Can someone explain to me pitch

[Godfirst]

Yes have read all about the terminiology of pitch but took off my old prop that was a 14.8x17 and replaced it with a 14.5X19 for clearance and more topend the blades on the prop look identical is the angle what is described about as pitch??
Thanks

 

[marlboro180]

Re: Can someone explain to me pitch

It is the theoretical number of inches the prop would travel with 1 rotation. Think of it as a screw, with the water being the wood. Less pitch = less travel. Yes, angle of the blades is described as pitch.
Hard to See difference in a minor change to the untrained eye.

 

[Bondo]

Re: Can someone explain to me pitch

the blades on the prop look identical is the angle what is described about as pitch??

Yes...

 

[Silvertip]

Re: Can someone explain to me pitch

The thing you really need to understand about pitch is you cannot arbitrarily decide you want more speed and therefore can slap a prop with higher pitch on the boat. Consider this -- IF the engine is already at the upper end of the manufacturers wide open throttle rpm band and you add pitch, you slow the engine down and may LOSE speed rather than gain it. The reason being once you pull an engine off its peak RPM it cannot spin that prop any longer so speed actually drops. On the opposite side of this fence is the situation where you remove pitch. Again, IF the engine was reving at the upper end ofthe rpm band it would now be overreving with the prop change. The best prop is one that allows the engine to rev at or very near the upper end of the rpm band with an average load. Top speed will be whatever that prop provides. The only way you know what the engine is doing is to read the tach. For all you know you may already have the best prop.

 

[Tommyfmu]

Re: Can someone explain to me pitch

I don't think this is exactly right because I don't think I understood the textbook explanation of it, but I had understood pitch to be the sum of the length of the blades measured fore to aft on the hub; i.e. 3 blades, 6" fore to aft for each blade = pitch of 18. It made sense since more pitch would be an increas in the fore to aft length, would grab more water and increase speed for the same rpm's, albeit the rpm's would be slower if the engine bogs or can't reach wot.

 

[Bondo]

Re: Can someone explain to me pitch

I don't think this is exactly right because I don't think I understood the textbook explanation of it,

Ayuh,... Exactly,.... It's Exactly Wrong....

It is the theoretical number of inches the prop would travel with 1 rotation. Think of it as a screw, with the water being the wood.

That,.... Is Exactly Right...

 

[Tacklewasher]

Re: Can someone explain to me pitch

I don't think this is exactly right because I don't think I understood the textbook explanation of it, but I had understood pitch to be the sum of the length of the blades measured fore to aft on the hub; i.e. 3 blades, 6" fore to aft for each blade = pitch of 18. It made sense since more pitch would be an increas in the fore to aft length, would grab more water and increase speed for the same rpm's, albeit the rpm's would be slower if the engine bogs or can't reach wot.

This is incorrect. Number of blades has nothing to do with pitch. What others have posted is correct.

 

[Robbabob]

Re: Can someone explain to me pitch

You are correct in your evaluation of which prop may give you more top end. However, what everyone has given for input is correct. Hopefully, my additional comments will help to clarify this a little more.

The only way you will achieve a higher speed with the new prop is if you could exceed your WOT RPM when using your old prop. If you could not achieve your WOT max with the old prop, you'll only be slowing yourself down with the new prop.

Volvo Penta has a great prop selection guide that may help you find what you're looking for:
http://www.4shared.com/account/file/107668525/de03e069/Volvo_Penta_Prop_Guide.html

Hope this helped! happy boating

 

[oceansbreeze]

Re: Can someone explain to me pitch

The only way you will achieve a higher speed with the new prop is if you could exceed your WOT RPM when using your old prop. If you could not achieve your WOT max with the old prop, you'll only be slowing yourself down with the new prop.

Ok, so as a case study.... let's use my baot 5300 RPM at WOT using a 17" pitch. I get 32MPH (GPS) after careful trimming. ... maybe 33MPH.

The book says 5500 RPM at WOT is peak performance. I cannot higher than 5300RPM... ever.

I want to go a LITTLE faster... top end only... (ie: 35/36MPH) with lower RPM ... for lower RPM.

a 19" pitch, won't do it ... even though theoretically it should ..... because ... ok this is where I kind of lose my understanding too... (but it may help him, and me understand) .... thinking out loud here....

the engine... won't... get to 5300rpm with a higher pitch? We have to assume it has enough HP to continue to get to 5300/5500rpm first?

