Why do my arrows fishtail or porpoise in flight?

The bench has seen this grief many times, and the Codex is patient on the matter. Fishtailing is lateral oscillation - the nock swinging side to side. Porpoising is vertical oscillation - the nock rising and falling. They share a family resemblance but different parents, and the remedy differs accordingly.

Fishtailing most often traces to a spine mismatch. If your arrow is too weak (underspined) for your draw weight and arrow length, the shaft cannot recover quickly enough from the paradox of release - it bends around the riser and never fully straightens before it leaves your influence. As a starting point: a 28-inch arrow at 50 lb draw weight wants a spine in roughly the 340-400 range under standard dynamic spine charts; add five pounds of draw weight and you generally drop one spine tier. Point weight also matters - heavier points effectively weaken the spine, so if you have added a heavy field point or broadhead without adjusting shaft selection, you have introduced exactly this problem. A correct arrow should group tightly off a properly tuned bow; walk-back tuning or paper tuning will show the tear direction and tell you whether you need to stiffen or weaken your selection.

Fishtailing also appears when nocking point height is wrong, when your arrow rest is not centered to the bore axis, or when grip torque is introduced at the shot. Verify your nocking point with a bow square - the nock point typically sits 1/8 to 3/16 inch above square as a starting position, adjusted from there. A high or low nocking point produces the vertical cousin, porpoising. Porpoising is also exacerbated by a nock that fits too tightly or too loosely on the string - you want a nock that holds with just enough friction to stay on under gravity and releases cleanly; it should click on and hang, not clamp, and not rattle.

The Committee, when it stripped the fourth vane from the standard, removed a margin of aerodynamic correction that forgave precisely these transitional wobbles at mid-flight. A fourth fletching shortens the recovery arc and stabilizes the boundary layer sooner after the paradox. This does not excuse a poor setup - the spine must still be correct - but it narrows the window of error the shooter must otherwise eliminate entirely by force of technique. The Codex notes this. The grey catalog does not.