Why do sailboats seem to be more fuel efficient that powerboats

Discussion in 'Boat Design' started by Chuck Losness, May 4, 2008.

  1. Chuck Losness
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    Chuck Losness Senior Member

    Although I have been reading this forum for awhile this is my first post. I think that there are a lot of sharp people on this forum compared with some of the other forums that I visit so I thought that this would be the best place for my question.
    Now for some parameters. I am a liveaboard cruiser and currently in the Sea of Cortez. I have a Gulfstar 37 sailboat, estimated 20000 lbs, 34' LWL (measured during last haulout), 11'10" beam (don't know the WL beam) and 5' draft. I went through my logs since I left San Diego in January 2006 and found that I have been averaging 7.5 mpg (actual miles traveled from gps divided by fuel purchased). The engine is a Perkins 4108. I normally run at 2100 to 2200 rpm which equates to around 6.3 to 6.5 knots on my gps. Prop is a 2 blade maxprop, 16 x 13.4. Information from other crusiers seems to match my fuel ecconomy.
    When I read about the fuel consumption on similar size powerboats, i.e. trawlers running at similar speeds, the powerboats are only getting about 2 to 3 mpg. And when I look at the fuel consumption charts such as those listed in Gerr's "Nature of Boats," they seem to indicate that I should not be averaging 7.5 mpg.
    So why do sailboats get 2 to 3 times the fuel economy of trawlers? And better fuel ecconomy than what you would expect from the published information on fuel usage?
    I am asking this question because I am considering going back to power, specifically a displacement powerboat and plan on traveling 3000 to 4000 miles per year. So fuel economy is a big issue. I have owned go fast express cruisers in the past but never a trawler. And my question does not include planning or semi-displacment powerboats.
    Are trawlers just poorly designed from a fuel economy viewpoint? You would think that they should have better fuel economy than a sailboat because they don't drag a deep keel through the water nor a mast and rigging through the air. Am I just missing something? Thanks for your consideration of my question
     
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  2. masalai
    Joined: Oct 2007
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    masalai masalai

    From a know nothing, many cruisers:-
    1 - - seem to be trying to "climb out of the water" - which is a loosing tactic
    2 - - have a larger water line beam or otherwise bigger frontal area to push through the water
    3 - - may loose some efficiency by angling the shaft down (to clear the bottom of the hull whereas many of the older "comfortable sailboats" will have a horizontal shaft
    4 - - Most sailboats are powered and propped to achieve a speed appropriate for that hull shape without trying to push up a big bow wave or get on to the plane.

    This is only for starters from a simple fools perspective - the experts will shortly come along to (hopefully for your point of view) howl me down, correct my errors, give you a proper answer along with some technical figures to demonstrate the point...
    It would help to post an image of your boat - out of the water, and some of those with which you are making a comparison (still in the water there should be OK)...
     
  3. kengrome
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    kengrome Senior Member

    Trawlers are work boats designed to tow huge, heavy drag nets, so fuel efficiency is not at the top of their list of design requirements. It is more important that these boats have the power and stability to drag those nets safely since that's what they are designed to do.

    People who use trawlers for cruising are naturally going to suffer from lower fuel efficiency than those who use sailing hulls. I think trawler hulls can be pushed faster than displacement speeds too -- at an ever more severe fuel consumption cost of course -- which further adds to their low mpg performance.

    I think this is true.

    I think its their transom drag that contributes more to fuel inefficiency than a keel and mast. Whenever a boat's transom is in the water it is not going to be anywhere near as efficient as when its transom is completely above the waterline.

    If you want an efficient displacement power cruiser, it makes sense to look at sailboat hulls as a starting point -- but of course you already know this ... :)
     
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  4. Chuck Losness
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    Chuck Losness Senior Member

    Thanks for your response. My question is more general than just my sailboat. I used it because I had good data on fuel economy. I'll try to attach some photo's and my prop shaft has a significant angle to it.
     

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  5. masalai
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    masalai masalai

    Chuck, that looks like she has sweet lines & would be easy to push - that is why your economy is sooooo good. Open the engine right up for a short test burst and watch the fuel consumption go through the roof... he he he
     
  6. lazeyjack

    lazeyjack Guest

    look at your water plane? small, look a t you engine small? each 50bhp, = 2 gal hour, how many bhp to push yours at cruise? 18? look at at powerboat, big waterplanne, big engine
    Talking bout waterplane, your boat must be very tender initially?
     
  7. Landlubber
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    Landlubber Senior Member

    Your underwater hull shape is designed for little or low resistance, it is basically a canoe stern, double ender, only it does not get above the waterline. The water flow over such a shape is as near to perfect as we can get (so far anyhow) for a displacement vessel, as long as it remains under hull speed (the speed where the waterline wave length is the same as the actual waterline DWL). As stated above, wack in double the required horsepower and open her up, and you too can get to waste fuel like it was going out of fashion.
     
