Choosing the Right Size Piranha Propeller
- If your current aluminum prop is performing to your satisfaction, select that size in the Piranha.
- Know your RPM at wide open throttle (WOT). Your reading should be in the upper end of your engine’s operating range at WOT. Consult your manufacturer’s website for the operating range for your engine.
- For every increase of 2" in pitch, your engine RPM will decrease by 400 RPM.
- For every decrease of 2" in pitch, your engine will increase by 400 RPM.
THE LONG VERSION
To some, choosing the right boat propeller can seem like a job for a marine service center. Anything that has to do with the back-end of the boat can be a scary thing. Despite popular belief, choosing the correct propeller for a particular boat and motor is actually a fairly simple thing to do, once you understand what a propeller really is.
First think of a propeller as a gear, whether it be on a car or even a bicycle. The lower the gear, faster hole shot, more power, but revolutions are much faster so at a point you can only turn that gear so fast until you over rev the engine (or your legs). Now think of your car stuck in only 5th or 6th gear, very slow out of the hole, little torque, but eventually it will catch up with itself so you can cruise to a good top-end speed.
The same basic concept applies to propellers, except with a prop you are committed to only one “gear". So what to do now? Find the prop size that will give the best overall performance. Sound difficult? It’s really not.
Your engine has a recommended wide open throttle rpm range. Let’s take an example of an 18′ boat with a 125 hp engine with a rpm range of 4500-5500. In most cases you purchase a boat with some sort of propeller on it. Let’s say this motor has a 21 pitch prop on it and at wide open throttle under a normal boat load, the engine turns 4900 rpms. Well it’s in the range so it’s the correct prop, right? Well yes and no. For the health and longevity of the engine it’s okay, but for overall performance, you want to run a propeller near the top of the range. Well how do you change the size to get that?
Remember this rule of thumb: For every 2" of propeller pitch, rpm’s will change approximately 400 rpm’s. As you drop in pitch, rpm’s increase, and as you go up in pitch, rpm’s decrease. So if we want to raise your rpm’s we should choose a lower pitch prop. Following the rule, going down to a 19 pitch should raise our rpm’s to about 5300. That would help the hole shot and also gain us some more speed.
As you follow the rule of thumb, remember that sometimes rules are meant to be broken. This is not an iron clad rule, sometimes varying 2" of pitch will only change rpm’s by 200. Switching prop brands or manufacturer, it can sometimes throw off the rule too. At this point the most important thing to understand is that less pitch means more rpm’s, more pitch means less rpms. For your boat to perform at its peak, you need to run at the upper end (80%) of your engine’s recommended rpm range. That’s it. Once you’re there and still want more speed, handling, lift, etc, that’s when choosing the right propeller brand comes into play.
Here’s a simple adjustment you can make to improve performance under varying boatloads: If you normally run with one other person on board, use that boatload as your measurement when determining your wide open throttle rpm’s. But if you sometimes load the boat with 2-5 more people or try pulling a water skier or tube, get another prop 2" of pitch lower than your normal prop. It will compensate for added weight and drag and make a huge improvement on performance in these circumstances. But remember to keep an eye on you rpm’s when running a lower pitch for these circumstances.
Most props have the size marked somewhere on the prop, either on the barrel, on the blades, or in the front or back of the propeller. The first number is the diameter and the second number is the pitch ( 15 D x 19 P ) Use that size that you’re currently running as your starting point. If it’s already running correctly then that is your size.
Boat propellers travel one of 2 directions in normal forward use: clockwise (right-hand rotation) or counter-clockwise (left hand rotation). Most single installation outboards and sterndrives use right-hand rotation propellers. Only on twin applications does one commonly encounter left hand rotation propellers since the boat will be rigged so the two drives spin in opposite directions. If there is a single drive, you can bet it is probably right-hand rotation – unless the drive is a Volvo.
Left-hand, single drive Volvos are common. The boats were set up that way from the factory. Why? We don’t know the official explanation. There is nothing inherently bad about the choice as long as the boat as a whole was designed with this consideration in mind. On the practical side, this will definitely affect the availability of the propellers you have to choose from.
If you still require assistance choosing the right size for your application please submit the form below. We will be happy to make a recommendation. Or call us at 1-866-621-7767.