The old "one inch of fender diameter per foot of boat length, minimum two fenders" rule was written for displacement hull sailboats in the 1970s. Apply it to a 6 m RIB or 5 m aluminum fishing boat and you end up with fenders too small to protect either the tube or the hull chine. Both boat types need fender sizing by weight and impact angle, not length.
RIBs: the tube gives you partial protection already, but the Hypalon or PVC is thin — 1.2-1.5 mm — and tears on concrete or steel pontoons. The fender's job on a RIB is not impact absorption (the tube does that) but abrasion prevention. You want long, flat fenders — not spheres — so they stay between the tube and the dock across the full contact arc.
Aluminum boats: the hull is stiff and thin (2-3 mm plate), with sharp chines at the waterline. Impact spreads poorly across aluminum — a small dent at the chine can become a crack in the weld the next season. Aluminum boats need cylindrical fenders large enough to absorb the impact, not just cushion it. Rule of thumb here: minimum fender diameter ≥ 1/25 of boat length (so 6 m boat = 240 mm fenders).
Fender count: 1 fender per 2 meters of boat length, minimum 3. On a 6 m boat that's 3 fenders (bow, midship, stern). 4 meter boats still need 3 — you can't protect bow, midship, and stern with 2. Spares matter: always carry one extra, because fender lines chafe and break at the worst time.
Line material: standard 6-8 mm polypropylene works but stretches under UV and loses 30% strength in the first season. Polyester (PES) braid is the upgrade — same diameter, no UV degradation, clean knots. Marine Kevlar-core lines are overkill for fender work unless you're docking in extreme conditions.
MARINAC Flexi EVA fenders address both RIB and aluminum boat needs with one shape: a flat-sided oval profile. The flat side sits against the hull or tube; the curved side takes the dock impact. This profile doesn't roll along the hull like cylinders do — staying put where you hung it. On RIBs it contacts the tube across the whole flat face; on aluminum it presents the curved surface to the dock for proper impact absorption.
Inflation pressure for pressurized fenders: 0.3-0.5 bar is correct. Over-inflation (above 0.5 bar) makes the fender rigid — it bounces instead of absorbing, and transmits shock to the hull. Under-inflation (below 0.3 bar) collapses it at first impact. Check pressure monthly; it drops 10-15% over winter storage.
Storage: when not in use, keep fenders out of direct sun. UV degrades PVC fenders in ~3 years; EVA in ~5. A simple canvas bag in a cockpit locker doubles fender lifespan. Don't leave fenders hanging overboard when the boat is moored long-term — barnacles attach to the line and destroy your cleats when you finally pull the boat in.