I put Issuma on the tidal grid yesterday to replace the prop zincs. The tidal grid is a pier that comes out from a sloping shore, with beams laid across the bottom beside the pier.

The boat is brought to the dock at high tide, positioned so it sits correctly between the beams (ie, best not to have the rudder on a beam, taking the weight of the boat), then one waits until the tide goes out to work on the boat. In the picture, the tide is part-way out–Issuma is sitting on the beams, but the propellers are still underwater. The tidal range in Sitka is about 3m/10′.