Celestial navigation practices


Robert T. Paige

Here is an article that describes the historical development of the sextant. Sextants
Not exactly on topic and with my apologies.
But question of the " 4 on and 4 off watches " Was this just something that was just common to WSL in 1912 ?

Robert T. Paige

I am apologizing for continually harping on this question on the "4 on and 4 off watch schedule" because it seems almost bizarre to this 21st Century "landlubber". I am just wondering if this was something that was in common practice in the Merchant Marine at that time and how long that practice was in use ?
David Allison

David Allison

" Resistance is directly related to the horsepower required to propel a ship" [Titanic]
Sorry David, but that is not correct.
Sam, That statement comes directly from the US Naval Academy document found here. Below is their example and model testing of a Navy-YP.

"Model testing is carried out over the expected speed range of the ship with resistance data collected at each testing speed. Effective horsepower is then calculated and plotted as shown in Figure 7.4. It will be observed from the figure that the doubling of speed of the Navy YP from 7 to 14 knots increases the power by a factor of 10! Speed and power are not linearly related."


Now compare a diesel powered Navy-YP with the same Navy-YP powered by steam. Both engines producing the same maximum 500 EHP.

At 14 knots they both have the same stopping distance using 500 EHP.

Now reduce their speeds to 7 knots. Both use 10% of their max power to maintain 7 knots, 50 EHP.

Now the diesel powered Navy-YP can go into reverse and increase its power to 500 EHP to be stopped from a speed of 7 knots. Its stopping distance is then considerably less, compared with its stopping distance from 14 knots.

Now the steam powered Navy YP cannot immediately increase its power to stop. It has only 50 EHP available to stop from a speed of 7 knots. Its stopping distance is considerably more compared with its stopping distance from 14 knots.

So if you’re out and about in your steam powered Navy-YP on some dark calm night, and worried about visibility and bumping into an immovable object, it’s a very dumb idea to run your Navy-YP at 7 knots.

This is very basic and easy to understand and is the fundamental reason why ship captains of the day, did not slow down in the vicinity of ice. In doing so, this increased their stopping distance considerably. They may not have been able to express why they operated their ships this way, (British Inquiry) however it was in their mariner DNA to do this.

Sam, I’m sure you are aware of the sea trial test data for Titanic/Olympic from Wilding. The faster their speed, the shorter their stopping distance for speeds 18, 20 and 22 knots. This seemed to puzzle Wilding particularly at 22 knots. Again, the basic reason for the shorter stopping distance as speed increase is the phenomena discussed above.

Anyone interested in reading about Titanic and for those who criticize Captain Smith for proceeding too fast, this criticism is totally unfounded. The perfect storm for Titanic was flat calm conditions. If Titanic wasn’t a new ship which they were breaking in, I’m sure Captain Smith would have increased her speed to the maximum possible in the vicinity of ice. It is the right thing to do and may have saved the day.

Titanic falls into a category the same as destroyers which have a final turning circle diameter of 4L, where L is the length between perpendiculars.

From the Naval Academy

“As shown in previous sections, the power required to propel a ship through the water is the product of total hull resistance and ship speed, and so engine power increases even more rapidly than resistance. Often, ship power is roughly proportional to the cube of the speed, so doubling (2x) the speed of a destroyer from 15 knots to 30 knots will require 2^3 =8 times as much power!”

The hull of destroyers and Titanic are more streamlined compared with a Navy YP and so rather than taking 10 times the power to double its speed from 7 to 14 knots, it only requires 8 times the power to double their speed and for the destroyer from 15 to 30 knots.

Steam ship handling 101
– In flat calm conditions in the vicinity of ice, operate your ship at the maximum speed possible to give the shortest stopping distance.
David Allison

David Allison

David, your question seems trivial, and the answer is obvious.
Hi Doug,
Yes I believe it is trivial. You may be familiar with the following information from Edward Wilding at the British inquiry. It is a gift to mariners as a result of the tragedy and should interest any curious ship handler.

Testimony of Edward Wilding, recalled

Does that complete the information?
- No, there is a little more information that I think the Court wishes to have. Since the accident, we have tried the "Olympic" to see how long it took her to turn two points, which was referred to in some of the early evidence. She was running at about 74 revolutions, that corresponds to about 21 1/2 knots, and from the time the order was given to put the helm hard over till the vessel had turned two points was 37 seconds. (2 points = 22.5 degrees)

This should puzzle experienced ship handlers in terms of the angular velocity. In degrees per second this is (22.5 degrees / 37 seconds = 0.61 degrees/second) Experienced ship handlers will immediately recognize the Titanic is capable of higher angular velocities at a speed of 22.5 knots. At minimum in the order of 1.25 degrees/second. The problem is if you put the rudder hard over, the rate of turn is very small. Count out 37 seconds for 2 points and it will emphasize just how small this rate is. You need to turn quickly to avoid an obstacle ahead and putting the rudder hard over limits you to 0.61 degrees/second. If you go gently on the rudder you will be able to attain a higher angular velocity. (rate of turn) This is for Titanic/Olympic in flat calm conditions and deep seawater. I haven’t looked at balance rudders on modern ships, but I suspect it is the same. As I mentioned to George there are two schools of thought among experienced ship handlers today. Only one is correct and it is the one where ship handlers pause at 20 degrees rudder displacement, before continuing to put the rudder hard over. One can’t really blame First Officer William Murdoch if by modern day standards and practice at least some ship handlers believe the right thing to do is put the rudder hard over to avoid an obstacle ahead.

Ship handling 102
-Avoid hard over turns