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Old 03-14-2004
jkumin jkumin is offline
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what mast?

I''m surprised this question didn''t generate a little more discussion, as conventional wisdom favors keel stepped rigs for off-shore work. A couple comments:

Jeff feels keel stepped rigs guarantee water in the bilges as it comes in through halyard boxes etc. Not necessarily - you can get a water stop in the mast with a drain hole just above deck level. Isomat used to do it with a glob of foam. Hall Spars does it with a sloped aluminum plate. Jeff - you should try this, you''ll like it. Dry bilges are nice. The advantage of deck stepped masts is they eliminate sealing around the outside of the mast, although Spar-tite has made this simpler.

The main reason for tie rods connecting the deck to the mast or the step with a keel-stepped rig is a force Jeff may not have mentioned - upward pull of lines led aft such as halyards. Crank hard on a halyard led to a winch on the cabintop and you are exerting a powerful upward force that can lift the deck right off the bulkheads. This isn''t a problem with deck stepped masts as the mast keeps the deck from lifting.

Mast are placed in enormous compression to off-set shroud tension. If you run the numbers this is very much more than any sideways kick. The limiting factor for most masts is buckling from this compression - popping out of column. This is governed by Euler''s formula. The big advantage of keel stepped masts is the added resistance to this buckling. A simple analogy is push down on a thin dowel with one end on the floor. It pops out of column easily. Take the same diameter dowel and clamp the bottom in a vise and push down. Even with the same length exposed it is much more resistant to this buckling. That''s why a keel stepped mast can be a lighter section for less weight aloft, always a good thing. Or if the same section it can be that much more resistant to getting out of column, which can lead to failure if uncontrolled.

There are other ways of getting this stiffness. Some of open class boats use a tripod setup to brace the mast above decks and Hunter has experimented with this. Jeff''s idea of using a base plate through-bolted to a matching plate on the jack post has merit, although it seems rarely done. This would be like a column on a moment resisting frame as used for buildings. It would require careful engineering to avoid localized buckling of the thin mast wall at the plate.

As for dis-masting, unfortunately I have been there with both. The deck-stepped mast popped out of column and folded, something that seems to happen from time to time. It was blowing about 90 at the time and the reefed main had blown out of the bolt rope. Without the stabilizing aft pull of a reefed main the center of the mast popped forward when we fell off a wave and stopped. The keel-stepped rig came down from a chainplate failure. It left 6'' or so of stump. They were both a pain to clean up.

Which brings me to the main point. There is no simple right answer. Both setups can work fine -if they are properly engineered, maintained, and sailed.
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