Help, bought used boat and need to know what size prop it should have

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I bought a used "92" 19' Merc Capri with a 4.3l vortec but it has a chipped prop blade. Apparently not the first time, they replaced it but after searching the web I've come to realize they may not have used the right one. It now has a 14'x19' prop on it and I really would love to know what props I can use to replace it. Any sizes and why they would be a good idea. Thanks in advance for any help.
 

alldodge

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Best way to determine which prop to use is first we need what is your current WOT rpm? Also tell us what is the max speed using a GPS?

If your turn close to 4400-4800 rpm at WOT or a bit more you probably have the right prop.
 

tpenfield

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14 x 19 is not a bad match for that boat. You will want to know the gear ratio of the outdrive, as it makes a difference in pitch that will be the best match.

Mercury has a prop selector program that you can use to put all your boat/engine information in and it will suggest a variety of props.
 
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How do I determine what the gear drive or my rpms are? Seriously newbie here..lol It is the original motor if that helps.
 

alldodge

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If you don't have a tachometer installed at the helm, you can get a hand held at an auto parts store

The easiest way to determine drive ratio if its not stamped on the drive, is to remove it and then put in gear. Then rotate the drive shaft and count the turns on the drive shaft and prop shaft.
 
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Ok.. I've found the gear ratio 1.81 and the WOT is 4400-4800. My question now is, is the 14x19 prop the only one I can use or are there others that will work as well?
 
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I have a tach but neither it or the speedometer work..lol This is a work in progress..lol But she looks pretty good for her age.
 

alldodge

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Ok.. I've found the gear ratio 1.81 and the WOT is 4400-4800. My question now is, is the 14x19 prop the only one I can use or are there others that will work as well?

Wow, you went from not knowing both to knowing both in 6 minutes, cool. OK now all we need to know is what was the speed at WOT
 

alldodge

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He says it's running between 45-4650 full out

OK, so you don't know and your making stuff up.

The reason we need to know what the GPS speed, is because this determines prop slip. A 19 pitch prop moves 19 inches forward for every revolution. The slip is calculated to determine how much less then 19 inches it is moving. There is NO such thing as a Negative slip. A Negative means the prop is move farther in the water then the picth

So if we use your provided numbers
Speed 45 to 46 mph
Ratio 1.81
Pitch 19
RPM 4400-4800 (comes from the manual)

Use 19 pitch and 46 mph at 4400 rpm = (Neg) - 5.17 percent slip
Use 19 pitch and 46 mph at 4800 rpm = 3.59 percent slip
Use 19 pitch and 45 mph at 4400 rpm = (Neg) - 2.88 percent slip
Use 19 pitch and 45 mph at 4800 rpm = 5.69 percent slip

Even if the Negative numbers are thrown out the 3 to 5 percent numbers are not realistic. I would expect to see 8 to 10 percent slip.
 

alldodge

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Will add, all the above numbers matter in finding exact issues. But even if we did not know the actual speed of the boat, we do need to know what the actual rpm is. If the motor is achieving 4800 or a tad more, then the 19 would be the best match. This tells us your motor is reaching max rpms. If the dings on the prop are fixe, this would bring the rpms down a tad and increase the speed and thereby reducing slip
 

tpenfield

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Yes, the numbers don't seem right, but not sure if the speed is GPS or the boat's speedometer.

Anyway, even using this added information, you still get a 14 x 19 propeller as the best fit. Funny thing is that boats usually have the right propeller on them.
 
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