The Engine is of no interest except as a power source, and we only need to know two things about it;

• Maximum continous RPM
• BHP at this maximum RPM.

It is very important to note that unless your vessel is fitted with a continuous duty heavy marine diesel, which is most unlikely unless you have a converted trawler or working boat, the maximum continous rating of the engine will be significantly lower than the maximum rating quoted by the engine manufacturer in its sales brochure.

A safe figure for maximum continuous RPM is 85% of quoted maximum RPM, so if your motor manufacturer specified a max RPM of 2,200 then you must use 85% of this figure, or 1,870 RPM as your maximum continuous RPM figure.

You then need to know your engines BHP output at this figure of 1,870 RPM. Ideally ask your manufacturer or look at the RPM / Power graph in the handbook. DO NOT just assume that is is 85% of quoted maximum BHP at 2,200 RPM as there is no linear relationship between RPM and BHP, this is actually a curve when plotted on a graph.

This is one of those areas mentioned on the Introduction page where you must not under any circumstances guess the answer. You MUST determine the correct answer or you cannot proceed any further.