VARIATION OF THE COMPASS The magnetic variation for 1934 and annual increase at points mentioned are as follows: A very important characteristic of the tides on the western coast of the United States is the large inequality in the heights of the two high waters and of the two low waters of each day. On the outer coast the average difference between the heights of the two high waters of the day is from 1 to 2 feet (0.3 to 0.6 m), and the average difference in the heights of the two low waters from 2 to 3 feet (0.6 to 0.9 m). It was because of this large difference in the low-water heights that the mean of the lower low waters, rather than the mean of all low waters, was adopted as the plane of reference for the charts of this region. This inequality changes with the declination of the moon. When the moon is near the Equator the inequality is relatively small; but when the moon is near its greatest north or south declination, the difference in the heights of the two high waters or of the two low waters of each day reaches a maximum. The tides at this time are called "Tropic tides." Off the outer coast the mean lunitidal interval for the high waters increases from 91/2 hours near San Diego to about 121⁄2 hours near Cape Flattery. The latter interval may be expressed also as zero hour when referred to the nearest transit. The mean rise of the tide above the plane of reference off the outer coast varies from 5 feet (1.5 m) off southern California to about 72 feet (2.3 m) off the coast of Washington. Extreme variations from 3 feet (0.9 m) below to 10 feet (3.0 m) above the datum may reasonably be expected. At the entrance to San Francisco Bay the mean high-water lunitidal interval is 111⁄2 hours and the mean rise of the tide is about 5 feet (1.5 m) above the plane of reference. At the southern end of the bay the tide occurs about 12 hours later, and the mean rise is about 212 feet (0.8 m) greater than at the entrance to the bay. Passing northward into San Pablo Bay, the tide occurs from 1 to 2 hours later than at the Golden Gate, with a mean rise of about 1 foot (0.3 m) greater than at the latter place. In Suisun Bay the time of tide is about 3 hours later than at the Golden Gate, with a mean rise about the same. It requires about 5 hours for high water to pass from Suisun Bay to Stockton, on the San Joaquin River, and about 51/2 hours from Suisun Bay to Sacramento, on the Sacramento River. The mean rise of the tide above the plane of reference at Stockton is about 312 feet (1.1 m) and at Sacramento about 2 feet (0.6 m). In Humboldt Bay the tide is from 12 to 1 hour later than on the outer coast. The mean high-water lunitidal interval is approximately 12 hours, and the mean rise is about 6 feet (1.8 m) above the plane of reference. In Coos Bay the tide is from 12 to 111⁄2 hours later, and the rise of high water about the same as in Humboldt Bay. In Yaquina Bay the mean lunitidal interval is about 12 hours, and the mean rise about 7 feet (2.1 m) above the plane of reference. At the entrance to the Columbia River high water occurs about 11⁄2 hour before the transit of the moon and the mean rise is about 712 feet (2.3 m) above the plane of reference. It requires about 6 hours for high water to pass from the entrance to the Columbia River to the mouth of the Willamette River. In passing up the Columbia River the range of tide decreases until it is only about 12 feet (0.5 m) at the mouth of the Willamette. Above this point the tidal range becomes too small to be of practical importance. There are, however, large fluctuations in the level due to meteorological conditions. An extreme variation of 241⁄2 feet (7.5 m) has been noted at St. Johns on the Willamette River. The river is usually highest during the months of May, June, and July, and lowest during the months of September, October, and November. The tide in Willapa Bay and in Grays Harbor occurs near the time of the transit of the moon and the mean rise is about 9 feet (2.7 m) above the plane of reference. Passing through the Strait of Juan de Fuca, the tide occurs about 4 hours later at Port Townsend than at Cape Flattery. The mean rise increases from 7 feet (2.1 m) above the datum at Cape Flattery to 8 feet (2.4 m) at Port Townsend. There is an increase in the average inequality between the two low waters of each day from 3 feet (0.9 m) at Cape Flattery to 52 feet (1.7 m) at Port Townsend, a smaller and less important increase in the high-water inequality.. In Puget Sound the tide is about 1/2 to 1 hour later than at Port Townsend. The mean rise increases from 8 feet (2.4 m) above the datum of mean lower low water at Port Townsend to 132 feet (4.1 m) at Olympia. In Puget Sound the average difference between the two low waters of each day is 6 feet (1.8 m). At Seattle an extreme range from 412 feet (1.4 m) below the datum of mean lower low