overpowering a sailboat

Discussion in 'Gas Engines' started by Paul No Boat, Jan 3, 2010.

  1. Paul No Boat
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    Paul No Boat Junior Member

    Most people agree that when designing a boat one should allow enough power for added systems such as alternators, pumps, etc. But if you overpower a boat does it necessarily effect fuel consumption? by much?

    I am thinking about back when we had lead sled cars that frquently got better gas milage with a big engine loafing along than with a small 6 cylinder working at its maximum. ie: Would an 8 horsepower engine running 60% open get the same as a 5hp running full speed? I know there are other variables.
     
  2. gonzo
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    gonzo Senior Member

    A small six cylinder working at its maximum can be more ecomomic. There are less energy losses, including hauling less weight. The extra weight alone of a grossly oversized engine will cause a loss of efficiency.
     
  3. Frosty

    Frosty Previous Member

    Grossly oversized possibly. Rule of thumb is a diesel consumes 5 galls per 100Hp used per hour.

    There are times when you really need to plow into it needing more power than you would normally use.

    The comfort of knowing you can get there rather than hide behind some island is nice.

    Theres is no excuse for under powering today.
     
  4. Paul No Boat
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    Paul No Boat Junior Member

    I understand that there is a point over overkill too. I was thinking of small engines where the difference in weight would be less than..oh say..20 pounds but give you the option of adding additional systems later. like a generator without becoming a drain when putting along.
     
  5. Frosty

    Frosty Previous Member

    20 pounds is more or less only 10 liters of fuel. If you have the space and the extra dosh I certainly would go bigger.

    When looking at the Hp output look at the displacement, bore X stroke of the motor, it may well be the same motor with different fuel injection calibrations.

    In this situation the extra weight may not be from a bigger engine but bigger ancillaries.
     
  6. Paul No Boat
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    Paul No Boat Junior Member

    I agree, Frosty. Engines in general across all modes of transportation have become more efficient over the years. and there are people who can extrapolate down to "Thermal efficiency" ie: work done per litre of fuel.

    It floors me when I look at cars with variable timing, 24 valves, cylinders that can cut in or out on demand etc. all this is well and good when new and working perfectly but when a vehical gets a little older and finicky I sure don't want to be stuck in some remote place where variable timing is something the mechanic read about in a popular mechanics article.

    I still see beauty in the old flatheads which may not have been as efficient, but darn, you could rebuild one with a tool kit off the rack from K mart.
     
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  7. PAR
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    PAR Yacht Designer/Builder

    There is an advantage to having more shaft power then is necessary to get through the average conditions you experience, but it's a diminishing point of return after a while.

    It's only an advantage if you're running the accessories you want, such as the extra alternator, generator, water maker, etc. If you're not running these things, then the extra fuel it takes to spin up the heavier reciprocating mass is wasted.

    How much bigger can you go? Who knows, but you can do a rough load check to see how much power you need to spin everything at once and work from there.

    Paul No Boat, you're correct that the mechanical systems previously used on cars to alter timing or drop cylinders out under light load, etc. are fraught with issues and the very reason these systems didn't catch on. This is much less the case now. Electronics have changed everything. Electronic controls have completely changed the way engines can be operated.

    In the days of the flathead, which was a grossly inefficient engine, but simple to fix (unless it cracked, which wasn't that uncommon). You changed out points and spark plugs every 10,000 miles, you refaced valves every 30,000 and at 100,000 miles, the rings were worn out, the engine burned oil and she was probably tapping pretty good. I too remember the "good old days" when getting flat tire was a common occurrence, setting the dwell was done every year, replacing plug wires, distributor caps, rotors and points was just part of the typical costs of a good tune up.

    This isn't the case any more. Electronic controls came in during the early 90's and have matured into fully integrated systems that literally have replaced whole systems. The distributor, which was the source of so many tune up parts is gone, in favor of a piece with no moving parts to wear out (a magnetic crank trigger), spark plug wires routinely last well over 100,000 miles and still fire reliably, tires are so tough now that you can drive them the full life of 50,000 miles with out a flat, but with several bits of metal and the like embedded in the tread.

    When people bring this up, I'm reminded of a cute girl with big boobs, I had the hots for as a young man. She had a Galaxy 500 and it tended to vapor lock on occasion. She would bring it to me to work on it and called if she couldn't start it. Well, I would ask where she was, how long the car had been sitting, etc. and I'd wait about 20 minutes and go find her. Arriving at the car, I would proclaim myself the car wizard and wave my hand over the hood, mumbling some ridiculousness as I motioned to her to attempt to start it. It would start and I would suggest it had no choice once I'd arrived and waved my magic hands. I actually pulled this off successfully over a several year span, because she had the car from high school through 4 years in collage. I was a God, that happened to understand vapor lock.

