what determines thickness of a helmet (mk v)

[ballastbelly]

Hi there, since the pressure was balanced on the inside & outside of the helmet, why did it need to be made of such thick (therefore heavy) metal?

any additional safety provided by the 'armor' protection of the helmet would be offset by its crushing weight,& huge buoyancy required to counter it, all which increases potential for accidents

thanks
B

 

[ofg-1]

A Mk V is not that thick in the shell. You can still dent them up if you try. The ports and attachments are thicker to allow for soldering the thing together. The breastplate, neck ring, and flanges are heavy to help offset buoyancy of the hat. If you look at the old Miller-Dunn hats, they had weights bolted to the breastplate. The fiberglass helmets are thicker in the shell.

BTW, even with the weightbelt and heavy shoes, you can grab the chin valve on the exhaust with your teeth, pull it closed, and you can inflate the thing enough to float the entire outfit, or blow out a cuff, whichever comes first.

 

[Akimbo]

The US Navy Mark V hat and breast plate weighs 54 lbs. and probably displaces about that much, though I doubt it was a design factor. They used 35 Lb. shoes and an 85 Lb. weight belt with it. Once in the water, it wasn't nearly as negatively buoyant as you would think. There is a LOT of variation due to the size of the diver and volume of underwear but I was probably only about 40 Lbs. negative when I dumped the max air from the suit. I could easily pick up 200 Lbs. by inflating the suit.

The shell of the hat was spun out of a single sheet of copper so they had to account for thinning inherent in the process. The cast bronze hardware for ports (windows), the bayonet threads, suit bales, etc. accounted for much more of the weight than the copper shell itself. Modern "lightweight" metal commercial hats like the Kirby Morgan 97 go to great lengths in engineering to make the assembly very nearly neutral buoyant. Saving weight in materials was never a consideration for the Siebe Gorman-style "heavy gear".

Edit: You might find this thread interesting:
What do you call this gear?

 

[ballastbelly]

thanks for your replies & links, i didnt realize the mk v displaced so much, probably they reasoned the bigger the helmet the better.

 

[Akimbo]

... i didnt realize the mk v displaced so much, probably they reasoned the bigger the helmet the better.

I have seen big guys that had a hard time getting their head in one. Murray Black, founder of Divecon, used to be a speaker at seminars in the 1960s and would cock a hat on his head so it was stuck on his forehead. He could get in one, but had to struggle not to scrape skin off.