Propeller 101:
 
 
1.) General Considerations
2.) Theory of Propeller Action
3.) Propeller Slip
4.) Selection of Efficient Shaft Speed
5.) Blade Contours
6.) Galvanic Corrosion
7.) Rules of Thumb

 

 

 

1.) General Considerations
There are few products in such common use as marine propellers that are often so little understood. We shall try to provide in non-technical fashion, an understanding of how a propeller functions.

  • The power developed by any marine engine is available at the propeller shaft in the form of torque, or "twisting effort." We need something to convert this twisting effort to thrust or "push" which can be used to drive the boat. The propeller does this job. It will be helpful to become familiar with propeller geometry and dimensions.
  • Propeller diameter is the 'diameter' of a circle circumscribing the tips of the propeller blades. It is equal to twice the distance from the shaft centerline to the tip of one blade.
  • Propeller pitch is a linear dimension usually expressed in inches, feet, millimeters, or meters, and is equal to the advance
    of the propeller in one revolution at "zero slip". It is exactly the same thing as the pitch of a machine screw if one imagines the propeller to replace the screw and the water to replace the nut into which the screw is threaded.
  • Propellers commonly have three blades, but may be built with two, four, five or more blades for special purposes.
  • Propellers are either right hand or left hand turning, depending on the direction of rotation of the shaft. Direction of rotation has no special effect upon performance as far as speed or engine load are concerned.

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2.) Theory of Propeller Action

  • The function of a propeller is to convert torque to thrust. Thrust is another name for force, and a basic axiom of mechanics tells us that Force (thrust) is equal to Mass times Acceleration. In other words, if we impart an acceleration to a mass of water, we will generate a thrust or push which will accelerate the boat forward while the mass of water is moved in the opposite direction.
  • The propeller blade is given a shape such that when rotated in water, it acts like a pump and pushes a mass of water astern. It is, in other words, a pump without a casing, operating submerged in the fluid it is pumping.
  • A propeller and an oar blade do the same job--both impart an acceleration to a mass of water, except that the oar blade does so intermittently whereas the propeller does so continuously.

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3.) Propeller Slip

  • We have shown that the propeller, in order to generate thrust must accelerate or move a mass of water astern. Now the propeller, being shaped like a screw might conceivably, as it rotates, slide through the water as a machine screw would into a nut, without displacing any water aft. If this happened the propeller (and the boat) would, in one revolution of the shaft, advance an amount equal to the propeller pitch. This would be called zero slip. But in order to produce a thrust,
    we must accelerate or move some water aft, and therefore it is apparent that the propeller will not advance the full
    amount of its pitch in each revolution, but will advance some lesser amount, depending upon how much water it accelerates astern in the process of producing enough thrust to offset the resistance of the boat to being driven ahead.
    If the boat were tied to a dock, the propeller would not advance at all but would generate maximum thrust because full engine power would go into accelerating water astern. This would be called operation at 100% slip.
  • The term "apparent slip" is used to indicate the difference between the theoretical speed that the boat would obtain on the propeller pitch and the rpm of the propeller shaft, and the actual speed of the boat.
  • Slip must not be confused with efficiency that is a measure of the percentage of engine power converted to thrust by the propeller. We have seen that we must have slip in order to generate thrust and the amount of slip will be proportional to the amount of thrust required by the boat.
  • High-speed runabouts and fast cruisers require relatively low thrust and therefore operate at low slip whereas tugs and other heavy vessels require high thrust and therefore operate most efficiently at high slip.

