When the Coho are feeding and you're fly fishing for them only a few things really matter with the fly type. Best not to fixate on only one type of fly being the god send.
Make it the size or a bit bigger than the bait they are chasing naturally. Make it ungulate if you can..ie trailers, hitches etc. Bucktails, lot's of bright blues, greens, blacks, mixed with some contrasting colors like pink/orange/red. Have some flashabou in there, metallics...
Some times just two contrasts work better though such as a baby blue/black etc. If your gonna cast and twitch retrieve you want some good weight up at the head. Let it sink twitch retrieve have the fly coming up and towards you after a bit of decent. But trolling in your prop wash and they are feeding I've had them smash so many different color combo bucktail fly types, tube flys, hitches, trailers etc.. Coho are voracious feeders when the bite is on. We've had them take fly's literally 5' behind the prop.
You want some killer fun, small skinny needlefish fly's on light gear stripped through the kelp beds while sitting still with engines off is a lot funner to me than the trolling prop wash fishing. Spend some time trolling for them first to find them then stop, motors off, and cast to them. Utopia