Counter Rotating Outboard Engines

What advantages are there, if any, to installing counter rotating outboard engines on a relatively slow boat such as a Rosborough 246 with targeted cruising speeds of 13 to 18 knots.

 

None, really.

 

I would disagree with Gonzo. There are advantages even at low speeds, perhaps particularly ar low speeds. Counter rotating props means that the prop walk created by either engine are cancelled out, and the boat will track better. With two props rotating the same way the boat will always be turning one way or the other (depends on prop direction which way). This will cause you to always have to have the engines turned the other way to compensate. Increasing fuel usage, and decreasing top speed.

 

Stumble, it's true but the penalty is negligible compared to the added cost.

There are no appreciable benefits only penalties which, interestingly, were not asked for.

-Tom

 

The disadvantages, by in large, seem (to me) to be self evident.
Reduced engine and prop choices. Need of a dedicated spare prop for each engine. Possibility of increased cost and complexity as well as reduced availability of spare parts for the counterrotating engine.
All responses, pro and con, are always welcome.

 

Are there any CCW rotating props available for outboard engines?
If there are, you could simply put one engine in reverse.

 

I don't believe there is any good reason to install dual engines on a boat like the Rosborough. Some of the reasons not to have been mentioned.

The pro's like manuverability and redundancy don't carry much weight against the cons on a small boat like that. Dual maintenence, dual steering, dual controls, dual batteries, dual fuel filters, increased complexity of fuel system, increased fuel burn, twice the drag cutting into efficiency, loss of space on a small boat, twice the spares

Reliability of modern outboards is quite good, stick a Honda on the back and if you need the piece of mind install a kicker bracket on the transom and hang a 10hp on it. You could use the 10hp for a dingy too. Keep your Sea Tow membership current.

$.02 Steve

 

Keys,

There are a number of discussions on the advantages of dual vs single outboards. That wasn't really the question asked.

Milehog

Other than experimental props I don't even own a spare prop. We did 15 years of cruising with the same prop. I just don't see having to have a spare for each engine as a real concern. And modern outboard manufacturers sell pretty much every engine in counterrotating designs, it might limit your options for a used engine, but not new ones. And other than the transmission, which I don't think anyone carries the parts to rebuild underway the parts are all the same.


In short I see no advantage to not going with counter rotating props and a number of disadvantages. Finally I question the prop steer effect as minor. Not every boat does it equally, but I have piloted one that was turning almost 10 degrees just because of the props.

 

No cant do that the reverse gear is not engineered for very much load at all and the thrust of the prop shaft that normally presses on the forward gear with no bearing will be running in the opposite direction
PS that boat looks like its needs a diesel

 

That's right Powerabout. To me the gears look like they cannot handle much thrust in any direction, but Mercury offers CCW for their larger engines and they do have some extra washers the CW models do not have. And there is of course a $ 300 price difference.

 

Ok what is the right direction of rotation for what side of the boat ?? Does that make any difference or not .
What about toe in and toe out with twin out boards does that have any affect on the set up and performance as well ??????.

Not a trick question just interested to here your opinions and what works for you !!

 

According to Glenn L. Witt in his book "Inboard motor installations" the starboard prop should turn clockwise (seen from behind) because this prevents air being sucked under the boat from the sides.

That applies to inboard engines only. For outboards the shape of the underwater housing with 2 or 3 plates determines whether or not air has access to the prop.

A few degrees of toe in is usual to prevent chatter caused by play in the swivel bearings of the tie bar. I've experimented extensively with stern drives and couldn't determine any effects on performance or stability.

 

My 22' boat with twin outboards (non counterclockwise) came with 1/4" toe out and was tricky to steer on a steady course. Adjusted them to 3/16" toe in and all was well.
I've heard of using the wake as a metric to tune toe in but have no idea of how that works. Per Evinrude and Boston Whaler the deeper the vee the more toe in is needed.

post edited for accuracy

 

Every boat behaves differently and same rotation or counter rotation props makes little difference ! . Toe in also helps to raise the nose of any boat . The amount of toe in is a individual thing depending on the set up and the distribution of weight within the boat it self AND yes you do have to play a little to get it right for your needs an feel !.
I asked this same question with the installation we did recently of a pair of Volvo IPS units as well because they to are set 2 degrees toe in !! Everyone looked at me blankly and could not give any sort of answer or a reason for this !!!
Its done to raise the nose of the boat because you normally need to fit Trim tabs to counter the action if need be . Pushing the nose down is not ever a problem its getting it up that can be a big issue if things are a little out of balance .

Yes with outboards and racing them we always had toe in and the power trim angle was another interesting issue as the port side outboard would be trim in slightly more than the starboard outboard !! this would stop the hull from rolling un-naturally from side to side
Always interested to hear other peoples theories and reasonings for things they do .
Never stop learning !!!!
Because when you think you have all the answers and you open the other eye you will be in for a surprise !!

 

Ok another point of interest !! tie bars whats best and what is the difference. Tie bar at the front of the motor or tie bar behind the motor ?? :?: