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16
Medens saaledes i 1877 og 1878 Skibets Hastighed i
Observationsøjeblikket kunde bestemmes med al ønskelig
Nøjagtighed, var dette ingenlunde Tilfældet i 1876. Saa
ofte der loggedes med den almindelige Log paa samme Tid
som de meteorologiske Observationer toges, erholdt man den
forønskede Nøjagtighed i Bestemmelsen af Skibets Fart.
Men denne Log benyttedes forholdsvis sjelden.
Patentloggen, der viser udløben Distance, giver kun under jevn
Fart et Maal for Hastigheden i Øjeblikket. Man var
saaledes ofte henvist til at bedømme Skibets Fart ved Skjøn,
og søgte at corrigere dette ved en senere Aflæsning af
Patentloggen eller ved de paa astronomisk Yej fundne
Positioner. Fejlen i Bestemmelsen af Skibets Fart kan
saaledes vistnok i enkelte Tilfælder gaa op til 1 til 2
Kvartmil i Timen eller 0.5 til 1 Meter i Secundet og derved
blive af samme Orden som Usikkerheden ved de samme
Aar med Vindmaaleren bestemte Vindhastigheder.
Thus, whereas in 1877 and 1878 the rate of the ship
could be determined with all desirable accuracy at the
moment of observation, such was by no means the case in 1876.
Whenever the common log was used and meteorological
observations taken simultaneously, we could obtain the
required accuracy for determining the speed of the vessel;
but this log was used comparatively seldom. The patent
log, which shows the distance run, does not give a measure
of the rate at the moment of observation, unless the
speed be uniform. Hence, we had often to estimate the
rate of the vessel, and . then try to correct the result by
a subsequent reading of the patent log or by means of
astronomically found positions. The error in the
determination of the ship’s speed can, for 1876, therefore, in some
cases amount to as much as 1 to 2 miles an hour, or 0.5
to 1 metre per second, and accordingly rise to the same
order as the inaccuracy attaching to the velocity of the
wind determined the same year with the anemometer.
Naar Skibet bevæger sig gjennem Luften, virker dets
Bevægelse som en Modvind, der sammen med Luftens
virkelige Bevægelse har, en Resultant, som er den Luftbevægelse,
der føles og maales ombord. Beregningen af den sande
Vindretning og sande Vindhastighed kan gjøres paa følgende
Maade.
Kaldes:
y den observerede Vinkel mellem Vindretningen ombord
og Diametralplanet. eller den observerede Vindretning
p. C. ombord minus Kurs p. C.
v Skibets Fart. regnet i modsat Retning af dets Bevægelse
(Kurs),
w den ombord med Vindmaaler maalte Vindhastighed,
u den virkelige Vindretnings Vinkel med Diametralplanet.
JV den virkelige Vindhastighed,
e . Vinkelen mellem den observerede og den virkelige
Vindretning,
saa har man Ligningerne *
W sin u — wsiny (1)
W eos u = w cosy—v (2)
w eos y—v
cotang u :
w sin y
W = y’w* + v* —2wvcosy
(3)
(4)
Naar y — o, det er Vinden ombord føles ret forind,
har man 2 Tilfælder:
1n y = o, u = o, det er, den virkelige Vind stik i Stevn,
da er
W — w—v (5)
2° y =. o, u — 180°, det er den virkelige Vind ret
agterind. da er
W = v—w (6)
Naar y = 180°, det er, Vinden ombord føles ret
agterind, har man kun 1 Tilfælde, nemlig:
The motion of a ship moving through the air acts
as a head wind, which, together with the true motion of
the air, produces as resultant the motion of the air as felt
and measured at the place of observation on board. The
computation of the true direction and velocity of the wind
can be effected in the following manner: —
If
y be the observed angle subtending between the direction
of the wind on board and the fore and aft line, or the
observed direction of the wind on board p. C. minus
the courso p. 0.;
v the speed of the ship taken in the opposite direction of
her motion (course);
w the velocity of the wind on board as measured with
the anemometer;
u the angle subtending between the true direction of the
wind and the fore and aft line;
W the true velocity of the wind;
e the angle subtending between the observed and the true
direction of the wind;
then we have the following equations: —
W sin u = w sin y (1)
W eos it — w cosy—v (2)
, w eos y—v
cotang u — ––(o)
«• sin y
W :
V w-
-2 tv v cosy
(4)
If y = o, i. e. the wind on board is felt to blow right
ahead, we have 2 cases: —
1° y = o, u = o, i. e., the true wind comes right ahead,
and then
W = tv—v-
20 y = o, u = 180°, i. e., the true wind comes right aft,
and then
W = v—w.
If y = 180°, i. e., the wind on board is felt to blow
right aft, we have only 1 case, viz.: —
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