Props 101

[Z3CO23]

So... I've seen some discussion on props over the last few days so I thought I'd throw this thread up here. I was curious as to what prop my boat had (I had never checked). My boat has a 1631 on it, so with the trusty ole google I was obviously able to find that the 1631 has a different pitch. From what I can find, I believe my boat had the stock 537 as many of our boats have... is this correct? I have also seen a lot of people swapping their props for 1235's. I obviously have absolutely no working knowledge of props, can anyone chime in and explain what different pitches and sizes of props do? Advantages and disadvantages of different sizes and pitches?

[boatwakes]

Best thing to do is google diameter and pitch for detailed explanation but here's the quick and dirty:
- Always size your prop to achieve the correct wide open throttle rpm listed on your engine, if you use a prop that allows the motor to fall short a couple hundred rpm, you're overloading the engine and will cause serious damage over a short period of time.
- If you undersize your prop, you'll end up over-revving the engine if you're not watching and/or listening to the rpms. Again possible serious damage to the engine. Would only recommend undersizing on a motor with over 150 hours on it, break in time.

Diameter of a propeller is its actual diameter measured in either inches or mm depending on brand.
Pitch is the theoretical forward movement through the water per revolution. 13"pitch means theoretical 13" of forward movement through the water per propeller revolution.
Our rule of thumb when we repower boats involves the vessel weight, engine hp and max rpm of engine being used and when sea trailed we want about 50-150 rpm above our max rated rpm with average load weight on the boat. We always want our diameter and pitch to be as close as possible to take advantage of propeller efficiency and equal blade loading not to mention transmission health. This is especially important with surfing boats using weight and relatively slow speeds. Keep the prop blades loaded at those low speeds or it will aerate and unload the blades and the engine.

[Ewok]

Here is a good start: http://www.acmemarine.com/tech_info.php

And if you want to get into understanding how the cup of the propeller describes the camber of the blade, which produces lift which pulls the prop through the water, look here: http://en.wikipedia.org/wiki/Lift_(force)

After reading that, we can have some discussions about how the size and weight and power of our boats can benefit from different props depending on your desired performance preference.