And am a little disappointed to say the least. I recently purchased a hand held GPS and decided to go out and make some test runs this weekend and here are the results:

with the original labbed 3 blade Turbo prop (14.25 x 24")
3000 RPMS--- 34 MPH (34% slip)
4000 RPMS--- 48 MPH (30% slip)
5400 RPMS--- 70.4 MPH (24% slip) WOT

with the new stock Revolution 4 4 blade (14 5/8 x 23")
3000 RPMS--- 40 MPH (19% slip)
4000 RPMS--- 54 MPH (18% slip)
5200 RPMS--- 66.4 MPH (23% slip) WOT

This is a 1987 Checkmate Ambassador (19'7" probably around 3200-3300 lbs during these runs with an 18 deg V and a 6" pad) with a 5.7 EFI and Alpha 1 SS, the motor was rebuilt and upgraded in 1995 and supposedly made 424 HP and 445 TQ (I have build and dyno sheets, supposedly from this motor). According to the guy that brokered the deal on this boat (he owns the marine service center that has supposedly maintained this boat for the last 10 years, and also sells used boats) this boat has a 1.32 gear ratio in the Alpha SS drive and that is the figure I used to calculate the slip values (of course he also claimed this boat would run 80+ MPH). What do you guys think of these numbers? The top speed seems way off for the HP this motor should be making (granted it is 10 years old, but even at 400 HP it seems off to me), and the slip numbers seem way too high. I guess I will try and verify that I do have a 1.32 ratio and not a 1.5... if that is correct then I think I either have issues causing excessive slip, or my GPS meter is way off, or my tach is way off (which I know is likely; however, I think it is reading low if anything which would make my slip numbers even worse). What do you all think; and if these numbers are correct, what do you think could be causing the high slip numbers?

424 horse and 445 TQ are HUGE numbers for 350ci engine
without forced induction and having the power being produced under 5000 rpms
You need some seriously worked heads and cam to achieve this.
And also will probably need to rev close to 6000 to aproach these numbers, maybe thats where its producing the TQ

In any event I have been made to understand that:
300 horse = 60 mph
400 horse = 70 mph

This matches your numbers.
Also the 4 blade prop will give you a better hole shot and mid range where the 3 blade will have a higher top end (less drag)
this also matches you readings.

In my opinion, your numbers look really good for a 350.

Thank you for posting the prop data looks like the 4 blade would be great for general use and saving gas mileage and the 3 blade for when you leave the passangers behind.

I'd double check the ratio of your drive. A 1.5 seems more likely with those numbers.

My friend had a Checkmate Enchanter with approx. 330 HP and he was running 68 GPS. He had a standard 1.5 alpha and a Mirage + (23P, I think). If you truly have 425HP, something is not right.

It is a 355 with forged flat tops, Dart heads with 2.02/1.6 valves (not sure if any additional port work was done), Crane roller with 222/230 duration at .050 and .509/.528 lift, roller rockers, and a few other goodies. According to the graphs it made peak corrected TQ of 450 ft lbs at 4500 RPMS and peak HP of 424 at 5500 RPMS.

First I would double check the gear ratio, Put the drive in gear and rotate the engine by hand and count the number of engine revolutions for each prop revolution.

The numbers don't seem to be that far off for a 1.47 ratio. The SS lower may be causing some problems due to the drive being too high. If you have an opportunity to try a standard Alpha lower I would do that as well.

The REV 4 didn't do too bad, if it was allowed to turn the same rpm as the Turbo and lab finished I think you would find the top speed to be about the same between the two props.

your engine specs sound good to me. (for what its worth)

Do you expect to see better than 75 mph?

Consider that you are running too high in your power band.
Since your gear ratio is pretty low already, try re-pitching to 26" or so.
This should bring you down in your torque curve.

Can the experts please tell us where we should be pitching for?
peak HP or Peak torque?

I would guess its just above peak torque or 4800 rpm in your case.
The couple degrees should give you a little and the fact that your closer to the center of your power band should give a little more. Hopefully good for 75???

I am just joe blow, but its worth a try.

I am in the same boat as you.
So, experts please chime in.

With the lower unit parallel to the pad, the prop center is almost exactly 5.5" below the pad. What gear ratios are available for the old style Alpha's??? The guy that used to work on the boat actually claimed it had either a 1.36 or 1.38 ratio in it, but when I started looking all I found was the 1.32 and 1.5 ratio's so I just assumed he must have meant it had the 1.32 ratio. I know he was specifically trying to make the point that it had a different ratio for higher top speed than most other Alphas. One thing I did fail to mention is that the original 3 blade does have one slightly tweaked (bent) blade which I am sure has some affect on the slip #'s but how much is ???

I have read many of their recommendations, and they have always suggested propping to hit your peak HP. I am actually propped fairly conservatively (probably why the motor has lasted over 10 years), the cam installed in my motor is an antique... it has been around forever and has an advertised RPM range of 2500-6000 RPMS and a max recommended RPM of 6200. In fact on the first dyno sheet they actually pulled it to over 6000 and it continued to make more power, but when they swapped out the ECU for the second dyno they made 13 more HP only spinning it to 5500 than they did on the first pull when they spun it to 6000. In time I would like to maybe spin the motor a little faster, but I want to do it by making more power not by reducing the pitch. This winter it will be getting a set of valve springs for sure being as these are 10 years old and not exactly the best match for this cam anyway. In addition to that they set them up about .060 over what they were supposed to be set up at (at least according to what they wrote down on the build sheet), so I would definately not want to spin it over about 5500 as it sits.

OK, I finally got around to checking my gear ratio, and what most of you suspected is in fact the case. It appears that I have a 1.50 gear ratio (not the 1.32 ratio the mechanic who was brokering the boat claimed it had). In a way this is a good thing, but in another it is not. At least now I know the props and everything else appear to be working well, in fact the new box stock Mercury REV 4 is kicking butt; but this also means no quick or easy fixes to get more speed. Here are the corrected numbers:
3 Blade Labbed Turbo prop 24" pitch
3000 rpms--34mph (24% slip)
4000 rpms--48mph (21% slip)
5400 rpms--70.4mph (14% slip) WOT

4 Blade Stock REV 4 23" pitch
3000 rpms--- 40 mph (9% slip)
4000 rpms--- 54 mph (7% slip)
5200 rpms--- 66.4 mph (13% slip) WOT

Now if I could just get the REV 4 worked to get me 300 more RPM and get the WOT slip down to 10% that would get me to 72 MPH and I would be happy with that (for now anyway).