How To Select a Prop (it's not magic)

[kick bass]

Prop selection
One of the most asked questions is What Prop?
The next is how to get the best fuel mileage.
You will need to do several things to get this done.
First make sure your hull is clean. Next make sure your engine is properly tuned.
Now you need to know the manufactures maximum engine RPM and the Gear ratio of your lower unit.
You will need a GPS, and a properly working tachometer.
Take your boat out with normal load (What is in your Boat most of the time) and check maximum speed with GPS, look at your tach and see what RPM'S you are running. If your tach shows you are running 5000 and the manufacture says you should be running 5500 then go down in pitch, figure 200 RPM's for each drop. Example if you were running a 24 pitch prop at 5000 then you could go to 22 pitch prop and get your RPM's to 5400.
A 22 Pitch prop would theoretically move your boat 22 inches in one revolution. But you have slippage (efficiency), and this is one number you need to watch. Here is the formula you can use.
RPM divided by the gear case ratio times Pitch of Prop divided by 12 times 60 divided by 5280 = MPH.

Example on a 150 Johnson Running 5500 RPM's with a 1.86:1 Gear case and 24 pitch Prop:
5500 ? 1.86=2956.99 X 24=70967.76 ? 12=5913.98 X 60= 354838.8 ? 5280 = 67.20 MPH
RPM ? Gear Ratio x Prop Pitch ? 12 x 60 ? 5280 = MPH
Then take GPS speed ? Formula MPH = Efficiency

The speed you found on your GPS and divide that by the speed you came up with in the calculation and this is your efficiency. Example 61(GPS SPEED) ? 67.20(formula speed) and you will find you have 90 % efficiency . This is a good number. The higher the efficiency number the better, you cannot have above 100%.This is where different design and materials in props come into play. Different design props raise your efficiency rate and bring you closer to theory speed. Example stainless compared to aluminum. If you read some posts you will see some of the numbers are not right. Example a boat turning 5500 RPM's with a 1.86 to one gear case and a 22 pitch Prop cannot run 70 MPH!
So according to this theory the bigger the prop the faster the boat, this is true except your engine will run out of power and not be able to turn max RPM's! So you have to find the largest pitch that will allow your engine to Run The RECOMMENDED MAX RPM's.
That is why you need to check what you have first!

OK now for fuel efficiency:
Check your boat at Different RPM'S and speeds. Write these down. Next take the formula above and find the highest efficiency number at a speed and this will be your best fuel mileage speed.
Example 3500 RPMS at 40MPH(GPS) divided by 42.7 (formula) equals 93% efficiency and at 3000 RPM's at 35 MPH (GPS) divided by 36.66 (formula) equals 95% efficiency. 35 Mph would be your best efficiency speed. Your engine fuel consumption verses speed would be the best. Try this at several speeds to get the best efficiency number, don't worry you can't get 100%! Anything above 90% is good and 95% is excellent!

Now don't run out and buy a prop using this! There are different design props......cups, diameter, pitch, material. Use this as a reference and call iBoats and let them help you!~ they sell a lot of props and you can't beat experience!!!
Gary Cooper

 

[Silvertip]

Re: PROP The Magic Propeller

Re: PROP The Magic Propeller

Before you begin spending money on any prop, especially if its just a few hundred RPM you are after, experiment with engine height (mounting height on the transom). Raising the engine will very often yield the results you are after and the cost is zero. Proper use of trim and good load balancing can also produce the results you are after without the need for prop changes.

Then there is the poor performing boat that has an engine that is rated at 75% or less of the boats maximum rated HP. If your boat is rated for 150 but is powered with anything less than a 110 HP engine, no amount of prop changing will make that boat perform with any reasonable efficiency so save the money and put it towards a larger engine. A too small engine running its guts out will cost more to operate than a larger engine loafing.

 

[kick bass]

Re: PROP The Magic Propeller

Re: PROP The Magic Propeller

I agree with the engine height and offset. I also agree with the engine/boat combination. I want to explain the best combo for what you already have. I wanted people to realize what props do and don't do. I have read posts where people give advice on props that are crazy! like the example I used for a 22 pitch prop. There are a lot of factors in setting a boat up properly, If you want I can give more formula's. The bottom line is, before you take a posts advice check the numbers.
Thanks for the reply.

