Pitch "vs" Diameter

[MOP]

If you reduce pitch by 3/4" how much would you have to increase pitch to stay at the same RPM's?

Edit**
Ok I'll do it right this time, I was typing in the middle of a pitched sword fight on TV!!

If you reduce "diameter" by 3/4" how much would you have to increase pitch to stay at the same RPM's?

[onesubdrvr]

If you reduce pitch by 3/4" how much would you have to increase pitch to stay at the same RPM's?
Just a guess, but 3/4" :D :D :D

Gotta love it when someone's a little off keel :D :wavey:

Wayne

[Ed Donnelly]

Mop; Love trick questions.
Now try it with diameter....Ed

[boxy]

Is the prop punched or bored ????

[smoothie]

Just a thought but I would say that it depends on the cup and the shape of the blade,as far as the smaller dia. goes,I can tell ya that it dont take much,I played with a couple 3 blade and 4 blades and just a 1/2" off the dia give me another 300 rpm and thats putting the cup back in.

[MOP]

Thanks smoothie

I was wondering what the effect the lessening the blade area but increasing pitch has on stability, speed, etc. I was passing ideas around on props and shaft angle, on a "MAY" be resurrected buddy's plywood SK. He is in the dreaming stage still and the boat NEEDS! But makes for good chilly night Bsing. I need to chat with FlatRacer :idea:

[d1mbu1b]

try playing with differnt prop diameters and pitch on:
http://www.speedboatperformance.com/

this should be able to tell you where the relative maximun RPMs and relative speeds lie.

I imagine that rpm will increase with a decrease in diameter
and rpm will decrease with increase in pitch
not quite sure what will happen to speed
I imagine that the pop slip and speed will change

[boldts]

MOP, just an observation I observed when I went from a Mercruiser Mirage to a Turbo 1 on my 22. I thought because I was reducing the diameter from I believe 15 1/4" (Mirage) to 14 1/2" (Turbo) that if I ordered the same pitch (23") I would gain the 3 or 400 RPMs I was looking for.
Once the Turbo arrived and I tested the prop, I found the the RPMs were almost the same. I think I gained about 100 RPMs with the Turbo. I ended up sending the 23" Turbo 1 back and using a 22" pitch Turbo 1.
So, my point is, efficency of the prop is going to play into your figures I believe. As you may know from previous posts, I gained 3 mph with the Turbo 1 even though it was 1" less pitch. Food for thought.

[MOP]

Thanks to all of you, Got into the prop thing with my buddy's flat bottom as I thought his shaft angle was to steep, and the prop to big. My reason for thinking that way was a flat bottom I had many years back, my shaft angle was much less and the prop was much smaller. We talked about it the other night he got some input that his setup was more for pulling skiers then speed, so he is now thinking of lessening the angle and going smaller with the prop. Geo had some good input on reduced prop size that go me thinking I need to chat with him again. I spoke with another fellow this past week about propping and gearing on I/O's, he said he swapped from 1:50 to 1:36 gearing and went down on pitch. His speed was only a tiny bit more but the boat was no longer a handful at speed, chine walk disappeared. His rational was it helped to eliminate the side walking force of the prop. It helped a little more with the thoughts I have had, it all still boils down to what is always said. Propping is not a science it is all experimenting!!!

Phil