By Tom Burden, West Marine Technical Editor
The right fenders in the right size and quantity can mean the difference between a clean gelcoat and a costly repair. This guide gives you a sizing chart by boat length, a simple formula for how many fenders you need, placement recommendations, and a breakdown of every fender type — so you leave with a clear answer, not just general advice.
- How Boat Fenders Work
- How to Choose the Right Size Boat Fender
- How Many Fenders do You Need?
- Where Should You Place Your Fenders?
- Vertical vs. Horizontal Hanging Boat Fenders
- Boat Fender Types
- Matching Fender Colors to Your Boat
- Boat Fender Accessories
How Boat Fenders Work
Corner dock wheels help you get into your slip when crosswinds or currents are a challenge.
Tuff End™ round fenders are great for raft-ups, and are often used by commercial anglers, as mooring buoys or starting line markers for sailing races.
Whether inflated or soft foam, cylindrical, spherical, or flat, fenders absorb the kinetic energy of a vessel moving toward a dock or another boat. They work by compressing under load, which decelerates the boat gradually rather than allowing a sudden hard impact. The larger the fender diameter, the greater the contact area between fender and hull, and the more energy is absorbed before the boat surfaces reach each other. This is why fender diameter — not just quantity — is the primary protection variable: a fender that is too small will compress fully under load and still allow hull contact.
Boats that tie up to floating docks usually set fenders just above the waterline to maintain a buffer zone between dock and boat. Boats that raft together place fenders at the point of contact — generally where the beam is widest. Docking next to pilings presents a special challenge: the pilings tend to push fenders aside as the boat surges, with damaging consequences. The solution is a fender board — a plank that spans the distance between two or more pilings, with fenders placed between the board and the hull. With the fenders and board in place, a boat can ride comfortably against a piling wharf without individual fenders slipping between the pilings.
All the Comforts of Home
Boats with permanent slips often customize their space, installing dock guards, dock wheels, or cushions so that the dock itself carries the protection rather than requiring the boat to carry fenders at all times. Since these forms of padding are fixed at the point of contact, they won’t swing out of position. Dock cushions can prevent damage when fenders aren’t lined up precisely. With a little creativity, you can defend any style of boat from any kind of docking damage using the combination of dock-mounted and boat-mounted protection that suits your slip.
Sizing Fenders by Boat Length
We like roughly 1” of diameter for cylindrical fenders or 2” of diameter for spherical fenders for every four-to-five feet of boat length. This is not a firm rule because fender size also depends on location, boat weight, and conditions. Boats in unprotected end ties on Lake Erie, San Francisco Bay, or other locations with nasty chop or tidal surge will need more protection than a vessel docked in a calm lake. When in doubt, choose the next size up — it is only the width of the fender that separates your hull from the dock.
Choosing the Right Size Boat Fender
| Boat Length In Feet | Cylindrical Fender Diameter | Round Fender Diameter | Flat Fender Size |
|---|---|---|---|
| Under 10' | 3” | NR | Small |
| 10'–15' | 3”–4” | 9” | Small |
| 15'–20' | 5”–5.5” | 12” | Small |
| 20'–25' | 6”–6.5” | 15” | Medium |
| 25'–35' | 8”–8.5” | 18” | Medium |
| 35'–50' | 10.5”–12” | 21” | Large |
| 50'–60' | 12” | 27” | Large |
| 60'–72' | 15”–24” | 27” | Large |
How Many Fenders Do You Need?
The general rule of thumb is: one fender for every 10 feet of boat length, plus one additional fender.
Breakdown:
- Small Boats (up to 20 feet): 3 fenders (2 based on length + 1 additional)
- Medium Boats (20–30 feet): 4 fenders (3 based on length + 1 additional)
- Larger Boats (30–40 feet): 5 fenders (4 based on length + 1 additional)
For a 30-foot boat, you would typically need 4 fenders: 3 for the 30-foot length plus 1 additional. This rule of thumb provides a good starting point, but the actual number may vary depending on specific docking conditions, the type of dock, and how the boat will be tied up. Add extra fenders for exposed docks, tidal surge, or rough conditions — these are minimums, not maximums.
Where Should You Place Your Fenders?
For most recreational boat dockings, position fenders at three points along the docking side of the hull:
- Bow: One fender near the bow to protect the forward section as the boat approaches the dock.
- Midship: One or two fenders along the midship area, where the hull is widest and makes the most consistent contact with the dock.
- Stern: One fender near the stern to protect the aft section, particularly when the boat pivots during docking.
For piling docks, add a fender board — a horizontal plank that spans at least two pilings, with fenders placed between the board and the hull. Without a fender board, cylindrical fenders will be pushed between or around the pilings as the boat surges, providing no protection at the actual contact point. The fender board transfers the contact load to a flat surface that the fenders can hold against reliably.
