Data for Yamaha F150 on Aquasport 222

[bripley]

I am still trying to get this discussion straight in my head so I went to BOATING Magazine's site for this quote. It seems by this definition that going from 19" to 17" is lowering the pitch:


"Pitch is defined as "the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 21-pitch propeller would move forward 21 inches in one revolution.
 
Think of a propeller as you would a car's axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the pitch, the better your hole-shot. However, this comes at a price: top speed.The lower pitch makes the engine reach maximum rpm at slower speeds.
 
Conversely, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with a propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts.
 
If you're changing pitch on a recreational boat, remember that each inch of pitch is worth about 200 rpm.Lowering the pitch will increase rpm and vice versa. For example, going from a 23 pitch to a 21 pitch will increase engine rpm by about 400 revolutions."

[Capt. Bob]

This is what I've always heard.

There is of course slippage in any prop. This may be why I'm having trouble comprehending "Slippery's" post.
viewtopic.php?p=73204#p73204

[slippery73]

This is what I've always heard.

There is of course slippage in any prop. This may be why I'm having trouble comprehending "Slippery's" post.
viewtopic.php?p=73204#p73204

Although the pitch is considered lower, the numerical number associated with the pitch is higher.  I worded my original post so it would be more understandable without going through all this.

I should have said that "the boat will have a better hole shot with a higher numerical pitch # than a lower pitch #."

[slippery73]

My planned change from 19" pitch to 17" pitch will result in higher rpm at a given speed and more power out of the hole.

You'll have worse hole shot but more top end with your planned swap.  I directed you to prop gods because there is way more to it than just diameter and pitch.  You can add cup, vented vs non vented, 3-4 blade, etc.  Depends on what your doing with your boat. I'd dial it in accordingly otherwise look for a used prop on craigslist, I picked up a couple powertech 4 blades for my skiff a while back cheap. Well less than half the cost of new.

[Capt. Bob]

I should have said that "the boat will have a better hole shot with a higher numerical pitch # than a lower pitch #."

I thought the pitch number was the actual "pitch displacement measurement" of the given prop. All these years I've been led to believe that say a 15 x 17 prop had a diameter of 15" and a pitch of 17".  If this is correct, then the theoretical movement of the boat with one revolution of the prop would be 17". Putting aside rake, cupping, and blade #s for ultimate performance, then logic would dictate that a lower pitch would allow the driving device aka the motor to rev more easily due to reduced water mass being moved by the prop turning. This would allow the motor to reach the designed performance rpm easier/quicker thus breaking the inertia of the boat sooner and that results in the ability to pull off a better "hole shot".

These facts aren't disputable. What is confusing me is the pitch # thing. I'm understanding you to say that a 19 pitch # is really a different pitch measurement. That measurement increases as the pitch # goes down decreases as the # increases. Is that correct?

If so then you would be correct that the higher # would allow for a quicker hole shot. I did not know there was a difference.

[Blue Agave]

My planned change from 19" pitch to 17" pitch will result in higher rpm at a given speed and more power out of the hole.

Going from a 19 to a 17 pitch will improve your hole shot and decrease your top end!  Plain & Simple!

I thought the pitch number was the actual "pitch displacement measurement" of the given prop. All these years I've been led to believe that say a 15 x 17 prop had a diameter of 15" and a pitch of 17". If this is correct, then the theoretical movement of the boat with one revolution of the prop would be 17". Putting aside rake, cupping, and blade #s for ultimate performance, then logic would dictate that a lower pitch would allow the driving device aka the motor to rev more easily due to reduced water mass being moved by the prop turning. This would allow the motor to reach the designed performance rpm easier/quicker thus breaking the inertia of the boat sooner and that results in the ability to pull off a better "hole shot".

These facts aren't disputable.

 

[gran398]

Hogwash!

Dagnabbit, when it's time to get with it...."Hogwash" will get'r done

(And you'll have the cleanest hog)

 

[slippery73]

I should have said that "the boat will have a better hole shot with a higher numerical pitch # than a lower pitch #."

