Dead Reckoning

Bob Murdoch
23rd December 2011, 13:59
Found the following explanation of Dead Reckoning recently. Hope you dont mind a Sparks inputing to the Bridge Forum.

Dead reckoning can tell you where you are at any given time, and it operates on the principle that if you know where you were, and if you know where you have gone, then by extrapolating from where you were and where you have gone, you can calculate where you are, or at least where you were when you originally started calculating. Since the position where you are now is not the position that you calculated, and the position that you were in then is not the position you are in now, it follows that you have introduced the error. (This is defined as the difference between where you are and where you aren't, but calculated you should be.)

This error can be minimised by calculating your next position from the position where you weren't but calculated you were, to the position that you now aren't, because you had just moved. Unfortunately, you still have an error but by this method it does not compound itself, giving rise to an accurate position of where you are not.

Unfortunately, things now get complicated. If the wind and tide are not equal and opposite then a calculation must be made which takes into account their cumulative force and direction. It is important to remember, however that the time interval is the time from where you were to where you no longer are. This calculation results in a correction that is to be applied to the position where you are not in order to accurately ascertain that you still are not.

This is the same method as used by Christopher Columbus when he set out to establish the existence of Australasia but discovered the Americas.

No wonder the Mates always had a slightly puzzled look.

Cheers and a Merry Christmas to you all (Pint)
Bob

sparkie2182
23rd December 2011, 14:07
"a but discovered the Americas"

Actually he was looking for a chip shop in Fleetwood.

:)

Keltic Star
24th December 2011, 04:18
Deadreconing, better known as naviguessing. Standard proceedure for 7/8 days across the N. Atlantic in winter before loran and GPS.

holland25
24th December 2011, 05:42
Is it true that its really short for DEDuced reckoning.

bugga divino
24th December 2011, 09:27
This is true - intended to be the short form for Deduced Reckoning, but can now be twisted into more morbid shapes. In a nutshell, the assumptions behind DR calculations are:
1) One assumes that one knows the direction in which one is headed
2) One assumes that one knows the speed (through the water) of the ship
3) One assumes that one knows the direction and speed of both wind and current.
A simple trigonometric operation on steps 1 and 2, with subsequent allowances for 3 should, in theory, give you a pretty exact position. In the real world, however, one is never 100% aware of the (gyro)compass error/s at all times - believe that watchkeepers are not very diligent in observing gyro and compass errors each watch. The ships speed is another assumption which is open to question.
At best, the DR is an approximation and should be treated as such....

Chris Isaac
24th December 2011, 09:40
Dead Reckoning - Is a very accurate estimation of where the ship probably isn't.
It is calcualted by using course and speed from the last known position.

Estimated Position - Is the DR plus allowing for wind, current etc.

True Position - Deep sea was calculated using sun stars etc and by the time it was calculated was definately not where the ship was currently situated.

They each had a purpose and providing the navigator remembered that if his ship missed the port it was very seldom the fault of the port, then all was well.

Cisco
24th December 2011, 09:44
#1 and 2 give a DR.... the addition of #3 gives an EP.....

If you run up from a good set of AM stars to a DR position at evening twilight then the difference between that evening DR position and the position derived from PM stars will be the set and drift over that period....

'Stars' of course would/could give a good fix.... Noon positions based on a morning sight and a mer alt were simply a running fix... no more no less....

NoMoss
24th December 2011, 10:12
Dead Reckoning - Is a very accurate estimation of where the ship probably isn't.
It is calcualted by using course and speed from the last known position.

Estimated Position - Is the DR plus allowing for wind, current etc.

True Position - Deep sea was calculated using sun stars etc and by the time it was calculated was definately not where the ship was currently situated.

They each had a purpose and providing the navigator remembered that if his ship missed the port it was very seldom the fault of the port, then all was well.

I thought the Bridge blamed the Engine Room for any error in dead reckoning.

woodend
24th December 2011, 10:25
When you read about the number of ships that hit 'fixed objects' I don't think they use it any more. They obviously know where they have 'actually' been far more recently than we did. As said in a previous post 'many days in the N. Atlantic winter'.

Cisco
24th December 2011, 10:31
I thought the Bridge blamed the Engine Room for any error in dead reckoning.

I never did quite understand the 'distance the ship went +/- the distance the propellor went/100 = % slip' business...

Chris Isaac
24th December 2011, 11:17
I thought the Bridge blamed the Engine Room for any error in dead reckoning.

Yes, and your point is?

Chris Isaac
24th December 2011, 11:19
I never did quite understand the 'distance the ship went +/- the distance the propellor went/100 = % slip' business...

