Can't create mast pre-bend

Moderator: GreenLake

Can't create mast pre-bend

Postby holstein » Mon Apr 11, 2011 11:12 pm

I'm still tinkering with my DS-I, trying to get it tuned up correctly.

It has become painfully obvious to me that I do not have any pre-bend in the mast. And with my limited knowledge of physics, I can't figure out what I need to tweak to get the mast to bend.

I have a DS variety in which the mast is stepped on the cabin top. The mast has a pin about an inch from the base, which catches in a bracket mounted on the deck.

I have adjusted the shroud lengths and forestay tension, but have not been able to achieve the desired results. The mast stays perfectly straight.

My thoughts were that the swept back spreaders would exert a forward push on the mast to create the bend, while the shrouds and forestay with a virtually connected at the same point, would hold the mast in place.

However, even with an extremely tight rig, still no bend :-(

Researching a little, it sounds like DS's with a keel stepped mast, to create bend you would chock the mast base slightly aft to induce the bend.

But with my setup, I don't think I can do this. The only thing I can think of is to shim the aft edge of the base of the mast to tilt if forward, and use the shrouds to pull it back and create the bend.

Does this sound reasonable ???

Any advice would be appreciated. I losing upwind performance and have a race this Saturday.

Thanks
Bill H
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Postby GreenLake » Tue Apr 12, 2011 12:12 am

With a keel-stepped mast, the shrouds pull at a very long lever and the mast-step at a shorter lever, both using the partners as fulcrum. I think it's the long lever that allows them to bend the mast. (The mast step can provide significant fore/aft forces, so the shorter lever doesn't signify.)

With a deck stepped mast, the only leverage is between the shrouds and the spreaders. The forces are significantly smaller there, and the lever arm is short. (The spreaders would contribute additionally to the pre-bend of a keel-stepped mast, of course).

Anyway, I'm not surprised to see that you don't observe an effect, and to me that would explain why I keep reading that desk-stepped masts are not considered competitive.
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Sail cut

Postby holstein » Tue Apr 12, 2011 8:38 am

Green Lake,

Sailing up wind in light air, I consistently see a crease in the main from the head to the clew, and an extra pouch of material near the luff. Considering I can't create any type of significant mast mend, I would image I would need a slightly different main sail cut. Do you agree ?

Also, is it possible that lengthening the spreaders would help to increase their leverage ?

Thanks
Bill H
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Postby K.C. Walker » Tue Apr 12, 2011 8:50 am

I have "tabernacle" mast on my boat and I've been able to get one inch of pre-bend easily and about 2 inches when desired. I looked to the Thistle class for inspiration on how to do this. They have essentially a deck stepped mast. The mast base fitting has a rocker on it. When they want more prebend they shim the back of the rocker.

The way I set mine up is a combination of the way a Daysailer gets prebend and the way a Thistle gets prebend. I used the mast stub that goes through the cuddy top and to the keelson with a system to be able to chock the base forward and back, like would be used to get prebend on a DaySailer. This tilts the "tabernacle" forward when I chock the mast stub base back. I only use the rear a pin in the "tabernacle" which allows the mast base to float. I can adjust the amount of gap at the front of the tabernacle to limit the amount of float that the mast would have, therefore controlling the amount of prebend or how easy the mast bends.

My Allspar mast is very bendy so I don't need a lot of rig tension to induce bend. I do use my trailer winch to tension my rig, though.

With a 20:1 boom vang and this mast base set up I can easily achieve overbend "speed" wrinkles in the mainsail. If I look at the pictures from the North Sails website and compare them to the wrinkles in my sail the distribution seems to be about right so I think the mast bend I'm getting is about right.

I was not able to get any control of my mast bend before I did this.
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Postby K.C. Walker » Tue Apr 12, 2011 8:58 am

As to spreader length: the length of the spreaders controls side bend mostly. The sweep of the spreaders controls front to back bend.
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Tabernacle pin

Postby holstein » Tue Apr 12, 2011 9:59 am

KC, Thanks for the reply.

There is one point you made that seems counter intuitive to me.

You said that you only used the rear pin on your tabernacle.
I would think that you would want to only use the forward pin so that the tabernacle hinge does not open when you pull the mast back with the shrouds. If you use the rear pin, doesn't the tabernacle lift ??

Also, do you know if it is possible to convert the configuration I have to use a tabernacle ?

With the mast stub moving, how do you keep water from entering through the cuddy top ?

Thanks
Bill H
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Postby Mike Gillum » Tue Apr 12, 2011 11:33 am

Regardless of whether your DS Mast is stepped through the Cuddy Cabin on the Keelson or hinged at the Cuddy Cabin the ability to create prebend and mast bend aren't too far removed from each configuration.
If your mast is stepped through the Cuddy Cabin on the Keelson the hole in the Cuddy Cabin significantly limits the amount of fore/aft travel of the mast as it pertains to prebend or bending under load.
The front of my mast contacts the front of the hole in the Cuddy Cabin the moment I start to crank down my jib halyard using the bow winch to connect the forestay.
My mast is now bending between the top of the Cuddy Cabin and the very tip of the mast through a combination of tension created by the shrouds and forestay to create 1-3/4" of prebend and the load from either the mainsheet, boomvang, or both in my case.
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Postby K.C. Walker » Tue Apr 12, 2011 11:43 am

Yes, I can see now that it might seem counter-intuitive.

