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By D. Clay Hays Your boat's prop may be the last component of its propulsion system but it's certainly not the least important. However, props can be a very difficult subject. There are so many variables and without some understanding of "Prop Basics", it can get very confusing. I'll do my best to simplify - try to stay with me! Props are measured in diameter and pitch (i.e. 14-1/4 X 21). Diameter is self explanatory, pitch has to do with the angle of the blades and is measured in inches. If a prop has a "21 pitch", then theoretically it should move the boat forward 21 inches in one full revolution. Simple enough, huh? Now, here's where it gets a little more complicated but this is an important thing to know before you go to switching props around! When you change to a different pitch, you also change the engine's RPM level. The theoretical change per inch of pitch is 200 RPM - in the opposite direction! Meaning, if you go down from a 21 pitch to a 19 pitch your engine's RPM will increse by 400 (2 inches X 200 RPM) at wide open throttle. And if that puts you above the normal operating range, you could literally blow your engine! Most late model engines, both inboard and outboard, are equipped with rev limiters that are designed to keep this from happening. Even so, it is still important to prop your boat correctly so that it runs in the desired range. Running an engine at lower RPM's than suggested can be just as detrimental as going over. I strongly suggest you consult a pro before you try a prop other than what came on your boat. I'll be glad to answer any questions you have but the "real" expert around here is Dennis @ Perfect Pitch Prop Shop. Now lets talk about the different materials used to make props. Aluminum is by far the most widely used but Stainless Steel is a very popular performance upgrade, and Composite props have become more prevalent in recent years. There doing some amazing things with plastics these days! Aluminum props are much less expensive and are easily repaired but are more easily damaged as well. Stainless Steel props can be repaired too, but you can count on the tab being 2 - 3 times that of an aluminum repair job. Switching to Stainless Steel can gain an overall performance increase but can be pricey. However, most boaters with a need for speed will agree the better performance is well worth the average $450 to $650 price tag. What kind of performance increase you ask? Stronger hole shot, better handling & control, and very often a couple extra miles per hour on top end. Yeah, yeah, some will say, with Stainless Steel, you risk damaging the lower unit if you strike a submerged object because the blades are so hard and don't give way like aluminum does. I agree that is mildly possible, but not as likely as some would have you believe. Especially since the advent of plastic hub inserts which are designed to self-destruct to save "the goods" in the event of contact with a stationary object. Composite props are still fairly new but have been coming on strong. One interesting concept with some Composite props is the ability to replace individual blades if damage occurs. I've seen some thought provoking test results in boating magazines but, for me, the jury is still out. There is much more to learn about propellers but studying this post will put you a head above the average boater. At least you'll know what "pitch" means and they'll still be calling their props "that thing down there that makes it go". Besides, gives me a reason to come back with "Props 102"! Next time.....Clay If you'd like to comment on this post please use the Contact form.
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Prop Basics - Props 101
