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Scary Inverted mast

Posted By: dave mosley

Scary Inverted mast - 06/03/03 12:54 AM

Have you ever seen a mast bend opposite of the spreaders?
I have, not too fun either. Hopefully you will have solutions for me.
I have only had this boat, a G-Force 21, for one month. Sailed it twice, both in races. The first time I sailed it the mast did everything correctly, so much so that I took the over-rotator arm off as it tangled on the Jib line every other tack. I never had problems with the mast over-rotating going to weather, or under rotating down wind. Now I only had 190 + 95 pounds on the wire, and it was blowing 15-22mph.
I took the rotator off before this weekends race.
When we were going to weather double trapped we noticed the jib flapping and the main twisting off in a very unusual way, and as gusts were hitting us, some as high as 20+ I cracked off the main. We looked up and to our horror the mast was over-rotated 180 degrees, and inverted opposite of the spreader configuration then bent back in the correct shape from the hounds up as we had a good bit of downhaul on.
I cant beleive, nor could my crew(Jake Kohl and Tommy Metts) that it did not snap in half. I have never seen any metal bend like that and come back straight. We rigged a rotation limiter and stayed off the wire, somewhat nursing the boat to the finish. It still didnt behave correctly when I cracked off the main sheet.
so what is the solution?
Tighter diamond wires?
Less rotation( I usually sail to weather with the rotator pointing at the shrouds)?

My understanding is this is the same extrusion as the NACRA 6.0- and 31.5 ft, one set of diamonds.

Any help will be greatly appreciated.

David Mosley
www.seacats.org
Posted By: thom

Re: Scary Inverted mast - 06/03/03 03:04 AM

hello David- Theres several reasons for controlled mast rotation...you may have found one.

thom
Posted By: Anonymous

Re: Scary Inverted mast - 06/03/03 01:02 PM

This also happened to my N6.0na. I have also heard of another 6.0 that had the problem. More spreader rake and more diamond tension are the only suggestions I have. After mine inverted, I straightened it, added some rake and tension, and have not had a problem since then.
Posted By: Kevin Rose

Re: Scary Inverted mast - 06/03/03 01:48 PM

Quote
. . . We looked up and to our horror the mast was over-rotated 180 degrees, . . .


David,

I'm trying to picture this. When you say "over-rotated 180 degrees", does that mean that the luff track was facing forward?

As for spreader rake and diamond tension, I've got my rake set to 1 3/4" and the diamond tension at 600 lbs, giving a pre-bend of 1 1/4 inches. (I run a bit less rake and pre-bend than others because I weigh in at 200 lbs and often have crew at 160 - 180 lbs.)

What does your pre-bend look like? (Should be greater than an inch, say 1 1/4 to 1 3/4, with spreaders raked to 1 3/4 to 2 inches.)
Posted By: carlbohannon

Common Factor - 06/03/03 01:59 PM

Interesting

I have now heard of 4 cases, 3 6.0's and a G-Cat. The interesting part is they all use the same mast extrusion.

Has anyone heard of a case of mast inversion that does not use the 6.0 extrusion?

It could be it is the only extrusion thick and soft enough to survive.
Posted By: dave mosley

kevin - 06/03/03 02:28 PM

yes, the luff track was facing forward. Funny thing is we didnt break any battens.

David Mosley
Posted By: Kevin Rose

Re: kevin - 06/03/03 03:40 PM

Quote
yes, the luff track was facing forward. Funny thing is we didnt break any battens.

David Mosley


Wow,

I'm still struggling to visualize what it would take to cause such an orientation. Looking forward to hearing other's input.
Posted By: SoggyCheetoh

Re: kevin - 06/03/03 03:55 PM

It would seem like the rig tension would have to be pretty loose to allow the mast to rotate so far around.
Posted By: Kevin Rose

Re: kevin - 06/03/03 04:24 PM

Quote
It would seem like the rig tension would have to be pretty loose to allow the mast to rotate so far around.


I agree. With my rig tensioned and without the sail up, the shrouds are pretty tight by the time I rotate the mast to 110.

You would also think that the forestay would be trying to prevent a 180 degree flip.

Seems like the mast would have to pop into an inverted position first (due to incorrect spreader/diamond setup). Then, could tension on the leech pull the mast around somehow?
Posted By: Jake

Re: kevin - 06/04/03 04:17 AM

I was on the boat and it was definitely startling to see the "S" shape the mast had presumed with two of us on the wire and one on the hull. The rig tension was not terribly tight and while the diamond wires were snug, there was not a whole lot of prebend when compared to my 6.0NA. Additional downhaul brought the mast closer to the 6.0 mast shape that I was used too. I think that the diamonds and the rig could use more tension to prevent that negative bend.
Posted By: dave mosley

Jake - 06/04/03 11:06 AM

Thanks for your handy engineering that saved the day and still let us enjoy 3rd.

David
Posted By: Mike Hill

Re: Jake - 06/04/03 05:02 PM

I would sweep the spreaders back at least 2 1/2" and make sure the tension is 450lbs or greater.

Mike Hill
H20 #791
Posted By: catman

Re: Dave - 06/04/03 10:35 PM

Hi Dave, I met you on the beach when you were picking that beast up.

That mast is not the stock G21 mast as you know. It's my understanding that it is a mast from N6.0. The stock mast for that boat was 34'. So you should be able to tune it like a N6.0.

