Little America's Cup 2010-C CLass-the real one..

Discussion in 'Multihulls' started by Doug Lord, Apr 12, 2010.

  1. Doug Lord
    Joined: May 2009
    Posts: 16,679
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Litlle America's Cup---Aethon

    More analysis from Steve on SA:

    I am not blaming anyone or anything.
    You engineer and build things. If the "fiber wasn't strong enough" it isn't the fiber's fault but the responsibility of those who asked not enough fiber to do a job that was too large for them. But in conversation it is always the "fibers broke" not the designers didn't get it right.

    The beam design is driven by the desire to reduce weight to the absolute minimum and still maintain adequate strength and stiffness.
    The main beam is one of the 7 dumbest places to save weight, but it is also one of the largest and heaviest structures on the entire catamaran, so one doesn't do things willy nilly. In 1995 we put strain gauges on Cogito in order to better understand the loads. We discovered that our calculated "worse case" load was about 30% higher than the highest loads we recorded sailing on a maximum day. So Cogito's mast amd main beam were WAY overbuilt. Given that there isn't much meat on the bone elsewhere, the mast and main beam were key areas to achieve significant weight reductions on the new boat.
    By using the real life load information and buy using high modulus fiber we were able to reduce the weight of the main beam by about 20 %. Pretty significant, but it comes at a price. The beam has a thinner wall and the high modulus fiber is less elastic. So the beam is more easily damaged.

    When the main beam was first assembled, I tested it in isolation to assure that it was strong enough to with stand the compression loads imposed by the mast. Once the platform was complete, we again tested the entire platform to 125% of the measured maximum loads. This qualified not only the beams, but the rest of the load bearing structures as well. We then went through a fairly testing work up in which we certainly went into the maximum load case more than once. I was very happy with the strength and stiffness of the platform. It performed really well, and if anything, I was thinking that it was probably slightly overbuilt.

    So when the beam packed it in off Toronto we had to establish if 1) Our understanding of the loads was flawed and the beams were fundamentally under specified. In this case we would be accepting that our load testing 115 years ago produced misleading data and that our understanding of the loads on a catamaran were fundamentally flawed. We would need to revisit all of the structural decisions, including mast, chainplate, standing rigging and beam chock designs. 2) We damaged the beams in Newport and didn't know it. In this case the beam design is fine, just not strong enough to take falling onto the mast. By concluding that we broke the beams in the Newport capsize, we have essentially said that we know that our design data is sound, that the beam design is correct but that you cant count on them being undamaged in the event of a catastrophic accident.

    What I meant by saying I could have changed the outcome by more testing is that if I had done a full platform stress test, we might have discovered the beam failure on land instead of 3 miles off shore. The damage to the platform would look about the same, but the wing would not be trashed.
    In any event we may do some sailing with strain gauges again. Just to refine our understanding of what is going on. We were pretty chuffed by what we did 15 years ago, but the modern world has introduced another level of tools to make this even easier.

    nzpaul. the lee shroud stays pretty tight, but it is not doing all the work keeping the hull from rotating with the board. There is =\-45 fiber aft of the main beam that transmits most of their rotational energy into the beams. Because we are picking the boat up as well as rolling it over, this load is higher than on a boat with straight daggerboards. Indeed the curved daggerboards have to be stronger than straight ones. We took all this into account in the laminate design and also in the configuration of the tackle that controls the top of the daggerboard.

    Beatings will continue until morale improves.
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