Cavitation - does it depend on depth?

[richardf]

Last year I changed my engine from a Kelvin P4 to a Perkins D3. My counter was always in the water by some way and I never suffered cavitation at all. The D3 is obviously lighter and although the counter was still in the water, the cavitation noise was really evident

I added some extra ballast (it's now deeper than ever) and things have improved, however sometimes it returns, when the only thing that will stop it is to slow down

I get the feeling that it is shallow canals that give me a problem.

Has anyone found it to be depth dependent?

I am probably spinning the prop faster, but a recent prop calc showing that I am not far off optimum.

Can I be too deep, could that cause it?

Any ideas folks?

[GUMPY]

Is it cavitation (water effectively boiling on the prop)

Or

Ventilation (Air being sucked into the prop)

[richardf]

Can't see it being ventilation as the counter at the rear is about 3inches under and will go down even further when underway. I also replaced the weed hatch seal in case it was sucking in air that way

[GUMPY]

Can't see it being ventilation as the counter at the rear is about 3inches under and will go down even further when underway. I also replaced the weed hatch seal in case it was sucking in air that way

A lot depends on the prop my counter is 3" below water level at the back however air is sucked in at the very front of the counter due to the wave shape generated when moving.

Cavitation is very unlikely on a NB as the prop just doesn't move fast enough to create the low pressure required.

[MtB]

What diameter and pitch is the blade? and how fast is your D3 spinning it?

 

All blades will cavitate at some point. Maybe you are just going too fast.

 

But to answer your question intuitively I'd expect the shaft speed at which cavitation begins to be a lot lower in shallow water than very deep with a NB. In shallow water the supply of water to the blade is along the sides of the swim only, but in deep water unlimited fluid is supplied from under the baseplate so a higher shaft speed will be needed to achieve cavitation.

 

But I'm just guessing. I know nuffink about it really.

 

MtB

[nb Innisfree]

We had cavitation on our naarrowboat, forced engine change meant we had to get our the blade area and pitch reduced on our Crowther prop as the new engine peaked at 3000 rpm instead of the old 2000 rpm. Crowther did a nice job, but, though it performed well it cavitated badly in reverse. Shame really as it was excellent in forward.

[richardf]

From memory, I think the prop was 20x17.

No rev counter, so unable to state revs. I'm running a 2:1 reduction if that helps

[Phil Ambrose]

I cruise on rivers with normally plenty of water, I swing a 22" prop not sure of the pitch, counter is 2" below, more when on the move and do not suffer from cavitation so maybe it is to do with depth of water.

Phil

[Murflynn]

What diameter and pitch is the blade? and how fast is your D3 spinning it?

 

All blades will cavitate at some point. Maybe you are just going too fast.

 

But to answer your question intuitively I'd expect the shaft speed at which cavitation begins to be a lot lower in shallow water than very deep with a NB. In shallow water the supply of water to the blade is along the sides of the swim only, but in deep water unlimited fluid is supplied from under the baseplate so a higher shaft speed will be needed to achieve cavitation.

 

But I'm just guessing. I know nuffink about it really.

 

MtB

this may be true for ventilation. I agree with Loddon - cavitation is very unlikely on a canal boat.

[John V]

While developing a diesel electric drive system I took a range of readings of shaft speed against power input

I no longer have my notebook from that period but I remember that I found a complication in that the depth of water had a major effect on the maximum shaft speed achievable with fixed power

The input power was fixed as 10Kw with the power input constantly measured, it was found that the maximum shaft speed achievable was variable over a range of nearly 10% depending on the depth of water below the bottom.

Due to inability to obtain accurate test data I could not measure the relationship between boat speed/shaft speed/depth of water with any accuracy.

 

 

ETA HA! brain fade....point of the post.....if depth of water effects energy requirement of the prop it is going to figure largely in any problem involving cavitation

Edited September 24, 2014 by John V