I have been given a starboard 159 formula board from a friend and i wanted to know what sails would be right to get me planing in 8-15 knots. Im 70kg and an experienced skiff sailor but haven't done much windsurfing before. I don't have much knowlage about windsurfing gear so anything would help. Thanks guys

let's start with what did your friend use ??
you realize 8 knots is NOT much wind
personally use 10 knots as my wind minimum ...
what is the BIGgest sail you can use now ??

One thing in your favour right away is that you are already an established mental case your 12 footer history firmly establishes your psycho street cred. The old 159's are a sweet board and one of the best entry level formula boards around. They are small and curvy in the outline compared to modern boards but they have a really nice rocker line that makes them a great all rounder. At your weight a 9 or 10 metre will get you going nice and early even and 8.5 would be okay that's the great thing about the 159's they carry smaller rigs very well. formula gear is kind of like a 12 footer they don't point very high but get nice and low downwind at least by windsurfing standards anyway. What we call going upwind is a very tight reach by dinghy standards but it's so fast you will love it anyway.

The fact that you sail a 12 footer shows that you are a sailor who commits to what he's doing and the formula board is no different so in my opinion if you're not really willing to commit to mastering this beast then probably just sell it but my advice is put the time in and master it you will be glad you did.

Here's a vid to help motivate you from about 3 minutes into the vid I get a gust in some flat water and you can see some of what a formula board can do upwind.

Thanks heaps guys... My friend hasn't used it much in the last 3 years but used a 9 metre sail in 10-15 and that pulled him well. Is it worth spending the money on new gear or is it best to look out for second hand gear? and if so do you guys have any recommendations as to what would be reasonably priced and work well?

I'd definitely go used. Guys that race formula tend the upgrade their gear often and there is a relatively small market for it. You can get very good used gear at good value.

There is good used gear on seabreeze. Severne, neil pryde and ka make great sails. There is a bit of used severne reflex in 10 to 12.5msizes available these sails have amazing wind range. Get matching carbon masts and carbon boom if you can. Fins are important too, not just speed but actually being able to sail em.

as i asked - what is the largest sail you have used in windsurfing ??
Cluffy suggests that board can handle many sizes and is NOT restricted to large sails
you may be better off starting a little smaller than 9 m²
depends on your strength, stamina , experience, and your weight somewhat
if you have not windsurfed and start with 9 m² = WOW

yep i get what you are saying,
the biggest sail i have used was a 7.4
is there a significant drop off in power if i was to go to trying say a 9m down to a 8m or 7.5m?
what im trying to ask is would it make it more sailable and easier to handle if i was to go for a 7.5 or 8 and then upgrade when i feel underpowered?

No harm in trying a small sail. Have a go mith a 6.5 if you want it will float you back. Just dont expect to fly upwind and sail deep downwind. Formula boards are made for vmg and to do that they have big fins and need big sails, usually 11.5 to 12.5m. With that setup they fly up and down but pretty much wont reach, too much fin and too much sail. With smaller sails you should be able to sail more cross wind. You might want to drop the fin down to say 56cm too. The big sails are easy once you are going, its just yhe water starts, tacks and gybes. Ffs learn how to sail without dropping the rig, if you do drop it, drop it on the right side and get it out quick. Wear a pfd to help lifting the rig. Id say definitely get out there with a 7 or 8m to get used to it but dont judge it untill you are out with the big gear. Absolutely have an uphaul and make sure you can in shsllow water. The big rigs are exhausting in the water. Not sure if i made my point. Exhausting but worth it.

Yea mate I definatly get your point. Worth it if you stay at it.