Cavitation/Ventilation

[DBA Forum (B) - Mark Johnson]

Here's the problem: Mallana is 68' x 12'6" with a 3' draught. She sits a bit high in the water but this is in order to facilitate navigation on the River Avon (Warks). So, I don't really want to take her down any lower at the present time - I may later. The propeller is only a couple of inches below the water line and the result is a lot of ventilation - the prop is churning air and sending water flying up instead of doing what it ought to. It also makes a lot of noise at the stern and with a few extra revs, when on the Severn for example, there is more vibration that I would like. I prefer not to think about fuel consumption, I'll cry!!

So, the question is: will a cavitation plate alleviate this problem? Any advice gratefully received.

[DBA Forum (B) - Edward & Pam Burrell]

Your barge is very similar to mine. A half tunnel concentrates the
water below the surface. There is an illustration on p84 of your copy
of A Guide to Motor Barge Handling. Worth doing, I would say.

regards
Edward.

Post by DBA Forum (B) - Mark Johnson
Here's the problem: Mallana is 68' x 12'6" with a 3' draught. She sits a bit high in the water but this is in order to facilitate navigation on the River Avon (Warks). So, I don't really want to take her down any lower at the present time - I may later. The propeller is only a couple of inches below the water line and the result is a lot of ventilation - the prop is churning air and sending water flying up instead of doing what it ought to. It also makes a lot of noise at the stern and with a few extra revs, when on the Severn for example, there is more vibration that I would like. I prefer not to think about fuel consumption, I'll cry!!
So, the question is: will a cavitation plate alleviate this problem? Any advice gratefully received.

[DBA Forum (B) - Paul Hayes]

Hi Mark

1/2 tunnels often fitted to French (and other European) commercials to prevent the exact problem that you have when running light (empty).

If it works for them it's a good indication that it's a good idea.

Also check how far your propeller is from the deadwood, there should be a substantial gap (from memory I think at least 15% of prop diameter ) I found this out the hard way, and got over the problem by inserting a 4" long spacer I had made between the faces of the coupling, made a huge difference, no cavitation since the modification.

Hope this helps

Paul Hayes

[DBA Forum (B) - David Beaumont]

I don't know why anti ventilation plates are referred to as anti cavitation plates. Cavitation is entirely different to Ventilation. You are experiencing ventilation, i.e. sucking in of air. Cavitation is the production of minute bubbles of water vapour by extreme low pressure which then collapse violently, damaging metal. You are only likely to get cavitation if you put a lot of power through a propeller that is too small or coarse pitched for your barge, if you are running it too fast or if the prop is too close to the hull/skeg/rudder. It should be pretty rare in a barge.

Anti ventilation plates, or better a 1/3rd tunnel, will stop ventilation and significantly reduce your prop walk. I've found about 1/3rd tunnel best as it left us with some slight prop walk which can be useful. When it was a 1/2 tunnel, prop walk went completely and we also lost almost a knot of speed.

David