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September 28th 2018. By Alex Smith.

Marine engines

Alex Smith runs through the basics of the modern marine engine market.

Driven partly by stringent emissions standards and partly by public demand, modern engines from the top manufacturers all now seem to be relatively light, compact, reliable, refined and economical – and yet the sheer variety of engines available to the modern boater is unprecedented. We now have outboards from two to more than 600hp, plus genuinely practical electric motors and a sturdy backbone of bulletproof inboards in petrol and diesel form that can provide decades of loyal service. There’s also tremendous consumer flexibility in terms of the options and extras, the variety of propulsive techniques, the customisation potential and the choice of interface, data display and operation that makes the marine engines market more exciting than it’s ever been before. But at its heart, the engine selection process remains as straightforward as it ever was. It’s all about finding a size, weight, output and performance profile that matches your boat, allied to features that match your lifestyle and a price that matches your wallet.

Modern engines from the top manufacturers all now seem to be relatively light, compact, reliable, refined and economical.

The basic distinctions

In the broadest terms, a marine engine comes either as an inboard or an outboard. In inboard form, it either sits right at the stern, operating through the transom by means of a sterndrive; it sits further forward and operates via a fixed shaft that exits through the bottom of the hull; or it is hooked up to pod drives – integrated steerable pods, comprising transmission, outdrive and props, which protrude directly through the hull. As the name suggests, an outboard differs in that it is positioned higher up and further aft, on top of the transom itself – and as you might expect, the various methods have very different strengths and weaknesses…

Outboard engines

The range of outboard power options now available is extraordinary. While Seven Marine has previously introduced engines like the 557 and 627 (with 527 and 627hp respectively), even mainstream outboard manufacturers are now building models for the recreational market that generate upwards of 400hp. Mercury now has a relatively lightweight 400R model which is highly favoured by the race boat market and Yamaha’s recent introduction of the XTO Offshore model is an even more significant sign of the times. This big block 5.6-litre V8 generates 425hp and massive amounts of torque, enabling it to turn big 17-inch props and to push far bigger, heavier boats than have ever previously fallen within the remit of outboard propulsion.
In fact, with North America’s vast leisure boating market leading the charge and the rest of the world following suit, it seems that inboard engines are routinely now being ousted from beneath the deck and replaced with transom-mounted outboards? Key factors behind this trend certainly include public perceptions regarding accessible purchase prices, increases in inboard space, reductions in noise and vibration and the simplicity of servicing. But of course, it’s the increased power capacities that are enabling the upper end of the market to expand into the world of 40 to 45-foot offshore fishers, voluminous mid-range sports cruisers and multiple rig 45 to 50-foot offshore cabin RIBs – and with a lot of the world’s premier boat builders presenting outboard versions of existing inboard models, that’s a trend that’s set to gather pace.
At the other end of the spectrum, smaller, less glamorous outboards tend to be used on compact, weight-sensitive craft at the hands of users who are restricted in budget, so price and weight parameters are particularly stringent. They also sit much closer to the driver (particularly when tiller-steered), which means that refinement also needs to be a major priority. And the outlawing of two-strokes in the UK has made their job even tougher, particularly as sub-10hp units need to be sufficiently lightweight for you to carry without breaking your back, colliding your shins or covering your legs in oil.
But while the upper end of the outboard market exhibits some strikingly divergent approaches, the similarities in the offerings of the various engine manufacturers at the lower power brackets has produced firm trends in the kinds of product the consumer can expect. Outboards from 2 to 6hp tend to be single-cylinder units weighing between 13 and 30kg; engines from 8 to 20hp tend to employ the twin-cylinder approach, with weights of between 38 and 55kg; and larger (usually three-cylinder) outboards continue their battle with the bulge by trying to dip beneath the magic 100kg mark as the 50hp bracket approaches.

