Like the idea of an outboard engine but not so keen on how it looks? Rotax may have the answer with its Rotax Stealth outboard…
The seemingly unstoppable rise in the popularity of outboard engines has caused a surge in recent innovations.
From new fuel types such as Cox diesel and Evoy electric outboards to fresh configurations like Mercury’s monster 600hp V12 and Mathwall’s hybrid Halfboard, everyone seems to be jumping on the outboard bandwagon.
Now Canadian engine builder Rotax is entering the fray too with an unusual new petrol outboard design featuring what it is calling Rotax Stealth Technology.
While that seems a bit of an exaggeration, the new design does have a much lower profile than a conventional outboard engine, allowing it to fit under the bathing platform of the boats it’s fitted to.
Not only does this keep the centre of gravity lower for better handling it also makes for a much cleaner appearance and leaves the bathing platform free of any obstructions for wakeboarding, fishing or simply lounging around at water level.
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It’s no coincidence therefore that the first three boats it is being fitted to are a Manitou pontoon boat where space is king, an Alumacraft Trophy aimed at tournament fishing and a watersports-friendly Quintrex Freestyler.
All three brands form part of the same BRP group that Rotax belongs to (as well as Seadoo and Evinrude) and were specifically designed around the new Rotax Stealth outboard to make the most of its unusual shape.
It’s not yet clear whether Rotax will make this engine available to third-party boatbuilders as well but for the moment at least it wants to focus production on its own craft.
The new Rotax Stealth outboard engine is barely visible on this Manitou pontoon boat
Rotax claims this new design allows it to have all the same benefits as a conventional outboard, such as low pricing, strong performance, ease of maintenance and built in trim, with a number of additional advantages such as being hidden out of sight, as well as quieter (due to the sound deadening effect of the bathing platform) and more fuel efficient (Rotax claims it is up to 20% more economical than a traditional outboard).
It’s not immediately apparent why it would be more fuel efficient than a modern four-stroke outboard, although with an all up weight of just 227kg (500lbs) including battery, steering and throttle cables it is marginally lighter than some (but not all) of its traditional outboard rivals.
Small is beautiful
The key to this new outboard’s svelte design are twofold. Firstly it uses a very compact three-cylinder direct injection two-stroke block instead of the physically bigger four cylinder four-stroke designs which most of its rivals use.
Unlike a conventional outboard the three-cylinder engine is mounted horizontally rather than vertically
Secondly, the engine block is mounted horizontally behind the driveshaft rather than vertically above it, keeping the overall height of the outboard to little more than the length of a single piston rotation.
This is reduced even further by submerging the bottom half of the engine under water so that only the top few inches protrude above the surface.
Because the outboard still needs to tilt up and down, the bathing platform has to be a little higher than would otherwise be the case but it’s still a far neater solution than having two tiny platforms either side of a traditional outboard engine and a big engine well that eats into cockpit space.
The bottom half of the Rotax Stealth outboard engine sits below the waterline at rest
In fact by mounting the whole engine behind the transom, every inch of the cockpit be used for accommodation.
Being a two-stroke engine, each of its three pistons delivers power every second stroke rather than every fourth, enabling it to generate as much power from its modest 1.9-litre capacity as bigger four-stroke rivals from Mercury, Yamaha, Honda et al.
It’s based on the same block as the former Evinrude G2 outboard engine and is available in either 115hp or 150hp versions but we wouldn’t be surprised to see higher power variants joining the range in the future.
The low cowling height allows for a full width bathing platform over it
Clean machine
Surprisingly, given that previous generation two-stroke engines had a reputation for being rather smoky, Rotax also says the new outboard is the cleanest combustion outboard engine in its class, quoting a 12% reduction in reportable emissions and a 98% reduction in carbon monoxide at idle thanks to an extremely precise direct injection oil system that helps minimise oil burn.
Other benefits include a comprehensive three-year warranty and extended service intervals – the first scheduled service is after 5 years or 500 hours.
Paul Klug, engineering project manager for the new engine, says the idea came out of a brainstorming session in April 2019: “The initial discussions were about where we can bring BRP’s boat offering in the future.
It’s an ideal solution for fishing and watersports
“We listen to our customers and create products that meet their needs. With our boat brands, we observed a major obstruction and loss of space in the stern caused by traditional outboard motors.
“We challenged ourselves to create sleeker, more distinctive and timeless designs. The new Rotax Outboard Engine, with the MAX Deck (bathing platform), allows continuous, seamless, holistic design from bow to stern, freeing up invaluable space for boaters and their families.”
We haven’t yet had a chance to sea trial the new engine but it will be interesting to see how it performs on the water. Previous generation two-stroke engines tended to be noisier and smokier than their four stroke counterparts but Evinrude’s G2 range managed to dispel that myth while keeping the positive aspects of strong torque, instant throttle response and good power to weight.
That said, the addition of an extra universal joint to transfer power from the engine’s horizontal output shaft, down a vertical leg and back out through the prop, adds complexity and friction.
The marinisation must also be suitably robust given that it sits below the waterline. Thankfully, Rotax already has extensive experience of designing small, powerful, reliable engines for its Seadoo PWCs while its former range of two-stroke Evinrude ETEC engines (production ceased last year) were very nearly as refined as their four-stroke rivals.
If this new engine combines the best of both with a few new tricks of its own, the Rotax Stealth outboard could be a real winner.
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