counter rotating engines?

[jim rosenthal]

Post on another forum: do any Hatteras Yachts have counter rotating diesels, especially Detroits?

Now- I have never seen this. I know that the Cummins engines we installed in my 36C both spin in the same direction. The 'reverse' is done in the gears, one gear runs in 'same rotation as engine', the other reverses it. The ZF gears are designed for that. I know that Twin Discs will do this, also, and I assume Allisons and Paragons are the same.

I know that BW gears, Velvet Drives, do NOT do this and that the 'reversing' gear in a pair of Velvet Drives is actually sl different- you can't run a Velvet Drive at full throttle in reverse. I have had two boats with BW gears, and one had gas engines with one reverse-rotation engine. The other had both engines turn the same, but one gear was specially set up to reverse the rotation direction.

Is anyone familiar with a set of DDs that were supplied in different rotations- one CW, one CCW? IIRC, virtually all marine diesels run in CCW, that's the normal rotation direction.

[oscarvan]

My engine model numbers are different P and SB. Some elements are "flipped" for accessibility, ie dipstick, oil filler on the valve cover. However the meat and potatoes is the same.

The M20's have plates on them with M20R and M20L.

This seems to indicate that the reversing is taking place in the gears.

[racclarkson@gmail.com]

My engine model numbers are different P and SB. Some elements are "flipped" for accessibility, ie dipstick, oil filler on the valve cover. However the meat and potatoes is the same.The M20's have plates on them with M20R and M20L.This seems to indicate that the reversing is taking place in the gears.

That’s incorrect. Model number code indicates rotation. I think you’ll find you have counter rotating motors.

[oscarvan]

That’s incorrect. Model number code indicates rotation. I think you’ll find you have counter rotating motors.

OK, ready to learn here:

Port 70823300
SB 70827300

From Wikipedia: "Boats equipped with two engines would typically use one Left Hand and one Right Hand, so that the torque from the propellers would cancel each other out, without the need for a complex reversing gear on one side."

But then I have the question: Why the M20L and M20R gears if the engine takes care of it ?????

Of course it will be easy to look at them later today when I fire them up..... so stay tuned.

[racclarkson@gmail.com]

OK, ready to learn here:

Port 70823300
SB 70827300

From Wikipedia: "Boats equipped with two engines would typically use one Left Hand and one Right Hand, so that the torque from the propellers would cancel each other out, without the need for a complex reversing gear on one side."

But then I have the question: Why the M20L and M20R gears if the engine takes care of it ?????

Of course it will be easy to look at them later today when I fire them up..... so stay tuned.

Wikipedia?

The motors rotate inboard viewed from rear (P-CW; S-CCW) to neutralize the torque, just like multi-engine piston aircraft. Boats with non-counterrotating motors/props will yaw to one side on power up/down, for example.

Both gears reverse the rotation in forward for outboard rotating props. OB turning props give slightly more speed. IB spinners throw more water across the rudders for better low speed handling. Both gears operate with engine rotation in reverse.

[rsmith]

Replace a starter and you’ll find out

[Lmcruiser]

I found a reference but can’t figure how to post directly. If you go to engine.od.ua website then go to diesels, then choose Detroit it will take you to a list of files and the first in the list has a free PDF down load that shows engine configurations by Model number. It appears the 5th number 0-4 is LH and 5 or more is RH.

[oscarvan]

Wikipedia?

The motors rotate inboard viewed from rear (P-CW; S-CCW) to neutralize the torque, just like multi-engine piston aircraft. Boats with non-counterrotating motors/props will yaw to one side on power up/down, for example.

Both gears reverse the rotation in forward for outboard rotating props. OB turning props give slightly more speed. IB spinners throw more water across the rudders for better low speed handling. Both gears operate with engine rotation in reverse.

OK, that makes perfect sense. Thank you.

Thanks for the link LMCruiser. I had that but forgot about it.

[Fanfare]

The early boats had contrarotating engines. In theory to balance torque. The original Allison transmissions could not take continuous full power in reverse. I think these were model H transmissions. Don't know about the later MH transmissions.

By the time I replaced my original model H transmissions and 8V-71 naturals with 6V-92TAs 25 years ago all new Detroit Diesels rotated in only one direction with Twin Disc transmissions converting the props to opposite rotations. Somewhat to my surprise I could tell no difference in handling. Still tracked straight.

The opposite rotating props are essential in walking the boat sideways in reverse, particularly in a slip. According to Chapman's the swirling water flow backwards pushes the deep bow sideways and "walks" the stern in the other direction.

[oscarvan]

The opposite rotating props are essential in walking the boat sideways in reverse, particularly in a slip. According to Chapman's the swirling water flow backwards pushes the deep bow sideways and "walks" the stern in the other direction.

Prop walk is real. Those of us that have lived with single screws, especially before the age of bow thrusters got to know, and utilize, it well.

So thinking about all this more and in keeping with Dr. Clarkson's statement that engines turn inboard and props turn inboard with them in reverse it follows that the turning moment in reverse is accentuated due the prop walk. IE SB prop pulls the stern to Port due prop walk AND asymmetrical thrust, and vice versa.

By the same token the props turning outward add to the turning moment in forward. IE SB prop walking to SB pulling the stern the same way and vice versa.

Thanks all for the info. Cool stuff. I love freaking science man.