Height matters as much as position. Hang fenders so their center is at or slightly above the widest point of the hull at the point of dock contact. A fender set too high or too low will ride over or under the dock edge when the boat surges or as tide changes. Fender whips or height adjusters make it easy to set and hold the correct height without retying every time.
Vertical vs. Horizontal Hanging Boat Fenders
Specialty fenders are made to protect specific parts of your boat, like this fence saver made specifically for pontoon boats.
We offer many specialized fenders to protect your boat, like this transom fender used on a swimstep.
Hang fenders vertically for most dock situations. A vertically hung cylindrical fender presents its full length as a buffer zone, protecting a broader section of hull against a flat dock face or dock edge. Vertical hanging is the standard configuration for floating docks and fixed docks with flat faces.
Hang fenders horizontally when docking against a high dock or bulkhead where the boat rises and falls significantly with tidal change, wave action, or surge. A horizontally hung fender spans a fixed height and allows the hull to slide up and down against it without the fender riding off the dock edge as the boat moves vertically. Horizontal hanging is also effective against rough pilings or dock edges with significant surface variation, where a vertical fender might catch and flip.
Center rope tube fenders (Big B fenders) allow you to hang the fender either vertically or horizontally using one line running through the centerline. A figure eight knot in the end of the line, or figure eights at both ends of the fender, keeps it from sliding. This versatility makes center rope tube fenders practical for boats that dock in varied situations.
Flat fenders are either modular or articulating and vinyl-covered. String the modular style together to create a custom-fitted system. Hinged vinyl-covered flat fenders wrap vertically around small boat gunwales and are particularly well-suited for boats with tumblehome — topsides that slant inward at the gunwale — where a cylindrical fender would roll off the hull rather than staying in contact with the dock.
Boat Fender Types
Round fenders are popular for larger powerboats with concave bows and larger commercial vessels, especially commercial fishing boats. Their spherical shape means they maintain contact with curved surfaces from any angle, making them ideal for raft-ups where two boats are lying alongside each other in open water with no fixed reference point to orient a cylindrical fender against. Orange round fenders are also commonly used as low-tech mooring buoys.
- Best for: larger powerboats with concave bows, raft-ups, commercial use
Two-eye cylindrical fenders have a molded-in fairlead or tab at each end. Tie a line to one end only and hang vertically, or tie both ends for horizontal deployment. This is the most common fender type for general recreational use and fits most standard docking situations.
- Best for: most recreational boats, versatile vertical or horizontal use
Transom-mounted fenders protect a boat’s transom or swim step when docking stern-to in Mediterranean-style marinas, where the boat backs into the slip and ties off at the stern to a dock or quay.
- Best for: Mediterranean-style stern-to docking
Rafting cushions are designed specifically for rafting boats together. They can also be tied to pilings using two rows of center holes. Their large surface area, six independent molded tubes, and versatile mounting options mean they won’t roll or ride up when the boats move relative to each other.
Low freeboard fenders sit high on the hull and hang inward over the gunwale. The smaller size protects boats that sit low in the water — bass, fishing, and ski boats. The larger size prevents bigger boats from riding up under high docks when tide rises.
- Best for: bass boats, ski boats, fishing boats that sit low in the water
Tuff End fenders are rugged, large-diameter, heavy-duty fenders with good protection for flared, deep-V hulls. They are popular for large boats and commercial applications. The Trivalve design can be inflated easily with any type of pump.
V-shaped Freedom Fenders have a self-centering shape that keeps them on a piling regardless of boat movement. They are sized for everything from small boats to mega yachts and eliminate the frustration of fenders that slide off pilings as the boat surges.
- Best for: piling docks where standard fenders slide off
Pontoon fenders — such as the Fence Saver II, Pontoon Curve Protector, and Pontoon Protector 90 — are available in specialized shapes that protect the delicate aluminum fencing and unusual corners on pontoon boats, which standard cylindrical fenders cannot reach effectively.
- Best for: pontoon boats only — protects aluminum fencing and corners
| Boat Length | Cylindrical Diameter | Round Diameter | Flat Size | Recommended Qty |
|---|---|---|---|---|
| Under 10' | 3” | NR | Small | 2 |
| 10'–15' | 3”–4” | 9” | Small | 3 |
| 15'–20' | 5”–5.5” | 12” | Small | 3 |
| 20'–25' | 6”–6.5” | 15” | Medium | 4 |
| 25'–35' | 8”–8.5” | 18” | Medium | 4 |
| 35'–50' | 10.5”–12” | 21” | Large | 5 |
| 50'–60' | 12” | 27” | Large | 6 |
| 60'–72' | 15”–24” | 27” | Large | 7 |
NR = Not recommended. Quantities are minimums — add extra fenders for exposed docks, tidal surge, or rough conditions.