 

[hwsiii]

Re: How To Select a Prop (it's not majic)

Re: How To Select a Prop (it's not majic)

That is exellent information Gary.


H

 

[kick bass]

Re: How To Select a Prop (it's not majic)

Re: How To Select a Prop (it's not majic)

Thanks, Glad to help!

 

[45Auto]

Re: How To Select a Prop (it's not magic)

You're confusing prop slip and prop efficiency and fuel efficiency. Prop efficiency doesn't have anything to do with fuel efficiency. Depending on the boat and usage, a prop with high slip and low efficiency may deliver better fuel economy than a prop with low slip and high efficiency.

Efficiency is also hard to calculate because the effective pitch that the prop delivers to the water is NOT what is stamped on the prop. Cup, rake, etc, all affect the mathematical pitch of the prop. For example, my 28" pitch Bravo III at 30 MPH shows an efficiency of 106%. This is impossible, what is happening is that the effective pitch (due to cup, rake, material, counter-rotating props, etc) in the water is actually equivelent to a 32" or 33" pitch.

Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency (see Efficiency, below). Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack (see Angle of Attack, above). For example, in Figure 4-28, a 10" propeller actually advances only 8-1/2" in one revolution. Eight and one-half inches is 85% of 10", leaving a slip of 15%. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust.

To create thrust there must be some angle of attack or slip. The objective of propeller design is to achieve the right amount of slip or angle of attack, which is around 40, give or take a degree (Figure 4-30). This is accomplished by matching the right amount of blade diameter and blade area to the existing engine horsepower and propeller shaft RPM. Too much diameter and/or blade area will lower slip but will also lower propeller efficiency, resulting in reduced performance. Figure 4-29 illustrates this point.

The above paragraph and pics are straight from the Mercury Propeller Basics web page.

http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL

It's about the best explanation I've seen about how props work without getting into a bunch of theory involving calculus and differential equations.

 

[kick bass]

Re: How To Select a Prop (it's not magic)

OK I agree. If you take the efficiency Number and subtract it from 100 (no slip) you have prop slip. I understand you have to have some slippage or it becomes a brake. There are 2 factors that effect the performance of an engine. one is hores power, the other is torq. to over simplify it torq is take off and horse power is speed. A prop is a design of compromise. You have to have slip at lower speeds to get your engine into the Power band, so you can have speed. I understand cup and rake that atually pulls more water than theorectically it can. That is where different design props come into play. Like I said check with people who sell props, they know what works and doesn't. Now for the efficientcy at given speeds, different engines perform better at different speeds. (fuel consumption, power, etc.) I understand this. But, If a prop is say a 22 pitch, and at say 3500 Rpms you are turning your prop at close to the output shaft RPM then as a rule you are not waisting fuel as compared to an over reved engine at the same speed. I see you are a very knowlegable person, And I thank you for your feed back. I did not want to write a book, just give some understanding. A lot of people, I'd say most, have no idea what is going on with their prop. Any how I did not mean to say this will be your best economy, I meant fuel consumed as compared to speed. If you could burn one gallon of fuel and it took you all day to get to your spot, what would be the purpose. Again, you have impressed me and I would like to talk to you some time. But, try the formula and let me know what you think.

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

Please exuse my typing as my "c" key quit working.

Gentlemen, you are both right. Prop effiieny itself really has nothing to do with prop slip, beause about the best effiieny rating that a prop an get is in the neighborhood of about 85% and I would bet that you have never seen any published literature desribing the effiieny rating of any ommerialy manufatured outboard propellers beause they don't publish it on purpose.
What Merury states is that when you pass a ertain prop slip that you are wasting power beause it is being used to turn a prop that has too muh blad geometry for the vessel it is trying to push. Thus the prop slip is very small and usually it will not be able to attain the orret RPM for the motor it is on. The orret prop slip for most boats is in the neighborhood of about 8 to 12% for the best effiieny of that partiular boat and motor ombination. Like 45 says the pith number that is stamped on the prop is an approximation used by the manufaturer so we have some idea of what is required to turn that prop. But that is not enough information to pik the right propeller, beause the blade geometry also defines what hull forms that prop is best suited for as well.
I have a hart that I am posting that shows three real props on a Yamaha 70 HP motor that exemplifies this exat problem that merury is talking about. The Performane series props in a 16" and an 18" pith both have way too muh blade geometry and annot be turned over 5,050 RPM on an 18' outboard Dory, but the 17" Painted stainless prop is able to go to 5,750 RPM but if you will notie at the end it is starting to have less prop slip as it runs out of power trying to reah 6,000 RPM whih is the manufaturers WOT reommendation.

