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Rich
02-24-2004, 08:02 AM
I am putting counter-rotating Alpha 1's on my 1969 7 Meter with 160 hp in-line 6 cylinder engines. WOT is 3900 - 4300 RPM. The existing standard rotating drives have 15-1/2 X 19 aluminum props. This seems to be the correct aluminum prop for a single engine boat. I've never run the boat so I don't know how good these are. All the prop charts I find say to increase pitch by 2 - 4 for twin engine boats. This will put the pitch at 21 - 23. My local prop guy says 19 pitch is about right. All the prop calculators I find want to know the current prop and performance info which I don't have. I'd appreciate any input.

Ranman
02-24-2004, 08:11 AM
I would say if your existing aluminum 19's are in good shape, start with those. They're free to use and will give you a good idea of where you need to be once you get her running. If you over rev with the 19's, move up to a set of 21's and see what happens. If for some reason you don't pull the full RPM out of the 19's (unlikely IMO) step down a set. I just don't see the sense of spending money on props when you have no clue what you need. Start with what you have and work from there. This is somewhat general prop advice as youi can also change prop styles, diameters, etc to achieve different affects. BTW, if you decide you like the 19's, you can always upgrade to stainless for better performance too. Good luck with your project.

Rich
02-24-2004, 08:32 AM
Thanks Ranman but the props I have are both RH. I will need a RH & LH with the new counter-rotating drives.

Ranman
02-24-2004, 08:50 AM
The existing standard rotating drives have 15-1/2 X 19 aluminum props.

Guess I need to pay better attention.

Now that I think about it:

What is the gear ratio of the new Alpha drives?

How much does the seven meter weigh?

Rootsy
02-24-2004, 08:59 AM
and what make are the existing drives and corresponding gear ratios?

then you can ASSUME it was propped correctly for the existing drive gear ratio and WOT operating range... for starters.. to do some bench building...

Rich
02-24-2004, 12:46 PM
The existing drives are standard rotation mercruiser #1's (pre-alpha) with 1.65 ratio.
The new drives are counter-rotating Alpha 1's with 1.62 ratio.
I'll be verifying the weight but it's about 4500 lbs.

Woodsy
02-24-2004, 04:30 PM
Where did you find a counter rotating (LH) Alpha? Bertram Boy has been looking for one of those for quite some time....


Woodsy Von Outboard

Rich
02-24-2004, 05:02 PM
Woodsy,
Counter-rotating Alpha 1 Gen 1 & II are available brand new from mercruiser in all different ratios. The only problem is they are expensive. I looked for a used / rebuilt one for over a year with no luck. Finally bit the bullet and bought a new one.
B.O.A.T. - Break Out Another Thousand