What I can't remember is if it had a boom or not. If it's boomless you might want to move the main sheet foward on the clew plate (if you can) resulting in a tighter foot. That might help the problem. Your gonna want to have the abilty to control rotation so you need to find the solution to the lines fouling.

Mike
Posted By: dave mosley

Re: Dave - 06/05/03 01:30 AM

I cant imagine having more sail area on this boat. It is a bear to handle with the area it has now. It seems to fly a hull in no wind, and I really have a time keeping t down when the winds up. The rig now is boomless, and doesnt look like it has ever had a gooseneck attached.
I was told by Glen that Hans used the 6.0 mast extrusion because it was readily available and in order to get an extrusion custom made he would have to buy 4000 feet. Maybe a load od BS, but thats what he told me. I didnt get any riggin tips, and man I sure need them, so the 6.0 tips are going to be my gameplan for now.
The rotation arm was basically an over-rotator , mounted forward of the mast and cleated to both sides of the main beam. I am going to switch it to a limiter, facing it aft. Hopefully that will solve some of my problems.

Thanks,
David
Posted By: catman

Re: Dave - 06/05/03 04:08 AM

Dave, a couple guys around here run that foward arm. They have a shock cord running from about where the diamonds attach on the mast out to the furler. This preventer for the jib sheet seems to work. If you haven't you might want to try it first.

I was just rereading your orig. post. Remember you set your rotator by sheeting in your sail while going to weather and looking at the lee side of the sail-mast junction adjust the rotator so the air flows smoothly into the sail. Depending on the sail you might find the spot to rotate to a little more or less then the shroud.

Have Fun,
Mike
Posted By: samevans

Re: rotators - 06/05/03 04:16 PM

Hi Dave,
RE: rotation limiter.
You could try the system that is on the Hobie 20.
It does not connect to the boom, so it will work on a boomless boat.
The line runs from the arm, through a grommet in the tramp and cleats on the hulls.
It is easy for the crew to adjust without moving to the center of the boat.
Even the Tigers and Foxes are switching to it.

RE: postive rotator
Many people like the forward bar and cleats, but I had it on my 18sq and felt it was problematic.
I prefer the bungee cord type, like on my 17. It works with the rear facing bar.
It is automatic and it always holds the mast steady instead of allowing it to flog around in light air chop.

Are you going to the Charleston regatta?
There will be many different variations of roatators on the different boats.
Posted By: Todd_Sails

Re: rotators - 06/05/03 05:42 PM

AS I have previously posted, my inverted N6.0 mast, is still working with manual bending back, then increased spreaker rake and increased wire tension.

Dave,

Do you use a jib sheet jam preventer?

Todd
Posted By: dave mosley

Re: rotators - 06/05/03 09:44 PM

Hi Todd,
I have always used a simple bungee system for jam prevention, but didnt really have time to work on the boat or find the correct settings. This is my plan.
prebend to 2 inches-400#s +/- tension on the Loos Gauge
Tighten my rig tremendously(As Sam can vouch it was pretty loose)
Jam prevention bungee
Mast rotator limiter(it will obviously rotate 180 degrees, so positive rotation is not a factor)
Replace all worn pins, rings, crappy rigging

Thanks for all the input!!
David Mosley
Posted By: Sycho15

Re: Scary Inverted mast - 06/09/03 06:47 PM

Dave, you got a G-Force 21? You lucky bastad. I'm sorry, but I lost your mailing address and forgot to get it again to mail you all that G-Cat literature. There's a brochure on G-Force 21s, too. By any chance do you have the 11' wide Grand Prix model? Those have to be my all-time favorite boat. There's a guy just south of me with one of those and a mast supposedly taller than a Tornado's rig
Posted By: Jake

Re: Scary Inverted mast - 06/09/03 07:06 PM

That 8.5' wide G-Force has a hard enough time tacking with those deep V hulls! I can't image trying to tack an 11' wide version. However, it's fast in a straight line and with PLENTY of room on board.
Posted By: Sycho15

Re: Scary Inverted mast - 06/10/03 02:37 AM

You've got to be pretty well forwards towards the section of the hull with the most rocker. Hans can get these things to tack pretty fast! Of course, with all that sail you've got you could just about tack it with one hull still out of the water....

That G-Force 21 Grand Prix beats up on all the boats at the Olympic Triangle races held in the bay. He finishes the race, furls the jib, and has a drink while waiting for everyone to finish. Hardly ever has to get out on the trapeze wire, either...
Posted By: Anonymous

Re: Scary Inverted mast - 06/11/03 12:33 AM

David,
The mast bend inversion is caused by the big overlapping jib on your boat in high winds. Here's what happens: The forestay sags; the mast leans forward due to the sagging jib luff and this pulls the jib leech tighter. In this situation the jib is just pulling sideways and backwinding the mainsail like crazy. This causes the mast bend to invert if the spreader tips are not 3 to 4 inches behind the mast. With the spreader tips well aft of the mast sailtrack and looking at the mast from the side, tall slender triangles are created due to the spreader sweepback which push forward on the central part of the mast and prevent mast bend inversion. It will not prevent jib backwinding; that takes a small high aspect ratio jib like you see on most beach catamarans. The rig, large overlapping jib, that you have on your G cat is a light air rig only.
Good Sailing,
Bill
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