Prop size question

N

newtons law

Cadet
Joined
Mar 3, 2018
Messages
13
Got this 1984 Galaxy runabout, I'm working on. It's 19.5' with a mercruiser 140, 3.0..the prop number is 18-78120-19. Could anyone tell me if this prop is anywhere close for this boat. Was thinking a 17 pitch might be closer,but that was on my pontoon boat with same engine . Thanks
 
bajaman123

bajaman123

Seaman
Joined
May 6, 2009
Messages
53
Pretty sure that first number is 48 not 18! Need to know what ratio your outdrive is...and if your drivetrain on the other boat was the same. What was your WOT rpm on the 'toon? I suspect you have around a 1.85:1 outdrive ratio, which if so, the 19" pitch prop should be good, unless you are just lugging the engine.
 
jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,631
What year is the Engine? What is the WOT Rpm range, and as asked for earlier, what is the Gear Ratio of the outdrive?
 
A

airshot

Vice Admiral
Joined
Jul 22, 2008
Messages
5,361
Most 140 up engines used a 1.98 drive ratio. With a typical load onboard, what is your wot rpm ? You need to know this, along with being sure your engine is running properly. Without both of these things, we can only guess what prop might work.....
 
jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,631
In the Early 70s the 140 used a 2:1 drive. The 1.98 while pretty much the same Ratio, reduced the Frequency of repeated Contact Pattern on the Gears, extending their Life
Year the Engine is Important, as the WOT Rpm ranges has varied over the past 50+ yrs.
1970-Mercruiser-12.jpg
 
J

jlh3rd

Chief Petty Officer
Joined
Jul 10, 2017
Messages
648
as stated, the starting point is what is the WOT rpm ?
And a V hull is going to run differently and give different #'s than a pontoon "hull" with the same engine.
That V hull is gonna run faster and can take a higher pitched prop than that plowing pontoon.
 
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