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Start with your boat’s length, then adjust for weight, hull-to-dock contact, dock design and exposure. As a starting rule, choose about 1 inch of cylindrical-fender diameter for every 4–5 feet of boat length. That puts a 20–25-foot boat at roughly 6–6.5 inches and a 25–35-foot boat at 8–8.5 inches. Use a larger size or additional fenders when the berth is exposed to chop, wakes, tidal surge or heavy boat movement.
Use length to choose a starting diameter
Boat length gives you the first sizing range, not a final answer. The commonly used rule is approximately 1 inch of cylindrical-fender diameter for each 4–5 feet of boat length. For round fenders, the comparable rule is about 2 inches of diameter for each 4–5 feet of boat length. West Marine and Lippert publish the following general leisure-boat chart.
| Boat length | Cylindrical diameter | Round diameter | Flat fender size |
|---|---|---|---|
| Under 10 ft | 3 in | Not recommended | Small |
| 10–15 ft | 3–4 in | 9 in | Small |
| 15–20 ft | 5–5.5 in | 12 in | Small |
| 20–25 ft | 6–6.5 in | 15 in | Medium |
| 25–35 ft | 8–8.5 in | 18 in | Medium |
| 35–50 ft | 10.5–12 in | 21 in | Large |
| 50–60 ft | 12 in | 27 in | Large |
| 60–72 ft | 15–24 in | 27 in | Large |
The chart is a selection guide, not an engineering calculation for commercial berthing energy. A boat at the top of a length band, a heavy displacement boat and a boat in an exposed slip may need the next larger diameter. West Marine’s guide expresses the practical rule as: “When in doubt, choose the next size up.” That advice is a sizing margin, not a guarantee that any fender will prevent damage in every berth.
Adjust the size for weight and conditions
Boat weight and hull shape
Two boats of the same length can impose very different loads on a fender. A heavy displacement hull has more momentum when it comes alongside than a light planing boat. Concentrated contact at a narrow chine, rail or hard hull feature also needs more cushioning than a broad, smooth side. Treat the length chart as the minimum starting point and move up a diameter when the boat is unusually heavy or the contact area is small.
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Exposure to chop, wakes and tidal movement
In a sheltered slip with little water movement, the chart size may be adequate. In an exposed berth, the boat can repeatedly load the fenders as waves, wakes or wind push it toward the dock. Tidal rise and fall can also move the contact point away from a fender that was correctly positioned at high or low water. A larger fender provides more stand-off and reserve capacity; additional fenders can protect contact points that move as the boat surges.
When to size up
- The slip is open to fetch, wakes or regular chop.
- The boat is heavy for its length or carries a substantial load.
- The dock face is high, irregular or difficult to keep aligned with a small fender.
- The hull has a pronounced chine or another small contact area.
- You cannot keep a fender from bottoming out or being pushed above the contact zone.
If any of these conditions apply, choose the next diameter shown in the chart before adding complexity to the rigging. A larger fender is harder to stow and handle, so confirm that your cleats, rails and storage spaces can accommodate it.
Choose the shape that matches the contact geometry
Cylindrical fenders
Cylindrical fenders are the general-purpose choice for most recreational boats. They hang vertically or horizontally from a line and present a long contact surface along a dock face. Their diameters are the figures in the main sizing chart, so they are the easiest option when you know your boat length and need a standard starting point.
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Round fenders
Round, or spherical, fenders concentrate buoyant volume in a compact shape. They can work where a boat needs a large cushion at a particular point, but their diameter ranges differ from cylindrical models. For example, the chart calls for a 15-inch round fender for a 20–25-foot boat and an 18-inch round fender for a 25–35-foot boat. Do not substitute a round fender’s number for the cylindrical diameter; they are different shapes with different contact footprints.
Flat fenders
Flat fenders are useful when storage space is limited or when the boat needs a broad, shallow pad against a surface. The chart labels them small for boats under 20 feet, medium for 20–35 feet and large for 35 feet and longer. Because the chart gives size categories rather than dimensions, use the specific maker’s measurements to verify that the pad covers the actual contact area.
