Rig Tuning for Cruising Boats

Rig tuning is one of the most commonly overlooked areas of maintenance on cruising boats. Having the mast and shrouds set properly is critical to your boat’s sailing performance. It’s the foundation of everything we do related to sail trim and sail shape. If the rig is not set up and tuned properly, a sail just won’t set right and the boat will be losing performance. In this month’s tip I am going to try and help take the mystery out of rig tuning. It is critical that the mast be centered in the boat. This is true whether you’re deck stepped, keel stepped, fractional rig or masthead rig. The best way to do this is by using a tape measure, but you can also use your primary genoa halyard. 

Step 1 – Centering the Mast.

The first step is to either measure back from the stem to a point – the same distance on the port side and the starboard side. This could be a spot on the rail of the boat or where the shrouds pass through the deck, as long as they are the same distance from the stem of the boat. Put a piece of masking tape down to mark this spot.  On a deck-stepped rig make sure the mast is seated on its step and go on to step 3. On a keel-stepped rig, start by making sure the mast is seated properly on its step then examine the mast where it passes through the deck or cabin house. This area is called the partners.

Step 2 – Centering the Mast in the Partners.

I will assume that the hole for the mast has been carefully and painstakingly centered in the boat by the manufacturer, although this can sometimes be a bad assumption. The mast needs to be blocked on all sides at the partners and with equal spacing on the port and starboard sides. If the space to port is ¾” and the space to starboard is a ¼”, by the time the tip of the mast projects up 45 feet into the air it’s going to be pretty off center in the boat, and if you try and compensate by adjusting the shrouds to straighten it out, you’ll just end up chasing your tail. The mast also gets blocked at the partners in the fore and aft direction. This is important for setting pre-bend in the rig. Pre-bend is the forward bend or bow in the rig when there is no backstay tension or other loads on the rig other than the shrouds. As a rule, all masts should have some degree of pre-bend. This is best determined by the mast designer and is important to discuss with your sailmaker. One big concern I have when I go aboard a lot of cruising boats is the lack of any pre-bend or worse, a slight inversion in the mast. Masts are designed to have the aft face in compression and the forward face in tension; a little pre-bend actually induces this state. Inverted masts create just the opposite condition and put the mast at risk of failure. By putting more blocks on the aft face of the mast and less in the front you increase the amount of pre-bend. In general 1 to 2 inches of pre-bend on the typical masthead rigged 40 footer works well, but different boats have different needs, for instance a modern day Hunter with the B & R rig will have 4 to 6 inches of pre-bend. Pre-bend is critical when designing and building a sail. If the sail is design for 2” of pre-bend and the boat carries 6”, the sail will always be too flat and will never set up properly. Conversely, if the main is cut for 6” of pre-bend and the mast only has 2”, the sail will always be too deep and overpower the boat and rob its upwind performance.

Step 3 – Centering the tip of the rig.

Now that the partners are set and you have a common point on port and starboard, it ‘s time to center the tip of the rig. Attach the end of your tape measure to the primary genoa halyard and hoist it to the top of the mast and cleat it off. Pull the tape measure down to the port side and measure down to the spot on the rail we marked earlier. Now do the same thing on the starboard side. Compare the measurements and adjust the shroud tension on the upper shroud only to center the tip of the mast. Re-measure both port and starboard sides again. When you can get the measures to be within a ¼” of each other, you’re pretty well assured that the tip is centered.

Step 4 – Bringing the mast in column.

The partners are set; the tip is centered; now we are moving to the final steps. Using your dominant eye, sight up the aft face of the mast. Use the mainsail track as a guide. You are looking to see if the mast is straight or in column. It probably is not and this is where some of the art comes in. Each shroud that attaches to the mast affects how the mast sets up. By tightening some and easing others, we will begin to bring the mast into a straight line. This is quite often a series of trial and error adjustments. You make an adjustment, then sight up the mast and make another adjustment and sight up the mast again. The whole time you are making baby steps towards having the mast become straight. The final step is to sight the mast. Verify that you are happy that it is in column, and now you are ready to tension the shrouds.

Step 5 – Getting the right shroud tension.

The mast is straight and now all you need to do is tighten the shrouds. You want to make sure you put the same number of turns on each side. You might start by putting 5 turns on the port upper and 3 turns on the port intermediate and the 2 turns on the port lower. Then, move to starboard and put the same sequence of turns on. Repeat this process until the rig is good and tight. As an aside, the upper shrouds will always feel tighter and actually be tighter than the lower shrouds. Because they run a longer distance, there will be more stretch and as such, they start off tighter to compensate for this stretch. Make sure you are turning the right way and make sure you have a good count on your turns. This is not a good time for your dockmate to show up and ask what you’re doing or discuss an upcoming cruise. You really need to spend about 30 minutes of total concentration. Your goal is to get the shrouds set to about 90% of the final tension.

Step 6 – Taking a sanity check.

Take a quick look at the rig make sure you are still straight/ in column. If everything looks good, then go ahead and pin the turnbuckles, but not for good; we still have one last step.

Step 7 – Final tuning.

The dockside tuning portion is called a static tune; it gets you very close to the final tune, but until the rig is looked at under load you can’t be sure of the tune. The dynamic tuning will be fairly straightforward and will allow you to accomplish two important objectives – confirm mast straightness under load and determine proper shroud tension. The process is simple; go out and sail on Port tack with the main and genoa out; get the boat on a good upwind heading with the sails trimmed and then go forward and sight up the mast track. You are looking for two things. One, make sure the mast is still straight. Sometimes when a rig is loaded it responds differently than it does at dock. When the boat is heeled over and the shrouds are loaded, you may see that the mast has some spots that are out of column; make a note as to which shrouds need adjustment and then repeat this same step on starboard tack.  Two, check to see if the tip of the mast is staying straight or dropping off to leeward. If it is dropping off to leeward you are going to need to tension the upper shrouds. When you sail along upwind, as a rule, your leeward upper shrouds should just start being slack at around 15 degrees of heel. If your upper shrouds are loose and are easy to shake around, this is a sure sign that the shroud tension is not tight enough and this will hurt your boat’s performance as well as present some concerns for rig damage. When you get back to dock, make the final rig adjustments according to your notes, then pin the rig for good this time.

Getting your rig set up and tuned properly is a pretty daunting task at first, but well worth the effort. Your boat will perform better and be safer to sail as well. Look for a future tip of the month to discuss mast rake and its impact on your boat’s balance and performance.