I thought the pitch number was the actual "pitch displacement measurement" of the given prop. All these years I've been led to believe that say a 15 x 17 prop had a diameter of 15" and a pitch of 17".  If this is correct, then the theoretical movement of the boat with one revolution of the prop would be 17". Putting aside rake, cupping, and blade #s for ultimate performance, then logic would dictate that a lower pitch would allow the driving device aka the motor to rev more easily due to reduced water mass being moved by the prop turning. This would allow the motor to reach the designed performance rpm easier/quicker thus breaking the inertia of the boat sooner and that results in the ability to pull off a better "hole shot".

These facts aren't disputable. What is confusing me is the pitch # thing. I'm understanding you to say that a 19 pitch # is really a different pitch measurement. That measurement increases as the pitch # goes down decreases as the # increases. Is that correct?

If so then you would be correct that the higher # would allow for a quicker hole shot. I did not know there was a difference.

Where your having the hang up is in the numerical value.  The pitch # is equal to the distance moved forward in inches with one rotation of the prop.  If you use an extreme example to better illustrate this its easier understood.  Take a prop with a pitch # of 1, vs a prop with a pitch # of 30.  The prop with the pitch # of 1 will travel 1" with one complete rotation of the prop.  The prop with the pitch # of 30 will travel 30" with that same rotation.   It is pretty simple to see which prop would accelerate quicker when its put like that.

Now,  decreasing or increasing pitch is the opposite of its numerical value.  Decreasing the pitch is actually a higher numerical pitch number and vice versa with increasing the pitch.

Think of it like car gears:  Big truck might have 4.56 gears to get big tires going. Those are considered really "low" gears.   A small car might have 3.08 gears, which would be considered "high" gears.  The high gears wont accelerate as fast but will keep a lower rpm at higher speeds compared to the "low" gears.

Even though the bigger number is considered low, they are numerically higher. Thats where it gets confusing.... thats why I worded my first post like I did, its hard to explain!

[Capt. Bob]

Where your having the hang up is in the numerical value.  The pitch # is equal to the distance moved forward in inches with one rotation of the prop.  If you use an extreme example to better illustrate this its easier understood.  Take a prop with a pitch # of 1, vs a prop with a pitch # of 30.  The prop with the pitch # of 1 will travel 1" with one complete rotation of the prop.  The prop with the pitch # of 30 will travel 30" with that same rotation.

OK then.
We have established that the pitch # is the actual distance traveled (theoretical due to slippage, hull type, general crap hanging off the boat) for one (1) rotation of the prop.

  It is pretty simple to see which prop would accelerate quicker when its put like that.

Here lies the true hang up and I'll show you using your example below.

Now,  decreasing or increasing pitch is the opposite of its numerical value.  Decreasing the pitch is actually a higher numerical pitch number and vice versa with increasing the pitch.

Maybe I need to check my medication but that appears to me as the exact opposite of what you first stated.

Think of it like car gears:  Big truck might have 4.56 gears to get big tires going. Those are considered really "low" gears.   A small car might have 3.08 gears, which would be considered "high" gears.  The high gears wont accelerate as fast but will keep a lower rpm at higher speeds compared to the "low" gears.

You are correct on the final drive ratio used to "get big tires going". This is where (based on the pitch # and distance traveled being equal thought) the understanding what is physically occurring gets fuzzy. Using the "4.56 gears" as an example, what is actually being done is the big tire is completing one revolution for every 4.56 revolutions of the drive shaft. That shaft is also connected to another set of gears (the transmission) which in turn is connected to the engine. I'm sure you fully realize these connections but the confusion is what is actually taking place. While a vehicle conveyance of power to move it is more complicated than a boat, the physical theory is the same. You must overcome the forces keeping both the car/truck and boat at rest. The energy needed to do that is supplied by the motor in both instances. You will agree that mechanical devices such as these have design parameters (optimum operating range) where each generates its highest energy aka HP and torque. The quicker/easier that operating range can be reached, the faster the system can achieve its intended goal. Again with the gear ratio example you quote, the "low gear" allows the engine to reach its best operational range faster (revs more quickly) and thus utilize that power to overcome the forces acting on the vehicle (inertia). This principle is the same for a boat. While the mediums traversed are different, the physical laws are the same.

Therefore, an outboard motor...
1). Has a given performance range
2). The faster it reaches this range, the faster the forces on the boat can be overcome.

