# Decca Navigator Anniversary



## Binnacle

66 years ago this month, Decca Navigator was first used operationally when Operation Neptune was launched prior to D-day. Here is a history of the system much respected by seamen :- 
http://www.jproc.ca/hyperbolic/decca_hist.html


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## bert thompson

Thanks very much for all that information
A great deal I had not been informed of but did spend thirteen happy years as a Decca Engineer
Best wishes
Bert.


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## Klaatu83

Decca was a good system. Range was shorter than LORAN-C but coverage was more thorough in North Western Europe. It was practically a must on ships operating in the North Sea/English Channel area. About the only down side was having to rent the receiver, and buy the navigational charts that went with it, from the Decca company.


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## cueball44

the first time i looked at a decca navigator was when i was 15, first trip to sea on 'tarnborg' a north sea siener 1959, 'cueball44'.


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## Ron Stringer

Some of the ships that I sailed on did not carry the DN all the time, just when in NW European waters. So, after a couple of weeks or more on passage, we would head in through the Western Approaches (or around Ushant) and up the Channel in dense fog without the navigators having much of an idea where we actually were. Having made it safely into port by the skin of someone's teeth, the DN would be placed aboard and a coasting crew would join, fresh from leave to take her round the coast. Sometimes we would be several hours between ports! Not to worry, they had an excellent navaid aboard that provided continuous, accurate position fixes.

Before we went deep sea again, the DN would be removed - no use paying for its rental while the ship was out of DN coverage and could not be used. To hell with the mates, trying to make a safe landfall on their return, think of the cost!


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## Binnacle

Years ago I attended a BOT inquiry into the stranding of a trawler on the NE coast of Scotland. The skipper was getting a very severe grilling into his competence. What impressed me was the thoroughness of the evidence presented. They had a Decca engineer give evidence that he had boarded the trawler in Granton , set up the Decca Nav prior to vessel's departure, tested it in good working order. Then they had another Decca engineer giving evidence, who had waded out to the wreck, recovered the DN and noted the reading which tallied exactly with the stranding locus. The skipper, quite rightly IMHO had his ticket suspended.
I believe during the Berlin Airlift, where relief aircraft had to fly in a severely restricted corridor, many aircraft made use of Decca Nav due to it's higher degree of accuracy. Whether US aircraft used the system or not I do not know.


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## Brushy

On a very very foggy night the captain of GSNC Fauvette, Freddie Mace OBE brought us all the way up the Thames to our berth at Irongate Wharf, next to Tower Bridge on Decca . That was the same night a Norwegian ship hit one of the "forts" in the Estuary with some crew members deaths .He swore by Decca and it came in useful too during the floods on the european coast in 1954/55 when many channel markers went astray.
Bob Broome


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## slick

All,
I remember (somewhat vaguely, as though through a glass darkly) Olsen's Fishermans Charts for Icelandic Waters had the Scottish Red Lane extended to give coverage way out of range and Loran also marked on them.
Can anyone elaborete on this?

Yours aye,


slick


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## RayJordandpo

The good old Decca mark 12 (with the turning vectors) and 21 if my memory serves me right.
I recall a trawler skipper telling me how they often used the red, green or purple lanes on a decca chart as an actual course line. Very handy in fog he said, all you have to do is keep an eye on the decca chain on the machine itself and adjust your course accordingly, saves a lot of messing about!
I don't think a DTI examiner would be too pleased to hear that, especially in dense fog. What would happen if a ship coming the other way had the same idea!


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## Peter Martin

Does any Ancient Mariner out there remember a system called 'Dectra'? This was supposed to be a longer range version of Decca. Also, at the back of my mind was the Decca 'Hi-fix' system. The mist of time seems to have obscured further knowledge from my mind.


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## Klaatu83

RayJordandpo said:


> The good old Decca mark 12 (with the turning vectors) and 21 if my memory serves me right.
> I recall a trawler skipper telling me how they often used the red, green or purple lanes on a decca chart as an actual course line. Very handy in fog he said, all you have to do is keep an eye on the decca chain on the machine itself and adjust your course accordingly, saves a lot of messing about!
> I don't think a DTI examiner would be too pleased to hear that, especially in dense fog. What would happen if a ship coming the other way had the same idea!


I've heard of commercial fishermen using LORAN in a somewhat similar fashion. Supposedly, they would note down the LORAN numbers of their favorite fishing spots, rather than the geographic positions, so that their competitors wouldn't be at able to locate them.


