RR or counter-rotating, reverse prop vs reverse engine?

[Tod duBois]

Big Dummie Here, wondering why the reverse rotating engines in twins? I get something about torque - but seriously - that much cost and trouble for that? Fill me in? Also, my current re-power suggests that two same rotating engines and a reverse pitch prop is about $10,000 less than rebuilding a counter rotating engine. Thoughts? I assume this has been discussed in the past but I don't see a search bar, or any bar right now that had alcohol. 

[MoreCoffee]

I read an article a while back about this... The gist of it was that they do not have counter rotating engines anymore. The reverse rotation is done with the transmission.  If memory serves the cheapest way to accomplish that was to change the one transmission to a gear set up rather than a chain. It was cheaper than buying a new trans...  it might have been change gear to chain.  

[Joel Leavitt]

MoreCoffee is correct about modern counter rotation capability shifting from transmissions to  engines. This is largely because it means manufacturers don't have to make, and dealers don't have to stock, two different engines. They (you) just flip a switch to change the rotation. Full disclosure - this comes from my experience with outboards, not inboards.

The "torque" angle is that counter rotating props help the boat track in a straight line. If both props rotate in the same direction, the boat will not track straight, even with the rudder centered. For modern outboards, this is a simple decision  because there is no additional cost.

What intrigues me about your question is the reference to counter rotating props. I've always assumed that this refers to the direction of shaft spin. Am I missing something? Is it possible to buy a prop that reverses propulsion from that of the shaft? That would be fantastic for retrofits!

Captain Joel

[Tod duBois]

Joel, if they have counter rotating props and regular rotating props then I'd assume you could just swap them...but again, maybe I'm missing something. Seems easy enough to create a brass propeller with negative or positive pitch. I can see that with a brand new high-end boat - buyers would expect it to track straight...but if you are working on a retirement dream on a fixed income it seems you just might not care if it tracked straight, I mean tracking, tacking - sounds like sailboat mumbo jumbo, if the boat doesn't go quite straight ya just blame it on the bar snacks and beer. Unless of course the boat went in circles then it might be something like Taquila. 

[aval...@fastmail.fm]

There is only two directions a prop can rotate, clockwise and counterclockwise. Same for pitch angle, it’s either pitched to move forward under a given rotation or reverse. (Pitch has multiple angle variations but can only be angled two directions) The counter rotating props you guys speak of do exist and are pretty common, they are what is required to go on a counter rotating engine or transmission. Put a counter rotating prop on a standard rotation output isn’t going to solve the problem, it’s just going to reverse when you want to go forward and vice versa.

Capn Jeb

[MoreCoffee]

If you currently have two transmissions and 1 of them was paired to a counter rotating engine I do not believe you can  change it  to pair with a standard engine and just change the prop.  You would be turning the transmission in the opposite direction it was designed for. Not sure exactly why this should not be done... but that was why the guy who I read about doing this changed the direction mechanicaly.  You need to have a chat with a velvet drive trans re-builder.  

[Tod duBois]

Capt Jeb, More Coffee, I'm not sure if you are negatively confirming my suspicions or not. Belief is also not a high enough standard with regard to boating...nobody wants a perfect storm. So if Capt says that reverse and forward are opposites and an opposite prop will take you the wrong direction but we have two-way controls, is it not up to the capt which way the boat goes? As far as specific transmissions, I'm thinking that transmission manufacturers are a tad smarter than engines manufacturers, which would imply that they sell the same transmission and tell the engine manufacturers to make an engine that goes backwards, therefore, maximizing their profits. Though I do admit not knowing if that is true or not, so one should verify rather than believe. 

[BoatRDavid]

I used to have a 1984 42' Aft Cabin with two Detroit 8.2 L v8s.  It came with the Double Clutch (I think) transmissions.  Both engines had the same rotation.  The port transmission was run in reverse.  It seemed strange, but it worked fine.

[Maggie Harbour]

In the old days the  engines determined the rotation  (ie) counterrotating! transmissions had  weak clutchs in reverse. What went in came out left, right .Now the transmissions determine rotation have same clutch pack either forward or reverse. I have twin 555cummins in 42SF one right one left. BW 73C 3:1 trans.

[Georgeviking]

Ok I do agree with all that's said but how is rebuilding a counter rotating engine $10,000 more than a standard rotation engine,,,??? I've done many gas so this must be diesel but still don't add up

[eric]

The main difference is the camshaft, as far as transmissions, on the BW velvet drives all that is required is to reverse the oil pump cover when used on a reverse rotation engines.
These days, manufacturers use standard rotation engines and simply run one transmission in forward and one in reverse using a right hand and one left hand prop. The gear ratio is usually a tiny bit off but that is compensated by slightly different engine rpm or prop pitch.

[Tod duBois]

Thank you, everyone, even those who caught me with some embellishment:) I got enough good information to investigate this project boat and I'll get back to you late July with "the rest of the story" Now onto ageism and what the hull matters. See new post if you have found this in any way entertaining. 

[Tod duBois]

Okay, time to get serious - she's on the hard and I have two motor home engines in the pickup. 

[Tod duBois]

Best I can tell from other forums the Velvet drive can be converted to run automotive rotation by switching the oil pump (not sure what that precisely means yet) and I assume the direction of the controls F-R, and then adding a automotive rotation prop. Anyone out there can verify? With powered trim tabs I can't imagine two automotive rotation engines at half power could cause much roll.