Borg Warner velvet drive question

** I have a velvet drive hooked to my Ford Lehman 120, it is a 2.57 to 1 ratio; the first question; is the round part on the aft end the reduction gear?**can the direction of rotation be reversed? I would like to put a 4 blade prop on and I can find right hands but not left hands. Currently there is a 26" dia by 19" pitch 1.75 shaft left hand propeller on her. Thanks, Ted

* www.kp44.org/ftp/VelvetDriveInstallationManual.pdf* This file answers the question about rotation. Ted

You are correct, the direction of rotation can be reversed, but it's not recommended as I remember. It's been a long time, but as I understand it, the reverse gear is only rated for intermittent use................Arctic Traveller

Someone explained to me that in a Velvet Drive there is only one set of drive plates or clutches or something for reverse gear but several for forward. Hence the reason for not asking a lot of a Velvet Drive in reverse gear.

There is an extra gear in the Velvet Drive with an output shaft that turns clockwise (when viewed from the rear). We have one of these on the starboard engine since the starboard prop has to turn the opposite direction of the engine. While the port and starboard Velvet Drives in our boat look the same physically, the fluid connections for the fluid heat exchangers are in different places on each one. Both transmissions are original to the boat from the model and serial numbers in the owners manual. Whether these different hose connections are due to the differences in the gear trains between the transmission is something I don't know.

Reversing the direction of an existing Velvet Drive by installing the proper extra gear on the output side of the transmission sounds like something perhaps best left to, or at least asked of, a transmission professional.

-- Edited by Marin on Sunday 21st of November 2010 05:44:19 PM

awpptdt wrote:

I have a velvet drive hooked to my Ford Lehman 120, it is a 2.57 to 1 ratio; the first question; is the round part on the aft end the reduction gear?**can the direction of rotation be reversed? I would like to put a 4 blade prop on and I can find right hands but not left hands. Currently there is a 26" dia by 19" pitch 1.75 shaft left hand propeller on her. Thanks, Ted

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I think we should ask you first why you want to change the prop. Your question about the Velvet Drive is a solution to some problem you are trying to solve.
If your Lehman engine is turning up full rpm under load, then your prop is correct.
No load engine RPM for the Lehman 120 should be 2650 rpm and under load is 2500.
Going to a 4 blade is still going to require you to meet those specs, so the pitch of the blade has to achieve 2500 rpm at full throttle.
R.

*

It sounds like you are trying to find a used prop.

If you are going new you should be able to get what you need. As R.Y. pointed out though the new prop must be matched to the engine and boat so it can still turn up the revs.

* Thanks to all for the answers, I've been real busy as to my late reply. I have been told the four blade would be more efficient. Currently the three blade performs just fine. I was also told by the prop shop that I should drop an inch in pitch going to a four blade. Thanks again.

If you are having the prop shop do something for you be sure you tell them you want to end up with 2500 RPM at full thottle in gear.

No load engine RPM for the Lehman 120 should be 2650 rpm and under load is 2500.

You also have to calibrate your tachometer before doing this prop test so you know exactly that you are measuring it correctly.

"I have been told the four blade would be more efficient."

According to Dave Gerr's book that is not true. 3 blade is more efficient than 4 blade.
But I am sure more than one theory exists on propeller efficiency.
Just make sure you do as Ralph suggested and calibrate your boat's tachometers before you make any changes...

awpptdt wrote:

I have been told the four blade would be more efficient.

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According to the prop shops we've talked to, as well as people in the marine engine inustry we know, the fewer blades a prop has the more efficient it will be. The most efficient prop would have one blade but it'd be a bitch to balance * So assuming the same diameter propeller and optimum blade design, two blades will be more efficient than three blades, three blades will be more efficient than four blades, and so on.

But, the more blades there are, the smoother (or less vibration) the prop will be.* And there may be some thrust advantages with a higher number of blades.* But in terms of basic efficiency, a three bladed prop will be more efficient than a four bladed prop of the same dimensions and turned at the same rpms.

Also, a three-bladed prop will back better (less propwalk) than a four bladed prop of he same basic dimensions.

We were going to change from four blades back to three blades (which out boat had when built) but when we learned our four bladed props were still good we kept them and had them completely reworked and repitched down.* Saved us several thousand dollars over going to new props, but the four-bladed props are not as efficient as the three bladed props would have been.* However they generate more propwalk which in a twin-engine boat is an advantage when maneuvering.

"I have been told the four blade would be more efficient."

Efficient can have a lot of meanings.

For slow cruising a 2 blade would be even better than the 3 blade , IF you could swing the prop diameter .

IF your current prop is not loading the engine properly at Flank, you may gain a bunch more Knots , if that is your "efficiency".

Yes, there were 1 bladed aircraft props , way back , with a balance weight , but a big diameter is easy on a small aircraft with a puny motor.