5 tips for buying a low-powered boat engine

In theory, you only need an outboard engine that is powerful enough to get a boat up onto the plane, where it will perform at its best in terms of speed versus fuel use.

But there are a few more things to think about when choosing an engine that more often than not is small enough for you to comfortably lift and lug around by yourself.

Key points:

The move to four-stroke engine technology means that there are no new two-stroke outboard engines for sale in Australia, although you can still buy a used one.

All the engines you’ll be looking at in this class will be tiller-steer, meaning you have a handle connected to the engine that you either pull towards you or push away to turn the boat.

Generally speaking, you’ll need a smaller outboard engine to power something that doesn't need a whole lot of performance.

In most cases, the engine is likely to be for something like a tender for a larger boat, that serves as the easiest way to head ashore and pick up a few supplies, or as a means of helping a sailboat in and out of moorings or boat ramps.

They’re not going to be overtaxed or running long hours at maximum revs.

The smallest outboard engines on the market are rated at 2.5hp, or producing about the same level of performance as an ordinary domestic air compressor.

These engines have a single cylinder displacing around 70cc, which is the equivalent of about a quarter of a cup of coffee – not much in other words. They weigh less than 20kg, or around the same as an adult kelpie.

At the other end of the scale are the larger portable engines producing around 25hp. These engines generally have a second cylinder and displace around 400cc, or a bit more than a can of soft drink.

While they’re still classed as portable engines, a 25hp engine will weigh around 60kg, so you’re going to need a second person to help you carry it, and use an outboard engine trolley. That makes them somewhat impractical to use on small boats that are constantly in and out of the water.

Realistically, the most engine you’re going to carry comfortably will be up to around 10hp. Even so, you’re still lugging around 30kg of weight, as it’s still likely to use a 200cc twin-cylinder block. 

The difference in performance between a 2.5hp outboard engine and a 10hp outboard engine is significant.

If you have an inflatable boat with a floor that you need to insert, it’s not going to handle much in terms of performance, meaning you only need something that is the equivalent of rowing to power it.

However, if your inflatable boat has a rigid aluminium or fibreglass floor in between the pontoons, it can handle more performance so can easily accept a 10hp engine because of the extra strength the hull brings.

One thing to keep in mind is how powerful an engine best suits your needs. People have been blown off course and even lost out at sea because their tender’s small outboard engine hasn’t been powerful enough to hold its own against tide and wind.

If you're going to be crossing unprotected waters with extra passengers on board, spend a bit more on both the boat and engine to ensure they’re both rated with enough performance to handle a range of conditions.

If you only ever need the tender to go a short distance from the boat to the shoreline in a well-protected marina, you can get away with the lack of grunt.

Boats come with a maximum horsepower rating, which in small boats is more an indication of how much twisting and pushing the part of the transom that the engine connects to can handle.

Generally speaking, when there’s not much horsepower involved you can step down to about half the maximum horsepower rating and still get decent performance out of the boat. However, factor into this decision how many people you plan on carrying as the smaller outboard engine could struggle under higher loads.

Have a look, too at where the exhaust comes out of the engine. Some smaller outboard engines vent exhaust above the propeller, which makes it easier to reverse as the propeller isn’t trying to back through a mix of water and exhaust gas like you get when the propeller hub is used as the exhaust.

If you’re going to be carrying the outboard engine, the weight will be an important factor. And while features such as electric start and power tilt are handy, they can significantly affect the overall portability of an outboard engine.

Starter motors and tilt motors add significant weight, not only to the engine but also to the boat as they need a battery to power them. Even a simple marine battery can weigh around 15kg.

Generally speaking, a low-power outboard engine is quite easy to tilt manually, with most also having a shallow-water setting that keeps the prop clear of the bottom as you’re running the boat up on a beach or crossing a shallow sandbar.

As for having an electric starter motor, it’s not an issue in this day and age. Outboard engine technology has evolved with electronic ignition systems that open the valves just enough to make turning over the engine with the pull cord very easy. Modern-day electronic fuel injection systems also deliver precise amounts of fuel to the cylinder to kick it into life every time on the first pull.

No more days of priming the engine to ensure the carby has fuel in it.

Not only that, automatic throttles make starting an outboard engine a very simple rip-and-forget procedure.

Smaller low-powered engines are generally all-in-one units that have a fuel reservoir built into the top of the cowling. However, as you step up in performance, you’re going to need a separate fuel tank to feed the beast.

There are benefits to having a separate tote tank. The first is that when you break the engine down to store onboard, it’s a simple matter of just unclipping the fuel line to disconnect the tank.

However, with fuel stored in a tank on the engine, you’re going to have to drain it before laying it down or else the fuel will leak through the breather pipe that allows the fuel to expand, and prevents a vacuum lock as the fuel is burned in the engine.

Other big benefits of tote tanks are you can easily manage stale fuel (use it to mow the lawn), refuelling is a simple matter of grabbing the tank and going to the petrol station, and when it is not in use there are no lingering petrol fumes.

One of the key requirements for an outboard engine to push a boat at its best is ensuring the propeller is low enough in the water to push it along efficiently. That means ensuring the engine’s leg that connects the engine with the propeller is long enough to suit the transom.

Boats all have slightly different transom heights, so it’s important to measure the depth of your transom before buying a low-horsepower outboard engine. There will be subtle differences between how engines are measured as short, mid or long shaft, so while one long brand’s shaft fits your boat perfectly, another’s may not.

Buy one too short and the propeller won’t be able to grab onto the water to push the boat along, even potentially sucking in air. Go too deep, and the boat will behave like you’re balancing a broomstick on your finger and awkward to handle.