Volvo engines - "range" of max rpm

[cmedsailor]

The volvo penta engines usually show in their specs a maximum range of rpm (eg 3200-3600 for the MD2030, 2800-3200 for the D1-30) whereas other makes such as Yanmar show a maximum output rpm and a continuous output which usually are very closed to each other (eg for 3YM30 3600 and 3500).
What does that “range” of maximum mean for Volvo engines? Does it mean that under any load (heavy, light, etc) the engine must be able to reach ANY rpm within that range at full throttle? But I guess that mean you could play with propeller sizes too since a change in diameter/pitch is roughly a drop/increase of 200-300 rpm, thus within the 400 range allowed. Is that correct?
And even more strangely, why not all manufactures specify a range rather than a maximum limit (eg the 3600 for the yanmar)?

 

[theoldsalt]

If you look at the power/torque curves for the engines you will see that the curve is almost horizontal over an rpm range near max power. To obtain the best performance from the engine it should be loaded to operate within this range at full throttle.

If the engine is "over-propped" or "under-propped" the engine will not be developing full power at full throttle.

For example :-

 

[cmedsailor]

If you look at the power/torque curves for the engines you will see that the curve is almost horizontal over an rpm range near max power. To obtain the best performance from the engine it should be loaded to operate within this range at full throttle.

If the engine is "over-propped" or "under-propped" the engine will not be developing full power at full throttle.

For example :-

Strange curve. Have a look at this http://www.marinepartsexpress.com/prodbull/md2030.pdf
I do understand the curves but still cannot understand the maximum rpm (3200-3600) instead of just saying max 3600.
And since you mentioned over/under propped what is the correct size prop for this engine? The one that loads the engine to 3200, 3400 or 3600 rpm? From my understanding anything within that range is OK, and that means there is more than one correct size of prop.

 

[theoldsalt]

cmedsailor,

My understanding of that Volvo power curve is that the engine has been derated. In other words it is capable of producing more power but is limited.

This is often done to extend the life of an engine by reducing the operating stresses well below the full design values.

If more revs were produced then more power would be available but would increase stresses and shorten the engine life..

 

[Bobobolinsky]

If you have a displacement hull, you are better off looking at maximum torque, than maximum revs

 

[richardbrennan]

I cannot give you the technical details, but having just had a new Volvo D1-30 fitted to my boat, the fitter needed to see on the sea trial that the maximum rpm was within the specified range, in this case 2800 to 3200 rpm, to validate the guarantee.