December 21, 2006

Round and Round - PROPS 101

I’m sure we can all agree that a Prop is a fairly integral part of the boating experience, but if you’re like me you don’t give them a whole lot of thought – they spin and make the boat move – good enough. All joking aside – there’s actually quite a bit to them and they’re more important than you might think. The prop is a critical element and having the properly (no pun intended) sized prop on your boat is key to performance and engine longevity. Folks have asked some great questions recently regarding how to select the right prop for their RIB, so I thought we’d look into it together.

Before we can even talk about selecting the right prop, I think it would be helpful to explore the anatomy of the prop, something that I like to call “Props 101”. So here are the basics.

Like people, propellers are either left or right handed. Left handed props rotate counter clockwise and right handed props rotate clockwise if looking aft. In addition, a prop has a leading edge and a trailing edge with the leading edge obviously being the part that first cuts through the water when spinning. The trailing edge is where the water flows off the prop.

Propellers are mostly measured in inches, for example 10” x 11” with the first number (10”) referring to the diameter and the second number (11”) referring to the pitch. So, what does that mean? The diameter refers to the distance the prop would make when spinning (distance traveled around the circle). This is measured by multiplying the distance between the end of the blade and the center of the prop or the hub. Pitch is a little more sticky (OK, pun intended this time); it is the theoretical progression that the propeller would make in one revolution. But, and this is where it gets sticky, water isn’t a solid so some slip occurs so the progression is actually slightly less.

Diameter is generally related to pitch. The lower the pitch the larger the diameter and the larger the diameter the lower the pitch. This makes sense because large heavy slow boats usually have lower pitched props with large diameters which provides the needed propulsion to move the boat. RIBs, which are faster and lighter generally need less thrust to get on a plane so as a result require smaller diameter props that have higher pitches.

Now that you have the basics covered, we’ll look at how to make sure you have the “right” prop on your boat in another posting. Stay tuned!

Posted by ribcraftusa at December 21, 2006 03:52 PM