December 27, 2006
Selecting the Proper Prop (Props 102)
Now that we have the basic anatomy of the prop covered, let’s look at how we can go about insuring that the optimal prop for your boat is installed.
Getting the right prop on your boat is very much a process of trial an error. The best place to start is to look at what you have on your boat now.
When your boat is at wide open or wide open throttle (WOT) what is the maximum RPM? Does this RPM fall between the manufacturer’s recommended range? If it does then you most likely have the best prop for your boat – if not, there’s work to be done. The goal when selecting the optimal prop is to have your maximum RPM for the engine fall between the range given by the manufacturer. The higher the RPMs you have within the range, the higher speeds you will most likely reach. Having the correct prop will deliver maximum speed and optimal efficiencies. However, if the RPM exceeds or falls short of the range, it will cause unnecessary wear and may damage the engine so it’s very important that your boat is propped correctly.
Once you have figured out what you’re getting for RPM with your current prop, it’s time to determine what size prop you have. Typically the prop size is marked on the side of the propeller, but you may need to remove the prop as manufactures sometimes put the markings on the hub or the closest part to the engine.
While you have the prop off I’d also recommend noting the number of splines there are on the propeller shaft as these can vary from manufacturer and between engine type (2-stroke vs. 4-stroke). You’ll also want to measure the gear case size. This is the outside dimension of the casing where the propeller abuts.
So, now that you know your current RPMs and prop size, what next? If your current RPMs fall within the engine manufacturer’s recommended range, than I would recommend nothing – stay with what you have. But, if it’s below, above, or you’re a “tweaker” then just focus on the pitch as the diameter is generally pre-determined by the engine manufacturer.
Propellers are sized in two inch increments; 17”, 19”, 21”. Generally a 2” pitch change will result in an average change of 300-400 RPM at full throttle. When thinking about pitch – think of it in terms of gears on a car. Selecting a higher pitch propeller will decrease RPM at full throttle, just like when shifting up to a higher gear in your car. Transversely, going to a lower pitch propeller will increase the RPMs.
Higher the Pitch = Lower the RPM
Lower the Pitch = Higher the RPM
Posted by ribcraftusa at December 27, 2006 12:57 PMBack To Index