A form of cavitation on certain tides?

CJS · 27 May 2012, 09:29

In certain tidal conditions I open up the throttle on my outboard but the prop doesn't seem to have any bite in the water. The engine revs increase but there is little acceleration. Yesterday coming back down the Hamble on the outgoing tide I experiences this phenomena. At first one thinks there is a problem with the engine or something tangled in the prop but upon inspection there is no problem.

There are also two other areas I experience this phenomena - two miles the Southampton side of Ryde and between Calshot Spit and Cowes usually riding the tide. It doesn't always happen but there appears to be areas of "dead water" so to speak.

I assume it is some tidal affect creating a form of cavitation almost water with to much air in it - anyone experienced this before?

Erin · 27 May 2012, 14:11

Does your prop rely on a split pin or rubber bush for gearbox protection? If its a bush then there is chance it is starting to fail. I had this once on a Honda prop on a 50. When load was put on it it started to slip giving the symptoms you describe.

lightning · 27 May 2012, 17:32

I had this on my SIB when going flat out on Loch Linnhe in Scotland, which is a sea Loch and had more waves.

I'd be on full throttle (on my Mercury 6hp) and the engine would sometimes over rev for a second or so, as if the split pin had failed on the prop.

Never had it happen anywhere else.

willk · 27 May 2012, 17:50

Might be picking up a bit of weed on the prop - it can cause cavitation and this won't stop until you throttle back. The weed will be long gone by then. Weed tends to float in bands along intersecting tidelines and where wind meets tide etc.

Cookee · 28 May 2012, 08:11

It's not going to be the tide for sure.

Dhf · 28 May 2012, 10:06

"The engine revs increase but there is little acceleration."

It could be tidal surge, The flow can dramatically increase in certain areas where the ground below differs from the surrounding.

For other areas,
You could also get this effect whilst climing over the back of a wave, when riding the tide, as the water gets drawn back into the trough of the following wave.