Pitch measurement as described is but one of several factors (as you stated earlier) that needs to be considered because the higher the pitch, the more the prop travels (and creates movement) and the more medium it must displace (water). Displace the water faster and the motor reaches its operating range faster and the forces at rest are overcome faster, equating to faster acceleration aka "hole shot". Outboard motors also have final drive ratios to help achieve this principle but what we are dealing with is how much water the prop has to displace and the highly scientific law of "less is more".

So if pitch # does equal theoretical distance traveled, then it is a given that in order to traverse a given distance, a given amount of medium must be displaced. As with everything in life, there must be compromise. Using your earlier example, a pitch #1 would cause the motor to reach its operating range very quickly but could not create the necessary displacement to move the boat. On the other end a pitch # 30 would require the motor to work much harder (displace more water) thus requiring more time to reach said range, thus acceleration would be slower.

This is just the way physics works.  

Even though the bigger number is considered low, they are numerically higher. Thats where it gets confusing.... thats why I worded my first post like I did, its hard to explain!

You're talking semantics here. The terms "low" and high" are speed related in such that high gearing is related to top end speed and low gearing is related to acceleration, in the pistonhead world. The actual gear ratios needed to achieve those goals are indeed opposite.

It seems to me you fully understand the principles but we are confused on the language.

Now everybody go back to bed. :(

[GoneFission]

One of these days we should probably do a "Propeller Facts & Fiction" section in the Resources forum, but that's a different story...

Here is some info on pitch, including pitch, slip, and progressive pitch:

Note - most aluminum props are straight pitch, because you can't cut aluminum thin enough to put a "cup" or progressive pitch into the prop.  Most stainless props are progressive pitch or "cupped" with thinner blades and are therefore more efficient than a comparable aluminum prop.  The pitch on a cupped prop is based on how it performs compared to a straight pitch - the actual pitch changes across the blade.  

But there's more than pitch - you also have to consider diameter, number of blades, and swept area.  Large diameter props have a larger swept area than small diameter, so they move more water. Lage diameter props are good for torque and getting the boat up on a plane, but they have more drag due to the larger swept area. The problem with large diameter props is cavitation (not ventilation) at high spin speeds or RPMs. The tips of a prop can only go so fast through the water before they start vaporizing the water, causing cavitation. The tip speed increases with the square of the diameter, so a little increase in OD makes a big difference in tip speed. Spinning a large diameter propeller at high speeds results in cavitation and a dramatic increase in slip. So if you have the engine torque to turn it, you will find a large diameter prop gets you on a plane faster, but you may find top speed is impacted (regardless of the pitch seleted) due to the tips of the blades starting to cavitate. Sometimes folks complain that the boat doesn't go much faster at 6000 RPM than it does at 5200 RPM and they think it's wind resistance. Wind resistance also is sqare factor - of speed - but often the problem is the prop is beginning to cavitate just over 5200, and is really cavitating and slipping at 6000. The fix is one of two things - reduce the diameter of the prop or reduce the gear ratio in the lower unit to reduce prop spin speed. It's usually easier to change the prop than the gear ratio...

This is why 4 blade and even 5 blade props are popular with heavier boats with higher HP motors. You can have the same swept area of a big-diameter 3 blade with a smaller diameter 4 blade. A 14" prop with 4 blades has a swept area like a 15" big-ear prop, but may keep the tip speed below cavitation level to acheive good efficiency at high RPMs. A basic rule of thumb is the faster you want to go, the smaller diameter prop you need, and if you want to go really fast, you have to deal with drag, so you start reducing swept area and blades. The fastest boats you will see (think unlimited hydroplanes with 12,000 HP turbine engines) are running little 2-blade props at really high speeds.

[slippery73]

Maybe I need to check my medication but that appears to me as the exact opposite of what you first stated.

Think of it like car gears:  Big truck might have 4.56 gears to get big tires going. Those are considered really "low" gears.   A small car might have 3.08 gears, which would be considered "high" gears.  The high gears wont accelerate as fast but will keep a lower rpm at higher speeds compared to the "low" gears.

