So, finding the time of the high and the time of the low is useful, but it'd be
nice to be able to find the height of the tide at a given time. For example, if you know that you'll be coming into port at
0830, but the high is at 1000 and the low is at 0745, you need to know what the height will be at 0830 so you know your overhead
and under keel clearances.
Step 1: Find the high and low tide that bracket (are on either side of) the time that you want for your
subordinate station. (If you haven't already corrected the height from the reference station to your subordinate station,
do that first.) For example, if you need the height at 0830, and the tide tables list a high at 0200, a low at 0815, a high
at 1414, and a low at 2022, select the low at 0815 and the high at 1414 and continue on with the problem.
Step 2: Set up an interpolation table like the one at right to make your calculations easier. This
table should consist of 2 columns and 3 rows, with enough space between each to allow for some writing.
Step 3: In the first column in the top row, place the time of the high or low tide occuring before the
desired time. In the 2nd column, place the height of that tide. Repeat this step, placing the time and height of the high
or low after the desired time in the bottom row. In the 2nd row of the 1st column, place the desired time.
Step 4: As per the directions in the interpolation table (table 3) of the tide table, find the difference
(range) of heights between the high and low, difference in time between the high and low (duration of rise or fall), and the
difference in time between the nearest tide (either high or low) and the desired time. These should each be noted along the
margins of your interpolation table just so you can keep track of them.
Step 5: Using the interpolation table (table 3) found within the tide tables and the values you
found in step 4 above, follow the table until you find your Height Correction.
-First find the row for the nearest value of the duration of rise or fall
-Move across the row until you find the time from the nearest high or low
-Move down that column until you come across the row that best describes your range of tide
-Where that row and column intersect, that is your height correction
Step 6: Apply that Height Correction to the height of the nearest tide (whichever you used to find the
difference in time). As it says at the bottom of the table, if the nearest tide is a high water, subtract the correction.
If the nearest tide is a low water, add the correction.
Step 7: That's it man, you're done. To find the clearance under your keel, add the tidal correction
you just found to the charted depth in the area you are passing through. From this, subtract your draft, whatever's left is
your keel's clearance. Finding the vertical clearance is a little more complicated, and look for it in a different lesson
