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Old 06-10-2021, 06:00 PM   #1
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Noob question about twin engine

Go easy on a newbie here please.


On a twin-engine boat, assuming you're bopping along at 8 knots or whatever, are both engines turning the same direction, and therefore "forward" for one transmission is actually the opposite of "forward" for the other so that the props turn in opposite directions?
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Old 06-10-2021, 06:08 PM   #2
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Greetings,
Mr. p. Depends. Some manufacturers offer engines that spin in opposite directions, others use the transmission to accomplish opposite rotations.
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Old 06-10-2021, 06:23 PM   #3
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Yes to everything.


Some boats come with counter rotating engines. Most boats now days both engines have the same rotation however most of these boats use the transmission to counter rotate one of the props. Now some transmissions do the counter rotating by adding an extra gear however some transmission achieves this by changing the shift linkage so that one tranny is in reverse while the other is in forward. Transmissions do not really know forward from reverse. They know clockwise versus counter clockwise and it’s the prop that determines forward or reverse.
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Old 06-10-2021, 06:24 PM   #4
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Another reason not to have twins there I go again starting that I realize that wasn’t your question, if my foggy brain is working most diesel boats use the transmissions or gearboxes which ever you prefer , although I did have a gas boat with twin V eights that ran opposite
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Old 06-10-2021, 06:32 PM   #5
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I LOVED my twins and would not have traded them for ANYTHING because I was a master with them in any maneuvering situation you could name..... and then I did trade down! Oh, well.
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Old 06-10-2021, 06:35 PM   #6
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We had a Trojan with gas engines and one engine was counterrotating. I think most now are done in the transmission.
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Old 06-10-2021, 06:38 PM   #7
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Old days there were left and right engines. Now just about all are left hand rotation. Gears are often the same also . Just one spins opposite the other. And since the wheels are left and right , everybody’s happy.
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Old 06-10-2021, 06:43 PM   #8
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In the old days our engine went both ways, we stopped the engine, changed the valve arm and restarted the engine in the opposite direction. We didn't have transmissions in the old days.
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Old 06-10-2021, 07:38 PM   #9
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I love twins....


... I think these are counter rotating.
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Old 06-10-2021, 08:21 PM   #10
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My two Detroits run "inboard" to cancel torque steer. (Port counter clockwise and SB clockwise facing the front of the motors ie looking towards the stern).

Then the transmissions reverse both rotations so as to have outboard turning props. Outboard/forward inboard/reverse turning props amplify the asymmetric thrust with prop walk and help in maneuvering. Inboard forward turning props can give a little more speed.

So I have a left and right turning engine and a left and right transmission.
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Old 06-10-2021, 08:42 PM   #11
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My two Detroits run "inboard" to cancel torque steer. (Port counter clockwise and SB clockwise facing the front of the motors ie looking towards the stern).

Then the transmissions reverse both rotations so as to have outboard turning props. Outboard/forward inboard/reverse turning props amplify the asymmetric thrust with prop walk and help in maneuvering. Inboard forward turning props can give a little more speed.

So I have a left and right turning engine and a left and right transmission.
That's an unusual setup.

With my gassers, the stbd spins backwards. Transmissions output same as input in forward, giving outward rotating props.

Some transmissions like a velvet drive have a distinct forward and reverse (although the velvet drive cr2 can be configured to rotate same or opposite of input in forward). Others have just an A and B position that are interchangable, so you just swap 1 shifter to rotate that prop the other way.
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Old 06-10-2021, 09:04 PM   #12
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Originally Posted by pjtemplin View Post
Go easy on a newbie here please.


On a twin-engine boat, assuming you're bopping along at 8 knots or whatever, are both engines turning the same direction, and therefore "forward" for one transmission is actually the opposite of "forward" for the other so that the props turn in opposite directions?
Except in rare instances, the props are always counter-rotating: in forward, viewed from astern, righthand (stbd) prop turns clockwise; the lefthand (port) prop turns counterclockwise. This provides dependable/predictable prop-walk in close quarters. For example, when coming into a side-tie to stbd, approach at a 30-45 degree angle, drop the outboard engine into reverse (inboard engine in neutral), and the outboard engine will now turn clockwise causing prop-walk to pull the stern towards the dock while simultaneously slowing the boat. This is a truism on >99% of the recreational twins built in the last 75 years.

In the old days, Detroit Diesel came in mirror engines and they actually turned opposite directions. This also allows two engines (or four) to be mounted to a single large transmission, a common design on WWII era yard tugs. For recreational boats, it allowed all service points to be inboard and easily accessible. Those days are long gone - the props are now counter-turned at the transmission.

I have been on a couple boats were the props both turned the same direction. My hunch was this was due to the original construction yard had oddball equipment laying around and they didn't know the difference.

Peter
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Old 06-10-2021, 09:28 PM   #13
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That's an unusual setup.
Factory original. They built a LOT of boats like this.
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Old 06-11-2021, 07:18 AM   #14
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Factory original. They built a LOT of boats like this.

