Determining buoyancy

[Pilikia]

I have noticed in a number of posts hints of a bit of foggyness surrounding questions of buoyancy. This is understandable enough. When you Google the subject, you find sites filled with undecipherable and eye-glazing formulas packed with weird mathmatical symbols, using units like "ergs" and "newtons." I don't know who they think they're talking to, but it certainly isn't me!

The more I've looked into it, the more I've come to realize how surprisingly simple it really is. It turns out to determine the buoyancy of any object, there are really only 2 variables: displacement and weight. Both of which are really easy to either measure or figure.

So, partly to clarify it for myself, and partly to help clarify it for anyone else who's interested, I've made a web page that tries to explain it, and uses an ROV example to demonstrate how it works.

You can study this web page at http://www.asis.com/users/jknope/ROV/RO ... yancy.html.

Anything there you find confusing, or you think is BS, don't hold back. I'm no expert in this, and it's more than a little likely I've got this or that wrong.

[sthone]

I tried to figure out buoyancy calculations once and the math part just confused me, but this makes sense. Very well written, I'm definitely going to bookmark that page for later reference. I'm sure it will save some time during the design phase over the TLAR (That Looks About Right) method I use now.

-Steve

[tazdevl77]

I had a system I was going to use that I think will work too. This is something I want to do PRIOR to mounting buoyancy pods, and is how I'm going to figure about what size pods I'll need. I was going to take a very large trash can and put it into a kiddy pool on my level garage floor (both of which I already have so it will cost me nothing). I was going to fill the trash can with water just to the point to where it was level with the top of the trash can and on the verge of over flowing. I would then submerge my ROV in the trash can and the water displaced would go into the kiddy pool. I'd then measure the weight of the water displaced and compare that with the weight of the ROV. Since I haven't added buoyancy pods yet, the ROV should weigh more than the water displaced weighs and therefor sink. So in theory, the ROV might weight 30 lbs, but displaces 20 lbs of water. So I need to do my math and figure out what weight and size my buoyancy pods should be to make the ROV slightly positively buoyant. In this case I would need 2 pods that ultimately have 5.25 lbs of positive buoyancy each, and then I could fine tune with weights in the skids. This method also saves me time in that I can figure out the ROV's displacement as a whole, and not have to try and figure out the displacement of every component. The only real math will be in figuring out the buoyancy pod sizes. Any Thoughts? Tim

[Pilikia]

Tim ~ This sounds perfectly good to me. Let us know how it goes. --Pilikia

[tStL]

Hey Pilikia,

I have been scratching my head for some time regarding the buoyancy issue, but thanks to your link it has all become clear.

Good stuff.

[ponynicker53]

Two ways to get your sub neutrally buoyant:

1) put it in water, keep adding weights (without adding volume) until sub is just about to sink. Drag it underwater ~5ft and see if it rises or sinks. Add/subtract weight accordingly.

And the more accurate way:

2) Calculate the buoyant force. For this you need to figure out the sub's volume. The easiest was is to submerge the sub in water, and measure how much the water level rises. A round trash can works well for this. Just remembered if it is significantly tapered, this will mess up your calculations. Figure out:
*diameter of round trashcan at widest point, and divide it by 2 to get the radius (in cm)
*and how much the water level rises (in cm)

Use the equation 3.14*radius*radius*height water rises

This will give you a certain number of cubic centimeters (mililiters) which is the volume of your sub.

Water has a density of 1.01g/mL, which your sub needs to match to be neutrally buoyant. So multiply the volume of your sub (in mL) by 1.01. This will tell you how much your sub must weight in grams. (divide by 454 if you want lbs.)

Example:
Trash can is 60cm diameter, so a 30cm radius.
When i put in my sub, the water rises 8cm up the side
Calculate: 3.14*30*30*8=22608mL, or about 22 and a half liters.
multiply by density of water:
22600*1.01=22830grams, or ~50.3lbs


Just remember, any time you change the volume or weight (add components inside, or add volume [e.g. a sensor] outside) you will change the buoyancy of your sub and will need to recalculate.

[Pilikia]

ponynicker53, You missed the most important point of my webpage. The idea is to know where you stand relative to buoyancy BEFORE you build your ROV. That way you can modify the design as needed while it's still "on paper." Both of your methods require that you've already built it, by which time you're stuck, especially if you need more positive buoyancy.

[Stargazerwa]

Nice website, your calculations assume that the skids and cross members are all sealed and thus displace water based upon their OD. Looking to build my first rov, I see that many folks seem to allow water into their entire frame which would dramatically reduce the displacement. I am looking to seal all or a portion of my frame to displace more water, is this how your rov is setup?