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PROPELLER DESIGN - HAVE YOU GOT THE CORRECT PROPELLER?

The first step in assessing whether an installed propeller is suited to the vessel and engine is observation. Does the vessel perform as well as others of similar power and design? If the answer is no, it is important not to jump to the conclusion that the propeller is incorrectly specified. Other factors must also be considered, such as the condition of the underwater surfaces of the hull. When was the vessel last cleaned and painted? What is the condition of the propeller - is it clean, undamaged and smooth? What is the power of the engine and what condition is it in - should it deliver the same amount of power?	From the desktop of Alejandro Aguilar

The propeller may be incorrectly specified if:

The engine fails to achieve designed RPM and is overloaded;

The engine passes designed RPM at full throttle, over-revs and is under loaded;

The propeller is overloaded and shows signs of cavitations and surface erosion.

Therefore, a preliminary check is advisable before consulting a propeller designer or naval architect for further assistance. A simple method for making a first estimate of what the basic parameters of selection of a propeller for their good operation is described next.

Engine overloading. Overloading of the engine through the installation of a propeller with too much pitch is the most common source of fuel inefficiency. Overloading can also result from the use of a propeller with too large a diameter, but this is less common. With inboard diesel engines, a sure sign of an overloaded engine is a lot of black smoke in the exhaust before reaching the designed RPM.
        Overloading can result in burnt valves, a cracked cylinder head, broken piston rings and a short engine life. It is important to remember that, with a diesel engine, it is the load and not the revs that determines fuel consumption.
        Therefore, continuous overloaded operation results in unnecessarily high fuel consumption and increased maintenance costs.

Engine under loading. Engine under loading from the installation of a propeller with too small a diameter or of insufficient pitch affects vessel performance. It can also result in engine damage if it is allowed to rev above its specified maximum RPM. Engine under loading is likely to be accompanied by low fuel consumption and, often, cavitations.
        If the preliminary check indicates that a change should be made to the propeller, it is worth remembering that some small changes to the pitch can be made without the expense of buying a new propeller. The repitching of a propeller is a specialized task, however, and the propeller will need to be sent to a manufacturer for reshaping. Rice Propulsion we counted on that service and we will be able to support it as far as its problems of propulsion.

Trawlers. The design of trawler propellers requires special attention, as the propeller has to perform under two completely different operating conditions - towing and "free running".
         With a fixed-pitch propeller it is impossible for the propeller to be operating at optimum design conditions while both free running and towing. The propeller designer must strike a compromise based on the time the vessel spends operating in the two situations.
        For vessels working a great distance from their home port, the benefits to be gained from designing a propeller with increased towing power (and therefore catching capacity in the case of a trawler) may well be outweighed by the increased cost of fuel for the transit journey, and the design will err towards a higher-pitched propeller. A day boat operating relatively close to its homeport would inevitably have a propeller optimized for towing.

Trawler vessel
Free running vessel
Towing vessel

Reference:

FAO - Food and Agriculture Organization
of the United Nations
_{FAO
FISHERIES TECHNICAL
PAPER 383}

If you have any suggestions or questions regarding these technical issues, or if you know someone who's willing to receive these technical letters, or if you wish to be removed from our mailing list, please send an e-mail to: technicalletter@ricepropulsion.com

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*Engine overloading.* Overloading of the engine through the installation of a propeller with too much pitch is the most common source of fuel inefficiency. Overloading can also result from the use of a propeller with too large a diameter, but this is less common. With inboard diesel engines, a sure sign of an overloaded engine is a lot of black smoke in the exhaust before reaching the designed RPM. Overloading can result in burnt valves, a cracked cylinder head, broken piston rings and a short engine life. It is important to remember that, with a diesel engine, it is the load and not the revs that determines fuel consumption. Therefore, continuous overloaded operation results in unnecessarily high fuel consumption and increased maintenance costs.
*Engine under loading.* Engine under loading from the installation of a propeller with too small a diameter or of insufficient pitch affects vessel performance. It can also result in engine damage if it is allowed to rev above its specified maximum RPM. Engine under loading is likely to be accompanied by low fuel consumption and, often, cavitations. If the preliminary check indicates that a change should be made to the propeller, it is worth remembering that some small changes to the pitch can be made without the expense of buying a new propeller. The repitching of a propeller is a specialized task, however, and the propeller will need to be sent to a manufacturer for reshaping. Rice Propulsion we counted on that service and we will be able to support it as far as its problems of propulsion.
*Trawlers.* The design of trawler propellers requires special attention, as the propeller has to perform under two completely different operating conditions - towing and "free running". With a fixed-pitch propeller it is impossible for the propeller to be operating at optimum design conditions while both free running and towing. The propeller designer must strike a compromise based on the time the vessel spends operating in the two situations. For vessels working a great distance from their home port, the benefits to be gained from designing a propeller with increased towing power (and therefore catching capacity in the case of a trawler) may well be outweighed by the increased cost of fuel for the transit journey, and the design will err towards a higher-pitched propeller. A day boat operating relatively close to its homeport would inevitably have a propeller optimized for towing.

Trawler vessel	Free running vessel	Towing vessel