Inboard diesel engines are common for sailing cruisers, but there are other ways to power a boat, as Stu Davies explains...


Rarely do we get to choose how our boats are powered. The usual scheme of things is we choose a boat on a myriad of parameters, of which one is the boat propulsion system. But having talked about diesel inboard engines and their replacement, perhaps a few words on other types of installation wouldn’t go amiss.

Conventional power units for cruising yachts, and I use the word ‘power’ to encompass electric power as well, usually drive through a propeller shaft or a sail drive. Of these two, a conventional propshaft drive is the simpler and cheaper to maintain.

Saildrives have bellows and seals that require regular checking and replacement and are principally made of aluminium that’s permanently under the waterline – requiring a degree of monitoring that a stainless steel propeller shaft with a bronze propeller on the end of it does not.

However there are other ways of putting that thrust into the water, especially when we consider vessels beyond cruising yachts, and these installations can be electrically powered too. They are namely:

  • Stern drive
  • Outboard
  • Jet drive

These forms of getting power into the water still use propellers but drive them through slightly different mechanisms. They’re mainly fitted to power boats.

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Stern drives

A stern drive is basically a sail drive but it is fitted to the transom of a boat with the engine hanging on the inside and the drive on the outside.

The drive is fitted on gimbals so it can be moved, usually by hydraulics, left to right for steering and up and down for trimming the propeller for the most efficient drive angle or for tilting it entirely out of the water for trailering or beaching.

These are well suited to power boats but a longer legged version for lower powered engines, called the Sillette Sonic drive was fitted to some Prout catamarans. They proved very useful for docking these high windage cats as the drive can be turned as well as the rudders for additional control.


A traditional yacht shaft drive with P-bracket –this one with a feathering propeller to reduce drag when sailing

They are installed behind a protective moulding in the bridgedeck in the region of the cockpit, so the propeller is near both the pitch and roll centre of the boat, remaining in the water in most conditions.

Depending on installation, on certain catamaran models they have the additional benefit of allowing the drive leg to be entirely lifted clear of the water to reduce drag while sailing. The Sonic drive is no longer in production, although Sillette Marine is still operating and offering some spares for the units.


An outboard, as we all know, is a standalone unit, clamped or bolted to the transom of the boat, originally used for small horsepower speedboat applications. They have got bigger and can be fitted in twin, triple, quad or even more numerous engine installations, producing horsepower up in the thousands.


This Mercury 3.3hp 2-stroke petrol outboard can be lifted off the transom with one hand, is powerful and quick to refuel… but no good for the environment

Outboards are usually manufactured from cast aluminium making them fairly lightweight for their power output. Electric outboards are the big growth area, especially integral battery models such as those from Torqeedo and ePropulsion.

Apart from being incredibly quiet in use, they require no difficult starting, are clean, greener and a pleasure to use. But they all have markedly lower range than their petrol equivalents, and ‘refuelling’ them is a matter of hours of charging, not minutes pouring petrol from a spare can.

When electric outboards get to the larger sizes, such as 10hp and above equivalents, they can no longer carry an integral battery, but due to the necessary battery requirement to achieve a usable range, must carry a much larger battery or batteries within the boat.


This new-to-the-market ThrustMe Kicker electric outboard is silent, emits no smoke, is half the weight (for half the power), but how green is it with its non-replaceable Lithium-ion batteries?

These can start getting quite expensive, quite quickly when you start to add up the costs of the motor, the battery and often a separate controller too.

For planing motorboats, the weight of batteries can become an issue too. They present designers with an unenviable mixture of variables and some unpalatable decisions (though strides forward are being made – see our test of the RS Pulse 63 electric RIB).

In comparison with petrol engined alternatives, the much lower energy density of electric boats means something always has to give. An electric outboard driven boat can match the range of a petrol equivalent, but only by cutting the speed right down – certainly to non planing speeds and really just trolling speeds.


For a small, high performance motorboat, the excellent power to weight ratio of a well tuned outboard is hard to beat

Or an electric boat can match the petrol on speed for a very short duration. What it cannot do is match the petrol boat on both parameters. And being the new technology on the block it can’t match petrol outboards on price.

Whether electric or internal combustion, outboards are a simple solution with no complicated stern tubes to be led out of the hull below the waterline and, for engines under about 20hp, no holes in the transom have to be made.

