Every retail fender size guide uses the same heuristic: one inch of cylindrical fender diameter for every four to five feet of yacht length overall. It's repeated by Jimmy Green Marine, Polyform US, Lippert, anchoring.com, and every fender retailer that publishes a chart. It's the industry standard.
It's also not an engineering specification. The rule has no documented origin, no ISO backing, and no equivalence to commercial berth fender design methodology (PIANC WG211). It works well enough for sub-60-foot pleasure craft where displacement and length scale together — and breaks down at superyacht scale where displacement, freeboard, and berthing environment matter as much as overall length.
This is the guide we use when sizing fenders for 30-metre to 100-metre-plus yachts. It cross-references the published charts from five major suppliers (SEARAFT, Fendequip, Polyform, AERÉ, Taylor Made), explains the engineering basis behind PIANC's energy-based methodology, and walks through the five variables beyond yacht length that actually drive the right answer.
Quick reference: superyacht fender diameter by yacht length
For inflatable yacht fenders: 18–30 ft yachts use 9 inch (23 cm) diameter; 30–45 ft use 10 inch (25 cm); 45–60 ft use 12 inch (30 cm); 60–100 ft use 18 inch (46 cm); 100–200 ft use 24 inch to 3 ft (61–91 cm); 200–400 ft use 4, 6, or 8 ft (122–244 cm). Final spec depends on displacement, freeboard, hull shape, and berthing environment.
Where the "one inch per four-to-five feet" rule comes from
Every published version of the rule traces back to the same wording: "A cylindrical fender should have one inch of diameter for every four to five feet of boat length, and two inches for a round fender." It appears in Lippert's Boat Fender Size Chart, Polyform US's Boat Fender Guide, Jimmy Green Marine's size guide, and several other retail sources. None of them cite a primary source.
Toprik puts it plainly in their own guide: "no official boat fender size guide exists, and even more so, the parameters of shock absorbers are not regulated by any provision of the maritime regulations." The rule is folklore — a heuristic that calibrates to the typical displacement-per-foot of recreational hulls built in the 1960s and 70s. At sub-60-foot pleasure craft size it's a reasonable approximation. At superyacht scale it's a starting point at best.
The companion rule for fender count is similar: "one fender per 10 ft of waterline length per side, with a minimum of three for even the smallest boats." Polyform US repeats it; so does Lippert; so do retailers across the marine trade. Useful, but not authoritative.
The engineering basis — PIANC and ½mv²
For commercial berths, fender capacity is sized using kinetic energy. Naval architects, shipyards, and port engineers use this; retail buyers don't. The current framework is PIANC WG211 (2024), which replaces the older WG33 (2002):
Ed = 0.5 × M × VB2 × CM × CE × CC × CS
Where M = displacement mass (tonnes), VB = berthing velocity perpendicular to the berth (m/s), and the four coefficients capture added mass of water (CM), eccentricity of impact relative to vessel centre-of-gravity (CE), berth-configuration cushioning (CC), and hull-fender softness (CS, typically 0.9–1.0; 1.0 for most yacht-fender combinations). The 2024 revision replaced the WG33 global safety factor with partial safety factors derived from statistical berthing-velocity data.
PIANC is calibrated for commercial seagoing vessels — typical berthing velocity 0.1–0.2 m/s with tug assistance or thruster control. PIANC WG211 advises that characteristic velocities below 0.10 m/s should not be assumed without site data — a practical lower bound for thruster-assisted superyacht berthing. The standard does not publish a velocity table specifically for yachts and small craft, which means any specific velocity number quoted for a yacht (for example "0.15 m/s for a 50m M/Y") is a working estimate rather than a standardised figure.
The only published yacht-fender datasheet that quotes absorbed energy in engineering units is Polyform Norway's F-13: 75 cm diameter, 188 cm length, 23 kg, rated to absorb up to 3.8 tonne-metres of energy, sized for vessels up to 1,500 tonnes deadweight. This is a useful sanity-check number: a 50-metre yacht with roughly 500–800 tonnes displacement, berthing at 0.15 m/s, transfers around 0.4–0.7 tonne-metres of energy on contact — well within the F-13's rated capacity. Anything smaller than F-11 size (roughly 18 inch diameter, 50 inch length) starts to look marginal on a 50-metre yacht.
Brand-by-brand size charts cross-referenced
Five major brands publish size guidance for yachts above 30 metres. Below the 60-metre bracket, they converge on the same diameter recommendations. Above 60 metres, they diverge — reflecting both the lack of an industry standard and the reality that fender selection on 50m+ yachts is typically a confidential build-spec decision between the yard, captain, and brand reps.