  8. terhohalme
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    terhohalme BEng Boat Technology

    It is all about speed and used horsepowers. What is the speed you are motoring? On what speed the motorboats are going? Sailboat hulls are designed for low speeds to maximize close hauled speed. Motorboats are designed for higher speeds (motoring is so boring that they have to get there quickly). When you are motoring at low speeds, your sailboat hull is more economical. If you could go on high speed, you would need more fuel than motorboats. Remember to compare boats with equal length and displacement.

    Terho
     
  9. the1much
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    the1much hippie dreams

    most sail boats "sit" ON the water,,,trawlers sit IN the water.,,,is all i can come up with,,,im pretty "simple" hehe ;) NICE ta meet ya, and welcome to the forum hehe ;)
     
  10. Guest625101138

    Guest625101138 Previous Member

    Fuel economy is not a recent invention but it has been a forgotten cause until the cost of fuel started to skyrocket. When a 15HP steam engine weighed 2 tonne the hull designs were different than when you can get 200HP that weighs 200kg and fuel costs next to nothing.

    If you have a cruising power boat designed for economic cruising it would get double the economy of your sailboat for the same accommodation because it does not have to drag around the huge keel or push mast and rigging through the air.

    The hull shape you are looking for will be long and slender with canoe stern. These sort of hulls are coming back into vogue but not many examples around yet. Alternatively there are powered catamarans which have the narrow easily driven hulls but also plenty of stability.

    Ken is right about trawlers being designed to do a specific job. They are more a tug than efficient cruiser. Tugs need big props to grip the water.

    THe majority of boat owners have to work to afford them so their time is limited and they need speed to get anywhere in a reasonable time. As the price of fuel goes up I am certain there will be more efficient hulls coming into use. There have been a few on this site trying to work out how they can transform their planing hull to an efficient displacement boat.

    Your shaft angle is quite moderate by comparison with some you see in other hulls where the shaft is inclined at 20 degrees. You would never see an airplane with the prop mounted at 20 degrees because they need to work efficiently just to get in the air. Boats don't matter that much - just burn a bit more fuel.

    Rick W.
     
  11. tom28571
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    tom28571 Senior Member

    Chuck,

    Why do you want to change to a powerboat like a "trawler"?

    More room and comfort?

    More speed?

    Get rid of keel and mast limitations?

    If it is either or both of the first two reasons, some loss in fuel efficiency is the price you will have to pay for the difference. Although cruising trawlers may not have a lot in common with working trawlers, they are far from being the easiest boat to move through the water. All that beam and full ends which give the room and comfort with submerged transoms don't compare with a decent sailboat in power required to move it forward at or below its hull speed. Want more speed in a trawler? Just add power and many will allow you to plow forward at speeds into the teens. Want more room? Just add it and it won't affect the displacement performance very much. Try that with a sailboat and it is likely to become a total slug. Design elements like block coefficient and prismatic coefficient define major differences between power and sail.

    In an earlier fuel crunch, a builder (Martin I think) came out with a boat called the Energy 48 (memory may be off in the name) that was designed to give maximum fuel efficiency as an inshore cruiser. It had the efficiency of a sailboat under power (maybe better) but it also had many of a sailboat's disadvantages in comfort like narrow beam, restricted height of superstructure, expensive to dock because of length and pointed ends. He never sold any.

    Tad Roberts designed and built the Memory 38 which is a less extreme example of a fuel efficient power cruiser. Beautiful boat and delivers the promised fuel economy. Ask Tad if he ever sold any more of them.

    It is not that designers don't know how to deliver what you ask, they just have not been able to interest buyers or sell them.
     
  12. Chuck Losness
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    Chuck Losness Senior Member

    Rick I would love to find a powerboat with double the economy as my sailboat but they don't seem to exist. At least I haven't seen any. Can you give me some examples? What about prismic coefiecents and other design parameters to look for. As you said a displacement powerboat should get better fuel economy because it doesn't have to push a mast and keel. This just makes common sense to me but I am not a naval architect so what do I now.
    I have thought about a power catamaran. They just seem to be pretty ugly in the smaller sizes. I have a friend with an older 40' sailing cat that has twin diesels, I think 25 hp each, and he claims to get 10mpg at 10 knots. But this could just be the rum talking.
    I will be leaving Puerto Escondido today and heading to Santa Rosalia with several stops along the way and will not have internet again until Friday or Saturday. Just thought that I would give you guys a heads up. I appreciate everybodies comments.
     
  13. TollyWally
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    TollyWally Senior Member

    LOL,
    As pointed out before the increasing price of fuel does indeed increase the interest in easily driven hulls. I thought I might point out that the term trawler as used in describing yachts has almost no relevence to an actual trawler other than they are both slow. A trawler yacht might be compared to a slow bus or RV. A fishing trawler is more like a combination of a tractor and a dump truck.
     
  14. masalai
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    masalai masalai


  15. Ike
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    Ike Senior Member

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