How much horsepower can a 71C or 72C handle?

Factory limits by model

The 73C, which most people never run into, is rated higher still: 560 hp gas / 400 hp diesel in the 1982 installation manual. See the 73C series page.

Duty classes

A flat hp number only tells half the story. The factory rates these transmissions by duty class:

• Pleasure: full throttle about 5% of the time, cruise at 80% of rated engine rpm or less, 400 hours a year or less.
• Light-duty commercial (LDC): full throttle about 5% of the time, longer running at 75% of rated rpm or less, 1,500 hours a year or less.
• Heavy-duty commercial (HDC): full throttle about 5% of the time, longer running at up to 80% of rated rpm, 2,500 hours a year or less.

The 71C's factory charts show no heavy-duty commercial rating at all. The factory rating charts (Form 1237) give each duty class its own input-torque rating, and the commercial ratings are lower than the pleasure ones. If your boat works harder than pleasure use, go by the chart for your class, not the maximums above.

Why the ratio and group matter

The factory doesn't rate every 72C the same way even at the same horsepower. Ratings are given as input torque, sorted by duty class and by assembly group:

• Group A: in-line 1:1, 2.57:1 and 2.91:1.
• Group B: in-line 1.52:1, 1.88:1, 2.10:1, plus several V-drive assemblies.
• Group C: only two V-drive assemblies, 10-05-000-003 (1.99:1) and -004 (2.49:1).

The selection rule underneath all of this: the engine, transmission, ratio and prop combination has to let the engine reach its maximum rated rpm. As a rule of thumb, figure about 100 prop-shaft rpm for every mph of boat speed you're targeting.

Older ratings (1980s)

The 1982 installation manual lists lower numbers than the 1988 table above: 71C 255 hp gas / 145 hp diesel; 72C 380 hp gas / 185-210 hp diesel; 73C 560 hp / 400 hp (the 1988 manual doesn't cover the 73C). Always say which document a rating came from. The two manuals don't agree, and neither one is wrong; they're just from different years.

Our experience

That's the practical version of everything above: match the ratio and the engine to the factory table before you commit to a repower, not after. If you're not sure which duty class your boat falls into, or whether your ratio and prop combination will actually let the new engine reach its rated rpm, call us before you buy anything.

Related reading

• Velvet Drive specs by series
• Velvet Drive 71C series
• Velvet Drive 72C series

Repowering and not sure your ratio still fits? Call (804) 333-3677, Monday through Friday 9 to 3 Eastern, or email sales@halemarine.com.