Velvet drive gear ratios?

[hooch]

Prepping to haul out a new-to-me pearson 390, primarily for an overhaul of the old perkins 4.108. The 71c Borg Warner velvet drive has ended up with water in the fluid... I'm betting on a bad oil cooler and am replacing both. My question here is the gear ratios; currently direct drive 1:1, and am wondering if there is any advantage to changing it up to a 1:1.52 which I assume gives me more revolution at the prop for the same energy from the engine. Or is this only an idea to turn a bigger prop?

[rbk]

Is the 4108 water cooled or heat exchanger? The transmission cooler on most 4108 use the secondary (fresh water/antifreeze) if you use antifreeze and there’s water in the tranny it may be from elsewhere. It’s surprising you have a 1:1 ratio. Most run the 1:1.9/2.1 for larger displacement boats 30’ +. I would recommend moving up to the 1.9 if possible to reduce wear in your 4108. Most with that ratio turn average sized props 16-18” fyi

[DougR]

When describing and calculating gear ratios, by definition, it is the number of gear teeth on the output shaft gear divided by the number of gear teeth on the input shaft gear. For example, a 20 tooth gear on the output shaft divided by a 10 tooth gear on the input shaft would result in a ratio of 2. This would be written as a 2:1 REDUCTION ratio, that is, the input shaft would rotate two revolutions for each revolution of the output shaft.

Most marine transmissions are reduction transmissions. They reduce the number of revolutions of the prop shaft relative to the engine crankshaft revolutions. Occasionally there are situations that call for transmissions which increase the speed of the output shaft relative to the engine crankshaft and these are called step up gears or step up transmissions. In the above example if the number of teeth on the output shaft was 10 and the number of teeth on the input shaft was 20 then the output shaft would rotate twice as fast as the input shaft and the ratio would be stated as 1/2 or a 1:2 step up.

In marine use, reduction transmissions are the norm, because larger, slower turning propellers are generally more efficient at putting power into the water than smaller faster turning propellers. The amount of horsepower on a shaft is a function of shaft RPM and torque. If we have a given amount of horsepower on a crankshaft, and we run that power thru a 2:1 reduction transmission, we don't change the amount of horsepower (gearbox losses excepted) on the output side of the transmission, but we increase the torque by a factor of two. This increase of torque allows the engine to spin a larger propeller slower which is more efficient and provides more propeller thrust, hence pushing the vessel better.

In the earlier posted case with a 1:1 transmission, a change to a 1:1.52 transmission will in fact give more RPM at the prop, but that is entirely the opposite direction in which one should move. He should change to a 1.52:1 reduction, or even a 2:1 reduction to slow the prop down, produce more torque and turn a larger more efficient prop.

There are, however two considerations....
1. On a Borg Warner 71c transmission with a 1:1 ratio, adding a gear reduction housing will increase the overall length of the transmission case by about 6 or 7", so the engine will have to move forward, or the shaft coupling will have to move aft by that amount.
2. Adding a reduction gear will almost certainly require a larger propeller, and it might be impossible to do that with the available space. Equally important, it might require a larger prop shaft diameter because of the increased torque.

Something to think about....

DougR

[rbk]

Originally Posted by DougR

When describing and calculating gear ratios, by definition, it is the number of gear teeth on the output shaft gear divided by the number of gear teeth on the input shaft gear. For example, a 20 tooth gear on the output shaft divided by a 10 tooth gear on the input shaft would result in a ratio of 2. This would be written as a 2:1 REDUCTION ratio, that is, the input shaft would rotate two revolutions for each revolution of the output shaft.

Most marine transmissions are reduction transmissions. They reduce the number of revolutions of the prop shaft relative to the engine crankshaft revolutions. Occasionally there are situations that call for transmissions which increase the speed of the output shaft relative to the engine crankshaft and these are called step up gears or step up transmissions. In the above example if the number of teeth on the output shaft was 10 and the number of teeth on the input shaft was 20 then the output shaft would rotate twice as fast as the input shaft and the ratio would be stated as 1/2 or a 1:2 step up.

In marine use, reduction transmissions are the norm, because larger, slower turning propellers are generally more efficient at putting power into the water than smaller faster turning propellers. The amount of horsepower on a shaft is a function of shaft RPM and torque. If we have a given amount of horsepower on a crankshaft, and we run that power thru a 2:1 reduction transmission, we don't change the amount of horsepower (gearbox losses excepted) on the output side of the transmission, but we increase the torque by a factor of two. This increase of torque allows the engine to spin a larger propeller slower which is more efficient and provides more propeller thrust, hence pushing the vessel better.

In the earlier posted case with a 1:1 transmission, a change to a 1:1.52 transmission will in fact give more RPM at the prop, but that is entirely the opposite direction in which one should move. He should change to a 1.52:1 reduction, or even a 2:1 reduction to slow the prop down, produce more torque and turn a larger more efficient prop.

There are, however two considerations....
1. On a Borg Warner 71c transmission with a 1:1 ratio, adding a gear reduction housing will increase the overall length of the transmission case by about 6 or 7", so the engine will have to move forward, or the shaft coupling will have to move aft by that amount.
2. Adding a reduction gear will almost certainly require a larger propeller, and it might be impossible to do that with the available space. Equally important, it might require a larger prop shaft diameter because of the increased torque.

Something to think about....

DougR

Yes I expressed the ratio incorrectly. BW does not make a step up transmission for marine use or any other that I am aware.

[hooch]

Originally Posted by rbk

Yes I expressed the ratio incorrectly. BW does not make a step up transmission for marine use or any other that I am aware.

Appreciate the replies guys, I'm going with the 1:1 on advice of the mechanic so it'll be an easier swap. Maybe when the rugged old Perkins dies I'll look at other options