Author Topic: Vang Tension Downwind - Keeping the mast up  (Read 6233 times)

Clifford Begnaud

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Vang Tension Downwind - Keeping the mast up
« on: July 21, 2009, 04:45:59 PM »
On the Yahoo group, Drew Harper made some comments about vang tension when going downwind and I thought it would be best to move that discussion here in private.

Here's part of the post Drew made:
"First off, the Euro rig IS .003 thicker than the US rig, making it slightly stiffer, however, the boat doesn't have a backstay and to kick the vang off with too little prebend is flirting with disaster.
Remember, you have four things keeping the rig in column with the kite up on a U20. Prebend, mainsheet, vang and cunningham. " end quote

It is my understanding that most of the mast failures have been in a sideways direction at or near the Main Halyard exit, this is how my mast broke. If the boom is out near the shrouds and you have a lot of vang tension, the boom will be pulled into the mast in a sideways direction. Would this not actually contribute to the mast failing? I can see how vang and mainsheet tension would help keep the mast in column when the boom is not all the way out, but when it's blowing hard, sheeting in the main would cause a round up, no?

I may be missing some key point here so I hope someone can straighten me out about this vang issue.
thanks,
Cliff Begnaud (Clifton is a famous U20 racer from San Diego)
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Phil Kanegsberg

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #1 on: July 22, 2009, 12:51:21 AM »
Cliff,

I believe that you are correct.  The failures happen due to the mast failing sideways, not forward.  The data collected by Van and the broken masts show this.  I am still running Jim's prototype 'high wind kit' and I often see one side of the kit loose while the other is tight.  When I start to get concerned about the sideways bend, I ease off the vang a bit.  I am not concerned about blowing the vang in any condition that I have sailed in.

I should point out that at some point during the 45 minutes that the half furled jib was flogging, the rig pumping wildly and we were repeatedly getting knocked down during the Trans Tahoe, the half hitches securing my 'high wind kit' came untied.  Once we got the jib sheets untangled, a reef line rigged and the main reefed, we sailed back across the lake with no issues.  I only noticed that the 'kit' was disconnected when we took the stick down.

We had much more fun at Huntington this last weekend...

Phil
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Trent Watkins

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #2 on: September 24, 2009, 12:46:55 AM »
Most recent regatta on UFO #27 in SF. Sarcoma cup where the small sport boats were put on what many of us call the big boy course right in the middle of the "slot" while the wabbits and other small boats sailed the small boat course over behind Angel island.

UFO and BreakAway were the only two U20's we had the T650 out there with us but they had a hell of a time in the 30+ knots and brutal wave action that put green water over the bow and out the cockpit while going up wind. UFO raced with a reefed main all day - full crew. My uppers were pegged putting the rig tension around 520lbs even with the reefed main I had to over trim the main to keep the head stay from developing excessive slop on the down wind legs - it might have slowed us a tad but when your skipping across the wave tops at full tilt boogie who cares about a half knot? ha ha

We don't have the high wind kit on UFO and yes it was a concern but we found that keeping the main over trimmed kept the mast fairly happy with the reefed main. BreakAway called it a day early as they were double handed and not set up with a reef they took one hell of a beating. UFO started the 3rd and last race the conditions had become much worse we watched an Olson 30 with its blade jib and 8 guys on the rail being blown sideways trying to go upwind as we too were doing more sideways than forward if I got even the slightest bit off kilter on the helm. We did round the mark and the boat exploded on to a flat out run under white sails we called it a day and went home.

Boomvang doesn't seem to have as much effect but in other classes with fairly flexible masts a tight boomvang is known for blowing out the mast if its not eased at the corners-  the main sheet or as Phil notes the highwind kit stays does help a bunch when you need to keep the mast happy.

The Tahoe event and the Sarcoma cup event both were very heavy winds and pretty rough water conditions not something I would intentionally choose to sail in and with precautions ie proper tension - reefing - paying attention to kite loads ie headstay sag the old alum rig will hold up fine. Were we faster with the reefed main - you bet! We lead the T650 around the course and could out point BreakAway by a good 8+ degrees on the upwind legs. We are sold on the reefed main approach anything reaching into the upper 20's our full main is a nightmare but reef that sucker and its game on till 40 knots.

Trent Watkins

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #3 on: September 24, 2009, 12:56:06 AM »
Picts of UFO racing SF Bay "slot" where the wind was blowing 30 before the first start.
By the end of the day many boats had dropped out - we saw enough big boats shred expensive sails and gear a fleet of U20's could have been bought with the replacement value.

Yes the Open 5.7's had a full on fleet of boats and were finally sailing in conditions the boat was built for with those little sails.

The close up shot we passed the photo boat by about 5 ft Peter the photo guy was scrambling for his throttle at that point.
http://photo.beatsarcoma.org/Events/Sarcoma-Cup-Sunday-On-The/9400515_gdpxV#P-18-12

Tac Boston

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #4 on: September 29, 2009, 07:42:28 PM »
In that photo sequence that Trent shows, I notice that that boat is reefed. Are "we" still breaking rigs when the main is reefed? (I would venture to say no but I really am curious if they do or not.) If not I have a thesis for you all.

