Round vs Square cross section tubing

In summary, it is important to look at the design criteria for scaffolding and boatyard cradles when deciding on the best tubing to use for yacht beaching legs.f
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I want to make some beaching legs for my 30ft, 9T yacht... these are supports either side of the yacht that keep it upright when the tide goes out and it sits on its keel without falling from one side or the other. My question is this, would it be stronger to use sguare or round tube to do this (ie: 80mm square vs 80mm round)
 
  • #2
I want to make some beaching legs for my 30ft, 9T yacht... these are supports either side of the yacht that keep it upright when the tide goes out and it sits on its keel without falling from one side or the other. My question is this, would it be stronger to use sguare or round tube to do this (ie: 80mm square vs 80mm round)
Welcome to PhysicsForums. :smile:

What is your background in Mechanical Engineering? What kinds of things could happen if this structure fails?
 
  • #3
I want to make some beaching legs for my 30ft, 9T yacht... these are supports either side of the yacht that keep it upright when the tide goes out and it sits on its keel without falling from one side or the other. My question is this, would it be stronger to use sguare or round tube to do this (ie: 80mm square vs 80mm round)
It depends on the details of the loads you’re expecting... but in this application you should be overbuilding enough that the differences between square and round tubing are too small to matter.
 
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My question is this, would it be stronger to use sguare or round tube to do this (ie: 80mm square vs 80mm round)
Strength will depend on wall thickness.
The tube will need to be securely attached somehow to the gunwale at the point of widest beam. How will that be done?
What is the footing? The foot of the tube must not sink into the mud, or slide sideways.
Tube profile is unimportant except that you must drill, weld or somehow fabricate the end attachment points.
 
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  • #5
My question is this, would it be stronger to use sguare or round tube to do this
I'd say the answer to this question can be found by simply looking up at the scaffolding that's used on all the construction projects in your local area. Thousands of structural Engineers can't be wrong.

Also, of course, you could look at your mast, spars and stanchions. Can you find any square section stuff? It has to be a no-brainer to use circular cross section.
 
  • #6
I'd say the answer to this question can be found by simply looking up at the scaffolding that's used on all the construction projects in your local area. Thousands of structural Engineers can't be wrong.
But what if you look at the cradles that are used in all the boatyards in your area? The two have very different design criteria, and the criteria for beaching legs are different again.

Also, of course, you could look at your mast, spars and stanchions. Can you find any square section stuff? It has to be a no-brainer to use circular cross section.
Again different criteria - in particular regarding torsional vs. compressive loads and weight.

As has been said upthread there are many more important design considerations for yacht legs than relative 'strength' of square vs round section - this is the last thing I'd be worried about. There is quite a lot of useful information on specialist boating sites and even from manufacturers sites.

Having said that, most legs I've seen 'in the wild' are either square section timber or circular aluminium, and they are a lot bigger than 80mm.
 
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But what if you look at the cradles that are used in all the boatyards in your area? The two have very different design criteria, and the criteria for beaching legs are different again.
That's correct, and there is a reason. When a boat is put on blocks in the boatyard, the weight is borne by the blocks, not the jack stands. A critical assumption is that the ground is not soft. If it is soft, some weight will transfer from the blocks to the jack stands. When you prop up a boat for beaching, it is probably over a soft bottom, hence different criteria for beaching legs.

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  • #8
That's correct, and there is a reason. When a boat is put on blocks in the boatyard, the weight is borne by the blocks, not the jack stands.
There's more reasons than that e.g. square section is easier to weld; weight unimportant, not immersed in salt water* etc. etc.

The point I was trying to make is that the proper design of anything for application A involves a lot more than copying elements of a design for application B. This is particularly true for marine applications, and particularly true for weight bearing applications. Here we have both and only a fool
would proceed without extensive background knowledge and research.

* Edit: square sections generally corrode quicker due to uneven electolytic effects and anodising.
 
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  • #9
Here we have both and only a fool
would proceed without extensive background knowledge and research.
Agreed. The only source of a bomb proof answer to this will be from (or via) the Insurance Company. They will recommend a design that's got plenty in hand. I wouldn't know where to start to calculate the maximum stress that legs may have to deal with at the worst state of the tide, wind and chop. I had the luxury of a bilge keeler so, as long as the ground was soft , I was happy.
 

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