Dive boat etiquette and buoyancy check

[TravisD]

Does the boat have a hang-bar setup for safety stops? If so, maybe figure out a way to clip off a bag there - when you're doing your stop, move some of your weights into the bag. Work this out with your buddy before hand so they can help if needed.

 

[Dave Whitlow]

Saltwater is about 3% denser than freshwater, so the upward buoyant force provided by saltwater is about 3% more than in freshwater.

When you're neutrally buoyant in freshwater, the upward buoyant force is equal to your weight... i.e. the weight of you and all your gear.

This means the increase in the buoyant force when you move to saltwater will be about 3% of your total weight (not the lead weight... your total weight, you and your gear.)

Many people weigh roughly 200 lbs with their gear... although some weigh more and some less. So adding 6 lbs (i.e. 3% of 200 lbs) is a rough guideline for going from freshwater to saltwater. You can get a better estimate if you know your total weight. Scuba gear (BCD, tank, regulator...) weighs about 50 pounds. If a person weighs 200 lbs without scuba gear, their total weight while outfitted for the pool (including fins, wetsuit, lead...) could approach 300 lbs. This person would need to add 9 lbs of lead going to saltwater.

Isn't physics wonderful! A bit of understanding, and a few measurements, and lots of annoying guesswork or sub-optimal experiences can be avoided.

I saw a great illustration of this last year when someone did a wreck dive using the same wight they'd used for a quarry dive. They left the 45m wreck (150') planning some stops on the way up but found they weren't able to stop and surfaced 25 minutes before they intended. A helicopter lift was involved (I got some good video of this) followed by a visit to a decompression chamber and a lot of embarrassment. As this was in the UK the evac and medical costs were covered so everyone was happy.

 

[boulderjohn]

Isn't physics wonderful! A bit of understanding, and a few measurements, and lots of annoying guesswork or sub-optimal experiences can be avoided.

I saw a great illustration of this last year when someone did a wreck dive using the same wight they'd used for a quarry dive. They left the 45m wreck (150') planning some stops on the way up but found they weren't able to stop and surfaced 25 minutes before they intended. A helicopter lift was involved (I got some good video of this) followed by a visit to a decompression chamber and a lot of embarrassment. As this was in the UK the evac and medical costs were covered so everyone was happy.

It's hard to believe someone could have the training and experience to do a planned decompression dive and not know this very basic concept of weighting. It's like a calculus student not knowing about division.

 

[Dave Whitlow]

It's hard to believe someone could have the training and experience to do a planned decompression dive and not know this very basic concept of weighting. It's like a calculus student not knowing about division.

I find it easy to believe. The more experience divers have the easier it is for them to become complacent. People make mistakes. People make a lot of mistakes and often they can be quickly corrected. In this case it wasn't just a matter of connecting the drysuit hose, or turning on the suit inflator bottle, as they had already started their ascent and when the error was apparent it was too late to do anything.