mast design question...

[MikeJohns]

Dont knock the science too much.
At least with the modern FEA computer packages we can do a good job on the static forces with non-linear analysis. This means we can give numbers for the actual forces as represented by sailing in static situations eg sheltered waterways. We can then guarantee that the rig is strong enough to not be broken by any sail/wind combination. (This has happened recently in Americas cup races where the envelope has been too finely pushed.)

Once we add ocean dynamics ie induced loads due to acceleration, we need a factor of safety to ensure the rig survival. At this point it becomes a little less empirical but measutements with strain gauges have shown that the rules of thumb tend to hold in that a safe rig has a breaking/yield strain at least 3 to 4 times the calculated static loads.

[catsketcher]

Thanks to Eric and Mike for the contributions. I hope I didn't sound dimissive of getting technical. I have great wads of paper filled with Euler column formula, simple beam formulae and the like. It is just interesting that in the field I am interested in namely multihulls, there is very little public domain load data available. This makes it hard for even good experienced designers to do a good job.

As a case in point I went to work for a very good Australian multi builder and designer for a few months to start understanding how to build and design boats(this was 12 years ago). I was into tris back then and would ask him often how to design the beams. In the end he told me "Just get something that works like Lock Crowther's Kraken 40 beams and use that"

It does work. It is a sort of evolution where the good designs are weeded out. Our structures are often forgiving of failure and materials like wood were very good at showing when they were approaching failure by cracking so you could run to shore and beef it up. Now structural failures are rare in cruising multis.

It seems a little easier in aircraft where a plane is made to undergo +5 and -3 g on its structure. In my aircraft design book they really cut the FOS low then, about 1.1. They have lots of data from many years.

Thanks Mike for the dynamic factor - that is just the type of thing I was wondering about. I would love to do the experiment myself. I have a few leads on load cells but first have a new boat to finish. It would certainly be fun to see for myself.

cheers

Phil Thompson

[Eric Sponberg]

Mast manufacturers these days often supply a hydraulic jack under the mast of racing boats so that you can tension all the rigging just by jacking up the jack. It should not be hard to put a load cell in the mechanics of the jack and calibrate it. I'd bet dollars to donuts that someone has figured it out and collected data somewhere.

Eric

[tspeer]

Originally Posted by Eric W. Sponberg

...It should not be hard to put a load cell in the mechanics of the jack and calibrate it. ...

Or just measure the hydraulic pressure in the cylinder. Pressure * area = force.