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North Sails continues to lead the way in Viper 640 sail development and product features. Our Viper team has focused our product line on creating the fastest sails using the best materials and design features to enhance performance and durability. The North Viper main and jib are constructed using Pentex fabric which, in concert with our radial panel construction, has proven to provide the best durability as well as superior shape holding which equates to faster, more durable sails.

Viper M-7 Mainsail

The Viper 640 M-7 mainsail has been designed to take advantage of a tuning system that emphasizes simplicity while optimizing speed! . This shape enables you to chock the mast in light air to induce the proper amount of headstay sag while enabling you to block the mast back and tension the lowers in fresher breezes to straighten the headstay and allow optimal use of vang sheeting. Our design has other new features as well including optimizing the batten positions of the full length battens enabling us to maximize the size of the sail at the measured girth points as well as relocating the jib leech window for better viewing of the jib leech telltale for both helmsman and trimmer.

Mainsails include:

  • Trim stripes
  • Sail numbers
  • Insignia
  • Vision window
  • Jib leech window
  • Sail bag

How to tension battens?

For prices and more information contact our Viper Experts or your

Viper Jib

The North Viper 640 jib has been changed as well. To start we have lowered the jib clew so that the jib lead adjustment is more effective and each change has a distinct effect on enhancing performance. Also, our jib features a full-length top batten which helps the top of the sail twist more effectively which allows more control with the jib sheet and the ability to trim harder! Combine all this with a design that features a fuller entry, flatter leech and a maximum length tapered bottom batten as well as additional panels in the head which assist in adding durability to the sails shape and you have a proven performance leader.

Jibs include:

  • Pentex load bearing fabric with radial construction for superior durability
  • Trim stripes
  • 2:1 Clew blocks
  • Luff telltales
  • Leech telltales
  • Sail bag

For prices and more information contact our Viper Experts or your

Viper Asymmetrical Spinnaker

The North Viper 640 spinnaker has been designed utilizing the knowledge we have gained on making championship winning sails for a variety of sportboats. Our first project was to maximize the size of our design which, in doing so, makes our sail class maximum size. Our next goal was to create a design that would twist and rotate better so that we could sail lower angles while sailing the same speed or faster. Our Viper 640 spinnaker enables you to do just that. Maximum size, lower and faster. The perfect Viper 640 spinnaker.

create a design that would twist and rotate better so that we could sail lower angles while sailing the same speed or faster. Our 2009 Viper 640 spinnaker enables you to do just that. Maximum size, lower and faster. The perfect Viper 640 spinnaker.
For prices and more information contact our Viper Experts or your

Tuning Guide for the Carbon Mast

Mast Tuning

1. Set spreader angle
Place the pins in the bracket to put the spreaders in the after most position.  The outer tabs have two or three different positions. Measure from the side of the mast to the shroud along the spreader. Set the end tab so this measures 685cm. Place a batten across the shrouds from one spreader to the other. Measure from the batten to the back side of the mast. We used the spreaders set at 29cm.

2. Mark your headstay
Lay the forestay along the front of the mast and mark where it is even with the top of the mast band at the gooseneck. This can be accomplished with the mast on the ground or stepped with the uppers attached. Set Forestay by measuring from the mark along the forestay to the pin in the stem fitting. We have used 52”.

3. Mast step position
Use the middle of the three positions. Measure from the top of the lip where the transom meets the cockpit floor next to the rudder pylon. Measure to the aft face of the mast at the top of the step. This should measure

4. Upper Shroud Tension
Find your base setting. Use a PT1 Loos tension gauge. Tension 36.

5. Mast Partners
In order to best tune your Viper, it is necessary to restrict the mast movement in the partners. You can do this with the line and cleat that comes with the boat. We recommend using mast blocks in front of the mast for setting up for over 8 knot in order to solidly restrict mast bend. This provides greater headstay tension to keep the jib flat and twisted as the breeze increases. The aft block which fits against the front of the mast needs to be filed away to accommodate the angle from the rake of the mast. Properly done this keeps the blocks from climbing up.

In this case we are using 1 ¾” thick block, 1 ½” and 2 ¼” thick blocks. This is what worked in this case to achieve 3 ¼” of pre bend.  The ¾” block has been filed out on the underside to accommodate the rake and spread the load on the front of the mast.

The use of the line led to hold the mast forward serves a s a safety precaution in heavy air. (over 20 knts) It will keep the mast from inverting and possibly breaking.

