Locking Prop, not the usual question

[Spyro]

I have discovered that when under sail although I put the engine in forward gear the prop and shaft will continue to turn at anything above 3 knots boats speed. If I put it in reverse the prop and shaft is "locked" is this normal on a Yanmar 2GM20?

 

[NorthUp]

Yes.

 

[ebbtide]

Yes, and for many other gearboxes too - and the only sensible way to proceed.
Please don't ask me why!

 

[co256]

Is it the same for a 1gm10?

And if so..... why?

 

[misterg]

[ QUOTE ]
Please don't ask me why!

[/ QUOTE ]

I *think* the explanation is that many gearboxes have permanently meshed gears, but use clutches to select the drive direction. The clutches in the Yanmar 2GM box (and possibly others) are designed such that they are engaged loosely by the 'gear' selector, then more tightly by the torque from the engine as the drive takes up. Putting such a box in 'forward' when the engine is stopped, but the boat is moving will tend to drive it in the opposite direction to normal, so instead of the 'forwards' clutch being engaged tightly, the torque from the prop tends to disengage the clutch, allowing it to slip and wear away the clutch cones and 'gear' selector mechanism.

If you put it in reverse in the same circumstances, the torque from the prop is in the correct direction to firmly engage the 'reverse' clutch, so the box locks OK.

(There may also be other considerations about whether oil pumps operate correctly, etc.)

Andy