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Max Speed VS RPM

N

NewCHKMATowner

Member
I took my boat out for the first time last week. After some initial cruising I decided to do a couple of speed passes to see what it could do. We used GPS to measure speed.

79 Eluder
79 Evinrude 2.5L 175HP V6. Compression test showed all 6 cylinders between 109 and 115psi.
14-1/4" 21 pitch prop, not sure what brand it was on the boat when I bought it last week.
No Jack plate installed.
With almost full tank of gas 2 large people( 205 and 320ish) and 1 kid 100 pounds or so we hit 50 and 51 around 5200 rpm on GPS today. Lake was fairly smooth.
In another thread I read that my engine should be good for 6000 rpm but 5000-5200 rpm was the most I saw all day.
I have some questions.
How sensitive are these boats to weight? Will the heavy load that I had in it limit the rpm's the engine turns?
Is 6000 rpm's an accurate number for this engine? If so what do I need to do to get max speed/rpm out of this engine?
I know that adding a jack plate will help top speed, but I am not ready to do any major mods to it just yet.
 
Are there any numbers on the prop? Assuming the lower unit is 1.86:1 like the newer OMC's are you would only be at 8% prop slip and that number is amazingly low. Something in the math just doesn't add up.
 
How do you figure out the amount of slip? I am new to boating and trying to learn as much as I can.
I took some pictures of the prop but I can only make out the size and pitch
14 1/4 x 21
There is another set of numbers on it 3878(could be a 6)20 I am not sure about the 3 the picture is not clear and the boat is in the warehouse at work .
 
You should be getting some more rpms than that with a 21 pitch.How low is the engine mounted?should be around 5800 or around that.you should try a solo run.
 
Using that calculator if my tach is off and I was running 6000 rpm's i have 22% slip and my GPS speed of 50mph would be correct. Maybe I need to invest in a new tach?
 
paulsboats said:
You should be getting some more rpms than that with a 21 pitch.How low is the engine mounted?should be around 5800 or around that.you should try a solo run.
Click to expand...
I will measure the depth of the prop when I get to work tomorrow but I am using the stock mount locations.
Next time I take it out I will try a solo run and see what the GPS says. I am using my phone with a gps app maybe I will borrow a car GPS and see if it shows a different reading.
As far as I can tell the gauges in the boat are original 1979 equipment and may need updating.
 
The phone gps is fine. my engine was mounted as low as it would go.(my mistake) I raised it 3-1/2 inches and it really helped wake the boat and engine up.
 
yes...depending where the holes are on the transom. on mine i drilled the holes to low so i added a set back to get it where i wanted.I did not want to drill new holes. You may be able to raise it a few holes ...get some pictures also so we all can take a look.
 
Yes. There's several holes on the engine bracket. Move the engine to higher (maybe the highest) holes. Make sure to seal the bolts back up, don't want any water getting in and rotting the transom.

John
 
I wouldn't buy anything until I figured out if the tach is right. In order to make an informed decision on which prop to get you need to know the current RPM you are at as well as the speed (which you know) and the intended use. Any chance you know someone who has a timing light with a built in tach? You might be able to use that to verify your RPM.

John
 
atc250r said:
I wouldn't buy anything until I figured out if the tach is right. In order to make an informed decision on which prop to get you need to know the current RPM you are at as well as the speed (which you know) and the intended use. Any chance you know someone who has a timing light with a built in tach? You might be able to use that to verify your RPM.

John
Click to expand...

I will ask around. If nothing else I will find a cheap V6 tach and hook it up to see if it reads the same.
 
I found a chart with some helpful information.
WOT rpm's for the 175 Evinrude 5250-5750 and the gear ratio is 1.86. So with the load I had in the boat and the current configuration I might have been topped out.
 
I took a couple of pictures of how my engine sits when trimmed down and the mounts. I also looked at the numbers on the prop 387520 and this crosses as a stock prop.
looks like my engine is mounted as high as it can be without a jack plate
enginemounts.jpg
With the engine trimmed down the prop is below the hull.
enginetrimmeddown.jpg
enginetrimmeddown2.jpg
 
Yep. Looks about as high as it can bee mounted. Maybe something around the same pitch in a stainless 4 blade would work better, especially if you're going to be carrying some weight all the time. Something like a Trophy or in 3 blade a Raker. Those props give lots of lift and will help get the boat out of the water but the best way to pick a prop is after a jack plate is installed if you are going to run one.

Still, something isn't adding up, 8% prop slip is incredibly low. I'm going to bet that tach is not accurate.
 
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