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(New boat owners only) Prop calculations For fastest possible prop

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  • 3 Post By Frontier2104Df200a
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(New boat owners only) Prop calculations For fastest possible prop

Old 07-13-2019, 08:59 PM
  #1  
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Default (New boat owners only) Prop calculations For fastest possible prop

The big problem for guys trying to understand props and how to work out the right prop. They are using these calculators instead of doing the math. If you do the math yourself, you can wrap your head around how a prop should be picked for best performance and you can start actually understanding what the numbers actually mean practically. Once you understand that, you become a guy who can dial in props with little to no help.

Lets start with the actual formula, then we will break it down
Understand your motorís max RPM IS constant it ďcanítĒ go any hire than that, and the props pitch is constant (if you use the most aggressive pitch recommended for the motor you can get a perfect world number that tells you the absolute top speed of that motor .if you always use the 10% slip number we discuss later. It is just a good number to use to give you a good ball park idea how fast this motor will actually be.....those 2 numbers RPM and Pitch are the only numbers that give you MPH once you account for gear ratio.
Formula
RPM x Pitch = (answer) x .000947 =(MPH answer) / gear ratio = FINAL MPH for that MOTOR on the perfect boat at 0 slip, perfect conditions (impossible in water) this lets you know what your motor and pitch of your prop can do in a perfect world ( you will use this number after you run your boat to decide what your slip is. Lets say it is 6000 rpm at 60 mph (so no matter what, that motor will never go any faster than this number if the max RPMs for the motor is 6000 with that certain constant pitch.

When dealing with a prop, you drive your boat WOT, take your max RPM. and your top MPH and pitch

Take your boats number letís say just as an example 6000 rpm at 51 mph real world numbers, and the 6000 rpm at 60mph perfect world numbers

take (real world number) divided by (perfect number)

51/60= .85. Take this answer and for non math people just drop the decimal and subtract 85 (or what ever number you get ) from 100 and that will give you the percent of slip, or your slip number, in this case slip is 15%. Which is a little below average for most good props. Try to at least hit 12%.

(Side note all Suzuki OEM props are 2 pitches above what is noted on the prop...trust me this is true.) do you should always calculate their pitch two above the marked number ex if it is a 20p you will calculate 22p when doing you equations.

So so the first problem you need to know is

RPM x Pitch is the real important numbers. First Pitch is set with any given prop. Diameter does not have anything to do with increasing speed directly, itís relationship is with RPM. Same with height of motor and itís drag, and the drag of the boat. They only make the motor work harder and can cause changes only to RPM and slip.

So the only way you can work with that prop Pitch is change RPM to get a higher Mph. The pitch is your constant.

Quick notes
raise/lower the motor - change RPM
lighten in the boat -changes RPM

What we can do with this knowledge, it makes boat shopping and motor shopping a little more open book. You can look for the max RPM a motor can safely turn, and check the specks for the motor to see the most aggressive prop it can turn and you can get some comparable numbers that will give you ball park ranges in picking a motor and get a good guess for speed. Always use 10% slip when doing these best case scenario calculations. This can let you know the top speed limit the boat and motor will ever be able to get to. It also helps you understand when you start testing your first prop how close are you to best case, if you hit 10% of best case for the boat with the aggressive prop, you can feel confident you can stop.


I will do step 2 to the process tomorrow, that is using diameter, and motor height, to change RPM to a point you are at max RPM and the highest pitch the boat can turn while maintaining a slip of close to 10%. This will give us max speed for this boat. Remember this first lesson is all about finding the fastest prop. We will discuss best over all prop on lesson 3 or 4



Last edited by Frontier2104Df200a; 07-13-2019 at 09:04 PM. Reason: Words mistake
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Old 07-14-2019, 01:49 AM
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In the end it’s still trial and error if the prop manufacturer is not putting the right numbers on there props.
alloyboy and Grunts N Grits like this.
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Old 07-14-2019, 07:48 AM
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Default Understand

It seems many props are off, you are correct. On another note it seems all Suzuki OEM Watergrip props might be very efficient at the 16 diameter in most boats. People say itís impossible, but we are getting 4 - 6 % slip, single motor applications.
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Old 07-14-2019, 07:51 AM
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Originally Posted by brownman View Post
In the end itís still trial and error if the prop manufacturer is not putting the right numbers on there props.
And there is so much more than the numbers. Think that every blade of every 15.5" diameter 17" pitch propeller are going to be exactly alike? I doubt that each blade of any three bladed propeller will be exactly alike.
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Old 07-14-2019, 07:53 AM
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Originally Posted by Frontier2104Df200a View Post
The big problem for guys trying to understand props and how to work out the right prop. They are using these calculators instead of doing the math. If you do the math yourself, you can wrap your head around how a prop should be picked for best performance and you can start actually understanding what the numbers actually mean practically. Once you understand that, you become a guy who can dial in props with little to no help.

