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For those unaware, the US army has been using canvas tarps to float jeeps across water for decades.

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Video http://www.criticalpast.com/video/65675028751_US-4th-Engineers-troops-with-Bantam-Jeep_tarpaulin_river

Canvas tarps are stronger than the blue plastic tarps you purchase for most uses today. Can I use a the standard blue plastic tarp to float my vehicle across water? Are there any limits or considerations?

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    How do you get the tarp under the wheels? Or if you drive it on to the tarp on land, how do you then get the vehicle into the water? – jamesqf Mar 22 '16 at 17:20
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    I'd guess an army jeep is pretty waterproof, diesel engine maybe a exhaust snorkel, etc. I'm not sure you should try this with a Toyota Prius... – user2766 Mar 22 '16 at 17:27
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    @Liam gas engine, most do not have a snorkel, Water proof distributer. If you get water in the fuel, or the carb it does not run well. I have driven both the army jeep M151A2 and the civilian TJ (2003 model) each have similar fording capabilities. – James Jenkins Mar 22 '16 at 17:55
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    Why do I get the feeling that very soon there will a question here "How do I get my Jeep out of the bottom of the river?" – cobaltduck Mar 22 '16 at 20:05
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    @James Jenkins: I don't do video. So you drive it on the tarp, then tie the tarp around it. How do you then get the whole thing into the river? (Though I admit I didn't think of the "wait for the tide to come in" solution :-)) – jamesqf Mar 23 '16 at 4:22
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If your tarp is heavy duty enough, I'd expect it to work. Most blue plastic tarps are fairly thin and weak, so while it might work, it would be risky.

The difficulty is that you will be abrading the tarp by driving onto it, probably subjecting it to abrasion from rubbing against the jeep's body, and risking the tarp catching on underwater debris. The thinner your tarp, the higher your risk.

Even a pinprick leak could be a problem, depending on how long you are in the water. A flimsy tarp of any material will develop leaks or rip relatively easily, maybe even while loading for your first test. A sturdy one would probably hold up to a few trips, although I've got no idea how many. The toughest waterproof fabrics are used for whitewater rafting vessels, which gives an idea of how sturdy a tarp (not polypropylene) can be.

Make sure before you start that you don't have any leaks in the tarp. I've never had to do this, but a possible setup that comes to mind is hanging the tarp in a cup shape and dumping in 10 gallons of water or more, then swishing it all over the tarp surface to make sure it doesn't leak. At the minimum, I'd spray one side with a hose, but that has a few problems: 1) it's hard to be sure no spray is reaching the far side, which makes leaks harder to spot, and 2) if the material is only somewhat waterproof, it might still seem impermeable when sprayed, while holding several gallons would show that it can't stand up to long, heavy exposure.

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The whole thing comes down to weight, if your setup can displace more than its weight then it will float, those jeeps were made to be very light. You could be in the situation where your "wrapped car" could float but its so deep in the water you wont be able to move it anywhere (or you would need an huge canvas and a mean to hold it in deep water as much as the soldier standing in the river is doing in the video). As for the durability of those blue tarps you can never tell, i have polytarps that are extremely sturdy and some that will rip apart very easily. I wouldn't trust them for these kind of things. I would probably end using a double or triple layer of polytarps.

But, that said, if you have a car from a scrapyard, a body of water to try and a mean to pull the car out if it sinks it can be a fun idea to try. (Often boatramps have a decently steep entry, so chances are that something like that will start floating one end of the completely wrapped car while the other end is still resting at the edge of the water on the ramp. That could give you a safety margin to still be able to drive out of the water in case things don't go the way you want. If it works that way then you can be more tranquil when you will try driving on the tarp further in the water like in the video)

If you calculate the volume of a box that has the dimensions of the car, then see how much water it displaces and compare that with the curb weight of the car you can have a very very rough idea on how high it will float. Then, obviously, you have to take in account that the cube doesn't have the tires sticking out etc but as I said its just to give yourself an idea. (a rough calculation using dimension and weight of an accent hatchback gives an immersion of about 8inch for the cube example) Or you could go high tech and use Delftship and a model built on the profile of the car you want to use to see its displacement ;)

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Ok to address your question: Can I use a plastic blue tarp to float my vehicle over water?

Yes

Limits and Considerations:

WILL YOU FLOAT?

First off I will describe the physics of the floating car. The force of buoyancy acting on your car must be greater than the force of gravity pulling it down. The equation for buoyancy is this Newton's equation for Force due to buoyancy. Where Fb is the force in the upwards direction, Vs is the volume submerged in meters cubed and rhoL is the density of the liquid/gas the object is in. g is the gravitational constant (9.8 m/s^2). This is my source.

