Stuffing box adjustment

[Guest]

I've got a 1988 Jeanneau Fantasia 27. I recently fouled the prop and this caused me to look at the prop shaft as it exits the gearbox and enters the inside if the hull. Here there is a sealing gland which is romantically called the stuffing box through which the prop shaft exits the hull. It seems to consist of 2 black plastic sections - one inserted into the other. There is no visible means of lubrication which I assume is done by the very slight ingress of sea water. On examination I found that the 2 bolts holding the 2 sections together were slightly loose - which seemed wrong! When I tried to tighten them slightly, the nut just kept turning on the bolt. i.e. the 2 sections of the stuffing box were compressing.

Can anybody tell me how to correctly adjust this type of stuffing box?

Thanks

Geoff