Prop sizing

mjsiega

New member
I have a 75 HP Mercury 2 stroke running an aluminum 13 1/4" X 17" prop. I need a new propeller and would like to go to a slightly lower pitch as I'm just barely touching the low end of the rpm I should be at at WOT. I get about 4600 rpm when WOT should be 4700-5200. The problem is when looking at props, they don't have multiple pitch sizes in a given diameter size, at least not in the 13 1/4" diameter that I currently have. The next "lowest" prop looks to be a 13 3/4" diameter with a 15" pitch. Supposedly each change of 1" in pitch results in a 200 rpm change. What about changes in diameter? I have enough clearance under my cavitation plate for the 13 3/4" diameter but I'm not sure about changes in turbulence with the bigger diameter and how changes in diameter effect WOT rpm. Also I have a strong pull to starboard on my outboard, even with my trim tab set at a good 20 degrees to push the outboard over the other way. I use the boat mainly for cruising (21 foot Larivee pilothouse) what's a good aluminum prop to get? Thanks for any info.

-Mark
 
mjsiega":3pbxazn7 said:
I have a 75 HP Mercury 2 stroke running an aluminum 13 1/4" X 17" prop. I need a new propeller and would like to go to a slightly lower pitch as I'm just barely touching the low end of the rpm I should be at at WOT. I get about 4600 rpm when WOT should be 4700-5200. The problem is when looking at props, they don't have multiple pitch sizes in a given diameter size, at least not in the 13 1/4" diameter that I currently have. The next "lowest" prop looks to be a 13 3/4" diameter with a 15" pitch. Supposedly each change of 1" in pitch results in a 200 rpm change. What about changes in diameter? I have enough clearance under my cavitation plate for the 13 3/4" diameter but I'm not sure about changes in turbulence with the bigger diameter and how changes in diameter effect WOT rpm. Also I have a strong pull to starboard on my outboard, even with my trim tab set at a good 20 degrees to push the outboard over the other way. I use the boat mainly for cruising (21 foot Larivee pilothouse) what's a good aluminum prop to get? Thanks for any info.

-Mark

Mark-

When they reduce the pitch, they also increase the diameter, and vis'-a-vis'.

The lower pitched props are for heavier loads (boats), and the increased diameter gives more surface area to reduce slippage for the heavier loading.

It's kinda like gearing down, then broadening the tire tread to get more traction. (without increasing the tire diameter, which would change the ratio again!)

I'd guess the drop from 17 to 15 would give you about a 400 rpm increase to 5000 (using the rule of thumb).

Joe. :teeth :thup
 
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