counter rotating drives benefit

[261 Lib]

I have a 1987 Liberator with twin 454s and alpha outdrives. The drives both go the same way. My question is what gain would there be in performace (MPH) and fuel consuption if it had counter rotating drives? One of the local dealers told me up to 5 MPH. This sounds like a lot to me.
Anybody out there have tried this change and can comment.
Thanks

 

[Don S]

Re: counter rotating drives benefit

Shouldn't have anything to do with speed. Same gear ratio same size prop, just one drive is counter rotating to balance the torque of the other standard rotating drive. They aren't both counter rotating, only one.

 

[tpenfield]

Re: counter rotating drives benefit

Yea, probably the only loss in speed, etc. with twin std rotation is fightling the torque steer . . .

 

[r.j.dawg]

Re: counter rotating drives benefit

I have twin Alpha's and my port drive is counter rotating. I doubt I get any more speed than I would with two standard drives. The counter rotating helps at idle/no wake speeds in that the boat steers straight and backing into a slip is a little easier. Maybe your dealer meant Bravo III drives (twin counter rotating)

 

[HT32BSX115]

Re: counter rotating drives benefit

Maybe your dealer meant Bravo III drives (twin counter rotating)

Some times people refer to "Counter-Rotating" meaning "Contra-Rotating"

Wikipedia is your friend here......

Counter-rotating propellers generally spin clockwise on the left engine and counter-clockwise on the right. The advantage of such designs is that counter-rotating propellers balance the effects of torque and p-factor, eliminating the problem of the critical engine.

The above is an airplane description AND for the most part, the above benefits also work for a twin engine boat....

Counter-Rotating: look closely at the propellers below, one is clockwise and the other is counter-clock wise.....like some boats.




Again from Wikipedia....

Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque.

Contra-Rotating:

Welcome aboard by the way!! Another Lib!!!

I have a 1987 Liberator with twin 454s and alpha outdrives. The drives both go the same way. My question is what gain would there be in performace (MPH) and fuel consuption if it had counter rotating drives? One of the local dealers told me up to 5 MPH. This sounds like a lot to me.
Anybody out there have tried this change and can comment.
Thanks

I don't think Counter Rotating drives would help much except for maybe a little torque steer. I don't think you'll get a LOT of speed out of it.

However, I would LOVE to have a 454 '261 with TWIN Bravo III's .

The hole shot would be GREATLY improved!

My 211 Lib comes out of the hole like a Ski-boat..... more than twice as fast as it ever did with the original 460 King Kobra

With twin Bravo III's you would be amazed at the low speed control around the dock too!

It wouldn't be cheap though, to change your gimbal mounts and drives but If I already had a Mercruiser powered 261 I would start acquiring the stuff to do it!


Cheers,


Rick

 

[r.j.dawg]

Re: counter rotating drives benefit

Some times people refer to "Counter-Rotating" meaning "Contra-Rotating"

Contra-Rotating:

That's exactly what I meant.

 

[HT32BSX115]

Re: counter rotating drives benefit

I love that picture!

Not because it's mine, but because my then teenage daughter used The GIMP: GIMP - The GNU Image Manipulation Program and 27 still pictures she took while I moved the props a little each frame to create an animated picture!


Oh, and by the way, you would absolutely use a little less fuel at the optimum cruising speed with twin Bravo III's. you reduce your "SLIP" a lot with contra-rotating prop drives.

SLIP definition:

?Slip The difference between the theoretical distance the propeller should travel in one revolution and the actual distance the vessel travels.
For example if you cruise at 2000 rpm and your vessel has a 2:1 reduction gear, a wheel with 24" pitch, your theoretical speed through the water should be 19.74 knots (the distance a 24" wheel should move in one hour). In reality your vessel only does 14 knots at 2000 rpm on a calm day with no current, the difference is slip.

You can usually find the slip using a prop slip calculator. You MUST have accurate speed, RPM, drive ratio and propeller pitch. Marine Propeller Calculator


Cheers,


Rick

 

[261 Lib]

Re: counter rotating drives benefit

Thanks for all the replys. If I installed Bravo III, the drives would be worth more that the rest of the boat. lol The guy that told me this is well known in the go fast boats. He was a Canadian Champion quite a few years in boat racing so he knows his stuff I would think. Sounds like he may have given me the sales pitch?

 

[HT32BSX115]

Re: counter rotating drives benefit

. If I installed Bravo III, the drives would be worth more that the rest of the boat.

Um, SAD but probably true.

You would have to scrounge a couple of gimbals AND drives to make it happen. .......it wouldn't be cheap but it would be cool!

I am guessing you would want 1.81:1 ratio drives and maybe 28p props.

You would probably top out at 65 MPH or so with that combination and the hole shot would be impressive.

 

[jerryjerry05]

Re: counter rotating drives benefit

1987 454's and Apha Drives.
Worst combo Mercruiser made.
All good equipment just the wrong combo.
The Alpha wasn't really made for the big block.
Even though they made special gearing (1.32) they werent really strong enough.
If I had the unit I'd change it to counter rotating.
Just to reduce the stress that the drives have while both turning the same direction.
Also install drive coolers and the resivoir system for oil.

