Hazards and precautions when working with very short towline on Large freeboard vessels
Keeping to our motto of understanding things in very practical and easy to follow method without bothering too much about the detailed theory I will discuss this topic here so that you can apply same in your day to day operations.
Due to very steep angle, towline will have a very sharp angle at fairlead of ship as well as tug, which will cause excessive chaffing, fusing/melting of line, which may even part the towline.
When tug is pulling at very close distance to ships hull, the propeller wash of tug will tend to push the ship, thus reducing the effectiveness of tugs pull. This called wash effect. Wash effect will be even stronger when there is very less UKC.
Most important: The force on towline will be very high even more than the SWL of towline when vertical angle is less than 30 degress. Ideally vertical angle should be more than 40 deg. As can be seen with below table, if tug is pulling with horizontal force F, The force on Towline(Tf) can is 1.5 time at 40 deg, 2 times at 30 deg, 3 times at 20 deg and 6 times at 10 deg. So if tug is at half pull(abt 30ton), the force on towline at 20 deg angle is 90 tons, at full pull(45ton BP tug) this force will be 45X3=135 tons, much more than SWL of line, hence line will part at full pull.
How to estimate towline length to be given for 45 deg angle
L=Towline length from ship fairlead to tug fairlead = Freeboard/Cos 45 = Freboard/0.7 = 1.4 X freeboard.
Hence total length of towline should include from fairlead to fairlead about 1.4 times freeboard plus additional length for ships fairlead to bollard and tugs fairlead to bollard(about 15 mtr)
Total line to be given by tug should be = 1.4 X freeboard + 15 mtr.
Ex" for 10 mtr freboard(183 m vl in ballast), total length reqd=1.4 X 10 + 15 = 29 mtr
Ex" for 13 mtr freboard(230 m vl in ballast), total length reqd=1.4 X 13 + 15 = 33 mtr
Ex" for 14 mtr freboard(250 m vl in ballast), total length reqd=1.4 X 14 + 15 = 35 mtr
4. While vessel is falling back or approaching jetty, tug master should remain very alert, tug should be ready for push/pull at 90 deg. Normally if vessel is swinging toward jetty of moving fast toward jetty, an experienced tug master should predict that now pilot is going to give pull order and he should be ready for the same. similar way if vessel is swinging to starboard, an expererinced tug master will predict that pilot will give pushing order and he should be ready for the same. Same way in NE'ly wind experienced tug master will always predict that pilot ids going to give pull orders again and again and and tug should always be ready for pulling. Little bit experience and alertness of tug master makes the movement much simpler for pilot and makes it a very good team work.
5. While changing positions from push to pull, Tug master must ensure there is no sudden jerk on towline. Tug should be steady and not yawing, if tug is yawing, the towline will chaff against the fairlead of tug and towline may part.