What size fenders should I use for my boat?

For a typical 10-metre cruising boat, start with cylindrical fenders around 200 to 250 mm in diameter and 600 to 700 mm long, choosing the larger size for a heavy boat or an exposed berth. Choose diameter first, then check that the fender covers the hull's contact area as the boat rolls and changes height against the pontoon.

Check the berth's contact points

Before buying, check where the boat touches the pontoon, piles or neighbouring hull. A fender must stay between those surfaces as the boat moves. Even a large one offers little protection if it hangs below the pontoon edge or rides up over it.

Boat length gives a starting point, but displacement, topside shape and exposure determine how much cushioning you need. A heavy cruising yacht needs more protection than a light sportsboat of the same length. A motorboat with high topsides may need longer, wider fenders because its windage pushes it harder against the berth.

If your existing fenders squash flat, check for leaks and correct inflation before replacing them. Persistent flattening can also mean the fenders are too small, too few are sharing the load, or the mooring lines allow excessive movement.

Choose diameter before length

Use these approximate cylindrical fender sizes for ordinary marina berthing. Dimensions give diameter first, then overall length; manufacturers' proportions vary.

• Boats up to 6 metres: roughly 120 to 150 mm diameter and 400 to 500 mm long.
• 6 to 8 metres: roughly 150 to 200 mm diameter and 500 to 600 mm long.
• 8 to 10 metres: roughly 200 to 250 mm diameter and 600 to 700 mm long.
• 10 to 12 metres: roughly 250 to 300 mm diameter and 700 to 800 mm long.
• 12 to 15 metres: roughly 300 to 350 mm diameter and 800 to 1,000 mm long.

These are starting sizes, not load ratings. Move up a size for substantial displacement, regular rafting or a berth affected by wash and swell. Beyond 15 metres, use the manufacturer's selection guide, taking account of the boat's displacement and berthing conditions.

Diameter gives the fender room to compress before the hull reaches the obstruction. Extra length mainly improves vertical coverage: a long, skinny fender cannot replace a sufficiently wide one. Polyform, Majoni and DAN-FENDER offer suitable ranges, but compare actual dimensions rather than model numbers.

Choose numbers and shapes

For an 8 to 12 metre cruiser, six cylindrical fenders make a useful cruising inventory: three for the working side, with the others available for the opposite side or an unexpected contact point. Four along the working side may suit a longer boat or an awkward berth. All six need not hang permanently on one side.