For accurate under-keel clearances, what must you do to gauge the reading?

To ensure accurate under-keel clearances, it is essential to add the depth of your keel to the charted depth of the water. The reason for this is that the charted depth only reflects the water depth above the seabed, not accounting for the vessel’s draft, which is determined by its keel depth.

Adding the keel depth ensures that you have a complete understanding of how much water is beneath the lowest point of your vessel, which is critical for safe navigation. This calculation helps avoid grounding by revealing the true clearance between the vessel's keel and the waterbed.

Other factors such as tidal currents and varying water levels due to tides may affect your overall navigation but are not directly related to computing under-keel clearance as defined by the keel's depth. Knowing the precise depth beneath the vessel is fundamental to maintaining navigational safety, and thus the practice of adding the keel depth is important for accurate assessments.