Optimal Cruising speeds for best gas milage

Discussion in 'General Discussion' started by Kevin Kidd, Dec 28, 2017.

  1. Kevin Kidd

    Kevin Kidd Member

    41
    Oct 16, 2017
    California Delta
    270 Sundancer 2000
    7.4 Mercruiser w/ Bravo 3 outdrive
    We have a 2000 270 Sundancer powered by a single MerCruiser V8 7.4 MPI with Bravo 3 outdrive. Only had her since late Oct. of this year, so we are still getting to know each other.
    I find that about 3400 RPM seems to be her sweet spot for fast cruising. My wife wants to cruise slower to enjoy the ride and conserve fuel. What are your opinions on speed? What do others "slow" cruise at? I will add that I have always had runabouts and this is my first "big" boat. I've been told by a mechanic that I drive her like a ski boat. Meaning that I get on the throttle and I do tend to take fast turns, something I am working on correcting. I understand that depending on the weight in the boat will affect mileage, as will the tank levels etc. I'm looking for what speed folks cruise at for a leisurely ride. We are on the California Delta so its a lot of channels and sloughs, so a lot of water that's not wide open.
     
  2. Creekwood

    Creekwood Well-Known Member GOLD Sponsor

    Apr 26, 2009
    Oakville and Georgian Bay, Ontario
    '97 330 Sundancer
    2X 454 w/ vDrives
    Once you are on plane the gas mileage (measured in miles per gallon) will be the same from on plane to about 3500rpm. At cocktail cruise speed, say 10mph the Mpg will be about the same, but you will be going a lot slower. Planing boats are not fuel efficient. So if you don't have an agenda or destination to get to, cocktail speed will save you money. If you are going somewhere specific, your 3400 probably is about as good as it gets in MPG. Maybe 3100 to 3400.
     
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  3. northern

    northern Well-Known Member

    Jan 17, 2007
    West coast Vancouver to Alaska
    380 Aft Cabin 1989 GPS and Charts by Nobeltec
    Twin 454 strait shaft
    Hull speed is the best you can normally do at low speeds.
    Maximum Hull Speed
    Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)
    When we had a 28 footer (water line) beam 10.5 feet we found at 24 knots we burnt 1 liter a minute IE 60 an hour or 15 US GPH. Boat had upper and lower helm. Weight was about 8,000 pounds at hall out.
     
    Kevin Kidd likes this.
  4. Gofirstclass

    Gofirstclass Well-Known Member GOLD Sponsor

    Apr 20, 2010
    Tri Cities, WA
    1995 550 Sedan Bridge,
    2010 Boston Whaler 130 Super Sport,
    1981 Boston Whaler 130 Sport,
    CAT 3406C's, 580hp.
    Kevin, one way to figure out what your best slow cruising speed will be is to watch your hull.

    If you are moving at idle through the water your hull will be riding flat. As you step up the speed the bow tends to ride up on the bow wave and the stern digs in.

    The more the stern digs in the less efficient you are running. Part of that comes from pushing a big bow wave, part from dragging the stern through the water instead of it gliding over the water as it would if the hull were flat.

    If your water line length is 22' (for example), using the formula northern provided (which is very accurate BTW) you'd have:
    Hull Speed = 1.34 x (sq root of 22)
    = 1.34 x (4.690)
    =6.28 kts
    =6.28 x 1.15 (convert kts to mph)
    =7.22 mph.
    At any speed above that you're going to find your stern settling down in the water and your fuel economy going down.

    To give an example of what I'm talking about, here's my boat at about hull speed, about 7-8kts....
    [​IMG]
    and here she is at a very INEFFICIENT speed, about 10-11kts....
    [​IMG]
    In the first picture the stern is being supported by a wave that is generated by the bow as it moves through the water. In the second pic the boat is outrunning that wave and the stern has settled way into the water. Not a good way to cruise.

