Power calculation

Discussion in 'Diesel Engines' started by sjoduvan, Jul 18, 2018.

  1. sjoduvan
    Joined: Jan 2008
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    Location: point roberts washington

    sjoduvan New Member

    need to get power calculation for engine replacement on 3 boats.
    Boat 1 = Shelborne Dory 20’ 6’. Flat bottom. Water line beam 3’6”

    Boat 2 = uniflite 40 ( cruise-a-home hull )
    Converted to a marine wood shop. Desired speed 8.5 kts or hull speed. Lwl= 40. Beam 12’. Weight 20,000. Planing hull draws 4’

    Boat 3 = Bugeye Ketch. Designed from a Chesapeake bay oyster dredger Loa 67. Lwl 50. Beam 13’6”. Weight 44,000 lbs wet draft 3’ ( center board) Would like to make hull speed. Current motor 50hp Diesel makes 5.5 knots.


  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    You need a more complete list of specifications in order to determine power.
    Boatdiesel.com offers calculators to help determine power, as well as other pertinent parts.
  3. Herman Wessels
    Joined: Nov 2018
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    Location: The Netherlands

    Herman Wessels New Member

    For boat number 3, the hull speed will be about 9.5 knots. Theoretically, you need about twice the power you have now to reach the hull speed. An engine of 120hp should suffice.
    For boat number two, the theoretical hull speed is about 8.5 knots. Theoretically you will require about 50hp to reach it (I assume the 20,000 is pounds, not kg). A 65-70hp engine should suffice, imo.
  4. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Herman, what formulas are you using for your calculations?
  5. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    It's amazing, in the end it will not be necessary to know what boat it is but we'll be able to know the necessary power.

  6. Herman Wessels
    Joined: Nov 2018
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    Location: The Netherlands

    Herman Wessels New Member

    It is important to note CapnD comments. To accurately calculate the amount of power you will need, you need more detail. I do have a number of useful rules of thumb with which you can get pretty close however. I am Dutch and you will notice I use a rather eclectic combination of metric and imperial units to do the calculations. To calculate the hull speed for a water displacement vessel, use the square root of the LWL, in meters, X 4.5. For boat number 3, the LWL is 50 feet (15.24 meters). The square root of that is 3.9; 3.9 x 4.5 = 17.5 km/hour = 9.5 knots. The approximate hull speed of boat 3 will therefore be 9.5 knots. To reach the hull speed, you need approximately 5hp/ metric ton. Boat 3 weighs 44000 pounds, i.e. 20 metric tons. 20 x 5 = 100 horsepower. You will therefore need approximately 100 installed hp to reach the hull speed, if, of course, your prop is well matched to the boat, the engine power and the reduction of the reversing gear. This is actually enough power because in a water displacement vessel, you´ll rarely actually cruise at maximum power and hull speed. Your most efficient speed will be the square root of the LWL in feet, expressed in knots. For a LWL of 50 feet, your most effecient speed should be in the vicinity of 7 knots. At that speed you will only use about 3 to 3.5 hp per ton, i.e. about 70hp. Going faster will cause the fuel consumption to rise sharply.
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