RPM change per degree of prop pitch?

[DEYS]

If I change from a 19p to a 15p with my 80' 115 Mercury on a 16' fiberglass boat, what would the rpm change by? 100 rpm-200rpm per degree? And what is the max rpm this engine can handle? currently with the 19p the boat planes-out good with a full load and goes pretty good (67 km/hr with 3 adults in boat), but struggles to "pop" a skier onto the water and revs out at approx 4600 rpm.

[Raceman]

Hard to say on the RPM because there are variables (boat itself, prop itself, etc) but in any case you need to turn that motor harder than 4600 at full throttle, or it's in danger of burning pistons. High 5's (or even more) won't hurt it. I kinda lost track of which engine was which after the 1500's went away and Merc factored the horsepower, but the mid 70's 115's seemed happiest at mid fives, while the 150's, and later 140's of the same era seemed best at 5800 to 6K. Assuming your engine is in good shape I would guess going from a 19 to a 15 would put you in the low 5's.

There's an online prop calculator that's fun to play with. It has blanks for RPM, Pitch, Speed, Prop Slip and Gear Ratio (your gear ratio is 2:1 in case you didn't know, and I think you just plug in 2). You plug in any 4 of the 5 blanks and it gives you the missing one. Maybe somebody will post a link to it here. You could play with the numbers from 10 to 15% to give you an idea of slip, (15 may be more realistic on your boat) but as long as you use the same number with both pitches it'll put you in the ballpark.

Here's one of the online calculators: http://www.rbbi.com/folders/prop/propcalc.htm

[afr]

i was always taught 500 rpm per inch of pitch
works both ways up or down
so if you go down in pitch the rpm will go up and vise versa

key largo prop shop 10 years hands on training

[Raceman]

i was always taught 500 rpm per inch of pitch
works both ways up or down
so if you go down in pitch the rpm will go up and vise versa

key largo prop shop 10 years hands on training

Too many variables to have a hard fast formula like that. Obviously if the engine was turning 9K RPM the difference would be more than on an engine turning 4600. Also, an inch of pitch would make more difference when fooling with 15 pitch wheels than when fooling with 30's. If you don't believe it, plug some variables like these examples in the calculator I linked.;)

[DEYS]

Tried the 15p today. Got 5300rpm and 57.6 kms / hr. Did 67.4 kms/ hr and 4800 rpm with the 19p.

[DEYS]

So what is a decent amount of "slip" 5% 10% 15% ... ?

[afr]

well he just threw a variable in
lol
he only lost 500 rpm with apparently a 4 " pitch change
lol
you need to double check your pitch on both props
with a local prop shop

[DEYS]

I call bs on the 500 rpm per inch of pitch. I might not be a boat / prop expert, but if 1 inch of pitch changed rpm by 500 rpm, there would be props for sale in 1/2" or 1/4" increments, would there not? ....I'm not talking about some custom $1500 one-off prop either. The torque peak on a two-stroke engine is barely 500 rpm wide. If the prop shop catalog only has a 15, 17, 19, 21, 23, 25 etc listed, and joe-blow buys a prop with the next highest inch of pitch, the engine would not run very well at all, as the rpm would have changed 1000rpm, and fallen way off it's torque peak.

[C-Webb]

+-500 rpm change with 1 inch change is way too much I think. I would guess 150-200 rpm change per inch. Merc prop brochure also tells similar numbers.

[afr]

maybe its been to long but as i recall 500 was the number
1/2 sizes are done with a cup
check with your local prop shop

[rils]

More like 200 per inch from my expercience.

[Wayne Cammidge]

Guys.
I think you are missing the point.
There is something seriously wrong with that 115.
Most of those 115's pulled a 19" prop quite easily unless the boat was an absolute barge. Also with it revving at only 4600 and then dropping pitch it should have increased speed. I bet you that motor is only running on 4 cylinders.

[150aintenuff]

GENERAL RULE is 200 RPM per pitch, and you subtract 1" when going from aluminum to stainless( same style blade), and add 2" pitch and work back to where RPM's are happy... when changing from a flat eared aluminum and go to a performance style stainless...


should have increased about 800-1000RPM... something isnt right with the motor.........

[rils]

Guys.
I think you are missing the point.
There is something seriously wrong with that 115.
Most of those 115's pulled a 19" prop quite easily unless the boat was an absolute barge. Also with it revving at only 4600 and then dropping pitch it should have increased speed. I bet you that motor is only running on 4 cylinders.
A 115 only has 4 cylinders atleast the one i had did. Ok I guess the old mercs had six mine was a newer johnson.Mybad seems like mine had a 19 pich on a 17fs. would run mid 40s. The one he has was proably rated at the crank the biggest problem I found with the 115 I had it was just too small.