A terrestrial beetle of unkempt grassland, the Sailor Beetle is less common than its close relative the Soldier beetle. Both species feature on the trout's menu during the summer and autumn months, when aquatic insect life is sparse and the fish have to rely heavily on terrestrial insects for their food.
The Welsh Coch-y-Bonddu Beetle (actually an imitation of the Garden Shafer Beetle but recommended as a match for the Sailor Beetle too, provided it is not tied on too large a hook) is an effective imitation of this little insect. A size 14 hook is a good size match for the natural Sailor Beetle, and in the turbulent waters of upland torrents don't worry unduly if the fly becomes waterlogged and sinks, so long as you know where it is and can either see the rise or detect the take via leader movement.
For more information about the Sailor Beetle, and tying details of suitable artificial flies to imitate it, see Pat O'Reilly's guide to Matching the Hatch.
These beetles are not good swimmers, and so when fishing with an artificial fly that is intended to imitate the Sailor Beetle, simply twich it occasionally as it floats on the surface; there is no need or justification for any kind of 'retrieve'.
O'Reilly, Pat. (1997; 8th reprint 2010) Matching the Hatch. Shrewsbury: Quiller Publishing.
Foster G. N. & Friday L. E. (1988) Key to adults of the water beetles of Britain and Ireland (Part 1). Taunton: Field Studies Council.
Harde K.W. & Severa F. (1984) Field Guide in Colour to Beetles. Littlehampton Book Services.