The basics of prop pitch?

[Geo2008]

HELP!


I have no idea what the numbers describing prop pitch mean?

I have no idea what the various types of props mean or how to select?

Why would a three arm prop be better or worse than a four arm prop?


I have the boat below, Procraft 17ft combo with a 125 Mercury ELPTO and the prop has P24 written on it. I think the name of the prop is Laser II.

What kind of metal is my prop made of?


Can someone educate me on these matters?

Geo

 

[steelespike]

Re: The basics of prop pitch?

Prop demensions are expressed in inches -- X -- (diameter X pitch)
Pitch is basically the angle of the blades yours might be something like
14 X 24 Pitch number is the distance the prop moves through the water
in one revolution,disregarding slip.All props have some slip.The higher number pitch the harder to push the boat but can produce higher speed sort of like overdrive or high gear.(P24 may be your pitch but seems high to me)Lower number pitch improves hole shot/accelleration
but may limit top speed like the lower gears in a transmission.Diameter is the
result of the prop design/application and generally not considered except to be sure it will fit.Prop selection should put the motor well within its reccomended rpm range.To make an educated prop change we need the present max rpm and speed.Typical load and activity would help as well.
Props are usually aluminum or stainless steel stainless is usually unpainted while aluminum is painted.Stainless appears refined and more sofisticated
aluminum somewhat less refined blunter edges etc.

 

[Silvertip]

Re: The basics of prop pitch?

Do an on-line search for "Marine Propeller Theory" or "Marine Propeller Basics" and you can read until you puke.

 

[fdmsiv]

Re: The basics of prop pitch?

Boat speed is a function of propeller pitch. For the equation below you can determine the pitch you are looking for by determining your required top speed.

In most cases a slip of 10% to 20% (0.1 to 0.2) is used, 20% is on the conservative side

rpm X pitch X 0.000947 X ( 1 - Slip ) = Boat Speed in mph

For example, if you wanted 60 mph with a motor with WOT rpm of 5,000 and assuming a 20% slip you would want a pitch of about 16" (15.8).

Propellers are grouped in families with similar Pitch/Diameter ratios, and they are usually related, higher pitch, higher diameter. The problem with larger diameters at high rpm is that the blade tip will cause cavitation and end up eating your prop.

As far as the material the prop is made of your choice really depends on where you are going to use your boat. There was a recent thread with the alum. vs. stainless debate.

 

[Silvertip]

Re: The basics of prop pitch?

Pitch is also very much "power dependent" meaning you can't slap a 23" pitch prop on a 40 HP outboard and expect to go 65 MPH. Conversely you wouldnt slap a 10P prop on a 200 HP motor. To understand that, next time you go swiming, place your hand in the water so it is straight up and down which would represent a very high pitch prop. Now try to rotate it quickly around your body. Lots of resistance right? Now tilt you hand so it is flat or horizontal with the water and do the same thing which represents a prop with very little pitch. Not much resistance. So the higher the pitch, the more power it takes to spin it at recommended wide open throttle RPM.