Fuel consumption, one or two engines

CLII

CLII

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Hi.
I do have a 31ft Coronet from 1980 with two VP AQD40 engines.
We can do about 20kts but usually just enjoying cruising at 7-8 kts.
My question is simple; is rhe fuel consumption lower with just of the engines running at 2000 rpm than two at 1700 rpm?
Any other apects?

Thomas
 
I may be wrong but I will assume from my understanding of mechanics that , if we are talking about the same specification engines, then 2 engines can rarely use less fuel than one , even at lower rpm . From your question , think you are assuming that 2 props at 1700rpm can hold you at the same speed as one prop at 2000 rpm , which in real life is rarely the case. As far as I know, the main advantage of having 2 engines is definitely not economic as you will be spending twice the resources to service and maintain them but
1. You always have a backup if one fails
2. You can maneuver better in tight spaces
3. You can shut one off and alternate between the 2 if at very low trolling speeds.
4. Your boat will likely perform better in very rough and unfriendly seas.
 
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I vaguely remember a thread about this some time ago.

There were concerns over gearbox cooling on the inactive engine/gearbox as the unused prop could still turn due to motion through the water.
 
Apologies, just read your post again. The above was with regards to shaft drive. You are probably on legs so it may not be such an issue.
 
A few years ago, part of a cruise was along a river.

Running on just one engine at displacement speeds definitely used less fuel, but "switching" engines to keep the hours even meant that half the time I had no power steering or was not recharging the domestic batteries, so I gave up and reverted to two.
 
FlowerPower said:
A few years ago, part of a cruise was along a river.

Running on just one engine at displacement speeds definitely used less fuel, but "switching" engines to keep the hours even meant that half the time I had no power steering or was not recharging the domestic batteries, so I gave up and reverted to two.
Click to expand...

Flower power has hit the nail on the head.

At slow speed you are using so little fuel and the charging and power steering hasle ( and driving in a straight line ) don't make it worthwhile
 
CLII said:
My question is simple; is rhe fuel consumption lower with just of the engines running at 2000 rpm than two at 1700 rpm?
Click to expand...
Simple question, simple answer: Yes.
But, is the difference worth the hassle? No.

The one and only way to have the most efficient fuel burn at displacement speed without any other drawbacks is also simple, in fact: using a single engine boat! :)
 
Thanks for the answers.
No i will not get another boat with just one engine. No, there's no problem with charging batteries or such, everything is setup so both engines can serve needed things. There's no hassle.
The situation is only when the weather is calm and the extra power is not needed. Cruiising at low speed in great weather is more relaxing with only one engine producing noice and rumble even if at higher revs.
And yes, our boat do 7 kts at 2000 prm with one motor and at just under 1700 with two.
But since we don't have any gauges for fuel consumption other than tank meters it wouldbe interesting to hear from thosse who have, about the difference.

Thomas
 
7 knots in a 31ft boat is probably not the most economical economical slow speed.
Would 5 to 6 knots consume less fuel overall ?
 
Single engine running is a PITA.You constantly have to correct for the thrust all being on one side and the entire thing responds so much more ponderously when changing direction....and when approaching locks etc you have to remember to start the dead engine...........
 
I cant stop thinking "it all depends".
Depends on how the unused prop behaves, depends on the specific engine rpm,torque -> efficiency graph. Which leads to a bucket load of research and calculations. Whereas you might try to find out for your self. Either fit fuel consumption metering or replace the fuel tank with a 5 litre bottle and see how far you get.
 
CLII said:
There's no hassle.
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Fwiw, when I said that running on one engine on a twin screw boat is not worth the hassle, I didn't have just battery recharging, power steering or whatever in mind.
As others already said, twin engine boats are a proper PITA to helm, when running on just one engine.
At least, that was the case with all the ones I came across, or heard of.
If your Coronet remains steady (or if you don't mind the difference, anyway), well, good for you! :encouragement:
 
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