BTW, I truly believe as do a few of the others that have broken rigs that they, GO FORWARD, then sideways.

There is a picture of a boat on Sailing Anarchy from late may or june. That rig is so far forward that it loses all stabilty and breaks.

Also I sailed in Kingston where it snorted, we never let our vang off more then 4", of line, and never came close to breaking the rig. Andre did. Talking to Andre, Jim Pearson and his crew, they all agree that the rig went forward first.

I understand that the Ottawa study shows that the rig bends more out of column sideways is true, can't argue the #'s. But what the study fails to show in my humble opinion is what happens when a boat goes from fast to slow. Lets face it most of these rigs do not break when the boat is going from slow to fast. What happens when a boat is going 14-17 and plugs into the wave ahead and drops to 7-9 knots?

I am in the middle of writing my thoughts for the Stirring the Soup posts so you will hear more from me. I have also talked in depth with Brad about this. Maybe when he gets home from the Sonar Worlds he will pipe to help as well.
Tac


Clifford Begnaud

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #5 on: September 29, 2009, 11:27:54 PM »
Tac,
Mine broke with a reefed main and from fast to faster. We were showing 11-12 knots going downwind with reefed main and jib, we launched the spinnaker and jumped up to 16+, then we got hit with a huge gust (we briefly hit 20 knots of boat speed !!). The mast tip went forward which weakened the lower section when it bowed out to aft and then the boom, which was at the port shrouds, blew the mast out to starboard during the gust. The vang was not tight and we did not hit the back of a wave. It almost seemed like slow motion as we watched it happen. The boat was in perfect control with a neutral helm when this happened. Upper shrouds were set at over 500 lbs. The break occurred at the lower edge of the main halyard exit.

Feedback from Jeff Canapa after this event indicated that the majority of the broken masts that he knew about occurred in this manner, meaning that they broke in the lower section in a sideways direction, not fore and aft. Though I have seen one break aft, but I believe it was because the shrouds were much too loose.

I think this is why Vans testing showed the chicken stays to be so effective. It really makes sense that they would prevent this sideways failure mode. Other things that can be done to help reduce tip movement foreward:
1. Switch back to stretchy lines for the spin halyard and tack line. High modulus line transfers the load in a shorter period of time reaching a higher peak load. (at least this is my understanding of it)
2. Switch to Dyform upper shrouds because they have less stretch.
3. (yet to be done) Install chicken stays
4. Sail with boom sheeted in a bit if possible.
5. Don't launch spinnaker when it's gusting 40 knots

One other contributing factor that we don't hear much about is boat flex. I believe that the boat itself is flexing at the chainplate's (yeah, I know they are not really chainplates but I don't know what else to call it) when under heavy load.

Tac, I look forward to seeing your comments on the Carbon Mast, in particular the issue of sail cut and if a bit more speed will be tweaked out of the rig with the carbon mast and reworked sailshape.
Cliff
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Dave Whyman

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #6 on: September 30, 2009, 03:21:23 AM »
"One other contributing factor that we don't hear much about is boat flex. I believe that the boat itself is flexing at the chainplate's (yeah, I know they are not really chainplates but I don't know what else to call it) when under heavy load."

I just noticed the other day once I had the rig cranked down (500 upper/320 lower) the gunnels are flexing slighting at the shroud bases.  If we can run with less tension on the new rig to keep it up then that would probably be a good thing.


Peter Juryn

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #7 on: September 30, 2009, 09:17:57 AM »
Try this...

Take a plastiic straw and using a pin, put a small hole in it about 1/4 of the way up.  Stand the straw on end. Push down on the staw with one hand, wait a moment and then with the other hand push on the straw at the hole. 

Voila!

Clifford Begnaud

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #8 on: September 30, 2009, 12:33:15 PM »
Great analogy Peter. Btw, could C-Tech add tabs to the carbon mast to replace the steel plate of the High Wind Kit, or as local racer Marty May calls it... the All Wind Kit.
Cliff

Albert

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #9 on: September 30, 2009, 01:16:57 PM »
The high wind kit is not needed for the Carbon Mast.  The alloy rig as per our mesaurements was three time weaker side to side than fore/aft.   C-tech was asked to match the fore/aft characteristics of the ALloy mast, and since the carbon spar is circular the side to side bend charactersics are the same as the fore/aft.  That means that the Carbon spar is a least three times stronger  side to side than the alloy spar.

Also, all the exit hole for halyard have been relocated the areas where there is little or no load.  The main exits at a sheeve box at the base of the mast.  The spin halyard exits six to eight inches below the gooseneck.  This need to be lowered three to four inches due interfence with Tactic mount.


Trent Watkins

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #10 on: October 01, 2009, 11:20:58 PM »
In that photo sequence that Trent shows, I notice that that boat is reefed. Are "we" still breaking rigs when the main is reefed? (I would venture to say no but I really am curious if they do or not.) If not I have a thesis for you all.

