Searching for fuel efficient powerboat

Discussion in 'Powerboats' started by Chuck Losness, Apr 29, 2011.

  1. Chuck Losness
    Joined: Apr 2008
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    Location: Central CA

    Chuck Losness Senior Member

    First I would like to thank all the posters here for all of the information that they so freely provide. It has been a real education.

    I have a Gulfstar 37 sailboat, 37 LOA, 34 LWL, 11.8 Beam, 18,500 lbs displ., 4.75 draft. Perkins 4108 40 hp spinning a 16 x 13.4 2 blade max prop. .8 to 1 gph at 6 knots or 6 to 7.5 miles per gallon. Attached is a picture of my boat underway at 6 knots. I live aboard most of the year and consider myself to be a coastal cruiser. Longest passage without easy access to fuel is Cabo San Lucas to Turtle Bay, 450 miles.

    So here is my query.

    I would like to switch back to a powerboat. But want to keep the same fuel efficiency that I currently have. My intuition tells me that a similar size displacement powerboat should use less fuel at the same speed or go slightly faster using the same amount of fuel because the powerboat doesn't have the resistance of the deep keel and mast and rigging. Yet in my searching on this forum and others about the best fuel economy that I see on average seems to be around 5 mpg. It also seems like displacement powerboats in the 35' to 40' range have engines with at least 125 hp which seems to be way more than you would need. Seems to me that you don't need anymore than 50 hp to 60 hp if even that much. So what am I missing?

    I would like to be able to travel at 8 knots burning no more than 1 gph with the capability to travel at 10 knots if I wanted to with of course a significant increase in fuel usage. 8 knots at 1.33 gph is the same miles per gallon as 6 knots at 1 gph. So 8 knots at 1.33 gph is the line in the sand. If I can't achieve better than what I have right know, why bother to switch. Are there any existing boats out there that might achieve this fuel economy? A custom built boat is outside the range of my financial capabilities. I am aware of the Willard's but don't care for canoe stern boats. Just don't like the styling. The Roughwater 35 to 37 come close but you are back to at least a 125 hp engine and higher fuel usage..

    Or am I just wasting my time looking for an existing boat that will realistically achieve at least 6 mpg at 8 knots? Thanks for any help you can provide.

    Chuck
     

    Attached Files:

  2. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    I would like to be able to travel at 8 knots burning no more than 1 gph with the capability to travel at 10 knots if I wanted to with of course a significant increase in fuel usage.

    You will need to build the boat.

    Most boats are ONLY economical if run at a low percentage of "hull speed".

    To be cheap to keep you cant make waves , the sq rt of the lwl times .9 to 1.15 is your cruise speed so the LWL will need to be 64 or more ft.

    Weight is also a dirty word , if you are requiring efficiency.

    The usual Rule of thumb is 2 or 3 hp per ton (2240pounds per ton.)

    A cheap to maintain diesel will only give 16 or so HP per gallon of fuel, and a complex turboed and aftercooled ,electronic injected will advertise 22hp but 20hp is closer to what you get installed .

    So that one GPH at 8K will need a boat that displaces 8 to 10 tons , food , fuel and all accesories (dink) anchors and beer aboard.

    Skinny is also a requirement for small waves , happily that fits well with light.

    6-1 Length to beam is the bottom of skinny efficiency.

    So your new cruiser will 70?loa, 64lwl, disp about 20,000 and have an engine that is efficient when asked to produce 20 hp.

    A custom boat is the only way to get close,


    FF
     
  3. whitepointer23

    whitepointer23 Previous Member

    chuck look up prowler power cats. i don't know if they would suit but very efficient according to the write ups. 10 mt long and 18 knots from 2 60 yamaha 4 stroke outboards.
     
  4. Chuck Losness
    Joined: Apr 2008
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    Location: Central CA

    Chuck Losness Senior Member

    Thanks for feedback. Yes 1 gph at 8 knots is pretty much a pipedream. But 1.33 gph at 8 knots should be doable.
    I am pretty much limited to 40 feet LOA. Just don't want to go any bigger than that. And building a boat isn't realistic for me. Not that I couldn't do it. I just don't want to build a boat from scratch. The most that I would be willing to do is take an existing hull and build my own cabin layout.
    As for catamarans, I don't care for their styling and would like to stay with one engine with a get home engine that doubles as the generator.
    I am one of the few cruisers out there that doesn't have an overloaded vessel and believe that less is more. I actually have some empty storage lockers on board my current boat and it sits on its lines. Using my current boat as a reference, if I take away the 8000 lb lead keel and 500+ lbs for mast and rigging then add back for a heavier engine and more fuel, I feel that a 6 ton (13,500 lb) displacement powerboat should be more that adequate for my needs and allow me to carry everything that I currently have on board. I'm not crossing oceans. The trouble is finding a powerboat that light weight with a long waterline and a relatively narrow beam. I figure that I need at least 36' LWL and no more than 11' beam. Waterline beam would be less than that.
    Fred, I know that you have a 50' navy utility boat that you have converted to a cruiser. Do you have any experience with the 40' Willard navy utility boats and what they are like? They are relatively narrow with a long waterline and around 8 tons displ. Also one of my friends down in Mexico thinks that I should go find an old lobster boat and convert it to a cruiser but they at not available on the West Coast. He has had several converted lobster boats and loves them. Any thoughts on that?
    Thanks again for your posts.
    Chuck
     
  5. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    I know that you have a 50' navy utility boat that you have converted to a cruiser. Do you have any experience with the 40' Willard navy utility boats and what they are like?

    We brn 7K at 2.5 or so gph in our Utility 50.

    It is a huge comfortable platform, ours is about 10Tons (24,000lbs ) in operation.

