Rhumb lines and Compass rose

[Randy Aitken]

I'm trying to create a small compilation of maps that may have been collected or made by an adventurer or pirate as a story hook for a new campaign. I love researching new things so I've been reading about the history on maritime mapmaking and such to find how the compass rose came to be and the purpose behind rhumb lines. The questions To which I cannot seem to find answers are:

1) how are they used practically by sailors to predict travel time and distance?

2) how was the location of the compass rose determined on a given map?

[LordEntrails]

My understanding of rhumb lines is;
- That typically they are drawn at one of the 32 points of a typical compass.
- They are used to indicate a constant meridian.
- That a navigator can assume (minus wind and current drift) that if they keep a constant compass heading, they will travel along (or parallel to) a given rhumb line.
- A cartographer would put rhumb lines on a map at key navigation places, such as from a tip of a peninsula (or other geographic starting point) to be used to travel to a given destination.

For example, say you had something like Italy, you might have a rhumb line originating from the southern tip along one of the compass points that take you to the coast of North Africa. Such a line would not lead diretly to a city, but it would indicate that if a navigator started at the tip and followed the line (say 3 points west of true south) that they would then arrive on the African coast to the east of xyz city.

Note, because of the aforementioned drift, a navigator would not want a rhumb line that would lead exactly to their destination, because in reality they are not going to follow the meridian exactly, so they would rather end up to one known side or the other of their destination so that they could then follow the coast in a known direction to get to the destination.

[LordEntrails]

Oh, as for the location of the compass rose, I've never heard of any method of placement other than artistic (i.e. where their is room for it and it looks good).