The DaySailer method for pre-bend is to bow the mast back by using the shrouds and the cabin top to pull against and then adding compression with rig tension. Many people use quite a bit of rig tension so that the mast is more flexible if they have a stiff mast section like a Proctor.

The method I described uses rig tension to compress the mast only and it is the compression on the backside of the mast that bows it forward. My "tabernacle" rear pin is about 1 inch behind the sail track which helps in bowing the mast forward.

The partner opening on the top of the cuddy is how most DS 1 and DS II's are made.
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Vang tension

Postby holstein » Tue Apr 12, 2011 11:59 am

Perhaps my problem is not shroud tension, but instead a lack of vang tension.

I'm set up with a simple 4:1 vang. Perhaps I need something more substantial.

Does applying vang tension to create mast bend do any harm to the main sail ??

Thanks
Bill H
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Postby Mike Gillum » Tue Apr 12, 2011 2:21 pm

We're attempting to trick the wind through a wider wind range using the same rig tension from 0-30 knots through sheeting and vangsheeting.
Tension from the the Shrouds and Forestay creates the proper prebend before sails are raised.
Mainsheet tension pulls the leech of the main downward and bends the upper 1/3 to 1/2 of the mast to bend.
The boomvang when applied also pulls the leech of the main downward bending the upper 1/3 to 1/2 of the mast but it also "drives" the boom towards the bow bending the lower 1/3 of the mast.
Bending the mast to flatten the Main helps to depower the Main allowing you to safely sail in windier conditions while Forestay tension helps keep the entry of the Jib flatter so that when you ease your Main in a puff the Forestay doesn't sag that powers up the Jib capsizing you.
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Postby K.C. Walker » Tue Apr 12, 2011 2:33 pm

Boom vang tension is not going to cause any damage to your sails. There is the possibility that it could, in time cause, damage to a weak gooseneck, though. My stock Allspar set up with a slide in gooseneck has held up okay for a season and I don't see any signs of damage... yet. Mike had the same set up and he was seeing signs of it mushing out so he went to a Ballenger set up with a more robust gooseneck.

With a 20:1 boom vang you would get a lot more control over mast bend. With 4:1 I was not getting any mast bend. I can get a lot more mast bend when I tilt the mast step than when I leave it flat, though.

So far I haven't broken anything doing this and it's been fun. I don't race so I can't tell you relative to other boats how much faster this set up is than it was before. I can only tell you that I've enjoyed the set up and I can tell that I'm sailing faster and with more control. With this set up I'm happy up to anything including force 5. I'll do force 6, if it's not too gusty, with the setup but I really want to have good crew to make it fun.

So, maybe it's not the vang that would be wearing out the sails but the heavier air that you would be sailing in. :-)
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Postby GreenLake » Tue Apr 12, 2011 3:27 pm

I had written:
With a deck stepped mast, the only leverage is between the shrouds and the spreaders. The forces are significantly smaller there, and the lever arm is short.

I stand corrected: K.C.'s technique of only supporting the rear of the mast creates another asymmetry that I hadn't considered. (I implicitly assumed the pivot point to be central to the mast). Having the pivot point a full inch behind the mast should definitely magnify the effect.

I'm only familiar with the stiffer mast profile for which this effect would be smaller.

Being able to observe the mast-bend by the remaining gap is an interesting technique.
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Postby Baysailer » Tue Apr 12, 2011 3:55 pm

http://www.jet14.com/articles/schwenk_vang.htm

On this topic mast bend and vang use, here's a pretty good article from the jet 14 site where they use a lot of pictures to help with reading the sails.
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Postby Mike Gillum » Tue Apr 12, 2011 5:45 pm

Like K.C.'s mast the mast that came with #2772 originally was a gold anodized non-tapered Allspar with a silver anodized foam-cored Dyer boom.
Being oval shaped the Allspar didn't have a lot of fore/aft bend but did bend side to side easily though that freaked me out all the time.
It wasn't that the Allspar wasn't competitive as we finished 2nd overall to Dave Keran in DS A Fleet at the 2007 High Sierra Regatta after being tied for 1st and losing the tiebreaker because Dave had a 1,1,3 series while we had a 2,2,1 series.
The real problem was the original Stainless Steel Sliding Gooseneck was slowly being compressed into a SS mass/mess and didn't really stand a fair chance from the 15:1 boomvang.
Pretty quickly I was looking for a replacement boom that had an internal outhaul and an end that would allow me to convert to a standard SS Thistle Gooseneck.
DS #1154 became available with a nearly new tapered Ballenger Mast and Boom. The price of #1154 was a little more than a new Proctor Boom shipped to the west coast with a nearly new tapered Mast thrown in for free closed the deal.
Last year I converted the stock Ballenger Gooseneck with a Thistle Gooseneck knowing the loads in a Thistle are probably 5-10 times greater than a DS.
The
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Postby K.C. Walker » Tue Apr 12, 2011 6:03 pm

Baysailer,

Thanks for the link to that article. It's a good rundown and great to have photo illustrations.
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