Mercruiser Alpha Gen II Gear Ratio Help

[CreoleIII]

I purchased a 19 foot bow rider with no engine on eBay. I recently purchased a used 1999 4.3 V6 carb with alpha gen II outdrive cheaply in Florida sight unseen. When it arrived here in Canada I discovered it needs a new lower unit (salt water has eaten much of the aluminum threads around prop housing and there is water in the oil due to oil seal surface being eaten) I think the upper unit is O.K. but it too was exposed to water in the oil... So... I have found many options but I am confused about gear ratios.

What gear ratio do I need? 1:65?

Why is there an upper and lower gear ratio? Do they have to be the same?

If I find a deal on a v8 ratio (1:1.5) can I use that and put a low pitch prop?

Are the after-market drives on eBay any good?

[gonzo]

The aftermarket drives are trash. The correct ratio is 1.65:1. They are very common. The engine doesn't have the power to turn a prop with a lower ratio. A smaller pitch will partially compensate but will be less efficient.

[havasuman]

I just a 25 ft pontoon boat by lowe with the 4.3 alpha 1 drive it has a 17 pitch prop on it but only goes about 26 at 44oo rpms .Is that the best it should do .Or should I re pitch to about a 15 pitch and get my rpms up to about 4800 to 5000.I'd like to get about 30 to 35 out of her.Thanks Jay

[marshmat]

Creole- Get the proper Merc lower unit with the 1.65 gearing. Mixing up the gear ratios in a rebuild is surprisingly common and generally just makes the boat perform like a pig.

Havasuman- A 25' pontoon is not going to go like stink; 30 might be possible but I think 35 is wishful thinking. The 4.3 should turn 4400-4800 at WOT, so you might be just a bit over-propped. See if your local prop dealer will let you test-drive a couple of 15" or 16" props- you might also consider 4-blade models for this boat.

[gonzo]

A lower pitch at higher RPM is going to produce the same speed, but give a better hole shot and use more fuel. The engine is running within the target RPM.