Recently I was talking with a local boat shop owner about this. He had a couple 1970's vintage boats in the 20-25 foot range using twin SBC with alpha drives turning in the same direction. He said at worst they only exhibited a very gentle drift to one side and was nothing to worry about. I didn't ask how fast that would go but, I would imaging 50'ish or so. Granted it's only 260 horsepower per side so not much torque there. I have no experience with this just relating what I was told.

I asked the question specifically about surface drives because it's what is currently on my mind but I am interested about the effects for any drive. Does it torque steer? Does it cavitate one prop? I assume there is some historical data on this?

Still pondering the idea, I am an engineer so this kind of **** boggles my mind! Someone at some point said these drives need to turn opposite directions, why? Is this true for all drive types? What does a boat do with triples?

Just like any other drive, the boat will want to list to one side, steer dramatically better in one direction than the other, have a harder time docking and more than likely due to the combined torque of the engine and propeller, provided they are rotating the same direction, will be slower than the alternative.

yep, thats pretty much what my Scorpion did with twin smallys and Aplhas. Wasnt horrible but it also wasnt much of a performer, I would suspect all this would be greatly exaggerated as hp goes up. There would be no reason to persue this type of set up. I will say this though just cruising it wasnt very noticeable but when on or off throttle or turning or docking you got some twitchy stuff go'in on . . .

What is driving the question?

Although not surface, fella I know has a 312 stinger with same rotation King Cobras because previous owner couldn't find counter rotating one. Wore his arm out pulling to one side.

WOW that is a huge loss.Comparatively what is the loss thru a velvet drive?

And can you use a velvet drive thru a single engine mounted Kaama drive?

I want to install a ASD6 in my 18 one of these days and occasionally see the Kaama come up for sale and they interest me also.

Back in 1986 I rebuilt a 1985 Kaama edition Formula sr1 302 with Kaama 425's in it that spent a night in the surf and than another day on the bottom of the Gulf .

:Long story short even after the motors had been upped to 500s after the sinking the boat did not seem very fast even with 4 bladed Rollas . Must have been the high parasitic tranny loss. I always felt it was slow because it was a heavy boat and also that there was a ton of sand left under the cockpit that could not be accessed..

Never had any experience with a surface drive before that but remember when the 1st article came out in Powerboat about the Arneson claiming 10 to 15 mph gains I think the test boat was a old 70s Pachanga?

Buddy put a asd6 on my old boat and saw the 10 to 15 gain no problem

Back when we had are old 26 foot Monza with the screaming 6 bangers in it back in 74 it strayed to the left because you couldn't hold onto the wheel hard enough because of all the feedback from the daul Rack non assist steering , That boat pulled so hard left i bet it only went right 1% of it's whole life.

6 years later when we popped a mold off it and started building the 26 with twin 260s non counter the power steering helped a bunch with torque steer . But if you had that thing riding on the props in a narrow passage like the narrows in Macatawa at full throttle you could see it was walking hard every time it got up on the props.

Saw it especially one day later on when the guy who took over our molds and was building a very light /cheap lay up of the 26. Raced him with my 18 from the Heinz Factory all the way up Macatoilet against a 20 mph west wind and chop He had me by a good 5 mph but he had to keep slowing down as he was walking so bad.Boat had a ton of rocker in it and was a handful at 65 though.