4 blade vs 3 blade

I currently run a 3 blade 17" X 18p three blade on my Gran Sport and wish to go with four blade props because I understand cruise speed is increased at same rpm (and fuel economy is increased) although top speed is diminished slightly. Also I have heard the time to plane and, speed to maintain plane is lower.

Anyone know what the comparable four blade is to the three?
I would like similar WOT and holeshot.
IE: If the three blade is 17x18, is the comparable four blade a 17 x18
or would it be 17 x17 or?

Any prop people out there?

 

Unless you are considerably underproped and expierencing High Slip with the 3 Blade, Cruise and Top Speed will normally be diminished by perhaps 5% with the 4 blade prop. Rule of thumb, decrease pitch 1 - 2" for an increase in Dia by 1" or going from 3 to 4 blades.

Normally going to 4 blades is an advantage when Extra pulling power is required - towing heavy loads.

 

Firstly, thank you for the reply!

So I am out for lunch! I have been told that top speed will decrease 1-2 mph because of drag, but cruise speed would increase because less prop slippage. More blade less slip. With less slip, increase in cruise speed 1-2 mph at same cruise rpm (3200).

My current prop is 17x18 cup 4600 WOT
13400 lb 12.6 beam 34ft Boat for cruising, would like to see cruise 25 mph at 3200 rpm (350hp X2 1.55:1 V-drive)
I was thinking to go 18X 17 to drop WOT to 4200
hoping also bigger blade, less slip, higher cruise?

Any thoughts?

WGC (junior enthusiast)

 

Based on the no's you provided, if you are currently getting about 23 MPH @ 3200 RPM, you ARE expierencing High Slippage, > 38%. If the Boat is anywhere near being on Plane, Slippage should be Less.

Increasing Blade area, either thru larger Dia and/or no of Blades will decrease slippage, but you will need to also decrease pitch. Less Slip causes increased motor load, which can only be overcome by less pitch.

Going from a 17x18 to a 18x17 (both 3 blades) probably will not appreciably change RPM at WOT unless slippage is currently extremely High at WOT.
What is your speed at 4600 rpm.

You're on the Right Track , "Bigger Blade, Less Slip, Higher Cruise"

 

Tachs are not 100% but at 4600 I am 32-33mph by GPS
Should the 18 X 17p slip less and thereby show increases overall?
What slippage should I expect for this type of boat?

I found a prop calculator at http://www.castlemarine.co.uk/pitch.htm and it says 18"X16p @ 13% slip goes 35.7 @4200
So am I looking for this 13% range?

 

Your Slip at 4600 RPM is extremely High, still 38%. Are your Motors 350HP or 350C.I. (if c.i., Hp ?) What is Boat weight with half fuel, two passengers and minimal gear, 13,400 lb ?

Difficult to pinpoint exact slippage for your setup, but it certainly should be less than 20% @ max speed w. twin 350's. 13% would be very good and probably difficult to achieve.

 

Using the numbers provided your max top speed, using Crouch's formula, calculates to 39.3 mph. This assumes your HP(700) is at the "Propeller" not Flywheel.

Prop slip is between 10 & 15% for a Cruise type vessel. If the desired rpm is 4200 with a Prop slip of 13%, the calculated Prop pitch will be 17, keeping in mind that a "full cup" adds one inch of Pitch.

Therefore the recommended Props with 700 HP at the props are 18x16 full cup, 4 Blade @ 4200 engine rpm and 1.55 gear.

Note: If your min.displacement exceeds 13400 lb and/or HP is less, Prop pitch will need to be reduced.

Jango

 

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