tensioning a fractional rig

[Justin_NSA]

Okay, according to the instructions that came with my new Loos 91 tension gauge, the shrouds should be adjusted to 20% of the breaking strength of the cable. That's approximately 700 lbs for 5/32" cable!So I was out on the boat today checking my shrouds. The weren't even close to this tension! I started tightening them and on the first lower shroud the handle on the rig tuning tool I bought from Blue Water Yachts split completely and one of the pins/bolts was bent.This rigging tool from BWY obviously wasn't designed for this kind of tension. This makes me wonder if the Loos instructions are applicable to a Mac even though they describe our setup exactly. (see quotes below). I plan to call BWY Monday, I was just wondering what your thoughts were. One important point: I don't trailer mine so I don't care for ease of mast lowering and raising. I'm looking for the best performance."Upper and Lower Shroud Tension - Fractional Rig: For most fractional rigs the correct shroud tension is the same as that for a masthead rig, i.e., a tension setting that will keep the leeward shrouds from going slack. However there is one exception. On certain fractional rigs , the upper and lower shrouds lead to chainplates that are aft of the mast. The spreader is swept back. For such a rig most of the forestay tension is balanced by the upper shrouds. A shroud tension of approximately 20 % of the cable strength may be required to achieve the desired forestay tension. Never exceed 25% of the cable breaking strength. (Refer to the breaking strength chart Table 1.)"

 

[vwjon]

loosen up

Let's face it. These boats are built light weight. They are not made to be strung beyond their limits. Whatever fraction of a knot you will gain by over stressing the rigging, you will continue to pay for in broken parts and tools. This is not great advice. More like farm boy logic. Take it easy on your stuff and it will last longer. Happy sails to you. vwj

 

[Greg Pecaut]

Tuning

When I worked at Malloy Boat Yard on Long Island, we would start off the small boats with 8-10% (on a Mac that woud be about 250-300 lbs) tension. That's on the upper shrouds, then about half that on the lowers. Then take it for a quick sail. Tighten the shrouds only enough to take any slack out of the lee ward shrouds, then come about and tighten the other side the same amount. (same hole if using the original adjusters, same number of turns if using turn buckels) This keeps the dynamic stress off the shrouds, while not over stressing the shrouds or mast (compression on the mast step). The Loos gauge can be a real help here in making sure you haven't over tightened any thing. (no more than 20%, 700 lbs) From there it can be fine adjusted for sail shape. Like appling tension to the back stay for bend, or tightening the lowers to reduce bend in the middle. The back stay will help very little with getting tension on the for stay. This is counter opposed by the upper shrouds on a fractional rig. You can use your Loos gauge to make sure the tensions on opposing shrouds are equal. A plumb line from the top of the mast can be used to check that the mast is straight (with a level set side to side on the deck) and to check the rake, on most older Macs the cockpit floor should be level fore and aft while checking mast rake. As for rake, I use the boats sailing characteristics. Too much weather helm/ reduce the rake. No weather helm/ increase the rake. A small amount of weather helm is wanted. You don't want the rudder off center to sail straight, but you do want the boat to round up into the wind if you let go of the tiller. Tightening up on the back stay and boom vang will flatten the main, while loosening the back stay and vang will give you a more full main for light air. On the Venture 23's the loos gauge is a real help on getting the 2 forestays the same tension, though a little more (but not much) tension can be preferable on the forward stay (yankee). Hope this helps. These are a little harder to tune that the big keel boats. The back stay confuses people a lot, and is usually set too tight.