Propulsion and fuel: outboard, inboard, sterndrive, IPS, sail-drive, and diesel vs petrol

Outboard

The engine hangs on the transom. It is easy to service, easy to replace and frees the space below deck. Most powerboats under 10 m use outboards, often twin. Outboards run on petrol, and they leave less room for the swim platform and tender storage. Check the legs for corrosion and the trim system for leaks.

Inboard shaft drive

A diesel engine sits inside the hull and turns a propeller shaft. This is the standard layout for displacement cruisers and for sailboats with a conventional shaft. It is reliable and long-lived: well-maintained marine diesels from the main makers commonly run past 5,000 hours. It also needs space in the hull, and shaft seals and cutlass bearings need periodic attention.

Sterndrive

The engine is inside the hull and the drive leg sits outside the transom. It frees space compared with a shaft and steers by turning the leg. It needs yearly servicing of the bellows and seals, and the leg is exposed to corrosion in warm, salty water.

IPS and pod drives

Volvo Penta's IPS puts forward-facing propellers under the hull, driven by inboard diesels. It gives joystick docking, a clean hull bottom and, according to Volvo Penta, up to 30% lower fuel consumption than a shaft on a comparable boat. Pods are expensive to repair, and servicing needs a specialist dealer. Ask for the dealer's service history.

Sail-drive

Many production sailboats use a sail-drive: a leg that passes through the hull, sealed by a rubber diaphragm. It is quiet and compact. The diaphragm is a service item, and manufacturers commonly ask for replacement about every 7 years. A torn diaphragm can sink a boat. Ask for the date of the last replacement and check the manual for the interval.

Diesel versus petrol

• Diesel: less flammable, more fuel-efficient, longer engine life. Heavier and more expensive to buy. Standard for cruisers with an enclosed engine room.
• Petrol: lighter and cheaper per horsepower. Petrol vapour is heavier than air and settles in the bilge, so a closed engine space needs a blower and a vapour sensor. Common in outboards and on smaller sterndrives.

Which to pick

For a day boat under 40 ft, outboards are hard to beat on cost and simplicity. For a Balearic cruiser, a shaft or IPS diesel gives range and safety. For a sailboat, the sail-drive is normal on modern production boats, so budget for the diaphragm.

Related guides

• Mechanical survey and oil analysis
• Engine hours, and whether they can be trusted
• The annual maintenance schedule and antifouling