Part 2: There’s an optimum running attitude for most vessels.

By Capt. Bill Pike — June 2002

Illustration: Jeff Grunewald  More of this Feature • Part 1: Trim Tabs • Part 2: Trim Tabs • Part 3: Trim Tabs
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Let's start with the basics: a couple of hinged panels mounted on either side of centerline, most commonly along the after extremity of your boat's transom. Whether we call these babies trim planes, after planes, flaps, or, most commonly, trim tabs, they typically move up and down via electro-hydraulic rams or actuators, although wholly electric, screw-type, mechanical actuators are increasingly prevalent these days. About the simplest action we can perform with these tabs is to manipulate them down equally, well below the level of our boat's running surface, via our dashboard switches. This increases the water pressure on the underside of each tab, which makes the stern rise, the bow drop, and the boat continue forward at a more moderate angle of attack.

There's an optimum running attitude for most vessels at speed, often somewhere between four and six degrees. Take a sportboat designed to run at approximately four degrees, load her up with passengers and gear in such a way as to increase the running attitude to eight or nine degrees, and all sorts of sorry things start to happen. With the bow higher than it should be, draft increases at the transom--never a good thing--and the boat plows inefficiently through the water rather than riding on top of it, which burns extra fuel needlessly. Moreover, visibility from the helm begins to suffer or disappears completely, and with the prop or props angled upward instead of relatively horizontally, a propulsion snafu results: increased slippage. Any prop that interfaces water at a substantial angle travels a more turbulent and compressed path. The greater the angle, the more turbulence and slippage. Deploying tabs can eliminate all of these problems.

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