Running on one engine?

[asteven221]

Normally, sea state permitting we cruise at 7.5knots, which equates to 1000rpm on our Volvo TAMD63P's. However, I do give them some higher revs regularly to give them a regular work out to try and avoid problems associated with running at low revs.

Last weekend I switched off one engine and was surprised that the boat only lost 1kn, so I was thinking of doing this more often. If I added 200rpm we were back up at 7.5kn. According to the ZF manual it's okay to let the prop from the unused engine run free in neutral. I was wondering, is this something that's really only ok for a shortish period, or is it safe to run on just one engine over a long period of time e.g. several hours, perhaps (probably) alternating between engines?

Does anyone else have experience of this?

 

[omega2]

Cannot speak for ZF gearboxes but our borg warner 1.1's put a drag on the boat if one is stopped, had problems with overheating on port engine, shut it down and kept going on the starboard at 1800 we were doing 6 knots, but the port shaft was going round creating drag, we then started the port engine but did not engage gear and our speed went up to 8 knots.

 

[scottie]

I did hear of a situation where prolonged running on one engine resulted in water being pushed up through the system into the exhaust and backfilling the engine causing hydraulicing.

 

[Solitaire]

Normally, sea state permitting we cruise at 7.5knots, which equates to 1000rpm on our Volvo TAMD63P's. However, I do give them some higher revs regularly to give them a regular work out to try and avoid problems associated with running at low revs.

Last weekend I switched off one engine and was surprised that the boat only lost 1kn, so I was thinking of doing this more often. If I added 200rpm we were back up at 7.5kn. According to the ZF manual it's okay to let the prop from the unused engine run free in neutral. I was wondering, is this something that's really only ok for a shortish period, or is it safe to run on just one engine over a long period of time e.g. several hours, perhaps (probably) alternating between engines?

Does anyone else have experience of this?

What boat are we taking here? Seems to me that you are not going faster enough. Where is your cruising ground? If on the broads or river then just alternate each engine. But then I always question why people have such big engines on rivers/broads. There are better options.

 

[capsco]

What boat are we taking here? Seems to me that you are not going faster enough. Where is your cruising ground? If on the broads or river then just alternate each engine. But then I always question why people have such big engines on rivers/broads. There are better options.

Seems to me that he is going at the speed that suits him, so why do you think that he is not going "faster enough", maybe he has noticed the price of fuel has increased since he purchased his boat but as he likes it, he does not want to change it for something that you "think" would be more suitable.

 

[Piers]

Running on one engine has a number of issues. I tried it for one season and my overall fuel consumption increased. Issues include,

1 You need a given amount of hp to drive the boat through the water, whether from one or more engines.

2 Running on one engine means you have a free wheeling prop which may damage the gearbox - contact the gearbox manufacturer to learn of any operational limitations.

3 A free wheeling prop creates less drag than a stationary prop.

4 To combat the assymetric thrust you will need to apply opposite rudder to correct the drift. This will create additional drag requiring more hps for the same speed.

5 The opposite rudder will create additional strain on the rudder stock. Check with the manufacturer that it can take the strain.

6 Whilst using radar / AIS etc, remember that your heading will not be accurate and therefore any MARPA will be inaccurate.

So, additional drag from one prop plus an offset rudder = more hps needed to drive the boat at the same speed. Overall, my fuel consumption increased. The only benefit was a decrease in engine hours.

Piers

 

[asteven221]

Running on one engine has a number of issues. I tried it for one season and my overall fuel consumption increased. Issues include,

1 You need a given amount of hp to drive the boat through the water, whether from one or more engines.

2 Running on one engine means you have a free wheeling prop which may damage the gearbox - contact the gearbox manufacturer to learn of any operational limitations.

3 A free wheeling prop creates less drag than a stationary prop.

4 To combat the assymetric thrust you will need to apply opposite rudder to correct the drift. This will create additional drag requiring more hps for the same speed.

5 The opposite rudder will create additional strain on the rudder stock. Check with the manufacturer that it can take the strain.

6 Whilst using radar / AIS etc, remember that your heading will not be accurate and therefore any MARPA will be inaccurate.

So, additional drag from one prop plus an offset rudder = more hps needed to drive the boat at the same speed. Overall, my fuel consumption increased. The only benefit was a decrease in engine hours.

Piers

Thanks for responses everyone. Interesting.

