The Offshore Voyaging Reference Site

How To Select The Best Power and Propeller Settings For Your Engine

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In the last chapter I wrote about how important it is to run our engines regularly at higher power settings. But what exactly does that mean? How can we select the right RPM setting to make sure that our engines are not underloaded and thereby wrecked by glazing?

And any time we talk about the best power settings for our engine, we need to think about the propeller too.

The good news is, that if we get all this right, we can save a lot of fuel, emit less carbon, as well as save our engines from early demise.

Read on for how to do that.


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Odd Arne

Hi John,

Very useful information again, and thank you for bringing this subject up to a higher level. This understanding can save us money but it also gives many frustrations on how to do it right.

Also frustration regarding propellers, feathering propellers has very low drag, but when you see the flat surface of the blades you don´t need to be a rocket scientist to see that the prop is not the most efficient, but that is maybe another discussion since you put load on the engine with this propeller also.
Bore glazing occurs at low speed and light load operation and if you are lucky you can get rid of it as you did. With a correct run-in engine I think it´s not so easy to get bore glazing, -if you don´t change the routine you had the first (50-100?) hours.
Cylinder bore polishing is worse and you can get it after wrong run-in and long-term under-loading with incomplete fuel combustion and loss of lubrication. Then the microscopic peaks and valleys in the cylinder are gone.

jim clifford

Hi john,
This has been a very interesting series of articles on engine/propeller installation and optimization.
Over the years I have reached much the same conclusions as your articles as to what constitutes an efficient engine installation and have installed a Perkins M65 (45Kw @2600 rpm) engine driving a 23 inch feathering prop through a 3:1 reduction gearbox on our current 15 meter 17 tonne cruising yacht.
At commissioning I set the propeller pitch to the manufacturers recommendation and sea trials gave 9 knots at 2600 rpm (wide open throttle) which was a little better than expected top speed.
With these settings we were running at about 2000 rpm to achieve a cruising sped of 7 knots but noise levels were a bit on the high side . A couple of years ago I increased the propeller pitch to a level which only allows the engine to achieve 2450 rpm at wide open throttle. This has brought the cruising speed revs down to 1800 rpm and resulted in a much quieter installation. Fuel use has improved a little to 5 l/hr at 7 knots.
I have noticed two minor changes to the operational characteristics of the boat which may be significant if substantial over pitching were to be contemplated:
The boat speed at idle is now 3.5 knots which requires frequent shifts to neutral in slow speed maneuvers in marinas.
The prop walk in reverse has increased and definitely needs to be allowed for.

Another aspect of engine installation which needs to be considered if substantial over pitching is contemplated, is the strength of the drive train .
On a previous boat I changed from a fixed three blade prop to a controlled pitch prop after several thousand hours of problem free motoring and soon ran into problems with the gearbox clutch packs slipping. The extra power that the CPP was capable of adsorbing was more than the gearbox could transmit.
I suspect that some engine manufactures size their gearboxes to suit a typical propeller curve ie they can easily transmit the power that a typical propeller can adsorb but can only transmit full engine power at the seldom used wide open throttle revs.
From an engineering perspective a far more demanding criteria is to size the gearbox or prop shaft to transmit full engine power at the revs where the engine puts out maximum torque . This closely corresponds to the bulls eye in Matt’s fuel mapping diagram so any changes which we make to the engine/propeller system to move towards this ideal, will result in increased loads on the drive train.

Regards
Jim Clifford Wonambi 2

Alan S

John, note the modern generation of folding propellors, eg Varifold (as fitted to my yacht) and Flexofold have twisted blades and their performance is typically as good, if not better than a conventional fixed blade prop,. Also plenty of thrust going astern.

Dick Stevenson

Hi John,
A very nice article and well, thought out. You have brought together and balanced an impressive confluence of inter-dependent variables. A question and a comment.
What lead you to determine that you had glazing in the first year of your new engine?
You asked about others experience over-propping. We did so on our last engine/drive train in roughly the way you describe: conservatively, and were very pleased with the results which were in line with what you experienced. It was hard to parse out exact results as we were looking for more “punch” in adverse conditions and had gone from a 17 inch 3 blade Max Prop to a 19 inch 4 blade. We now have 2 years on a new engine and to date have stayed within the warranty limits of the engine. I just re-pitched one degree which should be about a 200 rpm decrease at WOT.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John
With respect to fitting a larger diameter prop: most recommendations for prop to hull clearance figures (such as the ones in Dave Gerr’s excellent prop book) are overly conservative. At least they were in my case. I believe they tend to give clearances based on the prop being below a flat hull bottom, such as found on many/most power boats. Sailboats generally have a significant “V” in the hull configuration above their prop which minimizes the exposed hull to the forces from the prop and its wash. It also helps to have a good prop guy backing you up such as Chris at Darglow in the UK. He was prepared to shave my Max Prop blades and re-balance (not really a big deal as I understand it) were there any problems.
My best, Dick Stevenson, s/v Alchemy