    If you look at my avatar pic, I'm running well below hull speed, about 5-6kts. Note the very small bow wave and small wake. Very Efficient and you won't spill drinks at this speed. :)
     
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  5. Kevin Kidd

    Kevin Kidd Member

    41
    Oct 16, 2017
    California Delta
    270 Sundancer 2000
    7.4 Mercruiser w/ Bravo 3 outdrive
    Thank you, I love the visual.
    I'll be trying this out this weekend.
     
  6. Kevin Kidd

    Kevin Kidd Member

    41
    Oct 16, 2017
    California Delta
    270 Sundancer 2000
    7.4 Mercruiser w/ Bravo 3 outdrive
    Thanks!
     
  7. bobeast

    bobeast Dance the Tide SILVER Sponsor

    307
    Oct 22, 2017
    Isleton, CA
    2002 310DA
    350 MPI w/V-drives
    Kevin. I believe the operative term in your question is "My wife wants"
     
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  8. Kevin Kidd

    Kevin Kidd Member

    41
    Oct 16, 2017
    California Delta
    270 Sundancer 2000
    7.4 Mercruiser w/ Bravo 3 outdrive
    Yes, I refer to her as the Admiral, I'm just the skipper who drives the boat. Lol!
     
  9. Arizona Mike

    Arizona Mike Member

    41
    Oct 19, 2015
    Tucson Arizona
    1994 270 Sundancer
    Mercruiser 7.4
    I have a 1994 270 with the same engine and drive. I have a fuel flow meter. Here are the flows at different RPM:
    600 rpm = .01 gpm @2.9 mph or 4.8 mpg
    1200 rpm = .06 gpm @ 7 mph or 1.9 mpg
    3200 rpm = .26 gpm @ 23 mph or 1.4 mpg
    3400 rpm = .28 gpm @ 26 mph or 1.5 mpg

    7 mph is hull speed and is the most efficient in mpg on plane 3400 rpm is most efficient.

    Hope that helps.

    Happy New Year!
     
  10. northern

    northern Well-Known Member

    Jan 17, 2007
    West coast Vancouver to Alaska
    380 Aft Cabin 1989 GPS and Charts by Nobeltec
    Twin 454 strait shaft
    We had twine V6, 3.8s. To go 24 knots we ran at 3,800 RPM.
    Based on Mike's numbers seem the same.
    We now travel at 6 knots and burn 1 US gallon per knot. Mostly that speed to watch for logs. If wind picks up we go 8 knots and burn about 9 GPH.
     
  11. dcpringle

    dcpringle Member

    432
    Dec 13, 2012
    CT River
    2000 Sea Ray 480DB "Diamond Girl"
    Cat 3196 660HP
    You guys dont want to know what we burn on plane..... lol
     
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  12. carterchapman

    carterchapman Well-Known Member TECHNICAL Contributor SILVER Sponsor

    Mar 25, 2008
    Lake Chickamauga/Marietta, GA/Ft. Myers, FL
    2006 Sea Ray 58 DB
    MAN CRM V8-900s, Twin Disc Drives; Onan 21.5 Generator
    If I run hull speed (800 RPM - 10 kts), I burn 6 GPH total. At 80% load (2000 RPM) I burn 64 GPH Total for 24 Kts...
     
  13. JVM225

    JVM225 Well-Known Member

    Apr 8, 2008
    Farmingdale, NY
    2002 410 Sundancer, Monaco Edition.
    3126 Cats.
    Install a fuel flow meter so you can monitor usage in real time and it will likely pay for itself.
    You’ll find that different sea conditions, boats and power combinations have different optimal speeds.
    My rule of thumb is to operate at 3 basic speeds.
    1. No Wake Zone speed dictated by location and signage.
    2. Cocktail or Hull Speed which is generally around 1200 RPM on my current boat. My old boat was similar.
    3. Plane speed which is about 2350 RPM on my current boat, and was about 3100-3200 for the 5.7 gassers on my old boat.
    The first you can’t do much about, but you can optimize fuel useage at the other two with slight adjustments as dictated by fuel flow monitors.
    I had Garmin electronics on my old boat and installed their relatively inexpensive GFS 10’s on each engines fuel line. They were fantastic. I wish there was a relatively inexpensive option for the diesels in my current boat.
     
