Setting up your masthead standing rigging
Start by slackening all shrouds and stays until all feel very slack. Then, tighten the backstay (and therefore the forestay) to a few turns tighter than hand tight. Any runners, checkstays or other running rigging should be slack. Haul a thin line attached to your main halyard shackle to the top of the mast and tie on a plumb bob at the level of the boom gooseneck. Thin line and a symmetrical weight will minimise the influence of any halyard stiffness or shape, especially with wire halyards. Adjust mast rake by adjusting the turnbuckles on the forestay and backstay and referring to the plumb-bob. Most cruising boats have a mast rake somewhere between upright and a few inches back. Once the desired amount of mast rake has been set, tighten the turnbuckles on both forestay and backstay one turn at a time to achieve the amount of tension you want. The tension required for these two wires is quite considerable and the wires will not move more than inch or so to a good push or pull by hand. The actual value will be determined later when sailing. Check again that the rake is as you want it.
Determine two reference points on the toe-rail, hull-mounted chain-plates or somewhere well outboard, approximately in line with the mast fore-and-aft, exactly the same distance from the mast step and of equal height. Make no assumptions about symmetry, boats can often be different on each side.
With a keel-stepped mast, centre it at the partners by measuring from the reference points and chock it fore/aft at the partners to the maximum "J" value.
Now begin to centre the masthead by measuring from the main halyard to the reference points. This can be done using the halyard itself, but stretch is difficult to estimate. One way is to use a spring balance attached to the end of the halyard to give a constant value each side, perhaps 10 kg. The best way to do it is to attach a steel tapemeasure to your main halyard’s headboard shackle. Hoist the halyard and extend the tape measure aloft. You can now measure the distance from the masthead to each reference point. Adjust the cap shroud turnbuckles so that you get the same measurement on both sides of the boat, ensuring that the mast is centred in the boat. Now begin to tension both turnbuckles, keeping the mast straight up. Turn the turnbuckle barrels either one or half a turn at a time. The tension needed on the cap shrouds is almost as much as you have on the forestay/backstay. A useful guide is that they should produce a deep musical note when struck with the flat of the hand. Final adjustments will be made later.
Now sight up the mainsail luff track. Compression from the cap shrouds may well cause it to be slightly bent or bowed. Tension on the babystay will pull the mast forward, normally taking out either of these. If you have twin lowers, take up the one that will pull any sideways bow straight, then tension the other one to match. At this point, the mast should be raked the amount that you want and exactly straight up and down relative to port or starboard lean.
A decision now needs to be taken regarding pre-bend in the mast. Pre-bend has the effect of pulling some of the fullness out of the mainsail to flatten it. It is normal to have a small amount of permanent pre-bend on a masthead rig, whereas tension on the backstay of a fractional rig will apply bend when required. Increase the tension equally on both forward lower shrouds, or the babystay, until two or three inches of bend is pulled into the middle of the mast. A normal amount is about half the mast diameter. Sight up the mast as you pre-bend it to make sure that you aren’t getting an "S" or a bow left or right.
The aft lowers balance the forward lowers or babystay, fine-tune the rig and give additional support to the mast. All you need to do with the aft lowers at this stage is tighten the turnbuckles about one turn past "finger-tight". Sight up the luff groove to make sure that the mast is still straight.
With these steps complete at the dock, it’s time to set sail and make your final adjustments.
Sail the boat on a series of upwind tacks in 10-14 knots of breeze and examine the rig on each tack.. You are looking for three main points for the rig, and three more for the sails:
1. That slack in the leeward shrouds is not excessive. The wire should loosen but there should be no clearance in the swivel or other pin joints. This applies to cap and lower shrouds. If slack is excessive when hard on the wind, adjust by equal numbers of turns on each side.
2. Keep sighting up the luff groove to ensure that the mast remains in column. If the top of the mast is falling away tighten the cap shrouds, if the centre is bowing upwind loosen the lowers, if it is bowing downwind, increase tension. Increase backstay tension and check mast straightness with the added compression load. Check on both tacks.
3. Check forestay sag with maximum backstay tension. It should be no more than two to four inches for a 35 ft. boat in this wind range. If it is greater than this then tighten the forestay. If less than this, loosen it, otherwise your forestay will be too tight to provide proper power in the lighter air. Most jibs are cut with some forestay sag designed in. If you don't have an adjustable backstay, then do these tests with a moderately tight backstay
4. Pay attention to mainsail shape, especially down low, as the luff curve should be matched to the mast pre-bend. Pre bend is adjusted on a keel-stepped boat by moving the mast at the step and partners but on a deck-stepped boat be increasing babystay tension. . Make this adjustment if the mainsail is either too full or too flat in the lower third, with respect to the rest of the sail (i.e., the sail is out of balance).
5. Check helm balance with the main and jib sheeted for upwind sailing. If weather helm is excessive with the mainsail flat but not backwinded, jib sheeted in to maximum, and enough backstay on to give about three inches of forestay sag, then you need to rake the rig forward a bit. Too little helm, or poor pointing ability, and you need to rake it aft. This adjustment is done by tightening or lengthening the forestay, perhaps three to ten turns, preferably when moored alongside. Don’t forget that changing forestay length will also change babystay tension. After doing this, again check headstay sag, mainsail shape, and rig athwartships position in 10-15 kts. If you have added rake, be sure to check that the genoa clew is not too close to the deck or the sheet lead is not too far forward to be sheeted properly.
When you get back to your berth, carry out final checks. First, verify that the relationship in tensions between the forestay/backstay and upper shrouds is still about the same. The forestay/backstay should still be slightly tighter than the cap shrouds. Next, measure the distance from masthead to toe-rails again. The measurement should be the same to both starboard and port toe-rails. Sight up the luff groove again and make sure that it forms a straight line. Make the appropriate adjustments to the turnbuckles.
With everything set correctly, secure the turnbuckles so that they won’t back themselves off and loosen the rigging. With closed barrel style turnbuckles, there is a locking nut on both bolts. Turn these nuts down to the barrel and use a small spanner to lock them tightly. With open style turnbuckles; use split-pins or seizing wire through the openings of the barrel and the small holes in the bolt ends.
Over time, there is every chance that your rig will loosen somewhat. This may be due to sailing in strong winds, changes in temperature, or combinations of other factors. Periodically check your standing rigging, at least annually, for tension and condition. Make your checks of the system both while at your berth and while sailing. The combination of pre-bend and mast rake can change the weather helm feel of the boat; the boat’s pointing ability, and the boat’s speed and efficiency through the water. So, don’t forget to check the standing rig from time to time. As your sails age and become fuller, increased mast pre-bend will help your upwind pointing ability and will decrease the amount of heel your boat develops in stronger breezes. The boat may also develop more weather helm. Reducing mast rake will improve this tendency.