pitch vs rpm

[nissannoodles]

I was wondering if there is a rule of thumb on RPM change per inch of pitch?
I am 200 rpm over recomended @ WOT.
the motor is a honda 15 hp four stroke, currently with a 9.5 pitch
was wondering if an 11 pitch would be to much
motor is pushing an inflatable.


thx for any comments

 

[steelespike]

Re: pitch vs rpm

I feel a 11 would be way too much.Small motors it doesn't take much pitch for a sizable change.Running a 4 stroke on an inflatable I would tend to stay with the 9.5 and just watch the rpm.Inflatables can be difficult to plane especially if you start adding weight.

 

[reelnative]

Re: pitch vs rpm

an 11 would be fine you will loose 200rpm for each inch of pitch added

 

[trendsetter240]

Re: pitch vs rpm

I agree with steelespike, that 9.5 is probably your best bet. If you make a change look for a 10" pitch. 11p will likely drop 500RPMs From where you are now.

The rule of thumb for 200rpm per inch of pitch has never held true in my experience. There are so many factors that affect how the prop will perform pitch, rake, cup, material, number of blades that the 200rpm/inch is only a very basic guideline and will be wrong in many cases.

The closest estimate rule of thumb I've found is using Old RPM x (old pitch / new pitch) = new RPM. This only comes close when you stick with the same prop manufacture, blades, material etc.

I've run 5 different props on my boat in the past month and I'm waiting for my 6th. The differences between prop manufactures can be huge. For example, two 3 blade 17p aluminium props have WOT 500RPMs apart. Another 3 blade 15p prop and a 4 blade 13p prop gave WOT 800RPMs apart.

Welcome to the black magic art of prop selection

cheers~!

 

[reelnative]

Re: pitch vs rpm

if your running a honda prop now and change up pich with a honda prop you will drop 200 rpm, changing cup and rake will alter the 200 rpm rule also alum and sst will alter the 200 rpm rule but if you are staying with a factory prop you will loose 200 rpm

 

[trendsetter240]

Re: pitch vs rpm

if your running a honda prop now and change up pich with a honda prop you will drop 200 rpm, changing cup and rake will alter the 200 rpm rule also alum and sst will alter the 200 rpm rule but if you are staying with a factory prop you will loose 200 rpm

Well, agree to disagree.

Good luck to the OP.

 

[steelespike]

Re: pitch vs rpm

I'll disagree.The prop section lists 8 sizes for the Honda 15; 5" to 11"
the three largest sizes are 1/2" apart 11,10 1/2,10.I find it hard to believe a person would want or need to buy a prop to gain or lose 100 rpm. rest of sizes are basically 1" intervals.
Wouldn't seem to me there is enough props to get to the rpm you might need at 100 per inch. 10" to 5" = 500 rpm.??to use on a kicker on a 4,000lb boat.
The most common size is 9" 6 selections including the somewhat radical Turning Point Rascal. 10" I believe has 2 selections and 8" has 4.
Its fairly obvious the 9" should cover many applications.

 

[reelnative]

Re: pitch vs rpm

I'll disagree.The prop section lists 8 sizes for the Honda 15; 5" to 11"
the three largest sizes are 1/2" apart 11,10 1/2,10.I find it hard to believe a person would want or need to buy a prop to gain or lose 100 rpm. rest of sizes are basically 1" intervals.
Wouldn't seem to me there is enough props to get to the rpm you might need at 100 per inch. 10" to 5" = 500 rpm.??to use on a kicker on a 4,000lb boat.
The most common size is 9" 6 selections including the somewhat radical Turning Point Rascal. 10" I believe has 2 selections and 8" has 4.
Its fairly obvious the 9" should cover many applications.

1 inch of pitch is not going to change rpm by 100 but by 200 and hes not pushing a 4000lb boat he is running the 15 on a inflatable boat and he is over reving 200 rpm he needs to change to a 11 pitch prop just to get to the top of his WOT rpm range, this will give him more speed and save his motor from damage if run at full throttle

 

[steelespike]

Re: pitch vs rpm

My point is on the props with 1/2" pitch change (11,10 1/2,10") at 200 per inch your only changing 100 rpm at 1/2" pitch change. Why would anyone need to buy a prop for a 100 rpm change The likely hood is pretty slim.
I know hes not pushing a 4,000 boat
point is at 200 per inch there isn't enough change over 6 inches to progress from
4,000 boat to say a 200 lb boat. Actual rpm change wouldn't add up at 200 per onch.

 

[180shabah]

Re: pitch vs rpm

...but if you are staying with a factory prop you will loose 200 rpm

based on what????

 

[reelnative]

Re: pitch vs rpm

My point is on the props with 1/2" pitch change (11,10 1/2,10") at 200 per inch your only changing 100 rpm at 1/2" pitch change. Why would anyone need to buy a prop for a 100 rpm change The likely hood is pretty slim.
I know hes not pushing a 4,000 boat
point is at 200 per inch there isn't enough change over 6 inches to progress from
4,000 boat to say a 200 lb boat. Actual rpm change wouldn't add up at 200 per onch.

I see what you mean .5in changes are really kinda nitpicki hahahah but I guess if you are really anal and wanted to dial her in to the point then that would help, for 100rpm changes I use the old sandbar method

 

[180shabah]

Re: pitch vs rpm

I see what you mean .5in changes are really kinda nitpicki hahahah but I guess if you are really anal and wanted to dial her in to the point then that would help, for 100rpm changes I use the old sandbar method

You are missing the point completely. RPM change is MUCH, MUCH more than 200/inch. That is why the props are in 1 and 1/2 inch increments, not too satisfy the anal retentives.

The 200/in is a guideline not a fact. It is never exact, not even when sticking with the same prop manufacturer. You haven't accounted for hull design, prop slip or gear ratio. It certainly doesn't apply to these very small engines.