BTW. The boat is a Sealine F44 and we cruise around the Clyde. We are quite a happy at 7.5knots which I reckon is the maximum speed before the bow starts to lift. For us, sadly the days are gone when we would routinely travel all over the place at 20+ knots.
£250+ for a 50 mile round trip isn't my idea of financial fun, especially when when we do that kind of mileage nearly every weekend in the year.

I understand the points you put forward which make sense, but I am surprised if the rudder stock couldn't cope with stress due to the slight drag at 7.5knots.

Regarding MARPA and the radar. You mentioned that it's possible that the radar and MARPA might be rendered inaccurate because the rudder is slightly offset. What would cause the potential errors, as the boats heading would remain the same and the radar orientation wouldn't change? Unless of course the system is taking the rudder position into acount, which is possible as all the instruments are integrated and the autopilot is aware of the rudder postion. I would have thought that the compass heading would take precedence, although I may well be wrong in that assumption.

I note that you saw an increase in fuel consumption when using just one engine, due to drag etc, which is makes sense. Nevertheless I found it interesting that our speed only dropped 1 knot from 7.5 to 6.5 when I stopped an engine. To me that would suggest that there could be a decent fuel saving just by sacrificing one knot.

 

[scottie]

If you run your hot via a calorifier then you may find yourself favoring that engine this may also apply to winch battery ,thrusters ,and power steering

 

[Piers]

I understand the points you put forward which make sense, but I am surprised if the rudder stock couldn't cope with stress due to the slight drag at 7.5knots....

Regarding MARPA and the radar. You mentioned that it's possible that the radar and MARPA might be rendered inaccurate because the rudder is slightly offset. What would cause the potential errors....

I note that you saw an increase in fuel consumption when using just one engine, due to drag etc, which is makes sense. Nevertheless I found it interesting that our speed only dropped 1 knot from 7.5 to 6.5 when I stopped an engine. To me that would suggest that there could be a decent fuel saving just by sacrificing one knot.

Hi there. Taking the points in turn,

Given the boat will be crabbing due to the assymetric thrust, the rudder will need to be offset to keep the boat heading in the direction you want. This means a constant force on the stock. It isn't the drag that causes the strain, it's the constant offset.

Since the boat is crabbing and the track throught the water is therefore different from the boat's heading, the radar and AIS will be in error by the difference.

If you stayed at the reduced speed you would save fuel. It's if you increased the live engine power to keep the original speed that you will find you burn more fuel.

The more apart your props are, the more pronounced the above effects are.

As I said, one real benefit is the service interval by saving on time on the shut dowm engine.

 

[Dave_Seager]

Although stopping one engine only cost you 1 knot it may not have halved your fuel consumption per hour.

Remember that most marine diesel engines are controlled by a governor and not by a throttle. This means that the control lever is used to set the revolutions that you require and the fuel delivered by the injection pump is adjusted to maintain those revolutions as the load varies. When you stop one engine the load on the other engine will increase so the fuel consumption per hour for the other engine will also increase.

 

[Latestarter1]

Thanks for responses everyone. Interesting.

BTW. The boat is a Sealine F44 and we cruise around the Clyde. We are quite a happy at 7.5knots which I reckon is the maximum speed before the bow starts to lift. For us, sadly the days are gone when we would routinely travel all over the place at 20+ knots.
£250+ for a 50 mile round trip isn't my idea of financial fun, especially when when we do that kind of mileage nearly every weekend in the year.

I understand the points you put forward which make sense, but I am surprised if the rudder stock couldn't cope with stress due to the slight drag at 7.5knots.

Regarding MARPA and the radar. You mentioned that it's possible that the radar and MARPA might be rendered inaccurate because the rudder is slightly offset. What would cause the potential errors, as the boats heading would remain the same and the radar orientation wouldn't change? Unless of course the system is taking the rudder position into acount, which is possible as all the instruments are integrated and the autopilot is aware of the rudder postion. I would have thought that the compass heading would take precedence, although I may well be wrong in that assumption.

I note that you saw an increase in fuel consumption when using just one engine, due to drag etc, which is makes sense. Nevertheless I found it interesting that our speed only dropped 1 knot from 7.5 to 6.5 when I stopped an engine. To me that would suggest that there could be a decent fuel saving just by sacrificing one knot.

Not sure what all the other perceived issues, but running on one motor makes total technical sense, very common when in 'no wake' zones on Intercoastal Waterway is US.

There are a number of technical positives.

Forget about letting your ZF box troll, it cares squat all.

