Damaged Propeller: Repair It or Replace It?

Neutral, key out, lanyard off. Tilt the engine (or haul the boat, for inboards). Turn the prop slowly by hand and inspect each blade for:

• Small nicks and burrs (feel the edges with your thumb)
• Bent or curled tips, or one blade out of line
• Cracks, especially near the hub
• Missing pieces
• Corrosion pitting, or the eaten-away edges cavitation causes
• Blade tips worn to a point or rounded off, and a prop that looks smaller than it should. BoatUS suggests measuring from the hub center to one blade tip and doubling it: if that's well short of the diameter stamped on the prop, worn-away metal is costing you speed and fuel.
• On bronze props, a pink tint and a dull "thud" instead of a ring when you tap a blade. BoatUS says that's galvanic corrosion pulling metal out of the alloy; a prop that far gone usually can't be saved, and the anodes and bonding need checking too.

Mercury adds that even a soft-bottom run through sand or mud can bend blades slightly with no obvious damage. If you notice new vibration or lost performance afterward, have the prop checked. Some bends start right at the blade root and are nearly invisible; BoatUS notes most prop shops will check blade spacing and tracking for little or nothing.

A simple home tracking check from BoatUS's 2023 prop inspection article: hold a ruler or marked straightedge against a fixed point (the lower unit, or a block on the ground) so its end sits just off one blade's edge, then turn the prop slowly by hand and see whether every blade passes at the same distance. A blade that comes closer or farther is out of true. The same article warns that worn or flattened blades can let the engine rev past its recommended range, which can damage it, and that a motor running well but unable to reach its rated RPM is a reason to have the prop checked right away. A prop cover protects it on the trailer and in storage.

If the engine revs but the boat won't get on plane, the rubber or plastic hub may have slipped. Boating Magazine suggests drawing a line across the inner and outer hub with a marker or nail polish; if the line is broken after a run, the hub spun. See the spun hub guide for the full test.

Light edge burrs from sand or a ramp are a DIY job. Yamaha's method: a coarse file, then a fine file, with a wire brush to keep the files clean. Boating Magazine suggests a mill bastard file held at a steep angle, working from the back of the blade toward the edge. Remove as little metal as possible and stop as soon as you can't feel the burr.

Small bends can be eased back with two adjustable wrenches (one holding the good part of the blade, one on the bent part). Yamaha says a curled stainless tip can sometimes be tapped out with a brass hammer. Treat these as get-home fixes unless the bend was very small, and get the prop checked by a shop.

Usually worth a repair quote:

• Stainless steel props with bent blades or missing tips
• Expensive or hard-to-find props (high-performance, large inboard, counter-rotating)
• Inboard props, which Boating Magazine notes can take weeks to replace new

Usually replace:

• Aluminum props with heavy damage, especially if the hub also needs work
• Props with cracks at the blade root or large missing sections (ask the shop; some can't be safely repaired)
• Common, inexpensive props where a used or new one costs about what the repair would

Call or visit a prop shop (many dealers send props out). BoatUS suggests looking for a shop that belongs to the National Marine Propeller Association, which trains and certifies prop repair technicians. Ask:

• What will it cost, and is the hub replaced or reused?
• Will the prop be returned to its original pitch and checked on all blades?
• Is it balanced after repair?
• For inboard and high-speed boats: what accuracy class is offered? Boating Magazine explains that shops sell repairs by accuracy class, with tighter tolerances costing more, and that light, fast boats and ski boats are most sensitive to small errors.
• How long will it take?

Many outboard and sterndrive props use a pressed-in rubber hub; others use a replaceable hub kit. Yamaha notes that pressing in a new rubber hub is a shop job that typically costs much less than a new prop. Replaceable hub kits can be swapped by the owner. Discover Boating adds that rubber hubs can age and slip even without a strike.

A new prop must match your engine's shaft, spline and hub system, rotation (right- or left-hand) and the diameter and pitch stamped on the old one. Bring the old prop or its numbers to the dealer. Discover Boating stresses matching size, pitch and hub type to your engine spec.

Yamaha suggests treating a ruined prop as a chance to check whether a different model, pitch or diameter would suit your boat better. Your wide-open-throttle RPM should land in the range in your owner's manual; see the propeller guide for how to test it. BoatUS gives a common outboard rule of thumb: each 2 inches more pitch drops WOT RPM by roughly 400 to 500, and 2 inches less raises it about the same (typical; your dealer can confirm for your engine). A prop shop can often re-pitch a prop by a small amount, which may be cheaper than a new one; bigger changes usually mean a different prop.

Grease the shaft splines with the grease your engine maker specifies, reinstall the washers and spacers in the original order, torque the nut to the spec in your manual, and lock it with a new tab washer or cotter pin (Yamaha recommends a new cotter pin every time).

With a normal load, check for vibration at all speeds and note your WOT RPM. Compare it with your log from before the damage. A repaired prop that vibrates or lost RPM may not be back to spec; take it back to the shop.

Both Yamaha and Discover Boating recommend carrying a spare prop, plus a spare nut, washer and hub hardware. An affordable aluminum prop makes a good backup for a stainless one. Yamaha also suggests running an aluminum prop in unfamiliar shallow water with oyster bars or stumps, since it's cheaper and passes less shock to the gearcase if you hit something.