I’ve never heard such a lubberly pair of grass-combing buggers in all my life. You live by the Atlantic for Pete’s sake. Boats that have two motors generally have them in two different rotations for two important reasons, both having to do with the propeller: number one, propellers spinning in water move boats forward, but also very slightly sideways one way or the other depending on rotation. Having to turning in opposite directions counter-acts this effect. The propeller acting on the water also creates a torsion on the hull, tending to make the boat lean one way or another. Again, two propellers turning in opposite directions fixes this.

How you two dirt-fingered simpletons could know so much about cars but not even literally THE FIRST THING about motorboats is beyond me. I suggest you take the weekend off, go the the Cape, and soak your heads!


The two land lubbers have retired, so they are taking every weekend off, in addition to weekdays. You were listening to a taped (and endlessly repeated) program from…perhaps as long as 20 years ago.

I’d bet that many car guys don’t know these fine point of two-screw propulsion.

It makes sense, but I think there are easier ways to reverse the second propeller, such as installing the engine backwards. If that would work, that is.

Most motors don’t have to be "turned " if the torque steer stabilizing tab is adjusted properly. People don’t buy two motors instead of one to combat torque steer; it’s negligible with the right adjustments. They do it for security and additional steerage, heavy loads and control when necessary. It’s much more complex, can be less efficient and poorly balance in some boats with the added weight and a lot more expensive.

I know that the OP is referring to large boats. However, the most interesting outboard motor I used was a 2 stroke motor that was made in the late 1930s or early 1940s. I believe it was a Nepture. It had a rope start and the flywheel was notched so that it could be started to run in either direction. It was handy for backing out of the dock. The throttle control was moved one direction for clockwise rotation and the other direction for counter-clockwise rotation. If one worked the throttle carefully, the rotation could be reversed without stopping and then restarting the motor.

Regarding the boat propeller. The density of the water at the bottom of a propeller is greater than that at the top. Therefore, when it is moving in a clockwise direction (when viewed from the stern) it tends to walk to the right, forcing the stern to starboard.

However, almost all boat engines only run in one direction. The direction of the propeller is dictated by the transmission. This way any engine can be used on either prop. Imaging the problems in producing two different kinds of engines. Running the engine in reverse would require the water pump, oil pump, alternator to also be reversed.

I have seen some diesel engines in boats that are stopped and then started in the other direction – long ago.

@b7822 - “The density of the water at the bottom of a propeller is greater than that at the top”

Not in any measurable sense, water is almost incompressible at these conditions.

However, almost all boat engines only run in one direction. The direction of the propeller is dictated by the transmission.

Technically, you are correct because counter-rotating engines have all but disappeared in new designs due to the complexities they present. However, there are boatloads :wink: of inboard twins running around that use a LH and RH engine that spin in opposite directions.

Should we start the debate on twin props turning in vs out?

Counter-rotating props to make the boat go straight? Possible. But I would have guessed the main reason for opposite moving crankshafts with dual inboard boats would be to cancel out the rotational-inertial forces that the mass of the rotating crankshaft would cause. This rotational force could cause the boat to lean (right to left or left to right) during periods of acceleration and deceleration. Anybody who has gunned the engine a bit while doing a tune-up on their car notices the engine rotates left and right in the engine mounts during an increase in rpms. This rolling force isn’t enough to tip a powerful V8 equipped car over since cars are on solid ground, but on boats, sitting in the water like they are, it seems like this rotational force could conceivably – if the engine was very powerful – cause a narrow boat the roll completely over during rapid acceleration.

The same force would be present in dual engine airplanes. And even a more pronounced right/left tipping effect than in boats I’d guess. I wonder if the engines on the right wing turn a different direction than the engines on the left for airplanes?

And back to boats, the question was about inboard motors. But what about dual outboard motors? In board motors I think the crank is in line with the long axis of the boat, right? It would depend on the orientation of the crank in outboards. Anybody know the orientation of the crank on an outboard motor? Vertical? Horizontal in line with the long axis of the boat? Or horizontal in line with the short axis of the boat?

Yep, pretty sure that most, if not all, few twin engine prop planes have counter-rotating props.


Outboards can handle prop rotation in the lower unit just like I/O. The flywheels are usually on top and the rotating mass isn’t very large anyway. Hull design plays a big role in how the boat reacts to propulsion (bow lift vs sink, walking etc)

Counter rotation is common in boats and planes with multiple engines off centerline. Lots of online references to why this is advantageous…The planes I have flow in turned inward IIRC.

Not as common as you’d think. In Pipers it’s not terribly uncommon but other than that, only a handful of them do it (including the P-38 Lightning, which is weird because they counter-rotate outboard, which means both engines are critical - which sounds stupid until you learn that it was because it made the machine gun more stable - something most planes don’t have to worry about :wink: ) Of the non-Pipers that do it, only 1, the Piaggio Avanti, is still in production.

Seem to recall that the Beech Baron did have counter-rotating propellers, and the Be-1900D didn’t.

It would seem that you’d have to stock twice as many parts, etc for “left-handed” and “right-handed” motors. I’m surprised that the nautical world would go to this trouble and expense, unless the perfomance gain was sizable…

Boats and airplanes run the props at opposite direction from side to side. Even the Wright Brothers Flyer had the props running counter to each other.

Yes not all car guys no about counter rotation. A friend of mines Dad got old boat with twin 327 Chevys. One ran one way and the other ran the other. I tried to tell him. He put the wrong one in his Hot Rod he buliding. 1 gear forword and 4 reverse. The look on his face when he tried to back out of the garage was priceless. Also with IO twins most trun the same way.