Efficiency

Discussion in 'Downeast Boat General Discussion' started by 05bill, Jul 12, 2019.

  1. 05bill

    05bill Captain

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    Really do not know, but is it more efficient ,speed , gallons per hour running a de flat , cutting through water with bow or on her flat stern area ? Doesnt seem to make that much of a difference on mine but would be interested to know others opinions ? And maybe even a de designer can cjim in.

    Thanks

    Bill
     
  2. djmarchand

    djmarchand Captain

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    In general DEs are not very efficient. Also in general the slower you go the better you mpg will be. At higher speeds the most efficient speed is just where you get up on a clean plane.

    David
     
  3. 05bill

    05bill Captain

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    once again David thank you
     
  4. PatriciaLynn

    PatriciaLynn Senior Member

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    David, you have an interesting point there. When you say that downeast hulls are generally not efficient, how are you measuring efficiency? I would say my 30 Repco is pretty dang slippery up to about 12 knots, but depending on your measure of efficiency, different people will think differently.

    Greg
     
  5. PatriciaLynn

    PatriciaLynn Senior Member

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    I am not a naval architect, but minored in naval architecture in school. Still, really just an amateur with a fair amount of time on boats and ships.

    My experience says that running a mostly flat planing surface the most efficient speed/trim will be when you have the smallest flat surface area possible in the water without excessive angle of attack. You can judge this by watching the wake. Running at constant speed, trim the bow up until the wake starts to increase in size. This will be a bit too far, so trim it back down a hair. On a big DE hull, this is easier said than done. Try it out in a skiff sometime and see. Longitudinal trim and weight distribution has a huge impact on running characteristics.
     
  6. djmarchand

    djmarchand Captain

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    I hope this data below comes out ok. Some years ago when I owned a downeaster style Mainship Pilot 34, I asked owners on Trawlerforum for their wot speed, displacement, length, hp, etc and plotted them up. The Cape Dory entry shown at the top was my own "true downeaster" that predated the Pilot 34 which is shown on the bottom. The remaining entries were reports from others.

    The boatdiesel horsepower numbers were generated using boatdiesel's prop calculator set for slow planing hulls. Here is the data which tells me that most downeasters are slow relative to what boatdiesel's non downeaster specific calculator tells you.

    For the two true downeasters shown below, the the Cape Dory 28 takes significantly more hp than boatdiesel predicts and the Duffy takes a little less. The data is based on wot speeds near 20 kts. I believe it would still show the same thing at about 12 kts, but who knows.

    It isn't all that surprising that downeasters are not very efficient. They have full keels and often have larger rudders than planing hull boats, both of which add drag.

    As always, YMMV.

    Boat Make, LOA, Displacement, WOT- Kts, Hp Actual, Hp Boatdiesel, Pct Act/BD

    Cape Dory 28 9500 20 275 175 157%

    Mainship Pilot 30 12500 17 230 166 139%

    Mainship Pilot 30 12500 22.5 315 280 113%

    Duffy Atlantic 37 22000 22.2 450 497 91%

    Mainship Pilot- Bella 34 18000 19.5 370 314 118%
     
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  7. BillD

    BillD Captain

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    chatted with a former 35 RP owner. He added trim tabs to his 35.
    no lifting rails.
    Trimming the nose down a bit at "cruise speed" lifted the ass up out of the water, hull picked up some speed and additionally burnt a little less fuel.
    No math numbers, owner's perspective.
     
  8. 05bill

    05bill Captain

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    Yes I find that to be true and I believe that is what David is saying . When I plane out at about 2300 rpm, I increase to about 2500 ,2600 for cruise and put the noise down .

    Dont really pick up any speed but she is happier. I burn very little, may 5 to 6 gallons an hour, temp at 1000.

    I do have lifting rails and trim tabs

    Not scientific but practical application.

    Bill
     
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  9. Henry

    Henry Senior Member

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    I run my boat, a Duffy 31 with a Yanmar 440, with the fuel flow meter. Once it is up “on plane” at 2200 rpm +/- depending on load through 2800 rpm (max continuous rpm) I first trim the boat level port to starboard.
    ( I have a small inclinometer mounted in the pilot house) then trim the bow down slowly and watch the fuel flow meter. The gph will come down and the speed will go up. This is due to the fact that you are making the semi-planing hull hydrodynamicly cleaner.
    Imagine that the hull from bow to stern is an airplane wing. If the bow is up and the stern is down you have a high angle of attack which creates lots of drag because you are pushing water (or air) In an airplane as in the boat it requires a lot of power to maintain that trim.
    If you can eliminate drag speed will go up and fuel flow will go down.By using the trim tabs you are doing exactly the same thing an airplane pilot is doing when he lowers the nose of the airplane. He reduces drag and speed increases and fuel flow goes down.
    If you don’t have a fuel flow meter, you can still improve performance if you have trim tabs by trimming the bow down a little and watching the speed. It should go up. If you use too much trim it will go up and then back down as the bow starts to dig in. That is too much and dangerous.
    I can pick up as much as a gallon per hour and a half to a knot in speed using fore and aft and port and starboard trim.
     
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  10. PatriciaLynn

    PatriciaLynn Senior Member

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    Great post, very interesting. I would argue (hope you are open to a debate) that boatdiesel's calculator may be off based on the general variable used to define the hull's form and efficiency. Each hull has a different number and BD can't hope to be spot on for each.

    Let's compare to boats of similar size and see how they stack up based purely on MPG...

    26 GM
    200 Volvo
    2.5 mpg @ 18 knots

    30 Repco
    200 Volvo
    3 mpg @ 12.5 knots

    32 holland
    330 cummins
    2.5mpg @ 20 knots

    37 Duffy
    450 cummins
    1.1 mpg @ 19 knots
    ------------------------------
    235 Grady White
    350 Yamaha
    3.0 mpg @ 24.5 knots

    280 Luhrs Flybridge
    260 Crusader
    1.5 mpg @ 19 knots

    32 Pursuit Express
    Twin 250 Cummins
    1.5 mpg @ 22 knots

    38 Bertram
    Twin CAT 3116
    .52 mpg @ 20 knots

    OK, I have spent some time on each of the boats I listed. Like them all, for different reasons. I am sure I could have picked it differently, but comparing boats for the same uses, I found no clear winners. And was surprised by a couple comparisons. Lmk what you think...
     
    Last edited: Jul 14, 2019
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  11. leaky

    leaky Captain

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    Your boat is most efficient when your gph meter and speedo says so :)..

    Seriously though Ive heard a number of credulous ideas from guys Ive fished with about slowing down to 12 or using what their mis-propped engine burns based on a chart. Or my favorite- what the prop guy says your cruise speed or RPM is. You really just gotta have a meter and do the math with your boat.

    Most boats are most efficient below planing speed, there is a more technical explanation for it involing hull speed but downeasts are better than true planing hulls as far as having a higher hull speed (and true displacement hulls really kick ass at it). You very well might get 4 mpg at 7 to 9 knots out of a 30-something foot boat.

    Get up to a cruising speed and all bets are off - downeasts are partly efficient because they lift the bow up and ride on a flat surface at the stern. If the boat doesn't plane right on its own tabs can add surface area but they can also drag and slow you down (ie less speed at same load, what the other guy said)..

    Generally with trim tabs you try not to use them, if the hull is designed right they are for counter acting problems - ie running at a speed the hull doesn't like due to conditions (ie slower than good planing but over hull speed), running overloaded, and not so much with a DE but on a planing hull imbalanced port/starboard.

    JMHO
     
    Last edited: Jul 14, 2019

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