Sunday, October 03, 2021


 Although five of the six boats we’ve owned since the 1980s have been planing boats a large chunk of our cruising 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 good reasons for this philosophy including some of these:

- You’re on the water to relax 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, especially in 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 move gear around as 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 are generally 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 reduce your 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 become fully 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 with that 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 range even though we and most people we know aren’t greatly concerned about fuel costs, understanding this is one of the cheapest of boating costs.

Here are four examples 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 relevant than litres per hr as the former takes into account the shorter distance traveled due to slower speeds.

1. Nimbus 405 13.3m LOA planing vessel with 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 - so fuel usage per nm decreases and range increases by a factor of about 5x

2. Absolute Vavetta 14.9m LOA semi-displacement vessel 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, 26 litres/hr, 3.2 litres/nm, 1,280 nm range – so fuel usage per nm decreases and range also increases by a factor of about 3x

4. Circa 24 - 26m LOA displacement 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

These examples include displacement, semi-displacement and planing vessels and similar results apply to all standard vessels including single engine vessels and yachts under power (however I’m not sure if this applies with foils.)

The above results are 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 substantial increase in range as a result of cruising closer to 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.

However there are some downsides to cruising at low rpm and I want to mention these 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 suggested rule of thumb is to run engines at 60-75 per cent of WOT for 60-75 per cent of the time, this 60-75 per cent range being the 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 the normal), 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 as soot bypass the rings causing a harmful sludge to build up on the rings in the process and to enter the crankcase. Some blow-by is normal, but increased levels can contaminate lubricating oil forming a sludge that can partially block lubrication feed lines as well as acids that attack engine parts, often resulting not 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 their engines are mostly selected according to their intended operational speed 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 operate so well at low rpm. For example aboard Rapport which has 24V battery banks our 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 fans also run more slowly causing alternators to overheat particularly 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 theoretically resulting 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.

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