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Special hull and dock arrangements
A specialized hull, such as a pontoon, may need a fender designed for its rails or deck height rather than a generic cylindrical model. Likewise, a piling dock may require more than individual fenders: a fender board spanning the pilings can keep a fender from being forced into the gap. The right shape is therefore determined by where the boat touches, not by length alone.
How many fenders do you need?
Two common rules of thumb
West Marine recommends one fender for every 10 feet of boat length, plus one additional fender. Its examples are three fenders for boats up to 20 feet and four for boats from 20 to 30 feet. Lippert states a related rule using waterline length per side, with at least three fenders. These rules count differently, so neither should be treated as a universal formula.
| Method | How it is counted | Published example or minimum |
|---|---|---|
| West Marine | One per 10 feet of boat length, plus one | Three for boats up to 20 ft; four for 20–30 ft |
| Lippert | One per 10 feet of waterline on each side | At least three fenders |
Use the berth, not arithmetic alone, to settle the quantity. A boat that can contact a floating dock at the bow, stern and amidships may need three well-positioned fenders. A boat that lies alongside a long fixed dock, shares a berth with another boat or experiences strong surge may need more. Keep enough fenders to cover every likely contact point on the side that faces the dock, and add protection on the opposite side when maneuvering or rafting makes contact possible.
Where to place them
- Bow: Protect the forward section where the bow shoulder or flare can swing toward the dock.
- Amidships: Cover the broadest or heaviest part of the hull, usually the first area to settle against a straight dock face.
- Stern: Protect the aft quarter, especially where a current or wind pivots the boat.
Space the fenders so their protected areas overlap as the boat moves. A fender that hangs several feet away from the actual contact point contributes little, even if its diameter is correct.
Set the height and orientation for the dock
Vertical hanging for most flat faces
Vertical hanging is the normal arrangement against a flat dock face. Set the fender so its center and lower section cover the part of the hull that actually meets the dock. Do not assume the rub rail is the only contact point: wave action can lift the boat, and a falling tide can expose a lower section of hull.
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Horizontal hanging for vertical movement
Horizontal hanging can help where the boat moves up and down against a high dock or bulkhead. A horizontal cylinder presents a longer protected run and is less likely to leave a gap when the contact point shifts vertically. The correct orientation depends on movement and geometry; it is not a rating advantage by itself.
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Use fender lines or adjusters that let you change height quickly. Recheck the setting after a tide change, after loading the boat, and whenever the water level or dock position changes. Lines should hold the fender in place without pulling it so tight that it cannot roll or absorb movement.
Use a fender board at pilings
Individual fenders can be squeezed between pilings when a boat moves laterally. A board spanning two or more pilings distributes the load across the gap and gives the fenders a continuous surface to bear against. Secure the board so it remains aligned with the likely contact zone, and size the fenders behind it for the boat’s length and exposure.
Check the product’s specifications before inflating or rigging
Inflation pressure is model-specific
Inflatable fenders are not all designed for the same pressure. Taylor Made lists 2 PSI as the recommended inflation pressure for its Tuff End line. That figure applies to that product line; it should not be copied to another brand or model. Follow the pressure, valve and dimensional instructions supplied with the fender you own, and use a pressure gauge rather than judging by hand.
Verify actual dimensions and line attachment
Nominal sizes can vary in length, eye placement and usable contact area. Before buying, compare the maker’s outside diameter and length with the chart range, then confirm that your fender line fits the eyes and can reach the required cleat or rail. Polyform’s fender-line guidance and each manufacturer’s own instructions are useful for matching line size and attachment method.
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Balance protection against handling
A larger fender generally gives more stand-off, but it also takes more storage space, weighs more and can be harder to move while docking. Measure lockers, rails and the distance between cleats before committing to an oversized set. If the berth is exposed, the protection margin usually matters more than compact storage; in a sheltered berth, a correctly positioned chart-size fender may be easier to manage.
A step-by-step sizing method
- Measure the boat’s length. Use the boat length used by the manufacturer or the length relevant to the berth, then identify the matching row in the chart.
- Select a starting shape. Choose cylindrical for general dock contact unless the berth requires a round, flat or specialized design.
