Hi Peter,
I just replaced the burgee halyard on my 30Ultra.
The manual reads 100' of #4 Cord... as close as I could figure #4 was somewhere around 3/16' or 1/4" - so I went with 1/4" Polyester rope. A little thicker is easier to handle and fit no problem thru the burgee block.
I purchased it from Qualitynylonrope.com - their website speaks to the differences between nylon, polyester, and other fabrications. I chose the polyester because it had low water absorbtion, low stretch, and was UV resistant. They recommend it for flagpoles.
A 250ft spool cost just under $30 with shipping - a much better price than any other site, at least that I found.
I bought 250' because I re-strung all of the lines from my StackPack to the boom as well, as these were beginning to show wear too.
It has a very nice hand a good white color.
Peter Grabow
S/V CAKE WALK III
1987 30 Ultra 430
Jersey City, NJ