Velvet Drive rotation: LH vs RH engines explained

First, the naming trap (Velvet Drive vs ZF)

Before anything else: Velvet Drive and ZF describe the exact same engine rotation with opposite names. An engine that turns counterclockwise at the flywheel is what Velvet Drive calls a left-hand engine. ZF's own paperwork calls that identical engine right-hand turning. Nobody's wrong here, they're just two manufacturers using opposite conventions for the same physical rotation, and mixing the two up causes a lot of confusion on the phone. If you're reading a ZF spec sheet and a Velvet Drive manual side by side, translate the vocabulary before you translate anything else.

Cast-iron 71C/72C/73C

The cast-iron in-line units, 70C, 71C, 72C and 73C, can run behind either engine rotation. What changes is the pump: turn it 180 degrees and the arrow on top, which should point the same direction the flywheel turns, now points the other way. An odd-numbered assembly (a "C" unit, like 10-17-000-001) is built for clockwise input, and an even-numbered "CR" unit for counterclockwise, viewed from behind looking forward toward the engine. That number is the factory setting, though, not necessarily what's in the case today. A unit may have been re-indexed since it left the factory.

Re-indexing the pump only changes which rotation the transmission accepts. It doesn't reverse the output relative to the input. Forward output still follows the engine on a straight 1:1 or most reduction ratios. The exceptions are the 1.88 and 1.91 reductions and the 73C's 2.00, covered below.

Changing Pump rotation on 1017, 1018,1004,1005, 1013, 1014 transmissions

The CR-2 and 2.10:1 exception

Every cast-iron in-line unit can be re-indexed for either rotation, except two: the CR-2 (1013/1014) and the 2.10:1 in-line reduction (AS3/AS13: 10-17-000-009/-010 and 10-18-000-007/-008). Velvet Drive's installation manual says CR-2 units and 2.10:1 in-line units must stay at their factory pump setting. On the 2.10:1 the planetary parts are different for each rotation, and the factory didn't cover failures from re-indexing one. If you've got a CR-2 or a 2.10:1 and the rotation is wrong for your engine, call us before you touch the pump.

1.88 and 1.91 turn the output around

Almost every ratio sends forward output the same direction as the engine. The 1.88:1 and 1.91:1 reductions (Velvet Drive's AS7 and AS17 units) are the exception: they're counter-rotating by design, so the output turns opposite the input. In Chris's words, replying about a pair of AS17-72C 1.91:1 units: "1.88 and 1.91 give opposite of input....left input equals right hand propeller." That's specific to those two ratios (and the 73C's 2.00). It isn't a rule for the rest of the lineup.

1004/1005 V-drives: gear vs chain

The cast-iron V-drive (1004/1005) also takes either engine rotation through the pump, the same as the in-line units. What's fixed at the factory is the relationship between the engine and the output: a gear-drive V-drive turns the output the same direction as the engine, and a chain-drive V-drive turns it opposite. That's specified when the unit is built, and it's how twin-screw boats with V-drives can run two same-rotation engines and still get opposite-turning props. Forward is toward the engine on the shift lever for all 1004/1005 units, regardless of which build you have.

5000 and Liberty

The 5000 and Liberty (2001/2002/3001/3002) are a different animal. They only run behind a counterclockwise-at-the-flywheel engine, what Velvet Drive calls a left-hand engine, the same one ZF calls right-hand turning. There's no pump to re-index. What is fully flexible is which way the propeller turns: the unit is full reversing, meaning it delivers the same capacity in forward and reverse, and which lever direction you rig as forward sets the prop's rotation. Chris confirmed this directly when asked whether you set prop rotation on a 5000 at the shift cable rather than at the pump like a 71 or 72: "Yes."

Chris makes the same point in his own words: "The ZF's and the 5000 Velvet Drives only work for left hand engines. They are bidirectional. In other words, they have the same capacity in forward then reverse." Remember the naming trap above: that's Velvet Drive's "left hand." A ZF document describing that same engine would call it right-hand turning.

Twin-engine boats

Twin-screw boats generally want opposite-rotation engines, so the boat doesn't pull to one side under acceleration, a reasoning that goes back to the earliest days of reversible marine gearboxes. Re-indexing matters here because one cast-iron transmission design can be set up for either engine in an opposite-rotation pair: each box gets set to match its own engine, not to change which way the pair turns relative to each other. If you're stuck with two same-rotation engines instead, re-indexing the pump can't fix that by itself. That's what the CR-2's counter-rotating drop-center gear and the 1004/1005 V-drive's gear-versus-chain build, both covered above, are actually for. Some owners try a different workaround for a rotation mismatch: leaving one transmission in reverse so that engine pushes the boat forward. Don't. Velvet Drive warns that running an in-line box in reverse for forward causes early gear failure, and as Chris puts it, "you will cook your transmission."

What the pump flip doesn't do

It's worth repeating because we hear this one a lot: the pump setting only changes which way fluid moves inside the transmission so it can build pressure with that engine's rotation. In Chris's words, walking through a pump on camera: "The pump setting is only to move the fluid within the transmission. It does not change the output shaft direction... It only allows the fluid to pump." Nothing about the pump setting changes forward and reverse at the shift lever, or which position drives the boat forward. On a cast-iron unit, forward only happens with the lever in the forward position. As Chris put it plainly to one owner: "The transmission is not a Bi-directional transmission. Mean forward can only be engaged in the F forward position."

Related reading

• Changing pump rotation
• Running reverse for forward
• CR-2 series
• Velvet Drive vs ZF/Hurth