propeller basics part 1

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    Propeller Basics Part 1

    An in depth explanation on propellers includes calculation for speed and performance. A case-study helps to illustrate how one mightfind the potential speed of a boat for a given propeller pitch.

    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 formulafor calculating the theoretical maximum speed of a boat based on the propeller pitch and turning speed.

    PROPELLER HISTORYModern 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 propeller

    became sufficiently developed that it outperformed the paddle wheel, which had previously been the preferred method of propulsion.

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

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

    DiameterDiameter 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 islimited by the dimensions of the outboard lower unit. Because of this limitation, for a given outboard motor the range of propellersavailable will only vary slightly in their diameter. The larger the diameter of a propeller the more horsepower needed to turn it at a givenspeed.

    PitchPitch is an easily understood concept but difficult to measure. The pitch of a propeller is the distance it would advance in one revolutionwhen 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 indicatethe 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 suitthe needs of a particular combination of boat, motor, load, and intended use.

    Other FactorsThere 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 variableis the shape of the blade. Many designs for propeller blades have been invented, and each has its claimed advantages. The most commonshape in use is the so-called "elephant ear" blade. The most common variation in shape is the cupped blade, which imparts a slightchange in the blade surface near the edge which tends to improve the performance of the propeller. An almost infinite variety of bladeshapes, thicknesses, tapers, rake angles, and other variations has been developed for marine use. In addition, there are variations in thematerials used to manufacture the propeller. (Discussed in more detail in this artile.) The most common propeller material is aluminum.Stainless steel is also of requently used. Non-metallic materials, ranging from simple plastics to complex composite materials have also

    been used.

    PERFORMANCEIn 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 thedimensions 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 SpeedBoat speed is an easily understood and measured parameter. For most boaters, it is quite simple: faster is better. Accurate measurementof boat speed is now quite simple thanks to advanced navigational instruments like the GPS. If wind and current are present at the testsite, their effects can be reduced by traveling the course line in opposite directions and averaging the measured speeds. Also, when usingGPS to indicate speed, use only straight line courses, as the GPS speed indication is deduced by comparing the difference in boat

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    position between two measured positions and assuming a straight line course between them. GPS speeds shown while turning or following curving course lines will not be as accurate.

    Determining Engine Crankshaft SpeedEngine 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 measurethe engine crankshaft speed with a tachometer. Attaining proper engine crankshaft speed is important because an engine can onlydevelop 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 therange of 5000 to 6000 revolutions per minute.

    Optimizing Engine PerformanceIf 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 loadon an engine is too light, the engine will be able to increase its speed above its maximum rated speed, which can cause damage to theengine. Engines should never be operated at engine speeds in excess of their maximum ratings. The optimum engine speed at wide openthrottle should be the rated maximum speed, which is, again, also the range of speed at which the engine produces maximumhorsepower.

    Propeller As Determining FactorThe coupling of engine power to the propulsion of the boat occurs through the propeller, and thus it is the propeller which in a sensecontrols 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 beunable 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 becausethe 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 theengine to run at its maximum rated speed. This is the goal of "prop tuning." With the engine turning at maximum rated speed at fullthrottle, 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. Todetermine the pitch one must generally rely upon information from the manufacturer since there is no simple way to deduce the pitchfrom measurements of the propeller. It a prop maker says he has sold you a 15-inch pitch prop, there is little that can be done toindependently measure the pitch.

    Engine Crankshaft Speed vs. Propeller Shaft Turning SpeedThe 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 enginecrankshaft. 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 SpeedTo determine the speed potential of a propeller, the following data must be known: pitch and turning speed. To determine the turningspeed, the engine crankshaft speed and gear reduction must be known. The relationship is:

    Engine Speed (RPM)-------------------- = Propeller shaft turning speedGear 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 speed2.33

    Calculating Maximum Speed of AdvanceWhen the actual turning speed of the propeller is known, the maximum speed potential for the propeller can be calculated by multiplyingthe 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)

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    Units ConversionSpeed 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 CalculatedIt is unlikely the theoretical maximum speed of advance will be realized from a propeller due to imperfect coupling of the propeller to thewater. 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 isinteresting to make careful measurement of the boat's actual speed at various engine speeds, calculate the theoretical speed of advancethat should have occured, and then compare the two. In any experiment, the more accurate the measurements and procedure, the moreaccurate and valid the results.

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

    The SPEED OF ADVANCE was based on:

    the RPM observed on the engines' digital tachometers,the engine's reduction ratio of 2.33:1, andthe claimed pitch of the propeller, 17-inches (as inferred by noting the stamped number "17" on the hub).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 SelectiveAvailability 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 instantaneouslydisplayed speeds on the GPS instrument.

    The PERCENT EFFICIENCY was calculated by simply dividing the OBSERVED SPEED by the SPEED OF ADVANCE andexpressing 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 synchronization difficult above 5000 RPM. Some additional throttle advance still remained, althoughmost likely not more than 500 additional RPM would have been achieved. That places the estimated maximum throttle engine speed inthe 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

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    to making high speed runs.

    Interpretation of ResultsThe propeller tests show that the current configuration of boat, motor, load, and propellers 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 desiredcondition, and therefore the propellers were judged to be well matched to the boat and motor. The observed top speed of the boat wasapproximately 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, weare 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 thatcould contribute to this. First, as the speed increases there is less and less of the hull in the water, resulting in less drag working againstthe 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 asolid. 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 thewater 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 propeller 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 materialsfor making propellers will be discussed, and an speed prediction formula will be presented.

    CONCLUSIONFollowing 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 itssuitability can be made. Rather than simply offer a "plug-in" formula for this process, this article has attempted to explain the conceptinvolved and to demonstrate by an example. This tends to produce a deeper understanding of the principles involved and of the processdescribed.