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Author Topic:   Twin Engine Counter Rotation
coolarrow posted 09-15-2011 04:37 PM ET (US)   Profile for coolarrow   Send Email to coolarrow  
What is the advantage of counter rotating propellers? I am assuming they cancel each other out and eliminate steering [unclear], but is there more than that? I am new to the twin engine outboard scene. I have an [Boston Whaler OUTRAGE 22] with twin Mariner 90-HP tower-of power engines. Thanks Jarrod
contender posted 09-15-2011 04:46 PM ET (US)     Profile for contender  Send Email to contender     
Cost more to fix the left handed one in the gears, Harder to fine left hand props, You are correct about the canceling each other out to help with the steering, torque of the boat. Can not really think of anything else, someone else will chime in...good luck
L H G posted 09-15-2011 05:24 PM ET (US)     Profile for L H G    
[Counter-rotating propellers on twin engines] eliminate two problems, which get worse as propeller hub size increases, which is why engines with the 4-1/4-inch propeller hubs don't offer counter-rotation, and those with the 4-3/4-inch prop hubs do.

Steering pull to starboard, or hard steering to port.

Starboard hull lift when underway, or in other words, hull lean to port. This can be considerable and uncomfortable, making the boat harder to handle.

For Mercury towers like you have, Mercury indicates that their Trophy Plus props almost eliminate propeller torque, which is the cause of the above. But I have found it not to be too bad with Laser II props either.

Tohsgib posted 09-15-2011 09:03 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
I have driven many boats with low-HP twins and NO CR. Personally unless you have the torque of a V6...FORGETTABOUTIT! My bud has twun 115 Suzuki 4 strokes. No problem.
jimh posted 09-15-2011 09:48 PM ET (US)     Profile for jimh  Send Email to jimh     
I have a great deal of time underway on classic Boston Whaler boats with either twin 70-HP or twin 90-HP engines and without counter-rotating propellers. I never noticed any unusual tendencies in the steering.

Steering torque can be compensated with the engine trim tab. See

http://continuouswave.com/whaler/reference/manual18-25/operations. html#trimTab

Boston Whaler boats tend to lean into the wind. You probably won't notice any leaning tendency unless you are running in very calm conditions.

jimh posted 09-15-2011 09:50 PM ET (US)     Profile for jimh  Send Email to jimh     
L H G proposes:

quote:
[Counter-rotating propellers on twin engines] eliminate two problems, which get worse as propeller hub size increases....

I don't think that propeller hub size is the real factor. I suspect that propeller size in general and engine power in particular are more of a factor. It just happens that lower horsepower engines tend to use propellers with a smaller hub. It is not the hub size that causes any particular problem with twin engine installations.

Jeff posted 09-15-2011 10:57 PM ET (US)     Profile for Jeff  Send Email to Jeff     
Yes, the counter rotation does cancel the torque out from one motor to the other. That said, I ran our 22 Guardian with twin standard rotation V6 Mercury 150's for 5 years and never noticed any ill effects from two standard rotating motors. The hydraulic steering easily overpowered any torque steer at speed. At slow speeds the boat never felt like it "crabbed" while trying to maneuver. We were turning 3 blade solas props and then moved to 4 blade turbos.

Now we have counter rotating Yamaha 150's on our 23 Walkaround Whaler Drive and the one thing I do notice is the wake "looks" more efficient.

macfam posted 09-15-2011 11:06 PM ET (US)     Profile for macfam  Send Email to macfam     
All I know is that none of the twin outboard boats off the coast of Northern Africa have crounter-rotation. This obviously causes the turbulance caused by props to turn the same direction.
I have also noticed that this is where all huricanes originate........coincidence?
I don't THINK so.......
jimh posted 09-16-2011 09:41 AM ET (US)     Profile for jimh  Send Email to jimh     
I think macfam is on to something.

In the case of the ancient Mariner 90-HP, counter rotation is not feasible. I don't think the engine was ever made in a counter rotation model.

