RULES OF THUMB


There are X number of rules of thumb on the marine business, here are some about propellers:


From the desktop of

Leonardo Montoya



  1. One inch in diameter absorbs the torque of one to two inches of pitch. This is a good one used mostly on commercial vessels in the mid-power range. Both pitch and diameter absorb the torque generated by the engine. Applies for pitch or diameter tuning, this rule should not be used to select the best propeller for your boat, this requires a deeper analysis.

  1. The higher the pitch your engine can turn near top horsepower and RPM, the faster your boat can go. This is also accurate as far as it goes. The greater the pitch, the greater the distance your boat will advance each revolution. Since top engine RPM is constant, increasing pitch means more speed. Then, why aren't all propellers as small in diameter as possible, with gigantic pitches? The answer is simply that when the pitch gets too large, the angle of attack of the propeller blades to the onrushing water becomes too steep and they stall. This is exactly the same as an airplane wing's stalling in too steep a climb. There are optimum pitches and pitch ratios for each boat. Within these limits it is worthwhile, on high-speed craft, to use the smallest diameter and the greatest pitch possible.

  1. Too little pitch can ruin an engine. This is quite true if the pitch and diameter combined are so low that it allows the engine to race at speeds far over its designed top-rated RPM. Never allow your engine to operate at more than 103 to 105 percent of top-rated RPM. If your engine exceeds that figure, a propeller with increased pitch Qr diameter is indicated.

  1. Every two-inch increase in pitch will decrease engine speed by 50-100 RPM, and viceversa. This is a good rough guide for low to moderate speed boats, draggers and tug boats. Like all rules of thumb, though, it is no more than a rough guide.

  1. A square wheel (a propeller with exactly the same diameter and pitch) is the most efficient. This is not true. There is nothing wrong with a square wheel; on the other hand, there is nothing special about it, either.

  1. The same propeller can't deliver both high speed and maximum power. This is true. A propeller sized for high speed has a small diameter and maximum pitch. A propeller sized for power or thrust has a large diameter. For some boats you can compromise on an in-between propeller, but for either real speed or real thrust there is little common ground.

Base Source: Propeller handbook (Dave Gerr)

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