6000 RPM @ WOT 130 hp Mercruiser Alpha1

[matthewl16]

Good Morning,
First time posting so dont chastise me if I screw this up, I have a 17' Dynasty Regency with a 3.0L MerCruiser Alpha One. When running at WOT my tach is showing 6000 RPM, all the advise that I have found says that I should be at 4400-4800 I believe that my tach or sending unit is the culpriate I think that I would notce if I was 1200 above optimum operating range but maybe not. I was hopeing that somone could point me in the right direction for checking out the system, I called my local marina and they told me that I need to purchase a new tach and see if I'm getting the same indication. I was hoping that there are some multi meter checks that I could run but I'm not sure shere to start. Before somone says the obvious I have a manual on the way but it's not here yet. (work computer wont let me down load spell check so sorry if there are any errors)

 

[Texasmark]

Re: 6000 RPM @ WOT 130 hp Mercruiser Alpha1

Go to Google search and look up BAM. It's a marine racing site and has a prop calculator. Pull up the calculator.

Plug in the numbers except for rpm and calculate rpm's. That will tell you real quick.

The difference in calculated speed and actual is slip. You didn't give your speed (you need to get it) but let's say it is 40 mph (gps.....more accurate than speedometer). This is your actual speed.

Then let's say that your boat is pretty efficient so your slip (prop slippage in water) is 10% which would give you a 44 mph theoretical (calculated) speed.

The gearbox on the outdrive could be from 1.47 to 2.4 (according to my '97 brochure) so you need to find out what your gear ratio is....maybe some serial numbers to Merc or your nearest Merc dealer will get that for you if you can't find out otherwise.

Your prop should be marked with pitch. If cupped, (tail end of blade has slight curve upwards from surface) add 1" to number st amped on prop.

Now you have:

Gear ratio
Theoretical speed
Prop slip
Actual speed
Prop pitch

Plug those numbers into the calculator....leave rpm's blank.

All you do now is (select) calculate for your rpm's. If yours are off from what the calculator says, either your tach is bad, dialed to the wrong timing ratio, or your prop hub is slipping.

HTH

Mark

 

[matthewl16]

Re: 6000 RPM @ WOT 130 hp Mercruiser Alpha1

Thanks for the info Mark
I havent put any real hours on the boat I just purchased it last week and I'm trying to make sure that every thing works as advertised before I put to much into it. I'll grab the GPS and take her out this weekend then I'll use your link and formula
-matt

 

[ricksrster]

Re: 6000 RPM @ WOT 130 hp Mercruiser Alpha1

http://ww2.tflx.com/pdf/stdign~1.pdf

 

[matthewl16]

Re: 6000 RPM @ WOT 130 hp Mercruiser Alpha1

Thanks for the link, I think that I got it strightened out
-matt

 

[Silvertip]

Re: 6000 RPM @ WOT 130 hp Mercruiser Alpha1

If the engine idles at about 650 RPM the tach is probably ok. If it idles 1200 RPM higher than that (1850 or so) there is a calibration problem. There is a dial on the back of the tach that needs to be properly set for your engine.