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Do Aluminium Boats Need More Outboard Motor Horsepower?

Blog | 01 May 2019

True, aluminium boats are made of metal, but their hulls offer superior strength-to-weight ratios. That means their sheet metal parts can be surprisingly thin. Fabricated out of thin, lightweight aluminium alloys, they float shallowly, so there’s very little drag to stop a metal hull from cruising forth on a low wave. Speaking quite frankly, boats made from this particular alloy can skim across the waves when fitted with a large enough outboard motor.

A Cut abovethe Rest: Propelling Aluminium Hulls

Generally speaking, aluminium is the lighter hull material, at least when compared to fibreglass. However, a low weight build isn’t always a good thing. Propelled by a powerful outboard motor, a lightweight hull will bounce erratically over smooth waters. That effect will only get worse as the waters get choppier. With that thought in mind, think twice before you install a bigger engine. Otherwise, if you do end up purchasing a high horsepower outboard motor, perhaps while assuming aluminium boats needed more propulsion oomph, then the results could be quite disastrous. Who wants to speed around on an overly muscled tinny if they need to nurse the throttle all day long?

Selecting Smaller Outboard Motors

Ultimately, an aluminium boat can use a low horsepower engine, although it might lose its responsive edge if you take away too much horsepower. For example, a 9.8 horsepower outboard motor will propel a 4.5-metre aluminium boat across the water at speeds in excess of 25-kph, which is more than fast enough for most fishermen. All the same, it won’t be as responsive, as steerable as a slightly larger model. Loaded with fishing gear, and moderately loaded with fishermen, a 15-horsepower engine will deliver snappier performance. If that’s not quite enough power, you could opt for a 25-hp titan, with its propulsion overhead providing the best possible performance gains, at least in terms of cruising responsiveness. A maximum cruise speed of 40-kph isn’t unusual when an aluminium boat has this much raw power on tap.

At the end of the day, there’s not much performance difference. Aluminium vessels ride shallower, so they’re not anchored by drag. They’re also lighter than fibreglass, so the temptation to increase outboard motor power should be resisted. Remember, lighter boat hulls can bounce. At best, a calm school of fish will be startled by such surface-rippling action. At worst, the stabilization problem could reduce steering control so badly that the craft becomes virtually unmanageable. If you own an aluminium boat, don’t go more than one size higher than the optimally powered engine size. Stick with a lower horsepower model, and leave the one-size-up option open if steering responsiveness becomes an issue.

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