fin boyancy vs weight

[SteveFass]

Does the dry surface weight of fins = their negative boyancy?

For example, if you swich from your 5+ to 6+ lb fins (e.g., Apollo BioFin, S-P Jet Fins) to your 1+lb fins (e.g., OMS, Dive Rite), do you have to add 5 lbs to your weight belt?

just an observation - I would think moving 5 lbs from your ankles to your waist or visa versa would affect your trim enough to matter. Though taking 5 lbs out of your suitcase would be nice.

Thanks.

 

[spectrum]

Technically no since density is mass and volume. A heavier fin that displaced much more water could be more buoyant.

Many divers have arrived at trim with fin changes. I have read of numerous women with floaty feet that love their SP Jets for their negative buoyancy.

Pete

 

[Thalassamania]

Density is a measure of the weight of something normalized to a unit volume (well ... mass, which along with gravity determines the weight, but we can let that go for this discussion). So a single unit volume of a denser object has more weight than the same volume of a less dense object (e.g., a car battery that is the same size as a particular loaf of bread, will weigh more than the bread.)

Buoyancy comes into play whenever an object is immersed in a fluid (water, or for that matter air ... think hot air balloon). When something is immersed in a fluid, it is buoyed up by a force equal to the weight of the fluid that is displaced. If it displaces a greater weight of fluid than it weighs … it floats, if less … then it sinks. Simply weighting a fin without knowing its volume tells you nothing about its buoyancy.

 

[SteveFass]

okay, makes sense - I didn't think of volume.

But the volume of fins of the same type don't vary much. Then we can take volume out of the equation.

Can we then say that for two pair of fins of similar size & shape, their difference in dry weight = their difference in boyancy = the amount of weight you need to add/subtract to your weight belt?

 

[Thalassamania]

okay, makes sense - I didn't think of volume.

But the volume of fins of the same type don't vary much. Then we can take volume out of the equation.

Can we then say that for two pair of fins of similar size & shape, their difference in dry weight = their difference in boyancy = the amount of weight you need to add/subtract to your weight belt?

You'd be amazed at the difference in volume, similar is not good enough.

 

[ams511]

What matters is how much water the fins displace when submerged and the density of the fin vs. the density of the water. If you have 2 identical fins and one is lighter then it would be less dense (mass would be the same). So yes in that case you could go with dry weight, however as the above poster points out there is a difference in volume among fins.

 

[SteveFass]

I was afraid of that. Sounds like 5 more lbs to the weightbelt for me. Dang it.

 

[spectrum]

I was afraid of that. Sounds like 5 more lbs to the weightbelt for me. Dang it.

Eyeball a pint of displaced water as a pound.

Since fins have a lot of surface area a small change in thickness can mean a significant volume change. Then you are talking about a pair of them at whatever difference.

If you have a barrel of water and a scale you can figure it all out if you do enough math.

The density of plastics and rubbers can vary widely due to base compounds, additives and processing.

Pete

 

[SteveFass]

Okay, thanks Pete. I get the point. 8 oz of water spread out over the entire surface of a fin sounds like a pretty thin layer, too thin to eyeball in the dive shop.

I don't have a barrel of water handy. I guess I'll just have to do a weight check on my next dive and hope for the best.