Mercury - Power Trim Sender Starboard-Side Digital Trim Feedback 891777A01. Overview. A trim sender might be small, but when it does its job right, the whole boat feels more predictable. The Mercury Power Trim Sender (891777A01) is the starboard-mounted sensor trusted on SmartCraft-equipped engines to keep your digital trim readings steady and honest. It's the kind of part boaters rely on without thinking about it-until the day the gauge stops moving and the ride stops feeling quite right.. This