I’ve done a lot of reading on this subject and played with various rig tensions on my Catalac. What I'm writing on this page is the result of my tinkering.Initially look to see if the upper on the leeward side of the boat visibly slackens on a beam reach. If it does, this needs to be addressed. Also, with hands off the helm, does the boat round up? Is it gently or a sharp turn to either leeward or windward?
I’ve found that stiffer rigging is a generally better performing configuration for Catalac 27’s (8M), however tightening the rigging will add more stress to the rigging attachment points. When we look how the rigging attaches to the boat, the only weak area I can see is the foredeck, particularly the inner forestay. Depending on the condition of your boat you may need reinforcing in this area, especially if you notice the foredeck 'hogging up' in this area. If that's not feasible, I would say that a 20 year old boat should not be sailed with tight rigging. On the other hand, I've sailed with a loose rigging configuration and lost 1.5 mph of boat speed and what little pointing ability I had. Only you can decide what your goals are in setting up the boat.
As I've mentioned, the goal on this page is to tune the rig for the sweetspot. We are going to achieve this by slightly bending the tip of the mast aft so it affects sail shape, windward pointing ability, pumping in a seaway and weather helm. It's a must for a baggy mainsail.
The tuning procedure begins at the dock on a calm day, with the rig slack. By slack I mean all turnbuckles loose and the wire loose to the touch with the exception of the forestays. With a slack rig it's difficult to set either forestay, as tightening them will just bend the mast forward as every other rigging component is slack.
Before we do anything else, take a look at the way the mast sits in the tabernacle. Step off the boat and put yourself in a place on either port or starboard, where you can see the mast from base to masthead. Look to see if it's straight up and down. Make sure the base of the mast at the step is hard against the rear of tabernacle. I neglected to check this when I first began adjusting and never could get enough tension on my backstays. I called in our area's prominent rigger and he pronounced the rig fine, and cautioned against over tightening the rigging. It wasn't until I was in a conversation with the owner of a Catalac 9M about my problem that he indicated to take a look at the mast base. He was right, I had a 5 degree mast rake. I corrected this and my problem was solved. I don't blame the rigger, as I don't believe he gets many calls about catamarans with deck stepped masts. I actually ended up buying a long thick bolt that fits the mast lowering slot at the base of the mast and the coresponding hole in the rear of the tabernacle. This gets inserted and tightened after raising the mast.
Next, detach the topping lift from the boom and shorten it's length so that it's bitter end is just above where the main halyard attaches to the mainsail. Tie the end of a tape measure to the bitter end of the topping lift. Now mark a spot on the rail of each hull even with the mast. Stretch the tape measure to each spot you've marked and take note of the measurement. The cap stays (upper shrouds) are tensioned next and are tightened hand tightened first, keeping an eye on our tape measure. Then use a short wrench to tighten, until we feel increasingly difficult resistance. Stop at this point. Using the tape, insure that both port and starboard have the same measurement.
Next, the lowers are tightened hand tight. You will also straighten the mast using the lowers. If necessary, adjust the rigging screws for the cap shrouds without changing the cap shrouds tension (slacken one rigging screw by exactly the same amount as you tighten the other one). As the lowers are tightened, check that the mast is straight by sighting up the luff groove, or compare it to our handy main halyard. At this time, we tighten the baby stay. It's important not to overdo this. If the babystay is overtightened, a slight hogging upward will occur in the area of the base of the babystay. If this occurs, loosen it.
Backstays are now tightened hand tight. The backstays shouldn't be overtightened, as not to depower the sails in light wind.
We're ready to go for a test sail. It's important to sail as close to the wind as the Catalac will go and maintain speed, as this exaggerates any possible slackening in the leeward shrouds as well as checking the forestay sag. If there is any slack in the leeward shrouds, tighten them just enough to eliminate it. Assume the other tack and do the same. At this time, it's impractical to use a tape measure, and the mast is in column, so if they need tightening, just tighten port and starboard an equal number of turns.
While sailing close to the wind, take a look at the all important forestay. Forestay sag is identified by comparing the forestay to the furler jib halyard. In 5-10 mph winds the forestay sag shouldn't exceed a couple of inches, if at all. In 15-20 mph winds 6-8 inches of sag is acceptable. At this time, if necessary, reduce excessive slack in the forestay. This can be done by tightening either the forestay or the backstays. Your helm tells us which one to adjust. If your helm is balanced during the test sail, then adjust the forestay and backstays equally 1/2 turn at a time. If there is too much weather helm, tighten only the forestay. If there is any Lee helm, tighten only the backstays. The boat should gently round up with your hands off the wheel when you've completed your adjustments.
That's about all there is to it. The results are remarkable in that the boat feels balanced and more responsive.