on this I would expect an outboard to be rated for the lightest of light duties. But some outboards are used pretty constantly.

    I wonder what the opportunity cost, initial cost, and maintenance cost of an inboard would look like if the same working vessel had the same installed power but from opposite ends of that range. For example, a deep sea fishing boat that needed 300hp of installed diesel. One vessel gets a marinized GM turbo charged pickup truck V8, while her sister gets a yanmar straight 6 marine diesel.

    The yanmar will last much longer. Much longer. But the cummins is well under half the weight, under a quarter the cost. Much easier to install and remove, and the parts cost a fraction as much. Not to mention it's easier to find a mechanic. The weight savings might not translate to too much fuel savings, but the smaller installed volume would.

    ...But: enough cheaper, enough easier, enough fuel saved, to cover the cost of maybe 3 times as frequent rebuild or replacement?
     
    Last edited: Apr 16, 2022
  2. DogCavalry
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    DogCavalry Senior Member

    And maybe the easier work and repair means overall comparable opportunity cost. I've seen army mechanics replace an 800hp diesel engine and transmission in about an hour, in a leopard tank.
     
  3. jehardiman
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    jehardiman Senior Member

    Have you ever had a microeconomics course? This is one of those NPV, or lifecycle cost sort of things. As I have said before, I've seen main propulsors selected on lube oil cost, or how the refits fit into the docking schedule.
     
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  4. philSweet
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    philSweet Senior Member

    There used to be dive-cruise 60-100 pax cats by Goldcoast that operate twice a day nearly 365 days/yr out of Key largo. They used to lease their propulsion. Some had 2 OBs and some had 4 OBs. They had hoped to get a year out of them, but the lease guy ended up having to trade them out about every 9 months or so. This seemed to work for everyone with no-one being terribly happy about it. I know they looked hard at the comparisons to inboards, but they just didn't want to have a mechanic pretty much on staff to maintain them. I heard they got the OBs tweaked at the factory to improve durability, but I don't know the details. You can be sure that Yanmar has all the numbers worked out to a fair-thee-well. Marine diesels have a different set of options and auxiliary equipment integration options, and that will often drive selection and profit margins. The truck conversion will not likely have a smaller footprint or be much lighter when the entire equipment space is fitted out.

    Waterfront real estate is pricey and doing major maintenance in a state park is not desirable. So being able to carry the OB's 20 miles inland to a steel building on $700/acre land has real economic benefits. This wouldn't be as big a deal for commercial fishing, but it still is an issue. Remove and replace maintenance works best in a standardized environment, but boats aren't that standardized. If you had ten boats with the same engines, designed for easy R & R, and the discipline to stick to the program, you'd have a real option to look at other power options. You'd also have real options to negotiate with the likes of Yanmar. I suspect the biggest issue is that the bean-counters simply can't bring themselves to take a boat out of service to do major maintenance on a prescribed schedule. The US Navy has never managed to do it, so I doubt a guy with five tuna boats is going to.

    A reman Cat C7 marine longblock is $11,000. US Engine Production - a Worldwide Leader in Remanufactured Engines https://usengineproduction.com/products/CATERPILLAR-C7-CAT-C7-ACERT-MARINE-DIESEL-LONG-BLOCK-ENGINE-/244?gclid=CjwKCAjw9e6SBhB2EiwA5myr9sbspXjPti7k9Q_gYolpzvZMMIUQzdyHnpWuGLTKul-2CqSMaKZk4RoCdJsQAvD_BwE

    A reman Cat C7 industrial longblock is $7000. US Engine Production - a Worldwide Leader in Remanufactured Engines https://usengineproduction.com/products/CAT-C7-Engine-Long-Block-Caterpillar/34#linktohere

    The rest would all be the same. So a $4000 price difference for 300 hp. Even a 500hp pickup truck diesel won't make 300 hp for any length of time. Running 25,000 GCW, it won't run much over 100 hp average. A 6 mpg average would be about 100 hp average. 300 hp at 60 mph would be about 2 mpg.
     
    Last edited: Apr 17, 2022
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  5. DogCavalry
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    DogCavalry Senior Member

    Solid information @jehardiman , @philSweet . Thanks guys.

    I have been thinking outboards for 2 years now, but Transport Canada really doesn't like folks carrying people in gasoline powered boats. So I am reluctantly thinking I/O again. And now I have 2 alpha legs. The question is, what to stick in of them? Naturally the design spiral starts interfering with my sleep, and then I'm posting new threads here...
     
