PSModel a look inside



Quick Start

Quick Series


Data Input

Data Output

Importing a
Data file into
MS Excel

Main Menu

Main Screen

and Data

Screen Colors
and Fonts



with Data


Test Sheet


19' Ski Boat

34' Sailboat

41' Utility Boat


Blade Area

the Cavitation


Hull Speed

Kt Breakdown


Propeller Law

Wake Factors




Testing a propeller on any size boat--from outboard to mega yacht--can be fun and easy.


Test Materials:

-- a tachometer 

-- stopwatch or radar gun to determine boat speed

-- pencil, paper, and a clip board

-- if possible, a relatively clean boat bottom

A tachometer should hook up to the outboard or inboard engine directly; however, in a pinch you can try the dashboard tachometer.  The more accurate the RPM reading the better. 

For outboard engines, several companies sell handheld tachometers.  Search the internet or contact your local outboard dealer for sources.

For inboard engines, there are a number of accurate tachometers available in the marketplace.  Some hook directly to the engine, and others read light pulses from a reflective strip attached to the propeller shaft or rotating part of the engine.

Test Course:

The easiest and most accurate way to determine boat speed is with an inexpensive stopwatch and a measured distance on the water.  Navigation charts usually show buoys that are set and surveyed specifically for this purpose.  The distance between these buoys is typically a half nautical mile, but check the charts. 

On lakes or rivers, a range can be set up on shore by setting two markers, and measuring the distance between them.  One MPH is approximately 1.467 feet per second, so if your boats top speed is 30 MPH you will cover 44 (1.467 x 30) feet every second.  If you want at least 5 seconds between clicks of the stop watch, your markers need to be at least 220 feet apart.  For 10 seconds between clicks, you need to separate the markers by 440 feet.  In general, set the distance between the markers as great as possible, greater distances minimize errors.  Perpendicular to the course and approximately 20 feet behind each marker set a pole.  The pole and marker will serve as a range.  When the  pole and marker are lined up during testing, start or stop the stopwatch.

On the water testing:

Test conditions:

Try to pick a test day when the wind is below 10 MPH, and tides or water currents are minimal.  Wave action beyond a chop will have a significant impact on test results.  Usually the higher the waves the more thrust is required from the propeller for a given speed.  If the boat is normally used in waves, then it may be better to test in waves.  If you are interested in top speed in calm water, then definitely test in calm water.  

The boat should be loaded in the condition most likely to be encountered, or under the conditions that maximum propeller performance is desired.  This includes fuel, gear, and people.  For boats less than 40 feet in length, maintaining the location of these loads between runs is important.  This especially applies to people loads.  Slight changes in trim or heel can have a significant influence on consistency of test results.

Water temperature:

if possible take the temperature of the water, and note if it is fresh, salt, or brackish water.  This information is needed by PSModel.

Recording Test Data:

Prior to testing, create a test sheet for recording test data.  On the sheet, create columns with the following headings: Run Number, MPH, RPM, and Comments.  The Comment column is to record any additional information that may occur during a particular run.

Also, on the test sheet, allow space for recording the following information:

-- test date
-- wind speed\direction
-- fresh, salt, or brackish water
-- P/D (on small propellers, usually stamped on the hub)
-- propeller BAR
-- propeller manufacturer\model
-- number of propeller blades
-- propeller diameter
-- reduction gear ratio
-- water temperature
-- wave height
-- test course length

Obtaining the correct reduction gear ratio is important.  In some cases, the boat manufacturer's literature may be incorrect, so always verify this ratio.  Copy the model and serial number down from the reduction gear case and go to the reduction gear manufacturer's web site.  If you cannot verify the ratio there, contact the manufacturer directly.  If the manufacturer is out of business, try to verify the ratio by surfing the Internet with the reduction gear model number.

Finally, actual testing:

During testing, run the course at the same RPM or Speed at least six times: three times in one direction and three times in the opposite direction.  It is very difficult to repeat the exact RPM or speed six times, but try to keep differences small.  Preferably, use RPM rather than speed as the controlling parameter for each run.  During each run, record the RPM reading several times, the more times the better.  If using a radar gun to determine speed, record the radar gun reading several times for each run.

Test data for the six runs should be averaged before inputting it into PSModel.  Averaging speed and RPM for several runs minimizes errors due to water currents, wind, and waves.

If you are interested in generating graphs of speed versus horsepower, speed versus thrust, etc. with PSModel, run the test course at several different speeds always keeping RPM as constant as possible for a particular speed.  If the top speed of the boat is 20 MPH, make runs at 5, 10, 15, and 20 MPH.  If the top speed of the boat is 40 MPH, make runs at 10, 20, 25, 30, 35, and 40 MPH and so on.  When test information is entered into PSModel, and the columns of data are graphed, you will be able to see curves of propeller torque, thrust, efficiency, and horsepower verses speed or RPM