So in order for this to work you'll need to know the mass of your vehicle plus tarp entity to calculate the force due to gravity acting on this entity. Next, determine how much volume your entity will take up in the water (so consider taking 75% of the total volume of your entity as you'll need part of the car above water). Also know the density of the fluid you are traversing. Only after knowing this will you be certain your car will or will not float.

Should your tarp break and water starts engulfing the space that was filled with air, recognize that the volume of your entity will decrease (thus decreasing the buoyant force). So make sure your tarp doesn't let water in!

If someone that's actually good at physics could review this answer that would be tremendous.

Traversing bodies of water can be DANGEROUS!

Bodies of water can contain foreign, sharp objects that could jeopardize the ability of your car to float. For example, what if a stick tears a hole in your tarp? If we're assuming your traversing a pool, then the STRENGTH of your tarp shouldn't worry you. But if you do think the strength of your tarp is a consideration, look up the numbers for your particular tarp.

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    Just a quick calculation: At 5m length, 1.8m width and 4.5t weight (at least thats 1t above the max allowed weight for a personal car in Switzerland), the car will sink 0.5m deep. So I guess it will float if it doesn't leak :P – imsodin Mar 22 '16 at 23:43
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Based solely on the video you provided the link to, and my experience with blue thick tarp, I have a few things to address. I don't know what kind of tarpaulin you have, how heavy your vehicle is, and how deep is the river/pond/lake you're trying to cross through. Despite that, there are a few things you should keep in mind.

  • You should make sure that your vehicle's density is always equals to or less than: 1 g/cm3 so if you have an object that weighs 1000g it needs to have at least a volume of 1000cm3 to "Float" (1 to 1 means that the object will not stick out because it'll be as dense as water)

density = mass / volume

Example:

Let's say you have a Hybrid Prius:

Weight: 1325 kg or 2921 lb or 1325000 g

Volume: 12.68 M³ or 774317 inches³ or 12688798 cm³

Thus: 1325000g / 12688798cm³ ~= 0.10g/cm³

Will it float?: Yes, and 90% to 80% of your car will stick out of the water

So grab your measuring tape, get your vehicle's sizes, and get ready for some simple math

Calculator: http://www.calculator.net/volume-calculator.html it helps you with the conversion

Buoyancy: http://scienceprimer.com/buoyancy

But I still think you shouldn't risk sinking thousands of dollars because you didn't want to spend some hundreds of dollars on a canvas tarp, but it's just my two cents.

  • Much improved answer, the key points with the question is will it float if water stays out? you have addressed this, I don't think there is a factory car or SUV that won't float if you keep the water out. Getting the vehicle into the tarp is challenging but not impossible. The big question is will the blue tarp hold up? You have addressed the canvas tarp is proven, and the blue is questionable. – James Jenkins Mar 23 '16 at 12:39
  • Is it possible to show that the blue tarp will or will not hold up? If it does get a leak will it be manageable, or catastrophic? – James Jenkins Mar 23 '16 at 12:41
  • @JamesJenkins it would be catastrophic, since the rate of water getting in will be greater than the rate of water being put out, unless, of course, you carry some kind of machine to pump the water out, the other little details that might sink your vehicle would be places where you're not going to be able to pump the water out like the trunk (it's gonna be at least really hard to access it), I hope the pond is not that deep and not so wide so you can cross it as quick as possible – Kyle Mar 23 '16 at 12:52
  • @JamesJenkins, for what concerns cars floating, with no other consideration, they all do. Their dimensions displace enough water for their weight, if you ever saw one going in the water you can see it floats until it fills up with water. The immersion is variable, a 1950Buick will for sure float deeper than today's car of the same dimensions but it wont be as deep as the roof if its wrapped up. The result of the leak depend on how big the leak is, a pinhole could be manageable, a complete rip would be a disaster. You want to aim to a no leaks situation. – Erik vanDoren Mar 23 '16 at 13:46
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    @JamesJenkins Yes, its a tad more difficult with those because they are way heavier, but they have a rated depth at which they start to float. We have a few Amphicars that use one of the ramps in a lake here that are a beauty and they are like ducks in just a couple feet depth. But as said the polytarp is a risk, you have to make sure you get decent ones just to cover a firewood stack or the wind will shred it, go figure to float a car... If the canvas in the video is like the ones we had covering our trucks when i was in the Army they are very thick – Erik vanDoren Mar 23 '16 at 14:02
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In addition to previous answers:

Consider the center of mass. The "modern" car usually are quite front heavy (quite often the engine, gearbox, clutch etc. are all in the front) in contrary to the depicted jeeps. That means a "modern" car will by no means float as level as the depicted one, even if you have enough lift.

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    This is important. Cars aren't heavy (compared to water). But if it pitches over in any direction, the tarp will take on water. – Johan Mar 30 '16 at 12:12

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