You are correct on the final drive ratio used to "get big tires going". This is where (based on the pitch # and distance traveled being equal thought) the understanding what is physically occurring gets fuzzy. Using the "4.56 gears" as an example, what is actually being done is the big tire is completing one revolution for every 4.56 revolutions of the drive shaft. That shaft is also connected to another set of gears (the transmission) which in turn is connected to the engine. I'm sure you fully realize these connections but the confusion is what is actually taking place. While a vehicle conveyance of power to move it is more complicated than a boat, the physical theory is the same. You must overcome the forces keeping both the car/truck and boat at rest. The energy needed to do that is supplied by the motor in both instances. You will agree that mechanical devices such as these have design parameters (optimum operating range) where each generates its highest energy aka HP and torque. The quicker/easier that operating range can be reached, the faster the system can achieve its intended goal. Again with the gear ratio example you quote, the "low gear" allows the engine to reach its best operational range faster (revs more quickly) and thus utilize that power to overcome the forces acting on the vehicle (inertia). This principle is the same for a boat. While the mediums traversed are different, the physical laws are the same.

Therefore, an outboard motor...
1). Has a given performance range
2). The faster it reaches this range, the faster the forces on the boat can be overcome.

Pitch measurement as described is but one of several factors (as you stated earlier) that needs to be considered because the higher the pitch, the more the prop travels (and creates movement) and the more medium it must displace (water). Displace the water faster and the motor reaches its operating range faster and the forces at rest are overcome faster, equating to faster acceleration aka "hole shot". Outboard motors also have final drive ratios to help achieve this principle but what we are dealing with is how much water the prop has to displace and the highly scientific law of "less is more".

So if pitch # does equal theoretical distance traveled, then it is a given that in order to traverse a given distance, a given amount of medium must be displaced. As with everything in life, there must be compromise. Using your earlier example, a pitch #1 would cause the motor to reach its operating range very quickly but could not create the necessary displacement to move the boat. On the other end a pitch # 30 would require the motor to work much harder (displace more water) thus requiring more time to reach said range, thus acceleration would be slower.

This is just the way physics works.  

How can you say that a prop with a pitch of 1 would accelerate faster than a prop with a pitch of 30.  This seems like a simple comparison to me so I don't understand how your saying that it would accelerate faster.

The fact that the pitch is measured in distance travelled in one rotation gives you all the information you need to know.  All other variables aside they are given that constant.  Gear ratios, slip factor, hydrodynamics, etc. play no role for comparison purposes.

If i tell you that I travel 3 feet with every one of my steps you could extrapolate how many steps I took for a given distance. The only way it would be possible for someone with a 2 foot stride to be faster than me per every step would be to take more steps.  

Same principal with the boat prop.  

If I know that for every revolution I travel a given distance. I also know that at a given rpm I can calculate my prop rotations per second.  If you calculate your rotations per second, and multiply that by your prop pitch you will have the distance in inches you travelled per one second. Divide inches by 12 and you would have how many feet you would travel in 1 second with zero slip.

5,000rpm / 60 secs. = 83.3 rotations per sec.    

83.3 rps x 21 prop pitch =  1,750 inches travelled per second       1,750 inches /  12 =  145.8 feet travelled in one second.


Compare to a 19 pitch prop:


5,000rpm / 60 secs. = 83.3 rotations per sec.    

83.3 rps x 19 prop pitch =  1,583.3 inches travelled per second       1,583.3 inches /  12 =  131.9 feet travelled in one second.



Looking at the math, I don't see how a 19 pitch prop could accelerate faster than a 21 pitch prop without turning considerably higher rpm's.   I know the lower pitch will achieve more rpm's faster than the higher pitch prop, but it also has to have the necessary force to move the boat up on plane.

If you took a prop with a pitch of 10 compared to a prop with a pitch of 20 it may reach higher rpm's a lot faster than the 20 pitch, which may equate to traveling more distance in the same time in a mathematical equation. However, it will not have the required pitch to effectively bring the boat on plane and therefore never reaching the higher rpm abilities of the props potential.

If im wrong, I guess im just crazy.... but this makes sense to me.

[Capt. Bob]

How can you say that a prop with a pitch of 1 would accelerate faster than a prop with a pitch of 30.  This seems like a simple comparison to me so I don't understand how your saying that it would accelerate faster.

  I don't know how I could say that, if I didn't. What I posted was that the lower pitch would allow the engine to reach it's operating range faster.

