I was coming out of a long river in USA, having loaded a full ship of Styrene Monomer.
The river pilot, gave me his calling card which is unusual. Obviously he was proud of who he was. It was punched on the card , that he was the Chief Examiner for Pilots in that neck of the woods.
We started idle talk after a while , and then he tells me he has seen ice bergy bits floating in the river, very often and how mysterious the river is—blah blah. . Obviously the terminal / barges have been pigging their chemical pipelines into the river. As soon a Styrene hits the dirty water, it polymerises, and then floats like ice sheets.
Noticing the lack of awareness, I told him . “ I know you are the honourable Chief Examiner, would you like to submit yourself to an examination orals. I have been in command for 28 years, and at the end of the orals exam session I guarantee, you will be much wiser, and will be glad that you showed some humility and kept your ego in cold storage“
He agreed, which shows that he had professional maturity and was interested in learning from a experienced sea Master.
A sea Master is NOT a one trick pony.
The gist of a 1 hour session is as follows. Of an orals session, where i corrected his answers, with detailed explanations and diagrams.
On a ship with fixed right hand propeller, every pilot will ask “ Captain bow goes to starboard, when propeller goes astern “
I would say “ Nope, The stern goes to port”
Which is true , as the pivoting centre is always towards the bows. When ship squats and is down by the head the pivoting centre is on the forecastle.
The pivoting centre shifts forward as the ship picks up speed. This is due to the water pressure on the bows.
When you drop both the anchors at anchorage the pivoting centre is between both the hawse pipes. This information is very important to ships who drop their anchor , to prevent running aground. If only the stbd anchor is used, the pivot will be on the stbd hawse pipe.
Knowing the pivoting centre is crucial , to determine where you will tell the tug to push or pull. A tug pushing exactly on the pivot cannot achieve anything. The ship is a lever, which tips at the fulcrum or the pivot.
This is why the bow thruster does a good job when the ship has speed less than 3 knots. When the ship moves faster , the pivot shifts forward. The feeble bow thruster has a smaller lever on the forward part of the fulcrum. Now if you have a stern thruster it will have a longer lever aft of the fulcrum and will turn the ship easily. It is NOT about the bow wave build up etc --Blah Blah.
Now why does the stern go to port when the RH propeller goes astern. All the books written by the white man talks about Transverse thrust, Screw race, Wheeling action—blah blah.
If a propeller will wheel the ship stern to port, while going astern like a car wheel on terra firma, then what prevents it from doing the reverse , when the propeller goes ahead? Pray?
Now, if you are on all fours, supporting your body with your two hands and legs, and I creep up from behind, and give you a resounding kick on your stbd side ass, which way will your stern go? Simple—your ass will be thrown to port and your head will go to stbd. This is exactly what happens to the ship.
The RH propeller blades have a skew. When the propeller goes astern, the wash on the port quarter misses the hull and goes away . While the wash on the stbd hits the hull by the stern, or the stbd ass of the ship. This is why when you go astern you always go to the stbd bridge wing, to check out the propeller wash, to decide when exactly to let go anchor. The stbd wash gets channeled to the bridge front first.
When the propeller moves ahead, there is no wash hitting the hull. This is why there is NO wheeling action, when the propeller moves ahead.
When you manoeuvre it is important to know that the GPS speed is just the speed of the GPS antenna over the ground. It does NOT give ships speed.. For a long ship, with the pivot right forward, a swinging stern will show good speed on the GPS. It will show you the course of the GPS antenna of course.
When you take a tight turn is is most important to start swinging well. For ships with aft accommodation, it acts like a huge sail. When you have a fresh wind from the port beam , it is easier to swing the ship to port.
I remember coming out of the Parana river and just before dropping the senior most pilot of Buenos Aires / Recalada, he says “ I am passing astern of the pilot vessel, so that if the ships engines fail, the wind will blow the ship away from the pilot ship” . I said “ This is exactly the reason why you should pass ahead of the pilot ship. To prove a point, I stopped the engines, and the pilot was surprised to see the ship heading exactly on a collision course, like magic . He did acknowledge how wrong he was, all these years.
On large ships when two tugs pull you off the berth, you must know that as soon as the pilot gives a forward momentum, the pivot will shift forward, giving the aft tug a longer lever on the fulcrum, and hence more practical power. The tug at the bows , getting a shorter lever, will pull harder to make up and keep the ship’s bows from swinging away. This is the time the tug lines part. Ship making fast tugs with modern hawsers doing figure of eight on bollard must be very careful. These modern ropes do NOT have friction coefficient. It is better to take two round turns on each bitt and then put a figure of eight. Surge means the rope will part with severe injury or loss of life.
Ships coming out of Houston river, after dropping pilot, gives full ahead, and then puts the helm on auto pilot. During which time, the helmsman, will take down the flags etc. You would have notices that the ship yaws from one side to another, basically the stern swinging wildly like a pendulum , with the fulcrum on the bulbous bows, due to less water under keel and squat. Less water under the ship means that the rudder does not bite, due to the vacuum effect. This is the time that the wheel must be taken on hand, and hand steering done till the water is deeper.
While in rivers, make sure that your ship does NOT get jack-knifed by the forces of wind and current, catching ford and aft sections, selectively. The aft accommodation acts like a sail for the wind , and the deep bows catches the current under the water.
CAPT AJIT VADAKAYIL