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## Binnacle

Peter Martin said:


> Does any Ancient Mariner out there remember a system called 'Dectra'? This was supposed to be a longer range version of Decca. Also, at the back of my mind was the Decca 'Hi-fix' system. The mist of time seems to have obscured further knowledge from my mind.


My memory may be playing tricks, but I seem to remember reading that during a cable/pipe laying operation to a Scot's island Decca 'Hi-fix' was used, which involved Decca setting up additional local stations and specialised lattice charts being used for survey precision. Google confirms this :- http://alancordwell.co.uk/radionavigation/hifix/hifixint.html
Google came up with this for Dectra :- http://www.jproc.ca/hyperbolic/dectra.html


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## Ron Stringer

*Decca Hi-Fix*

Went aboard the pipelaying barge 'Saipem Castoro Due' in Peterhead to attend to a problem with the Sitor radiotelex installation. Found everything working fine and asked the bargemaster why they had called us out.

'Because every time that bloody telex system is on the air, our Hi-Fix goes off the clock and we can't see where we are.'' They were dependent on the Hi-Fix system to lay pipe accurately but it was not happy with its receiving antenna within a few metres of a 1kW transmitter radiating almost square-wave data signals. Never found out how they got round the problem, I was only being paid to make sure the Sitor installation was in spec., so I left them to it.


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## 5036

Decca had three hyperbolic navigation systems operational in the 70's and 80's. They were in order shorter range, higher accuracy.

Accuracy was a function of angle of cut and range.Decca Main Chain was in the order of 75-150 metres, Pulse8 was 25 to 50 metres and Hi-Fix ( and later Hi-Fix 6) were 5 to 10 metres. Repeatability of a fix, ie going back to the derived co-ordinates on an identical base station/ receiver set-up was usually in the order of 1/3 of the original fix accuracy. Main Chain and Pulse8 were systems with established beacons, HiFix had a couple of established chains but were more often set-up to suit the working area. An exploration rig going to a seismic derived drilling location was often put on location using an identical set-up of the positioning system, similarly a pipe route survey would require the laybarge to deposit the pipe using the same system set-up.

The big problem with the systems was sky wave effect which often caused them to become unusable at dawn and dusk and to have a reduced accuracy at night. Thus high accuracy work was done during daylight hours and a witness buoy was often laid to come back to to check the system accuracy and repeatability.

I positioned a rig in the late seventies using Main Chain in the Northern North Sea for BNOC. We laid a pattern of buoys that gave a gate through which the rig was towed, a heading and transit buoy, a lateral positioning and transit buoy and a buoy as a target for each of the eight anchors to hold her in location. The rig was towed in along a lane and the intersecting lane gave distance to run and from location. It was all very basic stuff with a hand plot. Later on plotters came in linked to computers but there was no interface between the receiver and computer until much later, rather the readings were input by hand to the computer and triple checked before the "enter" button was hit. Some may remember the PDP11 computer which was the size of a fridge freezer that you hand entered the program by a series of up/down switches before running a series of punched cards to record the program for power loss etc which happened most times an anchor winch burst into life.

The Decca engineer was vital aboard to keep the system running at top performance and often he would not be required at all, but on many occasions they saved the day.


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## Ginge

As I have recently aquired a Mk12 display box - does annyone have any idea on how to rewire so as to light up behind the pods?
All ideas or reccomendations gratefully received.
Many thanks, Barry.


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## tzinieres

we used to call the mark 5 "the pox box'.


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## Nick Balls

Bridge Picture in the North Sea in 1984 doing survey work with a decca system which I think was called 'sylidis' the Mk 21 set can be seen on the right with the high fix boxes on the left (lots of red lights!)


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## lakercapt

I seem to remember that latterly there were other makers that used the Decca system and you could purchase them instead of paying Decca a rental for their receiver.
Decca charts (with the time lines on) were sold and had nothing to do with Decca rental as a previous post indicated. When they first came out they were not considered safe for navigation and we had an another without the hyperbolic marks and were supposed the copy the ships position on it(or am I imagining that?)