The fact that the pitch is measured in distance travelled in one rotation gives you all the information you need to know.  All other variables aside they are given that constant.  Gear ratios, slip factor, hydrodynamics, etc. play no role for comparison purposes.

True, pitch (for this example only) is constant. What changes is the time required to reach a given rpm.

If i tell you that I travel 3 feet with every one of my steps you could extrapolate how many steps I took for a given distance. The only way it would be possible for someone with a 2 foot stride to be faster than me per every step would be to take more steps.

Not an apples to apples comparison. Try this. You and a physically identical twin brother. You are wearing those huge Bozo the Clown shoes and your twin is wearing track shoes. Strides are equal (think motor). Pitch is different (think shoes). On your mark. Get set. GO!
Travel the EXACT same distance. I wonder who will get there quicker?
 

Same principal with the boat prop.  
If I know that for every revolution I travel a given distance. I also know that at a given rpm I can calculate my prop rotations per second.  If you calculate your rotations per second, and multiply that by your prop pitch you will have the distance in inches you travelled per one second. Divide inches by 12 and you would have how many feet you would travel in 1 second with zero slip.

This is an example of constant speed

5,000rpm / 60 secs. = 83.3 rotations per sec.    

83.3 rps x 21 prop pitch =  1,750 inches travelled per second       1,750 inches /  12 =  145.8 feet travelled in one second.


Compare to a 19 pitch prop:


5,000rpm / 60 secs. = 83.3 rotations per sec.    

83.3 rps x 19 prop pitch =  1,583.3 inches travelled per second       1,583.3 inches /  12 =  131.9 feet travelled in one second.

Again, constant speed.
The original question was concerning a better "out of the hole" response, ie acceleration from start to a constant speed on plane.

OK, it's been a hell of a volley so here is my last stroke.

Slip, let's go back to the member's question. Please understand that I'm not going to read back and get all his info but this I remember. He stated he had a certain diameter prop with a pitch of 19 IIRC.  Let's just assume that his motor's operating range is 4500-5500 rpm. Using your equations above and assuming at WOT his motor turned 5000 rpm, you calculated his theoretical speed at that given point in time. That would be his top speed at that rpm. I think you agree with that point.

Next he stated he wasn't so concerned about his top end speed but wanted a better hole shot. His idea was to change props keeping the same size (diameter) and lowering the pitch to a 17. Lowering the pitch with all things remaining equal usually raises WOT rpm. For an example, let's say his WOT is now 5300 rpm
Using your same calculation you will find that his top speed is about 7 f/s slower but remember he wanted a better hole shot not top end.

Lastly, there are many variables to finding the right combination of prop to engine (as shown in CJ's post above and your prior postings). The thing here is that the 17 pitch prop (as you stated above) will in fact reach a higher rpm FASTER (this is the key) allowing the motor to get the boat on plane faster. I fully agree there is a fine line on prop science with many factors but under most circumstances, lowering the pitch will result in a better hole shot. Remember, it's not about constant speed but rather the ability of the engine to achieve it sooner. The reason this occurs was stated in my last post. This is exactly the same principle as you used in an earlier example with the "low" geared/high ratio differential. It's the ability to reach the motor's power band more rapidly.

[Blue Agave]

Slippery73 that is a well thought out and analytical response, but your conclusions are backwards.  A prop with the pitch of 10 will over rev and blow the motor.

[Blue Agave]

You are wearing those huge Bozo the Clown shoes and your twin is wearing track shoes.

Now that is some funny chit!

[slippery73]

Slippery73 that is a well thought out and analytical response, but your conclusions are backwards.  A prop with the pitch of 10 will over rev and blow the motor.

Yea, I know.... thanks for making my point for me again.

 Bob is saying that because a smaller prop reaches a peak rpm faster the boat will in fact accelerate faster.  Not the case, the prop will spin faster but it won't have the power to move the boat out of the water and onto plane.  Thats the key.... a boat has a huge amount of drag that it must overcome to get onto plane, a 17 pitch will run a higher rpm once up to speed and the boats on plane, that will result in a higher top speed.  

Its a tradeoff, you cant have a prop that increases top end RPM and also increases hole shot.  You get one or the other, low end torque or top end speed.  Most people want to meet somewhere in the middle of where those two curves meet to have the best all around combo available.