How do I choose the best outboard engine for my boat?

The best outboard matches your boat's approved power and engine weight, has the correct shaft length and delivers the performance you need with your normal load aboard. Check the builder's plate and handbook first, then compare suitable engines on local servicing, handling, range and the complete installed cost.

Check limits and working load

Find the maximum horsepower and permitted engine weight in the builder's documentation. The plate may show power without a separate engine weight limit. Check any outboard bracket's rating too, especially on a sailing yacht: its weight limit can be lower than the transom's.

Never exceed the boat or bracket ratings. Have any transom structure that is cracked, soft or moving assessed before fitting an engine.

Choose power for the hull and its working load:

• Small tenders: 2.5 to 6 hp commonly suits light displacement use, but getting several people onto the plane may require substantially more power and a suitable hull.
• Planing fishing boats and RIBs: use the builder's recommended range and seek performance figures with comparable passengers, fuel and equipment. The smallest permitted engine may struggle when heavily loaded.
• Sailing yacht auxiliaries: choose for displacement, propeller grip and thrust against wind and waves. Extra horsepower brings diminishing speed gains as the hull approaches its displacement speed.

Your existing engine provides useful evidence. Record whether it planes the boat easily, maintains a comfortable cruise and has enough reserve in chop. Do not size a replacement from boat length alone.

Match shaft length and weight

Common nominal shaft lengths are short (15 inches), long (20 inches) and extra-long (25 inches). Some larger engines use 30-inch shafts. Measure the vertical distance from the mounting surface to the hull bottom at the transom, then check the engine maker's installation drawing.

On a conventional planing hull, the starting position for the anti-ventilation plate is roughly level with the hull bottom, subject to the manufacturer's guidance. Transom setback and hull design can alter the required height. A yacht's lifting bracket needs a different assessment: the propeller must stay immersed when the stern pitches.

Too short a shaft allows the propeller to draw air and lose thrust; excessive immersion increases drag and spray. Compare the exact engine variants, because electric start, power trim and different shaft lengths change weight. On a tender, check whether you can safely lift and stow the motor.

Choose petrol or electric

A petrol four-stroke usually suits longer journeys, frequent planing and quick refuelling. Electric suits quiet tender trips and short auxiliary runs where charging is dependable.