    We don't have these issues any more, with engine and fuel management computers. Pressure is always constant, fuel delivery always precise (how may carburetors have you rebuilt?) and reliable. We don't bounce our heads off bare metal dashboards any more, there's a big 'ol air bag to break our fall. In general there's a lot more to like about modern engines to like, then not to like. Yes, I miss the simplicity of the flathead, but frankly, I'll take a modern engine over one of those hunks of crap any day. They're lighter, stronger, longer lived, more responsive, more efficient, don't leak all their internal liquids all over the bilge and the list goes on.

    Look at it this way. Would you prefer a well used, tried and true old school 50 year old hooker or would you like to deal with a new, fresh, vibrant 25 year old hooker that has a tongue piercing?
     
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  8. TollyWally
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    TollyWally Senior Member

    LOL,
    Anybody can cure vapor lock, in high school the baffling challenge is in defeating panty lock. That is the test of magic hands, mumbled ridiculousness can be the key if mumbled effectively. :)
     
  9. Paul No Boat
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    Paul No Boat Junior Member

    Par,

    I tend to agree having driven several recent pickups to 175000 with virtually no attention other than oil changes and a new set of plugs.

    The early computers were prone to gliches but I agree pretty reliable now.

    Might be nice to incorporate the new metals and electronic simplicity into a basic flathead. I remember all those tasks you speak of. But I also remember bringing a car home 500 miles using a lampcord to bypass a bad computer by going from the battery + post to the coil. early 70s computers were problem children in their own right.

    Are Marine engines built with simple access in mind? Owner Maintainable? Or like cars are they designed to be worked on by a special mechanic with special tools and special training working in a special environment at a special hourly rate?

    The unfortunate byproduct of more reliable engines is that we have lost the need to understand them. and while it's great to cruise for years with no attention at all I'd hate to have a simple malfunction 500 miles out and have to be rescued only to find a lampcord patch could have gotten me home.

    I have always felt that basic auto theory should be a part of driver's training. so I guess if you don't understand your engine you have no business out on the high seas in the first place. LOL

    Do you still have the girl? I hope she did not become the 50 year old hooker!!

    Perhaps the 21st century version of "Buy me a drink, Sailor?" will become "Configure my microprocessor, Sailor?"
     
  10. Paul No Boat
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    Paul No Boat Junior Member

    TollyWolly, yer right. along with engines getting more maintainence free, women have gotten smarter too.

    Reminds me of that scene where I drove a girl home after her car broke down and taking a remote route through the woods, I made my engine quit and announced "We are out of gas." She pulled a wine bottle out of her purse and winked and all excited I asked, "You're beautiful, Is that Chabli?"
    She said, "No, It's Exon 87 octane".
     
  11. PAR
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    PAR Yacht Designer/Builder

    Engines are just out of our realm of understanding, not the current crop of gearheads that play with them. What we used to do under the hoods of our cars, on Saturday mornings every week many decades ago, is still being done on Saturday mornings, though they have spent several hours on line Friday night, looking up what to do and where to install software patches.

    I've sort of kept up with most of the technology and understand it. The same stuff still makes them go fast, though the applications are different.

    So, it's not the engines or the computers guys. It's the dinosaurs that are talking about them. An engine management computer for a flathead would just prove how big a piece of crap it was, but it sure looked and sounded cool.
     
  12. Paul No Boat
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    Paul No Boat Junior Member

    Yeah, it's a mindset I guess. Those little flathead V8s sure looked cool at the carshows. I loved their simplicity but like the log cabins I mentioned in an earlier thread they were phased out for valid reasons.

    I guess finding "your engine", like finding "your boat" is a matter of many factors and variables. but us old guys do have a value in the industry. a friend of mine who has built more than 250 Ford 289 racing engines successfully won't touch an engine block with less than 150,000 miles on it. He says that's where the engine has tempered nicely. and he hones them like a samuri hones his sword.

    just trying to form a visual of what "My Boat" would look like.
     
  13. FAST FRED
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    FAST FRED Senior Member

    "Are Marine engines built with simple access in mind? Owner Maintainable?

    Until you get up to the size where a city bus would fit in a cylinder , there is not really such thing a as a "Marine Engine" .

    Most are marinizations of yard equipment , auto , light or heavy truck engines or perhaps earth moving equipment.

    Weather the engine is easy to service usually depends more on the installation than what donor was chosen.

    Some outboards are purpose built , but removable engines are easy to service.

    FF
     
  14. kistinie
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    kistinie Hybrid corsair

    Coming back to initial asking.
    Overpowering a sailing boat is ok if the extra power gives extra of everything, More speed, more mpg thanks to bigger prop, more electricity... for a more comfortable trawler use.

    If all these parameters are reached, then it is worth overpowering a sail boat


    Of course the same logic works for an Electric Motor choice.
     

  15. gonzo
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    gonzo Senior Member

    More power and a bigger prop does not give more MPH but less
     
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