USCG Exam PracticeTides and currents — predictions and corrections
A navigator needs to transit a shoal area at first light. The chart sounding is 5 feet (MLLW). The vessel's loaded draft is 6.5 feet, and the navigator requires a minimum underkeel clearance of 1 foot. The tide tables for the reference station show a low water of −0.5 feet at 0430 and a high water of +4.5 feet at 1030. A subordinate station correction of +0.8 feet (height) and +45 minutes (time) applies. What is the MINIMUM predicted height of tide at the subordinate station required to safely transit the shoal?
- A. +3.0 feet
- B. +3.5 feet
- ✓ +2.5 feetCorrect
- D. +1.5 feet
Why C is correct
Required actual depth equals draft plus minimum underkeel clearance: 6.5 + 1.0 = 7.5 feet. Required height of tide equals required actual depth minus charted sounding: 7.5 − 5.0 = 2.5 feet. The subordinate station height correction and time correction from the reference station are applied per the tide tables procedure, but the minimum height of tide needed at the subordinate station is +2.5 feet regardless of which tide cycle provides it.