| Yacht LOA | SEARAFT | AERÉ | Fendequip maxiStow | Taylor Made Super-Duty | Polyform |
|---|---|---|---|---|---|
| 5–10 m | 9" / 23 cm | 9" × 22" (2/side) | 30 cm cylinder | — | G-2/G-3, NF-3 |
| 10–14 m | 10" / 25 cm | 10–12" × 26–42" | 30–42 cm cylinder | — | F-1 to F-4 |
| 14–18 m | 12" / 30 cm | 12" × 42–58" | 42 cm cylinder | — | F-5 to F-8, HTM-3 |
| 18–30 m | 18" / 46 cm | 18" × 58" (3–4) | 60 cm cylinder + 100 cm sphere | 18" × 29–58" | F-7 to F-11, A-4/A-5 |
| 30–60 m | 24–36" / 61–91 cm | 24–36" × 58–72" | 90–120 cm cylinder + 150 cm sphere | 24" × 42–58", 36" × 48" | F-13 + custom |
| 60–120 m | 4'/6'/8' (122/183/244 cm) | 36–48" × 84–120" | 150–270 cm cylinder, 200 cm sphere | Custom only | Custom only |
Sources: SEARAFT 2025 Fender Brochure; AERÉ Fender Size Guide; Fendequip Size Guide; Taylor Made Super-Duty; Polyform US F-series; Polyform Norway guide.
Where they agree
Every brand puts a 100 ft / 30 m yacht at roughly 18 inch / 46 cm diameter, and a 200 ft / 60 m yacht at roughly 24 inch / 61 cm. These are the sweet-spot ranges where industry convergence reflects decades of operational experience.
Where they diverge
- AERÉ scales further than other manufacturers above 60 m, recommending 36–48 inch (91–122 cm) for 200 ft-plus yachts.
- Fendequip is the only brand publishing standard SKUs above 120 cm cylinder diameter — their catalogue extends to 270 cm diameter cylinder and 200 cm sphere.
- Taylor Made's Super-Duty caps at 36 inch diameter; everything above that goes custom-bid.
- Polyform stops publishing public size guidance above the F-13 / 25-metre yacht bracket — F-13 is the last public datasheet, anything bigger is custom-quoted.
For comparison content covering construction differences (drop-stitch vs welded seam) and warranty terms, see our pillar guide on SEARAFT vs Fendequip.
Five variables beyond LOA that actually drive the answer
LOA is the starting point. These five factors materially change the right answer, and at superyacht scale they often matter more than the headline number.
1. Displacement
Impact energy scales with mass, not length. A 50-metre steel-hulled explorer (~700 tonnes) at the same berthing velocity carries roughly twice the impact energy of a 50-metre composite sport yacht (~350 tonnes). The PIANC formula makes this explicit: doubling M doubles Ed at constant VB. Heavy-displacement yachts should size up one bracket from the LOA recommendation.
2. Freeboard
Two yachts of equal LOA but different freeboard need different fender lengths. A 50-metre explorer with 4 m freeboard needs a longer fender than a 50-metre sport yacht with 2.5 m freeboard, because the fender must span from the rail attachment point down to the contact line on the dock. AERÉ's table reflects this by holding diameter constant within a bracket while lengthening fenders for higher-freeboard configurations.
3. Hull shape
Flared bows defeat cylindrical fenders — a flared hull pushes the cylinder up and out of the way. Spherical fenders lock into the geometry and stay in place. Plumb-bow yachts have the opposite problem: cylinders ride well, spheres roll. The practical heuristic from captain forums is consistent: spheres at bow and stern for flared hulls and rafting positions; cylinders alongside for plumb-bow yachts against fixed piers.
The 1-inch-per-4-feet rule also appears in retail guides from Jimmy Green Marine and Toprik, which honestly admits "no official boat fender size guide exists, and even more so, the parameters of shock absorbers are not regulated by any provision of the maritime regulations."
4. Mooring environment
The standard rule assumes calm, average conditions. Exposed berths, lock walls, commercial piers, and Med-moor stern-to with surge all justify sizing up one bracket. A yacht berthing routinely in Vilanova or Tarragona doesn't need the same fender as one alongside a quiet IGY berth in Sète.
5. Tidal range and surge
Vertical fender travel matters in high-tide locations. Longer cylinders handle tidal range better than balls because they keep contact through more vertical movement. For locations with significant surge (Atlantic crossings, exposed Med winter berths) consider the bracket-up principle and use the longer length within a diameter class.
Operational reality
Most superyacht captains carry one set sized to the "average" use case and a second smaller set for tender-side and stern-platform protection. The five variables matter most when you're sizing your primary set, which sees daily berthing impact through a 22-week Med charter season.
How many fenders does a superyacht need?
The industry rule is one fender per 10 ft of waterline length per side, minimum three. AERÉ publishes the cleanest count breakdown:
| Yacht LOA | Fenders per side | Plus bow/stern balls |
|---|---|---|
| 18–30 ft / 5–10 m | 2 | 0–1 |
| 30–45 ft / 10–14 m | 2–3 | 1 |
| 45–60 ft / 14–18 m | 3 | 1–2 |
| 60–100 ft / 18–30 m | 3–4 | 2 |
| 100–200 ft / 30–60 m | 4–5 | 2 |
| 200+ ft / 60 m+ | 5+ | 2–4 |
For a 50-metre yacht (~165 ft) this lands at 4–5 cylinders per side plus 2 spheres for bow and stern — a working set of roughly 10–14 fenders carried on board. Charter yachts of 40–60 m typically carry one or two spare spheres on top of the per-side count specifically for rafting and tender-side use.