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## Jim S

The Shipwrecked Mariners Society present the Edward and Maisie Lewis Award in recognition of the outstanding air/sea rescue of the year.
The trophy is in the form of a brass model of a Decca Navigator display unit.
Apparently it was made by the company's apprentices to mark the retirement of Sir Edward Lewis. As you can imagine the award is normally won by aircrews from the Royal Navy or RAF but in the 1980's a civilian crew won the award for the rescue from a liferaft of the crew of the fishing vessel Whyalla in the North Sea. - I have written that it was a "civilian crew" in fact the two pilots of the Bond AS365N helicopter were ex Army pilots , the winch operator was ex RN while the winchman "the dope on a rope" was a BP Heliclerk and SAR volunteer.


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## 5036

Nick Balls said:


> Bridge Picture in the North Sea in 1984 doing survey work with a decca system which I think was called 'sylidis' the Mk 21 set can be seen on the right with the high fix boxes on the left (lots of red lights!)


Syledis was a French system with a really dirty signal that interfered with tv's near to land stations. It was operated in range-range (+/- 3metres) or hyperbolic mode (+/- 5metres) Range/range had limited users, hyperbolic was unlimited. It was operated by Wimpol in the UK and they had a southern, central and northern chain with beacons attached, in the main, to the top of drilling derricks on production platforms. Special chains were set up as required. It was the mainstay for seismic survey and site survey ships and rig positioning for many years until it was superseded by GPS.

We had a problem one year in the central north sea where one beacon went out for ten minutes every day at the same time. Turned out the cleaner was using the socket for the hoover. Another beacon went off air in the Norwegian sector every Saturday morning. Turned out it was interfering with a football broadcast.


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## 5036

RayJordandpo said:


> The good old Decca mark 12 (with the turning vectors) and 21 if my memory serves me right.
> I recall a trawler skipper telling me how they often used the red, green or purple lanes on a decca chart as an actual course line. Very handy in fog he said, all you have to do is keep an eye on the decca chain on the machine itself and adjust your course accordingly, saves a lot of messing about!
> I don't think a DTI examiner would be too pleased to hear that, especially in dense fog. What would happen if a ship coming the other way had the same idea!


This coined the acronym "DACs" = Decca Assisted Collisions" caused precisely by what you say. Ships also left half a mile at a buoy causing similar distress but many mariners put in the buoy as a waypoint and found it with their bows exactly where the chart said it was.


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## 5036

lakercapt said:


> I seem to remember that latterly there were other makers that used the Decca system and you could purchase them instead of paying Decca a rental for their receiver.
> Decca charts (with the time lines on) were sold and had nothing to do with Decca rental as a previous post indicated. When they first came out they were not considered safe for navigation and we had an another without the hyperbolic marks and were supposed the copy the ships position on it(or am I imagining that?)


SIMRAD produced a model for leisure craft and non-commercial use.


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## backsplice

when I was JOS on the coaster Lochee in 58/59 I learnt how to read the Decca Navigator it was right beside the wheel ...... it seemed to make things easy (Whaaa)


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## Naytikos

As no-one else has mentioned it, may I throw in the special chain used by VLCCs to transit the Eurochannel. The pilots would bring the special receiver when boarding by helicopter and plug it into the normal Decca box. It had a brown lane, as I recall, which gave the correct course along the channel to Maas Centre Buoy.


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## Davie M

[=nav;593863]SIMRAD produced a model for leisure craft and non-commercial use.[/QUOTE]

I had a Decca system on a yacht in the 1980's which only displayed Lat&long and was not a lot bigger than a twenty packet of cig's, apart from arouind sunrise and sunset it was fairly accurate.
Davie


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## RayJordandpo

I'm trying to get the old grey matter going here! As I recall there were a number of factors determing how accurate the position fix was (e.g. angle of cut of hyperbolic lines of position ) I remember using the plastic 'Decca Ruler' with the mark 12 and allowing for errors, the position would be somewhere inside a diamond which could be an area of a couple of miles. The mark 21 was a welcome step up and a vast improvement.


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## Essjay

I wonder what system Captain Cook and Vasco De Gamma used???


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## Pat Kennedy

backsplice said:


> when I was JOS on the coaster Lochee in 58/59 I learnt how to read the Decca Navigator it was right beside the wheel ...... it seemed to make things easy (Whaaa)


 I got a bollocking off the Mate on the Ivernia, for even looking at the thing when I was on the wheel.
Pat


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## John Dryden

*Decca*



Pat Kennedy said:


> I got a bollocking off the Mate on the Ivernia, for even looking at the thing when I was on the wheel.
> Pat


Very funny Pat,maybe this model was more up your street (Jester)


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## RayJordandpo

Essjay said:


> I wonder what system Captain Cook and Vasco De Gamma used???


