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Author Topic:   Interpretation of Mercury Propeller Part Numbers
kgregg posted 02-07-2003 10:23 AM ET (US)   Profile for kgregg   Send Email to kgregg  
I need to repair/replace the alumimum prop on my 1991 Merc 135. It is really nicked and gouged. What do the following numbers on it mean? I understand "21P" means a 21 pitch. Is prop diameter encoded in these numbers?

48 78122 A40 21P

More info... boat is 1991 Outrage 19. I can get about 38 MPH max (per handheld GPS) at around 5100 RPMs. (How much can I trust a tach that reads 1000 RPMs when engine is off?) Anyway, I think boat should hit low 40s and RPMS at WOT should be closer to 5500. Thanks.

doobee posted 02-07-2003 08:45 PM ET (US)     Profile for doobee  Send Email to doobee     
the number indicates you have an original Merc prop.

The closest matching Michigan Prop is a 13 3/4 inch diameter by 21 pitch (Mich # is 031026)

If you can get your hands on a comparable SOLAS prop I would recommend it. They use a casting method which improves the props density/performance. The closest SOLAS prop is a 14 x 21 (SOLAS #1511-145-21).

You should definately be getting low 40s at WOT. Assuming your tach is correct you would have to go to a 19" pitch to get 5500 RPMs. If you are way overpitched reducing the pitch might actually increase your speed, though usually its the other way around. Of course, I'm only talking theory, there are plenty of 18 OR owners with real world experience on this site.

If you need a 19" pitch, here are the numbers:
Merc 48-78120A40
Mich 031026
SOLAS 1511-148-19

Sal DiMercurio posted 02-07-2003 09:37 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
The advise is correct on the prop.
I would drop down to a 19 pitch.
As far as your tach reading 1,000 rpms when shut off, most tachs do.
jimh posted 02-08-2003 11:50 AM ET (US)     Profile for jimh  Send Email to jimh     
Those other numbers on the propeller are Mercury's part number. I don't know that there is a system to the numbering that allows one to deduce characteristics of the propeller from the number. It may just be a case of assigning a part number to a part.
lhg posted 02-08-2003 02:28 PM ET (US)     Profile for lhg    
Looking at my Mercury propeller charts, here is what I find for a 135 Merc on an your Whaler:

19" pitch Load range 1800-3000, speed 41-48

21" pitch, load range 1500-2400, speed 46-52

Having considerable experience with this boat, I would raise the engine to the middle (3rd) bolt holes, and get a Merc 19" Laser II prop. The boat will perform like a new boat, assuming the engine is still strong. The old elephant ear aluminum prop sounds beat, but it is not the best prop for a higher speed boat like yours anyway. No V-6 should be running aluminum.

A friend had a 130 Yamaha on same boat, and I clocked him at 42 MPH. And the Merc V-6 135 will walk away from that 4 cylinder engine.
It's basically a detuned 2.0 liter 150.

kgregg posted 02-08-2003 06:00 PM ET (US)     Profile for kgregg  Send Email to kgregg     

The engine is strong.... I'm keeping it. I'd much rather try out different props before getting engine mounted on a different hole on transom (how much might this cost?) Thanks to all for their reply. Kevin

lhg posted 02-08-2003 06:43 PM ET (US)     Profile for lhg    
BW recommended the engine be in the 2nd set of holes. I'm only recommending it be lifted one more, which is 3/4", for max performance. But the second hole is fine. If it's in the first, it's too low, and will hurt performance. It costs about $50 to have an engine raised.

Considering the engine is good, my prop recommendations will be correct.

Tom W Clark posted 02-08-2003 07:10 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Larry's advice is good. 19" sounds like just what you need.

Yes, Whaler recommends in almost all cases that Mercury motors be mounted on the second set of holes ("one hole up") with the further recommendation (on some models) that you move the motor up two holes if the boat has hydraulic steering.

Does anybody know why this is? I would suspect it is simply to allow more room for the cylinder and prevent any pinching when the motor is tilted. I cannot think of any performance reason.

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