Driving on one engine, good or bad?

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Mr Wolf

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A question to the panel:

My Friend has a Bavaria 32 Sport with DHP outdrives; when cruising on the slow parts of the river he likes to run on one engine (to save fuel and engine hours) is he doing any longterm damage to the un-driven outdrive?
All opinions greatly received.
 
Captain Ludd said:
I'm not really sure but as long as he swaps around I can't see how.
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Thanks for your comment. Yes he does alternate the engines, my concern is if the undriven sterndrive is having the propeller turned without power are forces at work within the mechanical components for which they are not designed?

As an example on my boat (shaft drive Sealine T50) if the boat is to be towed the shafts must be locked into gear to prevent damage to gearbox's.
 
Mr Wolf said:
Thanks for your comment. Yes he does alternate the engines, my concern is if the undriven sterndrive is having the propeller turned without power are forces at work within the mechanical components for which they are not designed?

As an example on my boat (shaft drive Sealine T50) if the boat is to be towed the shafts must be locked into gear to prevent damage to gearbox's.
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I may be wrong here, but as far as I know, where damage can be done on shaft-driven boats it the gearbox oil overheating as the oil cooler is not in use.

This is not an issue with out-drive boats as the gearbox is immersed in sea-water hence keeping cool.

I would still lock the out-drive in gear though (easy on a traditional throttle control, not sure about the new electronic throttles)
 
Done this for 5 years on my Volvo Duoprops for exactly the same reasons while going upriver as its 6 knots all the way,
Never done any harm and in neutral, did try it in gear once but noticed extra drag and it would not come out of gear unless the boat was stopped.
 
Ive done 1000s miles on one outdrive, used to raise the other one for even less drag, it saves engine hours for sure but no significant fuel saving as one engine is working harder than two.

There is a serious issue to be aware of, if you forget to lower the leg before you need to manoeuvre you run the risk of damaging the top of the leg against the transom shield.

Full lock with leg raised can cause water ingress as the seals are misplaced being crushed against the transom sheild.

Summary
after running for hundreds of hours on one engine and now knowing how much it saved/cost I wouldnt do it as routine.
 
Don't see how he could damage a drive by allowing it to freewheel. It is beneficial for the engine though, it puts more load on the one and could alleviate bore glazing issues.

Don't expect to save much fuel though, by and large it takes the same amount of energy to push a hull through the water whether you are on one engine or two, disregarding prop drag pumping losses and friction on the second engine which would hardly be noticeable at river speeds.
 
Thanks for all your replies guys!

The concenus seems to be: no damage likely to occur but don't expect much in the way of fuel saving!
 
Mr Wolf said:
Thanks for all your replies guys!

The concenus seems to be: no damage likely to occur but don't expect much in the way of fuel saving!
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Thats a fair summary however you need to specify type of 'no damage done'

The damage to the top of my outdrive cost about £300 (DIY) and I was lucky no damage to gears were done before I found the water ingress and puncture hole through the alloy.

Other possible damage should include collision as you couldn't get out the way quick enough on one engine.

certainly in shallow water its a good idea to raise one leg out the way, rest of the time best to run on both.
 
DAKA said:
Thats a fair summary however you need to specify type of 'no damage done'

The damage to the top of my outdrive cost about £300 (DIY) and I was lucky no damage to gears were done before I found the water ingress and puncture hole through the alloy.

Other possible damage should include collision as you couldn't get out the way quick enough on one engine.

certainly in shallow water its a good idea to raise one leg out the way, rest of the time best to run on both.
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Point taken ref your damage but the 32 sport legs are linked so you cannot lift one shaft and not the other so at least he won't be encountering that one!

I totally agree ref the ability to move quickly on one leg in an emergency - I refuse to take a turn at the wheel if he is running on one as I find it quite slow to respond and basically horrible to drive when running on the engine without the power steering pump!
 
Just dont forget when running with some engine speed on one engine the boat speed is slower making propeller slip and trust/torque higher.

That means the same as running a engine against a to heavy propeller.
So be careful speeding up the running engine.
 
It's always been an interesting debate, but here's my twopennyworth.

1 As has been said, running at any given speed requires the same hp, whether from one or two engines, and therefore the same fuel burn.
2 When running on one engine, the boat will tend to crab due to the assymetric thrust. This will induce additional drag and therefore increase fuel burn.
3 A stopped prop is likely to cause more drag than a free wheeling prop, increasing fuel burn.
4 You need to check with your gearbox manufacturer if there are limits on a free wheeling prop.
5 To counter the crabbing, the rudder will be at an angle, thereby placing an abnormal constant load on the rudder(s). You would be wise to check if the rudder stock(s) can take this. This will also make the autopilot work harder.

On the other side of the coin, the saving is in engine servcing if calculated by time used.

I tried single engine running on Play d'eau for a season, and my fuel burn increased from 1.67nm to 1.48nm per imperial gallon.

Have fun

Piers
 
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