Counter rotation twin OMC

[jim vogt]

II need to replace my port OMC outdrive. It is right hand drive like the starboard, but boat originally came with a left turning port Outdrive. Is there an advantage to going back to left turning (counter rotating)

 

[northern]

I had OMC out drives on a 1982 28 footer once. Both outdrives were original both turned the same way. Made keeping spare propellers simple all you need is one. I sold the boat in 2005. The mechanic said he could not get a lot of parts for the units. He said the legs on Johnson and Evenrude outboards had similar parts. His advice was to change the legs to Merc.

 

[89P270]

My Pachanga was Twin 350s. They were both right hand drive from OEM. I had it repowered in 2012 (see my thread 1989/2012 Pachanga). When I repowered I went with Counter rotating. The reason some same counter rotating is better for boats is to avoid chine walk among other things. The Pachanga wasnt at the top end of performance and the motors werent staggered -Pretty basic low end performance. I can tell you that the switch from two right to two counter rotating didnt make a noticeable difference. In fact, I actually preferred the right hand rotation to the counter rotating. The only real notice I had was the new power got me from 55ish to 61ish and at some points near 60, there would be cavitation creating a "hole in the water" affect. Unnerving at 60MPH. The techs at Mercury racing told me that Alpha drives are not suppose to be used above about 60 because the lower unit torpedo shape creates a cavitation bulb that moves along it and hits the back end around this speed. The other thing is if you do switch to counter rotating, there were significant measures need to ensure they are properly set meaning cambered. Its something like the jet streams should cross about X boat lengths behind the boat at X speed. Probably want someone who knows what they are doing to do the refit. And, I dont know its an easy thing to simply swap the outdrive -I think most engines come OEM in one rotation and to convert at the flywheel is not easy. I had two new engines with rotation changed from manufacturer so came ready to go. I think counter rotating is more relevant to HIGH performance boats like the 42 Fountain lightning with staggered engine set up that a sport cruiser type boat.

 

[Bt Doctur]

CR is now done in the lower unit not in the engine.
Install a standard drive and reverse the control cable in the control box
To remove the CR drive you must shift into reverse to align the shift shoes

 

[DaveKamp]

"The only real notice I had was the new power got me from 55ish to 61ish and at some points near 60, there would be cavitation creating a "hole in the water" affect. Unnerving at 60MPH. The techs at Mercury racing told me that Alpha drives are not suppose to be used above about 60 because the lower unit torpedo shape creates a cavitation bulb that moves along it and hits the back end around this speed."

What exactly does that 'feel' like? I've never experienced anything unnnerving at 60...
...but I don't usually slow down that far...


The lower half of the MerCruiser R/1R/MR/Alpha/Alpha One drives are hydrodynamically similar to the high-power Mercury outboards, and they haven't exhibited any problematic handling. I DO know (from first hand) that running a standard gearcase over 90mph without a nosecone causes efficiency to fall off fairly fast, but at that speed, most guys choose to run a 'non-shifting' drive (Like a QuickSilver hydro racing lower, or a MerCruiser 'Super Speedmaster"). I haven't seen anyone MAKE an R/1R/MR/Alpha lower like this, but the Alpha One SS was KINDA like it...

The biggest issue with any of the Alpha, or it's predecessors, is the RPM limit... they do NOT like going past 4900 for any length of time, as the circulation pattern of lubricant becomes disorganized, causing the upper gearcase to rapidly overheat and melt gears...

 

[Soulshine]

I had twin OMC Cobras in my previous boat and the port side was counter rotational. It was a 1990.

I had no trouble finding parts but you’ll pay more for most things related to the counter rotation.

It can be hard to find a mechanic that truly understands those funky drives these days.

 

[89P270]

What exactly does that 'feel' like?

If felt like the props lost water to push. Like they hit an air pocket- a Large one! It was only for a split second until I pulled back on throttle and re-initiated. It did NOT do that with the OEM set up. But the difference was the OEM set up didnt get past about 56 while the refit (357 4V alpha engines and Gen II alpha drives) set up got to 61ish. And yea, from research I did, if I could have found 2 decent SS drives I would have had those rebuilt and used them rather than new GEN II alphas.

 

[DaveKamp]

Hmmm...

Okay, well, on my little boat (the one you see in the avatar), I do NOT run into any cavitation or ventilation circumstances that exhibit any sensation like that. I AM running a cleaver, because this hull, as it was fitted up, has the X-dimension slightly short... the cavitation plate is slightly BELOW the bottom of the keel. Not surprising, since it was originally a 120hp 153ci four, and I did NOT raise it up with the engine change. What I DID find, is that the hull DOES like about 5 degrees of trim, and at that angle, the front edge of the cav plate would snag flow and pull the transom down aggressively amidst a little pitching. To reduce that, I trimmed off about 4" of the front edge of the cavitation plate, and blended it (crudely... with a 4" flapdisk). Running it with a substantial amount of prop rake DID make the bow lift excessively, and would result in the drive ventilating very predictably at about 55mph, and I couldn't get the nose down enough to flatten out it's planing attitude.

The solution was trimming that front edge back a bit, AND swapping to a cleaver with essentially no rake. I can now run just about any trim angle without it getting squirrly or hitting that ventilation point.

I'm certain you've already been through it, but for sake of anyone ELSE who happens to read through this, if you get a 'ventilation-like' circumstance, DO make sure you're not spinning the prop hub (it has an elastomer element that will absorb shock in a prop strike), and also on the engine coupler. When the latter lets go, the engine revs freely like you've lost the prop... and the engine compartment fills with acrid burning rubber smoke, and lots of sticky black goo everywhere... (don't ask how I know)

 

[DaveKamp]

The Alpha SS's were rather low production, which stinks. I have ONE of my drives fitted with Alpha One SS 1.34:1 gears... but it runs a standard lower.