Charlie, for a cheap trial find some sticky tape and dig into your old cassette collections and rip out some magnetic tape.
Cut some 6-8 inch lengths and stick them onto your sails and shrouds and check it out.
Once you find the best location on your boat (for accuracy and visibility) then decide which permanent type you want - wool (yarn) or purchased ones. (they won't break the bank)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)