Lets start with the actual formula, then we will break it down
Understand your motorís max RPM IS constant it ďcanítĒ go any hire than that, and the props pitch is constant (if you use the most aggressive pitch recommended for the motor you can get a perfect world number that tells you the absolute top speed of that motor .if you always use the 10% slip number we discuss later. It is just a good number to use to give you a good ball park idea how fast this motor will actually be.....those 2 numbers RPM and Pitch are the only numbers that give you MPH once you account for gear ratio.
Formula
RPM x Pitch = (answer) x .000947 =(MPH answer) / gear ratio = FINAL MPH for that MOTOR on the perfect boat at 0 slip, perfect conditions (impossible in water) this lets you know what your motor and pitch of your prop can do in a perfect world ( you will use this number after you run your boat to decide what your slip is. Lets say it is 6000 rpm at 60 mph (so no matter what, that motor will never go any faster than this number if the max RPMs for the motor is 6000 with that certain constant pitch.

When dealing with a prop, you drive your boat WOT, take your max RPM. and your top MPH and pitch

Take your boats number letís say just as an example 6000 rpm at 51 mph real world numbers, and the 6000 rpm at 60mph perfect world numbers

take (real world number) divided by (perfect number)

51/60= .85. Take this answer and for non math people just drop the decimal and subtract 85 (or what ever number you get ) from 100 and that will give you the percent of slip, or your slip number, in this case slip is 15%. Which is a little below average for most good props. Try to at least hit 12%.

(Side note all Suzuki OEM props are 2 pitches above what is noted on the prop...trust me this is true.) do you should always calculate their pitch two above the marked number ex if it is a 20p you will calculate 22p when doing you equations.

So so the first problem you need to know is

RPM x Pitch is the real important numbers. First Pitch is set with any given prop. Diameter does not have anything to do with increasing speed directly, itís relationship is with RPM. Same with height of motor and itís drag, and the drag of the boat. They only make the motor work harder and can cause changes only to RPM and slip.

So the only way you can work with that prop Pitch is change RPM to get a higher Mph. The pitch is your constant.

Quick notes
raise/lower the motor - change RPM
lighten in the boat -changes RPM

What we can do with this knowledge, it makes boat shopping and motor shopping a little more open book. You can look for the max RPM a motor can safely turn, and check the specks for the motor to see the most aggressive prop it can turn and you can get some comparable numbers that will give you ball park ranges in picking a motor and get a good guess for speed. Always use 10% slip when doing these best case scenario calculations. This can let you know the top speed limit the boat and motor will ever be able to get to. It also helps you understand when you start testing your first prop how close are you to best case, if you hit 10% of best case for the boat with the aggressive prop, you can feel confident you can stop.

I will do step 2 to the process tomorrow, that is using diameter, and motor height, to change RPM to a point you are at max RPM and the highest pitch the boat can turn while maintaining a slip of close to 10%. This will give us max speed for this boat. Remember this first lesson is all about finding the fastest prop. We will discuss best over all prop on lesson 3 or 4

This is certainly definitive. We should no longer be reading any further posts here on THT asking about what brand, model and size propeller(s) to be used.

Thanks for your contribution to THT and for clearing this issue up.
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Old 07-14-2019, 08:06 AM
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This post 100% was not for anyone that is an everyday boater, it is a basic understanding for someone who has never dealt with this before and is coming in blind. I had a guy ask me the other day if he put a 200 Yamaha on a 16 ft 1200 lbs boat why would it not do 100mph.

This post is for him. The problem people like this guy is having is he does not understand the basic math of my that is impossible with the motor, it has a finite top speed on any boat and new people need to understand they can easily find it.
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