Prop Pitch Verses Diameter

[CAP'N ROG]

how much does diameter affect prop pitch? im running a 13 1/4"17 pitch and its over propped mercs prop selecter says i should run a12 3/4" 21 i cant spin the 17 over 4100 rpm with a 115 inline does 3/4" make that much diffrence? thanks

[Liberator25]

Diameter has ZERO affect on pitch. The combination of the two will affect the motor and it's ability to rev (pull the prop). If your 17" pitch prop will only turn 4100 a 21" pitch prop will drop that by about 800 rpms. The slightly smaller diameter of the 21 will make that drop in rpms a little less. Assumeing your engine is healthy, I would say you need LESS prop pitch, not more.

[CAP'N ROG]

OKAY SAY you hava a 13" prop with 17 pitch and a 15" prop with 17 pitch would the 15"be harder to turn do to more blade area?

[Liberator25]

With the same pitch, the larger diameter prop will be harder to turn and will lower your engine rpm at any given throttle setting. It is a common practice to trim the diameter of a prop down to increase the rpm's of a motor. This is due to the increased blade area of the larger diameter prop. You are right on target. The "rule of thumb" is for every inch you change the PITCH of a prop the rpms will change by 200. This is not always perfect, but if you had an engine (like yours) that would only turn a 17" pitch prop at 4100 rpm then by going to a prop with 15" pitch (both with the same diameter) your rpms would go to 4500. There is a similar formula for diameter but I'm not sure what it is. I THINK that for every change of 1/2" in diameter you get a 200 rpm change in engine rpm - smaller gives more, larger gives less. But I'm not sure about the diameter "rule of thumb". Perhaps someone else can clarify that for both of us.

[Bill Gohr]

as pitch goes up, diameter goes down, if you incresed the pitch and kept the diameter the same, the engine wouldn't be able to turn it because increasing the load with 2 factors wouls be too much for most engines. Thats why you'll see you can't go through a family of props for your engine and buy a prop by a pitch with different diameters......

[Liberator25]

You're generally correct but I had to decide if I wanted my Merc Labbed Cleavers to be 15X34 or 141/2X34. They make both and I chose the 15X34 because if I couldn't turn 'em I could get them cut down. The folks at Merc Racing told me that if the 15X34's weren't turning enough rpm's that is I would tell them how many rpm's I wanted (say plus 300), they would know exactly how much to take off. This tells me that there is "formula", I just don't know what it is.

[Mark75H]

You need more diameter and blade area to push a heavier boat and a heavier boat will naturally go slower and use less pitch, so, prop makers usually make the choices include bigger diameters as pitch decreases.

Its more of a market/sales need thing than go fast performance orientation.

Going to performance different boats and different set ups will respond differently to changes in diameter.........that is why there is no chart that will give you an exact and direct recommendation or relationship (formula).


Liberator25's idea of starting out fat and trimming is a good idea if you are buying new props......but there are enough used props out there for Rog's inline 6's at reasonable prices to just collect 5 or 6 and sort out which is best.

[Bill Gohr]

When I worked with the prop guys in engineering, they new how much to reduce the diameter as the pitch went up. Whether that was a formula or experience I don't know. The generalization I was speaking of was in common propeller families. When we talk about surfacing propellers everything changes, you need the extra diameter when you run them in the air, thats why Merc makes surfacing props in different diameters depending on the height you run.

[G-Train]

A friend of mine asked me to sell his Michigan Wheel 13 x 19p Stainless prop. I think he'll yake $115 shipped (us48).
This prop was used on a merc 115 (inline 6), for skiing, etc.

Greg 877 713-3223

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[CAP'N ROG]

if you use mercs prop selecter the heavier the weight you enter pitch keeps going up i tried 2200lbs to 26 00 and pitch goes from 19 to 22 i cant spin a 15" turning point prop unless there is a diffrence in design it might try a 21 black max

[Bill Gohr]

Boat is this thing on Rog

[CAP'N ROG]

Originally posted by Bill Gohr
Boat is this thing on Rog
okay dont laugh"" before i tell you im saving for a hydrostream voyager. but the kids love my 1988 bayliner capri bow rider 18'
the heavier weight i enter in mercs prop selector the higher pitch its telling me to use unless engine is lacking power

[bigobass1]

Another thing to consider is how high your engine is mounted on the transom.If you are on the transom by raising the motor you can increase both speed and rpm as long as the prop has a good bite on the water.BC