Index of Articles in Frank's Corner of Sailing Texas
Chapter 8 The Art of Tuning (The New Anchorage)
Part 1 Straight Mast
Keeping a sailboat moving efficiently through the water is a combination of rig tune and sail trim. This chapter on tuning may help you understand "The Art of Tuning".
The mast is held up by opposing forces. The shrouds on the port and starboard of the mast provide opposing forces, preventing it from falling to either side. The fore and back stays provide opposing forces preventing the mast from falling fore or aft. There are exceptions to this setup on both large and small boats. E.G. some boats have no back stay and the shrouds are swept back of the mast step. (i.e. Hunter 280 or a Ledo 14)
You begin tuning by stepping the mast, not leaning fore or aft. Do not tighten the turnbuckles very much at this time. Note, some boats are designed to have "aft rake" (mast leaning aft). If you are not sure about your boat contact the manufacturer, check the web, or call your favorite sailboat dealer. I use "The New Anchorage" located in Lyons, Colorado. I have bought 3 sailboats from them and they have help me in setting up these boats. (http://www.thenewanchorage.com/) In another chapter I will discuss a mast raising system they made for my 18 foot boat.
To check the alignment of the mast port and starboard, use the main halyard or topping lift to measure to identical spots on both sides (e.g. stanchions, toe rail). If it appears to lean either way adjust the appropriate lower shroud. Again, tighten only enough to straighten the mast.
The next step is very important. The mast must be in a vertical column, this means no twists or bows side to side. Also no "S" curves. You can judge this by sighting up the sail track, using a tightened halyard as a reference. The curves may be straightened by adjusting the upper and lower shrouds. With the mast straight the turnbuckles can be snugged up, being careful to take up equal amounts on each side or new curves will be created.
This is where the "ART" comes in. It should be noted this is no precise science and everybody has an opinion on how tight is right. A loose mast flops around but being too tight can damage the boat. Be patient and eventually you will figure out how tight is right for your boat and sailing style.
Part 2 Pre Bend or Rake
Once the mast is straight port to starboard as measured by the main halyard, the mast rake can be set. Rake is the amount of lean forward or aft a mast has. The first thing to do is level the boat. After the boat is leveled it is necessary to determine the amount of mast rake and which direction. To do this, hang a weight from the main halyard suspended over the boom; the distance between the mast and the weight will give you a relative indication of mast rake.
The further aft the mast is raked the more weather helm a boat will have. Since the amount of weather helm a boat has can only be determined by sailing it, you may have to change the mast rake after sailing to adjust the weather helm for your sailing style.
To set the rake you use the forestay and shrouds. If the forestay turnbuckle is opened the mast will lean aft, if it is closed it will pull forward. Since the shrouds were snugged up when the port to starboard lean was set they will have to be readjusted if the rake is changed.
Once the rake is set it is time to pre-bend the mast. Pre-bend is the amount of bow that a mast has. To measure the amount of pre-bend you will need to pull down on the main halyard and measure the distance between the halyard and the mast at the halfway point. This should be around the spreaders. The amount of pre-bend should be specified in the boat owner's manual.
After the rake and pre-bend are set it is time to set the stay and shroud tension, which requires a special gauge. There are two basic types of rigs, Masthead and Fractional. If the forestay is attached at the top of the mast it is a masthead rig. If the forestay is attached below the top of the mast it is a fractional rig. The tension for masthead rigs should be set to 15% of the rigging cables break strength, for 3/16" cable it would be 705 lbs. (3/16" break strength is 4700 lbs. X 15% = 705 lbs.). Forestay tension for a fractional rig is set to 15% of the rigging's break strength as long as this setting does not induce excessive bend in the mast. The shroud tension for both a masthead and fractional rigs should be set to 10% of the cable's breaking strength. On a fractional rig the shroud tension may have to be greater to achieve the correct forestay tension. The tension should never exceed 25% of the cable breaking strength.
These adjustments can be made but it should be noted that with most sailboats the mast should be as straight as possible. As stated earlier, if your boat has a lot of weather helm you may need to move the mast to the vertical.
All of the adjustments mentioned are varied to match the boat's performance characteristics to the designer's intentions. If one of the adjustments is changed the entire feel of the boat will change. A word of warning on this point, it takes years of experience to change the tune of a boat without inducing any adverse handling problems. Do not attempt to change your rig's tune or adjust any thing arbitrarily without expert advice.
One piece of running gear that can change the tune of a boat is called a backstay adjuster. This is a system for increasing the backstay tension while under sail. This allows an individual to increase the amount of bend in a mast. This is an effective tactic for sailing in heavy air. As the backstay tension is increased, the forestay tension also increases; this will flatten the jib, de-powering it. As the bend in the mast increases it carries some of the mainsail forward and increase the tension on the luff edge; this de-powers the main sail. By de-powering the sails you can carry full sails for a longer time before having to reef.
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