Is it possible to make up this many RPM loss?

[hawkrdr]

2004 Monterey 248 LS Montura. 4900 lbs dry weight. 5.0 MPI 260 HP Merc.
Alpha 1 Gen II outdrive.
I just had to change my upper half of the outdrive and had to change the gear set in the upper with the swap. I went from a 1.62 to a 1.47 upper. Current prop is a 19 in pitch (cant remember the diameter now for the life of me..)
Before outdrive change I was able to get up to 4200 rpms full throttle with 4 adults and 4 kids full of fuel, and gear. At 4000 RPMs i was running around 40 MPH.
Today went out with same load and I am only able to push around 2800 rpms with full load. I am running a 3 blade SS prop. (dont know the brand).
My question is am I going to be able to re-pitch to gain this RPM/speed back or am I screwed? When I am by myself on the boat I havent pushed above 3000 RPMs yet, but I also haven't tried to.
Is there anything I am going to be able to do with the prop to get up to the 4000 range? I think factory I am supposed to be around 4000-5000 RPMs full throttle.
Any help would be appreciated. I am also going to re-check the throttle linkages to be sure it is able to move full range again.

Thanks in advance.

 

[craze1cars]

Re: Is it possible to make up this many RPM loss?

I don't understand why you "had to" regear the upper? You have now geared a very heavy boat like it has 350 HP....but you only have 260. That was indeed the mistake that has created your problem.

FWIW, your proper RPM range for that MPI is most likely 4,600 to 5,000. It is definitely not as low as 4,000 as you stated. So you need to get from your current 2,800 to approximatly 4,600....or higher. To answer your title question...No. There is not any prop on the market that will help you gain 1,800+ RPM if you're currently running a 19....

If you regear the boat properly, your problem will be partly solved. If you don't regear it and continue to run it as-is... lugging WOT at 2,800...you run a very high risk of destroying your engine quickly.

I say "partly" solved because it also sounds like you were overpropped to start with. 4,200 is not enough with that motor. So even after you regear properly, I suspect you'll need to reprop to maybe a 17 pitch...or maybe even as small as a 15 pitch...to get those RPMS up to where they should be. And as a reward the boat will run better than it ever has...

I suppose your other alternative is to keep your new gear ratio and replace the engine with something substantially larger....320 to 400 HP I'd guess could be propped with your current ratio.

 

[hawkrdr]

Re: Is it possible to make up this many RPM loss?

had to re-gear the upper was due to the failed gears on the 1.62 upper from SEI. The replacement gears were recommended by them. This is going to be a b*&*& if I have to send this back to them.....

Thanks for the info. I am going to be contacting SEI on Tuesday to follow up on this.

 

[hawkrdr]

Re: Is it possible to make up this many RPM loss?

Follow up-
got a 15.25x15 prop went out today
1/2 tank of gass, 2 adults, 2 kids
gets up on plane nice an smooth, without going to full throttle I was able to get up to about 4000 rpms pushing around 35-37 mph. Still had a little throttle left to give, didn't feel like pushing it too much.
Cruises comfortably at 3100RPMs around 25-28 MPH.
Didn't think it was possible to make up that many rpms, but it did just fine.
Granted, didnt have the load like I did previously, but even with just me I couldn't get the boat to push the 14.5X19 stainless enough to get over 28-3000 RPMS.
Probably going to just run this alum. prop for awhile, it works and that is all I need now.....

 

[QC]

Re: Is it possible to make up this many RPM loss?

I think the tach is wrong, but I always say that . . . Whenever I see a big, heavy boat like yours, with a top speed of 40 MPH, low WOT RPM and no complaint about planing performance I suspect tach. ESPECIALLY when I see 2800 RPM WOT. If that was accurate, I don't care what gear ratio or what pitch, she wouldn't plane.

 

[hawkrdr]

Re: Is it possible to make up this many RPM loss?

I am suspect of my tach now as well, it was hopping around a little earlier in the day, guess I will need to trace down the connections and see what is happening. Didn't feel as if it was mis-firing like a normal bouncy tach would show.