This method allows the dive professional to arrive at a fair estimate of the velocity, which translates into force (in the sense of a diver having to swim against it). Most knowledgeable aquatic experts estimate that a strong, powerful swimmer is able to maintain position (not gain position, but merely maintain) against a 1.5 knot current. SSI recommends that divers do not swim in a current faster than one knot. One knot is a speed of one nautical mile per hour which is roughly equivalent to 1.86km per hour.