A zero degrees shaft angle (parallel to waterline) is the most efficient in theory, since thrust is straight aft and water flows to the propeller from straight ahead. In practice, it is very difficult to install like that and leave enough room for the engine and gearbox inside the hull.

The performance or efficiency between a shaft angle of 0° and 5° is very little, so is the same from 5° to 10°, however, shafts with angles greater than 15° begin to introduce significant variable loading to the propeller blades, this is because the upper blade, as it rotates up, is actually receding from the onrushing water, while the lower blade, as it rotates down, is moving forward into the slipstream, the result is uneven blade loading that can cause vibration and/or cavitation.

In addition to aperture size, shaft angle affects the maximum propeller diameter. The higher the shaft angle for a given engine location, the further below the hull bottom the propeller shaft will emerge from its bearing. Thus, the greater the propeller diameter can be. This is a particularly important on twinscrew crafts.

In a new design, or in any major refit and repowering, some thought should be given to the possibility of increasing diameter by increasing shaft angle, within reasonable limits.

An example of performance losses regarding the shaft angle:

Based on a request from one of our clients in Ecuador, we elaborated a simple procedure to measure a propeller’s pitch.

Necessary equipment to perform it:

• Protractor (angle measurement)
• Level
• Plumb Line (not necessary on small props)
• Square Set or Squadron (mm, in or ft)
• Compass

Procedure:

On a leveled surface, make a layout of: a center point, a circle with a diameter equal to the larger propeller hub diameter, and a circle with an approximate diameter that pass through the widest part of the blade to be measured.

Put the propeller with the pitch side up, and center it with the larger diameter previously layout.

From the diameter that pass through the widest part of the blade, measure the perpendicular heights from the surface to the points located on both sides of the blade on the pitch side, that will serve to find the height differences between these two points.

In this procedure, layout on the surface and over the layout diameter, the two points that will help us to define the projected angle over the surface, which will be measured with the protractor as shown in the picture.

Confusion often exists between electrolytic and galvanic corrosion. The difference is quite simple: Whereas galvanic corrosion is caused by an electric current generated by two different metals in a conducting medium such a seawater, electrolytic corrosion is caused by a current from an external source, often the boat’s battery or a shore supply. In a technical sense, the term electrolysis means the process of decomposing a chemical compound by the passage of an electric current. This means that even two similar metals can form the cathode and anode of a cell, the anode being corroded.

The current that causes electrolytic action usually emanates from a poorly installed electrical circuit or a bad grounded arrangement, on power tools or a radio for instance, or current leak due to damp conditions. Short circuit is also a common situation.
Preventing electrolytic corrosion is a matter of good electrical installation. The wiring system should be insulated return (two wires) rather than earth return. A metal hull must never be used as the earth return.
A battery switch should be fitted on the battery positive terminal and turned off when the boat is idle.
Grounded (grounding) is required for safety if voltages are high, as when an onboard 240 volts generator is fitted or a shore supply arranged. Grounded in this sense is not to be confused with earth return. Earth return carries current; grounded or grounding involves a third wire which does not carry current. The earth or ground should consist of a sacrificial plate on the underside of the hull well away from the propellers.

Examples from a good and a bad ground connection:

The phenomenon of cavitation becomes present when a propeller rotates its blades and expels the water backwards; leaving emptiness that is immediately filled by new liquid molecules.

The blades create such depression (reduction of pressure or vacuum) in its front side, that water boils at room temperature; the bubbles that come out of the propeller aren’t air bubbles, but strictly water steam.

These bubbles are displaced very quickly to the backside until they find a higher-pressure zone in which they will become imploding (opposite to explode) water against the propeller blades and pulling out a microscopic particle of metal in each clash (Fig 1).

CAVITATION BY EXCESS OF RPMS

If the speed of the propeller blade tips (tangential speed) surpasses certain limits: 150 feet per second (fps) in 5 bladed propellers, 175 fps in 2-4 bladed propellers, and 100 fps for propellers in nozzles; the expelled water will carry such strength that it’ll prevent that any other water molecules occupy the formed vacuum (Fig.2), therefore, cavitation is produced by tangential velocity of excess of RPM’s. This erosion can always be seen in the tips of the blades.

CAVITATION BY LACK OF BLADE AREA

If the pressure on the blade is higher than 7 psi in open propellers, and 8.5 psi in ducted propellers, the cavitation is produced by lack of area (DAR).

The origin of the bubbles is in the propeller blades leading edge, but the damage is shown on the backside with the aspect of corrosion that goes backwards in its destructive process to the center of the blade.

The erosion produced by any type of cavitation is shown with more intensity when the cathodic protection is not adequate. And in some extreme cases the propeller is completely consumed within days or weeks.

When a propeller is being designed, it is very important to observe the relation Diameter vs. RPM’s, in order to avoid reaching the above mentioned tangential velocities where the cavitation by excess of RPM’s is produced, and at the same time the Relation Diameter-DAR to avoid cavitation by lack of blade area, that finally any type of cavitation will reduce the efficiency of the ship by increasing fuel and reparation costs, or have to replace the eroded propeller.