What size boat fenders should I use for my boat and typical mooring conditions?

For a typical 10 m cruising yacht, start with cylindrical fenders about 200 to 250 mm in diameter and 600 to 700 mm long, choosing the larger diameter for a heavy boat or a berth affected by wash. Carry six to start with, then increase their size, number or coverage to suit the contact points and movement at your berth.

Choose diameter, then length

Check where the hull touches the pontoon, piles or neighbouring boat, and how far that contact point moves as the boat rolls. Diameter provides separation and cushioning; length keeps the contact point protected as the boat moves vertically.

Use these cylindrical fender sizes as starting points for ordinary cruising boats alongside a reasonably sheltered pontoon. Dimensions are diameter × body length; manufacturers measure overall length differently.

• 6 to 8 m boats: approximately 150 to 200 × 450 to 550 mm.
• 8 to 10 m boats: approximately 200 to 250 × 550 to 700 mm.
• 10 to 12 m boats: approximately 250 to 300 × 650 to 800 mm.
• 12 to 15 m boats: approximately 300 to 380 × 750 to 1,000 mm.

Choose the upper end for a heavy displacement yacht, a broad motorboat or an exposed berth. Boat length alone cannot account for weight, topside shape and windage. A light 8 m sportsboat and a heavily laden 8 m cruiser need different margins.

The Hallberg-Rassy 48 Mk I equipment specification, for example, lists six fenders: two 10 × 25 inch and four 12 × 30 inch, approximately 254 × 635 mm and 305 × 762 mm.

Match fenders to the mooring

• Sheltered floating pontoon: cylindrical fenders usually work well because pontoon and boat rise together. Put the largest at the main contact area around maximum beam.
• Wash, swell or an exposed marina: move up a diameter size and add fenders where rolling creates new contact points. Longer fenders help prevent the pontoon edge escaping above or below them.
• Rafting alongside another boat: use generous diameter because both hulls move independently. A large spherical fender is useful where the hulls converge, but check that rubbing strakes cannot catch above it.
• Piles or rough harbour walls: use a fender board outside two suitably spaced fenders to spread contact and bridge gaps. Check it cannot snag as the tide changes.
• Swinging mooring: ordinary hull fenders are generally unnecessary while the boat swings freely. Secure buoy gear so it cannot rub the topsides.