Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

[rolmops]

Hello everybody.
My Mercruiser currently runs (labors) at 3600 rpm running a top of 26 MPH in flat water trimmed out the right way.
The current prop is a Mercruiser prop #48-78120-A401-19P. This is a 14x19.
To go up 800 RPM I should just go down from 19 to 15,but most props I find are closer to a 15 inch diameter. How much more, if any, should I go down to compensate for the one inch increase in diameter?
Thanks, Cornelis.

 

[Bondo]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

Hello everybody.
My Mercruiser currently runs (labors) at 3600 rpm running a top of 26 MPH in flat water trimmed out the right way.
The current prop is a Mercruiser prop #48-78120-A401-19P. This is a 14x19.
To go up 800 RPM I should just go down from 19 to 15,but most props I find are closer to a 15 inch diameter. How much more, if any, should I go down to compensate for the one inch increase in diameter?
Thanks, Cornelis.

Ayuh,.... Generally speakin', prop diameter goes up, as pitch goes down,...

Ya still shop by pitch though,...

Donno's you'll find any props for yer drive with less than 15" pitch anyways,...
most All of those smaller I've seen are outboard props, 'n the barrel diameter is smaller, causin' blow-out,...

 

[jestor68]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

I assume we're talking about a 19 ft Star Craft Islander?

That 2300 lb(?) boat with it's 1.98 gear ratio should be able to spin a 19 pitch prop to 4500-4600 rpm @ about 36 mph.

It seems as if you have engine power issues, especially if it's not been freshened since 1984.

As far as diameter versus pitch goes; consider an inch of diameter as roughly equal to an inch of pitch. A 15.5 X 17 will run 100- 200 rpm higher than a 14.5 X 19 of the same prop family. That has been verified by testing on my boat(s), as well as other's boats.

The prop section shows alpha 1 props down to 11 inches of pitch.

 

[rolmops]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

I assume we're talking about a 19 ft Star Craft Islander?

That 2300 lb(?) boat with it's 1.98 gear ratio should be able to spin a 19 pitch prop to 4500-4600 rpm @ about 36 mph.

It seems as if you have engine power issues, especially if it's not been freshened since 1984.

As far as diameter versus pitch goes; consider an inch of diameter as roughly equal to an inch of pitch. A 15.5 X 17 will run 100- 200 rpm higher than a 14.5 X 19 of the same prop family. That has been verified by testing on my boat(s), as well as other's boats.

The prop section shows alpha 1 props down to 11 inches of pitch.

My Islander is indeed old and tired. I am just trying to nurse her along for a few more years till my kids are out of college.

 

[jestor68]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

My best recommendation from the off the shelf props available(aluminum)would be a Solas Rubex 3 15 X 15; #9511-150-15; Hub RBX 102 for the most cost effective solution. Factoring in the inch of diameter increase, you should still gain 600-700 rpm and get up into your recommended rpm range; which I believe is 4200-4600 rpm.

It is normal to have to re-prop sometime in the life of any boat; as the motor's get up and go goes away. :blue:

 

[Bondo]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

I assume we're talking about a 19 ft Star Craft Islander?

That 2300 lb(?) boat with it's 1.98 gear ratio should be able to spin a 19 pitch prop to 4500-4600 rpm @ about 36 mph.

It seems as if you have engine power issues, especially if it's not been freshened since 1984.

As far as diameter versus pitch goes; consider an inch of diameter as roughly equal to an inch of pitch. A 15.5 X 17 will run 100- 200 rpm higher than a 14.5 X 19 of the same prop family. That has been verified by testing on my boat(s), as well as other's boats.

The prop section shows alpha 1 props down to 11 inches of pitch.

Really,.... Not that I've looked lately,...

Back 'round the turn of the century, I searched high, 'n low, Michigan wheel, 'n Mercruiser, 'n the smallest for the large gear case outboards, 'n the outdrives was 15",...
Anything less was the smaller prop body/ barrel, for mid-sized gear cases,...

I tried a 13", 'n it slid on, 'n fit perfect, except for the gap at the drive, which caused massive blow-out,...

The availability of the 25" props also seems to come, 'n go,...

 

[jestor68]

Re: Prop for a 1984 19 foot islander with a 3 liter (181) 140 horse mercruiser

Really,.... Not that I've looked lately,.

Haven't looked lately? So go look already.