Looking to optimize the performance of your boat? Want to go faster when you are up on a plane? Or perhaps you want to get up on a plane faster. Want to have the power to pull a skier or two without bogging down your engine? These are all problems that boaters face when trying to make their boat run as efficiently as possible. One of the easiest ways to fine tune your boat is by making sure you are using the correct boat propeller. the next logical question is How do you know what type of boat propeller to use? I’m glad you asked…..
To choose the correct propeller for your boat you will need to know some basic information about your boat engine WOT(wide open throttle)RPM, Propeller Pitch and diameter, and how you intend to use your boat. With this you can increase or decrease the pitch of the prop to bring your engine Wide Open Throttle RPM to the higher side of the engine recommended RPM range.
But this is not all that needs to be taken into consideration when choosing a prop for your boat. Read below to learn how to fine tune the performance of your boat by choosing the correct propeller.
What will you do with your boat?
Even before you get into the specifics of the type prop you want to get for your boat, you first need to know how you are going to use your boat. Are you just looking to get out on the water and go as fast as you can? Or are you more concerned about maintaining a good fuel efficient cruise? Maybe you are looking to be able to tow 2-3 wake boarders at the same time? Will you be the only one on the boat or will there be several people on the boat? Understanding what you want your boat to do plays a big factor in how you choose a propeller. For instance if you are mostly concerned with water sports like wake boarding you may need a prop with more pitch to give you the power you need get to a plane faster. While if you are looking for top end speed you may need a smaller pitch blade.
Now with this in mind your boat also plays a role in this decision. The size and weight and hull type of your boat can mean the difference between a 13 pitch prop and a 17 pitch prop. In other words a pontoon boat will not react the same as a speed boat with the same propeller. Now I have thrown the word pitch out there a couple times….. don’t worry we are getting to what all that means.
What is Pitch and Diameter
Now before we go any further you need to understand how propellers are classified by their sizes. Propeller sizes are shown as either a 2 or 3 number sequence that is either stamped on the hub or on the inner hub of the prop. The first number is the Diameter of the prop from blade tip to blade tip. the next number is the pitch of the blades. It there is a third number then it is the number of blades that the prop has. For example of what this would look like:
9.75 X 17 X 3 This example shows a 9.75″ dia. X 17 pitch X 3 bladed propeller.
Now that you can Identify a propeller, let’s talk about what it all means. First, Pitch is the the distance that a propeller will travel through a solid after 1 full revolution, in inches. Using the example above, the propeller has a pitch of 17 so after 1 full revolution it theoretically will travel 17 inches. I already know what you are thinking…. then the higher the pitch the faster I will go!! Now just hold your horsepower. You noticed I said you would theoretically travel that far. There is another term you need to know. Slip is the actual distance the prop travels though water minus the pitch. Once again using the above example, if after the full revolution the prop only travels 15 inches, this would give us a Slip of 2 inches. Obviously the lower the slip the more efficient the prop is. you will see how this works later. And finally, Diameter. If you were to draw a circle around your prop using the tips of the blades, then draw a straight line though the middle this would be the diameter of propeller. There are many different size diameter props sold. Usually the size of your engine determines the diameter of the prop. bigger motor the bigger prop you can have. For now we are just going to say that larger diameter will give you a better dig into the water which will give you more power. But with that you have to have the horsepower to turn those blades that are digging into the water.
Know how boat propellers work with engines
Now that we have determined how we are going to use the boat, and what pitch and diameter is, we can use the final determining fact when choosing a propeller for your boat. Wide open throttle RPM is the range at which your motor performs at its best according to the manufacturer. This is where we want your boat motor to run making it as efficient as possible. And everything we discussed above will determine what RPM your motor will run. Most outboard motors usually run at about 5000-6000 RPM, while most inboard motors will stay around 4500-5000 RPM range. Making our prop selection to stay within these ranges will prolong the life of your motor. Just like a car motor, if the revs are too high you can cause damage to internals. And if your RPM is too low you are making the motor work too hard and it will wear out faster than it should. So maintaining this range is very important.
