GearPort
Fender sizing: what size, how many, and where they go
Fenders are the cheapest insurance a boat carries and the easiest thing to under-buy. Too small and they compress to nothing at the moment they are needed; too few and there is always one gap exactly where the piling is.
The sizing question has a straightforward answer, and then a set of reasons to go bigger.
What size
The working rule of thumb for cylindrical fenders is one inch of fender diameter for every four to five feet of boat length. A 24 foot boat lands on roughly a 5 to 6 inch diameter fender, a 32 footer on 6 to 8 inches.
Round or ball fenders are sized differently because their shape does the work: budget roughly two inches of diameter per four to five feet of length. They suit bows, high freeboard and rafting up, where a cylinder would roll out of position.
Then size up, rather than down, if any of the following apply: a heavy displacement boat, a rough or exposed slip, high docks or pilings where a fender has to bridge a bigger gap, significant tidal range, or rafting alongside another boat. The rule of thumb assumes an ordinary slip on a calm day, which is not when fenders matter.
| Boat length | Cylindrical diameter | Round diameter |
|---|---|---|
| Under 20 ft | 4 to 5 in | 8 to 9 in |
| 20 to 25 ft | 5 to 6 in | 9 to 12 in |
| 25 to 35 ft | 6 to 8 in | 12 to 15 in |
| 35 to 45 ft | 8 to 10 in | 15 to 18 in |
| 45 to 60 ft | 10 to 12 in | 18 to 21 in |
Derived from the standard rule of thumb: one inch of cylindrical diameter, or two inches of round diameter, per four to five feet of boat length. Treat it as the calm-day minimum and size up for weight, exposure, high docks, tide or rafting.
How many
The common guidance is one fender per 10 feet of waterline length, with an absolute minimum of three. Three is genuinely the floor even on a small boat: two working the dock side and one spare for the corner, the unexpected raft-up, or the fender that goes over the side.
In practice most owners end up with more than the minimum, because docking is not symmetrical. The bow and the widest point of the hull take the load, and a stern quarter fender earns its place the first time you come in with wind on the beam.
A note on the spare: fenders are consumable. They abrade, they lose air, and they occasionally sacrifice themselves. Having one more than you need is the difference between a scuff and a gelcoat repair.
Cylindrical, round, and flat
Cylindrical fenders with an eye at each end are the default. They hang vertically against a flat hull side or horizontally against a piling, and they stow easily.
Round fenders are better where you need volume rather than a flat contact patch: at the bow, against a high wall, or between two boats rafted together.
Flat fenders and fender pads take up almost no locker space and suit boats with limited stowage, at the cost of less cushioning for the same footprint.
Dock-mounted cushioning solves the problem from the other side. If the boat lives in one slip, protecting the dock edge itself means less depends on getting fenders in exactly the right place every time.
Hanging them so they work
Height is everything. A fender is only doing its job while it is between the hull and the hard thing. Set it so the widest part of the fender sits at the point of contact, which usually means just below the rub rail rather than level with it.
Tie to something structural. A cleat, a rail with a purpose-made fender holder, or a dedicated fender line. Tying to a lifeline or a flimsy rail transfers the load to the wrong place.
Account for the tide. A fender set perfectly at high water can be sitting on top of a piling six hours later. In any tidal berth, either set them for the range or check them.
Use fender covers if the fender rubs against the hull. A bare fender collects grit and polishes a dull patch into the gelcoat over a season; a fleece cover prevents that and keeps black streaks off the topsides.
Take them in under way. Fenders hanging while running is the universal sign of a boat in a hurry, and they get lost that way too.
Fenders and fender covers
Polyform twin-eye fenders, fender covers to keep grit off the gelcoat, and rail-mounted fender holders.
Cleats, chocks and dock hardware
What the fender and the dock line actually tie to: folding and fixed cleats in 316 stainless, chocks, and dock edging.
Common questions
- What size fenders do I need for my boat?
- As a rule of thumb, allow one inch of cylindrical fender diameter for every four to five feet of boat length, so a 24 foot boat wants roughly a 5 to 6 inch fender. Round or ball fenders need about two inches of diameter per four to five feet. Size up for a heavy boat, an exposed slip, high docks, a big tidal range or rafting up.
- How many fenders does a boat need?
- The common guidance is one per 10 feet of waterline length, with an absolute minimum of three for any boat. Most owners carry more, because the bow and the widest point of the hull both take load and a spare is worth having.
- What is the difference between cylindrical and round fenders?
- Cylindrical fenders hang from an eye at each end and give a flat contact patch against a hull side or a piling. Round or ball fenders provide more volume for their contact area, which suits bows, high walls and rafting alongside another boat.
- How high should a fender hang?
- So that the widest part of the fender sits at the point of contact, usually just below the rub rail. In a tidal berth, set them for the range or plan to adjust them, because a fender set at high water can end up above the piling later.
- Do I need fender covers?
- If the fender touches the hull, yes. A bare fender traps grit and will polish a dull patch into gelcoat over a season, and covers also keep black streaks off the topsides.