Propeller Basics: Part 1

This article presents a very brief history of the propeller, gives basic information and description of its operation, and provides a formula for calculating the theoretical maximum speed of a boat based on the propeller pitch and turning speed.

PROPELLER HISTORY

Modern mariners are accustomed to seeing their boats driven through the water by propellers. This was not always the case. Development of the propeller was a relatively modern invention in marine engineering. It was only first around 1850 that the propellor became sufficiently developed that it outperformed the paddle wheel, which had previously been the preferred method of propulsion.

PROPELLER DIMENSIONS

The marine propeller is characterized by three fundamental measurements: number of blades, diameter, and pitch.

Blades

The number of blades is the most straightforward and easily understood parameter. The three-blade propeller has become the most common in recreational marine use. At very high speeds, the two blade propeller has shown advantages. Four-blade and five-blade props have also been used in applications where high power and weight are involved. As a general rule, as power and weight increase, so does the optimum number of blades on the propeller.

Diameter

Diameter measures the diameter of the circle needed to encompass the blades of the propeller. It can easily be deduced by observation of the propeller and physical measurement of its diameter. In outboard marine applications, the diameter of the propeller that can be used is limited by the dimensions of the outboard lower unit. Because of this limitation, for a given outboard motor the range of propellers available will only vary slightly in their diameter. The larger the diameter of a propeller the more horsepower needed to turn it at a given speed.

Pitch

Pitch is an easily understood concept but difficult to measure. The pitch of a propeller is the distance it would advance in one revolution when acting like a screw. There is no simple way to discern the pitch of a propeller from inspection of it, except that generally the maker has stamped or engraved a number on the hub which can be interpreted to find the pitch. If a propeller has a number it can either indicate the actual pitch or a part number which can be consulted to determine the pitch. Deduction of the pitch of a propeller by measurement of its blade shape is a complex task beyond the skill of most boaters.

Since most outboards are limited to a small range of variation in diameter, most often it is the pitch of the propeller that is adjusted to suit the needs of a particular combination of boat, motor, load, and intended use.

Other Factors

There are many other parameters of a propeller which affect its performance (which we plan to explore in future articles), but they will just be briefly mentioned to give an idea of what other variables may be applied to propeller design and selection. The principal variable is the shape of the blade. Many designs for propeller blades have been invented, and each has its claimed advantages. The most common shape in use is the so-called "elephant ear" blade. The most common variation in shape is the cupped blade, which imparts a slight change in the blade surface near the edge which tends to improve the performance of the propeller. An almost infinite variety of blade shapes, thicknesses, tapers, rake angles, and other variations has been developed for marine use. In addition, there are variations in the materials used to manufacture the propeller. (Discussed in more detail in Part 2 of this artile.) The most common propeller material is aluminum. Stainless steel is also frequently used. Non-metallic materials, ranging from simple plastics to complex composite materials have also been used.

PERFORMANCE

In marine propulsion, the propeller is an extremely significant factor in determining the overall performance of the boat and motor combination. It is through the propeller that the power of the engine is converted to propulsion of the boat. Small changes in the dimensions of the propeller can have significant effect on the speed and acceleration of the boat. From a practical point, it is also much easier (and more economical) to experiment with changes in propeller dimensions than it is to undertake modification of the hull or powerplant of a boat. These two principles have lead to a tendency to experiment with propeller dimensions in the search for optimum boat performance.

How does one know when the optimum performance has been reached? Two indices are used to calculate this: boat speed and engine speed.

Determining Boat Speed

Boat speed is an easily understood and measured parameter. For most boaters, it is quite simple: faster is better. Accurate measurement of boat speed is now quite simple thanks to advanced navigational instruments like the GPS. If wind and current are present at the test site, their effects can be reduced by travelling the courseline in opposite directions and averaging the measured speeds. Also, when using GPS to indicate speed, use only straight line courses, as the GPS speed indication is deduced by comparing the difference in boat position between two measured positions and assuming a straight line course between them. GPS speeds shown while turning or following curving courselines will not be as accurate.

Determining Engine Crankshaft Speed

Engine crankshaft speed is also easy to measure, perhaps even more so than boat speed. Most engines include an accurate tachometer as part of their basic instrumentation. No serious analysis of propellor performance can be made without being able to accurately measure the engine crankshaft speed with a tachometer. Attaining proper engine crankshaft speed is important because an engine can only develop its rated horsepower if allowed to advance to its rated engine speed. For example, an outboard motor may be rated as a 50-HP engine. It only produces 50-HP when the engine crankshaft speed reaches the rated RPM, which in 2-cycle outboards is typically in the range of 5000 to 6000 revolutions per minute.

