Safe low rpm for Engines

I suggest that if anything makes that yacht unsafe it might be the captain.

The turbo does not "stop" and the output of a turbocharger is related to engine load, not rpm. If he can maintain 18 knots while reducing rpm he has a planing hull, a magic boat, or is operating well beyond hull speed in which case reducing rpm would be good for the engine which is probably well and truly overloaded.

You should obtain the engine manufacturer's data on engine output vs rpm to determine the performance you should expect. Each engine maker publishes a performance chart based on the use of a "well matched" fixed pitch propeller. This chart will tell you how much horsepower the prop absorbs at a given rpm and tells you the fuel consumption at each point. The chart has data points for operations within the manufacturer's acceptable range.

As far as the actual speed and range of the vessel in which the engine is installed, that is a whole 'nuther animal that only sea trials and operational history can determine.

I've heard that man came to Earth on spaceships too... doesn't make it true.

Hi,

The turbo is fed on high speed diesels by the full Oil Pressure of the engine.

On a High Tech/Electronically Controled Engine the amount of unburnt fuel and other gunge that builds up in the cylinders of a low tech mechanically controlled engine is a lot less but it still doesn't hurt to give them a run once or twice a day if running 24 Hrs a day to clear them out.

If it is real range you are looking at and are not too worried about the cruising speed you could do a few trials running on one engine with the other shaft freewheeling. If you are going to do this for long periods there is some lube needed for the g/box but it should be ok for a few hours at a time as is.

You should not achieve full revs on the one engine and should be aware of exh temps etc if it's a mechanically controlled one as it is easy to overload one.

I went around the world on a 55m Yacht with a pair of 3512'S in her. These were Mechanical Unit Injectors so it was a case of trial and error to see exactly where we were with the load condition. We could do 9 kts and had an average consumption around 125 lts an hour (3000 a day) for a main engine and 1 genset. Running 2 engines at 12 kts the consumption was nearly double that a day.

We had Electric Lube Pumps fitted when she was being built to use for this operation and I used to start and run the non in service one every day at 1200 while I was doing my rounds and logs etc. We would generally run each one for 3 or 4 days and then swap depending upon the conditions ( In any sort of head sea it was hopeless) and how long the voyage was going to be.

The quote by that captain is totally false. How many yachts travel at low rpm's for days on the ICW because of bad weather and such and they don't blow up. Turbo's turn at all RPM's....but more so the higher the rpm's.

I found the opposite to be true on a lot of this on yachts under 100 feet. Running just one engine with the other one freewheeling, greatly increases the engine load on the engine in gear and that coupled with your rudders fairly hard over to go straight(more rudder drag)has shown more fuel burn to do say 8 knots, then running both engines at a lower RPM and doing the same 8 knots. According to the engine computers fuel burn read out on many different yachts that I've run. This may be different on large yachts.

Most of the ZF gearboxes have a shaft driven oil pump that circulates oil through the gearbox, in addition to the other oil pump, and they can be run without the engine turning without damage. This is not true with all gearboxes however. Another thing is, if the yacht has dripless shafts you need to install a cross-over if you are going to run the yacht with only one engine running as it uses the cooling water from the engine to cool the shaft.

On mechanical diesels you do get a lot of build up, more-so the 2 stroke detroits rather then a 4 cycle like a cat 3208 when running low RPM's. I run the detroits up to 2000 rpms for 30 minutes (1400 rpms for a few minutes to start baking the build up off then slowly up to 2000rpm's) to clean out the engines every 4-6 hours. Detroit actually recommended I run them at 2050rpm's for 30 minutes every 4 hours. As long as the engine temps stay up, it shouldn't hurt them to run low rpm's. The electronic diesels do run a LOT cleaner at lower rpm's but still wouldn't hurt to run them up twice a day. The electronic diesels also keep the engine temps right at temp throughout the rpm range and also tend to run a lot hotter then the mechanical engines (through design) which helps keeping them clean. I have noticed a HUGE fuel savings by keeping them at 1000 rpm's or under.

I run a 135' motoryacht with 2000 series MTU's. I've put about 2000 hours on the boat in the last couple of years most of it running at 1350 rpm (about 35% load). These engines are sequentially turbocharged so only one turbo is spinning at lower speeds (second turbo kicks in at 1800 rpm). We've noticed no adverse affects from running at lower revs. If anything, the oil stays cleaner, we burn less of it, & we have noticed fewer breakdowns than were previously logged at higher rpms. The fuel savings has also been considerable. We burn 3.4 gallons per mile at 1350rpm doing 10.5 knots; 7.8 gallons per mile at 1950 doing 14 knots; 12.3 gallons per mile at 16.5 knots (WOT). Hope this helps.