Headstay Tension Downwind

Started by Brad Buck, May 23, 2007, 01:36:36 AM

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Brad Buck

How tight is is the forestay supposed to be when going downwind?
Ours is very slack ( it starts to coil and kink) sailing down wind in 15kts - is this normal or do we need to add tension to the uppers?


Justin Scott

#1
I believe yours is an aluminum mast.

The uppers should be set at somewhere around "35" to "40" on the Loos gauge scale. Loos Pro Tension Gauge (LPT 1) .
35 is a good all round average setting. Crank up to 40 when it is in the higher echelons.

Forestay tension upwind is achieved by putting a chock in front of the mast. A chock should be wide enough so that the headstay does not sag upwind. Try 1/2 - 3/4 inch but experiment. Other opinions about chock size welcome (I have switched to carbon and sail with more chock than I did with alumninum).

Downwind in 15knts there will be some sag but not too much. The mast is bullet proof. Look up at mast and ensure it is not inverting (bending forward). If in doubt, add some more to the uppers (NOTE a mast chock does not help prevent inversion)

In high winds (25/30+) downwind. :-
1)The leach of your sail is your backstay (keep some tension on the leach with the vang).
2) You can put some prebend in with lots of vang before the weather mark, then haul on a bar tight cunningham to help keep some prebend in before you ease the vang and turn downwind. The cunningham and luff of the main tight between mast tip and boom helps keep the mast straight.(like the string in a bow) (Remember to ease it again as you go upwind).
3) 40+ on the uppers.
4) Its still a bullet proof mast but this will help it.

FYI For carbon mast owners....the cunningham is very effective downwind. I am sailing with less tension on the carbon than I did with aluminum so in high winds it is important to put in a good amount of prebend.We move the chainplates futher aft for the carbon rig so prebend is easier to achieve.

Good sailing. Its fun in 15 knots!
Viper - Mambo Kings
Right Coast Refreshments Committee

Paul Gillard

Justin-
Bit confused-in strong winds (20+) why would you ease cunningham going upwind? Isn't that the time when you want to flatten the main and move the draft forward? Easing the cunningham is going to affect pointing (by giving more draft) and create power when you are trying to de-power.
  Downwind -we tend to ease vang to open leach and make it easier to spill wind if we need to counter any tendancy to round up- although we are actively working the vang downwind most the time(our control comes back to helmsman and it is continually worked in conjunction with mainsheet).
   I dont even give any thought to possibility of inversion because our mast(alloy) just seems so bulletproof. With the outers wound  up in stronger winds - would have thought that was enough. I am sure the Carbon rig is different.
   Maybe I have got it wrong?

Justin Scott

Agree......the Viper rig is bullet proof and I have never heard of anyone losing a rig downwind without some failure in shroud attachment or spreader.

The use of the cunningham downwind is most effective with a Mylar mainsail (or reinforced Luff) and carbon rig. With a dacron luff (which stretches more) and an aluminum mast (much stiffer) it is probaly much less noticeable. However, I still put on loads of Cunningham in the big breeze downwind on the Ali rig. At the margin, it all helps stiffen the rig and transfer gust into power forward rather than mast bend.

Easing the cunningham upwind?  The cunningham has 2 effects on the main. (1) It brings the draft forward. This is more noticeable in a dacron sail and helps a dacron sail (especially an older one). It is less noticeable in a Mylar sail and less needed, due to the less stretch. (2) It twists off the top of the main. This is more noticeable in the Mylar sail and somewhat less in dacron (but still quite significant).  Obviously I have more cunningham in bigger breeze upwind for both those good effects. BUT everything is about balance. I whale on the cunningham downwind. I tend to ease a bit upwind (still with more than I have in medium breeze) If you have too much cunningham upwind:-
(a) The twist at the top can act like an airbrake
(b) The draft can be brought too far forward
(c) On a carbon mast......I want the rig less stiff upwind. Flex is good upwind. 

Having said the above...with an aluminum rig and a less new/older dacron main.....I admit that the cunningham is pretty much whaled on upwind as well.

Vang Downwind?
Even in huge breeze, I adjust the Vang for a straight leech. Releasing the vang is the last emergency measure to avoid a big broach and I try to put it back on as soon as possible. We do w/l courses, so if a puff hits we hike and bear away. I can see it would be difficult with a tight reaching leg to navigation marks, but I would try and depower in other ways first. We play the vang downwind as well, but really trying to keep close to a straight leech. The gnav controls are led back to the helm which is nice.

Aluminum Rig owners:-
I think the short answers to sailing in big breeze is
1) Check shrouds and spreaders. The Ali rigs are 8-10 years old now. The masts themselves have held up great. Some of the components are reaching useful life expectancy. Eg. Check spreaders where they are bolted onto the brackets. Those stainless steel bolts have been chafing and pressing against aluminum for 10 years, spreaders are cheap to replace. Check cotter pins. Check swages on the shrouds. Check all connections shrouds to mast and shrouds to chainplates.
2) Chocks in the mast partners to tension the head stay
3) Plenty of tension on the uppers. They are dynaform and they can take a lot of tension. 45 on the loos gauge is fine.LPT 1 loos gauge.
4) Fwd crew has the vang in hand at all time downwind......but only blow it if on verge of a broach.

Thats my 2 cents.       
Viper - Mambo Kings
Right Coast Refreshments Committee

Paul Gillard

Thanks for that- will play around with the cunningham a bit more- one of the problems with a dacron main with a sewn in luff cord is that it  bunches up and jams in the internal track, so once tension has been put on it only releases to a certain point. It might be my track ? Do put lube on the luff when hoisting. I find that it is really important to get max. hoist on main halyard even if foot seems quite high, because once you start applying cunningham it stretches down easily(might be my main!) and if it is bunching up by the cunningham it is limiting your adjustment.


Kay VanValkenburgh

Earlier today I wrote up my thoughts about rig tension in another thread (topic: rig tension - shrouds and chocks) and just now read this thread. Oversight on my part. I notice, in any case, that Justin and I have different opinions about a few things. Several others in common, though. Will suggest in the other thread that people read this thread, and vice-versa. Interested what consensus we come to. Please post your experiences.
first boat I ever owned was viper #28; it was a gateway drug.
my current viper is #98; I can't sail it enough.
the resulting sailing addiction's ok up to that point, but come Winter whydahell do I also have to frostbite a Laser?!

Alan#38

Quote from: Justin on May 24, 2007, 07:54:32 AM
We move the chainplates futher aft for the carbon rig so prebend is easier to achieve.

Justin,  What do you mean when you say you move the chainplates farther aft for the carbon rig?  Was this recommended by Rondar?  Is there a recommendation on where to move them?

Justin Scott

The carbon mast comes with a chainplate kit. It is very easy to install - takes about 30 minutes for someone with zero technical skills apart from ability to operate an electric drill. The new chainplates are attached to the gunwhale rail. They are aft and out from the existing chainplates. My recollection is 13ft 8 inches aft of the bow. Paul Z has the precise dimension.
Viper - Mambo Kings
Right Coast Refreshments Committee