When I am cruising and looking for a fuel-efficient speed, I do not use engine RPM to decide how fast to run.
I look at just two things:
- Speed over ground (SOG)
- Fuel flow (L/h)
The calculation is simple:
SOG (knots) ÷ fuel flow (L/h) = nautical miles per liter (NM/L)
What I want to know is not, “What RPM am I running?”
What matters to me is:
How far am I actually moving over the ground for the fuel I am burning right now?
That is how I choose my cruising speed.
Why I Use Speed Over Ground
The speed I use is GPS-based speed over ground.
SOG shows the actual speed at which the boat is moving over the Earth after the effects of current, wind, waves, and other conditions have played out.
So I do not correct SOG again for current.
With a following current, SOG rises. With an opposing current, it falls.
That is not an error. It is what is actually happening during that passage.
I am not trying to measure the isolated efficiency of the hull or engines under controlled test conditions.
What I want to know is:
Under the conditions I am actually cruising in, how effectively is the fuel I burn turning into real movement over the ground?
That is what I watch.
My Boat Has Two Fuel-Efficient Speed Ranges
My boat is a Jeanneau Merry Fisher 895 Sport.
I describe the boat in more detail in a separate article, My Boat: Jeanneau Merry Fisher 895 Sport .
After watching SOG and fuel flow repeatedly while underway, I began to notice that my boat has two relatively fuel-efficient speed ranges.
One is at low speed.
At very low speeds, the fuel-economy figure I monitor can be around 1.0 NM/L.
The other is around 23 knots.
At that speed, I often see about 0.67 NM/L.
In between, around 12 to 20 knots, the figure drops to roughly 0.5–0.6 NM/L.
My boat has twin 250 hp outboards. The fuel-flow figure I use here is the reading from one engine, because that is the figure I normally monitor while cruising.
So the NM/L figures in this article are based on the fuel flow of one engine, not the combined fuel consumption of the entire boat.
This is not a controlled performance test of the boat.
It is simply a record of what I look at and how I use those numbers when I am actually out on the water.
Probably the Hump
At around 12 to 20 knots, I can open the throttles and hear the engines working harder, yet the boat does not seem to gain speed in proportion to the extra power.
From the helm, it feels something like this:
“The engines are working hard, but the boat is not really going much faster.”
My guess is that this is the hump—the transition region before the boat gets properly onto plane.
Once I reach around 23 knots, the boat is clearly planing, and the SOG-to-fuel-flow figure improves again.
My boat is fitted with Seakeeper Ride, an underway attitude-control system.
Because of that, the bow does not rise dramatically around the hump, so the change in running attitude is not especially obvious.
But when I watch fuel economy, I can still see that something important is changing in the way the boat is moving through the water.
When I Am Not in a Hurry, I Cruise Slowly
When I am not in a hurry, I often cruise at low speed.
Fuel economy is good, and the boat tends to move more gently.
There is less pounding, and the hull moves with the surface of the sea to some extent rather than constantly striking it at speed.
Of course, it takes longer to reach the destination.
But this is a pleasure boat. There is nothing wrong with enjoying the time spent moving slowly across the water.
When I have time, I like this way of cruising.
When I Want to Get There Faster, I Use Around 23 Knots
When I want to reach my destination more quickly, I cruise at around 23 knots.
At this speed, the boat is clearly on plane.
On my boat, fuel economy is still relatively good, while travel time is much shorter than at low speed.
There is another factor that matters to me: how much attention I still have available at the helm.
At around 23 knots, I still have enough time to:
- Watch the sea surface
- Look for floating debris
- Track the movement of other vessels
- Check the route
- Make whatever control inputs are needed
When sea conditions are good and I am feeling fully alert, I use this speed range quite often.
Why I Rarely Cruise Above 30 Knots
Once I go beyond 30 knots, the way the scenery moves past me changes noticeably.
To me, it feels a little like switching from normal video to fast-forward.
As speed increases, the time available to cover the same distance decreases.
That means there is less time to notice a change in the sea surface, read the movement of another vessel, make a decision, and then act on it.
And in a boat of this size, even a relatively small wave can produce a hard impact at higher speed.
The boat itself is capable of more than 30 knots.
But I rarely use that range for normal cruising unless:
- Sea conditions are good
- I am in good physical condition
- I have a real reason to get somewhere quickly
Fuel economy also gets worse.
For me, around 23 knots gives a comfortable balance between speed, fuel economy, the boat’s behavior, and my own ability to observe, decide, and operate the boat.
Why I Occasionally Use Full Throttle
I hardly look at RPM when choosing my cruising speed.
But checking the condition of the boat and engines is different.
Once the engines are fully warmed up, and when:
- Sea conditions are good
- There are no nearby vessels or obstacles
- I have enough open water
I sometimes open the throttles fully for a short period.
The purpose is not simply to go fast.
I use it to check things such as:
- Whether the engines reach their usual maximum RPM
- Whether there is any unusual difference between the two engines
- Whether maximum speed has changed significantly
If maximum RPM or top speed is clearly lower than usual, something may have changed—bottom fouling, propeller condition, load, engine condition, or something else.
In Tokyo Bay, there are not many places where I can safely run at full throttle for an extended period.
In my case, even a full-throttle run is usually no longer than about a minute.
I Don’t Treat Cruising Speed as One Fixed Number
When I choose a cruising speed, I mainly think about three things:
- How much motion and pounding I am willing to accept
- Fuel economy
- Travel time
So my basic choice is simple:
- If I am not in a hurry, I cruise slowly
- If I want to get there faster, I use around 23 knots
The intermediate range, roughly 12 to 20 knots, is not something I deliberately use very often because, on my boat, it burns quite a lot of fuel for the speed gained.
Of course, the numbers are never exactly the same.
Fuel load, payload, air temperature, water temperature, waves, wind, current, and the condition of the bottom can all affect SOG and fuel flow.
That is why I do not think of economical cruising as a fixed statement such as:
“This boat’s best cruising speed is at X RPM.”
Instead, while underway, I keep checking SOG and fuel flow and mentally calculate:
SOG ÷ fuel flow
Then I adjust the throttles to find a better number under the conditions I am actually in.
The most fuel-efficient cruising speed of a pleasure boat is not necessarily a single fixed speed.
At least on my boat, there are two distinctly useful ranges: low-speed cruising and planing cruise at around 23 knots.
Which one I choose depends on what matters more at that particular moment.
That is the way I have learned to choose cruising speed after spending time actually using this boat.
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