What boat? Marine lifting eyes come in a bunch of sizes. I used one on the front of my lawnmower that will lift 5500 pounds.
example: https://usstainless.com/stainless-st...de-heavy-duty/
Last edited by Tom M King; 09-30-2023 at 11:41 AM.
I"ll bet there's a disclaimer on the packaging that says they're not to be used for lifting. If the straps fail while being used to hold a load down what's the worst that could happen? If they fail while your boat is over head what's the worst tha could happen? That's how lifting is different.
Confidence: The feeling you experience before you fully understand the situation
This is correct but I'll add one small caveat- two unconnected flat beams on top of each other will double the stiffness. If you fix them along their length (glue, nails, etc) then the stiffness will go up by 8x (the cube). Otherwise they will slide over each other.
Note, however, that the maximum load increases by the square of thickness, not the cube (the stress term is My/I; I increases cubically but y does so linearly, so the effect of the stress is squared instead of cubed). So if you're worried about rigidity, then use cube. If you're worried about strength, use squared.
Width is linear for both stiffness and max load.
As others have said, the doubled 2x6 on end will be a LOT stiffer than the 4x4 (5.5^3/3.5^3)*(3/3.5) = 3.3x.
Source for the math: https://www.engineeringtoolbox.com/c...ms-d_1848.html
If you compare lifting straps to ratchet tie downs, they are constructed differently because they handle load differently. Tie-downs are designed to handle lateral stress, not lifting. Lifting straps are designed for the purpose and generally have loops on the ends to fasten to a winch or other raising solution, such as a forklift's forks. My biggest concern would be failure of the ratchet when lifting, but as I mentioned previously, if you are working with a small fraction of the lateral weight handling capacity, you "likely" will be ok. But there is no guarantee. I actually have had a ratchet fail and fortunately it was just being used for tie down. And I mean the heavy 3" commercial type tie downs, not cheapies. (but I've had those fail, too, when cranked too tight)
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The most expensive tool is the one you buy "cheaply" and often...
If the straps fail holding down the boat while driving down the highway at 70 mph I wouldn't want to be riding a bike behind the truck. Both the bike rider and boat are toast. On the other hand, rule #1 when lifting is never get under the load so if the strap fails the boat may be toast but no one should be in danger. So you could argue lifting is the less dangerous operation.
As stated by Jamie Buxton the paired 2x6 have the same stiffness as the 4x4. However the paired 2x6s have greater strength. This is more important. And as you suggested they can overcome local defects.
Defects near the ends of the beams and near the vertical center line of the beams are not too important. Select wood where you have good wood at the top and bottom of the beam near the midpoint.
The 2x6s need to be fastened together along the length in order to be stronger than the 4x4.