This is a bit confusing, so read carefully...
I am using a crane to get my 15' 600 pound sailboat in and out of the water. The crane has two hoists on it. One goes to the bow of the boat and attaches to where the jib normally attaches. On the stern there are two cleats usually used for attaching to a dock. To lift it with the crane, I will attach the cleats to a 4x4 with Ubolts and to the hoist with a Ubolt in the middle. At least that is the plan. The first time we did it we used a strap wrapped around the two cleats in the bow. It worked, but was a little spooky. The 4x4 should be better.
Now for my question...
A doubled 2x6 (so 5.5" wide and 3" high) is actually cheaper than a 4x4 (3.5x3.5). I took some mechanical engineering courses 50 years ago and understand that a 3.5" beam is stiffer than a 3" beam, but I think the much wider doubled beam will actually be stiffer than the 4x4. My friend, who will be operating the crane and has more experience than I do, thinks the 4x4 will be stiffer than the doubled 2x6s because they are two pieces. I think that they will be connected at the 3 Ubolts and will flex as one piece.
It also seems safer. If the 4x4 has a defect, it could cause a failure. A defect in one of the 2x6 pieces won't necessarily cause a failure. Redundancy.
Any opinions? While saving a couple dollars is nice, the important thing is safety. I kinda don't want to drop the boat.
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Yes, having the 2x6s use on end, 3" wide 5.5" high, will be stiffer than either one, but the crane was designed for motor boats and not sailboats. My trailer is higher than motor boat trailers because of the keel and it just barely managed to get it in the water. Every inch matters.