Repowering with a Velvet Drive: ratio, HP, rotation

The factory matching rule

Velvet Drive's own selection rule is simple to state and easy to get wrong: the engine, transmission, ratio and prop together have to let the engine reach its maximum rated rpm. If the prop and ratio don't let the engine get there, it runs overloaded its whole life. As a rough rule of thumb, figure about 100 prop-shaft rpm for every mph of boat speed you're after. That number gets you in the right neighborhood for picking a ratio; it isn't a substitute for a real prop calculation.

Picking a ratio

Every 72C reduction runs about 10 percent deeper in reverse than in forward (a 1:1 becomes 1.10 in reverse, a 1.88 becomes 2.07, and so on); the 71C's forward and reverse ratios are equal. The 1.88:1 and 1.91:1 reductions also turn the output opposite the input, which matters if you're matching a ratio to a specific prop hand. On the 5000/Liberty line, the ratio stamped on the tag is nominal, and the actual gear ratio is a little different: a "2.00" is actually 2.040, a "2.50" is 2.454, and so on. If you're calculating shaft speed for a repower, use the actual figure, not the rounded one. See our ratio identification page for the full table.

Horsepower and duty class

Velvet Drive rates each model for a maximum input horsepower, and that number depends on which document you're reading and how the boat gets used. As of the 1988 service manual, a 71C is rated to 310 hp gas at 4,200 rpm or 182 hp diesel at 3,200 rpm; a 72C is rated to 475 hp gas or 274 hp diesel. Earlier 1980s literature gives lower figures for the same models, 255 hp gas and 145 hp diesel on a 71C, 380 hp gas and 185 to 210 hp diesel on a 72C, so check which document and era a number came from before you use it. Ratings also depend on duty class: a boat run at full throttle only occasionally and under 400 hours a year (Velvet Drive's "Pleasure" class) gets more headroom than one run harder or longer. If you're repowering with more horsepower than the old engine had, check the transmission's rating against the new engine, not the old one.

And if you're overhauling the engine, overhaul the transmission at the same time. Chris put it directly to a customer doing exactly that: "You MUST overhaul the transmission if you are overhauling engine! A strong engine will hurt a older transmission." An older transmission that was fine behind a tired engine can wear out fast behind a fresh one.

Rotation and twins

Cast-iron 71C/72C/73C and 1004/1005 units take either engine rotation, set by the pump; the 5000 and Liberty only run behind a left-hand (counterclockwise-at-the-flywheel) engine, full stop. Twin installations generally want opposite-rotation engines so the boat doesn't pull to one side under acceleration. Two exceptions never get re-indexed in the field: the CR-2 (1013/1014) and the 2.10:1 in-line reduction, both of which have to stay at their factory pump setting. See our full rotation guide for the details and the Velvet Drive/ZF naming trap that trips up a lot of repower planning.

Bellhousing and damper

As of the 1982 installation manual, every Velvet Drive transmission that was current production at the time could be mounted to the same sized bellhousing, which is good news for anyone repowering into an existing installation. The damper is a different story and matters more than people expect. A knock at idle or low rpm that goes away by around 1,000 rpm or higher is usually the damper or the engine, not the transmission, and the wrong damper spring rate for the application wears either the damper's windows or its splines depending on which way it's mismatched. Damper selection is the engine or boat builder's responsibility, and Velvet Drive says plainly it isn't responsible for system-level torsional vibration problems. On a 5000 or Liberty repower, match the input shaft's spline generation to the damper type: current production uses a longer spline meant for rubber-block (Centa or Vulcan style) dampers, and putting the wrong damper on an early short-spline shaft can bottom the shaft out and damage the engine's own thrust bearing. We stock a dual-spring damper ($385.00) listed for big-block and small-block engines; call us to confirm it suits your input shaft before you order.

Walters V-drive setups

We've mounted a 1017 direct drive 71 series transmission to a Walters V-drive using a step shaft and a direct-mount adapter, a setup common in ski boats and wakeboard boats. On one of these, a brand-new pump and good reverse pressure meant the coupling to the Walters unit was the only thing left to check before it shipped. If you're planning something similar, the transmission side of that pairing is straightforward; the fit and alignment of the step shaft and adapter is where the work is.

1017-004 with Walters V-Drive adapter testing

Related reading

• Identifying your transmission and ratio
• Velvet Drive rotation explained
• 5000 series