prop rpm help

[whatisup02]

First off I have a new to me 1983 all family sport 180v. Its 17.8 feet long and about 2100lbs. With a merc 140.

My ? Is what prop is this 48-78122-21

Then I got 33mph with 4 adults and 3 kid about 950lbs of people and gear. I think that's good but I was only getting 4200rpms. At idle it says about 300 rpms. Can someone do the math and tell me if the rpms are off?

 

[Bondo]

Re: prop rpm help

First off I have a new to me 1983 all family sport 180v. Its 17.8 feet long and about 2100lbs. With a merc 140.

My ? Is what prop is this 48-78122-21

Then I got 33mph with 4 adults and 3 kid about 950lbs of people and gear. I think that's good but I was only getting 4200rpms. At idle it says about 300 rpms. Can someone do the math and tell me if the rpms are off?

Ayuh,.... Welcome Aboard,.... I'd sooner think it's idlin' 'bout 650/ 700 rpms....

Ya oughta run it, by Yerself, with 'bout 1/2 a tank of fuel, on reasonably calm conditions, 4" to 6" chop, the Best....
It oughta be pullin' 4600/ 4800 rpms at Wot, in proper trim....

Btw,.... With a Boatload, 4200 rpms, ain't to bad....

 

[whatisup02]

Re: prop rpm help

I have been look at things here for some time befor we got the boat.

I was thinking that 4200rpm was good. It was also 1-2 foot chop. But that said I don't think my rpms are that high.

We plan on doing a lot of tubing and want a prop that would work good for that.

 

[steelespike]

Re: prop rpm help

Maybe its not 300 but 3 notches at 200 rpm per notch. Your pitch is 21",pretty near the top of the chart for a 3.0.
Probably would struggle a little with a load.If the motor is original 83 it may be a little tired.
Disregarding the motor a 19" prop should give better all around performance. But do a light load test first.
A few things to check:: Does the boat respond to up trim?Is the timing set right and advancing as it should.
Note the hole shot with the light load.

 

[chris.j.marshall]

Re: prop rpm help

I run a 19 pitch prop on my 140 in a boat of the same approximate weight. I'd say for towing a tube, you'll be much happier. She'd be tough to get on plane with a few people on board and a tube in back with your current prop.

 

[jestor68]

Re: prop rpm help

I vote for a 19 pitch for better all around performance, considering the age of the motor, and your intention of using it for water sports.

Rpm range is 4400-4800.

 

[whatisup02]

Re: prop rpm help

Maybe its not 300 but 3 notches at 200 rpm per notch. Your pitch is 21",pretty near the top of the chart for a 3.0.
Probably would struggle a little with a load.If the motor is original 83 it may be a little tired.
Disregarding the motor a 19" prop should give better all around performance. But do a light load test first.
A few things to check:: Does the boat respond to up trim?Is the timing set right and advancing as it should.
Note the hole shot with the light load.

It is 200 rpms per notch and it idles at 11/2-2 notches. The motor runs great but does sound like it wants to quit at such a low rpm. As far as trim goes it did make it go about 2 mph faster.

One other thing. It has a "wing" on it. I think its to help it plan faster. If I moved people around it could plan at about 15mph.

So I should get a 19 pitch prop. What size would I need? 3 or 4 blade? I don't want to spend a lot on a prop when that money could go to gas and more fun on the water if ya know what I mean.

 

[steelespike]

Re: prop rpm help

Very likely if you remove the "wing" it will give you more rpm and speed may lose some hole shot but the 19 should help that.
Don't change the prop till you remove and test without the "wing".
Allways make one change at a time and test.

 

[whatisup02]

Re: prop rpm help

Going fishing tonight and I have taken the wing off. I will see how it goes and report back.

One other thing old prop is 13 3/4 - 21P and the new one I was looking at is 14 1/2 - 19P does the diameter change matter?

Oh and I dont care about speed. I want to pull the kids around on a tube.

 

[steelespike]

Re: prop rpm help

Usually as pitch decreases diameter increases.Sometimes not, other times may increase.

 

[whatisup02]

Re: prop rpm help

Ok here is the update. 4500rpm and 39mph gps. Like I said befor the idles low, trolling at idle was 2.1mph. It was running great so Im leaving it.

 

[steelespike]

Re: prop rpm help

Just for reference your still running the 21" prop?
You gained 6 mph! I think you can see why some of us question the use of the "wings".
Usually if you gain speed you gain a little fuel savings.

 

[whatisup02]

Re: prop rpm help

Yes still using the 21P but the gain in speed was because I had 4 adults and 2 kids plus gear for a day on the lake. This time it was 2 adult and very little fishing stuff. The boat did take a lil longer to get on plan but not enough to worry about. The big plus was it steered easyer.

 

[steelespike]

Re: prop rpm help

Steering is another thing that "wings"foils can mess up.The boat may feel more responsive to trim as well.