Physics and buoyancy question

Okay this is a totally different question not really related robotics but still interesting.

The question is. How much floatation, or should I say buoyancy/lift in the form of pool noodles, (you know those nice things we use for bumpers), would be required to keep a radio controlled model boat afloat if the model weighted 8 pounds?

The models hull is made from fiberglass, so it does not have any floatation capability. All the other running hardware is made up of various materials such as lithium polymer battery packs, electric motor, electronic speed control, aluminum and screws. All the materials in the model have variable densities but everything that is used in the model would sink.

I know that when an object is submerged in water, it’s effective weight is less. With that said is there some sort formula that maybe used?

The easy answer would be 8 pounds of lift, which is nice, but I may not be able to stuff the hull with enough pool noodles to achieve that much lift.

So any thoughts, ideas?

Erm… why won’t the boat’s hull displace enough water to keep it afloat?

A diagram or picture would help a lot, something fundamental doesn’t sound right in how you described the issue.

Stuffing pool noodles inside the hull does nothing to increase bouyancy, since they would not be displacing any water - unless, of course, the boat is already sinking and the hull is filling with water… is that what you meant?

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Okay, lets assume that hull has a big hole in it and is full of water with no trapped air pockets, except for the pool noodles. The “big hole” is a result of the top hatch coming off and the boat barrel rolling, filling up with water :frowning:

That makes more sense.

You simply need to displace 8lbs of water, or 221in^3 plus a little bit for the weight of the foam itself. I would aim for a round 250in^3 of foam volume.

You already said the boat weighs 8 pounds. Now you need to know how much water it displaces when submerged when sunk with no air pockets.

You can try to calculate this, but it won’t be easy. Or you can easily measure it experimentally by submerging the boat in an appropriately sized tank and measuring the water displacement.

Once you know the volume of the water displaced, that is the volume of the boat. You can use that, along with the known weight of the boat (8 pounds) to calculate the required volume of pool noodle to keep the boat afloat. You will need to know the density of the the pool noodle material.

Does that help? Do you need help with that calculation?

Or you can use a fish scale to measure the “weight” of the submerged boat. This is the additional buoyant force required to keep it from sinking. Knowing this required buoyant force, and the density of the pool noodle material, you can calculate the required volume of pool noodle.

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I believe he meant that the boat weighs 8 pounds in air. It would weigh noticeably less when submerged in water.

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Simple calculation will be figuring out the amount of water displaced.
When you know that you will already know the bouyant force of the boat.

The volume that is displaced by the boat multipled by the density of the water you are operating in (salt or fresh) will give you the bouyant force.
Subtract this from the “air” weight…

If your bouyant force is equal to or greater than 8 lbs you don’t need flotation…if it is less…you can add off-board flotation…(to increase the amount of water displaced…)

It is really pretty simple… the “weight” of the fluid displaced is equal to the bouyant force…

To float… bouyant force must be equal to or greater than the dry weight.

You’ll want to use the density of a saturated (water filled) pool noodle. It’s probably a fairly significant difference.

Most pool noodles are made from closed-cell foam, are they not?

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I didn’t mean within the cells themselves, but rather the long-ways cylindrical hole through the noodle. It doesn’t increase the weight, but it does decrease the volume. Sorry, should have clarified.

Edit: Or rather keep the total volume but include the weight of the water inside. The former works better, I think, but I originally referring to the latter.

Good point.

You might also want to use syntactic foam instead of pool noodles. Pool noodles absorb a lot of water drastically changing its buoyancy.

Only on the surface would absorb water and only if the cells are punctured. A new, undamaged, smooth surfaced pool noodle should not absorb much water at all, if any.

Sorry guys, I became rather side tracked on a number of other items.

After a number of floatation tests with pool noodles and the blue insulation foam I found that the pool noodle will allow water to permeate into its’ cell structure after time (hours that is), where as the blue foam does not.

Using just simple floatation test using lead to represent the weight of the boat I was able to determine how much foam would be required to float 8 lbs. Then after some creative forming and cutting of the foam, was able to fit just enough foam in the un-used spaces of hull so in the event of flooding the model will stay afloat.

So many thanks to everyone for the input as it was greatly appreciated.