Data for Yamaha F150 on Aquasport 222

[Aswaff400]

am i on THT?

seems everyone agrees, a 17 pitch will accelerate faster than a 21 pitch... why else would i change my prop from a 23 inch pitch to a 19 pitch? better hole shot. the 23p wont allow the motor to turn the rpms up therefore is a dog out of the hole. the 19p lets the motor reach higher rpm  and it reaches those rpm quicker.

in my case, with the boat fully loaded:
23p, turns 4800 rpm, reaches 52 mph
19p, turns 5600 rpm, reaches 51mph

   

[Blue Agave]

You can lead a horse to water.............

[GoneFission]

Now - let's throw another variable into the discussion - a shifter propeller!  

Land & Sea and Quicksilver (Mercury) made these - but Mercury lost a patent suit - and that left Land & Sea alone in making these props.  The Land & Sea factory burned down years ago, and no one picked up the patent.  However, AeroStar Marine has now taken over manufacture of the shifter prop technology from Land & Sea.   I ran a Quicksilver shifter prop for a couple years - it was hard to get it dialed in just right, but it did work like they say - started as 13 pitch, then shifted to 19 pitch.  I now run a 19P Quicksilver HighFive that's the best overall prop I've ever run.  

Here's a link to AeroStar:  http://www.aerostarmarine.com/ppp.html  
One downside - shifter props are expensive!   :x

Here's an older video from Land & Sea demonstrating their shifter prop   it's pretty amazing:  
http://www.youtube.com/watch?v=C5niXCY-ikM

[Capt. Bob]

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.

Again,
It's not about the PROP reaching peak rpm faster but rather the prop's ability to allow the MOTOR to reach its peak power sooner. Remember, the prop is just another mechanical device in the chain that transfers the engine's power to the water. That's the key.

Agreed that a lower pitched prop can result in higher WOT rpm but that doesn't always equate to a higher top speed. That was the example I related to last night using your equations. In many cases, the mathematical increase in rpm does not offset the mathematical decrease in pitch thus resulting in a lower top end speed.

Trade off yes but it's not what you're understanding. RPM will experience change in two areas. First in reduced time, that's the ability to achieve it sooner coupled with an increase at WOT but as stated before, in most cases results in a decrease in top speed.

You are correct in the fact that when you factor in ALL variables to this equation, prop selection becomes a very personal rather than "one size fits all". You are also correct in that as you continue to reduce pitch, the prop begins to lose its ability to lift the hull  while increasing the rpm of the motor beyond its operational range. Economist call this the "Law of Diminishing Returns". This principle also works in the other direction when one increases pitch.

At the end of the day, you can use all the gizintas, computer models even a crystal or the ol' 8 ball to find a starting point but most of us end up using the tried and true science of "trial and error"

Stick a fork in me, cause I'm done.

[GoneFission]

At the end of the day, you can use all the gizintas, computer models even a crystal or the ol' 8 ball to find a starting point but most of us end up using the tried and true science of "trial and error"
Stick a fork in me, cause I'm done.

[fmaster360]

That shifter prop is freaking cool.

[dburr]

So after all this discussion I don't remember anyone bringing up cavitation due to excessive blade load, optimum blade load or the relationship of blade load to diameter and the maximum EFECTIVE tip velocity, power and bollard pull...  :|  

Otherwise extremely well thought out and argured ..

It's all about getting the power that you want to the water when you want it how you want it..  

Another variable is the actual blade area and resultant blade load.  Keeping constant diameter while reducing pitch will cause an increase in tip velocity if there is not enough load to keep the engine RPM at what would be optimum for the wheel load vs tip velocity. (Been a while since I have thought about it so I don't remember off hand what the number is..) If the blade area is increased with a lower pitch the wheel will be able to transfer more torque/power to the water. The resulting acceleration will be slower (less turns so less distance traveled, But more power due to more effective blade area to transfer the power at higher engine rpm).  The resulting hole shot may not be as fast (less fps traveled) as desired BUT the bollard pull will be higher..

No real value to the argument, just another observation.  In a previous life I was a sternman on a lobster boat that went to the races every year and the race wheel that we used was way different from the normal hauling wheel... Also got 100 hp more out of that old J&T 671TIA too...

[gran398]

Said it before, and will say it again..."That 8 Ball is one cool dude".

[seabob4]

Just goes to my thinking...prop "science" is anything but...

The way I would do it.  Start with the most "generic" prop recommended for the size of the boat and the engine.  Evaluate.  Hole shot OK?  Time to plane OK?  Cruise RPMs OK?   Top speed and related RPMs OK?  Fuel burn OK?  Evaluations done, what don't you like, and how do you re-prop to compensate.  The most I would play with is dropping the diameter by an inch and MAYBE raising the pitch by 4" tops (in 2" increments) from a "typical" 15 X 17 that one might find on a 150-200.

After that, you start to talk $$, whether it be cupping, 4-blade VS 3-blade VS 5-blade, variable pitch, REALLY leaning towards one desirable VS another...you get my drift.  All facets of engine and boat performance are critical, yet all will suffer at the expense of another, given what exactly the boat owner holds to be "the priority".  It's all a compromise.  So, given that fine-tuning costs bucks, pay a couple hundred to Prop Gods, that's what they do for a living, and they have that "science" almost down to a...science...

Just my $.02...

[Capt. Bob]

am i on THT?

I thought Slip and I were rather civil.

Exchange of ideas are always useful.  
It's when they turn into a pissin' match that they become worthless and a waste of what little time we all have. :x

[slippery73]

am i on THT?

I thought Slip and I were rather civil.

Exchange of ideas are always useful.  
It's when they turn into a pissin' match that they become worthless and a waste of what little time we all have. :x

I thought we were pretty civil as well..... there wasn't any weeny head comments or a--- holes happening.  Thats pretty good for prop talk!

[gran398]

You guys did great...and therein lies the beauty of our clubhouse.

This discourse was referred to a time or two nonchalantly as "the argument is"....

Far from an argument. It was good, intelligent, respectful discussion

This place will never be THT, and thank goodness. And guess what? The rest of the board will blow off a blowhard faster than you can say 5550 RPM WOT.