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With counter rotating props... how important is it to have similar gear ratios? for example, one side would be 1.88 ratio and the other would be 2.1 ratio. Can this be solved with more pitch in the 2.1 ratio? Is there a problem with different pitch and different rpm props on a twin engine boat?

What is every ones thoughts?

Also, how hard is it to convert to a zf or hurth gear from a velvet drive gear in a bert31? Opposite rotation gas engines are starting to drive me crazy!

Having different gear ratios on each engine will drive you crazy. The props will be spinning at different speeds making the boat go around in circles forcing you to increase the RPMs on one engine or pulling the faster engine back. The engines will not give you the nice smooth harmonic sound you get when the engine RPM's are in sync. Trying to sync them up by changing pitch will be a long trial and error and probably cost you more in time and dollars than replacing the transmission.

I assume you have gas engines. Which ones?

Replacing transmissions is not that difficult. If it is a 31, depending on how they are installed, you should be able to remove them without touching the engine. When I had the 454 Mercs we had to rebuild a transmission. It came out without touching the engine.

I toyed with the idea years ago and the prop shop told me NO WAY... too much difference between the ratios.

Sounds like you are toying with velvet drives. I think the 1.88:1 has an output that turns opposite the input (if I recall correctly unless it is the 2:1...I forget). I was thinking about using that one for the counter rotating prop.

Too bad velvet drive did not make 2 versions of that particular gear ration bc many conversions could have avoided getting new transmission. With that said the velvet drives prob will not hold up to the big torque and HP numbers that some of the high HP diesels muster (I could be wrong tho)


If you were not talking about Velvet drives than ignore my .02....


note: If you are looking to replace one of you gas engines to just make it non-counter rotating than the VD will handle the power obviously.

if you goto ZF Hurth it is not that big of a deal to replace. The ZF transmission may actually be a bit smaller than the VDs and lighter (depending on which you need). The ZF come in down angle and straight (which is what you would need).

ZF (and some others) do not know the difference between "forward" and "reverse". They just either turn in one direction or the other and can be run at full power in either direction.

The velvet drive on the other hand can only run in forward at full throttle. You can not run them in reverse or you will burn them up. On the velvet drive forward is when the output turns in the SAME direction as the INPUT. You can turn the pump to allow the input to turn in EITHER direction (to allow for the counter rotating engine) but at the end of the day the OUTPUT still needs to follow the input for your FORWARD running.

I know it is a little confusing but that is the way it is

dognduck wrote: ↑Apr 7th, '22, 12:47 With counter rotating props... how important is it to have similar gear ratios? for example, one side would be 1.88 ratio and the other would be 2.1 ratio. Can this be solved with more pitch in the 2.1 ratio? Is there a problem with different pitch and different rpm props on a twin engine boat?

What is every ones thoughts?

It's really important. Yes the boat will move and run, but getting the motors in sync, without one lugging will be a challenge at best. How do you know if one motor has an issue as they are not evenly loaded. If ones overly loaded your cutting down on its life. I've always used the performance of one motor to judge the other...with two different wheels they are no longer a matched set making the even go out the window.

dognduck wrote: ↑Apr 7th, '22, 12:47Also, how hard is it to convert to a zf or hurth gear from a velvet drive gear in a bert31? Opposite rotation gas engines are starting to drive me crazy!

The hardest thing is paying for it.

I have to ask- - why are you thinking of doing this? What are you trying to accomplish? Why not just use matching gear ratios? Not available?

Carl, I believe he wants to replace the engine that turns opposite with a standard rotation engine. Velvet drives are NOT capable of accomplishing this. Velvet drive made one gear ratio (1.88:1 or close to that) that the output spins opposite the input for forward and reverse running. They did not make a normal drive transmission in the same ratio.

I only chimed in here for future information on this type of topic.

Hopefully the original poster will chime in

I'm thinking if your going to go through the aggravation and cost of changing a motor as the counterrotation is a pain...you might as well change the gears to VF's.

They are not all that costly...in the scheme of changing a motor anyway.

I believe there is a demand for the velvet drives and can be sold.