A fit-and-forget system with all the controls etc provided by the engine manufacturer, it is also a system that is almost infinitely variable with lots of choices of power supply for an owner.

Outboards come with a range of shaft lengths usually measured, vertically in inches, from the horizontal part of the clamp to either the cavitation plate (the horizontal plate above the propeller), or to the centre of the propeller shaft.

In terms of internal combustion engines, outboards have traditionally been petrol powered, although spurred on by the US Navy’s single fuel policy that has effectively banned gasoline from its ships, there has been a recent resurgence of diesel outboards with a number of diesel models from 50hp-300hp released in the last five years.

Unfortunately for those of us with smaller boats diesel outboards do not scale down successfully to very small sizes. A diesel engine requires heavy items such as an injector pump that would be difficult and expensive to miniaturise.

So a diesel outboard of, say, 5hp, would be undoubtedly hugely fuel efficient, and its fuel would be much safer to carry aboard, but it would be unacceptably heavy to hang off the transom of a small boat and have a high purchase price.

Outboard motors: Pros and cons

✔ Can be cheaper, particularly, at the smaller sizes, than a comparably powered inboard engine.
✔ In the event of a breakdown can be swapped out for a replacement very quickly.
✔ For planing motorboats, a petrol outboard offers lighter weight and often higher performance than an inboard.
✔ Quickly removed so one engine can be regularly used on several boats.
✘ Usually lives ‘outdoors’ on the boat so can suffer from corrosion and rapid deterioration and be a target for thieves.
✘ Spare parts can be surprisingly expensive.
✘ Smaller outboards will not have an alternator so will often offer zero or limited battery charging ability.
✘ Struggle to provide propulsive power where it is needed for displacement sailing yachts.

Sailing with outboards

The biggest issue facing sailing yachts with outboard motors, whether they are electric or internal combustion, is getting the propeller deep enough so that when in rough water it can still drive efficiently.

Units mounted on the stern of a sailing boat suffer from hobby horsing where the propeller is lifted in and out of the water in rough seas as the boat pitches up and down with the stern, and therefore the propeller periodically leaving the water entirely.


For this Astus 20.5 trimaran a long shaft electric outboard on a transom bracket is a good solution – The boat can’t heel enough to lift its prop out

The last thing you want in a rough sea off a lee shore is intermittent drive, or even worse the engine overheating and failing due to loss of its water cooling pickup.

Some smaller yachts have an outboard fitted in a well inside the cockpit to overcome this by bringing the engine closer to the pitch centre of the boat. I’ve never liked them as they can suffer from the engine choking on the exhaust fumes which can sometimes gather in the well.

Jet drives

The first working jet drive boat was built in the 1930s by Italian engineer Secondo Campini, but the concept was developed and first commercialised by Sir William Hamilton in New Zealand in the 1950s.

Hamilton Jet, the company that bears his name, still builds jet drives in a wide range of sizes from those suitable for small boats to monster versions for ferries.

They come in three varieties; axial flow, centrifugal flow and mixed flow, which is a hybrid of the first two. By far, the most commonly encountered water jets are axial flow, which have one or two impellers (single or twin stage) and a stator (a fixed, set of propeller-like blades behind the impeller).


This jetboat has been designed with soaking its crew in mind by slamming the reverse bucket down at speed and swamping the bow

A jet drive sucks and scoops water up through its scoop grate mounted at the lowest part of the hull, forward of the boat’s transom. The scoop grate is, as its name suggests, a scoop shape, usually protected by longitudinal fingers to discourage detritus from being picked up.

The jet drive itself is little more than a curved tube with a shaft driven propeller, or more properly an impeller, in it and sometimes a set of stator vanes. The water is constricted and hence accelerated out of the nozzle at the transom of the boat at high speed.

This makes it rather different from a propeller in free water, or even a ducted propeller propulsion system. What lots of people, including many owners of jet drive propelled boats, do not realise is that the boat, when under power at high speed is not being propelled by the stream of water you see firing out of the back, acting against the stationary water surface, or by pushing the boat along due to lift like a propeller.

As Isaac Newton famously said, ‘every action has an equal and opposite reaction’, and therein lies the magic of the jet drive. It is simply the mass of water being accelerated and ejected backwards that causes the equal and opposite reaction of the boat being driven forwards.

