Why does my outboard vibrate or shake?

Outboard vibrates or shakes · Symptom: The motor, the tiller or wheel, or the whole boat shakes or buzzes, especially as you speed up, often after you hit something or ran in shallow water.

Quick answer: The most common cause is the propeller: a bent, nicked, or broken blade, or fishing line or weeds wrapped around it. Vibration that gets worse as RPM rises points to the prop. With the engine off and the kill switch lanyard pulled, inspect each blade, check the prop nut, and look for line behind the prop. If the prop is fine, check that the transom mounting bolts are tight, since loose bolts can damage the transom and eventually break. If it shakes at idle too, a cylinder may be misfiring, so check the spark plugs and fuel.

Likely causes (most to least common)

Damaged propeller: bent, chipped, cracked, or out-of-balance blades (often after hitting something)
Fishing line, rope, or weeds tangled around the prop or prop shaft
Loose prop nut, worn or missing thrust washer, or a worn prop hub
Loose transom mounting bolts or worn mounts
Engine misfiring on one cylinder (fouled plug, fuel problem), which usually shows up at idle too
Bent propeller shaft or internal gearcase damage

Safe owner checks & fixes

Shut the engine off, remove the key, and pull the kill switch lanyard before touching the prop.
Slowly turn the prop and look at every blade. Mercury says to look for nicks, chips, cracks, bent or warped blades, and uneven wear, especially after running in shallow water.
Remove the prop following your manual and look for fishing line or rope wound around the shaft behind it. Line can also cut the gearcase seal, so check the gear oil for water afterward.
Check the prop shaft for obvious bends and the splines for damage, then reinstall the prop with the correct washers, nut, and cotter pin or tab washer, tightened to the manual's spec.
Check the transom mounting bolts. Mercury warns that loose transom bolts can wear out the holes and eventually break. Tighten them to the torque in your owner's manual.
If it shakes at idle or misses, check the spark plugs and fuel system (see the rough idle and spark plug pages).
Test with a known-good or spare prop. If the vibration goes away, have the damaged prop repaired by a prop shop or replace it.

Parts / tools

Prop wrench or socket, block of wood to keep the prop from turning, new cotter pin or tab washer, torque wrench, spare propeller (if you have one), owner's manual.

Call a marine mechanic when

The vibration continues with a good prop; the prop shaft looks bent or wobbles; there's water in the gear oil; the transom bolt holes or the transom itself look damaged or soft; or the motor clunks or makes grinding noises. A loose or shaking motor is a safety risk, so don't run it hard until it's fixed.

Sources

Values not stated here are model-specific; check your owner's manual.