To increase safety at sea and in the air, communications procedures were developed for use in times of distress and the international distress frequency was 500 KHz. In routine communication, merchantmen contacted each other (or shore stations) on 500 KHz and then shifted to a working frequency. To make sure that routine traffic does not interfere with distress traffic, two SILENT PERIODS were designated. These periods of 3 minutes each were from 15 to 18 minutes and 45 to 48 minutes past each hour. Safety and urgent preliminary broadcasts could be started during the last 15 seconds of a silent period. Except for actual distress messages, all traffic on frequencies between 480 and 515 KHz ceased during the silent periods.
The table below, circa 1966 and likely created around 1950, shows what portions of the radio spectrum that marine bands could be found in.
Table 1 .
RADIOTELEGRAPH RADIOTELEPHONE BAND
W 110 to 150 KHz T 1,605 to 4,000 KHz X 405 to 515 KHz U 4,000 to 23,000 KHz Y 1,605 to 3800 KHz V 156 to 174 MHz Z 4000 to 25,100 KHz
These frequencies in the MF band, had specific uses.
410 KHz was used for direction finding.
425, 448, 454, 468 and 480 KHz were the working frequencies for merchant ships. They were assigned to various coastal stations.
500 Khz was for calling and distress only with silent periods from 15-18 and 45-48 minutes after the hour.
512 KHz was the intership frequency between ship’s radio rooms and was used when 500 KHz was tied up with a distress call. Anyone using 512 KHz would also observe the silent periods.
Canadian coastal station VCS was assigned to 446, 484 and 500 KHz
Canadian coastal station VFF was assigned 430 to 500 and 512 KHz
The lowest assigned working frequency in this band was was TAF in Turkey at 416 KHz.
Some additioanl notes about the frequencies in Table 1:
For X band, both ship and Coast Stations used this band.
For Y band, it was used by both ships and coast stations using either CW or phone.
Z was used by ships fitted with HF CW.
T was used by ships fitted with MF Phone
U was used by ships fitted with HF phone
V was used by ships fitted with VHF phone.
Guarding the distress frequency was an important function of shore stations. Some stood continuous watch while others kept a loud speaker watch. When a naval ship was operating singly at sea, the ship kept a continuous watch if operators and equipment were available. In any case, a receiver watch was always kept during the silent periods. When ships were operating in a group, the Officer in Tactical Command (OTC) arranged for a distress guard. Usually, one ship would act as the guard for the whole group, however, under certain conditions, the OTC could request a shore station to handle the guard for his ships when the group was in the area of the shore station.
SOS distress calls were sent in a standard format. A serious moral obligation was placed on the Commanding Officer of the vessel acknowledging the receipt of a distress message. A Radioman could not acknowledge the receipt for a distress message until ordered to do so the by the CO. Under the International law of the sea, the CO of the ship that received the distress call was obligated to render all possible assistance. If the CO felt that he could be of assistance, he would order the acknowledgment of the SOS message. If assistance could not be offered, another option for the CO was to have the Radioman relay the distress message. In the relay, the distress message was repeated word for word on the distress frequency with full power on the transmitter. Naval vessels that were in distress did not normally use the International distress signal SOS. Instead, naval communication channels were used.
Before the creation of the VHF-FM marine band at 156 MHz, 2182 kc AM was the frequency that ships and boats used for communications with the Coast Guard and coastal stations up to a range of 200 miles. There was boat-to-boat calling, too. Needless to say, at night time, that frequency had lots of interference due to skip. Sometime in the 1950's, the International Telecommunications Union implemented two silent, three minute periods for 2182 Kcs. This gave shore stations an opportunity to listen for any weak Mayday calls. To assist radio operators in observing silence, marine radio clocks had silent periods marked at xx:00 to xx:03 and xx:30 to xx:33. On some clocks, the 2182 period was marked in red while other clocks had it marked in green, just to differentiate between the silent periods on 500 Kcs. There was always a silent period every fifteen minutes starting on the hour. In the early 1970s, the VHF-FM band was created and single side band was introduced into the 2 Mc marine band. Those developments left 2182 Kc a ghost frequency.
