Cant get formuline here but i think this is the same
www.lanex.cz/en/d-f2-rope-25-4-2018-185110

question is what size for downhaul and both ends of a severne mast extension and boom? 3/16 or 5/32?

the mast clamp and outhaul rope severne supply has to be the worst.

Go for 4mm, this the diameter for Formuline/Dynema :)

Select to expand quote

mark62 said..
Go for 4mm, this the diameter for Formuline/Dynema :)

so 5/32 - 3.9mm then
theres also df-1 vs df-2, both dyneema 75 but the df-1 has a non abrasive coating and higher breaking point,

The D F 2 in 3.9mm (marlow formuline is 3.8 and 4mm in the UK) looks fine as its 12-strand braided same as formuline. But. I'm not sure what the "SK 78 special surface finish" is, wondering if it might slip in cleat.

Any of these places any good for you?

www.surfsailaustralia.com.au/shop

www.windsurfinggear.com.au/products/windsurfing-rope-formuline

www.ebay.com.au/itm/WINDSURFING-DOWNHAUL-MARLOW-FORMULINE-ROPE-standard-sized-5-32-4mm-/301624629714

Hi Ben

Where are you located?. Most windsurf shops should be able to supply the 3.8 Formuline by Marlow Rope.
I would not recommend anything else, especially for the downhaul line.

This rope is developed by a windsurfer. The other type of ropes might be very strong, but if they are to soft, they might be painful to thread and cleat.

I can't actually remember last time I broke / or needed to change my Formuline on any of my extensions.

As Mark pointed out this product should be easy enough to get your hands on (even posted) and is normally between $5-7 pr meter, depending on the shop.
I got it for $6 a meter here as a reference.

thanks, will try the stuff i can find here first and see how it goes.
Im in Canada but not near any shops. I can get it shipped from a store here but the shipping in canada is expensive, $2.75 per foot plus $17 dollars shipping.

Marlow say formuline is 3.8 or 4.5mm so for severne booms and extensions go for the 3.8mm?

Find a shop in Canada that will ship it in a envelope, not a titanium box. You cant be serious $17 even CAN dollars.

You can get that shipped from the UK for less than that.

Chinook distributes their own Spectra Double braid line which is not as smooth as the original Formuline but wears very well & holds well in cleats...

chinooksailing.com/collections/components-windsurf/products/4mm-spectra-rope