To V Or Not To V
June 15, 2012 Share

To V Or Not To V

Now that’s a question. Are you better off with a stern drive or a V-drive?

Lately, there’s been a tricky choice facing anyone buying a fishboat or cruiser in the 28′ to 34′ range – a choice that stumps even the pros. With so many boats being offered with the new-generation V-drives, is there any reason to still consider an I/O?

You hear that stern drives are faster. But how can they be with more gears eating up power? Also, aren’t stern drives less durable in salt water? And what about fuel consumption, which directly affects range – something of more importance to a cruiser or offshore fisherman than pure speed.

Like you, we’ve had the same questions, but no clear-cut answers. That is, until Sea Ray gave us the chance to uncover the truth. The company recently added a V-drive option to its 290 Amberjack, a versatile boat that can be set up for both cruising and fishing. Originally offered only with stern drives, the V-drive model gave us the perfect opportunity to put the two propulsion systems to a head-to-head test.

We ran three 290 Amberjacks on the same day at the Sea Ray facilities in Knoxville, Tennessee, to ensure similar conditions. The stern drive boat was powered with twin 260-hp Mer-Cruiser 5.7L EFI Bravo Threes, which Sea Ray tells us is one of its more popular I/O packages. To match the stern drive’s power, the second boat had twin 260-hp MerCruiser 5.7L (carbureted) engines coupled with ZF 63-IV V-drives. The third boat, with a pair of 300-hp 350 MerCruiser MAG MPI Horizons and ZF 63-IV V-drives, was there in case it turned out that stern drives are faster than V-drives for the same horsepower.

GEAR HEADS

V-drives still get a bad rap for what they used to be – gear-mashing consumers of power. Older designs consisted of a transmission attached to an engine’s bell housing by a short shaft. This shaft was fitted with a universal joint that connected it to the input shaft of the V-drive. This meshed with the output gear on the end of the propshaft. The design was highly inefficient. But today’s integrated V-drive units, such as the ZF units on our Amberjacks, have the transmission and gears in the same housing and let a lot more power reach the prop.

Theoretically, a stern drive, which redirects the power through two right angles, should be less efficient than a V-drive, which reroutes the power only once. A stern drive connects to the engine’s flywheel with a short splined shaft that has a universal joint at its end. This lets you raise or lower the drive. Within the drive there is an upper horizontal shaft that has gears linked with those on a vertical shaft. Then the gears at the bottom of the vertical shaft connect to the horizontal propeller shaft. A stern drive loses about 13 percent of its power compared to 8 percent for the V-drive, according to Daniel Clarkston, director of product engineering at Mercury Marine’s MerCruiser division. So we wondered if the V-drives might do pretty well in terms of speed and fuel consumption.

by Eric Colby
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