Bump's World

VANG SHEETING

Vang sheeting has become a common system for trimming a mainsail. This system has evolved over the years.

The mainsail trims by letting the main sheet in and out. Tightening the sheet brings the sail closer to the point of attachment and allows you to sail closer to the wind. As you bear away from the wind direction you ease the sheet to maintain sail trim most effective for the angle of the wind.

If you have a single point of attachment for the main sheet the boom will go up as well as out. The adjustment of the boom parallel to the water maintains constant sail shape but changes the angle of the sail to the wind. If the boom is allowed to go up as you ease the sheet the sail gets fuller and more powerful, often too much so.

A traveler is often added that allows the attachment point to move. It is still a single point of attachment, but it can move sideways allowing for the boom to be adjusted to the angle of the wind.

The most efficient sail trim is to maintain equal flow of wind on both sides of the sail. The shape of the sail forces the wind on the back side to accelerate to meet the wind on the inside of the sail. The acceleration of the wind on the back side of the sail causes lower atmospheric pressure on the back side of the sail and this pulls the boat along.

If the sail is too full the wind will stall trying to catch the inside flow and create little pressure. So the wind does not push the boat forward the lower pressure on the back side of the sail, sucks it forward.

So we have adjustments on the sail to affect its shape. Therefore creating the most efficient flow over the shape.

First adjustment is halyard tension. More tension flattens the leading edge of the sail. The halyard tension can even be more controlled with a Cunningham which pulls down while the halyard pulls up.

Next we have out haul. This tightens therefore flattening the foot of the sail.

We can also bend the mast which will pull the center of the sail forward, again flattening the sail out.

The biggest adjustment is the mainsheet. Ease the mainsheet and the sail will go out and up and get fuller and more powerful.

Years ago a block and tack was attached from where the mast reached the deck to about 30% back on the boom. This was the first boom vang. Now the boom could be stopped from going up, or up more than you wanted, as you eased the sheet.

On bigger boats the loads on the boom vang were huge and it became difficult to adjust the block and tack. The block and tackle also just held the boom down and there are times when you would want the boom a little higher.

To hold the boom up a topping lift was added to the out board end. This was another line and another adjustment.

Hydraulic vangs became popular in the 1980s. They were very strong and held the boom up or down in any location you wanted. Often the vang was in addition to a hydraulic backstay and even hydraulic outhaul. You have a panel and you select the function you want and pump away to create pressure.

Hydraulic systems are expensive, heavy, and can create a huge mess when they fail.

The spring loaded solid vang as much simpler. The spring held the boom up and the block and tackle compress it to hold the boom down.

By the late 1980s the buyers really liked having the traveler moved to the cabin top. This track and car system was annoying in the cockpit taking up precious social space.
The problem here is the boom is a huge lever arm. As you move the point of adjustment forward the loads go way up. Many cruisers would never trim the mainsail in hard because the load was so great. They suffered with poor sail trim.

The adjustment of the traveler would not work because of the huge loads. There is also a structure issue since the cabin top had to support the huge load of the sheet/traveler.

So because moving the traveler out of the cockpit creates loads that traveler systems cannot deal with, it is better to have 2 attachment points on the cabin top for the main sheet and control the boom level with the solid vang.

This is called vang sheeting. Work the vang and the sheet together for proper sail shape.

This system works very well in all but serious racing sail trim. There are situations in racing where a traveler will give you better sail shape. The race boat will have the traveler well aft on the boom and in the cockpit where it can be most effective.

Most modern cruisers and cruiser/racers have cabin top double attachment main sail sheeting systems. The sheets are led to cabin top winches. The boom is controlled with a solid vang. This is a very clean efficient system.

Please mail comments to me at Bump@newwaveyachts.com

Bump Wilcox