CAUTION Tide Predictions are based on averages. Other factors can have significant effect on local tides at any particular time. Consequently, you should always leave an extra margin for safety. Time & Height of Local Tides Table 2: record the Time-Diff and Height Ratios [col (3),(4)]. Identify Reference Station. Table 1: record the Times and Heights [col (1),(2)] of High and Low tides for Date. Compute: Time and Heights [col (5),(6)] of Tides at Local Subordinate Station. (1)(2)(3)(4)(5)(6) ` TimeHeightTime-DiffHeightTimeHeight h m H/L ft. Factor* ft. PREDICTION OF HIGH AND LOW TIDES LOCATION: REF. STA.: DATE: No. DESIRED TIME: REFERENCE STATIONSUBORDINATE STATION Table 1Table 2Calculate Actual Range: the difference between High and Low tides bracketing the Desired time. Height at HighHeight at LowActual Range ft. = - Time Difference to Desired Time: Determine time interval from preceding High (or Low) to the Desired Time. Round that interval to the nearest whole number of hours. Desired TimeTime of prior H/LTime Difference = - hhhmmm ROUNDED 1 – 2 – 3 – 4 – 5 – 6 RULE OF TWELFTHS: Look up the percentage of change from Table below under ROUNDED hours Calculate Height of Tide at Desired Time: apply that percentage to the Actual Range to get estimate of the change in Tide Height at desired time. Then add/substract that from prior high/low to get Height of Tide at Desired Time. PercentageActual Range Change in Height from High/Low = x %ft Vertical Clearance: determine vertical clearance at the desired time (if needed). Look on chart for Table of Heights above MLLW, or calculate, using one-half of the Mean Range plus the Mean Tide height from the tide tables. Charted Height ft Ht of MHW above MLLW ft Height above MLLW ft Vertical Clearance ft + - = Rounded Hours 5% 25% 50% 75% 95% 100% of change 1hr 2hr 3hr 4hr 5hr 6hr * If Table 2 has an * before height difference multiple Table 1 height by this factor, a +/- means add or subtract difference to get subordinate height. Graphics on Student Form Height of prior High/Low Change in Height Height above MLLW = ± ft Graphics on Student Form Rainsford Island831 Boston, MA 18 April EST H L H L0.0 Figure 7-2 & Figure 7-3 Classroom Exercise * * H L H L Charted Depth MHW Mean Range MLLW MLW Bottom Mean Tide ½ Mean Range Charted Height Vertical Clearance Height above MLLW Time/Ht of Ref Low Time/Ht of Ref High Reference Station Table 1 MLLW Time/Ht of Sub Low Time/Ht of Sub High Subordinate Station Table 2 Time Difference Height Factor Actual Range Time Dif to Desired Time MLLW Mean Tide Level 4.8ft Station Name Station Number Reference Station Distance 42° ’ N070° ’ W Rainsford Island, Nantucket Shoals 831 Boston, Massachusetts 5.1 NM Information… Tide Table… Print Close Help Low High ft 9.6ft 0.3ft 9.9ft Sunrise Sunset Moonrise Moonset Apr ^ ^ ^ ^ ^ ^ ^ ^ Station Local Time ^ <<Week <Day Tide Stations…Current/Tide Stream Stations…Day>Week>> 4/18/ /19/2004 Tide Graph X ft 8ft 6ft 4ft 2ft 0ft -2ft