4 blades vs. 3 blades

[dakotamike]

can anyone tell me the pros and cons of a 4 bladed vs. a three bladed prop of the same pitch.........thanks in advance

 

[steelespike]

Assuming the correct rpm It is usually suggested to drop 1" in pitch when going to a 4 blade.
The 4 blade will usually improve hole shot,feel smoother,stay on plane easier at reduced throttle,
retain its grip on the water and respond better around the dock.It usually costs a little of top speed.
The Merc. Spitfire 4 blade is said to exchange pitch for pitch and retain top end.

 

[fishin98]

Assuming the correct rpm It is usually suggested to drop 1" in pitch when going to a 4 blade.
The 4 blade will usually improve hole shot,feel smoother,stay on plane easier at reduced throttle,
retain its grip on the water and respond better around the dock.It usually costs a little of top speed.
The Merc. Spitfire 4 blade is said to exchange pitch for pitch and retain top end.

Going from say a 19p 3blade to a 4 blade you would go to a 17p....going pitch for pitch would result in higher top end and possable over reving, but also poorer low end performance, with higher fuel usage. Which happened to me when I went to a 19p 4blade Amita from a Solas 3 blade 19p....switched down to a 17P ...ALL WAS GOOD

 

[steelespike]

I read 2 different independent Spitfire tests Exchanged pitch for pitch. They improved the usual performance numbers yet maintained wide open
rpm and speed even on a boat with pretty amazing numbers to start with.
Going from a 19 3 blade to a 19 4 blade would usually lose more top end due to blade drag very unusual for it to over rev.

 

[jimbo_jwc]

Here is what it did on little 4 banger n 196 CC boat .December 10th, 2014, 11:38 AM
Prop results after acquiring 3 different one's over last winter and using on 1987 40hr 3.0 L MCM Mercruiser w/rebuilt original lower unit on 19'6 ' Cuddy Cabin boat .
Original Prop Mercury 3 blade 14.5 x 17 WOT lightly loaded 5000+ rpm
Mercury Prop 3 blade 14.5 x 19 WOT medium loaded 4200 rpm 36mph GPS
Solas? Prop 4 blade 14.25 x 18 WOT lightly loaded 5000+ lightly loaded
Mercury SS vented Prop 3 blade 14 x 19 heavily loaded 4600 rpm 39 mph GPS Is what my boat likes best .
Motor has had new plugs ,fuel pump,points ,carb rebuilt,gimble brg and motor temp at 145-150 running hard WOT over an hour .

Different brand of props and sizes is hit n miss because of blade size's profiles and boats characteristics .

Hoping I can try some of these props on Fiberform w/5.0l 1.48 that came with 15x15 as the 3.0 is 1.98 ?.

 

[steelespike]

I would expect different brands and designs to act differently.If you wanted to find the best prop you need to stick with a brand and try different designs
and materials.Then move on if required.You need to stick with new props as used or rebuilt are unknown.If you felt that say a 17" prop is best then you could try 17s from various brands and designs.Then of course there are the various boaters needs and hull requirements.You could go crazy trying to find the one
ideal prop out of the many designs available. You would need really deep pockets or find a dealer(s) that would loan props.
I would expect a healthy 3.0 to "usually"over rev with a 17" pitch. A 19" same brand/design should lower rpm by about 300 or 400.
A ss prop would out perform most if not all al props. An entry level ss prop might not outperform a high level al prop.

 

[Maclin]

dakotamike,

Is this query in regards to your current 40hp outboard setup? If so a 4 blade would probably not be in the cards. A 4 blade is for maximum bite in the water, usually when there is some "excess" horsepower to harness and keep prop slippage down.

 

[Starcraft5834]

4 blade offers overall best performance compared to 3 blade...likely true regardless of the hp, more thrust is more thrust...top end speed is compromised with a 4 blade by a few mph... not much disputing that... both will get the job done.. just that the 4 blade offers benefits the 3 does not...

 

[jhnmdahl]

That is a bit of an oversimplification, especially as conventional wisdom has usually (and also incorrectly) stated that three blades provide faster speeds than four. Now the difference is more likely to be a result of a specific propeller design and how it works with an engine's torque curve than one being automatically better than the other. A four blade propeller has more drag, for example, but has more blade are and bite to compensate. Which factor wins out is dependent on the extra torque available, and other such factors. See http://www.boattest.com/view-news/32...es-or-3-blades for example.