RIG TUNING GUIDE

CHUCK SKEWES RIG TUNING GUIDE

Rig Tensions for the San Juan 24 by Ullman Sails Loft Manager and three‐time SJ24 North American Champion, Chuck Skewes.

Uppers 3/16” diameter

Lowers 5/32” diameter

Babystay 5/32” diameter

Backstay 3/16” diameter

Requires both PT‐1 (5/32) and PT‐2 (3/16) Loos Gauges

Swing measurement using the Genoa halyard: While pulling with tension to remove slack in the line, mark halyard at the top edge of lower mast band. This mark should be 25” from deck at front of mast base. Swing halyard forward to headstay/foil and pull to same tension as before and mark the forestay wire (or foil). Then measure from the top edge of the mark on the forestay down to the center of forestay pin to determine Mast Rake *. See NOTE below.

Mast Rake should be measured with the backstay on and the forestay at a known tension using a PT‐2 Loos gauge.

NOTE - the forestay pin should be approximately 1" above the deck. See image below for an example.

If your Forestay pin center is more than 1” above the nominal deck, then simply subtract the difference and make note of this value. Then subtract this value from the Chuck Skewes forestay suggested length. For example, if the forestay pin center height above deck is 1.5”. Then 1.5” – 1” = 0.5” value. For light winds up to 9 knots, Chuck recommends 51.5”. With an offset of 0.5”, then adjust the Forestay turnbuckle to result in a measurement of 51.0” between the center of the forestay pin and the mark on the stainless steel wire from the genoa swing procedure.

Link to Chuck Skewes video - Unlocking an extra 1/2 knot San Juan 24

Loos & Co Tension Gauge

The LOOS “Professional Model“ tension gauge is designed to provide an accurate measurement of the tension in rigging wire and other types of cable used in recreational and industrial applications. It is particularly used for accurate and repeatable tuning of a sailboat’s standing rigging.

Model PT-1 Cable size 3/32”, 1/8”, 5/32”
Model PT-1M Cable size 2.5mm, 3mm, 4mm
Model PT-2 Cable size 3/16”, 7/32”, 1/4”
Model PT-2M Cable size 5mm, 6mm, 7mm
Model PT-3 Cable size 1/4”, 9/32”, 5/16”, 3/8”
Model PT-3M Cable size 7mm, 8mm, 9mm, 10mm

Each model covers a tension range of approximately 5% to 25% of the breaking strength of the wire and is designed and tested to provide an accuracy of plus or minus 3% at mid-range.

The “Professional Model” tension gauges will provide an increase in accuracy and convenience of use when compared to our popular standard gauges, Model A and B. The gauge may be hooked on the wire and will remain in while the tension is adjusted.

Contrary to popular thought, a slack rig is more punishing on a hull than a properly adjusted, tight rig. Insufficient tension will not reduce the loads transmitted in the hull.

Slack rigging will punish the spar and rigging needlessly by allowing excessive movement, chafe and shock loading. Modern fiberglass hulls should not be damaged by a properly adjusted, tight rig.

The diagram shown lists the rigging tension under different conditions for a typical boat with a properly tuned rig and with a slack rig. It will be noted that the maximum load is the same. However, for properly tuned rig the leeward shrouds will not go slack under normal sailing conditions.

The lateral stiffness of the mast and the fore and aft stiffness of the spreaders is reduced by a factor of 2 when the leeward shrouds go slack. This important structural characteristic is not generally recognized.

Rigging tension is becoming more important as a result of the trend toward the use of mast bend to control mainsail shape under different wind conditions. Mast bend will also affect the shape and trim of the jib, since mast adjustment generally affects forestay tension. The expert skipper will benefit by maintaining consistent rigging tension while developing the optimum sail shape and sailing tactics.

How To Measure Shroud Cable Tension Using the Loos Gauge

Hold the gauge with the left

hand and place the cable between

the two nylon spools as shown.

With the right hand pull the lanyard

and extend the spring until the hook on

the nylon slider can be hooked on the

cable as shown.

Release the lanyard to assure accurate readings make sure that the slider moves freely in the frame slot and does not bind, move frame back and forth to center it.

Also, lubricate the slider slot frequently with silicone lubricant. Read the needle position on the scale. Refer to the calibration table to obtain correct tension in the cable. The gauge can be left on the wire for “hands free” adjusting of the cable.

Safety and Performance

The failure of a fitting, shroud or stay could damage your boat, buckle the mast or even cause personal injury. To avoid such failure of the cable and fittings from fatigue or shock loading, it is important to set up your standing rigging with the proper tension. Too little tension in the shroud will permit the leeward shroud to go slack, only to fetch up with a jolt when the boat rolls or pitches. A less common problem is excessive tension. This can cause permanent stretch to the cables and possibly damage the mast.

The actual set of sail under load is determined by the cut of the sail and the shape of the structure which supports the sail. Rigging tension plays an important part in determining the set of the sails.

When the boat has been tuned for peak performance, measure the cable tension should be recorded. The stainless steel used to make the rigging can stretch a little bit over time under high loading. Thus, marking turnbuckles, etc. cannot guarantee that subsequent adjustments will provide the desired tension. Only by gauging is it possible to repeat the initial tuning or improve it.

Limiting the sag of the forestay is perhaps the most important benefit to performance from having the proper rigging tension. Forestay sag permits the jib luff to fall off to leeward, tightening to leech and seriously degrading the performance to windward.

Tension in the upper and lower shrouds will influence the mast bend and set the mainsail. This is especially important on modern, fractional rigs where the mast bend is used to depower the sail in heavy winds.

If the shrouds are not set up with enough tension, the leeward shrouds will go slack when the boat is sailing to windward. This can result in fore and aft pumping of the mast in a head sea. This mast movement will change the shape of the mainsail and can cause performance loss as well as possible structural damage. Specific tension requirements for your application must be obtained from the boat, mast, or sail manufacturer or the manufacturer of the product on which the cable is used.

How much Tension?

Table 1 recommends an initial tension setting, but there is no simple solution since the optimum rigging tension will be a function of the boat design, the rig (masthead or fractional, one or more spreaders, etc.), and even the cut of the sails. Many skippers use insufficient tension because of a fear of “breaking something.”

It should be noted that on America’s Cup contenders, where electronic state of the art tension instrumentation is available, the standing rigging is set as tight as is structurally feasible.

Loos Gauge Scale Reading to Pounds Force Tension

NOTE ! THE INTENDED USE OF THIS GAUGE IS TO BE USED ON 302 / 304 1x19 S.S. CABLE IN APPROPRIATE SIZES. IF USED ON OTHER CABLE TYPES, SIZES, AND CONSTRUCTION YOUR READINGS WOULD BE CONSISTENT (REPEATABLE) BUT POUND TENSION VALUES WOULD DIFFER FROM THOSE LISTED ON THE LABEL. GAUGE MAY NOT READ "O" WHEN AT REST, AS THEY ARE CALIBRATED AT MIDRANGE OF TENSION. NOTE ! IF FLAT SPOTS APPEAR ON NYLON SPOOLS AFTER EXTENDED USE, ROTATE NYLON SPOOLS 45°.

Scale face of Loos PT-1

Scale face of Loos PT-2

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