Dumb question.. transmission gears

[IRGuy]

This may be a dumb question, and in asking it I will reveal my ignorance.. but..

Is the reverse gear in most marine transsmissions the same ratio as the forward gear?

[scot]

It depends on the transmission. Typically yes, however different manufactures have different reverse capacities. The Twindisc and most ZFs are 100% capacity in forward or reverse. This affords a single engine rotation for both engines...ie one gear is always running in reverse to achieve counter-rotation at the props.

The Borg warner is not built this way. The Borg Warners cannot run in reverse as the primary driving gear without destruction, the reverse clutch packs ar too small (so I've been told)

[Mikey]

Think it would have to be the same as in most cases the engines rotate in the same direction and the props in opposite directions. Therefore one transmission is running one way and the other tranny the other way.

[Harv]

I can't remember exactly where I saw this, but for some reason it sticks out in my head. The Hurth & Twindisc, being able to run with the same direction motors, had a slight variance between the 2 trannies. If the starboard was rated at 1.56 to 1, in order for the port engine to go forward, there was an extra gear to facilitate that. With the extra gear, I think the ratio was offset on a factor of .02,(thereabouts). Therefore, starboard would be 1.56 to 1 and port may be 1.58 or 1.54 to 1. Not sure if this is correct, but it was placed in my head from somewhere.

[mike ohlstein]

I can't remember exactly where I saw this, but for some reason it sticks out in my head. The Hurth & Twindisc, being able to run with the same direction motors, had a slight variance between the 2 trannies. If the starboard was rated at 1.56 to 1, in order for the port engine to go forward, there was an extra gear to facilitate that. With the extra gear, I think the ratio was offset on a factor of .02,(thereabouts). Therefore, starboard would be 1.56 to 1 and port may be 1.58 or 1.54 to 1. Not sure if this is correct, but it was placed in my head from somewhere.

Correct

[IRGuy]

Thanks guys.. I suspected the ratios were the same, but was not really sure. The reason for my asking is that my B33 with 315 HP Cummins 6Bs runs about 1 knot per 100 RPMs with a trans ratio of 1.5 to 1. With an idle of 700 RPM that means a minimum speed of 7 knots when in gear.. I wish it was slower in reverse.

[mike ohlstein]

It WILL be slower in reverse, unless you have some new fangled, variable pitch propellers, and a double ended boat.

I would be shocked if you make more than 3 knots.

[IRGuy]

Mike..

Thanks again!

I was going to ask about the efficiency of a prop in reverse vs in forward, but I figured one dumb question a month is about all anybody should be asking.