Re: Props and rpm



It is totally "normal" when picking a prop for a diesel engine to do exactly as Felxofold suggests.  You pick a propeller that allows the engine to turn its full rated RPM at full throttle. (which is where the engine power rating is specified). The way a propeller power curve is shaped, there is no advantage to operating the engine at its "maxiumum torque."

Most engine makers will insist on speed matching as part of the proper installation.  In fact, Volvo does on their other engines.  If you installed most marine diesel engines propped to run at 1500RPM below rating at full throttle, they would struggle, have significant maintenance issues, and have a shorter than expected lifespan.

With THIS engine in particular, Volvo did not insist. They even suggested in the engine manual that there are advantages to "over-propping" the engine and having it peak at lower RPM than its full rating. In fact you have to look really hard to even find Volvo mentioning the 4500 RPM number! I am not completely sure WHY Volvo felt this appropriate for this engine, but Amel took advantage of this flexibility in full throttle RPM, and propped it to full-throttle at about 3000RPM.  

I think the history of this engine in Amels supports that decision.  We do suffer from some issues (minor carbon build up in the turbo, and constant black soot on the hull) that come from it, but but they seem to have a long and relatively trouble free service life, as a rule.  Mine just turned 8000 hours.  If I figure 12,000 hours as a reasonable lifespan, that gives me another... 15 years :)

This is one of those places where someone coming from a long history with other boats looks at the Volvo installation in an Amel and decides right away that it is wrong...  but it works exactly like it is supposed to.

My wild speculation with what happened on this engine, is that that Perkins specified the max RPM with an eye to the Automotive market.  It was a popular engine in Land Rovers, among others. A redline RPM means something very different in an automobile, where running at maximum rated RPM is an unusual and short term event.  (At least the way I drive.) In addition, automobile engines that last 5000 hours are the exception, not the rule.

In a marine installation, an engine operates very high up on its power curve--all the time.  Volvo probably felt that operating the engine at 4500RPM continuously wasn't a good idea for service and longevity, but for some reason didn't want to change the injection pump to lower the peak RPM and HP rating of the engine.  I suspect because they wanted to keep the 74HP rating as a sales tool--even if it was unrealistic for a typical marine installation.

Long and short of this is: Have flexofold specify a prop that will load your engine enough that it tops out at something between 2900 and 3300RPM.  You'll motor at 8 knots (full throttle), and be happy.

Bill Kinney
SM160, Harmonie
Rock Sound Harbour, Eleuthera, Bahamas

---In amelyachtowners@..., <sailw32@...> wrote :

I had flexofold suggest a prop and pitch that would allow the engine reach its max rpm , in my case 4500 . I now just read on the Max prop website , that their prop would also allow the engine to reach full rpm ,"in flat water." I wrote back to Flexofold that this did not sound correct , as my fixed prop does not allow me to get beyond 3000, and I have not read about anyone else able to reach rpms that high. I would think  reaching max torque would be the goal . Should a prop/ pitch allow an engine to obtain max rpms ? I don't know what to think or what is correct on this subject, and I may owe Flexofold a retraction.



SM #123

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