water to 15 feet (4.6 m) above the same datum has been observed. At Olympia, in the southern part of the sound, an extreme high water 18 feet (5.5 m) above the datum has been noted. In San Juan Archipelago high water occurs from 4 to 5 hours after the transit of the moon, and the mean rise of the tide is about 8 feet (2.4 m) above the plane of reference. In this sound an extreme range from 412 feet (1.4 m) below the plane of reference to 12 feet (3.7 m) above the same datum may reasonably be expected. Tide Tables for the Pacific Ocean and Indian Ocean are published annually in advance by the United States Coast and Geodetic Survey. This volume furnishes, at the nominal cost of 25 cents, full tidal data for the Pacific coast. It contains a table of full daily predictions of the times and heights of high and low waters for certain standard or reference ports along the coast. The use of table 2 of the Tide Tables should be known to every navigator. By means of this table the predictions given for the reference ports are extended so as to enable one to obtain the predictions for each day for a large number of intermediate places. Caution. In using the Tide Tables, slack water should not be confounded with high or low water. For ocean stations there is usually but little difference between the time of high or low water and the beginning of ebb or flood currents; but for places in narrow channels, landlocked harbors, or on tidal rivers the time of slack current may differ by 2 or 3 hours from the time of high or low water stand, and local knowledge is required to enable one to make the proper allowance for this delay in the condition of tidal currents. To obtain the times of slack water or strength of current, reference should be made either to figures given for various places in this volume of the Coast Pilot or to the Pacific Coast Current Tables. CURRENTS An offshore current with an estimated width of 200 to 300 miles or more flows nearly south-southeast, following the trend of the coast, from about 50° north to Point Conception, where it bends southward and westward. The estimated velocity is about 3/4 knot, but this is largely influenced by prevailing winds; prevailing northerly winds increasing its velocity and southerly winds diminishing it. Generally speaking, the offshore currents set southerly throughout the year, but during the autumn months, north of the forty-fifth parallel, the set is easterly. Except in northerly winds a weak northerly setting current will generally exist close inshore known as the Davidson inshore current. This is especially noticeable at Swiftsure Bank Lightship and along the southwestern coast of Vancouver Island, setting in a northwesterly direction. This current increased in width, extends southward as far as San Francisco in the 3 winter months when the prevailing wind is southerly, but in other seasons of the year there is little evidence of it except on the coast of Washington, where it exists on southerly winds and sometimes on winds from other quarters at all seasons of the year. Current Tables for the Pacific coast of North America are published annually in advance by the United States Coast and Geodetic Survey. This volume, which sells for 10 cents, includes the daily predicted times of slack water and the times and velocities of strength of flood and ebb for certain reference stations and a table of current differences and constants by means of which corresponding daily predictions can be readily obtained for numerous other places. These tables also include current diagrams for five bodies of water along the coast which show in a graphical form the velocities of the flood and ebb currents and the times of slack and strength over a considerable stretch of the channel of these waterways. Currents at lightships.-Current observations have been made on the five light vessels along the Pacific coast for several years. A discussion of these observations will be found in Special Publication No. 121, entitled "Coastal currents along the Pacific coast of the United States", published by the Coast and Geodetic Survey. A short discussion is given in the appendix to this volume. The greatest velocity observed at the light vessel was 3 to 311⁄2 knots. Wind currents. This subject is very complex. In general it may be said that along the Pacific coast of the United States at a distance of from 5 to 10 miles offshore, the wind brings about a current having a velocity about 2 percent that of the wind. The direction of this wind-driven current, however, is not with the wind. With winds from the northeast, southeast, and northwest quadrants, the current sets about 20° to the right of the wind, while with winds from the southwest quadrant the current sets about 20° to the left of the wind. It is evident, however, that these are but average values, for strong currents