BTW, I truly believe as do a few of the others that have broken rigs that they, GO FORWARD, then sideways.

There is a picture of a boat on Sailing Anarchy from late may or june. That rig is so far forward that it loses all stabilty and breaks.

Also I sailed in Kingston where it snorted, we never let our vang off more then 4", of line, and never came close to breaking the rig. Andre did. Talking to Andre, Jim Pearson and his crew, they all agree that the rig went forward first.

I understand that the Ottawa study shows that the rig bends more out of column sideways is true, can't argue the #'s. But what the study fails to show in my humble opinion is what happens when a boat goes from fast to slow. Lets face it most of these rigs do not break when the boat is going from slow to fast. What happens when a boat is going 14-17 and plugs into the wave ahead and drops to 7-9 knots?

I am in the middle of writing my thoughts for the Stirring the Soup posts so you will hear more from me. I have also talked in depth with Brad about this. Maybe when he gets home from the Sonar Worlds he will pipe to help as well.
Tac



Tac we were reefed all day for a couple of reasons. #1 our UK main is super full cut anything over 18knots and UFO easily goes sideways vs forward - hey its their first U20 main it might take a few cuts before they get it right :-) Having said that its great in light air and reefed in heavy air. I've seen the sideways lower bend on my current rig when the main is against the shrouds and the forestay has mucho slop.
Yes the rig goes forward thats easy to see but what the engineer types found is that the sideways push via compression and other factors like main on swept spreader and boom loads are what lead to the main failure point. Launching the kite in 38+knots is something I avoid after getting caught with ours up in 40knots. I found that 38+knots the boat isn't going to go much if any faster full white sails vs kite but you might spend a whole lot of time beating the crap out of the boat and kite when you eventually eat it. In our case the boat was doing great ddw hauling the mail where it went wrong - we rode off a wave top and at the critical moment only had about an inch of rudder in the water as a result we flopped over on our side quite fast. Under white sails we lost maybe 2 knots of speed but were totally wipeout free and still hauling ass. ha ha

That pict it was blowing mid 30's with heavy SF chop that day - down wind runs we never once eased all the way off on the main sheet we simply played the main sheet out till we started seeing lots of forestay slop and took in a bit to snug things up. Couple of factors we were coming off the top of some heavy chop occasionally with a pretty good impact while at speed this shook the rig pretty hard. So having the main sheet on tight helped keep the rig happy for sure.

 I was maxed out on the uppers putting them around 520lbs the lowers were around 250 creating lots of prebend. I do think the boat flexes some - add the stretchy wire which I have and the rig will move around a bunch in those conditions. I think having the ability to lower the rig tension due to less stretchy wire and a stiffer rig would make our boats last longer - not to mention offer a wider range for error regarding proper rig tensions and rig tension vs wind conditions etc.

I will say that we had a blast and the boat is fun when she skips over the waves we just need to improve the rig stability without making major changes to the boat design.

Jim Pearson

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #11 on: October 04, 2009, 05:03:16 PM »
We are now talking about rigs going forward initially before coming down. I agree. We are talking about boats flexing, which anybody racing sportboats, should realize, they are built close to the edge and will get tired and start to flex. Thats why so many of top racers in sport classes keep upgrading to new boats on a regular basis. More powerful sails are being built which only compounds program. Sounds like it is time to allow upgrades to to boat like Carbon if people want and maybe even a backstay preventer to help spread out increasing loads on your boats. It may help keep your boat competitive for a longer period of time. Just a thought.
Jim Pearson
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Drew Harper

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Re: Vang Tension Downwind - Keeping the mast up
« Reply #12 on: October 19, 2009, 04:45:31 PM »
The 'out of column' situation starts with a rig that isn't held in place. The more you keep the shape of the rig in column, the easier it is for the wire to hold it up.

Prebend is historically the way to prevent out of column events. Once the rig overcompresses, there's little you can do. The compression load travels to the weakest part of the rig (read main halyard exit) and fails. This is always accelerated by the bend cycles that the particular rig has experienced. Alloy only bends so many times and the curve of degradation is geometric by nature.

Tac is right in that this is somewhat exacerbated by deceleration issues...stuff the bow and you're greatly increasing the chance of an out-of-column event.

I don't know the answers to this. I do know that I've spent considerable time sailing the U20 and U24 out in the SF slot in big breeze and paid careful attention to rig tune, prebend and column issues. The U24's mast gets pretty damn wiggly and yes, overvanging can crush one but a combination of Vang, Cunningham and Mainsheet helps keep the mast loaded aft, against the pull of the kite. I've not had any issues with the U20 tin rig, but I have traditionally carried way more uppers than may have been required. This had a habit of affecting our boatspeed adversely.

One thing for certain, a well vanged main that is not overly eased won't lie up against the spreaders and impart considerable twist. Twist is the enemy of sailboat masts.