How much bend should I putin my mast with downhaul? Sighting up the mast with the downhaul on, it looks like I get about 5". I only have a 3-1 purchase that came stock on my 87 nacra 5.7. but I dont want to damage the mast. How much should it bend?

Seems like you are asking about 2 different bends;
Prebend is the amount of spreader rake/diamond wire tension that is induced in the mast in a fore/aft plane without the sail even being hoisted.

Max mast bend is induced beyond that by mainsheet and dowm haul tension on the sail. Take the battens out of your mainsail[or loosen them] and lay it on a flat floor and look at the curve of the luff - this is the max bend that you are trying to get to.
i know that the latest 5.7s are being rigged with prebend [ie very tight diamonds] but I'm sure that yours was not designed with this in mind. Someone will need to chip in here to let us know if it works on the older rig.

Paul

Best way to check luff is to hook the top or bottom to something and then pull to tighten. Don on a sail loft floor you can measure exactly.
If the boat came with only a three to one the sail wil probably not have a lot of strengthening. Check blot rope for sail stretch and maybe lenghten it. I have jut added an 8:1 to an original 4:1 to great effect but I have taken care to strengthen sail.
Cheers

not sure^ about the^ 5.7 but

pre-1983 5.2, have straight diamond wire spreaders so neither sail nor mast made for pre-bend

from about 1984 the diamond wire spreaders were raked back and adjustable. think those masts can be pre^-bent

As long as your diamond wires are in place, you do not need to worry about over-bending the mast.

Your main sheet will induce far more bend than what your downhaul can induce.

In the attached photo, I pulled downward on the top end of this 31' mast so hard that the front of the boat lifted from the trailer. <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

GARY

Jikes! You need to downgrade your downhaul, far to much purchase there. Or you are so strong that you can lift yourself by your bootstraps <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Seriously, did you intend to do that or was it "unforeseen consequences" which put that much pressure on the masttop? It is a very impressive picture!

I did it purposely Rolf. Cannot remember my motive. Perhaps I was fiddling with my diamond wires.

GARY

Gary,

Are you supprised that you needed a new mast a few years ago? Looks like you beat your boat worse in the yard than you do in the ocean. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Lee

Nice to hear from another 5.7 sailor. Warbird has the rihgt idea about prebend, make it match the sail, after you release old shrunken bolt rope at bottom of luff.
And yes, you definitely need more than stock 3:1 downhaul.
http://austinyachtclub.net/members/jimcasto/IMGP0316.JPG