We are finding it hard to physically hold the Spin sheets when sailing at tight angles, especially in over 10kts. Does anyone have an idea of a different sheeting system which maybe incorporates a purchase system?

I assume that you have a ratchet on the spinny block.
Put a second turning block on the sheet[ ratchet not necessary on this one] so that more of the sheet is in contact with the ratchet sheave - more friction.
Also is you are sailing with a blown out spinny, you will be going slow and taking more load.

And if the turning blk alone doesn't do it, make it a ratchet blk too. I run 4 auto ratchets on the SC20 spin sheet. Works well on a fairly big chute.

dave

Im with you on this I use 4 auto ratchet blocks and adjust the turning blocks sensitivity depending on conditions

With the old Nacra 60 + W1000 modifications, we carried huge spinnakers that were difficult to handle. There were two approaches that were sometimes combined:
1. Smyth designed Harken Build "mickey mouse" ratchet blocks. It was an autoratchet block with two smaller blocks that made sure the line made contact with the ratchet for at least 60% of the ratchet block. Personally, I found them awkward in slow wind and ultimately took them off and stayed with a normal ratchet system (even the autoratchet system was not necessary since you usually know if ratchet makes sense before you start the race)
2. Changeable 2:1 or 1:1 system. Put a small line and two small blocks attached to the clew. Then put a cap cleat to the back beam of the boat (or nearby the spinnaker sheet block. You feed the sheet (from each side the same way) from the spinnaker rachet block to the small spinnaker clew block and then tie a figure-8 knot. If you take the knot and put it into the cam cleat, you have 2:1. If you release it from the cam cleat, it goes to the end of the spinnaker clew block and stop, thereby creating a conventional 1:1. Note, you can only do this on one side at a time unless you have huge amounts of line-- not advisable.

I know some of this has been up before.

Still does anyone have suggestions for optimal spin sheets for F18s? I'm thinkin Maffiloli Swiftcord 8 mm but I'm open to suggestions. (We're using dubble Harken ratchet blocks.)

Also, what is in this aspect coming with Tiger, Cap and Infusion 2008 racepacks?

5/16 bzzz line works very well, or 5/16 salsa, apex is my new favorite
cheaper than the swiftcord

The Capricorn comes equipped with an 8mm x 16.5m single-braid sheet with a center tail sewn in. I don't know the brand of line - it had a good hand but took the shape of the blocks after a few races. After it wore, I replaced it with 6mm FSE Robline. I have had Swiftcord sheets before and the Robline seems to be a shade more robust. It is not yet taking the shape of the blocks and is not showing wear after several regattas now. I like it.