Calculate Engine Load

I'd be very surprised if your system doesn't provide engine load but I'm not familiar with Volvo's systems. However, meanwhile, while the true load is a bit more complex calculation than this, the simple approximation is the fuel consumption divided by the fuel consumption at WOT. So, if your maximum is 150 gph and at your current level you're using 90 gph, then you're at 60% load.

There are two "sweet spots" that are often useful to compare. One is the comfort spot which you've identified. The other is found in performance charts. If you don't have one for your boat, then build one. They're like the charts you see on testing and review sites.

Here is one off one of our boats:


We normally cruise at 20-23 knots which is 52-65% and gives us 0.27-0.24 nmpg. 24 knots is around 2125 RPM and about 76% load and still well within a reasonable load but beyond 2000 RPM, the load and fuel usage rise much faster than the speed. That extra knot just isn't worth it. From 2000 RPM down load changes are fairly much a normal curve and any of those speeds make sense. We can run slow at 15 knots and 28% load and get 0.36 nmpg. Many run 12 knots at 15% load and get 0.55 nmpg. Those speeds are ok as long as periodically you raise to 23 knots or so. I am a believer in varying speeds. We might run two hours at 23 knots and then 30 minutes at 20 knots, just to vary things.

Now on another boat we get something like this:


2000 RPM, 37 knots is 78% load. 35 knots is around 72% load. So we might run at 35 knots or so if we're making a quick run for lunch in Miami or across to Bimini. However, the performance chart on this boat shows very little to be gained by running slower. 29 knots is still 60% load and only 0.67 nmpg vs. 0.66 nmpg at 37 knots.

An interesting comparison on the two boats. Comparing 1500 RPM to 2000 RPM
Boat number 1, you're comparing 28% load to 65% load and decreasing speed by 35% while increasing nmpg by 129%. Boat number 2, you're comparing 44% load to 78% and decreasing speed by 41% while increasing nmpg by only 5%. This is consistent with the design and intent of the two boats. Number 1 is designed for long range cruising at various speeds. While we run often at 20-23 knots, most owners run it most at 12-15 knots. Number 2 is designed for short high speed runs. Most would run it at 30-37 knots and there's no real benefit to slowing down.

You need to get very comfortable with your boat and understand it's load at various speeds, it's nmpg, it's range, how it feels, how it sounds, and how it handles various sea conditions.