[MHml] mast woes

[Charles Magnan]

One way of looking at the problem is to completely ignore sail/shroud loads on the theory that the big, expensive (not to mention heavy) bit is the mast extrusion which you already have (and thus are not designing) and the spreader & diamonds' job is to make sure that it does not fail due to buckling. 

This is achieved by keeping the mast in column and the required strength of the spreader system can be thought of as a function of the compressive strength of the mast as a slender (euler) column. This is a straightforward calculation of  buckling based on the mast section & alloy properties and unrestrained panel lengths. What is not so straightforward is the calculation of the required transverse force to be provided by the spreader to keep the mast suficiently straight (and we aere talking about both strength and stiffness, both have to be adequate), so it may be wise to be conservative here. 

The required tension in the diamond stays can then be fairly easily calculated once this is known (or conservatively estimated). Any strength over and above that required to keep the mast straight (with a reasonable safety factor) is simply wasted (i. e. an upper bound on the required strength).

Regards,

Charles

----- Original Message ----
From: Bill Gibbs <billg at gibbsCAM.com>
To: Informed discussion of multihull issues <multihulls at steamradio.com>
Sent: Monday, October 16, 2006 12:41:29 AM
Subject: Re: [MHml] mast woes


Charles,
The mast designer made similar comments on this topic #2.  It may be strong enough.  He also commented that he/they were surprised at the stress levels measured by strain gages on the diamonds during tuning.  He also commented that calculation of the stresses is more an art than a science.
 
Bill




From: multihulls-bounces at steamradio.com [mailto:multihulls-bounces at steamradio.com] On Behalf Of Charles Magnan
Sent: Sunday, October 15, 2006 11:44 AM
To: Informed discussion of multihull issues
Subject: Re: [MHml] mast woes


My comments/suggestions, for what they are worth:
 
1. try the nitronic and see if the mast breaks   It will probably be fine when tested unless you test it in fairly extreme conditions, which will be when you find out what it is really capable of, not a pleasant experience if it is inadequate.
 
2. refit with larger nitronic rod Can you not first check the adequacy of the newly installed rigging by calculations and only replace it if it seems under strength? If the cobalt rods were surplus from a previous project they were unlikely to be exactly sized for your boat and may have been over the minimum required. I stress the word "may"!
Regards,
 
Charles


----- Original Message ----
From: Bill Gibbs <billg at gibbsCAM.com>
To: Informed discussion of multihull issues <multihulls at steamradio.com>
Sent: Sunday, October 15, 2006 6:40:28 AM
Subject: [MHml] mast woes


The riggers got my new rod diamonds installed Friday, mostly.  They had trouble getting the original tensions back.  So I rang up the mast's original designer in NZ, who suggested tensioning further on-the-boat, but also offered to do some research with others involved in the original project.  With typical yard efficiency and some high tide problems (we're a little too tall for the crane), we get the mast stepped at the end of the day.  That night there's an e-mail waiting for me with bad news.  The original mast rigger remembers cobalt rod rigging, not nitronic.  Cobalt rod is stronger and stiffer than nitronic, so a same size replacement is a significant downgrade in strength and safety, perhaps too much.
 
History - Afterburner was built in '87 and modified/re-rigged in '92 or so.  Her owner and builders were tight with the NZ racing scene and AC program. Many parts and sails were surplus from the AC program.  My first spinnaker supposedly came from Grant Dalton.  Remember the 1988 America's Cup, and the big white NZ1 versus S&S the cat?  After that loss the boat sat idle and the program was discontinued resulting in a lot of surplus goods being sold cheap.  Like the cobalt rod that was supposedly used in my diamonds.
 
I've asked the rigger to research a test to determine if my old rods are cobalt.  Any suggestions?
 
Assuming it is, I have a problem.  Options include:
1. try the nitronic and see if the mast breaks
2. refit with larger nitronic rod
3. find a source for new cobalt rod.  One opinion is that it is no longer manufactured.
4. refit the old diamonds and use the mast until it breaks from old age.
5. get a new mast  (1 & 4 end up here)
6. other?
 
All this because I act on the advise to replace 14 year old rod diamonds before they fail.
 
On the good news side, the track replacement went ok, as did the new mast winches, mast clutches, sail cars, and Precourt synthetic side stays.  So I have a refurbished mast that may collapse as soon as I raise sails.  Sheesh!  Guess I should have researched the details before replacing the rigging.  I "assumed" the rigger would replace with "same", whatever it was.
 
Suggestions?
 
Bill
Afterburner
 
 
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