Steven Shelikoff wrote:
On Thu, 15 Apr 2004 15:44:47 GMT, Rich Hampel
wrote:


I'm not so sure about that. If, when there are sideloads in squalls,
knockdowns, etc., the deck stepped mast failed by remining in one piece
but slipping out of it's step, then I'd agree that a keel stepped mast
would solve that problem ... if it didn't snap at the deck. But most
of the mast failures I've see are when it snaps somewhere aloft, like at
the spreaders. How it's stepped doesn't make a difference when it
breaks up there.


Mast failure (usually at midsection) is usually due to some rigging
failure that permits the mast to move 'out of column' and permits
catastrophic buckling failure when the compressional loads get off
center. Doesnt matter if its deck stepped of keel stepped, if the
rigging support fails and the mast deflects catastrophically .... the
latent compression load finishes the job.


Exactly. Which is why I don't think it makes all that much difference.

Steve

When my SC22 mast folded into a right angle and spiked into the
water beside the boat, it was because there was no compression box
inside the mast at the point where the spreader was through bolted.
When I built up a new mast from the extrusion, I put a piece of
square aluminium tube inside for the bolt to pass through. I
positioned it with a long stick and duct tape which tore off once I
had secured the box with 2 pop rivets one above, one below, one on
each side, just there to retain it in place when the spraeader bolt
was removed. I had single lowers, as installed by South Coast. I do
not know if the mast was original factory equipment or not.

The side load in a gust caused the spreader and lower shroud mast
tang stresses to crush the mast at that point. I was watching it
when it went. We salvaged the mast lashed it alongside and rescued
the mainsail, there was no other damage except the tabernacle was
partially torn off the deck and bent somewhat.

Lack of a compression box at the spreaders is the most common
failure in design that I know of. Check your spreader mounts.

Mast pumping may have been the root cause, the final straw, so to
speak. Keel stepped mast / deck partners migh have prevented some of
that, while providing a fulcrum to develop gooseneck loads and crush
the mast at the partners in a manner different from those expressed
in a deck stepped system without them.

It seems to me that the main difference between deck and keel steps
in some boats is that the tabernacle bolts passing through the deck
might shear, as most of them seem relatively flimsy. The tabernacle
would never let the mast base get away, as it was all secured
together with bolts. A wad of 1/2 round convex bog faired all around
the base of the tabernacle might help, there.

Terry K