Page 318 - statbility for masters and mates

306 Ship Stability for Masters and Mates
comparatively longer in length. When a ship is stationary in water of zero current speed these bulbs disappear.
Note the elliptical Domain that encloses the vessel and these pressure bulbs. This Domain is very important. When the Domain of one vessel interfaces with the Domain of another vessel then interaction effects will occur. Effects of interaction are increased when ships are operating in shallow waters.
Ship to ground (squat) interaction
In a report on measured ship squats in the St Lawrence seaway, A. D. Watt stated: `meeting and passing in a channel also has an effect on squat. It was found that when two ships were moving at the low speed of ®ve knots that squat increased up to double the normal value. At higher speeds the squat when passing was in the region of one and a half times the normal value.' Unfortunately, no data relating to ship types, gaps between ships, blockage factors etc. accompanied this statement.
Fig. 36.1. Pressure distribution around ship's hull (not drawn to scale).
Thus, at speeds of the order of ®ve knots the squat increase is 100 per cent whilst at higher speeds, say ten knots, this increase is 50 per cent. Figure 36.2 illustrates this passing manoeuvre. Figure 36.3 interprets the percentages given in the previous paragraph.
How may these squat increases be explained? It has been shown in the chapter on Ship Squat that its value depends on the ratio of the ship's cross- section to the cross-section of the river. This is the blockage factor `S'. The presence of a second ship meeting and crossing will of course increase the blockage factor. Consequently the squat on each ship will increase.
Maximum squat is calculated by using the equation:
Cb S0:81 V2:08
dmax Consider the following example.
20
k metres