The engine distance assumes the propellor to be 100% efficient but of course it isn't. The percentage difference between what a 100% efficient propellor estimates and what the ship actually does is "slip".

Nick Balls
24th December 2011, 11:24
Often used to cross the N. Atlantic during the winter months without a single sun sight/ stars. Just a best guess!
By the way Columbus knew how to obtain latitude using the pole star, he also used careful observation of the variable difference between his compass and the pole star (Variation) This is how, after his initial journey, on subsequent voyages he was able to predict the likelihood of landfall. Another interesting document is the boats log from Blight's pacific trip, this shows precisely how you can keep a good track of things using methodical recorded observations.
A good navigator is always lost! Knowing just how lost(or not) is the true 'art' of navigation and possibly one reason that despite modern technology we still see many serious navigation errors.

Split
24th December 2011, 14:01
#1 and 2 give a DR.... the addition of #3 gives an EP.....

If you run up from a good set of AM stars to a DR position at evening twilight then the difference between that evening DR position and the position derived from PM stars will be the set and drift over that period....

'Stars' of course would/could give a good fix.... Noon positions based on a morning sight and a mer alt were simply a running fix... no more no less....

Three or four stars always gave a good position, otherwise there would be a "cocked hat". If my sights had an acceptable "cocked hat" I trusted them implicitly. There can be no forging, or fiddling of these, the person who took the sights knows whether they are rĄght, or not.

John Cassels
24th December 2011, 20:26
Three or four stars always gave a good position, otherwise there would be a "cocked hat". If my sights had an acceptable "cocked hat" I trusted them implicitly. There can be no forging, or fiddling of these, the person who took the sights knows whether they are rĄght, or not.

You can have a rather large cocked hat even with 3 or 4 star sights,
which says nothing re the accuracy of the position.

Boatman25
24th December 2011, 20:36
Often used to cross the N. Atlantic during the winter months without a single sun sight/ stars. Just a best guess!


I admired our officers in the North Atlantic in the winter as said nobody saw the sun sometimes for days but we always got to our port on time

Jardine
24th December 2011, 20:41
Three or four stars always gave a good position, otherwise there would be a "cocked hat". If my sights had an acceptable "cocked hat" I trusted them implicitly. There can be no forging, or fiddling of these, the person who took the sights knows whether they are rĄght, or not.

Was there ever 'not a cocked hat'?

Split
25th December 2011, 09:10
You can have a rather large cocked hat even with 3 or 4 star sights,
which says nothing re the accuracy of the position.

No, a large cocked hat is suspect. However, if you take four stars three of them, out of the four, should meet. Sometimes, because of cloud cover, the star sights are a few minutes more apart than they should be and the ship is travelling in the meantime. Let's face it, it can never be mathematical.

How did you previously calculate the readings? I've still got my US airforce surplus starfinder. It consists of a few clear plastic discs which were selected depending on latitude. You turned the discs to the hour angle and I found the brightest stars as soon as twilight started.

John Cassels
25th December 2011, 09:46
Very seldom did I ever not have a cocked hat even with 4 stars.

Always used the US air navigation tables both in preparation then in
calculation. Gave you an intercept from a fixed degree of latitude.

Split
25th December 2011, 10:25
Very seldom did I ever not have a cocked hat even with 4 stars.

Always used the US air navigation tables both in preparation then in
calculation. Gave you an intercept from a fixed degree of latitude.

Ok. If we are both able to chat to each other here, I guess we both got there, somehow or other! (*))

Have a good Christmas. We are leaving for SIL's home in a couple of hours. My daughter's navigating!

NoR
25th December 2011, 10:43
As I remember DR was arrived at by applying course and distance steamed through the water only. EP was when you applied drift and leeway.

Remember going from Birkenhead to Seven Islands in 1967 on DR all the way in fog (and ice). I'm sure that wasn't uncommon. A 2nd Mate once told me that being a good guesser was part of being a good navigator and that if the horizon was dodgy you were better with no sight at all rather than adding to the general uncertainty.

Cisco
25th December 2011, 12:07
The engine distance assumes the propellor to be 100% efficient but of course it isn't. The percentage difference between what a 100% efficient propellor estimates and what the ship actually does is "slip".

My point Ed Zachery.... it totally ignored any current experienced during the day and one could end up with positive slip....

Re the cocked hat... I always used to run each p/l up to a set time.. small correction.. but gave better accuracy... if you didn't do that and stars were taken over..say... 9 mins at 15 knots then a fair sized cocked hat was inevitable.

China hand
25th December 2011, 19:29
Mid Pacific, 15 knots, 6 stars, 2 mile cocked hat, so what? Making a landfall, something else. But we all knew our fixes were good or suspect and used them accordingly, as did everyone until satnav, and a few of us after.