How much horsepower do I need for my boat?

With outboard engines now pumping out as much as 600hp, going quick on the water has never been easier, or more accessible. But how much power is too much?

Choosing the right size of outboard engine for your boat can be difficult to figure out. Too much power and your boat could be stern heavy and dangerous at high speed – an accident just waiting to happen.

Too little power and your boat could be slow to get up on the plane and dull and ponderous underway. No one wants a slug.

Boat makers will usually list a maximum power rating but they don’t always give you a minimum horsepower figure. So how do you determine how much horsepower you need?

The number one rule of course is to abide by the boat maker’s maximum power rating. Do not fit an outboard engine that is too big or powerful for the boat.

Ignoring the manufacturer's maximum horsepower rating may void your hull warranty, as well as your insurance.

If your boat maker lists only a maximum horsepower rating, then I would opt for an outboard with a horsepower output that is within about 20 to 25 per cent of that maximum.

So, if your boat is rated for an outboard engine to a maximum of 115hp, then an outboard engine of around 90hp should provide you with ample power and performance.

If there is a recommended horsepower range listed, then I would advocate selecting an outboard engine with a power output midway between the two extremes but erring toward more rather than less power.

It is always better to have a bit of power in reserve than not enough. This is particularly important if you will be crossing bars or towing people behind the boat, or even regularly taking on a few passengers.

To this point, be mindful that some boat dealers rig their new boat packages with the bare minimum power outboard engine to reduce the overall package price to make it competitive with a rival brand and boat dealer.

Buying the lower-priced boat package may sound attractive but it will be a false economy in more ways than one. As well as the cheaper boat package potentially underperforming on the water, you may struggle to sell it on the used boat market.

The the minimum recommended power is rarely enough for a boat to achieve satisfactory performance on the water.

It might be okay in smooth conditions with a light load but it will otherwise struggle; your boat will take longer to get up on the plane and may be sluggish and unresponsive while underway.

Achieving a high top speed is not all that important for recreational boating applications but you do need ample power and throttle response in the mid-rev range, and when accelerating onto the plane from a displacement speed.

Having power in reserve is particularly important for rough water boating offshore, and for when you need to cross an ocean bar to reach the open sea.

In rough water your boat needs enough power to be able to accelerate out of potentially dangerous situations; you can’t afford to have a boat that is slow and unresponsive when you hit the throttle.

Fitting the maximum allowable horsepower outboard to your boat should, in theory, be perfectly safe to do. However, I do not recommend it for novice and first-time boat owners.

Boat makers don’t always get their power ratings 100 per cent accurate either; I think there is sometimes a bit of guesswork involved.

Put it this way… I have tested plenty of new boats that have been a little sketchy at high speed with the maximum power engine fitted.

In this circumstance, an experienced boater will simply throttle back so the boat runs safely and securely. First-timers, on the other hand, may not even realise when a boat is right on the limit, especially when running in anything other than smooth water.

Fitting the largest possible outboard engine will also cost you more to buy up front, and the higher-powered motor may use more fuel if you’re heavy-handed on the throttle.

There’s also the risk that the higher-power engine will be a lot weightier, which may make your boat stern heavy and thereby offset any gain in power output.

To this point, I have often marvelled at the size of some of the outboard engines fitted to classic 1970s-era Haines Hunters.

By way of an example, the V146/445 runabout and cuddy cabin perform beautifully with a 70 to 80hp outboard engine, yet many of these terrific boats are grossly overpowered with 115hp to 140hp engines.

The weight of these engines make the little Haines Hunters sit so stern-heavy in the water that the outboard engine is commonly fitted with a hydrofoil to help get the hull up and planing in a reasonable time.

So, if you’re thinking of fitting the largest allowable engine to your boat, be wary of putting too much weight on the transom, as this will result in an ill-handling boat.

• Never exceed the boat maker’s maximum recommended power rating
• Ignoring the maximum horsepower rating may void your hull warranty, as well as your insurance should something go wrong
• It is better to have power in reserve, than not enough power, especially for rough water boating, bar crossings and more
• As a rule, choose an outboard with a horsepower output that is within 20 to 25 per cent of the maximum
• When there is a minimum and maximum power recommendation, choose an outboard engine that is midway between the two extremes, but errs on the side of more power rather than less
• Boat speed and mid-range performance will be compromised with an engine that just doesn’t have enough grunt
• An underpowered boat may be difficult to re-sell and may fetch a reduced price on the used boat market
• When opting for a maximum-power outboard engine, be wary of putting too much weight on the transom, as this will result in an ill-handling boat