Can a gearbox be my trouble?

[Bergie]

I've posted a bit about my troubles in other posts.

The basics is that my Century is running very well in a test tank. On the boat however its running very poor. I've checked the usual, i.e. not to deeply submerged, points, spark, fuel/oil and fiddled with the needle setting on the water. I suspect the gearbox may cause the erratic running since the pinion bush is pretty badly worn. With no gearbox to swop it with, is there some way of testing the motor in a tank under load without the gearbox?