Question about spa design and engineering - Shell

[Chris_H]

All,

This is something that I always wanted to ask, but always thought everyone would laugh when I asked it.  

In regards to the shell of a spa, how can you tell which shell is the strongest?  It is something that I just do not understand.

The shell has been historically the least replaced items on a spa, so why does it matter which one is the strongest?  

I would actually like Jim Arjuna to answer this as well, but he would just go into a Hotspring rant and he sometimes gets annoying when that happens.

Thanks in advance for your replies,

Chris

[Spatech_tuo]

I'm not sure we can tell which is strongest or even need to know which is necessarily the strongest. What we need to know is which brands are strong enough to take that issue out of the equation. If they're engineered properly they'll figure out how strong it needs to be, add on a certain factor for safety sake and anything beyond that is just added cost.

[ebirrane]

Agreed!

The only thing I would add is that instead of saying "anything extra is added cost" I would say "anything extra is added profit".  It seems to me that people who take any given part of a hot tub and overengineer it, make that overengineering part of the sales pitch, which helps sell tubs and, I am sure, increases the cost of those tubs.

-Ed

[ebirrane]

I would actually like Jim Arjuna

I didn't know his name was a bad word needing textual replacement!


What, did someone get a lawsuit threatened or something?

-Ed

[Dr. Spa™ Ret.]

I believe it was back in the 50's the US Air Force did testing on the human body to basically determine what forces one could survive. The reason was that they figured there was no reason to go to the expense (and added weight) of building a jet cockpit strong enough to survive a crash if there was no chance of the pilot surviving the force of the impact.

Is it worth paying for a spa in which the shell will last 1000 years, but the rest of which will only last 15 or 20?

[ebirrane]

Dr. Spa,

 Not a good example!  I believe you are referring to John Paul Stapp and his rocket sled studies in human deceleration undertaken in the late 40's and early 50's at Muroc Field (Edwards Air Force Base)

To quote from: http://en.wikipedia.org/wiki/John_Paul_Stapp

"As a result of Dr. Stapp's findings, the strength requirement for fighter seats was increased considerably (up to 32 gees or 310 m/s²) since his work showed that a pilot could walk away from crashes when properly protected by harnesses, and if his seat does not break loose."

His work debunked some of the theories surrounding what is and is not overengineered!  

In areas that are new and still not completely understood, such as, perhaps, ozonation systems, it is really not clear what is and is not overengineering and maybe we will look forward to the John Paul Stapp of the ozone world.  But in terms of shell structure, with the well-known materials we have today, I'd say it is completely possible to determine how much is "good enough" and mow much is marketing.

-Ed

ps:

For further geekiness, his project stemmed the famous "Murphy's Law".   From: http://en.wikipedia.org/wiki/Murphy%27s_law

Murphy, in frustration, blamed the failure on his assistant, saying "If that guy has any way of making a mistake, he will". Nichols' account is that "Murphy's law" came about through conversation among the other members of the team; it was condensed to "If it can happen, it will happen", and named for Murphy in mockery of what Nichols perceived as arrogance on Murphy's part.

Deep apologies for thread hijacking...

[Gary]

There are two basic types:

Fiberglass backed

&

ABS co-extruded to the shell with high density foam.

Both can be done correctly and have a strong spa and both can be done wrong and have a weak spa.

The big difference is with fiberglass back shells the acrylic part is only aesthetics, where as ABS are part of the structure. So is a fiberglass spa cracks it hurts nothing but the looks but when an ABS cracks you have a leaking spa.

Myself I would never have anything but a fiberglass backed shell.

Gary

[Russ]

I believe it was back in the 50's the US Air Force did testing on the human body to basically determine what forces one could survive. The reason was that they figured there was no reason to go to the expense (and added weight) of building a jet cockpit strong enough to survive a crash if there was no chance of the pilot surviving the force of the impact.

Is it worth paying for a spa in which the shell will last 1000 years, but the rest of which will only last 15 or 20?

This begs the question : How long should one expect a spa to last?... almost everything can be fixed ..to a point