Prop Shaft Diameter

You're definately going to want a reduction. I would guess around 2:1, but speaking with a good propeller shop and engine shops would get you the best advice. The counter rotation is done in the gear, and I would recommend having your port engine turn in an opposite direction of your starboard engine for both efficiency and maneuverability. Another possibility is mounting your engines backwards and using v-drives and might work better with the space constraints you have

I worked at the Viking Service Center Riviera Beach (as mechanic/captain) for three years ending in '04. The largest shaft at that time was 3 1/2 for the 1800-2000 HP engines. 1400-1800 had the 3 inch shafts. 1100-1400 had 2 3/4, and under that was 2 1/2's. Non-splined aquamet 22 IIRC. Alignments were with "string" and we did literally hundreds of strut replacements. My Roscioli/Donzi has 1500HP and has 4 inch shafts- but it's believed to be overkill as Bob R. likes to "overbuild". I have little experience with smaller stuff however.

You cannot reverse rotation in the 71C or 72C Borg warners by running 1 in forward and 1 in reverse like a Twin Disc gear. There are only 2 friction discs in the reverse clutch pack. There are 6 or 7 friction discs in the forward clutch pack. They did make a 1.97 to one that reversed in the box and the 2.1 to one did not reverse. That is how they got the counter rotating props with same rotation engines. That was the only ratio available.

I got the same probleme with my bertram and got no choice i live in thailand and for going to a descent shipyard i got to go around malaysia via singapoor 2000 miles as no tecnicien i got to replace my 1 3/4inche 316 stainless shaft by much more strong but of the same diameter. my engine twin detroit diesel -71 325hp and i want to used 28 inche propellers
May be also the cutlass bearing will allowed me a small increase???????????
thanks for your help and advice
if you know where to buy those shaft in asia you are great
kind regards

pascal

Stone Marine in Singapore and Rogers & Lough in Brisbane come to mind.

Greetings,
I was under the impression that my BIL's 6-71s ran in opposite directions and a neighbors "flat" Perkins (sorry, don't know model #'s) did so as well.
Peter