Finding an efficient speed

Discussion in 'Powerboats' started by Jules Martin, Aug 8, 2010.

  1. Jules Martin
    Joined: Jun 2008
    Posts: 2
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    Location: Kingston, ON

    Jules Martin New Member

    I have recently purchased a lovely old Albin 25 motorboat. It is a "deluxe" model with slightly flattened aft sections to better use the massive 36 hp available from its 3 cylinder Volvo diesel. It has a beam of 8'6" and weighs about 3500 lbs. It seems quite happy running at 2000 rpm, which is the top of the torque and bottom of the fuel curves for the engine. The boat is moving at a good jogging pace, water is separating from the transom but not making a rooster tail. There are no speed measuring devices on the boat.

    I've read enough to know that small speed changes can result in large efficiency changes. Are there any ways of determining efficient speeds by "looking over the side"? For example: should the water separate from the transom or doesn't that matter, or, should the water dip once from the bow to the stern, or the bow begin to lift, or some such.

    Perhaps I don't really mean efficiency but "best speed in terms of easiness on the boat, lightness on fuel, and getting somewhere"...
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    My advice is to start with buying a GPS, that will tell you exactly how fast you are going.
    On a quiet day, going straight ahead, start at lowest rpm, increase in 100 or 200 rpm steps and write down the corresponding speed.

    Then at home draw a graph with rpm vertical, speed horizontal. You'll see that the first part of the line is almost straight, then to the right it curves up to a point where 100 rpm more does not significantly increase the speed.

    Any point along the straight part of the line is suitable for cruising, where it curves you start wasting fuel. Mark that spot on your rpm gauge and enjoy boating.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Well put CDK.

    -Tom
     
  4. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Jules,
    I had an Albin 25 for about 6yrs. Mine had a 34hp Yanmar 3HM35F. My hull was the earlier type and I suspect your's is the later type. The later hull has a slightly hooked underbody that helps keep the slightly overpowered (for a displacement hull) from going bow high. The earlier more banana like stern bottom has a higher top speed (just a tad) at the expense of a bow high attitude. The later hull runs quite level but makes a bigger wake and is (I think) a bit less efficient. My engine peaked at 3450rpm and for the 1st year I had it I cruised at 2750rpm and 8.5 knots. Later we went to Alaska and back and by that time I cruised most of the time at 7.35 knots and 2400rpm. My fuel burn was about 3 quarts an hour.
    CDK's advice is (as usual) excellent but for openers I'd make sure the boat is propped right. Your engine develops 36hp at it peak hp rpm. When you go to wide open throttle (WOT) your tach should indicate that rpm + or - 50 ..maybe 75rpm.
    The Albin is a wonderful boat. It will take care of you and give you time to enjoy boating. That makes it an uncommon boat.

    Easy Rider
     
  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

    . My engine peaked at 3450rpm and for the 1st year I had it I cruised at 2750rpm and 8.5 knots. Later we went to Alaska and back and by that time I cruised most of the time at 7.35 knots and 2400rpm. My fuel burn was about 3 quarts an hour.

    If the hull was only absorbing 3quarts an hour worth of power , you were basically using 12 -15 hp for cruising.

    A cruising prop would bring the engine rpm down to perhaps 1800 ,or lower , and still provide the required 15hp.

    At higher loading the fuel burn might also go down , and the engine wear and NOISE would surely be much better.

    FF
     

  6. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    FAST FRED,
    We've been here before. There is only one way to properly prop-load a small boat and that's max hp rpm at WOT. There is no such thing as a "cruising prop" and you must think the boat I was talking about was a displacement hull. Not so. SD.
    I'm quite sure I was driving it w about 24hp and specifically the fuel burn was .85 gph.

    Easy
     
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