Rafting up adds requirements. Polyform's guidance: "more fenders at maximum beam, with two round fenders set fore and aft." Most superyacht charter loadouts plan for at least one rafting scenario per season — the spare spheres pay for themselves on the first deployment.
Charter operations vs owner trips — different sizing logic
An active Mediterranean charter season (typically May to October) puts each fender through roughly 150 impact cycles per year — Mercer's working planning figure based on captain feedback, not an industry-published number. Owner-only operations doing 4–6 weeks of summer cruising see 30–50 cycles. This drives both the size and the construction choice.
For charter operations:
- Size up one bracket from the LOA baseline — expected impact frequency justifies the margin
- Plan for 1–2 spare units per yacht to cover damage between charters
- Favour inflatable construction (SEARAFT, Fendequip) so the full set stows below decks during Caribbean transit
- Match cover colours and personalisation across the full inventory — charter visual standards matter
For owner-only operations:
- Size to LOA baseline; the bracket-up margin is unnecessary at 30–50 cycles per year
- Smaller spare allocation (1 spare across the fleet, not per yacht)
- Construction less critical if storage isn't a constraint — foam fenders remain valid if you don't need to deflate for transit
How Mercer Yachting sizes for clients
We don't read a chart and ship. We start with yacht length, displacement, freeboard, home port, typical berthing environment, and the captain's existing fender experience. We cross-reference the brand size charts above, apply the five variables, and come back with a specific configuration — usually with two options at different price points.
We purchase direct from the SEARAFT factory in Nijmegen and the Fendequip factory in the South West UK. No authorised-dealer commitments — the recommendation depends on the yacht, not the supplier relationship. Transparent pricing on factory cost. Mediterranean delivery within 4–7 days of dispatch.
Tell us the yacht and we'll come back within four business hours.
Frequently asked questions
What size fender do I need for a 50m yacht?
A 50m (~165 ft) yacht typically uses fenders in the 18 inch (46 cm) to 24 inch (61 cm) diameter range, with lengths from 58 inch (147 cm) to 96 inch (244 cm). Final spec depends on displacement, freeboard, hull shape, and typical berthing environment. SEARAFT recommends 18 inch for 60–100 ft and 24 inch for 100–200 ft brackets; AERÉ and Polyform sit in the same range.
How many fenders does a superyacht need?
One fender per 10 ft of waterline length per side, minimum three. For superyachts: 18–30m yachts typically carry 2–3 per side; 30–50m carry 3–4; 50–80m carry 4–5; 80m+ carry 5 or more. Plus two ball fenders for bow and stern rafting positions. Most charter yachts carry one or two spare spheres beyond the per-side count.
Is there an ISO standard for yacht fender sizing?
No. ISO 17357 covers pneumatic rubber fenders for commercial ship-to-ship operations, not yacht docking fenders. The relevant framework is PIANC WG211 (2024), which uses kinetic energy methodology, but PIANC does not publish yacht-specific berthing velocity values. The "1 inch per 4–5 ft" retail rule has no formal authority.
Does fender size scale with yacht length or displacement?
Engineering basis is displacement. Impact energy is ½ × mass × velocity², so a 60m steel explorer (~800 tonnes) needs fenders sized for 800 tonnes of momentum even though it shares LOA with a 60m composite sport yacht (~400 tonnes). Retail size charts use LOA as a shorthand — accurate for production yachts, inadequate for heavy-displacement custom builds.
What's the difference between cylindrical and ball fenders?
Cylindrical fenders work alongside fixed piers and quays where the hull contact area is broad and roughly vertical. Ball fenders work at bow, stern, and quarter positions where the hull curves away from the dock, and during rafting between two yachts. Most superyachts carry both: spheres at bow and stern for flared hulls and rafting; cylinders alongside for plumb-bow yachts against fixed piers.
Why do brand size charts disagree at the top end?
Below 60m, every major brand converges on the same diameter recommendations. Above 60m, charts diverge: AERÉ scales further, SEARAFT splits into 4'/6'/8' classes, Taylor Made caps at 36 inch and quotes everything above as custom, Polyform stops publishing above F-13. Build-spec fender selection on 50m+ yachts is typically a confidential decision between yard, captain, and brand reps.
What is PIANC WG211 and does it apply to yachts?
PIANC WG211 (2024) is the international working-group document for fender system design. It uses kinetic energy (½mv²) methodology calibrated for commercial seagoing vessels with tug-assisted berthing around 0.1–0.2 m/s. PIANC does not publish a yacht-specific velocity table, so the formula gives a framework rather than a precise yacht specification.
Sources: SEARAFT 2025 Fender Brochure; Fendequip maxiStow inflatable fender size guide; AERÉ Fender Size Guide; Polyform Norway F-13 datasheet; Polyform US Boat Fender Guide; PIANC WG211 Fender Design Guidelines (Trelleborg / ShibataFenderTeam summaries); Taylor Made Super-Duty product datasheet. Mercer Yachting purchases SEARAFT and Fendequip direct from factory and has no exclusive arrangement with either.