Prop omparisons

The following is a omparison of Motor urves and Power urves as well as the thrust uves for these props as well. The thrust and motor urves assume that the motor is ramped up to 3,000 RPM at takeoff.


Prop Motor and thrust urves

H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Thanks H for the input. I really liked the graph's. I tried to over simplify the formula. You will find most marine engines are most efficient (speed verses fuel used) at 3/4 throttle if the engine/boat combination and engine/prop are correct. I know someone will dispute this, so I put the bottom formula in. I also know some of you have gallon per hour fuel meters, and will say this is not true. There are to many factors in a boat/engine combination. I just wanted people to understand a little about a prop. I know some marine dealers that if you ask them about pitch and prop they can tell you to go up or down in pitch but have no idea why, other than a few RPM change. Now before someone replies and says they know a marine dealer who knows everything, please don't because I know there are some good ones. What I am saying is, the formula above lets you know something about what is going on with your prop. If I would have said prop slippage, people would take this as a horror. So I tried to explain it in a good way. kind of like the holes in a prop to let exhaust out so you can have slip at lower Rpms. This is bad up speed, but good at lower speed.
Again I thank you, and Mr. H I have the utmost respect for you. I know you are very knowledgeable, and courteous, I highly recommend anyone to ASK H IF YOU HAVE A PROP QUESTION!
as for me, I tried, I give up, gone bass fishing!

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Two guys were measuring a pole. One held it and the other would shimmy up, right before he got to the top, the pole would fall. Another man walked up and asked why didn't they lay the pole down to measure it. They looked at him like he was stupid and said we want to know how tall it is, not how long it is.

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

Gary, please do not stop this thread or any others you deide to post, as the information you gave people made it easy for them to understand and it an help them have a better understanding of what is happening with their props, that is why I omplimented you on your post.
When I try to explain things to most people here I am too tehnial with my explanations and it is very dry and boring reading, so they don't bother, and I understand why. What you posted was anything but boring, and all you have to do is look at how many views your post has gotten and you an see that people are reading it.
We need more information like this posted so people an understand the theory behind what is atually happening with their props on their partiular boat and why. If you will look at how many atual threads I have started on here you will find that is exatly 10. And I have removed the data in a ouple of them just beause there was no interest in them. I realize that I do not have the ability to transfer knowledge in writing very well, and there is a need for people like you to help people that ome here to learn more about what is happening when they pik the wrong prop and why that partiular prop is not a good fit for their boat or how they an hek themselves to see if someone is giving them the orret information.
So please keep posting your information so people an learn more about props when they ome here.
TRANSFER of KNOWLEDGE is what this site is all about, not just helping people find a better prop for their boat. Teahing a man to fish is MUH better than just feeding him.


