A guide to static tuning of the standing rigging of the Tanzer T22, T7.5, T26 masthead sailing sloops
By Paul Coppin.
Prior to getting out the wrenches to begin setting up the Tanzer rigs, an examination of the rig, particularly with regard to its weak points, is in order.
The Tanzer rig is a simple, single spreader Bermudan masthead configuration comprised of a fore and back stay (split in some variants), and a single upper and lower shroud each side. The rig has a moderate aspect ratio of just under 3:1, supported on a relatively short and thick, non-bendable spar. The shrouds terminate in swaged eye fittings, to a single chain-plate, more or less in line athwartships with the mast. The mast is supported by an aluminum tabernacle, on a compression post attached to the principle bulkhead and stepped at the keel. On most models of this group the rigging wire is 1x19 5/32 stainless steel with a book breaking strength of 3300 lbs.
The features that are of particular interest to tuning are the single lower shrouds in line with the mast, the non-bendable spar, and the swaged eye fittings. Given the size and sail area of these boats there is adequate strength in the wire and mast, however, the rig layout is a compromise. For simplicity of construction, the boats were equipped with a single lower so that there would not be a need to provide in the hull a load distribution network to accommodate a more desirable fore and aft lower shroud configuration. Given the strength of the wire and components at this size of boat, this was a "do-able" compromise. This move also reduced cost to the builder and the buyer. What this decision means to the sailor though, is that there is no redundancy in the rig, and generally, the rig lacks adequate fore and aft support below the masthead; meaning, that no matter what you do, you will be unable to eliminate mast pumping. Keep this in mind when tuning the fore and back stays and the upper shrouds.
The other principle weakness is the use of swaged eyes at the termini of the stays. Properly swaged and healthy, the eye fittings will tolerate the strain parameters of the wire. The problem is that there is no way to determine the failure load of a swaged fitting, without testing to destruction. It is equally difficult to assess the health of a aged swaged fitting, since corrosion issues are typically deep in the swage, and may not be visible at the interface. Swage failures are a frequent cause of rig failures. It is for this reason that you will find recommendations for replacement of rig components on a planned schedule based on so many years in service depending on the use and the environment. Given the above, rig tensions should set conservatively, providing necessary support for the rig and its loads, and no more.
The textbook breaking strength of 5/32 1x19 wire is 3300 pounds. The general case is that the maximum working load is 1/4 of the breaking strength, or 825 lbs. This figure then is the maximum to which a stay should be static loaded, based on new, healthy wire, properly swaged (the truck and deck fittings may very well be another matter). The factor of safety for shroud tension is 2.5-2.75 for main upper stays and 3.0 for main lower stays. These factors define the wire size needed to support the rig, and are consistent with 5/32 wire. 1/8 at 2100 pounds is too weak, and 3/16 at 4700 pounds is overkill, and only results in excess weight aloft.
Shroud tension should be more less equal between the upper and lower shrouds, and based on a standard of 10-12% of breaking strength, around 400 pounds, or a scale reading of 41-42 on a Standard Loos Gauge. Sailing tests may show that this is a little soft, requiring one more turn of the screw which should bring you up to about 500 lb or 43-44 on the Loos. This is enough tension for these rigs, and more would not ordinarily be justified. Tensioning the uppers significantly greater than the lowers does not make much sense in these rigs, given that there is no means to prevent the spar from being forced into an "S" curve. I would not tension the uppers more that about 10% over the lowers at the most, if I felt that the masthead was sagging off too much. The mast, by its robustness, will only let you compensate so much, before it begins to collapse downward. Have a problem with leaking chainplates? Like 700 lbs of tension on the shrouds? Hmm.
Fore and back stay tension is typically set at 15% of breaking strength of the wire, or 495 pounds, to start. Sailing tests can result in this increasing, but in no case should it ever exceed 825 lbs. or 47 on the Loos, and for most boats a maximum of 600 lbs. (45 Loos) would be reasonable. The stated load of 900 lbs in one of the T22 tuning guides is way out of line, and puts undue strain on the rig, excess compression on the mast base and is guaranteed to put a fore and aft serpentine bend in the spar. Equally, undue load on the shrouds will result in a transverse serpentine in the mast which can be clearly seen by sighting up the sail track. By pulling on each shroud individually while sighting the track, the effect of tightening that particular shroud can easily be seen. Remember that once the shrouds are approaching proper tension, when you tighten up on one shroud, you may have to loosen its opposite by the same amount.
Since there is only a single lower on the athwartships line, over tightening the rig only has the effect of pulling the mast down and out of column, since the lower shrouds cannot offset that. Therefore, preserve your hull, rig and fittings by not over tightening the rig. Rig tension needs to be set so that dynamic loading of any one fitting does not exceed the breaking strength of the wire. Setting it conservatively accomplishes that.
Aft mast rake is accomplished by lengthening the forestay and shortening the backstay, NOT by reefing back on the backstay until you have it back where you want. Blocking up of the the leading edge of the mast foot may be required if significant aft rake is wanted. As described above, undue tension will pull the mast back and also DOWN, bending the spar. Any bend in the spar weakens it, and the rig is not configured to support a bendy spar.
After the rig has been set, leave it for a few days and check it again. Odds are, the settings will have changed as the flexible hull re-orients itself to the new loads. This is particularly true if the boat has just been put in the water off the cradle. You may have to redo the tensions several times until the rig and hull are stable together. On the T7.5, if you can't close the door to the head, you may have the rig too tight!