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Displacement &Semi-Displacement (or Semi-Planing)
Boat Calculations (English Units)

Description: This popular boilerplate template performs powering calculations (power prediction methods) applicable to two modes of boat or ship operation.  The first is the displacement mode and the second involves semidisplacement or semiplaning mode of operation.  Result graphs are automatically generated and show speed in knots versus the brake horsepower required.  Because the input weight is important, this spreadsheet is setup to conveniently calculate the vessel's weight in various operational conditions.  This template also checks to see if the vessel will go it's intended range, if the installed horsepower is adequate and if the stern's quarter butt exit angle permits the input speed.

These calculations apply generally to calm water performance.  The first set of calculations are for most types displacement and semi-displacement, or semi-planing vessels.  They are based on procedures described by Dave Gerr in Reference D.  The second set of calculations applies only to displacement vessels (that are optimized for displacement length ratio and prismatic coefficient) and are adapted from methods presented by Robert Beebe in Reference K.  References D and K are recommended reading for understanding the processes automated in this spreadsheet.  The other references contain supplemental information, but they are not needed for implementation of these calculations.  This template is designed for use with English units.

The first set of output computations apply to displacement and semi-displacement vessels.  Semi-displacement, or semi-planing, boats normally operate at speed length ratios, V / LWL1/2, from about 1.3 to 1.4 all the way up to about three.  However, in order to reach speeds in excess of maximum displacement speed, a vessel must have the appropriate aft quarter butt exit angle. An example drawing of this exit angle is provided in the spreadsheet along within the output calculations.  If the aft quarter butt exit angle is too large, these calculations are automatically modified to only go up to the maximum speed that is allowable for the exit angle that is present.  However, the automated result will never be less than the maximum allowable displacement speed.  Since all this automated, a complete understanding is not crucial.  What must be understood is decreasing the hull exit angle will often let the spreadsheet evaluate higher speeds.

The second set output of calculations apply only to displacement mode.  Displacement ships can only go up to their maximum displacement speed which is calculated in this spreadsheet.  This maximum displacement speed is a function of both the weight and the length of the boat.  The maximum speed length ratio normally associated with displacement mode is usually between 1.3 and 1.4.  But light and long vessels can exceed this displacement limit and these effects are taken into consideration by this spreadsheet.    This second set of calculations assume that the boat has been designed with an optimum prismatic coefficient, that corresponds to it's displacement length ratio.  Beebe discusses these optimization techniques in References G and K.  Other references also discuss the technique.

There are several advantages and benefits associated with these calculative methods.  First this approach saves time.  Numerous computations are automatically and quickly done on your input data.  This approach is also cost effective because the calculations are already setup for you.  As a result your research time is minimized to familiarization of concepts when necessary and not in time consuming procedural development activities.

Electronic Document Type:  Microsoft Excel spreadsheet           Cost: $35 US funds

Number of Pages: Inputs Sheet, Outputs Sheet, References, Figure and Notes Sheet, F2 Function SLR Page, Use Terms Page and Instructions Page.      


  • Hull characteristics

    • Length Over All, LOA, feet (optional input)

    • Length of Waterline, LWL, feet

    • Beam on Waterline, BWL, feet (optional input)

    • Exit Angle, qAFT, of Aft Quarter Butt, an explanatory diagram is provided on the References Sheet of this template.  This value is critical for proper hull operation and it is therefore checked in this analysis.  For complet details about this requirement refer to Pages 12 and 13 of Reference D. 

  • Weight information

    • Vessel light ship weight, pounds, the weight without any people, luggage, personal gear, provisions, stores, fluids or cargo on board

    • Weight margin on light ship, percent.  This is optional input and default values are provided.

    • Cargo weight, pounds, usually can be left at zero default value.  Examples of cargo weight are fuel drums on deck, weight of fish and ice on board.