  14. RollerCoastr

    RollerCoastr Well-Known Member

    Nov 15, 2007
    Cedar Point, OH / Miami, FL / MacRay Harbor, MI
    1997 400DA
    340HP 7.4 Mercruiser Bluewaters
    Garmin 8212, 741 24HD, Intellian I2
    1999 280BR
    Twin 250HP Merc 350 Alpha Ones
    I agree. I refer to the best cruising speed as a “moving target.” My meters not only allow me to find the best speed in any given circumstance, but by calculating range and fuel onboard with incredible accuracy, I can carry less fuel.
     
  15. Gofirstclass

    Gofirstclass Well-Known Member GOLD Sponsor

    Apr 20, 2010
    Tri Cities, WA
    1995 550 Sedan Bridge,
    2010 Boston Whaler 130 Super Sport,
    1981 Boston Whaler 130 Sport,
    CAT 3406C's, 580hp.
    Dang, Carter, those Mans are pretty thirsty. I don't get to those kinds of fuel consumption figures (64 GPH Total for 24 Kts) until I'm running at WOT at 2150 rpm's at 30.3 kts. I love my kitties.
     
  16. dcpringle

    dcpringle Member

    432
    Dec 13, 2012
    CT River
    2000 Sea Ray 480DB "Diamond Girl"
    Cat 3196 660HP
    i'm with gofirstclass my cat are only about 38gph at cruise22kts and @60gph at wot at 30kts
     
  17. carterchapman

    carterchapman Well-Known Member TECHNICAL Contributor SILVER Sponsor

    Mar 25, 2008
    Lake Chickamauga/Marietta, GA/Ft. Myers, FL
    2006 Sea Ray 58 DB
    MAN CRM V8-900s, Twin Disc Drives; Onan 21.5 Generator
    I miss my 3126's in One Sweet Day...1.3 MPG - better than my gasser 340...

    Reward weighed in at 78,000 lbs during survey, so all in all, .4 MPG is not so bad.
     
  18. Gofirstclass

    Gofirstclass Well-Known Member GOLD Sponsor

    Apr 20, 2010
    Tri Cities, WA
    1995 550 Sedan Bridge,
    2010 Boston Whaler 130 Super Sport,
    1981 Boston Whaler 130 Sport,
    CAT 3406C's, 580hp.
    Dammmmm Carter, you should have warned me to be ready for that. You just about made me choke on my first sip of coffee. Mine's not a helluva lot better with 65gph at WOT at 30.3kts, but I just HATE seeing that tiny little number expressed like that.

    Ahhhh, the joys of owning a bigger boat. Thank heaven mine likes to cruise like it is in my avatar photo.
     
  19. boatrboy

    boatrboy Active Member

    Oct 3, 2006
    Orange, CT
    1999 270 Da
    7.4 MPI w/B3
    image.jpeg image.jpeg I have the same boat and a fuel computer so I can answer this based on my experience. I have 22 props and no a/c or Genny. The pic above was on a trip to block island with
    approx 2 footers.
    I find when I slow down I lose some mpg but keep in mind you will reduce some gph. My computer is hooked up with my gps so I get a real sense of best fuel economy. The truth is my gut was right on. You develop a sense of your sweet spot as you learn your boat. My motor has 1000 hrs so it's not in its prime but she runs perfect for me. If I remember correctly this trip was burning around 16-18 gph. As you can see I leave the display on mpg as that is what I monitor mostly.
    Let me know if you want more info.
    Mark
     
    Last edited: Dec 30, 2017
  20. carterchapman

    carterchapman Well-Known Member TECHNICAL Contributor SILVER Sponsor

    Mar 25, 2008
    Lake Chickamauga/Marietta, GA/Ft. Myers, FL
    2006 Sea Ray 58 DB
    MAN CRM V8-900s, Twin Disc Drives; Onan 21.5 Generator
    You don't even want to know what the annual service runs....
     

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