Observed conditionData confidenceTh

Observed conditionData confidenceThe reliability of the trial and vessel data. It is important not to guess at values as they affect the prediction.Trial speedThe actual trial speed of the vessel.Trial RPMThe actual engine RPM corresponding to the trial speed. (This is not to be confused with the Rated RPM.) If the vessel has more than one propeller and engine, use an average value.Engine/gearEng. modelA descriptive name for the model of an engine. For example, suppose the ABC Engine Company has a line of engines called PowerMax of various sizes and configurations (series), say the 2000, 3000 and 4000. The individual sales models (model) might then be some model number or identifier used for each rating. A PowerMax 2000 at a 250 kW rating might then be something like Eng. model: PM2-250.ManufacturerA descriptive name for the manufacturer of the engine.Rated powerThe maximum defined brake power for a given engine model. Rated RPMThe maximum defined RPM for a given engine model. Gear ratioThe reduction ratio of the transmission gear. This is the ratio of engine RPM to output propeller RPM.The selection of an appropriate reduction ratio is very important to a successful propeller selection. Various factors must be evaluated to choose the best reduction ratio. Generally, the lowest propeller RPM (highest reduction ratio) will produce the highest efficiency. The choice of ratio, however, should be determined in conjunction with the propeller diameter.If a PropExpert optimum propeller is larger than allowable (due to the stern dimensions), then a higher RPM (lower ratio) may be needed to provide the necessary thrust with the smaller diameter. Also, the combination of propeller RPM and the number of blades may be the cause of unacceptable vibration.Gear efficiencyThis accounts for power losses through a reduction gear (transmission).All engine power shown is brake power . The gear efficiency is the ratio of output shaft power to brake power. Typical values for high-speed diesel engine transmissions are from 95% to 97%. PropExpert uses 97% as its standard suggestion.PropellerProp. modelA descriptive name for the model of the propeller.SeriesStandard propeller types of given specific characteristics, such as blade profile, shape and thickness. Types found in PropExpert are:Gawn AEW - open-wheel, flat-faced blade (good overall performance)BSeries - open-wheel, air-foil blade, highest efficiency (unsuitable for low-speed towing)Kaplan19A - accelerating duct, Kaplan blade (designed for high forward motion efficiency)Kaplan37 - accelerating duct, Kaplan blade (designed for improved backing efficiency)BladesThe number of blades on the propeller.Generally, propeller efficiency increases with fewer blades. Remember, however, that it is more difficult to get the necessary cavitation-controlling blade areas on fewer blades.It is also wise to remember that the number of blades can be a factor in the control of noise and vibration. As each propeller blade passes behind a skeg or strut, a pressure pulse is transmitted to the vessel. If the RPM and number of blades combine to create a mode that matches the natural frequency of some of the stern structure, significant vibration may be encountered. Changing the number of blades, or using a skewed propeller, may help to resolve the problem.Blade area ratioThe blade area ratio (BAR) of the selected propeller. Expanded area ratio (EAR) is used by PropExpert.Lowering the blade area can increase efficiency. However, it will also increase blade pressure for a given required thrust, increasing the likelihood of cavitation. Also, lowering blade area too far can mean that thicker blades are needed for adequate strength, eliminating any efficiency gains.DiameterThe tip-to-tip diameter of the propeller.Cavitation can influence diameter selection decisions. Excessive tip speeds - a function of diameter and RPM - can generate tip cavitation. Since propeller RPM is often fixed, the diameter might need to be reduced to correct this.PitchThe average blade pitch of the propeller, usually measured at the 70% radius.The typically large range of available pitch usually means that one with maximum efficiency and thrust can be found. Face cavitation can become a problem if the vessel is operating for long periods at speeds greater than originally intended, such as for a towing vessel being used to ferry passengers.Cup typeA curvature applied to the trailing edge of the blade. It has the effect of an increase in pitch.The definition of cup varies between manufacturers and even among propellers from the same manufacturer. HydroComp has prepared a systematic definition of cup that has made it possible to reliably determine the effect of cup on performance. This definition is based on the amount of trailing edge drop as a percentage of diameter:[None]PX Very Lt