Mast Set-Up: Powering Up or Powering Down - Viper 640 International Class Association

By Geoff Ewenson

As a relatively new team to the Viper 640 Class the last couple of year, I have been on a steep learning curve. Being analytical about boat set-up helps and here are some of the things I’ve learned.

Read the tuning guide for your sails

The first step in learning any new boat is to become well versed in the tuning guide that your sailmaker has provided. From there, it is a process of slowly figuring out what makes your individual boat go fast. One great aspect of the Viper Class members is the willingness and openness among all of the competitors to share thoughts and best practices among the fleet.

The biggest mistake I have seen in my years of sailing in various one designs is when a team thinks they are going to outsmart the top teams by going a long way off the tuning guide and changing more than one variable at a time.  It seems that new teams meet some success initially when they join the class and simply set their boats up according to class standards. They might even find themselves in the top tier of the fleet in the first few events. They then gain confidence and get a bit bold on their setups and find themselves sliding back in the pack.

The methodical advancement of tuning is hard to be patient with and often is rushed.

The other large problem new teams have comes from an unwillingness to be proactive and make changes on the fly during the course of a day (not during races). They may read the tuning guide and actually go thru the process of setting-up the boat to base settings but then once on the water they make all kinds of excuses to keep the setup the same. (I think the wind is going to stay the same/drop/build…lets just leave it!)  Big takeaway—be proactive!

I sail primarily with North Sails so I read the tuning guide that is available online and double check every day that we begin the day at “base.” We then make adjustments from there based upon the day’s conditions.

Actual adjustment process

The first adjustment is done with the uppers and can be easily changed on the water between races. The base setup for the North sails is 26 lbs. on a standard loos gauge. That correlates to a wind speed of 8 knots. I measured our boat the first time and once I was satisfied that the mast was at base, I then took calipers to measure the distance between the pins in the turnbuckle for each shroud in order to quickly be able to replicate the settings each time I rerig the boat without need for the loos gauge.

At base setting, the lowers are visually very loose. The relatively nonscientific way of measuring the correct length for the lowers is to mark (with a marker or nailpolish) the uppers and lowers 4 feet up from the lower pins at the chainplates. Then measure the distance between the upper and lower at those points. A very light (finger tight) pull between the two shrouds should give you a distance of approximately five inches.

Mast blocks

The mast in the partners at the deck is front loaded with mast blocks that are purchased aftermarket from folks like Craig and Deb Wilusz at The width of the blocks between the front of the mast and the front of the forward-most block is 2 3/4 inches at base.

Mast blocks and lowers are used to control the straightness of the mid to lower mast section and thus the amount of power in the main. They also effect the amount of headstay tension you can carry for any given wind strength.

Most folks only change the mast blocks between 2 1/4” and 3 1/4” total between very light conditions – Top end of sanity sailing.

In general the width of mast blocks and tension on the lowers are increased as the breeze builds.

Adjusting the uppers and lowers on the water

The easy way to think of the adjustments for uppers throughout the wind range is to make sure that, when trimmed in and sailing upwind, your leeward upper shrouds are just barely on the verge of snug to very slightly wiggling—never swinging around.

The relationship between lowers and uppers as far as adjustments is roughly 3-1 or even 2-1 depending on your aversion to risk. For every 2-3 turns of uppers you turn one turn of lowers (3-1 is safer for keeping the mast from inverting downwind).

The lowers control the stiffness of the middle of the mast and therefore the overall bend characteristics of the mast, both fore and aft, as well as side bend. That control of the middle of the mast directly affects the amount of headstay tension you can carry in a given wind strength. Without enough control of the middle of the mast in windier conditions, the headstay compresses, the rig goes soft, and you lose point.

How much bend?

In general terms, you are looking for more bend in very light conditions in order to make the main slightly flatter to give you the best speed or fast forward ability in light air (looking for low drag). Reduce mast blocks by 1/4 inch from base to get more bend down low. As the wind picks up and you are looking for a stiffer mast, you must bring in the additional blocks and lowers to match the conditions.

As the breeze builds slightly and the team is borderline hiking, the goal is to power up the main and force more depth into the middle of the sail. By increasing mast blocks and making sure the lowers are giving some pull, you can effectively put some shape into the main and get the power to be able to sit on the rail. Look up the mast from along the mast track and see how much sag there is at the spreader root (typically this will be about ½ to ¾ of and inch).

As the wind increases more, you might even get to point where the tip of the mast appears to fall off to leeward and the spreaders root will poke to windward rather than sag in the middle of the mast.

As the breeze builds further, and you are looking to de-power, increasing the lower shroud tension and adding blocks at the partners allows you to have a stiff enough mast to pull on the gnav without over bending the mast.

The gnav

The gnav in many ways is counterintuitive and actually acts similar to a backstay in most other boats. As you tighten the gnav, more pressure is put into the mast bend and the top of the main gets flatter and more open. The typical vang on most boats simply puts more tension into the leach of the main and actually closes the leach. To see the gnav effect on the sail, take a look up at the main in 12 knots with a lot of Gnav on and then release the gnav completely. You’ll see the middle of the main gets more powered up and the top closes dramatically with the gnav eased off.

Fully depowering

When the breeze is on and you are looking to fully depower it is important to have the right setup and enough overall tension on the rig. If you are under tensioned, then the forestay is too loose which ends up being a big problem in puffy conditions. Having a tighter overall rig with blocks and tightened lowers means that when you ease the mainsail (like in a puff) the rig doesn’t go soft. If you are not tight enough then main leach is directly affecting the amount of headstay load you are carrying at any given time.


The bad cycle goes like this:  puff hits, you ease the main, and then the headstay gets soft. The jib gets more powered up when the headstay goes loose. This pulls the bow down and the boat heels more which means you need to jam the tiller to leeward and ease the main more to keep the boat on an upwind angle and keep from slipping sideways.

Having a tighter rig allows you to play the main in and out without a gross change in headstay tension and, therefore, a steadier jib setup.

In big breeze I actually hand off the mainsheet to the middle person on long tacks and really play the gnav aggressively.


The beauty of the Viper is that it sails well throughout a wide range of breezes. It handles light air well and it has enough power in the sail plan to keep moving along while many other boats seem glued to the water. As the breeze builds and the boat gets livelier, the range of controls allows you to depower the rig and keep the boat on its feet.

As I mentioned…one of the very best aspects of this class is the willingness of all the folks to get together and discuss what makes these boats go fast. The top talent in the fleet has been very giving of knowledge and has helped me get to a point where someone has asked me to give my 2 cents on tuning. I know that they will always be willing to do the same for anyone else and I am also. Please feel free to come chat with me in the boat park any time.