Can we call them watches and keep within the seafaring tradition? Shifts is fine if you're in the army or maybe a factory.
There are many different watch systems in use, depending on the situation/crew size/purpose of the voyage. The traditional way from the square rigger days is four-on (hours) four-off with the watch from four in the afternoon to eight at night split into two two hour watches and thus "dogged" (those are the dog watches - not the middle of the night watch which is often and incorrectly called the dog watch) so that the crew alternates the watches and if you have the mid watch one night you don't have it the following night. That's where the the system of bells came from. The end of the first half hour of a watch was marked by ringing one bell, the end of the first hour by two bells etc, till the blessed arrival of eight bells when your four hour watch ended and you could go below. Watch changes take place at 4, 8 and 12 - AM and PM with the dogged watch noted above. Breakfast is at the morning 8 watch change, lunch is at the noon watch change and dinner at the dogged change at 6 PM.
The problem with short - three hour watches - is that in cold or inclement weather struggling in and out of many layers of clothes and foulies in a pitching boat is a chore that takes a lot out of you. In the tropics, where you just roll out of your bunk, put your shoes on and go on deck it works fine. On the other hand, the fourth hour of a four hour watch, especially at night, can be really rough.
My favorite system is one that has 3 four hour watches at night and two 6 hour watches during the day. Watch changes take place at 7 in the morning (breakfast), one o'clock in the afternoon (lunch) and 7 at night (dinner) and then at 11PM, 3AM and so back to 7AM. That way you get a shot a decent sleep to catch up during the day and your night watches are split so that if you get the mid watch (11 to 3AM) you get four hours before that and four hours after. I you get the 7 to 11 in the evening and 3 to 7 in the morning, you have just had six hour off before starting and six more hours off after ending. Six hour watches during the day aren't too bad. This system, having an odd number of watches, automatically "dogs", or alternates from one night to the next.
Of course, these watch systems work well on a fully crewed boat, weather racing or cruising. On a boat that has just a couple or just a couple of people, the need for such formal systems is much less and the individual strengths and weaknesses of the crew plays a bigger role than the tyranny of the hourglass. That can only be determined by experimentation and practice.
Last edited by genieskip; 01-21-2009 at 10:06 AM.