- Read the starting diameter. For example, a 22-foot boat starts at 6–6.5 inches cylindrical; a 30-foot boat starts at 8–8.5 inches.
- Assess mass and contact area. Move to the next size when the boat is heavy for its length, has concentrated hull contact or repeatedly loads the dock.
- Assess the berth. Increase size or quantity for chop, wakes, tidal surge, a high dock, bulkhead or other geometry that changes the contact point.
- Count the likely contact points. Apply the one-per-10-feet-plus-one estimate or the waterline-per-side estimate, then add protection where the actual berth demands it.
- Choose orientation and height. Hang vertically against most flat faces; consider horizontal hanging where vertical movement against a high dock is the problem.
- Test the setup at different water levels. Confirm that bow, amidships and stern remain protected as the boat rises, falls and surges.
- Check the model instructions. Confirm dimensions, inflation pressure, line compatibility and attachment points for the exact fender, not just the category.
Worked examples
20-foot boat in a sheltered slip
The chart starts at 6–6.5 inches for a 20–25-foot boat. A cylindrical set with three fenders follows West Marine’s example for boats up to 20 feet. Hang one forward, one amidships and one aft, then adjust the heights so the rub rail and lower hull are covered at the slip’s normal water level.
24-foot boat in an exposed marina
The same length range still starts at 6–6.5 inches, but repeated wake or chop argues for the next larger available diameter or additional fenders. Use at least the bow, amidships and stern positions, and check the setup during wake events and tidal changes. A fender board may be necessary if the berth uses open pilings.
30-foot heavy boat alongside a fixed dock
The chart starts at 8–8.5 inches for a 25–35-foot boat. Because the boat is heavy and the fixed dock may create concentrated contact, select the upper end of the range or move up a size if the maker offers one. Four fenders meets West Marine’s 20–30-foot example; distribute them to cover both quarters and the central hull rather than clustering them at one end.
40-foot boat with a high bulkhead
The chart gives a 10.5–12-inch cylindrical range for 35–50 feet. A horizontal arrangement can maintain coverage when the hull moves vertically against the bulkhead. Use the manufacturer’s dimensions to ensure the fender is long enough to protect the contact run, and provide additional units if the boat surges along the wall.
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Common sizing mistakes and corrections
Choosing by boat length only
Length does not account for displacement, hull shape or exposure. Correct the mistake by reviewing weight and the actual contact geometry before ordering, then size up when those factors increase loading.
Using the wrong diameter for the shape
A 15-inch round fender is not equivalent to a 15-inch cylindrical fender. Use the row for the shape you intend to buy and compare the maker’s actual dimensions.
Installing too few fenders
One large fender cannot protect bow, middle and stern simultaneously when the boat pivots or surges. Count likely contact points and use the published quantity rules only as a starting estimate.
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Leaving fenders at one fixed height
A setting that works at one tide can leave the hull exposed later. Use adjustable lines and inspect the contact zone after water-level changes, loading or a change in dock position.
Letting a fender fall between pilings
Open gaps can trap or squeeze an individual fender. Span the pilings with a properly secured fender board, then place the fenders against the board.
Inflating by feel or copying another model’s PSI
Pressure recommendations are product-specific. Follow the exact maker’s specification; Taylor Made’s 2 PSI recommendation for Tuff End fenders is not a universal setting.
Final decision checklist
- Boat length is matched to the correct cylindrical, round or flat size range.
- Boat weight and concentrated hull contact have been considered.
- Chop, wakes, wind, tide and surge have been considered.
- The fender shape fits the dock face, bulkhead or piling arrangement.
- Enough units cover the bow, amidships and stern contact zones.
- Height and orientation can be adjusted as the water level changes.
- A fender board is planned where pilings could trap an individual fender.
- The exact product’s dimensions, inflation pressure and line requirements have been checked.
For most boats, begin with the charted cylindrical diameter, then move up when weight or exposure increases. Match the shape and orientation to the place the hull actually touches, and treat the quantity formulas as estimates that must be verified against the berth. That approach gives you a defensible fender setup without pretending that boat length alone can predict every docking load.
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