Rendezvous Involving Effect of Wind and Current

Rendezvous Involving Effect of Wind and Current

A life raft is 55 miles from a ship at 09:00 and is bearing 100° T. The life raft is drifting at a speed of 3 knots due to a northerly wind. The ship expects 6° of leeway due to the wind. Establish a course for the ship to steer by and the earliest time it will rendezvous with the life raft. The ship can go at a maximum speed of 17 knots.

Also, If a current of 155° T at 2.8 knots was setting throughout, determine the EP of the rendezvous as the bearing and distance from the ship’s start position at 0900.

Rendezvous Involving Effect of Wind and Current

The positions of the ship and liferaft should be drawn in relation to one another using a suitable scale on a plotting sheet, with a bearing of 100° T and a distance of 55 miles. The liferaft should be in the center position because its movement will be constant.

Plot the liferaft’s 3-knot southward drift from O, where the wind is blowing northernly. To plot a good triangle for more reliability, an appropriate time interval should be used, such as 3 hours to obtain a drift of 9′ (3x 3). WO is 180° Tx 9. Estimate the liferaft’s movement for the calculated drift (9′) and name it to position W.

Use the vessel’s speed of 17 knots for a similar time interval (i.e. 3 hours) and find out the distance to be covered by the ship in 3 hours (3 x 17 = 51′). Draw an arc to cut OA’ at A by measuring a distance of 51′ on the compass at point W. The course to rendezvous the liferaft is marked as OA. The route calculated from the plot is 110° T.

The course to steer will be as follows because the ship will have 6° of leeway with a northerly wind:
110° T – 006° Leeway
104° T (Course to steer)

Time of rendezvous = 0900 + [(OA’ / OA) x plotting interval)]
0900+ [(55+ 48.7) x 3] = 0900+ 3h 23m
= 1223 hrs

Both units would encounter a similar current because it was setting 155° T at 2.8 knots continuously. The best course of action in this scenario would remain unchanged. The total current would be applied to the RV position to calculate the actual position of rendezvous.

Time to RV x set = Total Drift experienced
3h 23m x 2.8 knots= 9.47 or 9.5′

To obtain the EP, this drift needs to be plotted from the rendezvous DR position. According to the plot, the rendezvous point’s bearing and distance from the ship’s start position at 0900 are 116° Tx 64.7.

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