What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

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rodgeml1

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Hey guys. I was troubleshooting a low RPM condition on WOT and found out I have an outdrive with a much lower gear ratio than I thought it had (1.47 vs. 1.81), so I'm hoping for some advice on which prop I SHOULD be running. The details are as follows:

1) Performance issue you are trying to correct: Low RPM and power at WOT
2) Current prop manufacturer, model, aluminum or stainless as a minimum: Unknown manufacturer or model. Geometry of the prop is good but the numbers on it have been obscured. It's a heavy prop so probably steel.
3) Current prop diameter and pitch (required): Unknown pitch (see #2 above)...diameter is 15.
4) Wide open throttle RPM and speed with an average load (very helpful): RPM at WOT is 2600, speed less than 20MPH.
5) Engine/drive make, model, year, and HP: 1995 Mercruiser 4.3LX (0F586331), Alpha one, Gen II, gear ratio 1.47.
6) Boat make model, year, length and weight 1995 Wellcract 218 Coastal, 21ft 8in long, 3300 lbs dry weight.

Thanks in advance.
 
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steelespike

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

We will need the prop pitch usually near the base of the blades or under the prop nut.You may have to have a prop shop measure it for you.
Is the boat on plane at less than 20 mph?
Have you manually confirmed the ratio? Seems unlikely on a 21 ft boat. You need to make up about 2000 rpm
at 200 rpm per inch thats a 10" pitch change. If your at 21 inches now that would be a 11" prop. I don't even know
if that is available. If the boat isn't on plane then the pitch change could be significantly less.
 
Bondo

Bondo

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

5) Engine/drive make, model, year, and HP: 1995 Mercruiser 4.3LX (0F586331), Alpha one, Gen II, gear ratio 1.47.
Click to expand...

Ayuh,.... Until you regear it down to where it oughta be,... Yer gonna have No Joy....

It's geared Way to tall, 'n yer boat is Way to big....

Ya need the 1.81:1 gears....
 
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rodgeml1

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

Figures.

I'm guessing it'd make more sense to buy the replacement SEI outdrive than it would to try and go through the exercise of changing out the gears on this one. Would that be y'alls recommendation?
 
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steelespike

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

If the boat isn't planed out it may be able to use a more normal prop to make speed.
Would depend on what prop is on it.Of course it might still struggle even if it can make its rated rpm.
 
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rodgeml1

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

steelespike said:
If the boat isn't planed out it may be able to use a more normal prop to make speed.
Would depend on what prop is on it.Of course it might still struggle even if it can make its rated rpm.
Click to expand...

I'm sorry I didn't answer your questions from earlier in the thread. The boat is very sluggish on the hole shot and never gets up on plane. The engine sounds great....just won't get over 2600 rpm.
 
90stingray

90stingray

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

rodgeml1 said:
Figures.

I'm guessing it'd make more sense to buy the replacement SEI outdrive than it would to try and go through the exercise of changing out the gears on this one. Would that be y'alls recommendation?
Click to expand...

Or you could swap for the V8 to match the drive :) No, if the drive is fully functional... sell it and recoup some cash. SEI would be an affordable replacement unit. Did you just buy this boat?
 
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rodgeml1

Petty Officer 2nd Class
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Apr 4, 2012
Messages
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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

90stingray said:
Or you could swap for the V8 to match the drive :) No, if the drive is fully functional... sell it and recoup some cash. SEI would be an affordable replacement unit. Did you just buy this boat?
Click to expand...

Yes, I just bought the boat. Because of extenuating circumstances I wasn't able to put it in the water for a test drive before I bought it. (I won't be doing that again :facepalm: ) but it started fine and ran great so I was willing to take the risk. The seller and I will be having a conversation though. The boat looked good on the outside but I really didn't know what to be looking for on an I/O so I had no idea how bad everything really was. The only good news is that now, thanks largely in part to this forum, I've really come to know the engine and drive system.
 
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steelespike

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Re: What prop do I need (1995 Mercruiser 4.3LX, Alpha One outdrive 1.47 gear ratio)

Ok if the boat isn't planing depending on the prop you may not need as big a prop change.Once it planes the rpm will climb easier.
Besides the mismatched gears the prop could be a mismatch as well.
Sure the boat may struggle a little but at least you may be able to use the boat while you ponder your next move.
 
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