Owtrayj25 posted 09-16-2011 09:56 AM ET (US)     Profile for Owtrayj25  Send Email to Owtrayj25     
I have owned and operated several boats with twin engine set-ups, both with standard rotating propellers, and counter-rotating engines. Generally, the effects of propeller torque can be overcome by adjusting the zinc trim anodes on lower unit of each engine. Adjusting the power trim of each engine will also enable the operator to overcome any lean of the boat while underway. Hydraulic trim tabs will also correct any unevenness of the boats operation. The only drawback to non-counter-rotating is in decreased maneuverability in tight docking quarters. Counter-rotating engines will spin on a dime.
Jeff posted 09-16-2011 10:38 AM ET (US)     Profile for Jeff  Send Email to Jeff     
I completely disagree that a boat with counter rotation props will spin more effectively on a dime than with standard rotation. Think about it, the standard rotation motors offset each other when one is in reverse and one is in forward because they are now counter rotating. This keeps the stern from crabbing from one side to the other due the prop torque. The props of counter rotating motors when put into the same forward and reverse situation will both rotate in the same direction. With our counter rotating Yamahas on our 23 WalkAround Whaler Drive I have found the counter rotation motors hinder low speed operation when trying to spin the boat on it's axis with one motor is forward and the other in reverse. Depending on which motor is in forward and which is in reverse the stern will crab one way or the other due to the props spinning in the same direction. It takes a lot more throttle jockeying on the 23 than with the 22 (twin standard rotation). Of course one reason is the Whaler Drive but, the other I feel is the counter rotating props.
coolarrow posted 09-16-2011 11:04 AM ET (US)     Profile for coolarrow  Send Email to coolarrow     
Love the vortice theory thing. I am in arizona and so far there have been no hurricanes. I will adopt the ancient mariner moniker for my twins. Thanks
Owtrayj25 posted 09-16-2011 11:09 AM ET (US)     Profile for Owtrayj25  Send Email to Owtrayj25     
That's really strange. My experience has been the exact opposite as that which Jeff describes. I had a older 25' Parker with twin 175 HP standard rotation engines. My docking arrangements were very tight at the time, and docking maneuverability with the standard rotation was terrible-no better than a single engine. I can walk my Outrage (Whalerdrive) with twin counter-rotating Merc EFI's into spaces with only inches of clearance without leaving the helm.

Trying to spin on a dime with the standard rotation engines was pointless...it just didn't happen without walking all over the place.

captbone posted 09-16-2011 11:52 AM ET (US)     Profile for captbone  Send Email to captbone     
The one thing that I really enjoy about two standard rotating engines was the stardardization between the two engines. One style prop, gearcase, linkage (depending on the engine), and even the engine itself. In the past OMC used to spin their engines backward for counter rotation.

In any commercial application, twin standard rotation is the way to go just from a cost, maintaince, and parts point of view. Counter rotation is a nice feature but not a neccesity.

On a side note, I have always found left hand props to be a lot cheaper. Most used ones sell for half the price of the right hand version. For the right price, I would not think twice about installing a left hand engine on my boat if that was only engine I was running.

Owtrayj25 posted 09-16-2011 01:13 PM ET (US)     Profile for Owtrayj25  Send Email to Owtrayj25     
Jeff's experience regarding counter and non-counter rotating twin engine set-ups got me curious...because my experience was so drastically different from his. I googled the topic and found links to several discussions whereby folks have shared their opinions on the topic, specifically low speed/docking operation. The responses ranged from not much noticeable difference to counter-rotating set-ups were far superior. But none of the opinions or experiences suggested favoring standard rotation twins for low speed operation/docking.

Here is one article I came across from THT;

http://www.thehulltruth.com/boating-forum/ 7712-boat-handling-dock-twin-counter-rotating-props.html


Tohsgib posted 09-16-2011 01:50 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Counter rotation outboards came out about the same time as VRO. OMC never ran their engines backwards, it was always done with the foot gearing. Mercury was the only outboard I know of that ran the their engines backwards and that was for reverse. There was a reason why the ole "Dock crashers" were only made a few years. Inboards on the other hand were designed to run backwards back in the days. I have never heard of an outboard engine doing so. If you decided to run your linkage backwards and use a LH prop, you can do that but the gears are cut the wrong way and it will last maybe 100 hours, probably less.
captbone posted 09-16-2011 02:01 PM ET (US)     Profile for captbone  Send Email to captbone     
OMC did run their outboards backwards in the mid 1980s to achieve counter rotation, the entire powerhead rotated the opposite way.