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4.) Selection of Efficient Shaft Speed

  • Since force (thrust) is equal to mass times acceleration, it would seem that we might get the same force whether we gave a large acceleration to a small mass of water (small propeller turning fast), or a small acceleration to a large mass of water (large propeller turning slowly). In practice, however, there are other factors such as the relation of the propeller pitch to its diameter and the energy losses due to friction between the accelerated water and the surrounding water that make a proper relation between boat speed and propeller shaft speed essential to an efficient installation.
  • In general, horsepower available and shaft speed determine the propeller diameter, while shaft speed and boat speed determines propeller pitch. The pitch of a propeller divided by its diameter is a term called "pitch ratio." For example, a propeller of 20" diameter and 20" pitch has a pitch ratio of 1.0, a diameter of 20" and pitch of 15" a pitch ratio of .75, etc. For best efficiency, the pitch ratio of boat propellers should be in the range of .55 to .80 for tugs and trawlers, .65 to 1.0 for heavy and average cruisers, .80 to 1.2 for medium and fast cruisers and .90 to 1.5 for exceptionally fast cruisers and runabouts. Pitch ratios outside these ranges generally will indicate an unsuitable shaft speed.

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5.) Blade Contours

  • The evolution of blade contour to its present high state of efficiency has followed the development of marine steam turbines, high-speed marine gasoline engines, marine diesel engines, and marine electric drives.
  • Constantly increasing pitch ratios have necessitated continual advances in blade contour and section design. The refinements, which came as a result of this work, have been responsible for the development of the marine propeller into the high speed, precision mechanism that it is today.
  • Some styles have elliptical blade contour, with the maximum blade width located from 1/2 to 2/3 the radius; both leading and trailing edges are symmetrical. These propellers have maximum blade widths ranging from 25% to 40% of the diameter, except over 50" diameter, in which case widths are usually not in excess of 33 1/3%.
  • Modifications of the elliptical blade contour are employed for specific applications. For extremely heavy work boats using low speed heavy duty engines, modification of the blade contour with a square end places the maximum blade width at approximately 3/4 the radius with the leading and trailing edges practically symmetrical.
  • Narrower blade types usually are the choice for four and five blade propellers, selected to avoid vibration. The total developed blade area of such a propeller is comparable to that of a conventional three-blade propeller. Special designs of blade contour have been developed for light high speed, high rpm craft.

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6.) Galvanic Corrosion

  • Galvanic corrosion (sometimes called dissimalar metal corrosion) is the process by which the material in contact with each other oxidises or corrodes. There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimalar metals present. Second, there must be an electrically conductive path between the two metals. And third, there must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal ( salt water for example ). If any one of these three conditions does not exist, galvanic corrosion will not occur.
  • Good Practice
    Select metals listed as closely as possible to each other in the galvanic table.
    Insulate between dissimilar metals.
    Locate dissimilar metals as far apart as practical.
    Never use fasteners of less noble material than the metal parts they are used to secure. For example, while bronze bolts might be used to secure an iron skeg, steel bolts would corrode rapidly if used to secure a bronze skeg.

Galvanic Table
Active
(Anodic)
Magnesium
Zinc
Aluminum
Cadmium (plated)
Steel
Cast Iron
Stainless Steel (active)
Lead, Tin
Nickel
Brass
Copper
Bronze
Monel
Stainless Steel (passive)
Silver
Gold
Noble
(Cathodic)

 

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7.) Rules of Thumb (Reprinted Courtesy of the Propeller Handbook by Dave Gerr)

  • There are countless rules of thumb floating around about propellers. Some are useful and others are worthless. We will take a brief look at a few of them.
  • One inch in diameter absorbs the torque of two to three inches of pitch. This is a good rough guide. Both pitch and diameter absorb the torque generated by the engine. Diameter is, by far, the most important factor. Thus, the ratio of 2 to 3 inches of pitch equals1 inch in diameter is a fair guide. It is no more than that, however. You could not select a suitable propeller based only on this rule.
  • 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. Within limits it is worthwhile, on high-speed craft, to use the smallest diameter and the greatest pitch possible.
  • 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 or diameter is indicated.
  • Every two-inch increase in pitch will decrease engine speed by 450 RPM, and vice versa. This is a good rough guide for moderate- to high-speed pleasure craft, passenger vessels and crew boats. Like all rules of thumb, though, it is no more than a rough guide.
  • 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.
  • 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.

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