 

[261 Lib]

Re: counter rotating drives benefit

I'm sure your right about the drives being on the small side for the BB. They do look like they are the original drives on the boat. The guy that had it before me owned it for 15 years without any problems. If I do have any problems with the drives I would go counter rotating. These drives are fairly cheap compared to bravo 3s. I'm not the kind of person who abuses my own stuff. I've been told if I'm not to hard out of the hole I wont have any problems.

 

[HT32BSX115]

Re: counter rotating drives benefit

I'm sure your right about the drives being on the small side for the BB. They do look like they are the original drives on the boat. The guy that had it before me owned it for 15 years without any problems. If I do have any problems with the drives I would go counter rotating. These drives are fairly cheap compared to bravo 3s. I'm not the kind of person who abuses my own stuff. I've been told if I'm not to hard out of the hole I wont have any problems.

I have heard the same thing.

If I do have any problems with the drives I would go counter rotating.

Remember, "counter rotating" means left-hand prop on one side and a right-hand prop on the other side.

If you're talking about going to Bravo III's, then they're CONTRA-rotating. (minor point)

OTOH, If you needed to, you could replace both of your Alphas with a pair of SEI's for less than the price of ONE Bravo III!! Although I don't know if they do left-hand drives...... (to give you counter-rotating drives) you would need to ask them.


My dream 261 would be twin V-8 350hp VW TDI's and Bravo III's


I dream a lot though......:facepalm:

 

[261 Lib]

Re: counter rotating drives benefit

SEI does not make a 132 gear ratio drive. I would have to change to a 147 ratio. Probably would need to change the pitch of the props with a gear ratio change. Anybody know if Mercury still makes 132 gear ratio alpha drive?

 

[Don S]

Re: counter rotating drives benefit

The lower units are what is counter rotating, not the whole drive (or the uppers), and you can buy counter rotating lowers for your drive.
The uppers you can still get 1.32 gear sets for from Merc, but the 2 gears alone are about $700 per set.

That ratio Alpha drive has not been needed since the Bravo drives came out in 88, rather they are Bravo 1, 2, or 3.

 

[261 Lib]

Re: counter rotating drives benefit

Thanks Don S for clairifying the drive ratio questions for me. I also now understand where the gears are located that are a 132 ratio and where the counter rotation is located in the drive. Can a alpha outdrive with 147 gear ratio have the 132 gear ratio installed in it or vise versa or are they different units? Is it just the gears ratios that are different?

 

[Don S]

Re: counter rotating drives benefit

The Mercruiser MC-1, R, MR, and Alpha I drives can have the gears changed. None of this ever had a 1.47 ratio. That ratio didn't happen until the Alpha I Gen II.
I did check, and SEI uses the 1.47 on those models.
Will the Mercruiser 1.32 gear set work in an SEI upper housing? I have no idea.

http://forums.iboats.com/mercruiser...cellaneous-mercruiser-information-329915.html

 

[HT32BSX115]

Re: counter rotating drives benefit

You can probably get away with using 1.47 SEI drives behind the 454's in your 261 but you'll need to run the engines at a bit higher RPM than "normal" and maybe switch to "taller" props (higher pitch)

I would consider 24p 4 blade props in that case.

You'll get a good hole-shot and still top out at around 60mph or so... (you probably should still "mind" your throttle hand though)

 

[Tail_Gunner]

Re: counter rotating drives benefit

One would think having two drive's would greatly reduce the stress on each drive. While two engine's wont push a boat much faster due to prop speed and low slip....Two outdrive's are going to share the stress of the push which should greatly prolong the life. I am in the same situation with a single drive smaller boat and learning that one drive will last if it is not pushed hard on launch...the come to jesus moment happens when the drive leaves the water and renter's under power...aka when it goes airborn unload and reload reentering.

The larger gears mean smaller prop's which should lighten the stress load at launch..think about it...big prop churning water.. small prop churning..the smaller prop is going to neeed less tourqe every time. Prop size can really effect both high speed and low speed...a 4 blade...just one more blade than a 3 blade and a 3-5 mph loss is common. I think your in better shape than you realize.

Not as good as twin bravo's..but much better that a single alpha with a 454 behind it and the stigma that has earned..

 

[Don S]

Re: counter rotating drives benefit

Props, are not going to fix a gear ratio problem. Doesn't matter what type prop you use.
You might keep your WOT rpm where it belongs, but you are going to loose out of the hole performance or top end with the wrong one. In this case, you would loose out of the hole performance, and top end.
These manufacturers wouldn't come up with different gear ratios if just changing a prop would work as well.

 

[Tail_Gunner]

Re: counter rotating drives benefit

Props, are not going to fix a gear ratio problem. Doesn't matter what type prop you use.
You might keep your WOT rpm where it belongs, but you are going to loose out of the hole performance or top end with the wrong one. In this case, you would loose out of the hole performance, and top end.
These manufacturers wouldn't come up with different gear ratios if just changing a prop would work as well.

Well said his current gearing will allow the boat to both launch quicker and a higher speed at wot with less tourqe issue's. But the right prop can make as much as 5-7 mph...its all about slip on larger boats and there are many variables...big eared pusher...high rake and cupping ..4 blade vs 3 blade and whats works best with the hull. But at the end of the day slip will make the final determination