‘Jet drive boats have a party trick that no other propulsion system can emulate’

That’s why if you look at the location of the jet nozzle of a personal watercraft (or ‘Jetski’ if it happens to be a Kawasaki), it exits the transom above the waterline when it is planing.

It’s also why you can connect a hose to the jet outlet of a PWC one end and to a flying surfboard the other end and the rider of the surfboard can fly as high as they want, only constrained by fear, balance and the length of the hose.

Steering is affected by swivelling the jet drive nozzle. Reverse is affected by hinging a reverse bucket down over the jet and deflecting a proportion of the exiting water forwards under the hull.

They are particularly effective in very shallow water, which was the appeal for William Hamilton who first popularised the technology for use in the fast flowing, shallow rivers of New Zealand.

All of these mechanisms are mainly fitted to power boats. Jet drive and outboard motor manufacturers have long argued the toss of where the benefits of the two systems lie, but the general consensus is that small jet drive planing boats can become more efficient than propeller driven boats at over (about) 35 knots, but then, at very high speeds more performance orientated propeller drive systems that only run the bottom blade of their propeller through the water then inch past the jet drives in terms of efficiency.

Where exactly these crossovers apply for different hull types will be hotly debated for many years to come.

What isn’t up for debate though is the benefit of the jet drive configuration when it comes to safety of people in the water around the boat. Spinning propeller blades can cause horrific injuries, but even if somebody was silly enough to stick their fingers through the scoop grate up into a running jet drive, nobody has the 18 inch long fingers they’d have to have to reach the turning impeller. This, along with shallow water performance, is the reason why many purpose-built search and rescue craft run jet drives.

Because a jet drive boat does not have a rudder for turning, or the whole leg acting as a rudder as in the case of an outboard or sterndrive, they can display the sometimes alarming feature of little or no steering when letting off the throttle.

As anyone that’s ‘had a go on a Jetski’ will tell you, seeing something ahead in the water that you don’t wish to hit usually elicits the novice to simultaneously turn the handlebars while letting go of the throttle.

But with a jet drive, you’ll only get it to steer the boat well if there is water accelerating out of the nozzle that can be re-directed, so when decelerating the water flow, the physics just doesn’t work your way.

For the best steering, the driver has to increase the throttle, and jet drive boats can be highly manoeuvrable in the right hands, as ably demonstrated by the river racing jetboats that make their way round sinuous, ditch-like courses at alarmingly high speeds.

Thanks to the reverse bucket and the reverse and somewhat upward thrust it provides, jet drive boats have a party trick that no vessel with any different propulsion system can emulate. They can be brought to a stop from full speed, very suddenly, often within a couple of boat lengths.

Providing the reverse bucket isn’t torn off by the fantastic loads involved, such antics are often accompanied by the bow of the boat being pushed dramatically underwater – so quite a hit for the shrieking hen or stag party. Jet drive manufacturers tend to sell this ‘emergency stop’ ability as a safety feature.


This solar-assisted electric tourism catamaran in Portugal hides a pair of Torqeedo electric drives mounted in wells aft – so no exhaust fume problems


But there is a sea change happening; a revolution in power units for our boats. Lithium batteries and electric motors are a game changer. Companies like Torqeedo are offering integrated electric power units which address some of my criticisms regarding outboard wells in smaller yachts, removing the fumes issue from the equation.

But of course, if you have any engine’s propeller situated well away from the boat’s pitch centre, you still run the risk of it popping out of the water at the very time you don’t want it to.

This, along with the already well-discussed range issues of electric installations means owners of serious cruising boats wanting to go electric, really have to think about an inboard motor, and these are not yet off-the-shelf items.

The circumstances where fully electrically powering a boat is an acceptable solution with the necessary safety margins of available power and range are still very few. Pure battery electric sailing boats are largely limited to day sailing from a charging pontoon, ideally where alternative charging locations exist and the repercussions of running short of auxiliary propulsion power are seldom life threatening.

Often, in the case of a cruising yacht, the solution will be hybrid power. With hybrid, there is always the diesel engine element of the system as backup. It’s not so great for the environment, but it is a move in the right direction.

Perhaps it’s the way we think about our auxiliary propulsion, especially in cruising yachts, that has to change. The energy transition will perhaps bring us back much more to thinking of our sailing boats as sailing boats again; that the engine or electric motor is only there for backup, emergencies and taking the boat on or off its berth.

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