Two more dive physics problems

[Rich Keller]

Thanks! Do you mind telling how you did the calculations?
I calculated the absolute pressure at 20' = .445 psi * 20 ft + 14.7 psi = 23.6 psi. But how do you apply this to the container? Based on my understanding I would have multiplied 23.6 psi by 100 sq.in of the top surface of the container that the water column pressure is acting on... but this clearly doesn't produce the right answer.

You need to calculate the total pressure being exerted on all six sides.

 

[n03]

You need to calculate the total pressure being exerted on all six sides.

So that means the total surface is 600 sq.in, and the total pressure is 14,160 lb? That still doesn't sound right. Are there different pressures acting on different sides?

 

[Akimbo]

So which answer would you pick?....

If this test were given at a military or commercial diving school then it would be “none of the above” because you are expected to factor in the appropriate safety factor when choosing the lifting line and related connecting hardware.

Your method of calculating the displacement of the object is correct. I suspect that the people that wrote the test are not conversant with rigging terminology. My guess is they intended to ask something like: What is the smallest of the following safe working load ratings would you choose to lift this object? In that case, you always round the weight up to so 1) 7,500-lb. However, it would be “safe working load” and not “test”.

Note that the average weight of sea water is closer to 64.1 Lbs/Ft³, often rounded to 64. Pure fresh water is 62.3 Lbs/Ft³ (rounded and not compensated for by temperature). This small difference can add up to a big deal when the volume and safety margins get really large.

… How much margin do I need to add for line safety? Are there specific regulations on this? ...

They “typical” rule of thumb in most industrialized countries is 4:1 or 4x the weight of the load assuming that nobody is inside and the lift is from dead-flat water off a stable platform like a pier. OSHA, ABS (American Bureau of Shipping), and the USCG all have regulations that need to be considered in the US.

That would put the minimum breaking strength of the lifting gear at 28,845 Lbs (112.5 x 64.1 x 4).

 

[Rich Keller]

So that means the total surface is 600 sq.in, and the total pressure is 14,160 lb? That still doesn't sound right. Are there different pressures acting on different sides?

Forget the 14.7lbs of ambient pressure at the surface and just calculate the water pressure. At 20' of sea water you get an increase of 8.9psi multiplied by 600 inches comes to 5,340lbs.

 

[Akimbo]

Deleted, forgot about part II

 

[n03]

Forget the 14.7lbs of ambient pressure at the surface and just calculate the water pressure. At 20' of sea water you get an increase of 8.9psi multiplied by 600 inches comes to 5,340lbs.

OK, got it! Thank you so much for the clarification!

 

[TN-Steve]

Rich

I believe that the “object” is assumed to be non-compressible because they didn’t indicate that the “object” was open-bottom or open to ambient pressure the bottom. They also only indicated that it was floating with 6" of freeboard, so it could be open at the top. If so, pressure wouldn’t be a factor.

I “assume” this this because it is an SSI test for divers rather than a university course for structural engineers. Good grief, now you have me thinking about hull distortion under pressure on submersible hulls.

That's the trouble with experts in a field. Sometimes your own knowledge gets in the way of the 'simple' thing that was asked.

I did the calculation the same way, figured it based on displaced weight. I read it to mean what was the lightest line that COULD lift it, but there is no way I'd want to be on the crane when you tried. Maybe 100 feet off shooting the video.

FWIW, I'm not an engineer, but I did study it just a bit. It appeals to my mind.

Steve

 

[agilis]

Question #1, unlike question#2, does not indicate that the 'object' is a sealed cube. It states that it is 5 feet square and 5 feet high, with 6 inches of the object above the waterline. Depending on material and construction it may be bottomless, partially or completely, or in some other manner not watertight, made of balsa, have flotation chambers within, any number of possibilities.

I would assume nothing, especially considering that the second question has far more detailed and definitive wording.

Of course, I've seen a great many of these agency tests. They tend to be adapted from different and unrelated sources with some technical elements that are unfamiliar to the people who put the tests together, so the structure, clarity, and specificity of questions may vary widely.

 

[mathauck0814]

... if only there were people out there with knowledge of diving greater than that of a students' who could help to guide eager minds to an understanding. We should endeavor to create this leadership role that can provide knowledge and wisdom. We shall call them instructors and we shall see that it is Good ...

 

[TN-Steve]

... if only there were people out there with knowledge of diving greater than that of a students' who could help to guide eager minds to an understanding. We should endeavor to create this leadership role that can provide knowledge and wisdom. We shall call them instructors and we shall see that it is Good ...

I thought the position of having all knowledge and wisdom was assigned role.

Steve