MAST FLEX

LCDR George · #1 (**permalink**) 03-16-2003

Looking for some information about mast flex. My mast is flexing in the fore and aft direction. It''s more noticable when the sails are down. The mast head and foot are stationary. The flex seems to be in the mid-section of the mast. Is this normal? The mast is on a 25 footer and it is approxiamately 28 feet in lenght. It is deck stepped mast with a support pole attached to keel. The deck appears to be solid around the mast base. Any thoughts? Thanks, George

BertV · #2 (**permalink**) 03-16-2003

Need a bit more information. What is causing the mast to flex? Pounding through waves? Chances are that on a 25 ft boat (like my former 25 ft O''Day) the lower stays (single per side) are parallel to the mast and not set for and aft. The stiffness of the mast keeps it from flexing for and aft. This should not be a problem. If it''s flexing for and aft while there is no load (like pounding into waves) then is sounds like the rigging is too tight causing the mast to bend in the only way it can. The lower stays (port / starboard) keep the mast in column from side to side but there is nothing to do the same fore and aft. But it would take a lot of compression to do that.

LCDR George · #3 (**permalink**) 03-16-2003

Bert,
Thanks for the reply. Yes, you are correct about the rigging. I have lowers that run parallel to the mast. There is no fore and aft support on the sides. The mast will actually oscillate in a good breeze tied to the mooring ball. I don''t notice it under sail. Could the mast be raked to far aft and to tight on the sides? Should I rake the mast forward and try loosing the sides to see if it stops?

LCDR George · #4 (**permalink**) 03-16-2003

BertV · #5 (**permalink**) 03-17-2003

Again, it''s a bit difficult to determine if there is a problem. With the main sail up, the mast is supported fore and aft by the mainsail in tension. How much tension is on the rigging? When you go to weather in a good breeze (10 - 15 knots) are the leeward stays just about slack? You should be able to rattle the stays and feel almost no tension. If not, the rig may be set up too tight. Books have been written on how to set up a rig. In a few words, when at the dock you tension the rig so that the mast is straight and vertical (if that''s the way the manufacturer made it). Not a whole lot of tension but just enough to hold everything in place. If you don''t have an adjustable back stay, the fore and back stays should be tightened to the point where the mast starts to deflect sideways (remember the loose stays port and starboard). Check to make sure the mast is still vertical (not raked fore or aft). Then go sailing! When going to weather, the leaward slack stays will be reeeeaaaallll loose. Tighten them up a few turns. Tack. Do the same for the other side. Keep doing this (and it could take a whole bunch of tacks!) until the leaward stays are just slightly slack. Make sure the mast is still in column (no bend at the spreaders when going o windward). If there is a windward bend at the spreaders, the lower stays are too tight or the uppers too loose. If there is a leaward bend to the lowers are too loose or the uppers too tight. To get it right you have to experiment and take a lot of time. Good luck!

LCDR George · #6 (**permalink**) 03-17-2003

Bert,

I think you are on to something here. When I am out in a good blow, I see very little slack in the leeward rigging. I am going to loosen the sides, check the mast for straightness, adjust the rake and then retension the side rigging to see if this helps. Thanks for the information. George

knothead · #7 (**permalink**) 04-09-2003

You will probably a different opinion about rig tuning from each person you ask but here is another one. I don''t like to see shrouds so loose (even on the leeward side) that there is no tension on them at all. I believe that leaves the possibility open for shock loading (snapping tight).
I agree that the best time to tighten a rig is under sail but even the leeward side should have a little tension.
As far as the mast pumping (flexing) when moored, I have seen the problem addressed in a number of ways. Sometimes simply taking the pole topping lift forward and putting it under tension helps. I have been told that hardening the vang might help. Because you have no forward lowers, you could add a baby stay. Bottom line, a little mast pumping is usually more of an irritation that a real problem. Good Luck

Jeff_H · #8 (**permalink**) 04-09-2003

I think you meant that the best time to adjust the rig is when you are *not* underway. You really do not want to set up a rig underway as it is too easy to overtighten a component of the rig and get the mast out of column. Of course it is important to check your adjustments underway.

Jeff

knothead · #9 (**permalink**) 04-10-2003

On the contrary, I meant what I wrote. The best time to tune a rig is when the boat is sailing. A close reach is good. The mast should have been adjusted dockside to where the mast is in column. If one is careful to count the rotations as the turnbuckles are adjusted, and to do the same thing on each side, the mast will stay in column. Any final, minor adjustments can be made later after the proper tention has been achieved.