Hi PD's......I have borrowed a  from RGY  ....which depicts a RED ball on the PORT side & a GREEN ball on the STDB side of the compass

So ...what are these balls for?....& why paint them different colours?

Answer 1 = these balls were painted so as to remind the captain which side of the vessel is what?
Answer 2 = these balls were actually leftover from old unused cannon shot?
Question 1 = do the balls as painted have some technical significance?
Question 2 = if the balls were unpainted would they still have the same technical significance?
Question 3 = if we needed to save top weight, could we use an alternate to old scrap cannon balls?.......maybe small soccer balls?
Question 4 = if we did use over grown ping pong balls as painted decorations.... ....would the odd 5 degree off course/plot really matter?

.......Derek 

found this online....
Navigators Balls: The Iron balls attached to either side of the main Compass to help counter the ships magnetic deviation. The Port one was often painted red and the Starboard one green. Also known as Lord Kelvin's balls.

I also found this definition on wikipedia....
"A Flinders bar is a vertical soft iron bar placed in a tube on the fore side of a compass binnacle. The Flinders bar is used to counteract the vertical magnetism inherent within a ship and is usually calibrated as part of the process known as swinging the compass, where deviations caused by this inherent magnetism are negated by the use of horizontal (or quadrantal) correctors.
Where the deviation from a compass point cannot be counteracted through the use of Flinders bar, Kelvin's balls, Heeling error magnets and Horizontal magnets, a deviation card, or graph, is produced. This card, or graph, lists the deviation for various compass courses and is referred to by the navigator when compass courses need to be corrected.
It is named after Matthew Flinders (1774-1814) who wrote a paper on ships' magnetism for the British Navy"

  PD's.....I was hoping Mick that it would last a little longer than 10 minutes................. ........

In OZ if we don't have our balls by our compass it's about 5 degrees onto magnetic north.........or around the corner......Derek

Still can't find any reason why they paint them red and green though! the one in the museum has them painted black...

Is this on a model or a full size paddler.
If a model, they probably represent the navigation lights, red to port and green to starboard, different colours so that an approaching vessel can see which side of a ship is which  in the dark, and so avoid a collision