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The distance between the propeller and the hull influences the efficiency of the propeller operation, as well as the flow of water
around the hull and affects the intensity of the vibration caused by the propeller. This generates a loss of fuel.
In general, the larger those distances, the better. However, if the aperture is small, the larger these distances are, the smaller the propeller diameter may be and the lower the
efficiency. If these distances are expected to be large in the design phase, the propeller arch should be raised and this could require the waterline to be blunter immediately ahead of
the propeller. This would increase the resistance of the hull in the water. A small aperture requires a small diameter propeller, which may not be able to efficiently absorb all of the
engine's power, resulting in inefficient performance, engine damage, or poor trawling capacity. An intermediate solution for a small aperture can be found, for example, by the
following:
- Establish a new angularity in the shaft (for which the engine must be remounted).
- Use a shaft extension (for which the rudder must often be displaced).
- Install a propeller with a higher Disc Area Ratio (DAR).
At Rice Propulsion we recommend the following gaps or minimum and maximum spaces for a propeller protection for its proper operation, shown in table 1. in relation to Figure 4.
These distances are closely associated with the number of blades and can be calculated as follows:
In general:
- The distances to the tips of the blades should be as small as possible within the standards, so that the propeller can be as large as possible.
- The distance between the propeller and the rudder must be small to maintain steering control.
- The distance between the aft dock and the propeller must be large.
In the design and installation of trawler propellers, the space between the tip of the blades and the hull can be as small as 8 to 10 percent of the propeller diameter. The disadvantage
of increased vibration is offset by increased thrust and efficiency of a larger diameter propeller. (J.D.K. WILSON, 2005)
The distance between the tip of the blades and the hull must never be less than 50 mm in any boat.
At Rice Propulsion we have the necessary experience and knowledge for the selection and ideal design of the propeller with the necessary requirements.
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