I'll bite, what is the pain with counter-rotating? I never gave it much thought except for having starter rebuilt and the guy put wrong Bendix gear in...and that motor was obnoxiously tough to get starter in and out. And the timing always had me thinking when trying to set.

Here is why I ask. Currently have two freshwater cooled carbureted 5.7 sbc that counter rotate with velvet drives c71 1.5 ratios. My engines are on borrowed time. I really like gas engines because I can always make them run my self and do 95% of the work my self, which I really enjoy doing. with counter rotation, I and limited in what I can do in term of building new engines and parts. I am also worried that parts for counter rotations are going to get harder and harder to get. For example, I am limited in cam shafts with counter rotating. I don't think there is even a roller cam for counter rotating. I cant go LS based with counter rotating... So I found one velvet drive that will have output the opposite direction as input, but i cant match the ratios. I would prefer to have the zf63a, but not sure how much surgery I would have to do to the boat to get them in. Its a stock 15* angle shafts, length's and 1 3/8" diameter. 15inch pitch props. I really want to stick with sbc or LS and have roller cams. I don't want to go big block or diesel. Any thoughts?

I think one alternative would be to leave the standard rotation 1.5 VD in place and only replace the other side with a new transmission in a 1.5:1 ratio.

This way you only need to buy one transmission and deal with any issues of shaft length etc on one side only.

I went thru the same thing when I put the cummins in my boat. I bought the velvet drive 1.88 but it was too different and ended up buying 2 new ZF transmissions.

In my case the Cummins are very tall engines and I really needed the down angle transmission that I got with the ZF. You dont have that prob and can really just deal with the one side only. Yeah it might be a little Frankenstein but it will work fine

My vote is to Change the trans to match the ratio you have.

ZF come in different down angles including none, as in straight like your BW. You'll most likely need to modify your mounting location...not a big deal. Take a look at other boats with gas and ZF trans

If so, its a matter of shaft length. If too long a quick cut and rekey of your shaft. If too short new shaft OR a spacer between couplers can be made. I'm not a fan of doing that, but it is an option to save.


For what its worth...I liked my gas motors too. Absolutely nothing wrong with staying gas, so no need to justify your thoughts. I was set to keep mine but they needed to be pulled to address a rear main seal leak. Needed new hoses. While out it would be good to check heads, bottom end, distributors etc etc. It was at that point I thought maybe dumping good money into 40 year old 440's wasn;t a good idea. But then what...used 454's? Pull good motors to install someone's headache. New gas??? Spend alot for new motors and boats value doesn't change.
That is where I was when a pretty good deal on diesel came along. AKA- I could swing the conversion with my limited budget.
If not, my gas motors were happy, old, kinda tired but happy and dependable.

There are counter rotating GM based engines that use all standard parts except the timing chain is replaced with gears, and the crank runs opposite. Everything else is standard rotation, except the starter. The only other oddity, is you have to run the plug wires in the opposite direction around the cap. Same firing order, but other direction.
I think its mercruiser parts.
Then you can use any parts you want and make either engine counter rotating just by swapping the timing set.

Geebert wrote: ↑Apr 26th, '22, 07:15 There are counter rotating GM based engines that use all standard parts except the timing chain is replaced with gears, and the crank runs opposite. Everything else is standard rotation, except the starter. The only other oddity, is you have to run the plug wires in the opposite direction around the cap. Same firing order, but other direction.
I think its mercruiser parts.
Then you can use any parts you want and make either engine counter rotating just by swapping the timing set.

I found the gears at a website called "bpi ebasicpower". Looks like they have them for any gm engine.

I dont know how to post a link yet either

https://bpi.ebasicpower.com/shop/

Geebert,
At the top of your page you will see something that looks like a chain link.
That is the link a url button.

Push that and you will see url boxes.
Put your web address between those boxes.

Bruce wrote: ↑Apr 26th, '22, 13:25 Geebert,
At the top of your page you will see something that looks like a chain link.
That is the link a url button.

Push that and you will see url boxes.
Put your web address between those boxes.

Ok,
Just like posting pictures.
Thanks Bruce.