H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Thanks H.
I guess there is no way to simplify anything. I would like to post verable ratio props, then maybe rake and cup, ventalation, bow/stearn lift and push/drag. the problem is trying to present it in a way that people will want to read it and understand. You would have to have a post on each one, and yes they will overlap. I chose to start with pitch (not varible) so people could get a grip on props. No, I did not say this was a fixed prop theory, because in general it will work and that sounded confusing. Like I stated I did not want to write a book. Oh yes before I am corrected, prop diameter also. The point is if you want to build a house you have to teach someone to drive a nail first. I just read your post about the black art of props. I enjoyed it! A lot of information! Each prop builder has their secrets, because they don't want knock off's. I grew up around boats all of my life. My grandfather was killed racing boats. I remember him telling how to change a brass prop pitch (he called it prop Bite) with an anvil and a hammer.(This was when he was young and boats were also.) He was really smart. I don't think you can get brass props anymore. (No I don't recommend changing the pitch of a prop at home! so don't ask!) My first boat was a 14 foot aliminum boat with an elgin motor. My first new boat was a 1976 Kingfisher with a 50 mercury motor on it. Later that year I traded it in on a 1976 Ranger (Spirit of 76) with a 150 merc. I thought it would fly! I love boats, and better yet, how they work. evidently you do also, or you would not spend so much time trying to figure what does what. This whole thing started over a post I read where a guy said his 150 johnson with a 22 pitch prop would run 70 mph at 5200 R's. People started asking what prop. I gave him the extra 300 rpms in my post and said 5500. That is why I used a johnson in the formula. No I don't recommend one engine over another, the brand I prefer is the one that does not have to be towed back to the landing! I now realized that some of the things we take for granted others are trying to understand.
While I have your attention, If I take a 5 foot prop off a tug boat and adapt it to a 200 merc, how would it perform? would I have to drill holes in my boat to let the water out? LOL

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

LOL Gary The way you presented your information made it easy for people to understand, and that is what we need.


H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Thanks
I meant RAKE not rack.

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

I only saw rake not rak.


H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

I changed it!
Thanks,

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

Gary just sent this to me and I wanted to share it with everybody, beause it is an easy way to get the EXAT gear ratio of your lower unit.


Default Gear Ratio
Boy Mr. H you are smart. I tried to understand how you said to check the gear Ratio. May I help? It is harder to get to the flywheel, but is more accurate:
Go the your local sew center and get a cloth tape measure. Also get a marker.
Call a freind, and have him come over.
Take the battery cable off. Take the engine cover off. Mark the prop to the foot, using the marker. (make a line from the prop to the foot). Now take off what you have to get to the flywheel. Take the tape measure and wrap it around the outside of the flywheel. cut it so it buts up. make sure it starts at 0. now mark the block where the tape starts at zero. Turn the flywheel, watch the prop. count how many turns the flywheel makes. Example when the flywheel turns one turn stop and look at the prop continue to turn the flywheel till the marks on the prop line up.
Lets say the tape measure around the flywheel is 24 inches. and you turn it 1 turn 6 inches. Ok the first number is 1 because of one turn. Each full turn of the flywheel is the first number. Devide the remaining inches by the the ouside inches of the flywheel. Example 6 devided by 24 equals .25
so the ratio is 1:25 turns to one turn of the prop so it is 1:25 to one gear ratio.
If you have an inboard, do the same thing with the harmonic balancer.
Now you have this figured out, take the tape measure off. Keep it. when you catch a fish you will find the tape will stretch. So that 10 inch fish can be 12 inches. what a deal!
Try This.
__________________
Things in motion tend to stay in motion.
things that are stopped are usually broke!
Gary C.

 

[Thad]

Re: How To Select a Prop (it's not magic)

This is good stuff!
And I am a complete idiot when it comes to props.

hwsiii has helped me and hopefully you guys will again.

I used to think that a higher pitch would give me more speed, lower...well.
Now I know that not always to be true.

I have two 19p props, one will top out at 48.6 @ 5200r's, the other 46.4 @4600r's. Can't get higher rpm's to save my life.
Reading what you guys write, I get a little better understanding of various mfg's and types, styles, etc. that go into props.

Hope to get mine right some day. As soon as I stop tearing them up anyhow.
The info here can help ma and a lot more people keep from spending $$ we don't need to.
P.S. hwsiii, I get what you're telling me, I just don't always understand it right away And not really looking for anything here, just wanted to say thanks and keep these posts coming.
I'll get it sooner or later.

 

[hwsiii]

Re: How To Select a Prop (it's not magic)

Thad, thank you very muh and Gary has some great ideas and he an explain it bettert han I an.


H

 

[kick bass]

Re: How To Select a Prop (it's not magic)

Let's start a fund to get Mr H a "C" key. I'll pitch the first ten dollars in!
And as far as the posts goes, Mr H is great! I'm just worried his head will explode, from overload!