Boat basics
What Is a Fender on a Boat? Types, Sizes and Uses
Published · Last checked October 2026
A fender is an air-filled or foam cushion hung over the side of a boat to stop the hull rubbing or banging against a pontoon, harbour wall or another boat. Most yachts carry four to six cylindrical fenders, plus perhaps a flat fender board or a large ball fender. As a rough rule, a fender should be about 2.5 cm in diameter for every 30 cm (1 ft) of boat length, so a 10 m yacht wants fenders around 20 to 25 cm across.
What fenders do
Glassfibre gelcoat scratches easily and a boat moving in a swell can hit a wall with surprising force. Fenders absorb that energy and spread the load. They are rigged before arriving at a berth, while rafting up or when going alongside another vessel, and taken in once you leave: sailing with fenders dangling is the classic sign of a careless crew.
Common types
| Type | Shape | Best for |
|---|---|---|
| Cylindrical | Long sausage with an eye at one or both ends | General use on pontoons and alongside boats |
| Ball / round | Large sphere | Bow or stern protection, heavy surge, big topsides |
| Flat | Thin rectangular pad | Boats with little freeboard, stowing in small lockers |
| Fender board | Plank hung outside two fenders | Rough walls and piles where fenders would roll off |
| Fender step | Fender with a built-in step | Climbing ashore from high topsides |
How many and how big
- Number: at least four for a small cruiser, six or more for a 10 to 12 m yacht, with extra if you plan to raft up.
- Diameter: about 2.5 cm per 30 cm of length is a common starting point. Go bigger if you berth against walls or in exposed harbours.
- Length: long enough to cover the gap between the hull and the dock at the widest point of the topsides.
- Lines: each fender needs its own line, long enough to tie off with a clove hitch or round turn.
Looking after fenders
Keep them clean, as grit on a fender acts like sandpaper on the hull. Fender covers reduce marking and squeaking. Inflatable fenders can be topped up with a pump through the valve. Replace frayed lines before they part, which they usually do at the worst moment.
See how to tie fenders to a boat for setting the height and knots.