If you’re looking to re-power your boat with a new outboard, there’s plenty to consider from outboard type to horsepower, shaft length, warranty and ongoing servicing. While some of these decisions come down to personal preference, the size and make of your boat will also dictate things like maximum horsepower and shaft size.
However, people often forget to consider how and where you boat, which can have a big impact on your boating enjoyment.
A good place to start is to decide whether you go with a two-stroke or four-stroke engine.
Two or four stroke – what’s the difference?
Two strokes have the reputation of being high on emissions and bad for the environment. That’s definitely true for older carburetted two strokes and some newer ones where oil and fuel are released into the sea. However, the newer two strokes have gone a long way to address the issue of fuel/oil escaping and are therefore much friendlier on the environment. These days two strokes are engineered with direct fuel injection (DFI) and Electronic fuel injection (EFI) models so they have closed the gap on emissions.
In contrast, four strokes are cleaner to run because oil is added to the engine and not to the fuel. They also tend to be quieter, longer lasting and more fuel efficient than their two- stroke counterparts. That means they tend to be more popular with fishermen, who spend a lot of time at cruising speed.
The downside with four-strokes is that they can be heavier than a two-stroke and therefore have a less favourable power to weight ratio. Again, four-stroke manufacturers have been working hard to counteract this and are increasingly producing lighter engines.
On the whole, outboard manufacturers of both two and four strokes have improved design and through technological advances, have closed the gap between the two engines.
What horsepower do I need?
The first place to start is to check the recommended horsepower rating for your boat or hull, as you cannot exceed this. Your boat should come with a capacity plate, or you can check the owner’s manual to get this information.
In general, higher horse power outboards will use more fuel, however a more powerful outboard can also be better on fuel consumption as you reach higher speed for a lower RPM and therefore go a greater distance using less fuel.
Also, think about the load that you will typically take out, such as the number of people on board and any extra gear you take like dive tanks. The more weight you carry the more power you’ll need to keep the boat at speed.
Plus, if you’re planning on water skiing or wake boarding you’ll need plenty of instant acceleration so lean towards a higher HP engine.
We generally recommend that customers favour the upper limit of the HP rating for their boat, particularly as the seas around Auckland can get rough with strong currents. You need to be able to accelerate quickly to get out of tricky conditions.
Twin or single rig
For those bigger boats whether you opt for twin or single rig, is a bit about personal preference as there are pros and cons for both.
As outboards are increasingly built with more power, the need for a twin-rig to power a larger boat diminishes, for example a 250HP can easily power a big 8m boat.
One of the main reasons for a twin-rig is probably the safety aspect. If one engine fails at least you’ll have a back-up to limp home. This may be even more important if you venture our far, or predominantly have a family on board.
Two engines can also provide better boat handling and control which is particularly useful for spinning the boat around or getting in to tight spots.
On the downside, you’ll have two engines to fit-up and maintain through servicing – adding to the overall cost of owning a boat. Plus, even if twin-rigs provide more horsepower, you’ll have more drag and weight affecting the power to weight ratio.
The other cheaper option is to have a larger engine and a smaller auxiliary motor as a back-up if you like the idea of a reserve engine.
To determine the right shaft size for your boat, measure the back of the transom in the middle of the stern. The shaft size should match this. Generally, the smaller the boat the smaller the shaft required. The standard shaft sizes are 15”, 20”, 25” or 30”. It’s critical to get the shaft size right, if not the propeller won’t sit at the right height in the water, which can cause issues with handling, fuel consumption, or worse - damage to the outboard.
The length of warranty the outboard manufacturer offers is an indicator of how much they stand by the reliability of their engines. You can’t really go past Honda Marine's 7 Year Warranty with no limit on engine hours, as this is the longest of any outboard manufacturer for domestic use.
An often overlooked but important consideration is ongoing servicing of your engine. You’ll want to make sure your local outboard technician is authorised to service your brand of choice.
Also, check out what the minimum servicing requirements are to keep the engine under warranty, as this is another cost to consider.
Talk to the team at GT Marine for tips and advice on re-powering with a new engine.