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Fuel Burn Rate on 38 Mediterranean Convertible

Discussion in 'Engines' started by Mark Williams, Jan 3, 2022.

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  1. Mark Williams, Jan 3, 2022
    #1
    Mark Williams

    Mark Williams New Member

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    Jan 3, 2022
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    Cottonwood, CA
    Hello, I recently purchased a 1996 38 Mediterranean Convertible and am going to be taking it on a 1000 mile trip down the baja. The boat has 2- 6BTA5.9 330 HP cummins.

    I know fuel flo meters are the best, but would anyone have a good guestimate on fuel burn rates on this boat at particular speeds? I am trying to figure out the "sweet spot" to maximize fuel consumption and was hoping someone had a similar boat that could offer some advise.

    I am not sure if this helps or not, but this is advertised speed

    Cruising Speed
    17.38kn
    Max Speed
    19.12kn

    Thank you,
    Mark
  2. ranger58sb, Jan 3, 2022
    #2
    ranger58sb

    ranger58sb Senior member

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    Chesapeake Bay, USA
    Can't help with "sweet spot" but you can probably find the engine data sheet on the Cummins site... and that'll give you the RPM/fuel consumption curve. A place to start.

    Most economical will probably be down around 6-7 kts assuming a waterline length of 36'. That hull form may not be comfortable at those kinds of (non-) speeds, and actual cruising grounds will get a vote about that too.

    -Chris
    Mark Williams likes this.
  3. Mark Williams, Jan 3, 2022
    #3
    Mark Williams

    Mark Williams New Member

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    Thank you very much for your input. I was able to find this. I am assuming that having 2 engines will have some impact on this but as a rough guess, I am thinking 1100 rpms would be very close to that spot whatever speed that may be? 8-10kts?

    Rougly, @ 2 gallons an hour per side = 4 gph total at ~10kts, which would give me about 2.25mpg?

    upload_2022-1-3_6-43-34.png
  4. ranger58sb, Jan 3, 2022
    #4
    ranger58sb

    ranger58sb Senior member

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    Location:
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    The charts don't really take into account resulting boat speed -- due to variables like length, shape, weight, windage, etc. -- so that's really only a starting point with only engine data.

    But I'd guess 8-10 kts on a 38' boat would maybe be just pushing water. The formula (which isn't perfect for your hull form anyway) suggests lower is better, or up on plane. In between, trying to overcome your bow wake, won't be great.

    Once you know your real water line length, the formula is 1.34 * SQRT(LWL) and then lower than that resulting number -- often somewhere between 1.0-1.2 * SQRT(LWL) -- is usually more economical. Depending on resulting real-world speeds.

    And then sea states, wind, current, tides, etc. all chime in to impact reality.

    Also, you'll want to be running the engines at acceptable loads/temps; Cummins can probably advise on acceptable upper and lower limits. It's often said diesels run best at 80% of load, or 90% of load, or "200 off the top" (varies depending on pundit) at the upper end... and someplace enough above idle at the lower end to get engine temps into their acceptable range and keep them there.

    -Chris