Your boat’s propeller is an essential part of your boating experience. It is an important part of your vessel as it determines how well your boat works and performs in the water as well as your boat’s safety level. When it’s time to purchase a new propeller, you have to be sure to buy one with the right pitch and diameter for the best performance. Before you can buy your new propeller, though, you really have to understand how to determine the pitch of a boat propeller. This ensures that you buy the right one and equip your vessel with a part that will last long and do the work it’s supposed to do without issue. Without further ado, let’s learn a little about propeller diameter and pitch to get started.
Understanding Diameter and Propeller Pitch
The diameter of your propeller is the measurement of the distance across the circle that is made by the tips of the propeller blades. This circle can vary depending on the speed of the propeller. According to Mercury Marine’s manual (Everything You Need to Know About Propellers) regarding propellers, the diameter increases for propellers use on smaller boats, and decreases for those used on faster boats. Bottom line, the diameter of your boat’s propeller will increase as boat power increases and diameter will also increase as the propeller rpm decreases.
Pitch, on the other hand, refers to the maximum distance your boat propeller moves in one revolution through a “soft solid.” Of course, the definition itself is pretty confusing because boat propellers don’t rotate through a “solid”, soft or otherwise. The best way to grasp this concept, then, is to think of it this way:
Imagine a screw going through a piece of wood. The wood, of course, is solid, but the screw is still able to rotate completely through it. Every time the screw rotates completely, it impresses a certain distance into the wood. This distance is known as the screw’s pitch.
Still confused? You’re not alone. In short, determining the pitch of a boat propeller can be pretty tricky and confusing. This is because the practice itself is highly theoretical. Though in this scenario the water would be like the wood for the screw, water itself is amorphous and moveable, making the distance the propeller impresses on it hard to measure. Fortunately, there are ways to determine the pitch of your boat’s propeller despite all the confusion.
As stated, boat propellers do not move through wood, making the measurement a little hard to translate. For example, if you have a pitch propeller of 21,” you would assume that the propeller will travel 21 inches in one rotation. Unfortunately, this is not the case due to boat propellers rotating in water. The water element changes the distance the propeller actually travels and introduces a number of other factors as well: hull drag, unit drag, ventilation, cavitation, and aerodynamics, among others. The change of the distance measured in water is known as “slip.” The slip itself really refers to the difference between what a propeller’s theoretical pitch ought to be (think the 21” in our above example), and what it really does in real time in real water. To help things along a bit more, let’s check out the math to help you calculate slip - it could come in handy:
Speed and Slip
To begin, think of the theoretical speed. The full calculation looks something like this:
(pitch x RPM)(gear ratio x 1056) = theoretical speed
Once you have this solution, you can use it to determine slip. The calculation is:
(theoretical speed - actual speed) / theoretical speed = slip
These equations are useful if you suspect that you have the wrong propeller pitch, which can cause many issues when out on the water.
Another important thing to remember about pitch is that it measures the maximum distance a propeller travels in one rotation. When thinking of boats, this means how far the propeller moves the vessel through water. Pitch, then, measures the revolution at 100% accuracy, which is problematic because the propeller can never perform at 100% while in the water. Due to elements already mentioned, such as drag, aerodynamics, and fluid dynamics, we already know that there is a certain loss of power when the propeller moves through the water, meaning a less-than-accurate pitch determination. In short, we can calculate that at least ⅓ of maximum efficiency is lost in the water. With this in mind, a 21” pitch will only move the vessel forward about 14 inches through the water in real time.// ]]>
For the most accuracy, you’ll want to measure your propeller itself. The best way to do this is to remove the propeller from the shaft and have it on a table or other flat surface where you can make clean measurements. If you cannot detach the propeller from the shaft, don’t worry. You can still do the measurements just with a little less accuracy.
To begin measuring, look for the widest part of one propeller blade. Once this has been identified, draw a line on the blade from one edge to another. From here, you will measure the distance from the front of the hub to where you line meets each edge of the propeller blade. This measurement is best taken when viewing the propeller from the side. Once you have your measurements, take the smaller one and subtract it from the bigger one.
Now you’ll need either a protractor, angle, gauge, or carpentry square in order to get the measurement of the triangle created by the two points at either end of the line you drew on the propeller. The triangle should be situated as thus: the point should be at the center of the hub and the two lines drawn out from the center of the hub to the edges.
Once you have all of your measurements, take the first one and multiply it by 360. Take the result and divide it by the angle of the second measurement. The result of this equation will be the pitch of your propeller.
If all these equations are making your head hurt, or if math has never been your strong suit, there are other tools to help you determine pitch. Prop gauges are super useful tools that help you find the pitch of your propeller without having to do calculations totally on your own, offering you some useful help. Whichever route you choose, you can now determine the pitch of your boat propeller, giving you a smoother sail next time you set out to sea.
You can also check out this video to get a visual representation of how to determine the pitch of your boat propeller:
A Word of Caution: When You Have the Wrong Pitch
When looking for a new propeller, you run the risk of getting one with a pitch that just doesn’t work for you boat. To explain: if your boat’s engine is running too many RPMs, it is known as “under-propped” and you need to increase the propellers pitch (i.e. purchase a new one). If you increase the pitch of your propeller by just one nch, you can reduce RPM considerably. In the same vein, if you reduce the pitch by an inch, you can also increase the engine RPM by the same considerable amount. If you think your boat propeller has the wrong pitch, speak to your dealership or the manufacturer - they’re just as dedicated as you are to finding the right prop fit for you boat.
Overall, determining the pitch of a boat propeller can be difficult, but it is not impossible. We hope this guide and its resources offer you the help you need and we wish you happy sailing next time you’re out on the water!