Engine revs but the boat does not move normally
Confirm the control cable moves the gearbox selector fully. Check gearbox or saildrive lubricant using the correct procedure. Look for a lost propeller, failed hub, folding propeller that has not opened, clutch slip or broken coupling.
Delayed or harsh engagement
Check idle speed, control adjustment, cable condition, lubricant level and specification. A hydraulic gearbox may need pressure testing. Continued slipping creates heat and friction-material damage.
Vibration after impact
Assume propeller, shaft, strut, bearing, coupling or saildrive damage until inspected. A minor-looking blade bend can generate significant cyclic load.
Poor reverse thrust
Folding and feathering propellers require correct deployment, pitch and hub service. Growth or stiff blades can prevent proper operation. Some fixed installations also perform differently astern because of propeller walk and hull flow.
Oil around a saildrive
Determine whether leakage is internal, from the upper gear, around the propeller-shaft seals or from another engine source. Sample lower-unit oil for water and arrange pressure testing. Do not operate with an unexplained falling level.
Whenever divers are working around propulsion gear, isolate starting and use an agreed lockout method. Neutral alone is not protection.