are sometimes experienced when the local winds are light. COASTWISE NAVIGATION Navigation along the coast of California, Oregon, and Washington presents to the mariner a problem of unusual difficulty. The courses in general are long and must be traversed during frequent periods of thick weather, with the vessel subject to the action of currents whose velocity and even direction are uncertain. A statement on the subject of fog, including a table showing the prevalence at different seasons of the year, and a general statement on currents are given in the preceding pages. An inquiry into the subject of coastwise navigation, including interviews with navigators and a study of the investigations made by the Steamboat Inspection Service into the causes of strandings, indicates the following: 1. As a preliminary, it may be stated that the currents are frequently blamed for disasters for which they probably are in no way responsible. In a large percentage of the above strandings, total lack of knowledge of the compass deviation was the most striking fact brought out in the investigation. The course was shaped from the log of some previous voyage, and no one knew what the corresponding magnetic course might be. The factors which cause deviation from the track are changing and uncertain. On no two voyages are they identical. Therefore, to rely blindly on a course merely because it was made good on some previous occasion is to invite certain ultimate disaster. Yet cases of this sort were so common as to justify special mention even in a publication which does not, as a rule, take into account the shortcomings of the navigator. 2. Although a knowledge of the compass error is essential, in thick weather the navigator should never rely on the compass alone. There are undoubtedly many periods when the currents are not in operation, and the magnetic course steered will be made good. But there is no way of telling when such periods occur. As a rule, navigators cannot count on making their courses and distances good, or assume that even though such courses lead, in general, from 6 to 10 miles off the nearest shores they have allowed an ample margin of safety. It is by no means uncommon for vessels to be set 10 or 12 miles off their courses in as many hours, and to have their speed made good, accelerated, or retarded by considerably greater amounts. 3. The majority of the strandings have occurred in foggy but comparatively calm weather. Indeed, considering the large number of strandings on record, it is surprising that the loss in lives and property has been so small. Various reasons may be advanced for the strandings, one of them undoubtedly being the failure of the navigator to realize that currents of considerable velocity are frequently encountered when there are no other unfavorable local conditions to warn him of their existence. 4. The commonly accepted rule among navigators regarding the currents has been that they follow the prevailing winds, setting, in general, southward in summer and northward in winter. Recent observations indicate that the current is apt to be from 20° left to 20° right of the direction of the wind. See page 22. 5. In the majority of the cases where the strandings appear to have been directly due to currents, the currents have been against the vessel. Most of the strandings have happened to deeply laden, southbound vessels to the northward of the projecting points like Capes Blanco, Mendocino, and Arena. The consensus of opinion among the navigators of the coast is that the currents follow the curves of the shore. If this is true, a vessel southbound against a northerly current would experience a tendency to set in to the northward of the points and out to the southward of them. As a specific instance, one navigator states: "If you have seen Blanco and Northwest Seal Rocks and find you have been set off a little and the speed made good retarded some, then you can be sure you will be set in toward Mendocino, or if you have set in toward Seal Rocks and your speed has been accelerated, then you can be sure you will be set off on nearing Mendocino." The general configuration of the coast tends to support this theory. As already stated, wrecks to southbound vessels occur to the northward of Capes Blanco, Mendocino, and Arena. More northbound vessels have been lost in the vicinity of Punta Gorda than at any other point along the coast. It is in these localities that the deviation of the coast from its general north and south direction is greatest. There is one serious objection to the theory that the currents follow the curves of the shore. It can readily be seen how a current flowing in a general north and south direction would be deflected to the westward by the points projecting in that direction, resulting in a tendency to set the vessel offshore; the set being experienced in |