The object of this paper is to describe how anticollision simulation is employed at H.M.S. Dryad, the Royal Navy School of Navigation. The methods described are those used in the Navy but this paper has no specific military slant, nor do the anti-collision problems studied at Dryad spring exclusively from the naval environment. Ship safety is one of the fundamentals of seamanship where the naval and mercantile problem is the same; the Navy does not however suffer the same manpower limitations on board ship.