Message 4 of 4
Posted by member Steve Russell on Sunday 22 December 2024

I too have had this on my Hardy 27. It only occurred at low revs. It started after the boat was re-floated following about 8 weeks ashore for its annual servicing. After a few weeks the problem ceased and we had peace and quiet for the rest of the season.

I found some online threads that discussed whether very sharp, very clean leading edges on the props might cause some form of localized cavitation. The problem goes away with a few hours of use and the start of marine growth. I am not convinced by this, but going astern at the same revs did not cause a problem. The trailing edge of any prop is set at an optimal pitch for forward gear and is not as efficient when working as a leading edge in reverse. Cavitation is caused by low pressure zones that are so low as to effectively result in low temperature boiling and are more often encountered at higher revs, or wave jumping, but it is not impossible that very localized effects might happen at lower rpms. At 'pootle' revs the blade shape is not well matched to the boat's forward motion, as is also the case at very high revs. In the latter case the power being transmitted makes the problem more obvious. As the revs increase from 'pootle' into the optimal design range, the extra bite exerted by the props and better matching of rpm to forward motion lifts the localized pressure fields clear of the risk of cavitation.

I have also removed the spurs cutters I inherited. These had two blades and fitted to a three bladed prop passing the single blade of the P bracket. A resonance nightmare and quite capable of triggering weird, counter-intuitive effects.

The other theory I have heard from an informed source, and you seem to be aware of it when you say you 'burped' the stern gland, is that lengthy periods ashore allow seawater to drain from the stern gland, (although we all imagine them to be packed full of grease), and it takes a surprising amount of time for water to penetrate and re-fill the gland. Meanwhile this allows the shaft to 'sing' and is why the effect is noticeable at the start of the season, but fades with time.

i am not wholly convinced by either theory but offer them for whatever value they might give.

Good luck

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Message 2 of 4
Posted by member Peter Cox on Thursday 31 October 2024

Paul - perhaps the stern gland packing has failed or possibly the cutless bearing. Regards, Peter.

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