Boats powered by petrol or diesel engines have one thing in common – high fuel costs, with some of the larger planing motor cruisers burning a gallon a mile, which is why many owners try to find a way to save fuel. On the upside, that same planing boat will only average 50 engine hours a year, about the same as a heavy goods vehicle will clock in a fortnight.

As a result, standing faults and corrosion from lack of use are a bigger killer of marine engines than excessive mileage. Incidentally, and in a complete reverse of automotive thinking, when looking to buy a used motorboat, high engine hours are usually seen as an asset. They suggest the owner enjoys using their boat and so keeps the engines in good condition.

The latest common-rail diesel engines are proving very efficient and with lower emissions. Credit: Marie Ullnert/Bilduppdraget

With energy efficiency a buzzword in all sectors of mobility, from aircraft to ships, significant efforts have been made to increase the range of vehicles for the same amount of fuel, while reducing emissions. In the marine leisure sector, there is a range of simple things boaters can do as a matter of routine to take much of the friction away from their hull, optimise their propulsion, and trim the boat to make the best use of every drop of fuel. Whether you have a fast-planing boat, a heavy displacement yacht, or a tender with an outboard, as soon as the engine goes on, here’s how you can save diesel or petrol.

Save fuel: clean hull and props

The less resistance a hull creates, the less fuel will be needed to drive it along. A pre-season scrub-off and application of antifouling bottom paint can reduce fuel costs by more than 8% over an average season.

The propeller, rudders and other stern gear (struts, skegs, trim tabs etc) should also be antifouled, and there are several highly effective silicone coatings available for these protrusions. The non-toxic, slippery surface of these coatings causes any fouling to simply slide off when the boat is underway.

Pre-season antifouling keeps the hull smooth and more easily driven. Credit: Jake Kavanagh

Optimal efficiency also comes from an undamaged prop. Cavitation (small, exploding air bubbles) can erode the leading edge of each blade, reducing efficiency, as can small dings from debris strikes or grounding. Give the propeller a visual inspection when the chance allows.

If the propeller is damaged or has misaligned blades, you’ll either see the damage during a visual check or feel an unusual vibration or ‘wobble’ through the floorboards. Not only is this reducing fuel efficiency, but it will also be putting an uneven loading on the shaft bearings. Early intervention could save a more costly repair bill later.

If the opportunity arises, such as being near some tidal scrubbing piles or a suitable wall for drying out against, a mid-season hull clean is often a good idea. Some boat owners even careen (lay over) their boat on a sandy beach, scrubbing one side at each low tide.

Initial fouling usually consists of slime, but this attracts larger animals and plants to attach to the hull. Barnacles also emit a pheromone to encourage passing larvae to settle. Removing the slime and any immature barnacles early will extend the useful lifespan of your coating and prevent wasted fuel.

Regular engine service to save fuel

With a marine engine usually doing so few hours when compared to a road vehicle, it’s tempting to extend the time between service intervals, but this is a false economy. A leading marine insurance company told PBO that avoidable claims have arisen due to routine servicing being postponed. This was a particular issue during the Covid lockdowns. A poorly maintained engine can fail when needed most, but usually just runs inefficiently due to slipping belts or blocked filters.

Modern common rail injection engines are more fuel efficient than their direct injection predecessors, but regular filter, oil and impeller changes are still needed to keep the engine running as smoothly as possible. Don’t forget routine oil changes for the gearbox or drive leg too – it’s just as important to keep these well-maintained so they work to their full potential.

Optimise the propeller to save fuel

Propeller sizing is still a dark art, but engine makers will be able to suggest – or confirm – the right diameter and pitch for a particular boat and engine combination. When it comes to pitch, (the theoretical distance travelled forward in each revolution, usually expressed in inches), propellers are most often configured to the maximum speed of a boat. However, downsizing the propeller to match your preferred cruising speed better will save fuel on a long passage.