Optimizing Engine Performance

If the load on an engine is too great, the engine will not be able to advance to its rated speed nor produce its rated horsepower. If the load on an engine is too light, the engine will be able to increase its speed above its maximum rated speed, which can cause damage to the engine. Engines should never be operated at engine speeds in excess of their maximum ratings. The optimum engine speed at wide open throttle should be the rated maximum speed, which is, again, also the range of speed at which the engine produces maximum horsepower.

Propeller As Determining Factor

The coupling of engine power to the propulsion of the boat occurs through the propeller, and thus it is the propeller which in a sense controls the engine, not the other way around. If the pitch of the propeller is too high, the load will be too great and the engine will be unable to develop enough power to turn the propeller to the engine's maximum speed. Conversely, if the pitch of the propeller is too low, the engine will reach its maximum speed without being pushed to wide open throttle, and potential additional speed will be lost because the engine throttle cannot be advanced further without over revving the engine.

When the propeller is properly sized, its pitch (and diameter) will produce a load that matches the engine's horsepower and allows the engine to run at its maximum rated speed. This is the goal of "prop tuning." With the engine turning at maximum rated speed at full throttle, the propeller will drive the boat to its maximum speed.

SPEED POTENTIAL OF PROPELLER

The potential speed of advance that can be produced by a propeller is a function of its pitch and the speed at which it is turned. To determine the pitch one must generally rely upon information from the manufacturer since there is no simple way to deduce the pitch from measurements of the propeller. If a prop maker says he has sold you a 15-inch pitch prop, there is little that can be done to independently measure the pitch.

Engine Crankshaft Speed vs. Propeller Shaft Turning Speed

The speed that the propeller will turn is a function of the engine speed, but in almost all cases it is not a 1:1 ratio. Generally a gear reduction is accomplished in the lower unit of the outboard, resulting in the propeller shaft turning more slowly than the engine crankshaft. The gear ratio is usually available from the engine maker, and it is often listed as part of the dimensions or specifications of the engine in the owner's manual, repair manual, or sales literature.

Calculating Propeller Shaft Speed

To determine the speed potential of a propeller, the following data must be known: pitch and turning speed. To determine the turning speed, the engine crankshaft speed and gear reduction must be known. The relationship is:

   Engine Speed (RPM) 
  --------------------  =   Propeller shaft turning speed
   Gear Reduction 

For example, if an outboard engine has a gear reduction of 2.33:1 and turns at 5,500 RPM, the propeller shaft speed will be:

    5,500 RPM
  ------------  =   2,360.5 RPM Propeller shaft turning speed
    2.33 

Calculating Maximum Speed of Advance

When the actual turning speed of the propeller is known, the maximum speed potential for the propeller can be calculated by multiplying the turning speed by the pitch:

  Propeller shaft (REV/MIN) X Propeller (INCH/REV) = Advance (INCH/MIN)
  Turning Speed               Pitch

For Example, a 15-inch Pitch propeller being turned at 2,360.5 revolutions per minute will produce an advance of:

  2360.5 (REV/MIN) X 15 (INCH/REV) = 35407.5 (INCH/MIN)

Units Conversion

Speed is generally not familiar to us in units of inches per minute and therefore must be converted into more familiar units like miles per hour. A series of conversion factors are applied:

   1 FT          1 MILE         60 MINUTE      1 MILE/HOUR
 _________  X  _________  X   ___________ =   ________________
  12 INCH       5280 FT         1 HOUR        1056 INCH/MINUTE
  
 

Using this newly calculated factor to convert our initial answer, the theoretical speed of advance of the propeller can be found in miles per hour:

  35407.5 INCH/MINUTE      1 MILE/HOUR
                      X  _________________ =   33.5 MILE/HOUR
                         1056 INCH/MINUTE
  
 

Now that the procedure has been demonstrated, the various terms and conversion factors can be aggregated into one formula:

   ENGINE SPEED (RPM)       PROP PITCH (INCH)
  ____________________  X  ___________________  =  SPEED OF ADVANCE (MPH)
   GEAR REDUCTION               1056
   
 

Plugging in the numbers from the earlier example:

      5,500 (RPM)               15 (INCH)
  ___________________  X  ___________________  =   33.5 (MPH)
      2.33                      1056
   
 

 

Actual vs Calculated

It is unlikely the theoretical maximum speed of advance will be realized from a propeller due to imperfect coupling of the propeller to the water. Some slipping will occur, varying as a function many factors, including the weight of the boat, the design of the hull, the design of the propeller, and the density of the water. To gauge the effectiveness of a particular propeller, motor, and boat combination, it is interesting to make careful measurement of the boat's actual speed at various engine speeds, calculate the theoretical speed of advance that should have occured, and then compare the two. In any experiment, the more accurate the measurements and procedure, the more accurate and valid the results.

