# Underwater Caches

## Recommended Posts

I'm working on a new cache that I want to hide underwater. Since I'm not a diver, I have no clue how to determine how much weight is needed to sink a very bouyant object.

Yes, I could just attach a lot of poundage, but I want folks to be able to pick the thing up.

So, does anyone know how to do this? Is there a math formula that will get me there?

I'm working on a new cache that I want to hide underwater. Since I'm not a diver, I have no clue how to determine how much weight is needed to sink a very bouyant object.

Yes, I could just attach a lot of poundage, but I want folks to be able to pick the thing up.

So, does anyone know how to do this? Is there a math formula that will get me there?

First thing I would try is to add ballast until it sinks in my bathtub.

Of course, I would have to adjust for salinity if I was going to hide in the ocean (or certain lakes).

-WR

I'm working on a new cache that I want to hide underwater. Since I'm not a diver, I have no clue how to determine how much weight is needed to sink a very bouyant object.

Yes, I could just attach a lot of poundage, but I want folks to be able to pick the thing up.

So, does anyone know how to do this? Is there a math formula that will get me there?

First thing I would try is to add ballast until it sinks in my bathtub.

Of course, I would have to adjust for salinity if I was going to hide in the ocean (or certain lakes).

-WR

Easy, one cubic decimeter (10 by 10 by 10 cm) which is one liter of displacement means 1 kilogramme force as one liter of water weights 1 kg ! OK a bit more for salt water.

So roughly speaking, a body will float if it weighs less than one kg per cubic decimeter and sink if more.

For a box, just compute its volume in liter/cubic decimeter, that gives you the weigh of water displaced, you substract the weigh of the box and that is you what you need to add in the box to make it sink.

Can I get an english translation, OOPS.. that was english....

How about attaching it to something like a cinder block with a cable and a carabiner. That way it is guaranteed to stay down and people can just unclip it to do their thing and replace it when done.

How about attaching it to something like a cinder block with a cable and a carabiner. That way it is guaranteed to stay down and people can just unclip it to do their thing and replace it when done.

If it's underwater, attached to one of Cindy's relatives, how are they gonna get it?

The thing needs to be attached to the pier, and hang from it. I was hoping for an English translation of Suscrofa's explanation......

Can I get an english translation, OOPS.. that was english....

Fill a bucket all the way to the top with whatever type of water that you are planning to sink your cache into(fresh or salt...it does make a difference. Then take your cache container and sink it in the water and weigh the water that spills . Then all you have to do is add more weight than whatever the weight of the water that you collected

For example, if your container displaced 10lbs, then add 11lbs(or any amount great than 10lbs) and it will sink.

Fresh water weighs approximately 62.4lbs per cubic foot and salt water weighs approximately 64lbs per cubic foot...both of these weights depend on where in the world you are planning on placing the cache. However, if you place an object in fresh water that weighs 70lbs, it is being bouyed up by the force of the water, so it will only feel like it weighs 7.6lbs underwater. Getting it out of the water may prove to be very difficult though.

Displacement of water is the key here. You could measure your cache and figure it out mathmatically, but that doesn't take into account the shape of your container. Trust an old divemaster and go with my route.

Edited by mikeslomka

Can I get an english translation, OOPS.. that was english....

Fill a bucket all the way to the top with whatever type of water that you are planning to sink your cache into(fresh or salt...it does make a difference. Then take your cache container and sink it in the water and weigh the water that spills . Then all you have to do is add more weight than whatever the weight of the water that you collected

For example, if your container displaced 10lbs, then add 11lbs(or any amount great than 10lbs) and it will sink.

Fresh water weighs approximately 62.4lbs per cubic foot and salt water weighs approximately 64lbs per cubic foot...both of these weights depend on where in the world you are planning on placing the cache. However, if you place an object in fresh water that weighs 70lbs, it is being bouyed up by the force of the water, so it will only feel like it weighs 7.6lbs underwater. Getting it out of the water may prove to be very difficult though.

Displacement of water is the key here. You could measure your cache and figure it out mathmatically, but that doesn't take into account the shape of your container. Trust an old divemaster and go with my route.

Thank you. This, I understand.

Or as my Daddy always said, A pints a pound, the world around.

But it is simple !

OK, lets take an example, a box that is 32 cm by 16 cm by 30 cm, weight 1.2kg. Volume = 32X16X30= 15360 CC that is 15.360 liter so a water displacement and therefore a force of 15.36 kg when submerged in water. Substract the box weight, you have to add more than 14.160 kg to have it sink !

OK, for a complex shape, you can determine the volume as in the above post but for simple volume, more mess than needed no ?

How about attaching it to something like a cinder block with a cable and a carabiner. That way it is guaranteed to stay down and people can just unclip it to do their thing and replace it when done.

If it's underwater, attached to one of Cindy's relatives, how are they gonna get it?

The thing needs to be attached to the pier, and hang from it. I was hoping for an English translation of Suscrofa's explanation......

I assumed you meant you would need to dive, snorkle or wade to the cache. Didn't realized it would be hanging from a pier.

If you do that conisder the fact that nearly anybody with a little curiosity who sees the rope will pull it up to see what is on the end.

instead of worrying about bouyancy, i'm going to suggest cabling the cache to a stationary object. even with sufficient weight, an object that can move will move if the currents pick up enough.

in any case you should put a bobber on your cable so that when people remove the cache they can find the cable again when they replace your cache. it's the kind of thing you don't necessarily think about until someone can't find the cable and therefore don't replace the cache.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.