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Ourlast posting detailed how cruising at around displacement speed dramatically decreases fuel consumption and increases cruising range. It also highlighted the problems which can be encountered through cruising consistently at low rpm. Running diesel engines for long periods at idling speed is particularly detrimental and engine “wear” is said to occur at about double the ratecompared to running them under normal loading. Ideally for that reason engines should only have 3-5 minutes of idling followingstart up and then be brought up to around 1,200 rpm withsome load applied.

Not only can idling cause a build-up of carbon in the engine but also causes mirror glazing, which is the creation of a mirror-like surface finish on cylinder bores, eventually allowing more oil to pass the rings and creating more blow-by (the adverse effects of which were detailed in Part 1 of this article). Mirror glazing can also be caused by constantly running engines at the same rpm, so this should be avoided. Before shut down a diesel should also be idled for 3-5 minutes to allow the turbo to cool down. In practice this is catered for when entering your marina or approaching your anchorage.

Now let’s consider someoptions for low speed cruisingand their relative merits.

Option 1 – run both engines at low rpm

If you’re wanting to do this, avoid running below 1,200 rpm andit’s recommended to at least run at 60-75 per centof WOTfor about 30 minutes after reaching full operating temperature, then again for about15 minutes every 4 hours and then for about 30 minutes about 1 hour before shutdown. This last one is considered to be especially important to reduce soot formation and to clean the turbocharger andit’s better to spend less than optimal time at higher rpm than none at all.

Pros 

There will be a considerable reduction in fuel usage and increase in range.

All ancillary equipment driven by the engines such as power steering, refrigeration compressors, hot water manifolds will operate (unlike Option 2).

Both gearboxes and drive trains will be cooled (unlike Option 2).

Full maneuverability is maintained and there is no rudder bias (unlike Option 2).

There is no potential problem with prop shaft couplings (unlike Option 2).

In practical terms this option is easy to manage.

Cons 

Somemonitoring and planning is required for the periods at higher rpm and it is difficult to achieve on short cruises.

May cause issues with alternators.

Hours-based service costs may increase because you are using more engine hours to run a given distance.

Option 2 – run on one engine at a time at higher rpm

Underthis system only one engine is used at a time, alternatingperiodically(eg every one to two hours).

Pros 

It will take more rpm on the one selectedengine to reach your chosenspeed than it would be using two, therebyeliminatingor at least minimisingthe problem of light loading.

Fuel saving and range increase will be considerably less than Option 1, but still in the order of 10 to 15 per cent.

Higher rpm will make your in-use alternator run more efficiently.

The process is relatively east to manage.

The frequency of hours-based engine servicing is reduced thus saving service costs.

Cons 

Maneuverability is considerably reduced using one engine, particularly at low speedso this option should only be considered in open waters and not for example coming into or out of marinas.

There will be a slight steering bias in the direction away from the in-use engine ie using only the port engine the vessel will veer slightlytowardsstarboard.

The not-in-use engine’s prop will still turn or “windmill” causing drag and the gearbox to operate.The inactive engine’s gearboxmust be kept in neutral so that the engine doesn’t turn over. Most gearboxes are water-cooled using its engine’s heat exchanger, so without the engine running this cooling will be lost and gearbox damage can potentially occur. Consult your installations Owners’ Manual to ascertain for how long you can windmill. They normally suggest running your engine for about five minutes before wind milling and will advise the allowed time interval before it needs to be started again to activate the heat exchanger and circulate gearbox oil. My Caterpillar manual recommends idlingthe engine every 12 hours for five minutes, however the Twin Disc gearbox manual recommends idling the engine for a few minutes every hour, so I willfollow that guideline.

If initiating this procedure it would be a good idea to check the temperature atthe rear of the wind milling prop’s gearbox using an infra-red thermometer to see how long it takes for the temperature to rise. The lower the boat speed, the less the wind milling engine’s gearbox temperature will rise. Bear in mind there’s a good chance that some time in the future you’ll have a problem with one engine and need to run just on the other one, so this is not a wasted exercise.

Take into account that engines often run ancillary equipment, for example Rapport’s port engine runs our refrigeration compressor while her starboard engine runs our power steering and heats our hot water supply.

Some stuffing boxes have no cooling system beyond the sea water coming into it, others have oil or grease lubrication to keep temperatures down, while others and more particularly most dripless shaft seals are cooled with sea water supplied from the engine’s sea water pump, so for this latter category no cooling will be supplied if the engine is not running.

Note that some vessels have a system where either engine can supply cooling water to both shafts. However if this is not the case it is best to comparethe temperaturesof the not-in-use shaft seals with the in-use shaft seals using an infra red thermometer to determine for how long you can allow wind milling. A temperature up to about 40dC should be OK, in fact as a general rule mechanics say if the stuffing box is nottoo hot to touch it’s OK (be careful doing this though). Another measure is that stuffing box temperature should be 7-22dC above sea water temperature.

