I've been thinking a lot about this lately.
I would respectfully disagree with Cotemar that for most people the best cruising RPM is the one at which you burn the least fuel. Since the energy required to drive a sailboat through the water
goes up geometrically with speed, the least-fuel RPM will be, for most boats, something not far above idle. In my opinion a better definition of cruising RPM might be that speed at which you achieve the optimum balance between speed and fuel consumption
in most cases according to your own particular values.
My Yanmar 4JH3HTE is redlined at 3800 with continuous power at 3700. It is sometimes said that cruising RPM's should be 75% (or 80%, depending on who you talk to) of redline. That would be 2850 RPM. My first year with this boat, I usually motored at 2500 to 2600 RPM
Unless I was in a hurry to make a tidal gate or something like that (in which case 3,000 RPM or even 3,200 RPM). Or another case for a higher cruising speed is if you are bucking an adverse tide -- the slower you go through the water
, the more miles you lose to the current
, so a false economy appears at some point of slowness.
So at 2500 RPM I get about 8 knots in flattish water. According to Yanmar, I should be consuming about 7 liters/hour at that speed.
At 3000 RPM, I get about 9 knots. But the fuel consumption is now 12 l/h, almost double. So obviously, the fuel consuption measured in liters per mile goes up in a very steep curve -- only 12% more speed for almost double the consumption.
So what about if, on the contrary, I slow her down? That's what I've been doing this year. At 2200 RPM I still get about 7 knots, or maybe even a little more. Fuel consumption is about 5.5l/hr, about 20% less fuel burned per hour for about 12% less speed.
At 2200 RPM I'm already getting into diminishing returns -- I'm burning a little less fuel per sea mile but nothing like the huge economy from throttling back from 3000 to 2500.
Another reason to throttle back is if you have a fair tide. This is the opposite case of bucking a foul tide. When you have a favorable current, the slower you go through the water, the proportionately more work the tide does, instead of your engine. So this exaggerates the fuel economy gain from lower RPMs.
Of course the biggest fuel economy gain comes from putting up the sails
, let's not forget
Concerning the curves in your Yanmar manual: Be sure to correctly understand what is "specific fuel consumption". Specific fuel consumption is the efficiency of the engine at converting grams of fuel into horsepower/hours of work. This is not all that significant to us, especially if you notice that the scale is very shallow -- the SFC curve for my engine goes from 152 to 175 g/hp*h -- not a very big range.
On the contrary, the number of horsepower needed to drive our boats varies over a huge range depending on speed -- from 20 horsepower at 2300 RPM to 95 horsepower at 3800 RPM according to Yanmar (I'm assuming that the engine puts out its maximum power for a given RPM at that RPM, but that's probably a reasonable assumption considering my Bruton self-pitching propellor). So it's optimizing the quantity of horsepower used which is really important, not optimizing the efficiency with which each horsepower is produced.
You should therefore get fuel curves which show liters/hour -- absolute fuel consumption, not specific fuel consumption. Again, these assume maximum power at each particular RPM. For a fixed pitch
propellor, you may be using somewhat less than maximum power and so will consume somewhat less fuel at lower RPMs, than the fuel curves suggest.