Probably the 'mark 12'. The 'mark 21 came' came much later [=P]


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## Nova Scotian

*Decca With Ellermans*

Decca Navigators were a great aid to navigation. During my time with Ellerman and Bucknall, in the early 70s, it seems to me it was a system that was only available for coasting around the UK and Continent. On arrival in the UK the system would arrive on the dock and be connected by the 2/0. When a vessel was ready to go foreign again it was disconnected. I believe a small number of systems served the whole fleet.


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## Klaatu83

lakercapt said:


> I seem to remember that latterly there were other makers that used the Decca system and you could purchase them instead of paying Decca a rental for their receiver.
> Decca charts (with the time lines on) were sold and had nothing to do with Decca rental as a previous post indicated. When they first came out they were not considered safe for navigation and we had an another without the hyperbolic marks and were supposed the copy the ships position on it(or am I imagining that?)


I was on a couple of ships that had Decca navigation receivers made by a Danish company which could be bought, rather than simply rented, as was the case with the Decca-made receivers. I don't recall experiencing any issues with their accuracy. Unfortunately i cannot recall the manufacturer's name, as that was well over 20 years ago. Prior to the advent of GPS, Decca was practically a must for ships operating in the area of the North Sea and English Channel. Although there were Loran stations available in that region as well, their placement was awkward, the stations being so far apart, and the Loran lines of position often crossing at such shallow angles, that accuracy was adversely affected. Decca was more accurate in the region than Loran because there were more Decca stations, and they were positioned in closer proximity to each other.


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## 8575

RayJordandpo said:


> The good old Decca mark 12 (with the turning vectors) and 21 if my memory serves me right.
> I recall a trawler skipper telling me how they often used the red, green or purple lanes on a decca chart as an actual course line. Very handy in fog he said, all you have to do is keep an eye on the decca chain on the machine itself and adjust your course accordingly, saves a lot of messing about!
> I don't think a DTI examiner would be too pleased to hear that, especially in dense fog. What would happen if a ship coming the other way had the same idea!


Not just trawler skippers either, quite a few coasting outfits regarded the practise as normal. That said about every five years or so the BOT/DOT/Dtp/DfT or whatever they're called these days issued M Notices warning against the use of Decca lanes as courses precisely because of the collision risk.


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## Basil

I used a Decca moving map display in aircraft in the seventies.
In order to function, it used two slaves at a time and the hyperbolic line of one slave drove the rollers and the other drove the lateral pointer.
The problem now was that the hyperbolic lines were plotted on the moving map as straight lines at right angles to each other so the map features and tracks were distorted accordingly.
A straight line track appeared curved but, so long as the pointer was on it, we were on track.
Another problem was that, because of our speed, it was easy to get lane slip (or was it zone slip?), and regular checks had to be made so looking after it was time consuming.



> What would happen if a ship coming the other way had the same idea!


Since the inception of GPS, it has been common for airliners to be unofficially flown a mile right of track on oceanic crossing in case of ATC or pilot error resulting in opposite direction conflict.


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## kevinmurphy

Re sitting my Nav Aids course up at Shields (1980) due to the intake of 6 pints between the writtens in the morning & my Orals in the afternoon on first attempt (group of Cadets ending Phase 1),

I was on the second course with a bunch of older hands doing there 2nd mates, one of whom was a Ghanaaan, we had spent about 2.5 hrs being told about the Decca Navigator that morning, when the Lecturer (smashing fella called Capt Amari, really helpfull) went and switched on a Decca Mark 6,

to which the chap from Ghana piped up with "excuse me sah, what is that machine?", Capt Amari was nearly in tears,

"well I have never sailed with Decca" was his excuse after we had picked ourselves up off the floor.

Dont think he passed!


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## ninabaker

*Gee as well*

I am researching a book about RAF radar operators and was trying to track down what the machines I was looking at in family photos were. Eventually, I found out that they were Gee machines from about 1945-6 when my father was involved in setting up a chain of them in Libya and Palestine.


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## janmike

Peter Martin
I may be able to help you. First saw DN in 1960's as 3rd. Mate with British Tanker Co. Then worked with Decca eng's at Heathrow airport. Decra was proposed and developed. But american money prevailed and we ended up with Loran. Shame!


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