Hydrostatic Weight

[TwoBurgers]

Hey gents!

I was watching a diving video and they mentioned the impact of hydrostatic weight - things weight 2/3 once submerged. That said, do our boats have a perceived weight of 2/3 rated on the water (vs when on the trailer)? And for those much smarter that me, does the equation mean the force under the water, or what is actually under the waters surface?

I mention this because using hydrostatic weight would change the equation of some calculation around power, speed, prop pitch etc. Our boats theoretically weigh 1/3 less. Hmmmm.

Apparently I have too much time on my hands today.

Merry Christmas to all,
Chris

[Big Block Power]

TB really!
I know I weigh less in water! I float!

[TwoBurgers]

Ha! I may weigh less, but I still use those water saddle things to float around.

[Brokentoy1]

Who are you who are so wise in the ways of science?

[CaptBob]

it sounds like it only counts if you fully sink the boat. A boat under water weighs 1/3 less. The amount of water the boat displaces (the "hole" in the water the boat is sitting in) weighs the same as the boat. Maybe submarines need less propulsion. I know I do when I dive, but I'm still an air pig!

[sailor55330]

it sounds like it only counts if you fully sink the boat. A boat under water weighs 1/3 less. The amount of water the boat displaces (the "hole" in the water the boat is sitting in) weighs the same as the boat. Maybe submarines need less propulsion. I know I do when I dive, but I'm still an air pig!

This is correct. items submerged in water do weigh less due to buoyancy, but I don't believe it applies to items that displace water above the surface as mentioned above. Try this to help explain it.... fill a 2 liter bottle 100% with water, then see how easy it is to move up and down under water. now remove enough water to make it float and see if it takes more or less effort to lift.

My basis is years of diving....FWIW, I was never an air pig! I could easily manage a 1 hr dive at 85-100 ft on a 80CF tank when I was diving regularly, including deco stops (well, the reverse profile by the computer helped with those). On the plus side, things look 25% larger underwater...this is why I recommend diving nekkid.....

[Big Block Power]

Sailor you still need more than 25%

[jhnmdahl]

The issue here is that the engine is pushing the same boat mass (which does not change), plus displacing all that water as the boat plows or skims ahead. Displacing the water is the biggie, and the boat's mass really only comes into play when accelerating (or when plowing to a stop). Same thing with the spinning props - it's the mass being spun up initially or brought to a stop that takes the work, not maintaining speed - that's water displacement again.

It still fascinates me that only 30 feet under water you've doubled the atmospheric pressure, and the typical vacation wall dive going down to 90 feet or so reached quadruple atmospheric pressure. Air is heavy enough for planes to fly, but it's nothing compared to water!

John

[sailor55330]

It still fascinates me that only 30 feet under water you've doubled the atmospheric pressure, and the typical vacation wall dive going down to 90 feet or so reached quadruple atmospheric pressure. Air is heavy enough for planes to fly, but it's nothing compared to water!

As my original dive instructor and long time friend used to say..."when you are under water, you are in a medium 8 times thicker than air, that our eyes can't focus in and we can't breathe in... in other words, we don't belong there.....diving is so cool".... I'll never forget that. Well, that and the fact he did the beta testing on the SEALS teams rebreather technology... for you divers...250ft and no deco on air. It's probably much greater than that now.

Hard to imagine that if you take a deep breath at 30 feet and held your breath as you ascend, your lungs would explode... He's right....diving is cool.

That increased pressure is enough to literally force nitrogen that is part of the air we breath on land through the membranes in our body and into our blood....yeah, it's cool....