Re: '95 Merc 3.0 Prop pitch?
 

well, yes, I agree mikeyt, Our opinions differ. Not knowing what prop madill21 has I was making an assumption that it was either a 17 or 19" aluminum prop. Tho there wasn't a "standard" it would not normally be a 21 (well, not if the orginal owner had 4 or 5 family members and actually wanted the boat to get on plane ....) nor can I fathom a 15" prop. So, I will tell you that with the 3.0 litre gear ratio, 19" is what most 18 ft Mariahs went out with. I'm just guessing.
However, I must back up and correct myself earlier. The SpitFire 4 blades are for 40 thru 115 hp outboards. I should have stated the "Alpha 4 " aluminum are designed by Mercury to give an cheap alternative to the 3 blades. Coming in 16,18 and 20 " pitches, they are used in-place of the 17,19 and 21" 3 blade aluminum props when better hole-shot and ability to stay on plane at lower rpms is desired. Since most people are not as much looking for blazing top-end speed the Alpha 4 sacrifices some of that.. It just does. Perhaps Mercury could tell you why. But at about $40 difference in price it is a very good alternative to the traditional 3 blade aluminum and alot cheaper than stainless. BTW, most Mercury dealerships carry a broad selection and will let you buy, try and re-try from their selection. Same with prop shops!
As to the 3.0 litre being emenic... well, If you've never run with a V-6, then you'll never have known the benifits and better fuel economy that a V-6 can have.(with the right/correct prop, that is...) It doesn't have to work as hard. That doesn't make it wrong. The 3.0 is/was cheaper and may be good for a small family and tubing.. when little Johhny grows up to be 6'2" (and his friends), a V-6 may have been "just a little bit better". But, Hey, I'd just just like to have a little bit of a cushion on power. I'm different. In the mean time, my suggestion is, try a 18" Alpha 4 prop. I think he'd like it.

Re: '95 Merc 3.0 Prop pitch?
 

I agree that it will likely be between a 17" & a 19" prop and there is a good possibility that an 18" 4 blade just might be the perfect prop but we need to find out what he has on it now first. And, the OP says it performs well so i cant imagine there being a 21" prop on it. My 4.3 doesn't like 21" props either. :)

Btw... my understanding is that the Spitfire prop is now available for Alpha outdrives. If it performs as well as claimed (& i've read some good reviews) then it would be a more economical choice & yet give performance very similar to a SS prop of the same configuration.

The other application for that 3.0 that's being overlooked is an older person or couple who boat in areas where speeds are restricted etc and maybe want to do a little fishing, cruising or just plain getting from point A to point B. Also the fuel economy for the 3.0 is better than the 4.3 in normal cruising so those boaters get just what they need from the 4 banger. The only time the 4.3 has an advantage with fuel is when its under load pulling things like skiiers/tubers etc. At that point that little 3.0 has to work much harder & then it negates any fuel advantage it had.

Re: '95 Merc 3.0 Prop pitch?
 

I had a 4 banger in an 18' bowrider once...(old Sea Ray)
In coastal waters it just wasn't enough!
Maybe on a lake it's fine, but against wind and tide it was just plain hard work!

Mine was fitted with a 19" Lazer II, which clearly is the wrong prop for a Mariah. (bow lift!)
but on that boat merc/alpha 1 it was about the best it would get.
Top speed was 38mph.

Re: '95 Merc 3.0 Prop pitch?
 

I'm an owner of an 18' Mariah with a 3.0. We are a family of 4 (two thinner adults, a 13 year old, and a 7 year old) I had a 21 pitch 3 blade aluminum Michigan match prop on it until a couple years ago when I started wakeboarding. With the 21 pitch and 3 adults on board it couldn't pull me (175lbs) out of the water, or maybe I just couldn't hang on long enough to see if it could. lol. I bought a Solas 19" pitch aluminum 4 blade. Night and day difference in planing speed, the sucker would jump on plane faster than i've ever witnessed a boat do before, and would stay on plane at 17mph.. Top speed suffered though, with the 21 3 blade our top speed was 38mph, with the 19 4 blade it was about 32mph, the comfortable (not spinning the engine to death) cruising speed was around 22mph, and it could be over revved if not careful. I then tried a Michigan Match 20 pitch 4 blade aluminum, that was a nice compromise, it'd do a top speed of around 34mph, comfortable cruising speed was up to 25mph or so, and it was still enough to pull me up on the wakeboard.

So now if we're going to have a load of gear or people with us, or we're going to a small lake I put the 20 pitch 4 blade on it, if we'll have a lighter load and we're going to a bigger lake the 21 pitch 3 blade goes back on it. The 19 pitch 4 blade is simply a spare that I keep in the boat.

I know many say the 4.3 does the same on gas as the 3.0 because it doesn't have to work as hard and here is what i've witnessed. We have friends with 2011 18' Four Winns Horizon with the 4.3 Volvo Penta power, same size family, almost every time we go boating they are with us so i've been able to compare our fuel usage between the two boats and I must say, they always use about 1/3 more fuel than we do, and that is compared at the gas dock after a day on the water following each other around, both pulling our kids on inflatables, making a couple quick WOT blasts, etc.

Also, twice now we've had our boats beached next to each other and had people come over to ask about the 16 year old Mariah.. Nobody has ever asked about their newer Four Winns. lol. :-)