  6. DogCavalry
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    DogCavalry Senior Member

    Most informative, @jehardiman . How much consideration is typically given at design stage to maintainance? There are certainly enough tales of woe on the forum to make it clear that, at least some of the time, the answer is none.
    Getting back to combat tanks again, I know that the 1500hp engine and transmission in a Leopard 2 can be completely swapped out in an hour and a half, because that is a design requirement.
     
  7. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    Power to Weight Ratio... but aren't there a lot more variables?
    Including dollars, and that always seems to be a common theme in your threads and posts.
    So, it's power, weight, maintenance, fuel, Transport Canada, SoR, time, etc, etc, etc vs cost.
    Cost seems to become the leading variable and the rest secondary really.
    But wait, you've got assets: a $12 000 BinLiner, a 150hp outboard that doesn't run well.
    Hmmm... does it come down to what's the cheapest?
    Now would be a good time of year to sell the BayLiner, that might free up some money.
    Then you could put that toward another 150 o/b and try to get them running, together.
    When do you want the SeaSled to be in the water by?
    And how do you make that happen, or do you, and by when?
    But it's getting moved soon, or is it, do you have a mover booked?
    So many questions and, really, so few answers.
    Windows, seats, doors, bilge pumps, wiring, outfitting, paint, stocking, scheduling, oh, and lining up work.
    Who are your customers.
    Oh, and future dollars vs present dollars, cost effectiveness.
    But it all still needs to be designed, on the fly.
    How will you get there if you don't know where you're going.
    Man, overwhelming really.
    More threads showing more questions in sort of a "round-about-designing" technique.
    Maybe you have all your answers already, you just need to apply them.
    I certainly don't know.
    I'm wondering if more focus and less questions may be really helpful here.
    I don't know.
    If cash is the bottom line, then the choices should be easy?
    Not necessarily the best choice but an easy choice based on cost.
    Just do it if that's what you want is to get the boat in the water, running and making money.
     
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  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Sell those legs. Not worth the headaches to switch to i/o from a ob transom plan.

    Nothing like the reliability of a 150 Yamaha.
     
  9. DogCavalry
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    DogCavalry Senior Member

    Ah, Bluebell, right as always. And of course the reality is, I'm running in circles because I haven't been able to make much progress on Serenity in months. I've been in Edmonton this last week taking care of my father's estate. Once I can get back to work, I'll stop these irrelevant flights of fantasy.
     
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  10. jehardiman
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    jehardiman Senior Member

    It gets back to when the vessel was built, who designed it, and the intent. Remember that vessels are all about space and weight, so any work to make repair access is a volume, weight, and construction cost.
    So like an AFV, major warships with 50+ year lifespans prior to ~ 1970 had bolted access hatches and overhead trolley rails for equipment maintenance. You could pull and replace the turbines, STG's, all the pumps, shafts, etc. In the IOWA class battleships, even the 12 inch belt armor could be replaced if damaged. After about 1970, the way US warships are funded changes; there is construction money and then there is maintenance money and, in a broad brush, it is downhill from there till about 2010. I personally tried/did to design in maintenance access based on the lifespan I expect (i.e. there are expendable items), and recent (2010+) US Navy design practice is pre-plan all maintenance to generate a "life cycle" cost for the warship. This change came after the major failure of the LCS classes from a maintenance perspective.
    For commercial vessels, it is a little different. Commercial vessels are all about costs vs risk and the amount you pay for the insurance. The design/insurance life of a commercial ship is 15-20 years, or for some fishing vessels it is the term of the licensing. For the original constructor, there is no need to consider beyond that. That means there will only be 2 or 3 dry dockings before you sell the vessel and it will be someone else's problem. Since a drydocking is a non-profit time, you want to make sure that all periodic maintenance falls at these time or that there is access to preform maintenance underway (i.e. LSD cylinder replacement).
    The one area that doesn't fit these models is the state owned ferry. These vessels need to be designed for rapid maintenance and long life.

    Edit to add: After reading back the thread and your question, I will answer it for the small boat/automobile perspective. Small boats and automobiles are commodities, there is no advantage to the producer to design in anything more than the most basic maintenance access. So oil and filter changes only for the typical life of the vehicle.
     
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