I took a 2000 mm length of four different alloy mast sections, and clamped them in a work vice so that exactly 1000 mm projected freely. I then hung a series of weights from the free end, and measured the deflection. The table shows the deflection at the heaviest weight I used, 300 gm, because the deflections due to the smaller weights were linear and proportional, so they do not need to be reproduced here.
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.
It was interesting that only the 12.7 mm dia groovy showed a difference in stiffness when measured sideways as opposed to fore and aft. It was less stiff, overall, than the round section. The 11.1 mm groovy gave identical results in either plane, and was also less stiff than the round section.
In talking to Graham Bantock about these results, he pointed out that the groovy section has a piece "missing" from the section at the outer perimeter, and that this is "replaced" by the internal reinforcement, so the groovy section would usually be less stiff than the equivalent round section. Further, the groovy sections are also disadvantaged because of their thinner walls. This is good, of course, if you need to be very concerned about mast weight, but not otherwise.
Within experimental and sample error, these results tally with the modulus values shown in the SAILSetc catalogue for the groovy sections. I couldn't get on with the modulus values, however, since I couldn't relate them to the actual mast deflection I might see when the mast was loaded. Now I know...
John Rowley has some comments on mast stiffness, and in particular on the difference between 6061 and 7075 alloy tubes.
©2011 Lester Gilbert