    • Number of crew and number of passengers, integer

    • Additional equipment not included in light ship, pounds

    • Duration of voyage, days, for calculating provisions and stores, this value can often be left at it's default value which is equal to zero

    • Personal crew gear and passenger luggage, pounds, can be set to zero

    • Portions of total fluid capacities to be applied in analysis, default values that are set at 2/3 are provided, however these values may be specified as inputs.

    • Main propulsion fuel tank capacity, gallons, & fuel type (diesel or gasoline).  This is a highly recommended input value, if provided an estimate of the vessel's range will be automatically computed by this spreadsheet.

    • Potable water tank capacity, gallons

    • Sewage holding tank capacity, gallons

    • For catamarans an input factor is provided to facilitate calculating the maximum displacement speed length ratio based on the displacement of one hull.

  • Propulsion Plant Details:

    • Installed brake horsepower, if unknown guess an amount, this value can be easily changed once these calculations are done.

    • Parasitic loads, horsepower, for pumps, generators, etc.

    • Continuous operating rpm, percentage of maximum, guideline values are given on the input sheet, simply select the applicable rpm percentage

    • Shafting efficiency, ratio, guideline values are given, select appropriate value

    • Propeller efficiency, ratio, a default value is present, however another value may be substituted in the calculations

    • Vessel range required, nautical miles, optional input.  If this value is specified this spreadsheet will automatically determine if the tanks provided are adequate to make the voyage distance specified by this input.


  • For each of the input speeds the following is output are computed by this template:

    • Effective Horse Power, EHP

    • Brake Horse Power, BHP

    • Continuous Horsepower, BHP0

    • Estimated Range, nautical miles, from fuel consumption rate and tank capacity

  • The total operational weight is automatically computed based on light ship margin selected, cargo present, persons aboard, voyage duration which affects stores and provisions, selected operational tank capacities and gear on board.

  • Supplemental values like displacement length ratio, estimated maximum possible displacement speed, estimated fuel consumption rate, speed length ratios.

  • Calculative Checks are made for the following criteria:

    • Check if installed horsepower will meet the required horsepower needed for continuously operating at the speed computed.  It is important to know that If the speed involved is not a continuous operating speed this check should be ignored.  These speeds include those used for burst mode operation (operations only for short periods at this speed) and transitory speeds (when accelerating or decelerating towards a continuous operating speed).

    • Check if range requirements will be meet at the speed evaluated.  If this check is met the fuel tanks are adequately sized, if not the fuel tanks should be increased in size, but only for continuous operating speeds.

    • Check if hull bottom aft exit angle at quarter butt is appropriate for speed computed.  If this check is not met the bottom shape at the aft end of the hull should be flattened more so this speed can be reached by the hull.

Recommended Reading:

  • Reference D: Dave Gerr, Propeller Handbook, International Marine, 1989, Camden, Maine.

  • Reference E: Dave Gerr, Nature of Boats, International Marine, 1995, Camden, Maine.

  • Reference F: John Teale, article entitled "Teale on Trawlers," Motor Boating and Sailing, June 1981 issue.

  • Reference G: Robert P. Beebe, article entitled "Trawlers - Designing by the Numbers," Motor Boating and Sailing, January 1977 issue.

  • Reference H: Alfred D. Isaacson, paper entitled "Sewage Polution Control: A Guide for the Ship Owner & Design," SNAME Marine Technology, July 1977 issue..

  • Reference I: S9086-C6-STM-010/CH-096R1 "Naval Ships' Technical Manual Chapter 096 Weights & Stability," NAVSEA, 2 August 1996.

  • Reference J: Detroit Allison Diesel, article entitled "Calculating Hull Speed," Motor Boating and Sailing, February 1978 issue.

  • Reference K: Robert P. Beebe and James F. Leishman, Voyaging Under Power, Third Edition, International Marine, 1994, Camden, Maine.

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Download Now: click the following hyperlink to pay $35 fee and then immediately download the zip file containing the template.

Minimum System Requirements: Windows 95/98/NT/2000/XP/Vista/Windows7

Sample: A sample of an output page is shown below.