Knots

[Pirate Petee]

Ok, I got a question. I'm pretty sure the knots on the rope when they throw it overboard are 42 or 48 feet apart. But how much time is in the hour glass?

[Leatherneck]

I'm guessing about a minute or less because the rope would have to be pretty freakin long foranything more then that

[Leatherneck]

Look what i just found

How do sea navigators measure their ships speed

and here is what i found

Records from 1917 (Bowditch) indicate that the official U.S. Navy sand glass measured 28 seconds, and that knots were spaced out exactly 48 feet (or 8 fathoms - a popular length unit of that time). With this setup, ship's speed could be measured with an error of about 1.5%. This is speed relative to the water and assuming that the wood panel does not get pulled significantly from the place where it was initially dropped and that the rope does not stretch - all of which actually does happen and should be accounted as sources of error. Not to mention the problem of sea water currents adding or subtracting from the actual ship's speed relative to land.

[Fox]

I've not seen any evidence that either the glass time or the knot distance was standardised before the 19th century. The important thing is that the line and the glass match each other.

[Duchess]

I seem to recall reading about ships have several different glasses with various amounts of time, 12 and 28 seconds are the two values I remember. But unfortunately I have no recollection of where I came across the information.

I seems to me that any moderate amount of seconds would work just fine as long as you made the proper mathematical compensations and conversions. From a purely statistical point of view you would want the longest possible time for your line length and speed.

[Coastie04]

Long time doesn't necessarily make it more accurate for this. It could be that a longer time would pull the chip log more and it could gain some momentum. Also, there's the issue of more resistance for the spool of line and the extra force required to get a larger, heavier spool to start spinning. Generally, I've seen 15, 28, and 30 seconds used. If the boat's really going fast, they'd use the shorter glass instead of a longer line.

The other method of telling how fast the ship's going is to drop something that floats at the bow and time it until it passes the transom. Then, the proper conversions can be used to find speed.

Coastie

[blackjohn]

It's pretty easy to figure out the distance between knots/time in a glass for today's units of measure.

1 nautical mile per hour,

just about 6076 feet per hour,

6076 feet per 60 minutes,

6076 feet per 3600 seconds,

1.688 feet per second

Thanks for bringing this up Petee. I was thinking of making a chip log for demos, and the length of line/time in glass ratio hadn't crossed my mind.