Ch 08.qxd 8/25/04 11:27 AM Page 8-3 5. To determine the swell length or distance between crests in feet, multiply the square of the swell period by 5. For example, using a 6-second swell period, 6 2 multiplied by 5 equals 180 feet. [Figure 8-4] Figure 8-4. Rules of thumb to determine swell period, velocity, and length. Figure 8-3.Wave interference. Swell Period Swell Velocity  Swell Period  x 3 knots Swell Length  Swell Period$^{2}$  x 5 Feet Time in Seconds Number of Waves Counted Resultant Wave Wave A Wave B Resultant Wave Wave A Wave B LOW RECONNAISSANCE SWELL SYSTEM EVALUATION HIGH RECONNAISSANCE SELECT LANDING HEADING 8-3 4. Determine the swell velocity in knots by multi- plying the swell period by 3. In this example, 6 seconds multiplied by 3 equals 18 knots. 3. Obtain the swell period by dividing the time in seconds by the number of waves. For example, 5 waves in 30 seconds equates to a swell period of 6 seconds. 2. Time and count the passage of the smoke or float light over successive crests. The number of waves is the number of crests counted minus one. (A complete wave runs from crest to crest. Since the timing starts with a crest and ends with a crest, there is one less wave than crests.) Time and count each swell system. 1. Drop smoke or a float light and observe the wind condition. During the high reconnaissance, determine the swell period, swell velocity, and swell length. Perform the high reconnaissance at an altitude of 1,500 to 2,000 feet. Fly straight and level while observing the swell systems. Perform the observation through a complete 360… pattern, rolling out approximately every 45…. Fly parallel to each swell system and note the heading, the direction of movement of the swell, and the direc- tion of the wind. To determine the time and distance between crests, and their velocity, follow these directions: The wind direction and speed can be determined by dropping smoke or observing foam patches, white- caps, and wind streaks. Whitecaps fall forward with the wind but are overrun by the waves. Thus, the foam patches appear to slide backward into the direction from which the wind is blowing. To estimate wind velocity from sea surface indications, see figure 8-1. If the direction of the swell does not agree with the direction noted at 2,000 feet, then there are two swell systems from different directions. The secondary swell system is often moving in the same direction as the wind and may be superimposed on the first swell sys- tem. This condition may be indicated by the presence of periodic groups of larger-than-average swells. Perform the low reconnaissance at 500 feet to confirm the findings of the high reconnaissance and obtain a more accurate estimate of wind direction and velocity. The purpose of the swell system evaluation is to deter- mine the surface conditions and the best heading and technique for landing. Perform a high reconnaissance, a low reconnaissance, and then a final determination of landing heading and touchdown area. When selecting a landing heading, chart all observed variables and determine the headings that will prove the safest while taking advantage of winds, if possible. Descend to 100 feet and make a final evaluation by flying the various headings and note on which heading the sea appears most favorable. Use the heading that looks smoothest and corresponds with one of the pos- sible headings selected by other criteria. Consider the position of the sun. A glare on the water during final approach might make that heading an unsafe option.  Use caution in making a decision based on the appear- ance of the sea. Often a flightpath directly downswell appears to be the smoothest, but a landing on this heading could be disastrous.