noisy propellers ?

[rufus2015]

As you all know when a boat passes yours you can hear the propeller turning, big props on vintage engines make hardly any noise, but modern 3000 rpm engines turning smaller props make a load roaring sound when they pass, I was thinking, the noise must be cavitation, and that noise must be a waste of energy, ie ineffecient. Am I right or wrong ?

[Dalslandia]

RIGHT, noise is energy.

 

Propeller can make noise, and poor inflow to propeller can make noise, small low speed, low power propellers should not cavitate.

[Sea Dog]

Not likely to be cavitation, since this is essentially a high blade speed leaving "holes" in the water behind it which then collapse to cause noise and can erode the blade. Narrowboats passing you when you're moored up are not going to have such blade speeds.

 

Cavitation is more of an issue on higher speed craft - although even on a fairly slow craft an outboard with a small prop rotating very quickly could generate it. I think you're just noticing the effect of a smaller prop thrashing the water more than a slower moving larger prop for a given speed.

 

Serious design effort is put into props for submarines and some warships to address cavitation for noise reasons, and there are also systems for filling those "holes" with air to combat the collapsing effect. This is of course the F1 standard compared to the agricultural machinery we employ. Other vessels sit somewhere between, depending on the desirability versus cost of reducing cavitation.

 

You could always stick your head out of your side hatch and shout "slow down... or buy a better prop!"

[dmr]

RIGHT, noise is energy.

 

Propeller can make noise, and poor inflow to propeller can make noise, small low speed, low power propellers should not cavitate.

 

Noise can certainly be an indication of an inefficient process, but interestingly making the argument that "noise is energy" is a bit misleading.

The "Sound Power" associated with a very loud sound indeed is still only a fraction of a watt so is pretty insignificant in terms of wasted energy where internal combustion engine powers are concerned.. We are often misled into thinking that sound requires a lot of power by HiFi systems producing hundreds of Watts, but this is only because modern loudspeakers can be spectacularly inefficient..

 

............Dave

[richardf]

I think I know the noise you mean - we always call it 'singing'. You can often hear it on craft coming past when you are moored up. Ours seems to do it sporadically and it seems to be linked to shallower canals somehow.

Its definitely not cavitation as in our case as the counter is well underwater and we don't rev that high - although it gets worse at higher speed

It used to drive me mad, but I ignore it these days!

[DHutch]

It can be a poor prop, but it can also be the prop being too close the the hull/rudder, as well as dents/bends due to the being too thin and light for canal use, and the gearbox.

 

 

 

Daniel

[Sea Dog]

 

Its definitely not cavitation as in our case as the counter is well underwater

That would be ventilation - air getting down to the whizzer. Cavitation inception speed is still a noise consideration in submarines, even when dived to several hundred feet.

[Dalslandia]

I think I know the noise you mean - we always call it 'singing'. You can often hear it on craft coming past when you are moored up. Ours seems to do it sporadically and it seems to be linked to shallower canals somehow.

Its definitely not cavitation as in our case as the counter is well underwater and we don't rev that high - although it gets worse at higher speed

It used to drive me mad, but I ignore it these days!

Singing can be cured by angle of the trailing edge on the suction side (called backside on boat propellers) faced forward

[blackrose]

I can't help wondering why anyone who worries about prop noise and inefficiencies would be interested in anything as hydrodynamically inefficient a canal boat?

Edited August 22, 2015 by blackrose

[Dalslandia]

The phenomena?

 

Will add, the distance between blade tip and hull to sources of noise.

 

if anyone want better efficiency go with the biggest practical propeller and proper propeller RPM.

between thumb and index finger, the distance from tip to hull should be 17% of diameter on a 3 blade, 15% on a 4 blade, 13% on a 5 blade.

[ditchcrawler]

The phenomena?

 

Will add, the distance between blade tip and hull to sources of noise.

 

if anyone want better efficiency go with the biggest practical propeller and proper propeller RPM.

between thumb and index finger, the distance from tip to hull should be 17% of diameter on a 3 blade, 15% on a 4 blade, 13% on a 5 blade.

OK why can the gap be smaller the more blades you have? Mine is officially too big for the boat without enough clearance

[Keeping Up]

Our prop used to be too big so that there was not enough clearance from tip to hull and it sang horribly. A new prop, 1" smaller but coarser to compensate for that, made a world of difference. 10 years later it was singing but cleaning up the trailing edge with a grinder cured that. Last year, after another 10 years, it had to be done again and is now quiet once more.

[Dalslandia]

OK why can the gap be smaller the more blades you have? Mine is officially too big for the boat without enough clearance

more blades make smaller impulses for each blade.

An side? effect of more blades is that the diameter usually get smaller too, with same or increased BAR

 

I seen 20% 15% 10% and 2" as minimum clearance, the 17-15-13% I think I found from a good propeller maker site.

also the 3 blade should have 27% of diameter clearance forward to hull/skegg/keel

Edited August 22, 2015 by Dalslandia

[Peter-Bullfinch]

Is it cavitation when leaves swish around your prop in autumn and there is a loss of power? Usually cured by a quick burst of reverse.

[Top cat]

No that's accumulation.

 

Top Cat

[Dalslandia]

Is it cavitation when leaves swish around your prop in autumn and there is a loss of power? Usually cured by a quick burst of reverse.

it is called leavitation

[bizzard]

Change the propeller for a Repeller.