    The boats are very light for their size due to magnificant construction , but unless you remove the flotation , not very roomy inside.

    The 40's are similar .

    IF your ego can handle it , and you are done with world cruising, a 40ft or 50 ft with a 26ft (the tongue is counted so the living is about 22ft) Airstream would be both inexpensive and a quick build.

    Lowered into the hull with most of the frame removed and a small PH , after painting no one would notice.
    The RV interiors are the product of thousands of units being sold , so have as much utility and space as the best boat layouts.

    If this is too radical, a sailboat hull is always the easiest to push at SL1, but its a live in , rather than live on lifestyle.
    Power boats are mostly live on.

    A hurricane hull, no mast, sunken engine , could have most of the ballast trimmed for shallow water operation.

    A John Deere from the tractor folks (1/3 the marine cost) with a rebuilt Twin Disc should get you started .

    FF
     
  6. sabahcat
    Joined: Dec 2008
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    Location: australia

    sabahcat Senior Member

    5.03 GPH to do 8nm
    so 0.62L /nm @ 8 knots

    I'd like to see that

    I take it this will not be a cruising vessel?
     
  7. Frosty

    Frosty Previous Member

    What flavour gallon are we talking about the American gallon or the 4.4 liter Uk gallon.

    Its liters now guys come on.
     
  8. Submarine Tom

    Submarine Tom Previous Member

    It's actually 4.5 litres to the UK gallon and 3.8 to the US when rounded to the nearest 1/10th.

    -Tom
     
  9. sabahcat
    Joined: Dec 2008
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    Location: australia

    sabahcat Senior Member

    Correction
    That should have been LPH

    1.33 US gallons = 5.03459767 litres

    So says google
     
  10. Chuck Losness
    Joined: Apr 2008
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    Location: Central CA

    Chuck Losness Senior Member

    Sabahcat,
    This would be a cruising boat. I live aboard most of the year. Is 1.33 gph at 8 knots doable? I think so. My ideal boat would have at least a 36' LWL and displace no more that 13,500 lbs. 8 knots with 36 LWL would be a S/L of 1.33. According to Gerr's The Nature of Boats if my memory serves me right you need 1 hp per 500 lbs of displacement to travel at an S/L of 1.3 thus 27 hp for 13,500 lbs should be sufficient to achieve 8 knots. Drop the speed to 6 knots and fuel consumption would drop dramatically. Increase to 10 knots and hold on to your wallet. All of this assumes a hull design optimized for 8 knots. My problem is finding an existing boat that comes close to these numbers. I am hoping that someone can help me find an existing boat that comes close to what I want. Then you compromise.

    Fred,
    I don't mind living in the hull like one does on a sailboat. This helps dramatically to keep the center of gravity low and control roll IMHO. I totally agree with you about using a "tractor" diesel instead of a "marine" diesel. I buy virtually all of the spares for my 4108 from tractor supply houses. The savings are significant. Thanks for your input.
    Chuck
     
  11. sabahcat
    Joined: Dec 2008
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    Location: australia

    sabahcat Senior Member

    [​IMG]
    http://www.multihulldesigns.com/designs_stock/38tri.html
     
    1 person likes this.
  12. Chuck Losness
    Joined: Apr 2008
    Posts: 239
    Likes: 12, Points: 18, Legacy Rep: 135
    Location: Central CA

    Chuck Losness Senior Member

    Sabahcat
    Thanks for the suggestion. I took a look and it wouldn't work. Doesn't have enough displacement. Where would you put the extra 3000 lbs or so of stuff that you need a on a cruising boat? By the time you loaded all the cruising gear and spares on board you would probably double the displacement. Also no diesel engine. I know that I need around 13000 to 14000 lbs displacement to allow for the actual weight of the boat with sufficient displacement left over to carry all the cruising gear.
    I can't speak for your area but where I'm at multihulls have the biggest problem with being overloaded. They all seem to be way down on their lines. Typical is 6" to 12" inches down. I have looked at lots of multihull designs and haven't found one yet that seems to me to have sufficient displacement in the size that I am looking at. I would never have a sailing cat or tri but a power cat is appealing because of their fuel efficiency if you could get a decent interior layout. Most catamarans are filled with enough bunks and heads to take the whole neighborhood with you. My personal belief is that boats should sleep 2, dinner 4, maybe 6, and party 6 to 8, maybe 10, and only have one head. A double bunk and head in one hull and the galley and storage in the other might work.
    Anyway, thanks for the suggestion.
    Chuck
     
  13. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    Chuck, you are quite correct re the overloading of multihulls, this is however not the fault of the type,rather than a problem of the attitude of the owner, and as you have said you do not overload your monohull so you probably wouldnt overload a powercat either.
    Steve.
     
  14. Chuck Losness
    Joined: Apr 2008
    Posts: 239
    Likes: 12, Points: 18, Legacy Rep: 135
    Location: Central CA

    Chuck Losness Senior Member

    Steve,
    I agree with you that a lot of overloading is the fault of the owner. The owner should have bought or built a boat that could carry what he or she wants to carry on their boat. But the boat whether mono or multi still has to have sufficient displacement for its intended purpose and the multi's that I typically see are marketed as light and fast with what seems to me IMHO as having barely enough displacement to support the weight of the boat with little left over for cruising gear. This is a design issue that's solvable. I just don't seem to see it in real life.
    Chuck
     

  15. Chuck Losness
    Joined: Apr 2008
    Posts: 239
    Likes: 12, Points: 18, Legacy Rep: 135
    Location: Central CA

    Chuck Losness Senior Member

    Found this boat by Bieker Yacht Design. Similar to Tad's Passage Maker Lite 39 design but lighter displacement. What do you guys think?
     

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