Renew anodes every season, and make sure you check your propeller regularly. This one probably needs replacing…Credit: Jake Kavanagh

The engine performance curve from the manufacturer’s data sheet will reveal the most economical cruising speed, which should be obtained for 70% of the maximum rpm. Overpitching, where the blade angle is too coarse, will cause the boat to reach its cruising speed quickly and then labour, often producing dark smoke. Underpitching, or setting the pitch too fine, will cause the engine to over-rev before cruising speed is reached.

Some river boats deliberately fit finer-pitched propellers so the engine can run faster under load, while keeping the boat within the usual 5mph speed limit. This faster running allows the most economical fuel use and a healthy loading, which keeps the cylinders and bores in good shape.

The high-torque Sharrow propeller is a new twist to a proven shape and is now available in a contra-rotating version. Credit: Sharrow Marine

There are now several types of self-pitching propellers available, some intended for motorsailers and electric boats, such as the Brunton’s Eco*Star. These help to optimise fuel use by pitching themselves to the ideal setting for the engine output, while also reacting to the sailing conditions. Feathering versions can also be reversed with an identical pitch for hydro-regeneration.

Although the modern propeller hasn’t changed much in concept since its invention in the 1830s, new versions have appeared that promise fuel savings for motorboats by increasing torque. The Sharrow prop is a good example.

Save fuel by having the correct hull trim

The longer the waterline length, the faster a displacement boat can go. By trimming the boat’s internal weight distribution evenly, so as much of the hull as possible is in the water, less fuel will be needed to drive her along. I experienced this when loading my Corribee 21 with several crates of bargain wine from a hypermarket in France.

The less effort an engine must exert, the less fuel it will use. By moving the crew forward and trimming the drive leg inboard (image below), this RIB’s top speed dramatically increased for the same throttle setting. Fuel consumption per nautical mile also went down. Credit: Jake Kavanagh

By stacking it into the forepeak, it brought the nose down, offsetting the weight of the motor, fuel tanks and two crew in the cockpit. At the same revs on the outboard, I gained an extra knot on the voyage home, equating to a 20% improvement in fuel efficiency.

Moving the crew forward will save fuel. Credit: Jake Kavanagh

On fast motorboats, being able to use mechanical aids such as power trim on an outboard leg, or trim tabs on a shaft drive, can make significant fuel savings by keeping the hull at its ideal angle of attack. A motorboat wallowing along with its nose up will burn a lot of extra fuel. Generally, for the best performance, you want the flat sections of the hull to be as parallel to the surface as possible. Think of how you skim a flat pebble across a pond – it’s the same principle.

Modern electronic stabilisers, such as the dagger-like Humphree Interceptors, can take the guesswork out of hull trim. Able to move in milliseconds, they not only keep the hull at the optimal angle for the speed selected, but they also smooth out the ride. Processors based on drone technology handle the whole operation automatically, with manual override as an option. It thinks quickly – up to 120 decisions a second – and doesn’t get tired.

Use weather routing

This is a weather routing app from SailGrib, which opens
on most mobile devices. Credit: SailGrib

Bashing into a head sea under motor is not only tiring, but it also wastes a lot of fuel. For the sake of delaying a departure by a few hours, a trip can become much smoother and faster, especially if you work with a situation of wind with tide, as opposed to wind against tide. Commercial shipping uses all the latest weather routing software – also available for the leisure yachtsmen via chartplotter apps – to find the path of least resistance. Shipping companies reckon they can save around 10% in fuel costs by running their vessels at their most economical speed on flat seas.

Reduce windage to save fuel

Making a motorboat more aerodynamic is a good way to save fuel. Windage, a term that refers to the amount of surface area that can catch the wind, can be significantly reduced by folding or stowing several items that protrude from the superstructure. These include temporary fixtures such as biminis and sprayhoods, although these are sometimes needed on passage.

Anything that can affect windage will slow a boat down – dinghy stowage being an example here. Credit: Jake Kavanagh

Clearing the decks can also help to streamline the boat, especially for a long passage directly into a headwind. Collapsing and stowing dinghies, for example, and removing and stowing sunshades will present less of an airbrake to the wind.

As with all things boating, there is always a compromise, and some will argue that it’s better to have a more functional boat than a fuel-efficient one. Windage can, however, make a significant impact on fuel consumption over time, so ways to mitigate the drag are worth considering.

