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Blade Runners

Interceptors can sharpen up a boat’s performance and efficiency.

For decades trim tabs have been the popular way to adjust a boat’s running angle. They work well on boats up to about 45 feet but on larger boats, you need some seriously big plates, which can create drag and don’t always deploy quickly. That’s why lately many manufacturers are turning to interceptors, such as those manufactured by Humphree and Naiad Dynamics. These blades mount to the transom and drop down vertically into the flow of water coming from beneath the hull to create lift without producing the drag of conventional tabs.

[Interceptors are] an upgrade because you don’t add surface friction [like you do] with trim tabs,” says Dave Newcomb, engineering manager at Sabre Yachts in Casco, Maine. “They’re more sophisticated, and they’re precise.”

Sabre uses Humphree’s 850-mm-wide (about 33½ inches) Interceptors on its 54 because the Volvo Penta IPS II pods that are standard power on the boat don’t have integrated tabs. They are mounted in the same position as trim tabs. The standard series lowers two inches while the extended series drops three inches. The farther the Interceptors extend, the more lifting force they generate.

They create more lift efficiently with less drag,” explains Greg Platzer, owner of Platzer Marine in Fort Lauderdale, Florida, the exclusive U.S. Humphree distributor. Platzer says that because they can be custom designed, Interceptors can be installed in locations where trim tabs don’t always work, such as at the aft end of a propeller tunnel. They can also be retrofitted to a boat that has conventional trim tabs or had none at all. While some critics question the effectiveness of interceptors at slower speeds, Platzer says he has installed them on trawlers where they work just fine.

Because Humphree Interceptors are made out of composites, there’s no corrosion and no need for sacrificial anodes. Electrically powered servos—either 12- or 24-volt—inside the boat raise and lower the blades. Interceptors come in two sizes: standard, for boats from 45 to 150 feet in length, and extended for boats 60 feet and larger.  Both 2.5- and five-inch shafts are available to accommodate different transom thicknesses. Naiad Dynamics offers its own verson of interceptors called Active Motion Interceptors that deploy in a similar manner. Volvo Penta has licensed the Humphree technology to produce a version for smaller boats.

Humphree Interceptors come in various levels of sophistication. The basic system offers manual control and comes with an LCD display that provides trim and list angles. The Automatic Trim Optimization System (ATOS) lets you preset up to five blade positions for automatic operation. The next step up is the Bi-Trim Optimization System (BTOS), which automatically corrects for a boat’s listing. Set the target list angle, and the system adjusts the blades to maintain that orientation.

If you want to correct for heel and list, you need CTOS (Coordinated Turn Optimization System), which continuously monitors rudder position and adjusts heel by positioning the Interceptors to ensure a comfortable running angle during turns. For total control, the Humphree ACTIVE ride-control system automatically responds to pitch and roll motions, as when the boat is in rough conditions. Naiad offers similar control with its DATUM system.

One downside is price. Platzer charges about $7,000 for the basic Humphree system, which includes a pair of standard blades and everything you need to operate them. Upgrading to ATOS adds $2,000 while BTOS adds $4,000. The Volvo TS-450, which is designed for twin-engine boats up to 50 feet, retails for $799.94 on the company’s Web site. To compare, you can find Lenco electric trim tabs for boats up to 80 feet at West Marine for around $2,000. But for the reduced drag and improved efficiency, some people will consider interceptor blades as a worthwhile alternative to conventional trim tabs.

Humphree (+46) 31-744-3577.

Naiad Dynamics (203) 929-6355.

Platzer Marine (954) 377-0124.

This article originally appeared in the February 2012 issue of Power & Motoryacht magazine.