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Propeller PitchRPM

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chris_v

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"Is there a basic rule of thum

"Is there a basic rule of thumb for how many r's are gained for the next pitch on prop. I want to gain 1000 r.p.m and not sure how far to go from a 21 degree. What difference will I realize with a 19? I understand the differences have many variables, but is there a basic change to go by?"
 
"Hi chris,
About 200rpms for

"Hi chris,
About 200rpms for each 1" of pitch a 3 blade prop.You might want to post a bit more info ,Boat type ,motor type,max rpms vs speed,intended use ,Plus what you want to achieve by gaining 1000rpms."
 
"Chris, David is bang on. You

"Chris, David is bang on. You should shoot for your "optimum" rpm which is 200-250 rpms below absolute max.

So say on a motor that has a WOT range of 5000-5600, I would shoot for 5400. And as David stated every 1" change in pitch (assuming the Diameter stays very close) you will gain/lose 200 rpms.

Cupping may affect it by another 50-100 rpms up/down and the material the prop is made from (due to less/more flex) will be a minor factor..."
 
"I'd still attempt to get

"I'd still attempt to get it to run over max by some small degree, lugging the motor is hard on every mechanical part of it, and a fiberglass boat gets heavier as the season goes on. Not to mention atmospheric losses. In fact, some Mercury publications have recommended switching a lower pitch prop as the weather warms up. This is to counteract "secondary losses": the rpm drop as the air loses density, and the prop is too heavy to develop full rpm, which further degrades performance. If you will only be using one prop all year, consider if you will ever be sking or loading up a bunch of buddies, beer coolers, etc. Use the lightest possible load in cool weather with very little fuel onboard. Under this condition, hundred or 2 over max is not a huge issue."
 
"I'd still attempt to get

"I'd still attempt to get it to run over max by some small degree, lugging the motor is hard on every mechanical part of it, and a fiberglass boat gets heavier as the season goes on. Not to mention atmospheric losses. In fact, some Mercury publications have recommended switching a lower pitch prop as the weather warms up. This is to counteract "secondary losses": the rpm drop as the air loses density, and the prop is too heavy to develop full rpm, which further degrades performance. If you will only be using one prop all year, consider if you will ever be sking or loading up a bunch of buddies, beer coolers, etc. Gat a tach. For you trial run, use the lightest possible load in cool weather with very little fuel onboard. Under this condition, hundred or 2 over max is not a huge issue."
 
"Robert, you are absolutely co

"Robert, you are absolutely correct - during the hottest/most humid part of the summer you lose between 10-14% of horsepower and the corresponding loss of rpms's simply because the motor can not "breathe" properly.

That's why there is no "single" correct prop for any set-up. Two different pitch's, one for spring/fall and one for summer is the only sure fire way to keep your motor running at it's potential and in some extreme cases, prevent internal damage from occuring..."
 
"It's possible then I have

"It's possible then I have gotten some bad info. I looked up recommended prop on line and found what I expected to be a reasonable start to my needs but I am unable to achieve my desired rpm's. I have a 79 Merc. 150hp on a 18' fiberglass open bow. It could have, at times, a full capacity but for the most part 2 adults and 2 to 4 children. The prop I bought according to the chart I looked up said 19 to 21 degree 14.25" and 14.50" respectively. I settled on a 21 degree 14.3". My max rpm is around 4500 and merc. recommends 5500. Any one have a different recommendation?"
 
"Chris, it's nearly imposs

"Chris, it's nearly impossible to determine what prop to use off a chart - there is just way too many variables.

For example, the chart says use prop "x" for 17-19' boats - a 17 foot aluminum may way 900 pounds dry, a 19 foot tri-hull fibreglass may way 1700 pounds dry - there is no way the same pitch prop will push both effiently.

To properly calculate what prop is suitable for a particular rig you need to know hull type, waterline length, total displacement (including boat, motor, gear, rigging and load), horsepower and max prop diameter. That will allow you to calculate a reasonable prop to start with.

And the year of the motor is important as well. Your 1979 model was rated in BHP (Brake Horsepower at the crank). By today's standards you have a 135 horse SHP (Shaft Horsepower) motor. Most charts do not make that distinction, so you would be turning "too much prop" by their guidelines regardless of how accurate they made them.

From there, it's a matter of running the prop selected and then fine tuning using a tach on your rig as you actually run it. A decent calculator will normally select a reasonable prop. A paper chart is best used for starting the fireplace when the boat is in winter storage...

With the numbers you have posted, you should be looking to move to a 17 pitch to get into the rpm range where the motor should be running.

And to me a 17 pitch on an 18 footer sounds about correct with the load you describe..."
 
"What about the diameter? I ha

"What about the diameter? I have found a 17 degree propeller available in 12", 13" and 14.8". With this size motor and boat is bigger better?"
 
"Chris, through some rough num

"Chris, through some rough numbers (best guess) into my prop calculator based on the specs of your motor, your info on normal load and a few guesses as to hull weight etc - it spit out 14.46 Diameter, 16.98 pitch (that's the rough numbers that you round up).

Given that, I would go with the 14.8D 17P although, either the 12 or 13 diameter would not make a very noticable difference.

A 1" difference in diameter will have significantly less effect on rpms compared with 1" less in pitch."
 
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