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The Correct Prop For Your Boat! How to Know! [VIDEO]
Video Transcription
It's Technicians Tuesday everybody and we hope you had a Merry Christmas and we wish you a happy new year tonight. This week we are going to learn all about propping out a boat, why it's important, how to do it, and what finding the correct prop for your boat means for your wallet. This week's contest prize is some boat cleaning supplies from the Better Boat so smash that subscribe button, get the bell on and let's talk about some props.
They are the wheel of the boat and they come in all kinds of different styles and sizes from two blades to six blades from little props to big props. It's important to understand how they work because they affect the performance and the fuel economy of your boat. A propeller is measured in diameter and in pitch. The diameter is how big around the prop is and the pitch of the blade is defined as the theoretical distance the prop will move through the water in one revolution or turn. So a 19 pitch prop is theoretically going to move 19 inches through the water in only one turn. Now think about that at 6,000 RPMs, that's slicing through the water.
The keyword here though is the theoretical distance. If it wasn't theoretical then we would say that the bigger the pitch the better the prop but that's not really the case. We need to consider the physics of the boat, propeller and the water. The more of the boat that is in the water the more drag. Yes the more contact that we have with the water the better control we have but it also slows us down. The same goes for the prop. The bigger the prop, the more drag we have. This goes for the number of blades that the prop has as well. The more blades that the prop has the more drag it will have and the less top end wide open throttle speed we will be getting.
It's important to note that we are talking about the average recreational boating and fishing application here. We aren't talking about racing though the physics don't change there are way more variables to consider for racing than what we are going to talk about here today. For us if we go from a three blade to a four blade without changing the diameter or the pitch of the prop we will lose top end speed. What we'll be gaining though is our whole shot. The time it takes to get the boat out of the water will be way quicker because we have that extra blade which gives us more grip from the prop to the water. With that extra grip we also gain better control and handling. Understanding these things are the base foundation for us to begin the propping procedure to get the right prop for your boat.
Now each engine is going to have a top end wide open throttle RPM range. Each engine that has been manufactured in the last 20 years is also going to have what is called a rev limiter. The rev limiter is going to stop the engine from going over a certain amount of RPMs. We'll take a Verado for the first example. The RPM range for most Verados is 5,800 to 6,400 RPMs and the rev limiter is set to 6,500. On a 300 horsepower Yamaha the RPM range is 5,000 to 6,000 RPMs and the rev limiter is set to 6,300. The rev limiter setting isn't as important to know it's the RPM range of your engine that you need to know about.
If we put a small diameter prop on an engine with a low pitch the chances are that the power of the engine is going to power through the ability of the propeller to grip the water and push the boat. This results in the engine RPMs going above the specs and hitting the rev limiter. It's exactly like what happens when the prop hub gets spun. For all of you that have experienced spinning a hub before you can remember what that's like. On the opposite end of the spectrum though if we put a prop with too big of a diameter and pitch on then the engine won't have enough power to spin the propeller up to the right RPM range causing the engine to never get to wide open throttle and if ran like this for a long period of time can eventually wear out internal components of the engine prematurely.
There are ways to get propellers to turn up higher though without changing the prop. This is done by venting the propeller. Some propellers have venting holes already on them towards the bottom of the prop. These holes can either be plugged or have the plugs taken out of them. What this does is allows water to flow through the prop without having to be pushed off of the blades of the propeller creating force against the prop. This is useful in a wide range of applications but we still need to talk about how to properly choose the correct prop for the boat.
We know about the RPM ranges, the pitches and the diameters of the prop as well as the ability to vent a prop and go from three to four blades or vice versa but actually propping out the boat is a process of trial plain and simple. We have to make an educated guess of a prop to start with, take the boat out and try it and then go from there. Before doing anything though the conditions of the boat make a huge difference. Things like the amount of fuel in the tank, how many people are on board and what kind of gear we are carrying as well. You can prop out a boat with an eighth of a tank of gas with just you on the boat.
The RPM may come out correctly but when it comes time to use the boat normally it will definitely be different. That is because for adults weighing 200 pounds a piece, a hundred gallons of fuel weighing 600 pounds, two coolers full of ice, food, drinks and bait weighing about 150 pounds, eight rods and reels, the live well full of bait and water being roughly 250 pounds now the boat weighs almost 2000 pounds more and this could be a conservative number depending on the situation. That prop that was spinning let's say 5,800 is now only turning up 55, 54 or maybe only 5,200 RPMs which means that we are no longer propped out correctly. The more weight that the engine and the propeller has to push is ultimately what decides what prop we should use.
We can pick a prop based on the recommended propeller from the factory or manufacturer of the boat. We can also use the previous prop that we are running in a repower situation or for just trying. We'll install the prop and take the boat out for a test drive to try it. Let's say we have a 15 inch diameter prop with a 15 pitch.
Our engine RPM range is 5,000 to 6,000 RPMs and we've got close to the right amount of weight onboard that we will normally have. At wide open throttle our engine turns 6,300 RPMs and is hitting the rev limiter. This means that our prop is too small and isn't big enough to push the weight of the boat so we can change out that prop and put on one with say a 15 and a half inch diameter and a pitch of 23. So after taking the boat back out this time we only turned up to 5,400 RPMs. Now the prop is too big. Given these numbers we can keep that diameter and go back down to the 21 pitch and we will be looking at getting roughly 58 to 5,900 RPMs which means that we have found the right prop to use.
We need to talk about trimming up during this process though. Normally you won't have the boat fully loaded when propping it out. It's best to prop it out towards the top end of the RPM range without trimming the engine. We want to be able to add some trim to the engine and come really close to hitting the rev limiter under these conditions. That is because whenever we load the boat up and take it out under normal conditions, there will still be some room to add some trim at wide open throttle and have the RPMs come all the way up to the top end of the RPM range. If we have to trim up the engine to get to the top of the range then after we load the boat we won't be able to get to the top of the range because of the weight that has been added to the boat.
Now that you know how to prop out your boat let us know about your experience in the comments below. Make sure to include a hashtag clean boat for the contest prize, follow us on Instagram and let's talk about what having the right prop means for your wallet. Running the correct prop is going to save you some money because it improves your mid range cruising performance. If running a small prop you'll burn more fuel running higher RPMs just to stay on plane at a good speed. Running too big of a prop means that you'll never hit wide open throttle and that will lead to carbon buildup along with poor performance. With the right prop though we can cruise at an optimal mid range RPM with a little trim burning the least amount of fuel as possible.
As far as the three blade versus four blade debate that is ultimately up to you and what you are looking to get from your boat. Would you rather have a better whole shot or top end speed? The decision is up to you. Running larger boats close to the 30 foot or higher range generally running the four blade is best because of the size and weight of the boat. That extra blade really makes a difference in getting the boat up out of the water and controlling it at higher speeds. Don't forget to hit that like button and introduce us to your boating community by sharing us with a boater you know. Thank you for hanging out with us today. We look forward to seeing you next week. Now we hope everybody has a happy new year.