Cruising - 1 engine or 2

[robertmmcdermot]

Hi,
I'm looking for info regarding the efficiency and/or tradeoffs associated with, when motoring, whether to use one engine or two. That is, to achieve 5hp, I can run one engine running at high rpm, or two engines running at lower rpm.
Theoretically, a certain power consumption is required to propel the boat at a given speed, so it doesn't matter whether one engine is providing all of the power, or each of two engines is providing half the power.
Does this theory hold up in practice?
Thanks,
Rob

[Diesel Bill]

I'm leaning towards thinking 1 engine running at 3/4 load is more efficient than 2 engines running at 1/4 load each.

[Diesel Bill]

I probably opened a can of whoopass on myself but it's what I believe. And my boat has 2 engines.

[pumpcity]

Efficiency has a lot to do with Efficiency of upkeep, efficiency of speed, as well as efficiency of the prime mover.

If your boat is a planing boat, then generally you need a lot of horsepower, and dividing it up into two engines will provide you with maneuverability as well as a backup engine.

If, however, you are traveling only at hull speed, then a single engine will provide plenty of horsepower, and you will achieve efficiency of upkeep as well as efficiency of prime mover. Most engines originally installed in most boats are oversized for hull speed, so unless you are re-powering, a single engine is more efficient with regards to prime mover efficiency. RPM generally is superfluous.

I'm a single engine boater traveling at hull speed, and the engine provided with the boat when first built is oversized for the work it does, which means it isn't overworked but operates a just slightly less than best efficiency.

[Dsanduril]

Look in the engine manual for a fuel curve or fuel consumption table. This is ours for a Volvo D2-40:



The engine is most efficient (specific consumption) around 2600 rpm. We can motor at ~5 knots on one engine at 2400 rpm, two engines at 1800 rpm. If you look at the fuel usage at light load this is about the same usage (3.8 lph vs 2 x 1.9 lph) but at full load the one engine is way more fuel efficient (7.2 vs 2 x 5.7 lph). When motoring we're generally above minimum but a bit less than full load, so somewhere in the middle.

In the best case two engines uses about the same fuel but also results in twice as many engine hours and in lightly loaded diesels - not their favorite operating point. So, we run one most of the time, and in observation our fuel usage is about 80% of two engines for the same speed.

[thinwater]

It will also depend on whether the unused engine has a folding prop or lifts, such as an outboard. A fixed prop that is not turning is just drag.

[captstu]

On a crossing from Cape Town, we ran engine(s) to maintain 4kt minimum speed, making water and doing laundry with the power generated. Hard to say we used less fuel, our flow meter showed similar with very light winds at time.

The dominant factor, it turned out, is we ran the engine of the on watch crew. The sleeping crew got the quiet of being away from the engine.

Since we alternated in 3 hour rotation, we racked up about even engine hours.

Inshore, we always tan both engines for maneuverability. We always started the off engine when traffic was present,

[FSMike]

I can't remember the figures but on my boat using just one engine results in more fuel consumption for that engine at the same speed we cruise at using two engines. It's a good bit less than the total consumption using both engines. I imagine the relative narrowness of multis hulls and the rpms used by individual drivers affects this.
We have Campbell Sailor 3 blade fixed props, and the unused engine is left to freewheel.
And, as someone pointed out, it results in half the hours put on the engines.

[rom]

Originally Posted by thinwater

It will also depend on whether the unused engine has a folding prop or lifts, such as an outboard. A fixed prop that is not turning is just drag.

Yes. It is also critical to look at rudder feedback (angle). If the AP is struggling to maintain course with 20° rudder angle, then you probably better start the second engine.

[valhalla360]

At low speeds or particularly motorsailing, it likely will be more efficient. (motorsailing because you can choose the motor that favors the tack you are on)

At low speeds, you don't need a lot of power and the motors may be running well below their most efficient power output range. By using a single motor, the efficiency gains of running the motor closer to peak efficiency outweigh the drag and rudder input.

As you go faster, the demands are enough that both motors are close enough to peak efficiency that the losses due to drag or constant rudder input to counteract the turning force outweighs getting a single motor a little closer to peak efficiency.

[clownfishsydney]

Our 38' Lightwave has 30 hp Yanmars. At 2400 rpm using one engine we make 6 kts (no wind no current). Running both at same revs normally only gives us 6.5 kts. Of course our fuel consumption is double as is the engine maintenance.

The only time we run two is anchoring/mooring, bar crossings or when running into 20 kt or more winds. In that case, the speed actually goes from 3.5 kts (for example) to 5 kts.

I do not know a single catamaran owneer who runs both engines at a time as a norm.

[Pete7]

It could also depend on what ancillaries are attached to the engines. Power steering just off one engine might cause some issues for example. Do both engines charge presumably two battery banks? or is the house bank just charged off one engine?

Pete

[Bean Counter]

Generally one engine is more fuel efficient for say 5 - 6 knots. The boat tends to crab so there is some rudder drag. When motor sailing we run the windward engine, this counteracts the weather helm so that the rudders are usually straight ahead. In really light wind though with wind aft of 90 degrees we run the leeward engine so the exhaust stays clear of the boat.

[Cotemar]

Cruising with 1 engine on a Catamaran with folding props is the way to go.

[rabbi]

Originally Posted by Bean Counter

Generally one engine is more fuel efficient for say 5 - 6 knots.

+ less wear & tear on the engine
+ fewer engine hours
+ diesel engines like to run with some load, so one engine doing some work is better than two iddling along

Only reason to use both engines is for docking, anchoring, or when motoring dead into the wind.

Originally Posted by Bean Counter

The boat tends to crab so there is some rudder drag. When motor sailing we run the windward engine, this counteracts the weather helm so that the rudders are usually straight ahead. In really light wind though with wind aft of 90 degrees we run the leeward engine so the exhaust stays clear of the boat.

We do exactly the same.