Hose Clamps on Boats: Choosing, Installing and Inspecting Them

Go through the boat with a flashlight and note each hose: raw-water intake and engine cooling, exhaust, fuel fill, vent and supply, head and holding tank, sinks, cockpit scuppers, livewell and washdown, bilge pump discharge, sterndrive bellows and stuffing box. Put the below-waterline ones at the top of the list. BoatUS has a claim where a sportfishing boat sank because a corroded clamp on a cockpit scupper hose let go, so don't skip drains just above the waterline.

Hold a small magnet against the band, the screw and the housing of each clamp. Good 316 stainless barely reacts. If the screw or housing grabs the magnet hard, the clamp has non-stainless or low-grade parts and should be replaced. Practical Sailor found magnetic parts were usually the first to corrode. A magnet can't prove a clamp is 316, though: look for "316" stamped on the band and buy known marine brands.

BoatUS says corrosion usually starts on the bottom of the clamp, where water collects and where you can't see from above. Use a mirror and a light. Even all-stainless clamps aren't immune: surveyor Steve D'Antonio, writing in Cruising World, describes crevice corrosion in stagnant, oxygen-starved bilge water eating the band on the hidden side, especially on stuffing box hoses, so a clamp can look perfect from above and simply fall off when tugged. He also advises keeping T-bolt clamps out of areas that stay continuously wet for the same reason. Replace any clamp with:

• Rust stains, pitting or flaking on the band or screw
• A band that's thin, cracked or broken
• A stripped screw that spins without tightening
• A long sharp tail sticking out, or a kinked or bent band

BoatUS suggests pulling on hoses to find clamps that are ready to fail. Do this on below-waterline hoses only with the boat hauled out, or with the seacock closed. A hose that moves on its fitting needs attention now.

Each clamp is marked with a range of diameters. Pick one where the hose diameter falls in the middle of the range. BoatUS warns a clamp that's too big leaves a long, sharp tail and can tighten into an oval that grips poorly; one that's too small may barely catch on the screw threads and slip under load.

• Slide the clamp onto the hose before you push the hose onto the fitting.
• Push the hose fully onto the barbed fitting (hose tail), then slide the clamp over the part of the hose that sits on the barbs.
• Leave about 1/4 inch of hose showing between the clamp and the hose end (BoatUS gives 1/4 to 3/8 inch).
• The band must sit completely on the barb, never hanging over the end of the fitting where it can cut the hose.
• Never clamp a hose over a threaded fitting (liquid or exhaust can leak along the threads) or a plain smooth pipe (the hose can creep off). Use a proper hose barb.

If the barb is long enough, add a second clamp. BoatUS's surveyor advice: keep at least 1/4 inch between them (1/2 inch if you can) and put the screws on opposite sides of the hose so they don't line up. If there isn't room for both without one hanging off the barb, don't force it.

Use a nut driver, not a ratchet. BoatUS reports an informal test where an average person tightening by hand with a nut driver came close to the maker's spec for common sizes, while a small ratchet easily overtightened the clamp. Tighten until the hose is snug and the clamp no longer turns on the hose; you should see the hose compress slightly, not bulge out of the band or split. T-bolt clamps are tightened with a socket; follow the clamp maker's spec.

Put a small paint-pen mark across the screw and band of each new clamp. If it ever loosens, you'll see the mark has shifted. Log the date and what you replaced in your BoatMend maintenance log.

Hoses settle after installation, especially new hose and hot hoses like exhaust and cooling lines. After the first engine run, let everything cool and check the clamps again; snug any that loosened. Look for drips with the engine running and the boat in the water.

Carry a few clamps in each size used on your boat, plus a nut driver. A clamp that breaks far from the dock is a two-minute fix if you have the spare. Store them dry.

Check clamps at the start of every season, at every haul-out, and any time you're working near them. Check below-waterline and fuel clamps more often on older boats. Any corrosion, kink or damage means replacement, not tightening.