3 or 4 bladed prop?

[muckypup]

Looking through the Vetus catalog, the section on props. It shows a pic of a 3 bladed prop and claims "suitable for displacement craft", a 3 bladed prop with a much different profile "suitable for planing craft upto 30knots" and a 4 bladed prop "Suitable for semi-displacement craft upto 16knots". Harlequin falls into the 4 bladed prop category, yet she only has a 3 bladed prop (and seems to go fine).

Are there any advantages for a semi-displacement with a 4 bladed prop over a 3 bladed prop?

Steve

 

[andrewbarker]

Supposed to be smoother with less lumpy power delivery. Also smaller dia poss where prop/hull clearance is an issue.

 

[hlb]

Think there talking rubbish.

If it aint broke dont fix it!!

 

[Gludy]

props are a big subject and a bit of a black art.
<y advice is if it works leave it alone. The Vetus advice is very general - you hare not suffering anything because of your prop.

 

[muckypup]

Wasn't thinking of changing it.. /forums/images/graemlins/smile.gif (there are much more pressing issues like getting that diesel heater working grr).

Was just wondering why a 4 bladed prop was more suitable - I did notice your new trader has 4 bladed props.

Steve

 

[steverow]

Agree with Paul, the props on your boat are almost certainly a match for the engines.
Leave well alone.
Dont believe everything you read, and stop worrying about trying to investigate and learn about everything all at once. You'll burn yourself out.
Props are very very specialised and you'll need an expert to size them.
If the boat goes OK, doesnt overheat..then leave them and spend your time and money on something more pressing.

Steve.

 

[ccscott49]

Four bladers are quieter, thats about the only advantage. Leave well enough alone.

 

[Medskipper]

Re: I remember reading an article................

a few years ago stating that the most efficient prop was a two bladed prop! I'm no expert even though I've been messing around with boats nearly 20 years. Any experts out there care to comment?

Barry

 

[Artie]

I have no idea concerning the relative efficiency values of three or four bladed props but as a user of both over the years on semi displacement vessels I have to say that, using rope cutters, any debris left on the prop is more difficult to disperse from a four blader than a three no matter how much tooing andd froing one does. Probably because the angle between blades is larger on the three blader and therefore more room for the debris to fall away

 

[fisherman]

Re: I remember reading an article................

I always understood that 2 blades was best: I think the Turbinia had multiple 2 blade props on one shaft.

Interesting stuff in my reference book: Miss England II had a prop 14 in dia, 14in pitch turning at 12000 rpm; Bluebird, in 1939, shaft speed 9000 rpm.

One small point. If, like me , you have a 3:1 reduction and a three bladed prop, the blades have the same frequency as the engine revs: does this magnify vibration?

 

[jimg]

Re: I remember reading an article................

Nah! One bladed are best, I think that is the logic behind surface drives only having one blade in the water at a time?

 

[marcusgillies]

Re: I remember reading an article................

Let me try and put you streight 3 0r 4 bladed props provided they are matched to the boat are fine. The less blades the bigger the blade can be so more bite in the water hence slightly higher top end. More blades give faster acceleration of engine rpm and boat speed but typicaly loose out top end.
if you take two of the same boats same weight same lenth same engine hp,prm,gearbox reduction etc you will find the one with 3 bladed props will go that tiny bit quicker. on a broom 38 a semi desplacement the difference was about 1.5 knots. however the 4 bladed proped boat was smoother and had slightly faster acceleration.