Prop replacement

9rock · Post by **9rock** » Thu Mar 11, 2010 9:04 am

I think you can only repich a prop once so i would see if that hs been done to that prop

9

Rodman · Post by **Rodman** » Thu Mar 11, 2010 10:06 am

rossjo wrote:Per BoatDiesel.com (sign up for $25/year):

> Average Planing Hull
> 5400 lb
> 24' Waterline Length
> 1 engine
> 240 SAE HP
> 4,500 RPM
> 2,500 Cruising RPM
> V-Drive
> 1.1:1
> 3-Blade, Cupped
> 14" Diameter

RESULTS
MAX PERFORMANCE
Available Power: 232.8 SAE hp ( 173.6 kW) @ 4500 RPM
Max Speed: 32.2 kts
Fuel: 14 US Gall/Hr (typical)
CRUISING PERFORMANCE
Cruising Power: 48 SAE hp ( 35.5 kW) @ 2500 RPM
Cruising Speed: 14.6 kts
Fuel: 3 US Gall/Hr (typical)
RECOMMENDED PROPELLER
Diameter: 12.8 in. Pitch: 11.5 in.
Blades: 3 Cupped: Yes
Transmission Ratio Acceptable
PROPELLER BASED ON INPUT DIAMETER
Diameter: 14.0
Pitch: 10.0
Blades: 3

So, a 13x11.5 3-blade cupped is recommended, but a 14" diameters would be a 10" pitch.

I think you're engine is running weak - new points? Timing right? Good distributor? Carb opening? 4-BBL Carb? Motorcraft Carb?

The problem is, right now there is a 16x18 LH prop on the boat. Thats why I am trying to get a different prop.

Paul · Post by **Paul** » Thu Mar 11, 2010 10:14 am

How much clearance do you have between the blade tips and the bottom of the hull?

Rodman · Post by **Rodman** » Thu Mar 11, 2010 10:36 am

Paul wrote:How much clearance do you have between the blade tips and the bottom of the hull?

Less the a half of inch.

Paul · Post by **Paul** » Thu Mar 11, 2010 11:27 am

That's not enough clearance. There's a percentage of clearance that your supposed to have based on the diameter of the prop. I dont' remember what it was but I'll look it up. Check out this site below. Click on the prop it right tab and fill out the info and they'll tell you what size they'd recomend. Keep in mind that their recomendation might only be close and that you may have to adjust a bit for your specific application. They recomended that I run a 13 x 10 NiBrAl cupped which I did however a 14 x 9 NiBrAl cupped is the one that worked best for me even though it privides a little less clearance between the blade tips and the hull than recomended.

http://www.miwheel.com

Rodman · Post by **Rodman** » Thu Mar 11, 2010 12:32 pm

I found this site that has Prop Calculator.

http://www.vicprop.com/displacement_size.php

rossjo · Post by **rossjo** » Thu Mar 11, 2010 7:41 pm

Get rid of that HUGE 16x18 proip, and go with the 14x10 (or 13x11).

You need about 15% clearance min - and then go with the largest dia prop that would fit.

Danny Bailey · Post by **Danny Bailey** » Fri Mar 12, 2010 4:21 am

If slippage is excessive with a three blade, and you don't have clearance for more diameter, a four blade may perform better.

RWS · Post by **RWS** » Fri Mar 12, 2010 6:10 am

the 4 blade will GENERALLY provide more lift than speed.

With the twins on the International the 4 blade is more efficient.

This was easily proved as my boat came wearing 4 blades and had a set of brand new still in the boxes 3 blade Nibrals.

Getting the props dialed is is part science part math and part feel.

You need to get close with the math & science first, then see about making a fine adjustment by feel working closely with a good prop shop. A little added cup can make a big difference.

RWS

prowlersfish · Post by **prowlersfish** » Fri Mar 12, 2010 6:40 am

4 blades are smother but in many case's the 3 blades will preform better ,
( here we go again )they both have pros and cons the right size is the most important thing

Danny Bailey · Post by **Danny Bailey** » Fri Mar 12, 2010 9:56 am

It depends on the percentage of slippage. If a three blade does not have sufficient blade area to generate enough tons of thrust to shove the hull through the water without a lot of slippage, vibration and spinning water a four blade will perform better. To figure slippage multiply prop pitch by RPM divided by 12 times 60 divided by 5,280 to obtain speed in MPH with no slippage (theoritical). To find slippage divide this number by your actual speed at this RPM and subtract it from one. Or you can just find a Prop guy (50 yrs experience) like Bulls Prop Shop in Beaufort NC and ask him what to put on it. Thats how I 'discovered' four bladed props on my last boat.

Paul · Post by **Paul** » Fri Mar 12, 2010 10:15 am

Can you get a four blade in a 14" diameter?

rossjo · Post by **rossjo** » Fri Mar 12, 2010 10:16 am

With 250HP pushing only 5,400 lbs with 24ft WLL, the 14x10 3-blade will be very close.

That 16x18 is WAY oversize. I bet it makes a lot of noise with turbulence between it and the hull - there can't be more than 3/4" clearance!

Viva out ...

foofer b · Post by **foofer b** » Fri Mar 12, 2010 1:18 pm

My boat and Rodman's by extension, both were sold with 14x10x1 3 bladed nibral props. Mine does not hav standard power , so I repitched the 14x10 to a 14x12. It was still too little prop for my 454 crusader. Then I had a 14x16 cupped prop made, and it is too much prop. I think taking two inches out of it will be about right. (14x14) But anyway 14" is the largest I can run under my boat for clearance sake.

Rodman · Post by **Rodman** » Fri Mar 12, 2010 1:30 pm

foofer b wrote:My boat and Rodman's by extension, both were sold with 14x10x1 3 bladed nibbler props. Mine does not have standard power , so I re pitched the 14x10 to a 14x12. It was still too little prop for my 454 crusader. Then I had a 14x16 cupped prop made, and it is too much prop. I think taking two inches out of it will be about right. (14x14) But anyway 14" is the largest I can run under my boat for clearance sake.

I was thinking about coming get that prop this evening or early in AM in ST.Augustine What every day is best for you.