Pre-Bend

I had a bit of an eye opening experience yesterday. I've always set my spreaker settings (2") based on what Yves recommended for my sail and general recommendations from others about diamond wire tension.

I never realized that there was a method to tuning this setting. Yesterday, a friend showed me a thing or two.

we laid the sail on the ground and held the luff taught with some screwdrivers run through the grommets and into the ground. We then ran a thin line from the leading edge of the bolt rope from the top all of the way to the bottom. This helps to see the overall curvature of the sail, as the straight line will traverse through the curvature along the bolt rope and you can measure that distance from the bolt rope to where this line lays.

After taking some measurements, it appears that my sail has a maximum of 4.75" curvature at 14.5' from the top of the sail.

How do I make the mast have this kind of prebend? Does this mean my spreaker rake should be set to 4.75". That sounds pretty severe???

My friend suggested laying the mast across some saw horses, hang some weight off the center and then adjust spreader rake and diamond wire tension until I get the 4.75" gap. Is that the correct thing to do? Any other suggestions?

I am running about 2" of prebend now. 4" of bend will really depower the boat in everything maybe to much but you can always try .Just sail with a Ravens helmut on. Seriously though don't you get the rest of the bend in the mast when you sheet in and downhaul the main. because you are really not using the bend off the wind I do not think>>.....Bob G.

Duh . . . that makes perfect sense. So I'm assuming it's a bit of an art form in setting the on-shore prebend. You want enough to fit the natural fit of the sail, but you want to leave enough for downhaul and mainsheet control to deal with all kinds of conditions.

Makes perfect sense. Perhaps 2" spreader rake is right?

At any rate, I think I'm going to bend it back a little more as a start. I'm not getting the performance out of the sail (the top 1/3rd in particular). It always seems a little hooked and I can never flatten that area (even with loose battens).

I'll post what I learn.

Are you running light on your boat cause that's what your setting up . Also are the battens at the top tapered and are they in the right way, taper toward the mast usually there is only one tightening point already made but if you ended up with a weak or reversed batten it could hook more than normal.......

I'm definitely not running light and I'll take that into account when I dial in pre-bend. My problem is that my prebend is just plain off. I need to dial it in closer before I can make any detemrinations about it being too much or too little.

The battens appear to be a bit softer than they should be. It's almost impossible to flatten the sail near the top. Even worse, there is alsmost always a hooked leach. Some have advised that I needen't worry about the hook (that it will blow open), but looking at some newly installed leach tell tales tells a different story. Getting proper control is going to take some work on prebend as well as battens (for sure).

Basically, I've missed the mark on prebend and batten tuning every since I've owned the boat. With little specific guidance from Mystere, I've just kind of winged it. After reading your responses,I don't think I need to make drastic changes to prebend. It definitely needs some adjustment, though. A touch in the right direction and things will start to fall into place.

Right now, the sail shape at the top is just plain aweful.

Well I would loosen everything except for the diamonds and start to haul everything in on whatever tack you feel is the problem.Remember though you are not sailing in a straight line( I constantly forget this )when the puff hits drive off even to weather a little with it the boat will rocket ahead also your wieght forward I feel is important as this boat does naturelly balance out . It is the constant serpentine with the puffs to weather that support speed to speed aspects. Otherwise the speed-stall-speed triangle will squash your day. Anybody else in on this?

mmurphy384

I think your looking in the wrong place. The Sabre I have has a greater curve than yours and my spreaders are set at 1 3/4".

Flatting the top is more a function of main sheet tension.

How old is your sail??

Something else to think about. This boat like any cat needs to be brought up to speed before you can sheet hard. Try footing and get the boat moving, once it's moving start sheeting a little at a time. A few clicks. Wait about 15 seconds then 4 more clicks. Keep doing this. If the hull is popping up add downhaul and then more sheet. The point is keep the boat moving fast. The faster you go the more apparent wind you make and the tighter everything goes, the higher and faster you go to weather. If you lose the speed you need to ease and start again.

If your just sheeting in hard and trying to go fast to weather you won't.

Back to the diamond wires. Set them at 1 3/4". Get a loos gauge and set the tension to 400 lbs( make sure the mast is tensioned so it's straight) and forget about it and move on to the next thing.

I've been busy finishing a overhaul of my boat so I haven't been around much. Hope to have a little more time now to spend here.

Also I sent you a PM with my PH# so if you like call me and we can talk about some of this. I hate typing. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Hey Mike,
Sorry I haven't had a chance to call. I'd still like to talk and will call in the next few days or over the weekend.

I finally got my mast back up and the difference was incredible. I opted to leave a fair amount of spreader rake (probably a 3" gap) in there and then tightened the diamonds until about a 2.5-3" gap could be measured between the luff track and a straight line held taught along the luff track (picture the mast with a line tied from the tip to tip along the luff track. As the mast bends,you can measure the distance and use that to gauge your static prebend.

Having the extra leverage has made a huge different in the performance in the sail. Granted, all sails are different, so this might not be for all. Over time, I might back off the spreader rake as the downhaul is a bit too sensitive. It doesn't take much to fully flatten the sail.

At any rate, I'm much more in the ball park then I was before. I have something I can work with now (and tell tales start flowing when I make adjustments . . . so I guess that's a good thing!).

Anyway, thanks to everyone for the suggestions. I did the best I've ever done in my Tuesday night races last night and I attribute much of it to the prebend tuning.

Hey Murph,

If you need more flattening in the top of the sail, I suggest you try adding some mast rotation. The unstayed portion of the mast, above the hounds, bends sideways on its minor axis. The more the mast is sideways to the leech, the more the leech tension (ie: sheet tension) will bend the top of the mast. More bend will give more flattness.

GARY