How to Size a Boat Bumper
Most people shopping for fenders think about one number: how big is my boat? That gets you partway there, but a bumper actually has two dimensions that matter, and they do different jobs.
Big Bumpers come in four lengths: 2', 4', 6', and 12'. And in three diameters: 12", 18", and 24". Understanding what each dimension is for makes the choice a lot simpler.
Diameter is first
Diameter determines how far your boat sits off the dock. That's the whole job.
A 12" bumper holds you a foot away from whatever you're tied to. A 24" bumper holds you two feet away. When the wind pushes, when a wake rolls through, when someone ties up next to you and their boat swings, the amount of cushion between your gelcoat and the dock is set by diameter, not length.
The question to ask: how far off do I need to be held?
More freeboard, more flare. A boat with high sides or a hull that flares outward needs more standoff, or the widest point of the hull will reach past a small fender and contact the dock anyway.
Rougher water, more standoff. Busy marinas, wake zones, fuel docks, and tidal slips all mean more movement, and more movement means you want more room to absorb it.
Fixed structures, more standoff. A seawall or fixed piling doesn't move with you. Floating docks do, at least somewhat.
Length is coverage
Length determines how much of your hull is protected and, just as importantly, how much room your boat has to move without sliding off the bumper.
This is the part people underestimate. A boat doesn't stay put. It shifts fore and aft with wind and current all day. A short fender protects a spot; a long bumper protects a zone. When the boat slides two feet forward, a 2' fender is now behind the contact point and doing nothing, while a 6' bumper is still between you and the dock.
We've written about this before in The 6-Inch Problem; small movements are where most dock rash actually comes from, not dramatic impacts.
Longer also means the impact load spreads across more surface. Less pressure at any single point on the hull.
What to measure before you buy
Three things, and none of them take long:
Boat length. The starting point for the chart.
Freeboard at the rub rail. From the waterline up to your rub rail or widest point. This tells you how much bumper needs to be in the water versus above it.
Dock height and type. Floating or fixed? How high does the dock edge sit relative to your rub rail — and how much does that change with tide or seasonal water level?
That third one catches people. A bumper that's perfect at your home slip can sit too high or too low at a marina with a different dock height.
Start with the chart
The Size Matters chart is organized by watercraft type, wakeboard and surf boats, pontoons, center consoles, runabouts, personal watercraft, and docks, and gives boat length, recommended bumper length, recommended diameter, and how many you'll want. If you're not sure where to start, start there.
The chart is a solid baseline for typical conditions. The next section is about when your conditions aren't typical.
When to size up
The general rule with fenders is that going bigger is rarely the mistake. A bumper that's larger than you strictly need still works fine. A bumper that's too small fails exactly when you need it, under load, in wind, with a wake coming through.
Situations where it's worth going up a size:
You raft up. Boat-to-boat contact involves two hulls moving independently, in open water, with no dock to reference. This is the single most common reason people wish they'd bought bigger.
Your slip is wake-heavy or high-traffic. Constant movement means constant repositioning if the bumper is undersized.
You tie to a seawall or fixed dock. No give, and water level changes shift your relationship to the wall.
Your boat has high sides or a lot of flare. Diameter matters more than usual here.
You use the same bumpers at multiple locations. Size for the toughest spot, not the easiest one.
The traditional argument against oversizing is storage; big foam fenders are a nuisance to stow. That's much less of a factor with an inflatable. Big Bumpers deflate down to a fraction of their inflated size and roll up, so choosing a 6' over a 4' doesn't cost you a locker.
Two notes on getting it right
Inflation is a feel thing. There's no single correct pressure. Air level is a matter of preference; try a few and see what you like. What matters is not going overboard: overinflation stresses seams, and it isn't covered under the warranty. A bumper with a little give absorbs impact better than a drum-tight one anyway.
Size and quantity are separate questions. Picking the right bumper is one decision; deciding how many to hang is another. We covered that in Two Bumpers or One; the short version is that one bumper protects a point and two protect the movement zone.
The bottom line
Diameter sets how far you're held off. Length sets how much room your boat has to move while staying protected. Measure your freeboard, look at what you're actually tying to, and check the chart.
And when you're genuinely torn between two sizes, the bigger one is almost always the one you'll be glad you picked.