Can somebody give me some recomended string diameters for kicker, cunningham,downhaul,spinnaker halyard and, with lengths too for jib, main and spinnaker sheets, for a vintage Merlin to be used on the river?

For a more recent boat I last used 4mm for all except jib main and spinny. General opinion on length varies according to boat/design, but 9m x 6mm (Marlow/Excel taper)to 4mm for the jib, 16m x 6mm (Marlow/Excel taper)to 4mm for the spinny. Main needs to be virtually twice as long as your mast, depending on where the halyard exits and how high up you want to reach to tie on the main, so 16m of 4mm Excel Racing or Vectran would be enough? Cunningham and OUThaul could be 4mm excel control and the kicker was 3.5mm D12 or V12 (Dyneema or Vectran). Can't recall how much I used for a 16:1 cascade, but might be a bit less than 8m. Main was bought at 8m I think, polylite 7mm

I would get an old bit of string and try them out on your boat first to check, with spinny up, crew behind the thwart 'Ben Hur' style, Jib too, can measure the main halyard with the mast on the ground, mock it up and measure twice, cut once as I'm frequently told (usually after I cut it) The lengths I've quoted might only be any good for our Thin Ice set up the way we like it. I doubt there is a one size fits all for all boat/crew combos.

You might prefer to do it in string more in keeping with the boat's age and intended use of course....  HTH

That's great - thanks. I'll try for vintage Dyneema!!