Cruising a twin screw Semi-displacement boat for minimum fuel consumption

Discussion in 'Propulsion' started by rostan, Aug 12, 2011.

  1. rostan
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    rostan New Member

    In order to reduce overall fuel consumption on a twin boat I'm thinking about running a twin screw SD boat using one engine at low RPM for 4-6 hours and then at 75% for 1/2 hour before switching to the other engine? I understand that there will be losses due to the rudder angle and un-powered prop drag but I'll be able run the one engine closer to it's 75% sweet spot. I would also assume that an autopilot will be able to hold when running on that single engine since the operating prop will not be that far offset from centerline. Has anyone cruised this way and if so have you got any hard fuel consumption numbers (twin vs. single)?
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    FAST FRED Senior Member

    IF the boat is small, (can be pushed with a 24 or smaller prop) you might consider a pair of 3 blade sail boat feathering props.

    These feather to reduce drag , and many can have the pitch changed with an adjustment.

    This solves most of the prop drag hassle , and assures the tranny on the unused engine will not be destroyed.

    Proper CPP props would be better , but the retrofit cost might never be reclaimed.

  3. Stumble
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    Stumble Senior Member


    Maxi feathering props are available in 2,3, and 4 bladed props, in sizes up to 40".

    I am not sold on only using one engine, but this is an interesting option instead of dragging a wheel through the water. I would guess the drag from a large prop will cost you most of the efficiency gain of not having one engine on.
  4. Willallison
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    Willallison Senior Member

    As FF eluded to, you would need to check with the manufacturer of your transmission... some will cope with freewheeling, some wont. If yours is the latter, then a common solution is to lock the shaft.
    Feathering props have a habit of being woefully inefficient... so whilst you may gain on the one hand, you might lose on the other. Some are better than others, however....
    In answer to your questiona about whether you will see any efficiency gains, the answer is simple. It depends...;)
    Generally I'd say yes, but I've been on a number of boats where it makes little or no difference.
    It's also worth considering what kind of engine(s) you are running. The later model, electronically controlled, common rail diesels aren't as prone to cylinder glazing as older mechanical engines were, so are happier rolling along at lower revs (loads).
    Of course, the designer in me would tell you that if you are after an efficient displacement speed motor boat, then a relatively high powered semi-displacement boat is the wrong choice in the 1st place... but that's just being pedantic, I guess....;)
  5. Tad
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    Tad Boat Designer

    A few years ago a guy ran his twin engined semi-displacement boat from the West Coast to Hawaii on one engine. Before starting he removed one prop, ran halfway on the other engine, stopped at sea, dove overboard (singlehanded) and pulled one prop and installed the other, and finished the voyage. I assume he had either scuba or hookah gear......:D

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    FAST FRED Senior Member

    "I assume he had either scuba or hookah gear..."

    And a very good system of lifting.

    Our 32x32 prop is far beyond what a team of swimmers could hold up.

    Rob White of Rescue Minor fame used a splined shaft , similar to what an outboard would use.

    This alowd prop changes without using a puller tool.

    A custom splined shaft and props might out weigh any fuel savings.The cost of a prop for an Arninson drive is huge!!

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