Note that some cruisers have adopted measures to eliminate wind milling. At an extreme level one cruiser crossing the Pacific decided to remove oneprop until half way across, then replacethe prop and removethe other one so the in-use engine could be changed. This was done at sea using a block and tackle to support the prop’s weight. At a less extreme level it’s not uncommon for long distance cruisers to install a mechanical or hydraulic system enabling either prop shaft to be locked so it cannot rotate. I have discounted the use of such a system based on the inconvenience and practicalityof changing over engines and the compromise to maneuverability in the event of an emergency.

When an engine is driving your vessel it is trying to push the prop shaft and coupling flangetowards the engine, therefore not putting any load on the securing bolts. When the prop shaft’s wind milling it’strying to pull away fromthe engine and therefore your coupling flange, soconnections should be checked initially and at regular intervals thereafter.

Option 3 – run both engines with one engine at higher rpm than the other

Another option isto run one engine at high rpm and the other at low rpmso that all engine-driven equipment is operating, then interchange every couple of hours or so. If adopting this option avoid running the low rpm engine below1,200 rpm for the reasons outlined in the opening comments.

Pros 

The issue of light loading is eliminated.

Economy gains similar to running two engines at low rpm are achieved and range is increased.

There is littleloss of maneuverability.

There is no issue with cooling of gearboxes and shaft seals.

There is no issue with prop shaft flange connections.

The process is easy to manage.

All engine-driven ancillary equipment will operate.

Cons 

Both engines are stillrampingup engine hours, so no servicing costs are saved.

There will be a very slight steering bias in the direction away from engine operating at higher rpm.

The alternator’s efficiency is compromised for the engine running at low rpm.

Conclusion

As mentioned early on Di and I prefer to cruise much of the time off the plane, even when cruising long distances,so considering all of the above options here’s a practical solution based on Option 2 for Rapport.

-Start both engines and leave the marina using bothat low rpm (although preferably above 1,200 rpm wherever possible)providingmaximum maneuverability.

-When in openwatersshut down the starboard engine and as temperatures rise, gradually increase rpm on port to about 1,850 = 66 per cent of WOT. This will operate refrigeration and efficient alternator operation and battery charging at higher rpm. The power steering willnot operateso hand steering will be necessary, however this is not much of an issue in open waters. Any time that power steering and autopilot is wanted I can start the starboard engine.

-When the freezer reaches its operating temperature (after roughly three hours on first day outand on subsequent daysafter about an hour), runthe starboard engine at about 1,850 rpm and shut down port

–Then continue to alternate engines as required about hourly.

For subsequent days wenormally use the genset every morning so the batteries are fully charged at that time and the alternators don’t need to run at high outputs. Every several engine hours I’ll run both engines at about 2,200-2,400rpm for 15 minutes or so as well as doing this for about half an hour an hour before shutdown.

Happy Slow Cruising

 Although fiveof the six boats we’ve owned since the 1980s have been planing boatsa large chunk of ourcruising has been in the Med aboard Envoy at around 6 knots. During that time we really grew to enjoy life in the slow lane and now find that even though our current boat, Rapport, is capable of about 20 knots we prefer to cruise mostly around 8-10 knots.

Most our time aboard Envoy we cruised at about 6kn. Max speed was about 8kn

Most cruisers we speak to own twin-engine planing vessels and many of these choose to cruise on the plane when going some distance to their destination, but then cruise off the plane in the general area around their destination. There are some goodreasons for this philosophy including some of these:

– You’re on the water torelax so why not enjoy the journey as well as the destination

– Helming at slower speed needs less attention so you can leisurely enjoy the scenery at your leisure and have more time to navigate safely, especiallyin what may be an unfamiliar area

– Many skippers prefer to tow their RHIBs at slower than planing speeds

– You’re often close to shore where in any case speed is limited to 5 knots (within 200 metres)

– At slower speeds you generally don’t have to movegear aroundas you often need to in all but calm conditions when going on the plane

– At slower speeds you’re not generating so much engine noise or causing so much wake

– Slower speeds aregenerally more comfortable for crew and it’s easier to undertake activities like making cups of coffee, using the head or having lunch under way

– At slower speeds your journey will take longer allowing more time for battery charging, for engine-driven compressors to reduceyour refrigeration temperatures and for manifold hot water heaters to heat up. This is important because if for example we leave our marina for Oneroa and cruise at planing speed after the engines are up to temperature the journey will take about 90 minutes and this is insufficient time for the refrigeration to becomefully effective. At 8 knots or so the cruise will take about three hours which allows plenty of time. This is not so much of an issue on the following days when refrigeration is already cold

– At slower speeds you can troll and catch a kahawai or kingi on the way (good luck withthat one!)

– Due to lower rpm at slower speeds you’re saving a considerable amount in fuel costs and increasing your cruising range between fueling stops

In addition to these factors by nursing your engines along at low rpm you’re looking after them right? Actually NO – this is quite wrong so read on.

Although Rapport’s top speed is about 21kn fully-loaded, we prefer to cruise at 8-10kn

So let’s focus on reduced fuel consumption and increased rangeeven though weand most people we know aren’t greatlyconcerned about fuel costs, understanding this is one of the cheapest of boating costs.

