Propeller Testing: What Data to Collect; How to Evaluate that Data

Optimizing the performance of Boston Whaler boats
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Propeller Testing: What Data to Collect; How to Evaluate that Data

Postby jimh » Sun Jun 05, 2022 2:54 pm

The goal of propeller testing is to find a propeller that allows the engine to accelerate to the optimum full-throttle engine speed range when the boat is loaded in its normal or somewhat heavier than normal condition, and the propeller SLIP is in the range of 5 to 10, and the boat speed obtained at full-throttle is at or close to the target speed estimate based on Crouch's method and the engine mounting height is producing proper flow of water under the A-V plate.

In order to evaluate a propeller that has been tested, the data obtained from the sea trial and data about the boat, the engine, and the propeller need to be analyzed. An approach to analysis of the data is described below.

To properly evaluate a propeller, certain data must be collected. Some data must be obtained by measurement, such as weights, engine speeds, and boat speeds. Some data is just static data about the engine that can be found in the engine owner's guide. Data about the propeller can be taken from the manufacturer's specification.

The total weight of the boat during the sea trial must be either measured or known by very good estimation. The boat weight typically will be the sum of these components:
  • the manufacturer's dry weight for the hull
  • the weight of the engine and propeller and all fluids added to the engine such as lubricating oils
  • fuel volume aboard in the fuel tank or tanks
  • crew weight aboard
  • additional equipment weight, such as batteries, canvas, boarding ladders, engine brackets, radar arch, and so on added to the hull
  • gear aboard, including normal gear like lines, fenders, anchors, fishing gear, and so on.

For the engine, the static data that must be known are:
  • engine weight
  • engine power (HP)
  • maximum engine speed (Max RPM) permitted
  • recommended speed (RPM) range at full throttle for optimum operation, which typically means engine full throttle speed range (Optimum Full-throttle RPM Range) for producing rated power
  • engine gear reduction ratio

The measured data will be engine speed, taken from a tachometer with excellent accuracy and precision, and boat speed, usually taken from a GNSS receiver.

Sea trail data should be collected at several engine speed ranges. Typically the engine speed should be set progressively in 500-RPM increments of engine speed from idle speed to full throttle, and the boat speed recorded for each engine speed. Engine speeds in the 1,500 to 2,500 RPM range which usually represent a boat speed in transition to on-plane speed from displacement speed are not essential. Engine and boat speed data from the lowest engine RPM which will sustain the boat on plane to the full-throttle speeds are important. If available, also collect information about the rate of fuel consumed at each measured engine and boat speed.

With regard to units, all weights are in pounds. The speed of the engine is in rotations per minute or RPM. The gear ratio of the engine is expressed as a reduction ratio of n to 1, where n is the number of revolutions of the engine shaft needed to create one revolution of the propeller shaft. The speed of the boat is in statute-miles-per-hour. The fuel economy is in statute-miles-per-gallon. If any variation from these standard units is employed, the change must be clearly stated.

To measure distance on the water in nautical miles is generally only useful when operating completely away from land as in the middle of an ocean. When operating a small boat on inland waters or along a coastline while in sight of shore, there is little need to pretend one is in the middle of a vast ocean: use statute miles as the distance unit. Using metric distance units like meters or kilometers is also possible, but only if you think your data will only be read by someone outside North America.

Weights measured in kilograms are similarly much more familiar to boaters who are not American.

With the data collected and in the standard units, some analysis can be undertaken. The boat performance is compared to a target speed. The propeller performance is compared to the actual speed obtained compared to the expected speed of advance of a propeller minus its slip

The first assessment of a set of data about boat performance is to compare the measured engine speed at full throttle to the manufacturer's recommended full-throttle engine speed range and to the manufacturer's maximum allowed engine speed. The measured engine speed should never be allowed to be higher than the maximum permitted engine speed, and the measured engine speed should be in the manufacturer's recommended range of engine speeds for full-throttle operation. For some engines, a more narrow range of full throttle engine speeds, called the optimum full-throttle engine speed range, is provided; achieving an engine speed at full throttle in this narrower range is preferred. For example, a particular engine may have a maximum permitted engine speed of 6,000-RPM, a recommended full-throttle range of 4,500 to 5,800-RPM, and an optimum full-throttle range of 5,500 to 5,700-RPM. A careful reading of the engine manual should provide these values.

The measurement of engine speed should be done with precision and accuracy. Modern outboard engines transmit data about their engine speed using NMEA-2000 with very high accuracy and to a precision of 10-RPM. Measurement of engine speed with an analogue tachometer with a dial pointer that can only be read to about 200-RPM precision and of questionable accuracy will prevent proper assessment of the data obtained.

If the measured engine speed is not in the recommended engine speed range, the inference is the engine is not producing its rated power output. There can be several causes for this:

  • the engine is not in proper tune and needs maintenance
  • the engine is in proper tune, but the throttle linkage is not allowing the throttle to open to the full-throttle position
  • the engine is in proper tune, the throttle is opening to full, but the propeller load is excessive.

If the engine is known to be in good tune and the throttle linkage is known to be opening the throttle to full-throttle position, then failure of the engine to accelerate to the recommended engine speed range for full-throttle must be due to the propeller load being too great. The remedy for this problem is to reduce the load on the engine created by the propeller. This typically is done by reducing the propeller pitch or the propeller diameter or the number of blades in the propeller or the size of the blades on the propeller. The most common process is to reduce the propeller pitch.

