Impeller Placement on Counter Rotating Drive

[rcmpvet]

First off I have been lurking and learning for awhile now and have learned quite a bit. I up-sized to a bigger vessel this summer with twin 4.3 and Mercruiser Alpha Ones (1987). Being forever skeptical of former owners I decided this winter to remove and ensure that the stern drives are up to snuff. So far I found that either the previous owner or his mechanic were incompetent. (Fortunately my son is an automotive mechanic and he can help me with general parts and maintenance). So far we have found inverted thermostats on both engines and the worst paint job ever on the stern drives (painted over mussels and corrosion). (Note: I have put on about 15 hours on both engines since I bought and as the vessel was in the water I could not physically inspect the drives, but no problems other than the over heating and some other minor things).

That said I searched the Forums for this topic and can find no answer. In removing the impeller pumps I found both to be improperly installed in that the vanes on each one are going both ways, that is on each impeller half the vanes point clockwise (CW) and the other half point CCW, otherwise they are new looking, this is looking at the impeller when it is in the housing and you look into the bottom of the housing.

Now I know that in the starboard drive the impeller vanes should bend to the CCW position and that is with the drive in forward gear and turning the propeller CW. The port drive must be in reverse, hence the propeller will then also turn CW. What I have not found but what I suspect is that the port drive's impeller vanes must also have the same CCW bend.

Can anyone confirm this??

 

[Bondo]

Now I know that in the starboard drive the impeller vanes should bend to the CCW position and that is with the drive in forward gear and turning the propeller CW. The port drive must be in reverse, hence the propeller will then also turn CW. What I have not found but what I suspect is that the port drive's impeller vanes must also have the same CCW bend.

Ayuh,.... In the Alpha drives, Everything but the propshaft turns with the motor's crankshaft,.....

I believe you'll find the impellers should be identical, as they both turn the same way,.....

 

[Scott Danforth]

As bondo stated, I in your setup, one drive is running in reverse while one is in fwd. This is done in the lower. Both impellers will be the same

 

[smokeonthewater]

Further, the vanes of the impeller would instantly flop to the correct orientation when the motor was started.

Also, are you SURE you have a counter rotating drive???? Not saying it isn't but it would be pretty rare... Most are the same on both sides...

You would have two different props...

As stated above the CCW drive simply runs in reverse gear... The only difference is in the lower unit... Basically different bearings that will survive extended running in reverse... (Housings and etc are not interchangeable)

 

[thumpar]

The only time the impeller would rotate differently is if the engine was counter rotating.

 

[rcmpvet]

Thank you all for the confirmation.

Smokeonthewater - One point is that the vanes did not all flop to one direction and this showed after at least 15 hours running time by myself. (perhaps faulty impellers?). In case I am misinterpreting your comment and to confirm, yes the port engine does have a counter-rotating drive. "Also, are you SURE you have a counter rotating drive???? Not saying it isn't but it would be pretty rare... Most are the same on both sides...
You would have two different props..."

I have two different props as well. You say rare, but are not the majority of vessels with twin engines have the port side as a counter rotating drive, in that when in forward the propeller will actually turn CCW. Otherwise you would have both drives turning CW in forward and then the operator would be fighting the vessel's natural inclination to go right.

 

[JoLin]

Thank you all for the confirmation.

Smokeonthewater - One point is that the vanes did not all flop to one direction and this showed after at least 15 hours running time by myself. (perhaps faulty impellers?). In case I am misinterpreting your comment and to confirm, yes the port engine does have a counter-rotating drive. "Also, are you SURE you have a counter rotating drive???? Not saying it isn't but it would be pretty rare... Most are the same on both sides...
You would have two different props..."

I have two different props as well. You say rare, but are not the majority of vessels with twin engines have the port side as a counter rotating drive, in that when in forward the propeller will actually turn CCW. Otherwise you would have both drives turning CW in forward and then the operator would be fighting the vessel's natural inclination to go right.

Kevin, twin drives that rotate in the same direction is the rarity in my experience. In fact, I've never seen it. Mine are counter-rotating, too. The Volvo's in my last boat were counter-rotating, and 2 friends with twin-drive boats (one Merc, one Volvo) have counter-rotating drives in their boats. The props ARE different- mirror image.

My .02

 

[smokeonthewater]

I've had a couple and worked on several others (all merc alpha's) and have yet to come across a counter rotating alpha.

Yes I do know they exist...

Btw they all handle just fine... There is no fighting prop torque..

That said as long as the o/p has a counter rotating drive this is all irrelevant.

O/P I can't explain your pump vane issue without examining but my guess is that either the engines turned backwards a few degrees to cause some of the vanes to reverse or something weird going on..

I have repeatedly experimented with an impeller pump trying to find a condition where the vanes don't automatically orient themselves when installed backwards and they have always straightened out right away...
Either way looking down from the top both impellers will turn clockwise when the engine runs.