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Ginner
24-12-2019, 05:58 PM
Bought my self a new wetsuit, a 6mm O three semi dry. Went diving today and even with 12kgs of lead I was struggling to stay submerged. Before the new suit 9kgs was plenty. All my gear is the same apart from the wetsuit, my previous suit was a 5-4-3. Where am I going wrong?

bottle maker
24-12-2019, 06:06 PM
In my experience new neoprene seems to loose a lot of bouyancy once it has done a couple of dives. Possibly things will get better after a couple of dives.

Assuming you had no trapped gas stopping you sink and that was why you needed the extra lead

Graham.

Nickpicks
26-12-2019, 10:23 PM
The human body has a skin area of just under 2 square metres. assuming the previous suit was an average of 4mm (being 5-4-3 and probably a bit compressed with age). That gives an extra thickness of 2mm, which gives a volume of around 4 litres. Assuming the neoprene is 1kg/litre (neoprene is 1.2kg/litre, minus whatever bubbles they've put in), means you'll need 3kg extra to sink, so going from 9kg to 12kg isn't excessive.

graham_hk
27-12-2019, 06:29 AM
...

shapeshifter
27-12-2019, 03:39 PM
I'm always amazed when I hear about people who need double figure amounts of lead to get themselves to sink.

I mean, we're all made of basically the same stuff with basically the same density so how on earth can there be that much variability?

Tel
27-12-2019, 03:58 PM
I'm always amazed when I hear about people who need double figure amounts of lead to get themselves to sink.

I mean, we're all made of basically the same stuff with basically the same density so how on earth can there be that much variability?

Because of what's on the inside :)

I've seen many a diver that will sink like a stone in the pool and others exactly the same physical size to look at that need loads.
The former has little internal body fat and loads of muscle. the latter the other way around. Makes a suprisingly big diference.

shapeshifter
27-12-2019, 04:08 PM
I got intrigued enough by the question to start following some links and ended up with the following very interesting paper:

https://pdfs.semanticscholar.org/0236/b9277fed227a7bacc5f335eae4c532bd0b7f.pdf?_ga=2.242 525829.628177048.1577462203-2039543391.1577462203

Long story short is that measured body density does indeed have a surprising amount of variability to it: in the sample of 173 adult males, densities from 1.017g/cm3 up to 1.060g/cm3 were found.

I remain somewhat sceptical that these numbers explain the difference in (perceived) amount of required lead though, since the measured variation in density within the same age group is much, much smaller.

If we assume that two guys have identical equipment then we can measure the difference in their weighting requirements my dropping them in a pool in their swimming trunks and handing them 1kg weights until they sink. For me (scrawny), that'd be no more than 2kg. Without going to the extremes of BMI are there really people who'd still be floating with 7kg of lead in their hands?


Because of what's on the inside :)

I've seen many a diver that will sink like a stone in the pool and others exactly the same physical size to look at that need loads.
The former has little internal body fat and loads of muscle. the latter the other way around. Makes a suprisingly big diference.

Energy58
27-12-2019, 06:46 PM
I got intrigued enough by the question to start following some links and ended up with the following very interesting paper:

https://pdfs.semanticscholar.org/0236/b9277fed227a7bacc5f335eae4c532bd0b7f.pdf?_ga=2.242 525829.628177048.1577462203-2039543391.1577462203

Long story short is that measured body density does indeed have a surprising amount of variability to it: in the sample of 173 adult males, densities from 1.017g/cm3 up to 1.060g/cm3 were found.

I remain somewhat sceptical that these numbers explain the difference in (perceived) amount of required lead though, since the measured variation in density within the same age group is much, much smaller.

If we assume that two guys have identical equipment then we can measure the difference in their weighting requirements my dropping them in a pool in their swimming trunks and handing them 1kg weights until they sink. For me (scrawny), that'd be no more than 2kg. Without going to the extremes of BMI are there really people who'd still be floating with 7kg of lead in their hands?

Between someone of my size - 5'11" with normal BMI - to super skinny person will have a variation of about 4kg in buoyancy. The rest is how much breath they are holding. The guys that need masses of extra lead are either really fat lads or do not exhale...

Jen - Winged Blob
27-12-2019, 07:32 PM
And don't forget that the whippets will need correspondingly less neoprene to go round themselves.

shapeshifter
27-12-2019, 07:49 PM
I'm 5'7" and fairly scrawny, and in my trunks, 2Kg would sink me. A back-of-the-envelope calculation tells me that each 10Kg of extra fat would add about 1kg of lead to that (this coincides with my personal rule of thumb for weighting beginners).

I'm guessing that - except for edge cases - these double digit weightings are majoritarily caused by unconscious breath-holding during weight tests.






Between someone of my size - 5'11" with normal BMI - to super skinny person will have a variation of about 4kg in buoyancy. The rest is how much breath they are holding. The guys that need masses of extra lead are either really fat lads or do not exhale...

DaveBarber
29-12-2019, 01:45 PM
I can tread water without kicking with both arms above my head.

My BMI is 24!

steelemonkey
29-12-2019, 02:50 PM
And don't forget that the whippets will need correspondingly less neoprene to go round themselves.

OK Jen, now I cannot free my mind of a dog in a wetsuit. Thanks. :)

Tim Digger
29-12-2019, 04:53 PM
OK a bit of physiology/anatomy! The human lung is the most buoyant bit of the human body. During exercise one breathes deeply and at the top of lung volume, until learning how to breathe best for that activity. Breathing in and out occurs at a variable place in your total lung volume, the further up the lung volume the more air there is in the lungs and the more buoyant you are, the more you struggle to get down the fuller your lungs become as you fight to submerge.
There are issues with new suits but trying to overcome them is often self defeating. Take the lead you need but be prepared to shed it quickly.
Best of all Relax