CASE STUDY

Having recently purchased a new (used) boat, I was curious to see how it performed and if the propeller(s) were properly sized. We made a series of speed runs and collected the data shown below. The boat is a Boston Whaler 20-Revenge, powered by twin 70-HP 2-cycle outboards with three blade "elephant ear" propellors. Initially we thought the propellors were aluminum, but I have since learned that this engine maker (Yamaha) sometimes includes steel propellors which are painted black.

The SPEED OF ADVANCE was based on:

These numbers and the formula described above were used to calculate the values for SPEED OF ADVANCE.

The OBSERVED SPEED was measured using a GPS navigation instrument. During the time of these observations Selective Availability dither of civilian GPS accuracy was not in use. The GPS speed measurements are believed to be absolutely accurate to a few percentage points, but their relative accuracy may be much higher. The numbers recorded represent some averaging of instantaneously displayed speeds on the GPS instrument.

The PERCENT EFFICIENCY was calculated by simply dividing the OBSERVED SPEED by the SPEED OF ADVANCE and expressing the result as a percentage.

It should be mentioned that the engines were not run to maximum throttle during the test because of a slight problem with the tachometer on one of the engines which made synchonization difficult above 5000 RPM. Some additional throttle advance still remained, although most likely not more than 500 additional RPM would have been achieved. That places the estimated maximum throttle engine speed in the top of the specified range of engine speeds (4500-55000 RPM) that the maker advises is the "Operating Range."

During the testing the boat had an approximate half-tank of fuel and two adults were aboard. Water conditions were calm and well suited to making high speed runs.

Propeller Test 11/2000 BW-20 Revenge
PITCH=17 & GEAR REDUCTION 2.33:1
ENGINE RPM SPEED OF ADVANCE OBSERVED SPEED % EFFICIENCY
4000 27.6 24.3 87.9
4500 31.1 29.0 93.2
5000 34.5 32.5 94.2

 

Interpretation of Results

The propellor tests show that the current configuration of boat, motor, load, and propellors are well matched. The engines achieved speeds at near Wide-Open-Throttle that are close to those suggested by the maker as being the "operating range." This is the desired condition, and therefore the propellers were judged to be well matched to the boat and motor. The observed top speed of the boat was approximately 32 MPH, which may be a bit on the low side for some, but was judged fast enough to be satisfactory for us. After all, we are former sailors!

It is interesting to note how the efficiency seems to be rising as the speed rises. This is not totally unexpected. There are two factors that could contribute to this. First, as the speed increases there is less and less of the hull in the water, resulting in less drag working against the propeller's forward thrust. The progressive removal of the hull drag would tend to increase the efficiency of the propeller.

A second factor may also contribute. At higher speeds the characteristics of water change from those of a liquid to properties more like a solid. This would tend to improve the efficiency of the propeller which is trying to advance through the water by screwing itself forward. The result would be the rise in efficiency with speed, as observed. Of course, as the speed increased this same factor (the tendency of the water to act more like a solid at high speed) would tend to produce greater drag from those parts of the boat remaining in the water, i.e., the lower unit and propellor At some speed an equilibrium would be reached or efficiency might begin to reduce.

In Part 2 of this article I will explain in more detail why the propeller become more efficient as boat speed increases. Different materials for making propellers will be discussed, and an speed prediction forumula will be presented.

CONCLUSION

Following the procedure described in this article, a boater can measure and predict the potential top speed of his boat for a given propeller pitch. If careful measurements are made of boat speed at various engine speeds an analysis of the propeller efficiency and its suitability can be made. Rather than simply offer a "plug-in" formula for this process, this article has attempted to explain the concept involved and to demonstrate by an example. This tends to produce a deeper understanding of the principles involved and of the process described.

COMMENTS OR QUESTIONS

If you have a comment or question about this article, please post it in the Whaler Forum.

This article continues in PART 2.


DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 1999, 2000 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared March 29, 2001.