Now you are wondering how your propeller will determine your engine RPM’s. Here is an experiment that I think will explain what happens in its most fundamental terms. Take a yo-yo and unwind it. give yourself about 10 inches of line and spin the yo-yo around your hand as fast as you can. Now got to about half way up the string and spin it again. now go to the end of the string and spin it one more time. So what happened? The further you went out on the string the slower the RPM’s became and you had to work harder to get it to spin. For the most part the same thing applies to propellers and Engine RPM. Different sizes will determine your RPM based of the engine horsepower. Here are two general rules to remember:
For every inch of pitch your engine RPM will increase or decrease by about 150-200 RPM.
For every inch of Diameter your engine RPM will increase or decrease 500 RPM.
Example:
Say your engine sweet spot is 5000 – 6000 RPM and we want to be a little on the high side of that of that range to get more top end speed. And your current prop has your WOT RPM at about 5400 RPM. Let’s say your prop is currently 12 X 17 X 3. If we want to increase the engine RPM by 400 RPM we would reduce the pitch to a 15. this would get us to 5800 WOT RPM. Less pitch increases RPM and improves speed, but, you lose lower end power to get up on a plane. More pitch will reduce speed but will give you more low end power. See, this is why we needed to know what the boat is going to be used for. If you lower your pitch too much you may go faster on the top end, but if it takes you half the distance of the lake to get up onto a plane is it worth it? Or vise versa, if you have a ton of power to pop you up on a plane, but you can only go half the speed the boat is rated to go, are you really running efficiently? you have to choose how you like it to run.
Boat Propeller types
There are mainly 2 types of materials that props are made of, Aluminum, and Stainless Steel. Each of them have their purpose, pros, and cons. First, if your boat motor has more than 150 horse power you should be using a Stainless Steel prop. The Short answer to this is that with the higher horsepower aluminum props will flex more and it will lose efficiency. Remember when I was talking about Slip earlier? Because aluminum is a softer metal it will flex in the water. It will increase you slip and it will make it less efficient. You will not get this with the steel prop. One of the draw backs from this is that Stainless steel is at best, twice as much as an aluminum prop. But you will also get twice as much durability out of it. If you are not careful though that durability can hurt you if you are not careful with it. One of the biggest problems with a Steel prop is that it will not break like an aluminum prop will if you hit something. It could very possibly break your lower end instead. And that will cost you big time. That is why Stainless steel props are not recommended anywhere other than open clear water.
3 or 4 blade prop?
This is another one of those questions that is up to the preference of the boater. Three bladed props are more efficient than a four bladed prop. But the Four blade prop is better balanced and is smoother in the water, causing less vibrations. That is why 4 blade, or even 5 blade, props are the choice of speed boat owners. So to recap, the few the blades the more efficient the propeller is but the more blades better balanced and smoother.
FINALLY!
I know this is a lot of information to take in. And I know that a lot of what was said in here was really back and forth. As much as I would like to tell you that there is a magic number for a prop, there isn’t. Each boat is different, and each boat is used for a different reason. The best I can do is give you the information you need to have an educated guess on where to start. From there it is just trial and error to get the boat to respond to how you like it. Now with that being said, there is no rule that says you can’t have multiple props. You can a prop for each situation that you will be using the boat for. Not to mention it is always a good idea to have a spare prop on the boat for emergencies.
Quick Boat Propeller Definitions:
WOT – Wide Open Throttle: This is the recommended RPM that the engine manufacturer says the engine runs most efficient at. It is usually between 4500- and 6000 RPM depending on motor type.
Pitch: Is the the distance that a propeller will travel through a solid after 1 full revolution, in inches.
Diameter: Is the distance in a straight line though the center of the Hub from blade tip to blade tip.
Slip: is the actual distance the prop travels though water minus the pitch.