Here are fourexamples of fuel savings and increases in range (taken from Pacific PowerBoat magazine boat reviews). Note that fuel usage expressed in litres per nm is more relevantthan litres per hr as the former takes into account the shorter distance traveled due to slower speeds.

1. Nimbus 405 13.3m LOA planing vesselwith twin 200hp Volvos and shaft drives:

At 3,000 rpm = 17kn, 95.9 litres/hr, 4.4 litres/nm, 200 nm range

At 1,000 rpm = 7.3kn, 6.5 litres/hr, 0.9 litres/nm, 1,000 nm range – sofuel usage per nm decreases and range increases by a factor of about 5x

2. Absolute Vavetta 14.9m LOA semi-displacementvessel with twin Volvo Penta IPS650 “Pods”, each 480hp:

At 3,000 rpm = 18.7kn, 112 litres/h, 6 litres/nm, 272 nm range

At 1,500 rpm = 7.3kn, 21 litres/hr, 2.9 litres/nm, 564 nm range

At 1,250 rpm = 6.1kn, 9 litres/hr, 1.4 litres/nm, 1,137 nm range – so fuel usage per nm decreases and range increases by a factor of about 4.2x

3. Maritimo S55 17m planing vessel with twin Volvo D13 each 400hp and shaft drives:

At 2,100 rpm = 23.8kn, 226 litres/hr, 9.5 litres/nm, 430 nm range

At 900 rpm = 8.1kn, 26litres/hr, 3.2litres/nm, 1,280nm range – so fuel usage per nm decreases and range also increases by a factor of about 3x

4. Circa 24 –26m LOAdisplacement vessel with twin Scania DI 090, each 250hp @ 1,800rpm and shaft drives:

At 1,500 rpm = 12.3kn, 39.2 litres/hr, 3.2 litres/nm, 3,234 nm range

At 1,000 rpm = 8.7kn, 13.3 litres/hr, 1.52 litres/nm, 6,809 nm range – so fuel usage per nm decreases and range increases by a factor of about 2.1x. Note that at 6.5kn the range increases to over 10,000nm

Theseexamples include displacement, semi-displacement and planing vessels and similar results apply to all standardvessels including single engine vessels and yachts under power(however I’m not sureif this applies with foils.)

The above resultsare based on running both engines and we can see that reducing rpm results in a substantial decrease in fuel consumed per nm combined with a substantialincrease in range as a result of cruising closerto the vessel’s displacement speed where the boat’s hull becomes wonderfully efficient. That’s why long distance cruisers are nearly always displacement vessels or faster vessels cruising at displacement speed. The figures would be even more impressive if I’d compared maximum rpm with idling rpm, but I wanted to compare realistic speeds.

Howeverthere are somedownsidestocruising at low rpm and I want to mentionthese as well as suggesting several alternative options to minimise their effects. These thoughts are based on our own experiences and some internet research as well as discussions with four diesel mechanics over the last several years.

Diesel engines are not designed to be run for long periods at light loading, which is defined as rpm less than 40 per cent of wide open throttle (WOT). On the contrary the suggestedrule of thumb is to run enginesat 60-75 per cent of WOT for 60-75 per centof the time,this 60-75 per centrange beingthe range of mechanics’ varying opinions.

So what happens if you do consistently run at light loading rpm?

At low rpm and therefore lower than optimum engine temperature the piston rings don’t seat so well resulting in faster wear, additional blow-by (more than double thenormal), oil fouling of components such as turbos and carbonisation.Blow-by is the phenomenon whereby combustion chamber gasses consisting of unburned fuel and water vapour as well assoot bypass the rings causing a harmful sludge to build up on the rings in the processand to enter the crankcase. Some blow-by is normal, but increased levels cancontaminate lubricating oil forming a sludge that can partially block lubrication feed lines as well asacids that attack engine parts,often resultingnot only in later engine problems but in significantly reduced engine life.

This is one of several reasons why engines used in commercial vessels generally have a longer life span than in pleasure vessels, that is theirengines are mostly selected according to their intended operationalspeed and therefore rpm.

Another cause of increased blow-by is over filling lubrication oil so never add oil beyond the dipstick marking.

Additionally alternators don’t operatesowell at low rpm. For example aboard Rapport which has24V battery banksour approx 50amp alternators charge at 23 amps at 1,170rpm and 36 amps at 1,510rpm – a 57 per cent difference. At low engine rpm alternators’ cooling fansalso run more slowly causing alternators to overheatparticularly in the early stages of charging when the battery banks need for charging is greatest and the alternators are working their hardest. Leaving the marina this should not be too much of a problem as most vessels have shore powered chargers.

A negative for running slower is it results in more engine hours accumulating for the same distance cruised theoreticallyresulting in an increase in service costs, though practically many vessels have an annual service without reaching their hours of service threshold.

But don’t despair as there are several options available to run vessels at lower speeds without compromising engine wear or longevity, each option having its own pros and cons. 

Read about these options in our next posting.