The range of outboard power options now available on the market is extraordinary

Inboard engines

Despite what appears to be an unstoppable surge in the popularity and relevance of outboard engines, there are still those who swear by inboards – and whether in shaft, sterndrive or pod drive format, their merits remain very pronounced. While the modern diesel inboard offers all the benefits in terms of low-end torque and extraordinary long-term reliability that has always been the hallmark of an inboard diesel, the traditional noise, weight and sluggishness of old-fashioned units is all but gone. From a practical perspective, an inboard engine is also virtually impossible to steal and, despite the popularity of outboards, a well-maintained inboard diesel craft will still tend to command a more robust residual value on the used market.
For the keen cruise boater, there’s even more to enjoy. After all, marine diesel is much more readily available in the UK than petrol and it enjoys obvious safety benefits in terms of on board storage. Better still, a well-matched diesel engine tends to provide significantly better economy than either an outboard or inboard petrol unit. While fuel flows at cruising speeds might exhibit only a 10% saving in relation to comparable outboards, the upper end of the performance spectrum can often increase those savings to almost 40%.
More to the point, while diesel inboards might still be unjustly saddled with a reputation as bombproof powerplants for big, sluggish chuggers, they also offer plenty of natural dynamic advantages for fast planing craft. A sports boat needs its weight as low and central as possible, to assist both with stability at sea and high-speed handling – and in this respect, an inboard engine has natural advantages that half a tonne of outboard hanging off the transom can never hope to match. Take a look at how many endurance race craft and commercial vessels are rigged with inboard diesels and it becomes plain that, despite the extraordinary power, refinement, flexibility and sophistication of modern outboards, the inboard engine’s combination of reliability, performance and economy still has a great deal to recommend it.

Electric motors

While once, electric propulsion for the leisure market seemed to be more about the flattering of eco-conscious sensibilities than self-sustainability and increased cruising independence, there are now some very practical electric and hybrid solutions around. In particular, with its range of outputs, systems and drive formats, Torqeedo’s Deep Blue model (which is now available in inboard, hybrid and sail drive variants, as well as original outboard form), has brought the prospect of electric propulsion to those who want to enjoy watersports, as well as to those with heavyweight sailing vessels of up to 80 feet.
At the more affordable end of the market, the modest weight, compact size and unmatched cleanliness of electric outboards make them excellent both as auxiliary motors for larger vessels and as primary motors for inland runabouts. They sidestep the main issues of the type by offering easy portability, near silent propulsion and cheap running costs. They still carry a price premium and the batteries are likely to need replacing every three or four years, but that is offset by the fact that you don’t need to buy any fuel. Modern electric outboards are reliable and low-maintenance and come in every conceivable form, from simple trolling motors to remotely operated units, making them ideal for small boats, inland fishers, portable tenders and quick-access auxiliaries.
Even today, however, the problem is the fact that battery banks tend to be quite large, heavy and expensive so, while they are often favoured by commercial operators who put endless hours on their boats, most recreational users continue to view internal combustion as the most efficient means of primary propulsion. Having said that, it’s true to say that, in almost all cases, electric propulsion is a great deal more effective when the host boat is designed and built specifically for purpose. Happily, that is now happening more and more by means of commercial partnerships between electric engine specialists and often quite mainstream boat manufacturers – and the results in terms of refinement and efficiency can be truly astonishing.

There are now some very practical electric and hybrid solutions around.

Additional marine engine considerations

  1. On an inland boat, a displacement craft or a small tender, make sure you consider the electric options.
  2. On a ‘portable’ outboard, look for an easy-to-carry shape and anti-spill features as well as an easy user interface and low weight.
  3. While modern electric outboards are very effective, purpose-built electric boats tend to be much more refined and efficient than retro-fitted platforms.
  4. In addition to discounted prices, look out for added value accessories, like carry bags, fuel tanks and tiller extensions.
  5. The theft of outboards is a major issue in the UK, so investigate the security features of each engine.
  6. Take advice from your boat builder about the best engine options for your specific craft.
  7. Don’t forget the ‘peripherals’ (including connectivity, data displays and service backup).
  8. Be aware that a more powerful engine used at lower revs is often a better option than a smaller, more highly stressed unit.
  9. If you need high performance as well as reliability, it’s worth taking a look at some of the more common military, commercial and endurance rigs.
  10. There are now lots of cheaper Chinese-built options (particularly on the outboard market) but think carefully about warranties, spare parts and both the quality and proximity of after sales service.


Alex Smith is an ex-Naval officer, with extensive experience as a marine journalist, boat tester and magazine editor. Having raced as a Pilot in the National Thundercat Series and as a Navigator in the inaugural Red Sea RIB Rally, he has now settled in the West Country, where he lives and works as a specialist marine writer and photographer from his narrowboat in Bath.