Symptom diagnosis
Sailboat vibration at cruising RPM
A vibration with an RPM address is information. The band it lives in, and whether it changes with load, points to the culprit with surprising precision.
1. Dynamic shaft misalignment
most likelyFlexible mounts deflect under thrust and the hull changes shape afloat — a driveline aligned on the hardstand runs misaligned at cruise. No amount of static shimming chases it.
Do thisThe engineering fix is a constant-velocity coupling that absorbs the working angle.
How SigmaDrive kills it →2. Worn cutlass bearing
commonGrab the shaft near the bearing and try to move it — perceptible play means the bearing is letting the shaft whip at speed.
Do thisReplace the bearing before judging anything else; it's consumable.
3. Blade damage or fouling imbalance
commonA dinged blade edge or a barnacle colony on one blade unbalances the wheel — vibration follows shaft speed everywhere, worse under load.
Do thisInspect and clean; a workshop re-balance restores a damaged wheel.
Workshop re-balance →4. Tip clearance pressure pulses
worth checkingTips running closer than ~10% of diameter to the hull drum against the plating with every pass — felt as vibration, heard as the aft-cabin thrum.
Do thisMeasure your clearance; blade design (skew, tip unloading) is the cure at the next prop.
Measure tip clearance →
Before you spend money
Run the free checks in order: shaft play, blade inspection, engine-mount condition. If vibration changes with engine RPM in neutral, it's mounts or engine — not the propeller at all.
Want the numbers checked properly?
Tell the Builder about this symptom — it screens for it, and an engineer reviews every recommendation before anything is supplied.