If the engine accelerates to an engine speed greater than the manufacturer's recommended maximum engine speed, the inference is that the load produced by the propeller on the engine is too little, and the engine is in danger of being damaged by running at too great a speed. The remedy for this problem is to increase the load created on the engine by the propeller. This typically is done by increasing the propeller pitch or the propeller diameter or the number of blades in the propeller or the size of the blades on the propeller. The most common process is to increase the propeller pitch.

The second assessment of a set of data about boat performance is to see if the maximum boat speed achieved is reasonable for the ratio of power-to-weight for the particular hull design. The influence of the hull design on the speed of a moderate-speed planing hull boat can be represented by a numerical value called the hull factor or hull coefficient. An empirical formula to derive boat speed from power, weight, and hull factor was proposed by naval architect George Crouch. For boat weight in LBS, power in HP, and hull factor C (which includes the a scalar for units of speed), the formula is

    SPEED = C × (HP/LBS)^0.5

For certain hull designs, the hull factor has become known from many reported test results. For the hulls of interest to this website, the classic Boston Whaler moderate V-hull design generally known as the Outrage hull, the hull factor is in the range of 180 to 190 for speed to be calculated in statute-miles-per-hour (MPH).

Before any propeller testing begins, a target speed should be calculated. For example: assume a classic Boston Whaler Outrage-type moderate V-hull boat has a weight of 4,000-lbs and is powered by a 250-HP engine. Using a conservative hull factor of 180, the target speed should be

    SPEED = 180 × (250/4000)^0.5
    SPEED = 45-MPH

To facilitate estimates of speeds for power, weight, and hull factors, I have implemented Crouch's method in a calculator:

Measurement of boat speed should be done using speed-through-water. Direct measurement of speed through water is possible using a Pitot tube method, but the accuracy of such measurements is questionable, unless an expensive and carefully calibrated speedometer system is used. A GNSS receiver measures speed over ground, but it can give a good approximation to speed-through-water if there is no current effects in the water. A simple way to remove current effects is to take measurements over the same distance twice, using two course lines that are 180-degrees apart. This can remove most of the influence of the effect of current in the water.

The winds and seas will also affect boat speeds, so for best testing and consistency among test results, the boat speed data should be collected in calm winds and calm seas as much as possible.

A propeller has many characteristics, and among them are:
  • location of the propeller relative to the waterline when the boat is on plane and the engine is trimmed to vertical; this position can be judged by observation of the location of the Anti-Ventilation (A-V) plate relative to the flow of water around the plate;
  • number of blades;
  • diameter of blades;
  • pitch of blades;
  • rake of blades;
  • general shape of blades;
  • blade trailing edge treatments such a cup, increasing thickness, sharpness of edge;
  • material;
  • hub size and length;
  • propeller shaft size
  • propeller shaft splines
  • coupling of propeller shaft to hub
With so many variables in propeller design, construction, and use, when testing propellers and looking for a suitable propeller, the only reasonable method for subsequent testing is to only change one variable with each subsequent test. There are two variables that should be candidates for the initial change: the engine mounting height or the propeller pitch.

The engine mounting height should be set so the engine A-V plate is running just at or just above the flow of water around the engine gear case, while maintaining proper flow of cooling water and proper engine operating temperature. If the engine position is not correct, the engine mounting height should be changed before any other parameter is changed. Once the engine mounting height is properly set, then variables in the propeller can be changed.

The most effective variable to change in the first phase of testing is the propeller pitch. The propeller pitch should be changed to permit the engine to be able to accelerate into its optimum speed range. A change in pitch of 2-inches will typically result in an engine speed change of about 400-RPM. This rule of thumb can guide the amount of pitch change.

Once a propeller has been found that permits the engine to accelerate into its optimum speed range, the propeller itself can be assessed. The general method for evaluation of a propeller on an initial basis is to compare the boat speed produced by the propeller to the calculated speed-of-advance of the propeller based on its pitch and propeller shaft rotation speed.

The propeller shaft rotation speed is calculated from the measured engine speed and the known engine gear ratio reduction. Because the time interval of propeller shaft speed is in minutes and the time interval in boat speed is in hour, a conversion factor must be applied. A propeller calculator can simplify the analysis.

A propeller calculator performs very simple calculations. With an input of engine speed in RPM, the propeller shaft speed is calculated by the reduction in speed created by the GEAR RATIO. The propeller's speed of advance is then calculated by propeller shaft speed multiplied by PITCH, with a conversion of units into MPH. This calculation gives the speed of advance if the propeller were a screw turning in a solid medium. This does not happen in water; the propeller advances less distance in water. The difference in actual advance compared to calculated speed of advance is expressed as a factor SLIP, a percentage of lost distance.

All propellers have slip, but typically as a propeller approaches the speed range at which the propeller designer intended the propeller to operate, the amount of calculated slip reduces. The typical propeller operating at its optimum speed should have a calculated slip of about ten or less. To facilitate calculation of SLIP, I have implemented a propeller calculator. See

Propeller Calculator: MPH