Sea sickness on a cruise


Sea sickness on a cruiseOn ship the motions are exceedingly complicated. There is a partial to-and-fro rotation about the long axis of the ship; it is called rolling. For each direction there is a start, movement, and stop. These are all different; they affect the subject differently, and require different adjustments in order to harmonize with them. Aside from starts, and stops, the movement is further complicated by being in each case accelerated in the first half of its length and retarded in the second half. To a single round of the rolling movement, we therefore have eight different motions, namely, start, accelerated movement, retarded movament, and stop, for half the round; and four more such motions in the opposite direction complete the sea-sickness round to point of beginning. The curvature of this movement is still another complication.

This very brief consideration of the rolling movement applies also to the pitching, which is similar to the rolling, and corresponds to it in every detail. Its axis of rotation is the transverse axis of the ship. It rotates the long axis of the ship through a lesser angle than the transverse axis is rotated by the rolling, but the extreme ends of the ship rotate through a greater space.

The pitching and rolling then give us sixteen different kinds of motion, requiring as many different kinds of harmonizing adjustments on the part of the successful sailor; and the sailor executes these adjustments perfectly, automatically, unconsciously, and apparently without any deleterious expenditure of energy.

There is another class of ship movements, very similar to those of the elevator. This includes the up and down movements. Each round consists of downward start, accelerated movement, retarded movement, stop, upward start, accelerated movement, retarded movement, stop. These eight, with those already given, constitute twenty-four movements of the ship, exclusive of progression and certain tremors.

Of tremors there are at least two varieties present on steamships. One is caused by the operation of the propelling machinery and is always present and nearly always uniform. It is customary for trial purposes to run the machinery slowly for a time, a few hours prior to departure on the voyage. The ship resting otherwise quietly at the wharf in still water, the tremor thus set up, though gentle and uniform, is quite sufficient to make many persons sick.

The other variety is variable and not always present. An understanding of it enables us to appreciate. the cause of the greater prevalence of sea-sickness with winds ahead than with winds astern. With winds astern conditions vary with the relative velocities of ship and wave; velocity of the wave depending on the velocity of the wind. With wind, and therefore wave, directly astern, and velocity of wave and ship equal, there are no wave impingements against the ship, and for this reason the ship in this case is free from what we may call the wave­impingement tremor.

The ship is also free from this tremor during calms, when there are no waves because no winds. When velocity of wave is greater than the velocity of the ship, every wave that overtakes the ship makes an impingement, and the series of impingements gives rise to a tremor. And so, if the velocity of wave is less than the velocity of the ship, there will be a tremor caused by the impingements of ship against the waves which it is overtaking.

If we suppose two cases, in the one (a) a series of impingements by ship overtaking the waves, in the other (b) a series of impingements by waves overtaking the ship, and suppose that in each case the number of impingements in a given time are equal, then the violence of the tremor will be greater in case (b). as will appear by comparison of the cases thus : In (b) ship is struck by the leeward and tuerefore more abruptly inclined side of the wave, and the stern of the ship presents a greater area of surface more nearly perpendicular to direction of the force of the impinging wave. In (a) the ship strikes wave on its windward and therefore less abruptly inclined side, and the bow of the ship presents its surface more obliquely to the direction in which it impinges upon the waves.

With winds and therefore waves ahead, because the ship and waves are moving in opposite directions, there is a greater number of impingements in a given time than with winds astern. The ship is also struck by the more abrupt side of the wave. In this case the momentum, and therefore the force, with which waves strike the ship is always great, and sometimes enormous. If wave and vessel, for example, are moving at the ordinary steamship rate in opposite directions, the momentum and force with which the wave strikes the ship are equal to that of a like wave of twice the velocity striking the ship at rest. What the force of such an impingement is, can best be appreciated by those who have seen what destruction is sometimes wrought by a fragment of such a wave coming on deck.

Tremors are varied by changes of winds to other directions in respect of the vessel. With winds and therefore waves abeam, the ship rolls. When high and long waves "swells") pass under it, the ship keeps its keel toward the centre of the wave. When, as in times of strong beam. winds, there are smaller waves upon the "swell," such smaller waves impinging against the ship's side give rise to a tremor. Other, things equal, all that are at all susceptible are decidedly more sea-sick with winds ahead than when they are in any other direction. It is my opinion, based on observation, that on steamships, sea-sickness is least prevalent with no winds at all, and that the next most favorable condition is to have the winds directly astern.

The actual movement of a ship is a complication or resultant of nearly or quite all of the twenty-six details described. When all or nearly all of the details of movement are present the resultant is exceedingly complex. The intensity of sea sickness and the extent of its prevalence, among passengers and even crew, are proportional to the degree of complication of the motion. In other words, the greater the variety of details of movement, and the greater the number of variations of movement in respect of curvature, velocity, and direction, the greater, the prevalence of seasickness.

Many persons believe that there is a difference between rolling and pitching in respect of their efficiency in producing sea-sickness. This difference is believed to exist irrespective of their relative amplitudes and violence. The difference that such people observe is only apparent. When a ship has been out several days or longer, rolling all the time, the passengers about well, and the motion then changes in a short time to pitching, the passengers will in as short a time suffer a relapse of sea-sickness, and conclude that pitching is the worse motion. A similar change from pitching to rolling, with a like result, convinces other passengers that rolling is worse than pitching. In general that motion seems worst to which we are least subjected.

So far I have briefly indicated the condition upon which physiological disturbances depend, and have detailed the movements which under that condition cause such disturbances. We next wish to know how these motions cause these disturbances, and what these disturbances consist of.