Originally Posted by Alan Wheeler
No, you have to understand the different terms being used and sadly, not ALL the story is told in the specs with some. But youy specs shoudl tell you several different key points.
is a short term unloaded rating. It is the Maximum RPM
that the engine
will rev to before the Govoner cuts in a limits the engine
from going any further. This speed is not something you want the engine to be able to achieve normaly. You then have a "short term" maximum rating. This is where the Power peaks on tha Graph and is what most manufacturers will use as the HP rating. But this RPM is still not a continuous rating. The continuos rating is the RPM where the engine will happily run all day every day with out over heating
the engine. Which spec is being used, depends on the engine manufacturer.
So the easiest way of determining all this is a simply industry rule
of thumb. Reve the engine unloaded and see what the rev counter tells you. Place in gear
and do the same. The max differnce should be around 10% by rule
of thumb. If you sit within 10%, the bet is you are pretty close to spot on. It's not me that came up with it. It is a general inudstry recognised rule of thumb that brains bigger than mine have used for years.
I have heard and understand that theory. Here is the flaw.
In order to utilize max HP, one must be able to achieve the rated RPM for that HP. That takes torque.
If you look at the torque ratings on that engine, you will find that it drops off dramatically after 2500RPM (which is the ideal cruising RPM):
If the salt water
hits the fan and you need the HP (for short periods of time) you must be able to reach that HP by the use of torque. The dirtier the bottom of the boat or the harder the wind
is blowing or choppier the seas, the less ability a vessel will have to reach the RPM that will deliver needed HP when it is needed most.
I delivered a Catalina 36
to San Diego
via Panama) with that exact same engine. Before I left, I insisted that the owner re-pitch the prop so that I could attain max RPM. The vessel had just had a bottom job done and I could only get 3200RPM out of the engine in calm conditions and it smoked like a pig.
The prop was re-pitched and I was able to get (almost) 3700RPM out of it. The governor kicked out at 3750 (out of gear). There was no more smoke.
As it turned out, I had zero wind
from the Bahamas
and I was forced to motor for 900 miles. That motor was quiet and smooth at 2500RPM and it sipped fuel
. The owner told me that he got 3/4G per hr @ 6.5kts out of that engine. After re-pitching the prop, it burned just under 1/2G per hour @ 7kts.
I was extremely impressed with that engine.
The real problem is, the dirtier the bottom of the vessel is, the less apt it is to be able to reach the potential that the engine has. If it is slightly over-propped, this problem is worse.