This was the title of a previous thread that got deleted due to a poster's attitude problem. For the rest of our readers, the following is the Bombardier response to a different post that about E-TEC performance reports and why the "prop formula math" did not jive with the recorded results.<br /><br />-----------------<br />Bombardier<br />Boats and Outboard Engines Division<br />250 Sea-Horse Drive <br />Waukegan, IL 60085 <br /><br />December 17, 2003<br /><br />Subject: E-TEC Performance Reports, Propellers and Gearcases<br /><br />We have received several requests for information regarding E-TEC performance reports as well as questions relating to the accuracy of data contained in those reports and some questions regarding our propellers and gearcases. The following is the result of our review.<br /><br />We asked our Application Engineers to review their field notes and compare the results to their spreadsheets and to the published Performance Reports. They have verified there are no mistakes or typographical errors in any of the documents. The data provided is accurate. Our Application Engineers use highly accurate instruments, such as digital tachometers, fuel flow meters, as well as gps and/or radar guns to collect the data for the Performance Reports published by Bombardier. The figures recorded in the Performance Data table are the average of several runs. These are accepted measurement devices and test standards used by other marine engine manufacturers as well as the boating media.<br /><br />We also invited our Propeller Engineer to address the propping concerns and slip numbers which have been derived from the performance reports: Slip is the most misunderstood of all propeller terms, almost certainly because it sounds like something thats unwanted. Propeller slip is not a measure of propeller efficiency. Prop slip is the difference between actual and theoretical travel, resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but then there would be no positive and negative pressure created on the blades and therefore, there would be no thrust. To create thrust, in theory, there must be some angle of attack or slip. <br /><br />The objective of propeller design is to achieve the right amount of angle of attack. This is accomplished by matching the blade diameter and blade area to the existing engines horsepower and prop shaft rpm. Too much blade area or diameter will lower slip, but also lowers propeller efficiency, resulting in reduced performance (remember this last thought). <br /><br />When designing props, we start with: 1. what horsepower must it absorb, 2. what propshaft rpm do we want it to run at (based on gear ratio and operating range of the engine), and 3. what is the target boat speed for the application it will used in. We then measure or estimate the drag of the boat and the gearcase combined to give us the thrust that the prop must produce at the design speed. These values are then run through a computer using a propeller design program that calculates the optimum diameter, pitch and the blade loading for a specified blade area. From the output of the program, advance coefficient and prop efficiency can then be calculated.<br /> <br />This determines correct prop loading and allows us to manipulate the blade design, in very specific locations on the blade pressure face if necessary, to achieve the engine rpm and thrust we are targeting. That manipulation could be in blade flat, regression or progression and the difference from one area of the blade to the next could be .050" or less. It has a substantial effect on how the blade does its job.<br /><br />So, what is propeller efficiency? In simple terms, the power coming out of a prop divided by the power going into it, expressed as: Efficiency (%) = hp out / hp in x 100. <br /><br />To calculate hp out: hp out = boat speed (mph) x prop thrust (lbs) / 375<br />To calculate hp in; first calculate prop shaft speed: prop shaft rpm = engine rpm / gear reduction<br />then prop shaft torque must be measured: hp in = prop shaft rpm x prop shaft torque (ft.lbs.) / 5250<br /><br />Example: A boat powered by 120 hp outboard with 2.0:1 gear reduction, runs 47.5 mph @ 5500 rpm. With sophisticated equipment, measured prop shaft torque is 235 ft.lbs. and measured prop thrust @ 47.5 mph is 750 lbs. <br /><br />Prop shaft rpm equals: 5500 / 2 = 2750 rpm<br />Hp Out equals: 47.5 x 750 / 375 = 95 hp<br />Hp In equals: 2750 x 235 / 5250 = 123.09 hp<br />Propeller Efficiency equals: 95 / 123.09 x 100 = 77.17%<br /><br />Its not common for prop slip or speed calculations to show zero or negative slip percentages. Occasionally an application engineer encounters it. Heres why it occurs: In the performance reports discussed, the application engineers used Bombardier SST propellers. These props (part number 176***) which are marketed as a swept blade design were developed specifically for the Eagle V4 90/115hp. Since the current V4 prop line fits all V4 engines (including the old cross-flow), the three cylinder carbureted engines (both the old 49 cu.in. & the 56 cu.in.), the commercial in-line engines and our new E-TEC engines, they may be somewhat a compromise in some applications. In the case of the E-TEC performance reports, the hp & torque curves used in the advance coefficient (which was for the Eagle V4) was not actually reached for the particular props used in this test. Remember: Too much blade area or diameter will lower slip, but also lowers propeller efficiency, resulting in reduced performance. The props could potentially be brought back into their design limits by shaving some blade thickness or perhaps some diameter. If they were brought back to the 10% slip range (using the conventional prop slip calculation method), the operator may actually experience some improvement in performance. If this boat test had been conducted with a 75 hp V4, the slip % would be expected to be within the 10 to 6% range. Reviewing several V4 performance reports in which the 176*** props were used, substantiates this conclusion.<br /><br />Bear in mind, for the recreational marine industry, there is no industry standard for measuring pitch. Although there are some broad guidelines, the method for measuring propeller pitch varies from manufacturer to manufacturer and is probably an exercise in futility in all but the most sophisticated of propeller shops.<br /><br />The pitch number stamped on the hub (of BRP props) is what the engineering department dictates. Pitch changes constantly across the face of the propeller blade and we measure pitch at numerous points and at different radii from the center of the prop shaft, using advanced computer-aided equipment. We do not consider cupping, nor do we average all our measurements into the pitch designated on the hub. Loosely speaking, its an approximation of the performance you might expect to see with other propeller lines.<br /><br />Regarding the 40/50 E-TEC gearcase, it is the same gearcase used on the commercial in-line engines. By using the 2.67:1 gear ratio we are able to use the V4 line of propellers and although the 40/50 E-TEC does not have the displacement of the bigger (V4) engines, the stall speed with a large prop is perfect match for the 40/50 E-TEC power band and is an excellent choice for large pontoon boats or other applications where high thrust is desired. This is a very robust gearcase, failures are very few and even farther apart. It has exactly the attributes we want for E-TEC.
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