Attach our fender rack to a stanchion.
Matching Fender Colors to Your Boat
We have a wide variety of fender colors that will complement your boat’s color scheme, available in multiple fender styles. White is the most common choice for white-hulled boats, but colored fenders — navy, red, black, and others — are available to match or accent virtually any gelcoat. Fender covers are available to protect the fender surface from UV degradation and keep them cleaner between uses, and can be embroidered with the boat’s name for a personalized finish.
Boat Fender Accessories
There is a use for all of these on somebody’s boat:
Fender covers keep your fenders clean and protected from UV. Some boats have theirs embroidered with the boat’s name for a posh accent.
Fender racks allow you to store one or two fenders on lifeline stanchion tubing (7/8” or 1”), keeping them accessible for rapid deployment without cluttering the deck.
Hand-operated fender pumps keep your inflatable fenders at optimum pressure. Check fender pressure at the start of each season and any time a fender feels noticeably softer than usual — an under-inflated fender compresses fully under load and provides less protection than a properly inflated one.
We also carry a complete assortment of fender accessories: hangers, lifeline clips, fender adjusters, fender straps, fender cleats, locks, hooks, and suction cup straps. A dedicated fender cleaner removes grease, oil, and grime without damaging the vinyl.
Frequently Asked Questions
What size fender do I need for my boat?
Use roughly 1 inch of diameter for every 4–5 feet of boat length for cylindrical fenders, or 2 inches of diameter for spherical fenders. A 25-foot boat typically needs a 6–6.5 inch cylindrical fender or a 15-inch round fender. These are minimums — if your dock is exposed to wave action, tidal surge, or frequent vessel wake, size up to the next diameter. It is only the width of the fender that separates your hull from the dock.
How many fenders do I need?
The general rule is one fender per 10 feet of boat length, plus one extra. A 30-foot boat needs 4 fenders minimum; a 20-foot boat needs 3. Add additional fenders for piling docks (which require a fender board setup), exposed slips, or rough conditions. Carrying one or two extra fenders aboard for unexpected raft-ups or unfamiliar docks is good practice for any cruising boat.
Where should fenders be placed?
One near the bow, one or two amidships where the hull makes most contact with the dock, and one near the stern. Height matters as much as horizontal position — the center of each fender should be at or slightly above the widest point of the hull at the contact zone. Use fender whips or height adjusters to maintain correct height as tide changes. For piling docks, use a fender board spanning the pilings so fenders cannot be pushed between them by the boat’s surging movement.
Should fenders hang vertically or horizontally?
Vertically for most dock situations — a vertically hung cylindrical fender presents its full length as a buffer zone against a flat dock face. Horizontally when docking against a high dock, bulkhead, or fixed structure where the boat rises and falls significantly with tide, wake, or surge. A horizontal fender allows the hull to slide up and down without riding the fender off the dock edge. For piling docks, a fender board with fenders behind it eliminates the question entirely.
What is a fender board and when do I need one?
A fender board is a plank — typically 6 to 8 feet long — that spans two or more pilings, with fenders placed between the board and the hull. Without a fender board on a piling dock, cylindrical fenders get pushed aside between the pilings as the boat surges, leaving bare hull against bare piling. The fender board transfers the contact load to a flat surface that the fenders can hold against effectively. Any time you are docking alongside a row of pilings rather than a flat dock face, a fender board is the right solution.
What is the difference between inflatable and foam fenders?
Inflatable vinyl fenders are adjustable for firmness, store compactly when deflated, and are the most common type for recreational boats. Foam fenders require no maintenance, cannot go flat, and are genuinely maintenance-free — making them more reliable in situations where you cannot monitor inflation, such as a mooring ball setup or a boat left unattended for extended periods. Foam fenders are also used as dock bumpers and permanent dock-mounted protection. The tradeoff is that foam fenders are bulkier to store than deflated inflatables.
How firm should my inflatable fenders be?
Fenders should be firm enough that you cannot compress them significantly by hand but not so hard that they feel rigid. An overly hard fender transmits impact rather than absorbing it, reducing its protective value. An under-inflated fender compresses fully under load and allows hull contact. The correct pressure varies by fender manufacturer and model — check the manufacturer’s specification, which is typically 2–3 PSI for most recreational cylindrical fenders. Use a fender pump with a pressure gauge for consistent results. Check pressure at the start of each season and after any extended period in the sun, as UV and heat can cause fenders to lose or gain pressure.