Prop size question

[newtons law]

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]

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]

What year is the Engine? What is the WOT Rpm range, and as asked for earlier, what is the Gear Ratio of the outdrive?

 

[airshot]

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]

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.

 

[jlh3rd]

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.

 

[CMoonS]

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

Nice setup! That Mercruiser 3.0 is a solid and dependable engine. The prop you mentioned (18-78120-19) is a 19 pitch, and while it might work fine depending on your load and how you use the boat, you’re right to question if it’s optimal.

For a 19.5' runabout with a 3.0L engine, a 17 pitch is often a better match, especially if you're looking for quicker hole shot and better performance with passengers or gear. A 19 pitch can give you more top-end speed but might bog the engine down a bit if the boat is heavier or you're pulling skiers/tubes.

Best way to tell is by checking your WOT (wide-open throttle) RPMs. The 3.0L Mercruiser should be running between 4,400–4,800 RPM at full throttle. If you’re below that range with the 19p, switching to a 17p could help get you in the sweet spot.

Are you using it mostly for cruising, watersports, or fishing? That might help fine-tune the recommendation too.

 

[airshot]

No one knows until you have the engine running perfect, then testing a prop to see if the motor reaches the max rpm at wot. You adjust the pitch to reach max rpm at wot. Many like myself, adjust pitch to allow the engine to run a little over max rpm with just one on board, then when extra people and gear are on board the max rpm should be closer. But...you need to watch your fact when running lite so you don't over rev. If your pulling skiers or tubers, then drop an inch or two in pitch to get a better holeshot, but....watch the teach so you don't over rev your engine. Many folks carry extra props for special ocassions. Only takes about 10 minutes to swap props.