With the price of petrol on garage forecourts and in marinas regularly rising, there has never been a better time to think about boat fuel economy.

Owners and prospective buyers of large modern outboard motors can be assured that the engine manufacturers are doing all they can to make their motors as clean, quiet and economical as possible.

The bigger engines now have more computing power than the first Apollo space mission, and they use it well.

Alive with sensors, they automatically adjust for ambient temperature, sample the exhaust for the correct burn, and add as much free air as they can.

But the engine can only do so much by itself – there is a lot the motorboat skipper can do to help.

Boat fuel economy: Engine set-up

For an outboard engine to work efficiently, it needs several things. Most important is a clean, undamaged propeller of the right diameter and pitch.

The engine has to be properly serviced, and needs to be set up on the transom at the right depth and angle (as a general rule of thumb, the main anti-cavitation plate should be level with the bottom of the transom).

A clean underwater hull, either through dry storage, a good antifouling or regular scrubbing, will make a big difference, and if your boat is out of the water for winter, now is a good time to get all this done.

It’s the way you drive

Assuming the outboard is properly set up, you can make substantial miles-per-litre savings simply by the way you drive the boat.

We took two slightly different boats (a 2003 6.5m Ribcraft and a military spec 1999 Zodiac 6.0m SRMN; the tubes on the Zodiac permanently touch the water, so increases drag) fitted with identical Honda BF150 4-stroke engines to see how they’d fare if we committed some cardinal sins including excessive handling, incorrect trim, excessive throttle play and poor loading and balance.

Conditions on the day were: water temperature 5°C, air temperature 7.8°C, wind 10 knots and surface smooth.

Helping us with the results was Honda’s technical expert Mick Horsfall, who bought along a Navman 3100 fuel monitor.

Continues below…

The right trim

The first and easiest way to improve performance is with correct trim.

Most big engines will have a power-trim facility.

The engine is trimmed in to lift the stern and help the boat onto the plane, and then trimmed out again – usually to a neutral position – to make her angle of running as level as possible.

If the trim is incorrect, then normally the speed will drop and fuel consumption will rise.

At 4,000rpm, we found a difference in consumption of about 2lt/h for badly set trim. Our speed varied from 14.7 knots with the bow trimmed right down, and 17.4 knots with the trim correctly set.

At higher revs, 5,800rpm, we saw a more dramatic change in the consumption, from 53.9lt/h with the nose trimmed right down, to 57.1lt/h with the bow up and the stern dragging.

Older and less “intelligent” engines than the Honda would almost certainly show far bigger variations in fuel use.

Poor weight distribution

You’ve often seen boats motoring out to sea with several people sat at the stern.

The bow rides high and pushes a big wave as the engine tries to drive the boat upwards.

All this is wasted fuel.

The skipper would be better to simply throttle back or move some of their passengers forwards. She or he will go nearly as fast, but save several litres of fuel an hour.

We proved this by moving our heavy crew aft so the stern dug in.

Fuel consumption immediately went up by 2-3lt per hour – despite the engine’s electronics attempting to compensate – and the speed fell by three knots.

Moving the crew forward, the nose went down, the speed picked up and the consumption fell.

More important still, poor weight distribution may prevent you from getting ‘over the hump’ to the plane if horsepower is marginal for your size of boat.

One trailboater I knew could only plane if he sent his wife on to the foredeck.

Excessive helming

Every helm movement disturbs the efficiency of the propeller.

Obviously, this can’t be helped in restricted waterways, but on the open sea, avoiding over-correction will save fuel, especially if the steering is powered.

At very low revs and displacement speeds, we noticed a jump of 2lt/h (nearly a 30% rise) as we over-corrected.

Excessive throttling

In a lumpy sea you may need to work the throttle to maintain decent headway.

Powering up the back of a wave, and then reducing speed as you run down the other side prevents you burying the nose, and gives a safer ride.

Unfortunately, this also increases fuel consumption, and so reduces range.

If fuel is limited, a safer option is to cut back on the revs and then set the throttle so the engine can properly optimise itself.

We noticed that the longer the throttle was left alone, the more efficient the engine became.

Best cruise

Some experimentation will show what your most economical cruising speed is.

Our figures proved there is little point going everywhere at full throttle, as the extra speed is completely disproportionate to the extra fuel being used.

Gaining those last 4 knots of speed at full throttle needed an extra 16 litres of fuel per hour.

Conclusions

We were suitably impressed with how the two Honda 150s rationed their fuel, especially as both of them had seen a lot of hard use.

Even our deliberate mistakes didn’t throw them out too much.

Our 1999 Zodiac didn’t perform as well due to her dirty bottom and deeper tubes – she’d been afloat for six weeks due to constant booking so hadn’t been scrubbed.

Once an engine is correctly set up, our figures proved that gentle driving at the optimum setting with good trim and loading will save several litres an hour, and hundreds of litres each season.

It is worth investing in a fuel flow meter.

We also saw how modern engines calculate the best fuel return for given rpm, and just need to be left alone to get on with it.

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Enjoyed reading Boat fuel economy: how to save petrol?

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