3.0 Prop help

sparragh

Recruit
Joined
Aug 13, 2014
Messages
1
I have an older19.5 Spectrum with a 3.0 AlphaOne. I cant make out the numbers on the prop other than 18 and 120. Its kinda gutless getting me only 35mph at 4000 rpm. The engine also does not seem to want to go past this rpm. Also at 4000 the engine seems to bog out to about 3000 rpm then picks up again. I just bought this boat so do not know much about it. I did put new plugs in it
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Welcome aboard sparragh. I started your own thread. To keep things clear we recommend you always start a new topic for your specific issues. With that said, I am sure we can get you helped out.

Sounds like you may have two or three different issues going on, although 35 MPH with a 19.5 footer and a 3.0 is not that bad. So you may need to adjust your expectations some. However, if your tach is right, the RPM is too low. And the way you increase RPM is with prop selection. Take a read of the stickies at the top of this section. Also, your 3000 RPM comment sounds like a specific engine issue. Please post that issue separately in the Mercruiser section. Maybe link to this thread, so that others know you are messing with props too. But a prop cannot cause the hesitation or momentary drop in RPM that I think you are referring to. Pretty much your engine should smoothly pull up to the point where it runs out of horsepower (WOT RPM). Typically the 3.0 shoudl be between 4200 and 4600 RPM at that point, so you may indeed be over propped. I would:

a) Clarify the engine issue and post a thread in Mercruiser if indeed sounds like the engine has an issue

b) Verify your speed with GPS

c) Verify your tachometer

d) take a good look at the prop hub near the prop nut for any other numbers. # of blades would be helpful too. Aluminum? Stainless?

e) report back.

Good luck!!
 
Last edited:

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
I think this is more of a "Mercruiser" thread than a prop thread, probably get more responses over in the Mercruiser forum.

^^^ Agree with QC, I wouldn't expect much more than 35 with a 19.5'er - 3.0L combo. I have a heavily loaded 18.5' fishing rig with a 3.0L and can only squeak out 35MPH at 4600RPM,,, 18P 4 blade prop. Your sped sounds correct, so I wouldn't monkey with the prop at this point.

I'd check the tach as the RPM's don't sound correct. Is the speed by GPS or the boat's speedo, GPS is best.

For the bogging at 3000RPM and picking up again, and only going to 4000RPM,,, check to see if the distributor is advancing all the way. The dist curve should be all in by 3000RPM, but.... The weights and springs could be rusted and hanging, not allowing the dist to advance all the way,,,, if it's points....

Post up the engine serial number for better help.
 
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