6. Lower Shrouds
n less than 8 knots the lower shrouds should be set to be slack when sailing. Above 8 knots the tension on the lower shrouds becomes critical to control Fore and aft mast bend, side band and to some degree the headstay tension. Because the tension needs to be adjusted precisely under load on the water we recommend putting turnbuckles on the lowers. The tension setting in the 8-16Knot range is barely sufficient to register 5 on the Loos gauge. (model PT1) Only 1 more turn should be sufficient tension for stronger wind.

NB: It is critical to not overtension the lower shrouds in heavy air. If the lower shrouds are too tight, it could contribute to inverting the mast while sailing downwind and causing breakage.

Upwind Trim

1. Jib Lead Position
Your North jib is marked with a trim line near the clew drawn from the clew grommet toward the body of the sail. Your lead should be positioned so that the sheet is a direct extension of this trim line. In stronger breeze move it aft a hole or two, or move the sail lower to the deck by using a shorter shackle at the tack.

Generally the jib lead should be set more forward in light air to allow the foot to have 6” of round in it. This will have 3 or 4 holes showing in front of the jib lead. As soon as the breeze is strong enough to justify hiking, the lead should come aft as much to flatten the foot nearly to a straight line when the leech is properly trimmed.

2. Jib Sheet Tension
Your North jib has a leech telltale ¾ of the way up he leech. This telltale can be observed by the jib trimmer from the leeward side in light air. In any hiking breeze it can be observed through the luff window in the main. This is to use in judging sheet tension. In most conditions trim the jib in until this telltale no longer streams aft, then ease it out until it just streams aft.

Generally the middle and upper battens in the jib will point straight aft when it is properly trimmed.

The top jib batten is full length. In light air removing this batten helps the jib to be more powerful. As soon as the breeze is over 8 knots it is important to have the full length  batten in. This helps the leech to stay open while trimming the sail hard.

Mainsail Trim

1. The main bridle
We prefer the using the bridle in the back of the boat. Set it so the top of the block is __” above the cockpit floor.

2. Mainsail Outhaul
Easing the outhaul in light air powers up the main. Judge the outhaul tension at the mid point of the boom. Look at the distance between the side of the boom and the foot of the sail. In rough water light air conditions easing the outhaul to open the foot as much as 10-12” can be fast. In smoother water generally keep it tighter. As soon as there is enough wind to hike, tighten so the sail lays against the side of the boom.

3. Mainsheet tension
In light and moderate conditions when heeling can be controlled with hiking, the aft end of the upper battens (on either side of the Viper insignia) should be parallel to the boom. This may require quite a lot of sheet tension.  In stronger breeze

4. Gnav (rekcik)
Under 12 knots make sure the gnav isn’t applying any downward pressure on the boom. Upwind and downwind. As the wind becomes stronger, apply progressively more tension to keep the boom down as the mainsheet is eased in the puffs. The Gnav bends the mast to flatten the main and tensions the headstay as long as the lowers are properly adjusted and the mast is firmly blocked aft at the deck.

 Windstrength 0 - 8 8 - 16
 Headstay Length 52" 52"
 Upper Shroud Tension -10 turns 35 on Loos's Model 'A'
 Lower Shroud Tension - 7 turns 5 on Loo's
 Pre-bend 1" 3 - 31/2"
 Mast Partner pulled forward producing
 1/2 - 1" pre-bend
 blocked back to restrict
Jib Lead 2 holes showing behind 2 holes showing behind
Jib inhaul w/weather sheet over deck joint
Headstay Tensionslack hangs with 2" of sag26 on Loo's
Jib SheetLeech telltale just flowingLeech telltale just flowing
Outhaul 6" 2" - 0
Jib Tack twist shackle plus shackle just 1 short tackle

Viper 640 Targets

 TWS Upwind Tack> Downwind AWA Jibe>TWA
 6 4.5 90 4.4 90 90135
 8 5.3 85 5 110 70145
 10 5.6 80 5.8 125 60150
 12 5.8 78 6.2 130 40160
 14 5.9 78 9 85 100 130
 16 6 79 10.5 90 90 135
 18 6.1 80 12 93 85 138
 20 6.2 82 14 95 80 140

For more information on Viper sails, contact the North Viper experts.

Sail Numbers & Letters

300mm (12”) Sail Letters & Numbers

Please contact your local North Loft or Agent to make your order or for further details.

Contact the Viper Experts:
Ruairidh Scott
+44 (0)1329 443 430 Work
Paul Young
+44 (0) 1380 831138 Work
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