Which beam will work better?

[Wade Lippman]

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.

=============
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.

[Jim Koepke]

Can you lower the trailer by letting air out of the tires?

jtk

[Jamie Buxton]

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

[Wade Lippman]

Ooo this is embarrassing. I thought it was the square of the thickness. Thanks. I suppose it is for the best that I never actually went into engineering.

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

[Lee Schierer]

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.

The correct answer is none of the above. Those mooring cleats are not attached sufficiently to take the weight of the boat. If you try it you are likely to need a new boat.

I would suggest using lifting slings under the hull in both locations.

[Wade Lippman]

The correct answer is none of the above. Those mooring cleats are not attached sufficiently to take the weight of the boat. If you try it you are likely to need a new boat.

I would suggest using lifting slings under the hull in both locations.

I don't like using them myself, but the guy I bought the boat kept it at the local yacht club on a trailer and used 4 cleats and a harness to put it in and take it out with a crane everytime he sailed it. I wanted to replace the cleats with Ubolts, but my friend, who used to build fiberglass boats, thought they were plenty strong.

[Kevin Jenness]

Doubled 2x6's on edge will be far stiffer than either used flat or a 4x4. If it's been done before I guess the cleats are strong enough, though slings under the hull would inspire more confidence. A pair of 2" rachet straps would serve and be useful in other situations. 600 lbs is not that much, unless you drop it.

[Cameron Wood]

I agree with Lee. It doesn't matter which of those is stronger if they are both stronger than the cleats.

As Kevin says above, 2x6s on edge would be strongest, but why even have a beam? A longish strap would be simpler, and probably more stable as well.

[Scott Winners]

My first impression here is Lee is the sailor. I am pretty sure he made captain (steel chicken for the ground pounders) and his suggestion to use under hull straps is very likely right on.

I did play with a beam calculator on another website for the fun of it, plugging in utility grade SYP 4x4 and #3- stud grade for a single 2x6. I don't know if the OP (in upstate NY) has access to Southern Yellow Pine, but it is a reasonable guess. I used design values for SYP from southern pine dot com. I estimated the cleats on the stern to be 40" apart. Loaded to 600#, neither beam presents enough section modulus to pass. Both will fail when subjected to a 600# load. Break. Splat. Boat on the hard catastrophically.

This is not a cheap boat, but these are cheap boards. I could maybe look at SYP 2x10, but we are ass/u/me/ing the stern cleats are up to supporting the entire weight of the boat. I think underhull strapping, as originally suggested by Lee Schierer is a much safer idea, and probably inexpensive. I don't know the underwater profile of the hull, the straps will probably need to be placed judiciously. Seating a cleat to carry the weight of a 150# boat requires significant technology. I don't see hanging a 600# boat from a single cleat without very high quality welding into a steel hull by an experienced pro.

Expecting doubled 2x6 to be twice as strong as a single 2x6 is not prudent within beam calculators. Tom King can probably explain this more succinctly than I can even attempt. PVA glue is not stronger than wood. Just get over that idea for beams. The one crack in my current benchtop is where a holdfast hole crosses a glue line. If you need to come to my house and seat a hold fast in every hole in my benchtop to prove it to your self, my home zip code is 99701, my nearest airport is code FAI, and we are in peak season for Aurora viewing this month. PVA is good stuff, and I use plenty of it, but Doug Fir is stronger than PVA, in aggregate, for the defined engineering design values.

If you really need a "doubled 2x6" for a deck joist or window header, something common, no problem. When you are hanging an expensive boat from a crane jib, go find a 4x6. Or put straps under the hull.

M2c.

[Wade Lippman]

The boat is 600 pounds. The balance point is about 10" back from middle of the boat, so lets say that the front is 250 pounds and the back 350 pounds. That would put 175 pounds on each of the back cleats. I can't try it, but I would be comfortable hanging my 185 pounds from one of the cleats. They have been used for that purpose many times by the previous owner and obviously held. I was inclined to replace them with Ubolts, but my neighbor who actually built boats was confident they were adequate.

I put a 2x4 flat across 66" supports, the distance of the boat cleats. I measured the deflection at something less than an inch. The boat would be nearly twice that, but the doubled 2x6s would be 3 times as strong. So it ought to be okay.

I contacted the manufacturer. He advised me to put a loop of rope through the bow eye rather than using the jib plate. That makes sense.
He thought the rear cleats ought to be okay, but a sling would be better. 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.

[Wade Lippman]

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

I took some mechanical engineering 50 years ago and I remember it as the square of the thickness. Maybe I am getting senile, but after a great deal of thought...
The strength is proportionate to the square of the distance from the center line to the material. The sum of all the individual measures is the integral of D squared dD from the center to the surface, or 1/3 D cubed. It is also 50 years since I took calculus. Does any of this make any sense?

[Doug Garson]

I'm a retired mechanical not structural engineer and I've forgotten most of my structural design knowledge but one thing to keep in mind is calculators like the Sagulator and building codes are based on designing to limit deflection not failure. So a beam could be unacceptable based on the Sagulator (which is intended to design shelves with acceptable deflection) and still successfully lift the load.

[Scott Winners]

I was at team blue homestore today. They had 2 inch x 27 foot ratcheting tie down straps, rated at 3333 pounds per each, for under ten bucks each. Two of those is less than $20 and provide 10x required capacity if both straps are working. If one strap breaks, OP still has 5x required capacity to lift the entire boat.

I just can't see fooling around with beam tables and cleats and lumber grading when a ten dollar off the shelf item can handle 5x the weight of the entire boat. The straps will need need to be positioned judiciously, but if there is any kind of production quantity on the boat there is very likely already discreet marks on the inside of the hull where lifting straps should be placed.

Good luck and best wishes.

[Jim Becker]

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.

[Bill Dufour]

Use the beams as a load spreader with the lifting slings. Prevents inward crushing of the hull. Horrible Fright has tow straps for not much money. two inch by 20 feet for 12.98
Bill D.

[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-steel-316-lifting-eye-bolt-5-16-unc-marine-grade-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/cantilever-beams-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.