Introduction Power Ratings A great number of environmental factors, such as bar- ometric pressure, ambient temperature, humidity, the quality of fuel, and exhaust back pressure can affect engine performance. When it comes to quoting and comparing ratings, it is important that there is a unified set of standards for measurement. Load Condition (Speed of Planing Hull) The overall weight of the boat is another important factor in performance. Any increase in boat weight will slow down the boat speed, particularly on boats with planing and semi-planing hulls.
For example, a new boat tested with fuel and water tanks only half filled, and without any load, can easily drop 2 to 3 knots in speed when tested fully fuelled and loaded with all normal equipment and supplies for safe and comfortable cruising. This is because the propeller installed originally is frequently one that is designed to give maximum speed when the boat is new. For this reason it is often advisable to reduce the propeller pitch by as much as an inch or more in order to counter the effects of the increase in overall weight encountered in normal cruising, particularly in hotter climates.
Although this will reduce top speed some- what, overall ride conditions will improve and you should achieve greatly enhanced acceleration. In September 1989, all major marine engine manufac- turers agreed to quote engine power output according to a common set of conditions. These conditions are referred to as ISO 8665. All Volvo Penta engines meet the ISO 8665 standard. This ISO standard outlines the following fixed values or common conditions for deter- mining the rating of the engine.
Condition Value Air temperature .........................25°C (77°F) Barometric pressure..................100 kPa (14.504 PSI) Relative humidity.......................30% 7743722 - Downloaded from www.volvopenta.com 13/05/2008 20:17:06 A gasoline engine operates with very little surplus air. When conditions deviate from the standard values, the result can be a loss of power at full load. It can also cause a rise in exhaust emissions due to incomplete fuel combustion. In considering the influence of weight, it is worth re- membering that fiberglass boats absorb a significant amount of water into their hulls while left afloat for any length of time and so become progressively heavier.
Another negative influence on boat performance is marine growth beneath the water line–a problem that is often overlooked. Marine engines can be rated according to one of sev- eral power standards, but power output itself is quoted in kilowatts (KW) or horsepower (HP), for a given en- gine speed, usually at maximum revolutions per minute (RPM). VPA 7743722 English 10-2004 15