Weedhatch Test

Wednesday 30th November, 2011.
We have continued to have problems with the stern gland leaking and squeaking.
Axiom, who make the prop say its not because of their prop.
Vetus say its not their stern gland. However, their fitting instructions for the gland state that the pipe that is connected to the inboard end of the gland should be connected to a fitting below the waterline. Apparently, despite these instructions, it is common practice for narrowboats to terminate this pipe in the weed hatch above the waterline.
Logic would expect the area immediately in front of the prop to be a low pressure area and so this area would produce suction. It seems possible that with a 'normal' prop, which is very inefficient, this may not be the case. However, with the Axiom prop perhaps this low pressure is causing the stern gland to suck air down the pipe - thus causing friction, squeaking and then leakage.
To test this idea, and to see what was actually happening, I built a clear perspex weedhatch cover.
With this in place, my grandson Jamie and I watched what happened at various rev settings in forward and reverse, both moored up and in motion.
What became very clear was that in forward gear no water was coming out of the pipe in the weedhatch - so no water was flowing through the stern gland. In reverse gear a considerable amount of water was flowing through the pipe into the weedhatch.
Next we disconnected the pipe from the weedhatch and put it into a glass of water. In forward gear the glass gradually emptied. This clearly showed that there was suction in the pipe and, when connected above the waterline, it would have been sucking air into the stern gland.