Existence of frequent traffic of convoy in the area between Porto Alegre and Rio Grande, not issued by Notices to Mariners. Characteristics of the convoys: maximum length of 211 meters and estimated speed of 4 knots. Mariners should be aware of the area. Small ships should be kept away from large vessels when sailing in narrow channels, even if they have a preference in traffic. The movement of vessels in narrow and shallow channels causes swirls and abnormal water effects that can result in collision of ships with propellers.