Your vessel requires around 40 Hp to drive it along at 7.5 knots. Your 63's have waste gate turbochargers which makes the fuel curve a little screwy, however provided that you stay below 1,300 rpm you are in real sweet part of the fuel curve. At 1,000 rpm one motor has the potential to develop 50 kW or 67 Hp. Because propellers move boats not the engines your prop is absorbing a good percentage of engines potential power, cylinder temperatures will be close to optimum and life is good, diesel engines love to work.

We did sea trails on repowered vessel with electronic engines a year or so back which was due to go though French canals to the Med. Single engine running was what repower was all about. We recorded difference in % engine load with one engine shut down, drag of free wheeling gear never increased % engine load any more than 7%. Fuel log showed a small but measureable reduction in fuel consumption running on single engine vs twin engines.

Building motor sailers with a Gardner main engine and Lister wing used to be common practice. The Lister wing had a shaft brake to prevent propeller spinning, would you believe there is little difference between drag of spinning prop and one which is locked up. On long passages the Lister was given airing for hour or so, off with shaft brake, decompressors on, into gear, bump bump bump, close decompressors and away, who needs a starter.

As to strength of stern gear, if any vessel is of such flimsy build that this is an issue at displacement speeds then it should be avoided anyway.

Other than swapping engines to even out hours....no issue

 

[capsco]

Would the same apply for Twin Disc 502s, ie happy to troll?

 

[Piers]

Would the same apply for Twin Disc 502s, ie happy to troll?

Play d'eau has Twin Disc gearboxes, and the manufacturer advises the following,

1 Do not exceed normal propulsion speed of the vessel, and run the engine with the transmission in neutral at normal fluid pressures for 5 minutes, every 8 hours.

2 Maintain the oil level at the full mark on the dipstick, or lock the propeller shaft. Note that locking the shaft will increase drag.

Another note is to check if your shaft seals require pressurised water. If not, the engine can be shut down.

Piers

 

[asteven221]

Fuel log showed a small but measureable reduction in fuel consumption running on single engine vs twin engines.

Great reply thanks. I am a little confused by the statement above.

Are you saying that at the speeds I am talking about i.e. 7.5 knots @ 1000rpm with two engines running vs 6.5 knots at 1000rpm with one engine running, the fuel saving is only "small but measurable"? I guess if it's very small there isn't any great point in running on one engine.

 

[Latestarter1]

Great reply thanks. I am a little confused by the statement above.

Are you saying that at the speeds I am talking about i.e. 7.5 knots @ 1000rpm with two engines running vs 6.5 knots at 1000rpm with one engine running, the fuel saving is only "small but measurable"? I guess if it's very small there isn't any great point in running on one engine.

#1 Every vessel is slightly different, however basic rules are the same.....

Remember propellers move boats not the engines. Your vessel requires say 40 Hp to move it at 7.5 knots. You can take either 40 Hp out of the one motor or 20 Hp out of the two, power equals heat, heat equals fuel burned. Which is why potential gain can never twin engine fuel burn at given speed divided by two.

With throttle set at 1,000 rpm each one of your motors have the POTENTIAL to produce 50 kW of power or 67 proper Hp. Governor reponds to power demand from propeller. Taking just 20 Hp from engine which has potential at this throttle setting to produce 67 Hp means that cylinder temperatures are not optimum, diesel engines love work. Taking 40 Hp out of motor which has potential to produce 67 Hp is far better, power cylinder is now earning its eggs and bread, temperatures sweet, and life is good. Any gains by shutting down one motor, eleimination of engine pumping losses, gearbox losses etc etc are completely offset by drag from stern gear.

Therefore your gain is simply that you are producing our nominal 40 Hp more efficiently and your motor is a far happier bunny in the process.

 

[pappaecho]

Bought my P33 with twin Mermaids, and regularily run them on a single engine basis. The port engine is connected to the calorifier, but an hour on the port engine is enough to heat the water, so we alternate the engines. Frankly if we run two engines at 1300 rpm we get about 7.5 knots and 6.5 knots on one engine and half the fuel consumption. I had never given any thought as to freewheeling props, because the same effect takes place on sailing boats, and debates have raged for decades as to whether to freewheel the props, and wear out the bearings/gearbox, or to lock the prop in gear and suffer drag.

My boat handles perfectly on one engine, and the autohelm is fine, and does not have to make correction. The boat has two screws, which are contrarotating