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November 04, 2008

The Skinny on Jack Plates

Should every skiff come with a jack plate as standard equipment?

If you run any kind of skiff in shallow water, this column just became a must-read!

Lots of people get by without jack plates - those add-on  brackets that provide two more adjustability dimensions to your outboard engine trim, straight up and down. Are you one of those people who ask, "Why bother?"

The jack plate accomplishes several significant functions:
It enables you to run in shallower water than a standard engine mount.

It allows you to get up on plane in shallower water than a standard engine mount.

When trimming an engine for best  performance, a jack plate affords you better fuel economy thanks to channeling thrust in the most efficient direction.

Without a Jack Plate
When you're in shallow water, your propeller represents the deepest part  of your boat. Whether you're already running on plane or need to get up on plane from a standstill, your likely deterrent stems from the prop and skeg hanging below your hull.
When you're ready to throttle up in shallow water - without a jack plate - the general procedure involves "spinning" your boat up onto plane. This process consists of trimming the tabs all the way down, turning the wheel hard over and jamming the throttles forward. The thrust from your prop pushes it up and away from the bottom. Once you build momentum, you straighten out the wheel and (hopefully) continue on your way at speed.
Simply trimming up your drive actually hinders performance, as it alters the direction of your propeller's thrust to an inefficient angle. Additionally, tilting your engine degrades your performance as the water approaching the prop gets blocked by the cavitation plate as the angle of the engine changes outward.
A better way to "shallow up" your prop and skeg is to lift them straight up rather than tilt them. Mounting your outboard to a hydraulic- or electric-powered jack plate affixed to the transom of the boat allows you to lift your engine vertically rather than change the angle of thrust.

How Does It Work?
On standard engine mounts (with no jack plate), the outboard must be mounted low enough so as not to cause cavitation when running into chop or executing turns. In calm water on a straight course, it could be mounted higher, but when it's bolted tightly to the transom, moving it up or down obviously can't be accomplished. That's where a jack plate comes in.