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What about the stiffness of high strength aluminums? Larry Robinson, a member of the Seattle East Coast 12 fleet brought up this question, and what follows is more than enough information on this subject. Larry was also good enough to review this for accuracy, which I appreciate. The following is courtesy of a Senior Metallurgical
Engineer at Tennalum, a division of Kaiser Aluminum, and closely follows
Larry’s thinking. There are really two issues, one of them is "stiffness"
and the other is "strength". What about variations in manufacture or
specifications? Larry adds that he found that for one commonly used
extruded mast section; there was a modest difference in measured
stiffness, which also corresponded to the weight of the section. It
turns out that the dies used in the extrusion process wear
substantially, thus the differences. With high grade tubes, such as
those Easton sells for arrow shafts and such, one has to make sure that
wall thickness as well as diameters are the same, in order to compare
apples with apples. What does Duck Soup think
of all this? What we understand from this is if you want to
avoid permanently deforming the tube, 7075 is a good choice, price and
availability being equal, however if you want to put in some prebend the
7075 is nearly twice as hard to make take a set, but it will never
become unbent. This is due to the fact that the yield strength of
7075 is nearly twice as high as for 6061. Our extremely rough calculations show that the point of maximum
stress is where the jib stay attaches to the mast and the maximum static
stress is between 20% and 25% of the yield strength of the 6061
material. This of course means no unforeseen additional forces such as a
collision where mast tips of boats going opposite directions hooking, is
taken into account. Of course if this hypothetical collision were to
take place the forestays would probably break first. TESTS:
Some samples of 7075 T-9, 1/2" .66 mm wall thickness, and 7/16" .76 mm wall thickness, (12.7mm and 11.1mm), were
obtained and compared with a sample of SAILSetc 6061 T-6
And as a comparison a 7/16 inch diameter section:
It seems the stiffness is definitely a factor of diameter and wall thickness, not material, so, when purchasing our next mast, we will probably look at availability, delivery, price, color, and then look at the material. All things being equal, we would opt for the 1/2-inch diameter for the #1 rig. However, having said that, the location of the attachment point of the shrouds to the mast are a matter that needs study, if you want the rig to bend off in puffs, you need the top to bend off, so the stiffest mast might need lower shroud attachment points, or maybe the smaller diameter mast is the best choice. It is a puzzle?
There are some additional tests on Lester
Gilbert's web site that are rather interesting, the section is
Mast Stiffness, and are reproduced below. Note,
Lester says, exactly 1000 mm projected
freely (please note that in the tests above only 600 mm was projected
for the weight). I then hung a 300 gm 11.1 mm dia SAILSetc "groovy", fore and aft deflection
(ie in the plane of the groove): 62 mm 11.1 mm dia SAILSetc "groovy", sideways deflection (ie
normal to the plane of the groove): 62 mm 11.1 mm dia SAILSetc plain round, deflection 54mm 12.7 mm dia SAILSetc "groovy", fore and aft deflection
(ie in the plane of the groove): 43mm 12.7 mm dia SAILSetc "groovy", sideways deflection (ie
normal to the plane of the groove): 39mm 12.7 mm dia SAILSetc plain round, deflection 39 mm The 12.7 groovy has a wall thickness of 0.60 mm, while
the 11.1 mm groovy section is 0.50 mm. These are both significantly
thinner walls than the round tube, both of which have 0.70 mm wall
thickness. |
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01/09/2006 |
