Run 1 or 2 engines?

[rms95835]

I captain a Saba 50 cat. My personal boat is a 42 Jeanneau, so no experience relative to this question. When motoring, do you run just one, or both engines? It would seem to me, that there is a certain amount of energy to move the boat through the water at a specified speed. That the energy is divided between one engine or two, shouldn’t make a difference relative to fuel consumption, but I have no experience to verify this. In my mind, there may actually be an advantage to running both, as there is no steering correction to make up for the thrust of a single engine on one side of the boat. Thanks in advance for your input. Heading shortly from Puerto Rico to Saint Martin , and anticipate it being a mostly of wind over experience, show want to make sure I do show as efficient as possible

[PippaB]

With our similar sized cat we find that running one is most fuel efficient. It does come at the cost of about 5° of standing rudder. However, we mostly motor when there is no wind, so we're trying to get the most out of the fuel we have. When motoring against the wind, especially any significant breeze we find two works better. If the breeze is off to one side a bit you can select one engine to run that is counter-balanced by that breeze, but then keeping the fuel usage equal from port to starboard is a bit more difficult if it is a long run on the same tack.

A rough guide for us, one engine at very low cruise (most fuel efficient) gives us 4 knots. Adding the second engine at the same RPM gives us 5 knots at double the fuel usage. If we want to get into the 6-7 knot range it isn't as clear, to get there on one engine requires a much higher RPM than two engines getting us to that speed. Once we get in that speed range the fuel benefit of a single engines fades away quite quickly.

[Cheechako]

In my cat I ran the lee engine at high idle often. This was motorsailing. Doing this in light air, when on a close reach or to windward, the cat pointed up a good 15 degrees or more, sails were full, and got going fast , quiet and smooth.
So basically; charging batteries, making water (engine driven), doing 7-8 knots and headed toward the destination, not trying to tack all day.

[ItDepends]

There are a couple misconceptions about fuel use in this thread.

First error: If you run the boat at the same exact speed through the water with one engine you will always use less fuel than if you were running two. There is a certain amount of fuel that is needed just to turn the engine. There is a BIT more drag from the rudder angle needed to hold a straight course, but not much.

Second error: If you run one engine at a fixed RPM, and then add a second engine at the same RPM, the boat will go faster, and you will use more fuel, but you will use LESS than twice as much, because each engine works a bit less to maintain the same RPM.

Remember, the “throttle” control on a diesel engine does NOT control the fuel usage directly. Rather it sets the SPEED of the engine, and the fuel use adjusts up or down to hold the same RPM as the load changes.

[rms95835]

Good information, and makes sense. Thank you all for your responses.

[boatman61]

I run both engines leaving the harbour then once clear switch one off for 6hrs and run at the revs needed to maintain
5-6kts sog.. I then start the resting engine, bring her up to revs then turn off the first for 6hrs.. repeat.
Saves on fuel and engine hours.

[StuM]

It depends

A major advantage is only 50% of the engine hours meaning longer intervals between services/oil changes etc.

There's a cut off point where the extra fuel burn at high revs on one engine uses more fuel than lower revs on both engines to get the same HP. That depends very much on desired speed and size of engines.

[PippaB]

Originally Posted by ItDepends

There are a couple misconceptions about fuel use in this thread....

No misconceptions on this end, from here it looks like a lack of reading comprehension.

[chrisr]

if motoring we always prefer to run one engine with a good load - say 2500rpm, rather than 2 engines at light load - say 1500rpm

believe this is much better for the engine/s

cheers,

[Lancerbye]

I have twin engines on a 45 ft mono motorsailer. I only run both engines when maneuvering in a marina. I have also found that the difference between running 1 to 2 engines at 1400 rpm is from 6.5 kts to 7.5 kts. I also alternate between engines. Big savings on fuel consumption.

[Baby tug]

Originally Posted by chrisr

if motoring we always prefer to run one engine with a good load - say 2500rpm, rather than 2 engines at light load - say 1500rpm

believe this is much better for the engine/s

cheers,

Hey sorry it has been debated and debated and debated , a pet peeve of mine is running an engine with a light load is bad for the engine that is a myth , I think that got started with some of the old school two-stroke Detroit engines , but other than that running engine a lower RPM Is always better , ask a commercial engine rebuilder and they will tell you if they know what you’re talking about , it isn’t the hours it isn’t the miles it’s how much fuel you burned is the best indicator of Life left , If given a choice I would choose an engine that will give me needed horsepower at 1200 to 1600 RPMs , that engine will last much longer than an engine rated with the horsepower needed at 2500 or above

[Ifitsworthdoing]

Following - is this any different for cats with electric propulsion? Asking as we have just ordered an all-electric non-sailing powercat (which can only ever travel at hull speed or below) ...

[boatman61]

Depends.. have you two separate battery banks and the ability to recharge one set while the other is being used.

[Chotu]

Originally Posted by Baby tug

Hey sorry it has been debated and debated and debated , a pet peeve of mine is running an engine with a light load is bad for the engine that is a myth , I think that got started with some of the old school two-stroke Detroit engines , but other than that running engine a lower RPM Is always better , ask a commercial engine rebuilder and they will tell you if they know what you’re talking about , it isn’t the hours it isn’t the miles it’s how much fuel you burned is the best indicator of Life left , If given a choice I would choose an engine that will give me needed horsepower at 1200 to 1600 RPMs , that engine will last much longer than an engine rated with the horsepower needed at 2500 or above

This is a fact.

It’s also why an 1800 RPM generator lasts much longer than a 3600 one, to put it into something more tangible to boaters.

[sailjumanji]

We have a Seawind 1160 Lite (38 ft cat) with twin Yamaha 25 hp high-thrust outboards. Have good records on hours and fuel usage, both with one and two motors running:

1. Both motors, 3500 RPM each, fuel burn 1.7 gal/hr. To travel 100 nm would require 15.4 hours and use 26.1 gal

2. One motor at 3500 RPM, fuel burn 1.0 gal/hr. Travel 100 nm in 18.2 hr and use 18.2 gal.

Generally, it's not a big issue for us, and we just run both motors at the same time. But for long distances, we can't carry enough fuel to run both motors. For instance, in June we were crossing the Gulf from Clearwater, Florida to Matagorda, Texas. About 750 nm. And winds were very light out of the west - our direct course. We had to make a fuel stop in Grand Isle, Louisiana, about 400 nm from Clearwater. Running one engine, we'd burn 73 gal. Two engines, 104 gal. We have a 70 gal fuel tank and carry another 40 gal in jugs - for 110 gal total. We opted to take the slower route and motor with one engine at a time, alternative every 12 hours.

Thankfully, the boat sails really well, and that's what we do primarily. Sometimes you have to take the ICW because of bad offshore conditions (and a wife that doesn't care for overnight passages), and that means motoring. And occasionally you get stuck crossing the Gulf to Texas in a light west wind that lasts five days, when predominate is southeast! It could be - and has been - worse!