Why are 4 blades better than 3?

Rivers & creeks

Rivers & creeks

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The Kiwi K4 hasn't been on the market long, they say it's better to have four blades instead of three for larger diesel engines. If so, why aren't most 60hp+ engines given a four blade prop from new? The K4 is they say the best way of a tubby 70hp boat like ours fitting a feathering prop.

http://www.kiwiprops.co.nz/cms/index.php/therange/k4-info

k4-render.jpg
 
Check-out this work of art. I saw & touched ir at SIBS
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Max-Prop Whisper' is a 5 bladed feathering propeller. The Whisper has the same advantages of the Classic model and works as the Easy model with the external pitch regulation with calibrated screws. This prop is probably the quietest feathering propeller on the market.
 
Lazy Kipper said:
So is a big three blade better than a not quite so big four blade?
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sailorman said:
with a conventional multi blade bronze fixed blade prop`s the blades over lap as viewed from astern so more efficient with less slipping giving the full circle as above. that Kiwi has a lot of space between the blades so less efficient when under pwr
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According to this article: http://www.fao.org/docrep/x0487e/x0487e04.htm -

"Number of blades. In general, at a given shaft speed (RPM), the fewer blades a propeller has, the better. However the trade-off is that, with fewer blades, each one carries more load. This can lead to a lot of vibration (particularly with a two-bladed propeller) and contribute to cavitation. When the diameter of the propeller is limited by the size of the aperture, it may often be better to keep shaft speed low and absorb the power through the use of more blades.

Blade area. A propeller with narrow blades (of low blade area ratio, see Figure 8) is more efficient than one with broad blades. However, propellers with low blade area ratios are more prone to cavitation as the thrust that the propeller is delivering is distributed over a smaller blade surface area. Cavitation considerations invariably require that the chosen blade area ratio is higher than the most efficient value."

The rest of the article is worth reading as well.
 
Lazy Kipper said:
OK. Do the Kiwis throw blades or is that a myth? If they do throw a blade, can they still be run at low revs to get you home?
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I have still not lost a blade on my Kiwiprop, have used it for four years..

My father lost one blade on a fixed bronze prop may years ago - newer understood how it happened the boat was in the middle of deep water when he got extreme wibrations from the prop.
He was able motor to nearest harbor (It's a motor boat - motoring 60 nm to get home was not an option) for haul out and replacement of prop
 
Lazy Kipper said:
OK. Do the Kiwis throw blades or is that a myth? If they do throw a blade, can they still be run at low revs to get you home?
Click to expand...

Had my Kiwi for 7 years. Never lost a blade but fitted new ones last year as there was a bit of play developing on the shafts. Not an expensive job to replace the blades but the pitch must be set again. Simple to do though.
 
The explanation in post #10 covers it. 4 blades are used where there are constraints in prop diameter and the ideal 3 blade size cannot be fitted to absorb the power of the engine. You rarely see them on sailing boats because there is rarely a need, although some of the Oysters for example which have very large engines have used them. Commonly used on bigger planing boats because of the amount of power and restrictions on prop size, but often suffer from cavitation.

Suspect the reason why Kiwi have gone for 4 blades is to keep diameter down and use the same blades as on the 3 blade but be able to absorb more power.

No advantage in a displacement boat to use 4 blade instead of 3 if you can fit a big enough 3 blade.
 
Tranona said:
The explanation in post #10 covers it. 4 blades are used where there are constraints in prop diameter and the ideal 3 blade size cannot be fitted to absorb the power of the engine. You rarely see them on sailing boats because there is rarely a need, although some of the Oysters for example which have very large engines have used them. Commonly used on bigger planing boats because of the amount of power and restrictions on prop size, but often suffer from cavitation.

Suspect the reason why Kiwi have gone for 4 blades is to keep diameter down and use the same blades as on the 3 blade but be able to absorb more power.

No advantage in a displacement boat to use 4 blade instead of 3 if you can fit a big enough 3 blade.
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Oysters all have feathering or Folding props as std. i have yet to see a fixed prop on a new boat
 
Lazy Kipper said:
OK. Do the Kiwis throw blades or is that a myth? If they do throw a blade, can they still be run at low revs to get you home?
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I took a ruddy great chunk out of one of my blades when I hit something. The blade stayed on. I changed it in 90 seconds at the next opportunity.
Mine has worked well for the last 800 hours motoring.

There would be nothing to stop you running it with 2 blades, but the vibration may give you a cheap thrill. It's not highly recommended though, as it is transferring loads into the system which will damage bearings etc over time.
 
sailorman said:
You didnt mention folding either ;)
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So what? The question was about the number of blades, not about folding or feathering. The reference to Oysters was to illustrate circumstances in which a 4 blade prop might be used on a sailing boat. Does not matter if it is folding or fixed.
 
jwilson said:
Couple of weeks ago a boatyard owner told me 3 Kiwiprops had lost blades amongst his customers last year.
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How did that happen?

Was it single blade loss or the whole prop?

If you can point me in that directin, I will call Alan Pollard of Vecta and ask if he knows about that, as they are keen to have data.
 
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