Backstay Power & Efficiency

One of the best places to look for more boatspeed is the back of your boat - specifically your backstay tensioning system. Many boats have underpowered tensioners that cannot tension the headstay adequately, or systems that are awkward to use, requiting crew to leave their normal station to make adjustments. Some simple Improvements will make adjustment easier and more effective.

Backstay tension affects many aspects of performance. On H28s it is the primary control for headstay tension, which affects genoa shape. Ease the backstay to add draft to the genoa for more power in light-to-medium conditions; tension the backstay to remove draft for better pointing in heavy air.

Mast head rigs need a powerful backstay to tension the headstay. Boats up to 33’ (10 m) tend to favour block-and-tackle systems while larger boats generally use mechanical or hydraulic systems. A simple 6:1 tackle on is the most common used on H28s.

The split backstay with ‘pincher" blocks is one of most common methods used by H28 owners. With this system, the backstay resembles an inverted "Y’ with two lower legs attached to chain plates at the corners of the transom. A pincher block rides on the legs of the backstays, and is forced down with a purchase. Split backstay systems offer many advantages, including safety (even if the tackle is released completely, the mast stays up), good power, speedy adjustment and simplicity.

The power of these systems comes from both the pinching together of the legs (where the backstay splits into two) and from the tackle itself, in effect multiplying the power. Paradoxically, the power of this pinching is greatest when the legs of the backstay are nearly parallel, and diminishes quickly as the blocks are drawn down - in other words, greatest under light loads and diminishing as the stay is tensioned.

Looking at an example, with the angle between the legs (under the pincher blocks) at 15 degrees, the purchase is 117.1. Multiply that by a 6:1 tackle and the result would be a power of 700:1 — enough to allow a strong man to break a 1/2" 1 x 9 (12 mm) cable! Fortunately for our boats, but unfortunately for our backs, the power drops off dramatically as the pincher blocks descend and the angle between the legs of the backstay increases. By the time the angle is 45 degrees, the power is only 13:1, and at 90 degrees it has decreased to only 3.4:1 - a final power of just 20:1 with the same 6:1 purchase. Consider efficiency loss to friction and the same crew would be lucky to tension the headstay.

To increase power in a split backstay system, increase the height of the split so that the legs are closer to parallel when the system is unloaded. In other words, if your split is very low, shorten the upper portion of the backstay and buy two longer lower legs. The only disadvantage to this is that your tackle will be longer, adding weight and windage.

Another key element of a split backstay system are the pincher blocks. Because they ride on stiff 1x19 cable, the sheaves on the pincher blocks should be as large as possible. Forcing cable to bend in a tight radius uses a great deal of energy, and using small sheaves hurts efficiency. If your wire has a kink from the pincher block, you’re damaging the cable with undersized sheaves. Larger sheaves protect the wire and reduce friction.

One way to increase effective sheave size without excess weight is to use a sheave canter that has two small sheaves riding on the wire of each leg. Good beatings in all sheaves are also mandatory, because they operate under high loads. If the sheaves aren’t rolling properly, too much energy will be wasted moving the pincher blocks.

One form of split-backstay adjuster uses a tackle with a cleat mounted on one side of the transom. Depending on the tack being sailed this is can be ineffective because someone still needs to move from their normal position to reach it when adjustments are needed.

An alternative system is where the tackle is double-ended, and led forward to where they can be easily reached by the helmsperson or trimmer on either tack.

On boats larger than an H28, a split backstay rarely provides enough power. Mechanical or hydraulic adjusters are required. Mechanical adjusters include those that use winch handles, ratcheting handles, flip-up handles or wheels. While powerful and simple, they tend to be slow, and require a crew to move to the backstay to make adjustment. Some mechanical adjusters feature calibrated scales that make returning to a known adjustment easy. For those that don’t have a scale (or to supplement one), apply a stick-on scale to a batten and tape it to the tensioner.

Hydraulics were once the obvious backstay adjuster, as most race boats also had hydraulic vangs, and the system featured a centrally located master panel with valves that allowed adjustment from a convenient location. Solid rod vangs have mostly replaced hydraulics so the backstay is often the only hydraulic control aboard. Rather than fit remote panels, most hydraulic adjusters today are self-contained, requiting crew to move to the transom to adjust the backstay.

But hydraulics still offer advantages for backstay adjustment. They are generally faster than mechnical adjusters, especially in the release mode where pressure can be bled off in seconds by opening a valve. They also offer a gauge for repeating settings. Most hydraulic adjusters also have a relief valve that prevents overloading the rig.

When fitting a backstay adjuster to an H28 you need to keep in mind the incredible load that can be applied to your rig. The combined effect of the power of the wind on the sails, the pumping action on the mast in a heavy sea, and the extra load applied through the backstay adjuster is considerable.

A check should be made to ensure there is not too much deflection in deck around the mast step. Cracks around the deck where the mast is stepped or cabin doors that jamb while sailing are usually an indication of this problem. Using a plumb bob hung inside the cabln under the mast step can test this how much the deck deflects with the backstay tension applied. The deflection from the cabin sole to the bottom of the plumb bob should be no more than %" (5mm). If there is too much deflection then it would pay to strengthen the area under the mast.

Consideration should also be given to the forward side stays. Most H28s have two lower shrouds - one forward and one aft of the mast. A significant load on the forward side stay can occur as a result of the leverage effect due to tension being applied on the mast from the backstay. (The distance from the mast to the backstay is a lot greater than the distance from the mast to the forward side stay). It is possible to break the forward side stay if you are not careful! A check should be made on the tension of the forward side stays as the backstay tension is applied. About 1/2" (12mm) play sideways is acceptable on the side stays with the rig under load.

Don’t forget that at the end of the day’s sailing the backstay adjuster should be eased to released the load on the rig.

A well-designed backstay adjuster will make your boat faster, and allow you to sail closer to the wind. If you race without an adjustable backstay, you need to add one. If you already have an adjuster, spend some time analyzing it - it may be easy to increase the power or route the tackle to a more convenient location.