2182 kHz was the international Radiotelephone [RT] distress and calling frequency. CW was never used on this frequency. In 1927 the International Radiotelegraph Convention of Washington adopted the word "Mayday" as the radiotelephone distress call. At this time, there was no RT Mayday frequency but all marine RT transmitters could transmit on the international CW frequency of 500 kHz [using RT, of course]. In 1932, the International Telecommunications Convention of Madrid established 1650 kHz as the international RT distress frequency. Individual countries could decide when to adopt its use [i.e. instead of 500 kHz RT]. Many countries were still using 500 kHz distress RT up to the 1940s. 1650 kHz remained the RT distress freq. right up until 1950 when it was replaced by 2182 kHz as decreed by the International Telecoms Convention of Atlantic City 1947.
500 kHz * International Distress 2182 kHz HF Marine Distress 8364 kHz HF Maritime Safety 121.5 MHz VHF Distress 243 MHz UHF Distress
* As of 1999, 500 KHz is no longer monitored by the world's coast guards.
Modulated CW (MCW) Transmissions
The were also regulations which also defined MCW standards for merchant ships.  A main radiotelegraph transmitter aboard a compulsorily equipped ship must be capable of (a) a minimum of 160 watts A1 (keyed carrier) radio-frequency power output to the main antenna, (b) a minimum of 200 watts of A2 (keyed modulated) RF output to the main antenna, and (c) break-in operation.
The A2 emission required when transmitting an SOS on 500 kHz must be at least 70 percent, but not over 100 percent, modulated. The frequency of the modulating tone must be between 450 and 1,250 Hz.
The transmitter must be capable of transmitting on at least 4 frequencies in the MF band. Two of these must be 500 kHz and 410 kHz (the direction finding frequency) plus two working frequencies between these two. Other common frequencies for MF transmissions were 425, 454, 468, 480, and 512 kc/s. Some specials were 432 and 444 kc/s. Normally 410 was used for DF bearings.
A simplified schematic of the Mackay 2009A transmitter (410 khz to 500 kHz, 250 Watt) shows that in MCW mode, the audio oscillator is unkeyed but the other three stages (oscillator, buffer and RF amp) are keyed.
UPDATES TO THIS DOCUMENT
The IMO ship identification number scheme was introduced in 1987 through adoption of resolution A.600(15), as a measure aimed at enhancing "maritime safety, and pollution prevention and to facilitate the prevention of maritime fraud". It aimed at assigning a permanent number to each ship for identification purposes. That number would remain unchanged upon transfer of the ship to other flag(s) and would be inserted in the ship's certificates. When made mandatory, through SOLAS regulation XI/3 (adopted in 1994), specific criteria of passenger ships of 100 gross tonnage and upwards and all cargo ships of 300 gross tonnage and upwards were agreed. US and Canadian Coast Guard ships have IMO numbers.
Digital Selective Calling (DSC) is a feature that allows selective calling on VHF Ch70. To make a digital call each radio must have an identity, a 9-digit Maritime Mobile Service Identity (MMSI) number. MMSI numbers are assigned by national jurisdictions. The radios that are now required on board ships are also required to be DSC capable and every ship must have one or two licenced operators on board. This is also international.
A bulletin issued by the US Coast Guard in December 2006 regardinga121.5 and 243 MHz EPIRB's (Emergency Position Indicating Radio Beacons) prohibition.
"The U. S. Coast Guard reminds all boaters that beginning January 1, 2007, both 121.5 and 243 MHz Emergency Position Indicating Radio Beacons (EPIRBs) are prohibited from use in both commercial and recreational watercraft. Boaters wishing to have an emergency rescue beacon aboard their vessel must have a digital 406 MHz model.
The January 1, 2007, date to stop using 121.5 MHz EPIRBs is in preparation for February 1, 2009, when satellite processing of distress signals from all 121.5/243 MHZ beacons will terminate. Following the termination date, only the 406 MHz beacons will be detected by the International Cospas-Sarsat Satellite System which provides distress alert and location data for search and rescue operations around the world. The regulation applies to all Class A, B, and S 121.5/243 MHz EPIRBs. It does not affect 121.5/243 MHz man overboard devices which are designed to work directly with a base alerting unit only and not with the satellite system".
 The regulations shown here came from the text "Electronic Communication" (1959,1967) by Robert Shrader. It was also understood that any emission on any frequency which might attract attention was legal in a time of distress.
Contributors and Credits:
1) IMO: Paul Beesley <paulbeesley75(at)hotmail.com>
2) Spud Roscoe <spudroscoe(at)eastlink.ca>