This is a boat, but it's missing some pieces. That's because it's really rusty, so parts are falling off of it and it looks rust colored. In ancient Chinese literature, this rusty red color represented sincerity, and you'll often see this kanji take that meaning in jukugo words.
The rust colored boat has inspired you, so you sit down next to it in the sun and tan (たん). You want to be the exact same rust color as the boat, and in a few hours you're sure you'll match.