selecting a new prop for mercruiser 5.0, 240 hp

[lostriver]

I have a 20 ft Hurricane deck boat with a 240 hp Mercruiser 5.0 L engine. It has an alpha outdrive.

Currently the boat has a Michigan Wheel prop on it. Part # 992203, 14.5 x 18 RH 4 Blade prop. The prop is a bit dinged up but not too bad. I would like to replace it with a prop that will get me out of the water fast (that is why I have a 4 blade) and get me a decent top end speed. Currently with this prop the boat runs about 41 mph WOT. The WOT RPM's are only 3000 though. The bottom of the boat is very clean. I do not leave this boat in the water.

Any recommendations on getting the WOT RPM's up?? The books says I should have 4400 to 4800 rpm's at WOT.

Is this the right prop??? Also, anyone like any particular brand of props??

 

[hwsiii]

Re: selecting a new prop for mercruiser 5.0, 240 hp

Lost, what is the gear ratio for your drive.

H

 

[lostriver]

Re: selecting a new prop for mercruiser 5.0, 240 hp

The drive ratio is 1.62R.

The drive serial # is OL788504, the transom serial # is OL884895.

 

[JJJ]

Re: selecting a new prop for mercruiser 5.0, 240 hp

Is your tach accurate? Only 3000 rpms at WOT yet 41mph?

 

[lostriver]

Re: selecting a new prop for mercruiser 5.0, 240 hp

The gauge "appears" to be operating correctly. I have not idea if it is accurate. Do you know a good way to check it??

 

[tmh]

Re: selecting a new prop for mercruiser 5.0, 240 hp

yeah, I don't think those numbers "work"; either the mph or rpm is not correct. I can't find the online prop calc I usually use but no way 3000 rpm, 41mph, 18P and 1.62 ratio.

 

[lostriver]

Re: selecting a new prop for mercruiser 5.0, 240 hp

The speed was taken with a GPS, i think it is pretty accurate. May not be exact but I think it was in the correct range. Boat speedometer does not work right now. Hose is bent up.

I know the pitch is correct and the ratio I took right off of the lower unit so I trust that.

That only leaves us the rpm's to debate. I guess I could get another gauge. Is there any way to check the current gauge. How does it read?? Is it based on voltage??

 

[JJJ]

Re: selecting a new prop for mercruiser 5.0, 240 hp

Most any automotive inductive tach will work. Or even the type that go on the coil, maybe some of the auto parts have them as a loaner?

 

[hwsiii]

Re: selecting a new prop for mercruiser 5.0, 240 hp

Lostriver, if you are doing 3000 RPM with your 1.62 gear ratio and an 18" prop you would only be doing 28 MPH. At 41 MPH you are doing right at 4,300 RPM. Your tach appears to be off about 1300 RPM.

H

 

[JJJ]

Re: selecting a new prop for mercruiser 5.0, 240 hp

Also check to see if your tach has a way to set the # of cylinders, maybe it thinks it is a 6 cyl.