prebend?

Moderator: GreenLake

prebend?

Postby bulldog39 » Sun Jun 14, 2020 12:03 pm

dumb rookine question, I am setting up my new standing rigging and need to know if you measure the mast prebend on the front of the mast or rear? it should be obvious but I cant get my head around it right now. thanks for the help
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Re: prebend?

Postby tomodda » Sun Jun 14, 2020 4:04 pm

Rear. Hunt down North Sail's tuning guide for Daysailers, they'll give you some detail on the procedure. Their exact measurements are for THEIR sails though, so don't take them as gospel.
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Re: prebend?

Postby GreenLake » Sun Jun 14, 2020 4:09 pm

Good suggestion about the tuning guide.

I've always understood that the mast should be bent so the tip moves back, not the other way around. (So, hollow on the back).

But I have the massive, stiff one that doesn't take any pre-bend.
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Re: prebend?

Postby lemsteraak » Tue Jun 16, 2020 9:52 pm

Hi Bulldog, - Short answer, prebend is determined by the sails, not the boat.

Not a rookie question, prebend affects your sail's camber or the amount of power it produces.. If you take a look at your mainsail and sight along the luff (the leading edge), you will notice it isn't straight but has a slight arc to it. When hoisted, the sail will induce a bit of bend in the mast. As the wind picks up the pressure in the sail will make the mast match the arc of the mainsail. If you have a very flexible mast then it can overbend and you will see a large wrinkle from the clew (the back corner) diagonally right up the middle of the sail.

Some sailmakers, like Hank Jotz, an old time sailmaker, would give you instruction to put in a forward pre-bend to your mast to counter the bend the sail will induce. You peg the foot of the mast forward so the front of the mast is touching the front of the mast partner, the hole in the deck, then tighten the forestay to force the mast into a forward bend. Your spreaders would also be set in a forward position to allow the mast to bend forward. I talked with Hank when I ordered a set of sails and he asked me what mast I have. He wanted to know how flexible it is so he can calculate the camber of the sail so it matches the bend of the mast.

Daysailers have less control over mast bend than boats like keelboats where they run backstays. By tightening the backstay you can depower or flatten the mainsail in high winds. It also pulls the top of the mast back which has the added benefit of tightening the forestay so the jub;s shape is better. We can't do that with a Daysailer because we don't have a backstay so the sailmakers have to be creative.

This is just scratching the surface, prebend and the theories behind it are about as advanced as they get.
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Re: prebend?

Postby GreenLake » Wed Jun 17, 2020 12:11 am

@lemsteraak Thanks for the details. (Hank did not make any suggestions like that to me at all, which confirms my impression that my mast is simply too stiff.)
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Re: prebend?

Postby lemsteraak » Wed Jun 17, 2020 12:59 am

I don't know enough to comment on the stiff vs flexy mast thing.

but .... I have seen first hand how if your mast doesn't have a consistent bend, not good. If you can get your sails match the bend in your mast, that is a very good thing.

Yes, I do have a story..... In SF Bay, every other year, totally under the radar, there is an international invitational regatta with identical keelboats, just for the fun of sailing, no pros allowed. I went because they borrowed my boat and I delivered it to the SFYC in Tiburon. I had a beer and watched the top German team, absolutely torture my friend Dave's boat. They had six guys hanging off one of the shrouds while they sighted the old wooden mast. Christoph, the German skipper, told me "de boot, she is slow" and they were trying to determine why. How can that be? my buddy Dave takes perfect care of his boat, it sparkles and besides they are all identical. The Germans placed 12th in the practice race, and they were not happy, so while everyone was partying at the club, they took the mast off this really crappy old boat and swapped it with the really pretty mast that was on Dave's boat. They were much happier the next morning when they were busy tuning the boat. First race - 1st place, and I had a lot of questions for Christoph. They had one of the crew go up the mast and inspect everything on Dave's mast. There was a section of the mast the the wood was a slightly different color and they couldn't get an even bend so it was impossible to tune that mast. They brought two sets of sails from Germany, one for a stiff mast, the other for a flexy mast and they didn't like the set of either. The crappy old mast that they swapped in, it looked really bad, hadn't been varnished in years and had moss growing off of it, but they loved it, because they could tune it. When I caught up with Dave, I told him the story and he fessed up and said, yes, there was a repair done to his mast years back. He had a new mast made.

Here is a small article I wrote up years ago - http://www.sfbayfolkboats.org/archives/ ... ER658.html
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Re: prebend?

Postby GreenLake » Wed Jun 17, 2020 2:37 am

Very nice story.

Well, I'll have to see that I sail with someone who can take the tiller so I can observe how much bend gets introduced when I sail. All I know is that Hank quizzed me about both mast and sailing area (we have much less wind on typical day that SF Bay) and built the sail based on that. Next time, I'll (have to) go with a different sailmaker, so I'll make sure to really get their input.
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Re: prebend?

Postby tomodda » Wed Jun 17, 2020 9:39 am

As far as I know, and have observed by testing on the trailer, I have two controls to bend the mast on my DS1. My jib halyard and my vang.

Let's start with the vang. It's very simple and very obvious when you do it on the trailer - when you crank on the vang, you're pushing the entire boom into the mast, bending the bottom 1/3 forward and the top backwards. If you raise the sail, it's even more pronounced as the leech helps pull the masthead back. And I can see/feel it on the water, as the wind picks up and I start getting overpowered while close hauled or reaching, I'll trim the vang HARD. The mast bends, the sail flattens, the draft goes forward and the boat gets back under control. I'm sailing instead of fighting to stay upright. I have a 20:1 cascading vang, per someone's suggestion on this forum, and it makes all the difference.

On to the jib halyard, I have a wire luff and a fairly low-stretch halyard (though not dyneema). If I really tighten the halyard, then the middle of the mast bends forward (2/3 of the way up, right?) and tip goes back (again pulled by leach of the mainsail) and in theory the sail gets flatter/depowers. Of course, extra halyard tension is also affecting the shape of the jib, pulling the draft depth forward, so it's a balancing act. Testing on the trailer I can see it. Working with it on the water, actually sailing, I haven't noticed a lot of useful bend/main flattening from really cranking hard on the jib halyard, but the jury is still out on this as a method of controlling mast bend. When it really counts - high wind - I'm too busy sailing plus I've already cranked on the vang. I did set up a jib halyard tensioning system (3:1) and use it quite a bit, but it's for getting a nice luff line on the jib and helping me control its shape.

Anyway, lacking backstays, that's how I control my mast bend while underway. Any other bright ideas?
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