Cavitation.....

[Nostrodamus]

When motoring head on into short rough seas I tend to suffer bouts of cavitation. All I can do is either wait a few moments for it to try and get a grip or reduced and increase the engine revs again. If I change the angle I go into the sea it tends to goe away.
It is a long story but there is little I can do about it at the moment.
I just wondered if this problem is specific to me or if others ever suffer from cavitation?

 

[sailorman]

When motoring head on into short rough seas I tend to suffer bouts of cavitation. All I can do is either wait a few moments for it to try and get a grip or reduced and increase the engine revs again. If I change the angle I go into the sea it tends to go away.
It is a long story but there is little I can do about it at the moment.
I just wondered if this problem is specific to me or if others ever suffer from cavitation?

the boat has slowed / stopped the prop is trying to regain grip but is spinning in the same place so hence cavitation.
less rpm would help or "tack" to give a better angle to the seas

 

[sarabande]

is the prop lifting out of the water ? Possible if the wave length is roughly the same as the boat length.

Or the prop has growth on it or is 'dinged', causing the onset of early cavitation.

Or the prop pitch and size, or engine speed, are not optimised for bashing into short seas.

When was the last time you managed to clean the prop ?

 

[McKimm]

I get the same symptoms in short, lumpy seas if I'm going straight into it. My prop is undersized so am hoping a new prop for next season will sort it.

 

[cmedsailor]

When motoring head on into short rough seas I tend to suffer bouts of cavitation. All I can do is either wait a few moments for it to try and get a grip or reduced and increase the engine revs again. If I change the angle I go into the sea it tends to goe away.
It is a long story but there is little I can do about it at the moment.
I just wondered if this problem is specific to me or if others ever suffer from cavitation?

Welcome to the Med!!!

 

[Daydream believer]

I had a brunton auto prop fitted to my Hanse 311. On flat water the boat would do 6.9 kts against 6 kts of the old fixed prop.
Trouble was that in a 1 metre chop in the Thames estuary it would do what you describe. I took this to be a result of the prop changing pitch
When I told Brunton & they took the prop for examination. Fair doos to them because they apologised profusely & said that they had supplied a prop with too little pitch. They increased the initial pitch & it now works OK but does mean the engine will only rev at max 2500 instead of the old 3500 of the fixed prop
I would suspect that you may have a pitching problem. If you did not get it earlier in the season then it is also hindered by possible fouling of the hull increasing the load on the prop. This can be noticed by increased prop wash causing a greater weigh on he helm

 

[NormanS]

I had a brunton auto prop fitted to my Hanse 311. On flat water the boat would do 6.9 kts against 6 kts of the old fixed prop.
Trouble was that in a 1 metre chop in the Thames estuary it would do what you describe. I took this to be a result of the prop changing pitch
When I told Brunton & they took the prop for examination. Fair doos to them because they apologised profusely & said that they had supplied a prop with too little pitch. They increased the initial pitch & it now works OK but does mean the engine will only rev at max 2500 instead of the old 3500 of the fixed prop
I would suspect that you may have a pitching problem. If you did not get it earlier in the season then it is also hindered by possible fouling of the hull increasing the load on the prop. This can be noticed by increased prop wash causing a greater weigh on he helm

How did they alter the pitch on a (self-pitching) Autoprop? I'm interested as I have one too, and with a boat that is susceptible to pitching, if motoring into a headsea, I find the same. I have always just put it down to sea conditions.

 

[Pye_End]

When motoring head on into short rough seas I tend to suffer bouts of cavitation. All I can do is either wait a few moments for it to try and get a grip or reduced and increase the engine revs again. If I change the angle I go into the sea it tends to goe away.
It is a long story but there is little I can do about it at the moment.
I just wondered if this problem is specific to me or if others ever suffer from cavitation?

Props tend to be matched for boats doing hull speed. If you want to improve low speed grip against punchy seas then you can look at reducing the pitch, and increasing the surface area (probably diameter depending on what you have).

 

[Daydream believer]

How did they alter the pitch on a (self-pitching) Autoprop? I'm interested as I have one too, and with a boat that is susceptible to pitching, if motoring into a headsea, I find the same. I have always just put it down to sea conditions.

If you look at the prop each blade has a sort of angle built into it to start with. They just removed the 2 blades & fitted 2 with a slightly different angle to start with. When it spins it is this angle that determines what amount of pitch it applies when it spins. They are fair people to talk to & will readily discuss this with you

 

[NormanS]

If you look at the prop each blade has a sort of angle built into it to start with. They just removed the 2 blades & fitted 2 with a slightly different angle to start with. When it spins it is this angle that determines what amount of pitch it applies when it spins. They are fair people to talk to & will readily discuss this with you

Thanks for that, but on balance I am very happy with the Autoprop in almost all conditions, so I'll live with it.

