tach reading

willard887

Seaman
Joined
Jan 30, 2012
Messages
64
OK I got 1996 Johnson 50 h.p. remote steering. Tach is reading 6000 by my self on a 1648 Lowe Roughneck. One battery one 6 gallon tank of fuel, and one full cooler of course. Operating range is 5500 max. With 4 people and gear I'm getting 5500 on tach. The prop is 11-3/4 ? 17 stainless. I think this is acceptable. Does anyone have a diffent opinion? BTW my top speed is 45mph by my self and 32 mph loaded.
 

gm280

Supreme Mariner
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Jun 26, 2011
Messages
14,585
Re: tach reading

Those are pretty respectable MPH readings I'd say. Are they from a gauge or GPS? I'd run it like it is if it were mine...but that is just me. Your loaded RPMs are right there...
 

steelespike

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Apr 26, 2002
Messages
19,069
Re: tach reading

Prop calculator says you can't go 45 with the information you gave.
2.42 gear ratio,6000 rpm,17" prop,using a low 5% slip nets about 38 mph.
I would suspect your speedo is off.
My info says rpm is 4500-5500. Motors after 77 rated 50 hp at 5000.
I don't think your motor is happy at 6000.
 

willard887

Seaman
Joined
Jan 30, 2012
Messages
64
Re: tach reading

That 45 mph is off phone GPS . And going down river.With 2 people I read 40 mph. Again that's GPS off my phone. So are you thinking I need a 15 pitch to make motor happy or an 19 pitch? Sorry its 42 mph, my bad.
 
Last edited:

willard887

Seaman
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Jan 30, 2012
Messages
64
Re: tach reading

I think i just answered my own question. Prop calculator says i need to try a 19 pitch. or maybe the old tach is not reading right. But will i lose power with a loaded boat? I know i will lose holeshot.
 

steelespike

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Apr 26, 2002
Messages
19,069
Re: tach reading

I'm not sure but I think if you drift and check the speed then you can add or subtract the current.
Yes you would need 19" prop but will lose some power but might gain a touch of speed.
You can run the 17 but just watch the rpm with a light load.
I have a 50 tiller on a 18ft Crestliner bare utility about 1000 lbs with one aboard. Unfortunately no gauges.
Running a 19" prop it has an amazing hole shot seems really fast and has never had a problem even with heavy loads of construction materials.
 

steelespike

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Apr 26, 2002
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19,069
Re: tach reading

Yes I have checked drift speed. It is 3 mph
So are you saying your actual speed is 39? 42 minus 3 mph.That is closer to what is possible.
My original figures using 6000 and a low 5% slip indicated 38 mph.
According to the NADA guide the Lowe 1648s weigh about 275. thats about 100 lbs lighter than my Crerstliner.
There seems to be 3 or 4 versions of the 1648 all listed at 275.
Is your motor a 2 cylinder or 3 cylinder?
 

gm280

Supreme Mariner
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Jun 26, 2011
Messages
14,585
Re: tach reading

If you have a verified tach and know your final drive ratio, you can mathematically figure best world top speed of any prop selection. It isn't rocket surgery. BUT, you will never make those mathematical speeds because there is always slippage. So depending on the percentage of slippage you figure into the equation, you can guesstimate what would be real world speed... Too many true variables with engine height, boat design, boat friction, weight, and so many others too numerous to list, you can never get theoretical speed ever...
 

willard887

Seaman
Joined
Jan 30, 2012
Messages
64
Re: tach reading

It is a two cylinder. And yes drift speed is 3 mph. So it is 42 mph with drift added in. My concern is the 6000 its turning but I can always back off throttle just a touch.
 

steelespike

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Apr 26, 2002
Messages
19,069
Re: tach reading

I would definitely say avoid 6000 if you stick with the 17.
Based on your speed I think your rpm may be a little higher than 6000.
A Solas 19" may get you more speed and avoid the over rev.
Only draw back with your boat is its a little narrow at 48" My Crestliner is every bit of 5 ft at the transom
And of course 2 ft longer than yours. This kind of helps get a load up on the water.
Your flat bottom should help you get up.
 
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