Originally Posted by festivus
For a large cat frame, is there any real reason to use 1 1/2 over 1 1/4, as far as strength or other intangibles. Is the larger pipe overkill, or are they worth the extra 10% weight.
The 1.5" nps offers roughly 35% increase in strength over the 1.25" pipe size alloy for alloy. Long span i.e.72" 1.25 nps cross bars will bend every time when a 200#dynamic point load is applied. Do the arithmetic. The dynamic load will produce a moment of PL/2 or 7200 in lbs. For 1 1/4" pipe the section mod is around .2 in ^3. This puts the 1 1/4" pipe on the yield plateau...the 1 1/2" pipe in in the working stress range.
However, A boat pinned sideways in a strainer needs all the strength it can get. Look in the archive of utah rafters at the 1 1/4" frames that were ripped apart by boats caught in strainers. The force of the water on the fabric decks shreaded the boats with the 1 1/4" frames.
Do your homework and you'll build a better boat.