I'm not surprised it's factory, it's just a bit odd. Usually you get the counter rotation with the engines or the transmissions, but not both. I'm kinda curious how they ended up deciding to do both.
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Old 06-11-2021, 07:33 AM   #15
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I'm not surprised it's factory, it's just a bit odd. Usually you get the counter rotation with the engines or the transmissions, but not both. I'm kinda curious how they ended up deciding to do both.
Like I said: Inboard turning engines to counteract torque steer, outboard turning props for maneuverability.
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Old 06-11-2021, 07:37 AM   #16
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Like I said: Inboard turning engines to counteract torque steer, outboard turning props for maneuverability.

Why would the engine rotation cause torque steer though? You should only notice the effects from the direction of shaft / prop rotation.
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Old 06-11-2021, 10:25 AM   #17
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Except in rare instances, the props are always counter-rotating: in forward, viewed from astern, righthand (stbd) prop turns clockwise; the lefthand (port) prop turns counterclockwise. This provides dependable/predictable prop-walk in close quarters. For example, when coming into a side-tie to stbd, approach at a 30-45 degree angle, drop the outboard engine into reverse (inboard engine in neutral), and the outboard engine will now turn clockwise causing prop-walk to pull the stern towards the dock while simultaneously slowing the boat. This is a truism on >99% of the recreational twins built in the last 75 years.

In the old days, Detroit Diesel came in mirror engines and they actually turned opposite directions. This also allows two engines (or four) to be mounted to a single large transmission, a common design on WWII era yard tugs. For recreational boats, it allowed all service points to be inboard and easily accessible. Those days are long gone - the props are now counter-turned at the transmission.

I have been on a couple boats were the props both turned the same direction. My hunch was this was due to the original construction yard had oddball equipment laying around and they didn't know the difference.

Peter
I highly doubt that they counter rotate propellers for prop walk I’m sure the reason they counter rotate is to offset all the forces going on when you spin a propeller I’ve never found using prop walk to be all that useful there’s always something in the mix that screws things up wind current. Propellers are simple and complex at the same time but I don’t like thinking about it too much because I might hurt my brain . takes away from the simple enjoyment of boating If i have to put too much thought in it
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Old 06-11-2021, 11:00 AM   #18
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I highly doubt that they counter rotate propellers for prop walk I’m sure the reason they counter rotate is to offset all the forces going on when you spin a propeller I’ve never found using prop walk to be all that useful there’s always something in the mix that screws things up wind current. Propellers are simple and complex at the same time but I don’t like thinking about it too much because I might hurt my brain . takes away from the simple enjoyment of boating If i have to put too much thought in it
Leaving aside the risk of damaging your brain from too much thought.....

Twin props counter-rotate to offset their rotational forces which is, by definition, prop-walk. Prop-walk exists in forward as much as in reverse but the force is overwhelmed by wash over the rudder which makes prop-walk in forward inconsequential - a "whisper in a rock concert."

With rare misguided exceptions, twin screw recreational boats have the right screw rotating right, the left screw rotating left (viewed from astern). As an ex-delivery skipper who used to move boats around in prep for boat shows - the zenith of close-quarters-maneuvers, I can tell you that having a reliable 'truth' about prop-walk and counter-rotating props is invaluable and, for me at least, simplifies boat docking vs complicates it. I think relatively few twin-screw owners use prop-walk to turn a boat. Instead, they split the gears which often lands them 3-5 feet off the dock instead of gently landing against the dock. A recent post by another TF member (rslifkin??) put it well - driving a twin is better considered as driving two singles. Subtle but important difference.

I have to say Scooby, I seem to recall you've made dozens of references to the many single-screw trawlers you've owned. I am surprised you find that prop-walk is not that useful. You must be a wizard with fiberglass repair to compensate.

Peter
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Old 06-11-2021, 11:07 AM   #19
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With rare misguided exceptions, twin screw recreational boats have the right screw rotating right, the left screw rotating left (viewed from astern). As an ex-delivery skipper who used to move boats around in prep for boat shows - the zenith of close-quarters-maneuvers, I can tell you that having a reliable 'truth' about prop-walk and counter-rotating props is invaluable and, for me at least, simplifies boat docking vs complicates it. I think relatively few twin-screw owners use prop-walk to turn a boat. Instead, they split the gears which often lands them 3-5 feet off the dock instead of gently landing against the dock.

Agreed. I see many people struggle with handling twins because they feel like both have to be in gear at the same time (either same direction or opposite). Instead, I tend to treat it like 2 singles with really bad prop walk tied together. But my style also is to use the boat's momentum and just make adjustments, rather than the too-common style of trying to force the boat to move a certain direction immediately and jerking it around.
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Old 06-11-2021, 11:20 AM   #20
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Prop walk will help you control the stern obviously however on my single screw boats I have used the rudder to control my stern . controlling the bow with prop walk isn’t too effective forward motion will have control over the bow if you have a room for it. As far as twins One in forward and one in reverse using the throttles to control your forward or backward motion works quite well but I guess I’m not as sophisticated as you guys no I don’t worry about damaging the boat on the dock I’ve been running boats for over 40 years you do it your way I’ll do it mine I guess your way is your only way that matters anyways works for me may not work for you don’t know don’t care
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