120 Twin Force Engines

Just purchased a pair of 1992 Force 120 HP motor, the guy stated that one works great and the other had a gasket leak.

I plan on using only 1 of these motors for a ski boat, somebody ask me if one has a reverse prop on it.

How would I tell and does it make a difference on which one I use?

Thanks for your help.

Jerry

Re: 120 Twin Force Engines

Here's what I know on the subject:
A friend of mine has this setup on his boat. I believe they are called Contra-rotating propellers. They apply the maximum power of a single engine to drive two propellers, mounted in-line, in opposite rotation. I understand they are common in some marine transmission systems, in particular for medium to large size planing leisure crafts. The two propellers are arranged one behind the other.
When throttle speed is low, the water flowing through the propeller disk (thrust) causes a significant amount of rotational water flow to be created by the spinning blades. The energy of this water flow is wasted in a single propeller design. To use this wasted effort, the placement of a second propeller behind the first takes advantage of the disturbed water flow.
If it is well designed, a contra-rotating propeller will have no rotational water flow, pushing the maximum amount of water uniformly through the propeller disk, resulting in high performance and low energy loss. It also serves to counter the asymmetrical torque effect of a conventional single propeller. Hope that helps.

Re: 120 Twin Force Engines

Force never made a counter rotating engine. All force engines rotate clockwise at the flywheel and clockwise at the prop. Even if both engines had been mounted on a twin engine installation, they would have been both exactly the same. So choose to use whichever you wish.

Re: 120 Twin Force Engines

I think what the person ment was if the motors were set up for opposite rotation. One rotates to the right, one rotates to the left. This is usually the case if they came off the same boat. Check and see if the props are the same.
Counter-Rotation
Steering can be a real chore with a dual engine power setup. When the props turn in the same direction, the boat tends to list and steer off course. Keeping an even keel and true course requires constant attention, especially in choppy water and high winds. Much of that problem can be solved by having two props turn in opposite directions. In other words, counter-rotation. The major advantage of counter-rotation is its ability to enhance performance by reducing steering effort in all RPM ranges.
Most single engine boat setups normally operate in forward motion using clockwise rotation of the engine and gearcase. Although counterclockwise rotation setups have been used since the creation of the outboard engine, the use of counter- rotation has become more prevalent in the last decade. This increase is largely due to the manufacturing increase of larger twin engine recreational boats and performance boats.

To better understand how counter-rotation enhances performance, you must first understand what happens under normal conditions using clockwise rotation propellers. Clockwise rotation propellers, when turned in the same direction, will tend to list or walk to the right side of the direction they are moving forward in.

Left-handed or counterclockwise rotation is usually accomplished by using a special gearcase configuration that rotates opposite of engine rotation under normal operation. Counter-rotation is mechanically achieved by driving the gear clutched to the propshaft in counterclockwise rotation when shifted into forward gear. A counter-rotation gearcase is completely different than a standard gearcase, as it is comprised of special parts that provide strength and durability to accomplish this reversed rotation.

Re: 120 Twin Force Engines

This web site has photos that go with previous explanation.http://www.boatfix.com/how/props.html

Re: 120 Twin Force Engines

Thanks for the reply, yes this is exactly what I was referring to. Thanks for the link with the diagram, very helpful.

Now I assume that the lower units are interchangable, is that correct?

Or will it matter which direction the prop turns if I'm only use 1 motor?

Jerry

Re: 120 Twin Force Engines

If they are the same model number no problem, and no problem just do a compression check and pick the best.

Re: 120 Twin Force Engines

Jerry: As I wrote above, the engines are EXACTLY the same. Lower units are interchangeable as is everything else. They will only turn clockwise so there is nothing to worry about in a single engine installation. Most boats have standardized right hand pilot seating to counter the starboard lift from prop torque.

SO: Mix and match, or choose whichever engine you want, mount it on the boat and don't worry.

Re: 120 Twin Force Engines

Thanks to all who replied, I had a chance to check a few things today and you guy are correct. Both motors had the same rotation.

Checked the compresion onboth motor and found 1 motor with 130Lbs or better in each cylinder, the other motor ranged from 5, 60, 120 120.

So i'llload the good and start the wiring process. May be back for more help later.

Thanks again.

Jerry