 

[haydude]

When motoring head on into short rough seas I tend to suffer bouts of cavitation. All I can do is either wait a few moments for it to try and get a grip or reduced and increase the engine revs again. If I change the angle I go into the sea it tends to goe away.
It is a long story but there is little I can do about it at the moment.
I just wondered if this problem is specific to me or if others ever suffer from cavitation?

Cavitation is caused by excessive water pressure in contact with the blades that rises the water temperature above the evaporation point (thus the typical bubbles that are in fact water vapor). The high temperature coming with cavitation can damage the propeller's blades.

I do experience the same, but it is not cavitation, it is actually the prop loosing grip because of water turbulence. Motoring into the waves in rough seas can rise stern and propeller above the water ...

 

[nigelmercier]

Cavitation is caused by excessive water pressure in contact with the blades that rises the water temperature above the evaporation point (thus the typical bubbles that are in fact water vapor).

Surely it is due to a low pressure causing the water to vaporise, due to it falling below the vapour pressure.

 

[VicS]

Surely it is due to a low pressure causing the water to vaporise, due to it falling below the vapour pressure.

Beat me to it! ...

But then I have found a good explanation by experts in the field of propeller design and testing in the video HERE

 

[Daydream believer]

NormanS

I found fuel consumption went from 1.66 litres per hour fixed prop to 2litres per hour at 6 kts. If i push it it rises to as much as 2.3 per hour if I go flat out at around 6.9 kts which my fixed would not do unless in a canalWhat did you find. Bunton claim a fuel saving but this is not my finding
I also find manouvers in the marina less easy than a fixed prop due to the time needed to pitch up & extra prop wash whilst it is doing this. Did this happen to you?

 

[NormanS]

NormanS

I found fuel consumption went from 1.66 litres per hour fixed prop to 2litres per hour at 6 kts. If i push it it rises to as much as 2.3 per hour if I go flat out at around 6.9 kts which my fixed would not do unless in a canalWhat did you find. Bunton claim a fuel saving but this is not my finding
I also find manouvers in the marina less easy than a fixed prop due to the time needed to pitch up & extra prop wash whilst it is doing this. Did this happen to you?

Sorry, I don't have comparison figures for fuel use before and after Brunton. When I bought my current boat, she had a three bladed fixed prop, which had not nearly enough pitch, so I had to do something.
I only sailed with the old prop for a couple of months.
I don't berth in a marina, but on the occasions that I do go alongside, or have to manoeuver accurately, I've never found it a problem.
I know that she motors faster than before. I feel that she sails better, presumably there is less drag when sailing.
The biggest benefit is probably when motor-sailing, when the new prop really comes into its own. I'm very happy with it, apart from the price of anodes.

 

[vyv_cox]

Surely it is due to a low pressure causing the water to vaporise, due to it falling below the vapour pressure.

And the damage has nothing to do with temperature, it is the impact due to collapse of the cavities that causes it, metal loss mechanism depends on the condition of the metal. This shows how it affects a ductile metal such as a prop would be made from - work hardening followed by pitting. The numbers are Vickers hardness, bigger number = harder.

 

[david_bagshaw]

Beat me to it! ...

But then I have found a good explanation by experts in the field of propeller design and testing in the video HERE

Thanks for the video link, most interesting

 

[Daydream believer]

The biggest benefit is probably when motor-sailing, when the new prop really comes into its own. I'm very happy with it, apart from the price of anodes.

Dead right i would buy the prop for that alone

 

[fishermantwo]

Korts nozzle is the answer. Not always practical I know. I have a catamaran yacht tender powered by a Tohatsu 3.5. Made a Korts nozzle out of a section of plastic tube and epoxy carbon fibre and cloth laminated around it. Eliminates the disturbed water from the hulls and gives greater thrust and brilliant steering.

 

[Nostrodamus]

Korts nozzle is the answer. Not always practical I know. I have a catamaran yacht tender powered by a Tohatsu 3.5. Made a Korts nozzle out of a section of plastic tube and epoxy carbon fibre and cloth laminated around it. Eliminates the disturbed water from the hulls and gives greater thrust and brilliant steering.

Alright, someone's got to ask..what is a Korts nozzle and do you have any photos?