Certain engines would also require you to hook up the left hand outboard backwards at the linkage. The early 3.0 liter Mercury cases (mid 1990s) had to have the left hand reversed at linkage even though the gearing was different.

Binkster posted 09-16-2011 02:30 PM ET (US)     Profile for Binkster  Send Email to Binkster     
All mercury racing outboards from the 1950's at least the early 60's used left hand props. The reason for this was that race boats only turn left, and with left hand props there is more bite in the corners of the race course. Not sure about modern race motors though.
Tohsgib posted 09-16-2011 03:12 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Not according to my 1985 Evinrude brochure. It is in the foot.
captbone posted 09-16-2011 03:20 PM ET (US)     Profile for captbone  Send Email to captbone     
They didnt even offer counter rotation is 1985? When OMC did offer it in 1987 it was reverse rotation of the powerhead.
Tom W Clark posted 09-16-2011 06:08 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
OMC did have a reverse running motor or maybe a few. I know the original V-8s were offered that way, but whatever, the idea did not last long.

Getting back on topic, I happen to own a Whaler with twin outboard motors (1989 Mercury 150s) which were both standard rotation when I bought the boat six years ago. I converted the port motor to counter rotation after a few seasons by swapping out the gearcase for a used counter rotating one I bought used for $500.

When I made this swap, the shift cable had to be reversed at the control form push to pull and vice versa. This is true of most Mercury outboards but not all outboard gearcases work this way.

I can tell you there are several benefits to having the counter rotation pair.

- Steering torque is largely cancelled out. With both motors standard rotation there was a lot of torque to overcome at high speed. Yes, the hydraulic steering does not allow direct feedback, but it took two hand to turn to port at full throttle with the motors trimmed out during propeller testing. It was scary.

Remember, the trim tabs on the motors can only cancel steering torque if they are in the water. With higher motor mounting height and trimmed way out, they lift clear of the water's surface and do nothing except maybe produce a lot of spray. Which leads to the next benefit:

- You can loose the trim tabs on the motors and substitute the flat plate anodes. Less drag, less spray and no need to worry about adjustment or propeller blade clearance.

- With counter rotating motors, you do not suffer any list from propeller torque. The forces on the hull remain symmetrical.

- While it is perfectly true that a Whaler with standard rotation twins can turn on a dime (with one motor in forward and the other in reverse) just as easily as one with counter rotation motors, there is a nice benefit of being able to pull up to a dock and use one motor or the other to pull the stern into or away from the dock as desired. With standard rotation motors the tendency to always pull the stern to port.

- It is actually EASIER to find propellers for counter rotating twins, at least if buying used. Nobody sells pairs of standard rotation props, you have to buy a single prop...twice. It is a common myth that left hand props are somehow harder to come by or more expensive. If there is a right hand outboard pro, there is (with only a few exceptions) a left hand version. This is true of intermediate gearcase (40-115 HP) motors too (even if Mercury does not offer such a motor). Left hand props are difficult to sell by themselves so they are very often DIRT CHEAP and can be a real bargain.

- One additional benefit I discovered is that if the connection between your twin outboards fails, and one motor has no steering control at all, the boat is still operable and controllable at speed because the motors splay out in equal but opposite directions when accelerating. The steering arm extension broke off of my starboard motor the day before my wedding a few years ago and we were using the boat heavily as a ferry that week. With no time to deal with it, we just used it as is. It was amazing to see how the motors turned out when accelerating but then flowed back into alignment as the boat came on plane. This is actually one method of adjusting toe angle; you disconnect one motor and see where they settle for the speed and trim you normally cruise at, then reconnect the tie bar.

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