A marine gearbox selects ahead, neutral or astern and usually reduces engine speed before it reaches the propeller. A 2.5:1 ratio means the engine turns about 2.5 times for one propeller-shaft revolution. The ratio helps match a relatively fast-running engine to a larger, slower propeller.
Gearboxes may use hydraulic multi-plate clutches, mechanical cone clutches or other designs. The required lubricant may be engine oil, automatic transmission fluid or a dedicated gear oil. The colour of the old fluid is not a reliable specification. Use the exact manual.
Routine checks
- Correct fluid level using the specified hot, cold, running or stopped procedure
- Fluid condition and smell
- Leaks at input, output, selector and cooler connections
- Shift-cable travel and lever detent
- Cooler and hoses where fitted
- Coupling bolts, mounts and unusual movement
- Delay, chatter or slipping when selecting gear
Some gearboxes permit a sailing yacht to freewheel; some specify locking in ahead or astern; others warn against particular positions or prolonged freewheeling. Folding and feathering propellers add their own instructions. Never use a generic rule.
If the engine accelerates but thrust fades, stop testing before a slipping clutch creates more heat and debris. Confirm the control actually moves the gearbox lever through its full travel, then obtain a pressure or mechanical assessment as appropriate.
Related: Gearbox and propeller troubleshooting · Propellers