Twin Outboard Question 1976 22-2

[strouc]

What are the advantages and disadvantages of a counter rotating OB in the set? Was it a stock feature of the boat? Is it necessary? Thanks BB

[Georgie]

I normally hate using Wikipedia for information, but thought these two links were helpful.  The descriptions and explanations pertained to planes, but the principles are the same.

http://en.wikipedia.org/wiki/Critical_engine

http://en.wikipedia.org/wiki/Counter-rotating_propellers

[wingtime]

Well those give you a good idea a critical engine doesn't really exist on a boat since the engines are much closer to center line on a boat (yes I know cats don't count).  P-factor is a non issue on a boat too since there is no fuselage or tail fin for the slipstream to act upon nor are the props blades long enough to have a gyroscopic effect etc. An airplanes engine don't turn for steering like an outboard.  Also if you loose an engine on a boat your not going to crash... your just going to go slower or be stranded.  No the real reason for counter rotating engines is canceling out torque.  On a single engine boat the torque the engine puts on the prop has to be counteracted by some force... that typically is the hull of the boat.... that why it lists to port.  Trim tabs help level the boat out...  but that torque makes the motor and boat harder to turn in one direction and easier in the other since it helps the boat to turn one way while it makes it harder to turn the other direction.  Now if you add twins that are both standard rotation than the torque causing the problem is doubled.  But if you have one of the engines that turns the prop in the other direction than the torque it creates cancels out the torque of the other engine.... the net result is  basically zero torque is acting on the hull of the boat.... so it doesn't list over while under way and it is MUCH easier to steer and handle.  Thats it.

[Blue Agave]

Is it necessary? Thanks BB

It is not necessary, after all in 1976 CR did not exist.

[wingtime]

Is it necessary? Thanks BB

It is not necessary, after all in 1976 CR did not exist.

Yup Fernando is right... it's not needed.  Like he said CR outboards didn't exist back in 1976 also they did not have the high HP outboards back then like we do now.  What were they originally rigged with? Twin 75 HP?  The only difference in the CR motor is the lower unit anyways.  It is nice to have though.

[gran398]

Yep, great to have but impossible to find in anything under 115 HP.

[fitz73222]

The whole counter rotating advantage really doesn't start to come in to play until you get into higher horsepower installs and boats that are capable of running in the 50 mph+ range anyway. I also think that the greater the deadrise the greater the effect of counter rotation as a postive influence. As far as torque issues are concerned; with hydraulic steering it isn't an issue. I've run twin non counter rotating twins with mechanical steering and it was ridiculous. You could not turn the wheel with any positive trim and it felt like you were going to break the helm or the cable. Really felt unsafe. On my boat with twin 115's non counter rotating I can turn the engines with the center hub of the wheel with positive trim at cruise speed, very nice actually. I will get a slight list to port when at cruise so I just bump out the starboard engine trim and she levels right out. What's is really weird is running on plane with the starboard engine and port engine tilted up she runs a perfect attitude and will cruise with 4 people, 50 gallons of gas at 24mph at 3700 and 31 mph at 4850 WOT. Now run on the port engine with the same throttle settings and she lists bad to starboard; enough to where you can't trim it out so you have to shift passengers to correct it. Perhaps the 4 blade props have a greater influence on this but work very well in my mid range application which is all I care about. Until you run twin engine boats it's hard to realize how great they are in many other aspects from maneuvering at idle to much greater torque and I never felt the urge to need trim tabs since I have the control with adjusting trim settings to correct attitude. I just really like them and would not go back to a single in my application.

[Aswaff400]

the main advantage of counter rotating twins is slow speed docking. generally the boat is operated without using the steering wheel and only the shifters, anyone who has tried to back a single screw in a straight line knows the boat will spin one way or the other the same will happen on a twin screw non counter rotate. if both props oppose eachother the boat will back down in a straight line which makes docking a whole lot easier.
if you look at most triple and quad powered boats, the 2 outer motors are ran opposite of eachother