Which beam will work better?

[Scott Winners]

I can't use a regular sling for space problems, but I can run a sling from one end of the board, under the boat to the other end of the board.

I have a couple weeks to think about all this.

Space problems are cold hearted witches for sure. What if you got a piece of 8/4 white oak >66 inches long? It would be a relative lot more expensive than SYP construction lumber, but it would also be hella stronger.

[Ken Combs]

Space problems are cold hearted witches for sure. What if you got a piece of 8/4 white oak >66 inches long? It would be a relative lot more expensive than SYP construction lumber, but it would also be hella stronger.

Very true. Also true is it should be easy to locate at least two 66" pieces of clear 2x6 #2 or better at the lumber yard even if it means cutting them out of a longer piece. Defect free SYP or DF is much stronger than the standard 2 or 3 grades.

[Tom M King]

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/

[Wade Lippman]

While ratcheting tie downs are not supposed to be used for lifting, using them that way far below their rating or for a "safety catch" is probably ok.

The ones I am looking at have a working limit of 500 pounds and a breaking limit of 1500 pounds. I plan on using two of them to support 300 pounds.
My understanding is that working limit is what they can safely support. How is that different than lifting?

[Jerry Bruette]

The ones I am looking at have a working limit of 500 pounds and a breaking limit of 1500 pounds. I plan on using two of them to support 300 pounds.
My understanding is that working limit is what they can safely support. How is that different than lifting?

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.

[Bert McMahan]

Beam stiffness goes up as the cube of thickness, and linearly with width. Your two beams have the same stiffness within 1%.

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

[Jim Becker]

The ones I am looking at have a working limit of 500 pounds and a breaking limit of 1500 pounds. I plan on using two of them to support 300 pounds.
My understanding is that working limit is what they can safely support. How is that different than lifting?

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)

[Doug Garson]

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.

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.

[Jerry Bruette]

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.

Either way, you buy a new boat.

[Doug Garson]

Either way, you buy a new boat.

But, in scenario one, that is the least of your worries.

[Mike Nolan]

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.