I think I'm thinking too much like a car. In first gear you can make the RPM go as high as you want, and the RPM are ALWAYS climbing, no matter what gear you're in ... until you hit the top end... So... the engine's top end is 5300/5500.. argh... confused now! lol

 

[QC]

Re: Can someone explain to me pitch

Most cars are geared so that they run out of horsepower in top gear at top speed. In that way they are sort of like a boat and prop at recommended WOT RPM. But stop with the car and boat comparisons, it will only make you crazy . . .

Also, please show us your spec if it says "peak performance" that would be unique. Normally it is expressed as a range. IF your combination (engine and prop) reaches the top of the WOT RPM range MAYBE a higher pitch prop will result in a faster top speed. It will also usually result in slower acceleration or what we typically call "hole shot". All of these answers are variable due to prop design and because the world is an imperfect place . . . No sarcasm intended, ALL of this stuff is a crap shoot unless you are in a test cell with absolutely perfectly repeatable ambient conditions, air temp, water temp, water density, engine tolerances, propeller balance add in additional perfection etc. etc. etc. None of these things are perfect so "your results may vary" . . .

 

[Robbabob]

Re: Can someone explain to me pitch

OK, look at it as though it is a car, for the sake of an example.

If you could put multiple props on while you are cruising on the water. Your first gear might be a 6" pitch (get you off the line VERY quickly but you'll blow up the engine if you don't shift). Second gear would be something higher like a 10" pitch (allows you to go faster but you can still blow the engine if you keep the pedal to the floor). You continue through the gears (increasing the pitch) traveling faster until you have a gear/prop that allows you to keep your throttle wide open without red lining your engine (exceeding max WOT rpm).

So in my previous post, if the 17" pitch with his WOT did not get to his max rpm range, then a 19" will only make the boat go slower by turning even fewer rpms -- for his set up of HP and boat drag, etc.

Some people ONLY want the quick starts for skiing and wake boarding, so they might choose a prop pitch for only that function. Shorter pitch for quick off the line. They are then careful not to attempt WOT as they will easily exceed max rpm.

Others only care about how fast they go from point A to point B. They give up the hole shot by having a longer (higher number) pitch, but they will not have to worry so much about exceeding the max WOT rpm. Of course, trim will play a huge factor here as an over trimmed drive at WOT can easily exceed the max rpm.

Several other factors must be considered along with items I've pointed out. This example was purely for attempting to clarify the thread direction. I hope this example of car gears helps.

Good luck and happy boating

 

[Robbabob]

Re: Can someone explain to me pitch

sorry QC, I was busy typing when you wrote your response. maybe it will still help :redface:

 

[QC]

Re: Can someone explain to me pitch

Agree ^^^^^ and decent car/boat comparison

 

[45Auto]

Re: Can someone explain to me pitch

So in my previous post, if the 17" pitch with his WOT did not get to his max rpm range, then a 19" will only make the boat go slower by turning even fewer rpms

Not necessarily true. It depends on where the horsepower peak is in the RPM range. If the horsepower peak of his engine is at just over 5000 RPM, then it's possible a 19" pitch prop would drop him right onto the horsepower peak and deliver more speed.

Too much trouble to type out again, this old thread covered it a couple of years ago:

http://forums.iboats.com/showthread.php?t=225803&highlight=secrets

 

[hwsiii]

Re: Can someone explain to me pitch

First, everything you need to understand about PITCH has been discussed very succinctly by everybody in the posts above, and they did a very good job of it in explaining it in different concepts. But there are MANY other variables with different hulls and motor combinations and blade geometry that affect your Top Speed than just prop pitch.
In one prop that is a 17" pitch and another 17" pitch prop there could be as much as a 7 MPH speed difference between the two props, even though the maximum speed is only 40 MPH.

You might try reading this for a better understanding of changing props for better performance.

http://forums.iboats.com/showthread.php?t=365653

And if you really don't get bored very easily you might try reading this .

http://forums.iboats.com/showthread.php?t=303221&highlight=black+art


H

 

[Godfirst]

Re: Can someone explain to me pitch

Thanks for all the info...
The post above makes sense I have a deep v hull the old prop was a solas aliumimiun 17 pitch with no cup on the blades, the new prop I bought was a michigan wheel same good price 19 pitch with a good cup on it, problem is now I have a couple of dead batteries and it is threating rain here in southern cal..