Minimise extra weight to save fuel

As electric boats have become more popular, a great deal of attention is turning to weight reduction. The lighter a boat is, the less energy is needed for propulsion, so battery range can be improved. Weight is less of an issue on a sailing yacht, although many modern designs are still built to be as light as possible to improve sailing performance. When it comes to planing motorboats, however, extra weight can make a big difference.

A lot of weight can be saved by keeping the water and fuel tanks below optimum. A litre of diesel weighs around 0.85kg (1.87lb), and water weighs 1kg (2.2lb).

So, by leaving a 100lt water tank half full, you’ve already shaved 50kg (110lb) off your displacement. Do the same with your diesel tank and that’s another 42kg (93.5lb) saved. They can both be filled up before a long passage, but do remember to keep a 20% reserve for any planned journey.

A lighter trailboat will also save fuel for the tow vehicle. Experienced trailboaters often return to the slipway with tanks almost dry, and only fill them again at the last opportunity when they return to their cruising area. Not only is the boat lighter on the road, but the fuel and water are always fresh before an outing.

Consider stabilisers

A lot of energy is wasted when countering wave action, but thankfully a new generation of stabilisers is on the market. They come in three main types: fin, rotary and gyro, and can make a big difference to fuel consumption and crew comfort – but they are expensive.

Gyro stabilisers can be fitted to boats as small as 22ft and run off a 12V power system. They exert a counter force to rolling, helping to keep the boat on an even keel in a seaway, which makes the propulsion more efficient. They are much less effective against pitch, so often work in tandem with interceptors or trim tabs.

Stabilisers and interceptors keep the hull at an economical angle of cruise while reducing fuel-wasting movement. This is the new DMS All-In-One system (from left) stabilisation at anchor or not moving; trim tabs at high speed; articulating fins at mid-speed. Credit: DMS

Fin stabilisers have traditionally only been the remit of much larger motorboats, but advances in actuators and fin design mean they’re now available for boats down to about 12m (40ft). They often work in conjunction with trim tabs or a fixed ‘spoiler’ at the stern. The new All-In-One from DMS could also prove a game changer (see above).

Rotary stabilisers use the Magnus effect to create lift, but also have a degree of drag. Although there will be some payback in terms of fuel saving, the real bonus is the comfort these systems give, making boating far more enjoyable in marginal weather.

Closer monitoring

Advances in common rail diesels, fuel-injected outboards and DC electric motors have led to very accurate monitoring of fuel or energy use. The data can be presented on any modern chartplotter or compatible mobile device, and companies like Actisense and Maretron can also provide plug-and-play modules that digitalise analogue readings. This means older engines with legacy gauges and dials can have their performance displayed as colourful digital data, and in real-time.

Advanced sensors in engines and battery systems allow mobile apps like Yacht Sentinel to keep a real-time check on fuel and energy. Credit: Vanemar

Careful monitoring of the boat’s fuel consumption in various sea states, degrees of bottom cleanliness and different loading will help a boat owner assess the optimum settings for engine and trim. There is even software available to detect subtle changes in performance and fuel consumption that may indicate a fault.

These programmes and apps were originally intended for charter fleet managers, to help with preventative maintenance, but examples from the likes of Yacht Sentinel are available for individual boats. Performance data can also help assess if the correct propeller is fitted, especially as the speed is calculated with a high degree of accuracy by GPS. Not only will the sensors detect the speed through the water, and compare this with the exact number of revs, but they can also relay other indicators such as engine, water and exhaust temperature, fuel flow rates, and other relevant factors.

Shop around

The price of fuel can vary significantly from port to port, so it pays to shop around. Boats with a wifi connection can tap into fuel price comparison sites to get the latest fuel prices, and some quayside refuellers will often offer discounts for larger quantities.

Cruising clubs that arrive at a port with a flotilla of motorboats have even made big savings by ordering a fuel delivery by tanker, effectively buying it wholesale. Some sailors take advantage of discounted petrol from supermarkets to top up their tanks, rather than pay premium prices at the quayside.

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