The access policy to ports for deep-drafted ships making use of channels which are subject to waves, tides, currents, wind and other complicating factors can be based on either deterministic or probabilistic principles. ProToel is a calculation tool for determining tidal windows for marginal vessels arriving at or departing from ports located at the Belgian coast or in the Scheldt estuary. The paper provides an overview of possible criteria to be fulfilled for a vessel to have access to approach channels. The outlines of the software are described and applications for short-term decisions and long-term accessibility studies are discussed.