A question for any scientists out there

[CaliforniaDreamin']

I have a 10' Zodiac with an air floor. My question is this: After you inflate the tubes to the proper air pressure, then add the engine, put it in the water and add the passengers, does the air pressure in the tubes increase? The reason I ask this is that I've made some leak repairs which seem to be fine when checked overnight at home, but I'm wondering if any added air pressure when the boat is in use might adversely affect the patch repairs. My guess is that the air pressure in the tubes will increase due to displacement of water.

 

[GA_Boater]

Re: A question for any scientists out there

Put a gauge on it. That will tell you. Also flexing might affect the patch. But from the wording of your post you haven't tried the patched boat on the water, right?

 

[dennis461]

Re: A question for any scientists out there

A question for any scientists ( or small children out there...)

If you blow up a balloon and tie it off, then sit on it, why does it pop?

Is there a pin on the floor? No.

So pressure must go up to the breaking point.

 

[CapeAnn]

Re: A question for any scientists out there

P1V1 = P2V2. If you decrease volume pressure goes up. Boyle's Law.

Additionally, Archimede's Principle is at work when you place the boat in the water. There is a force being exerted by the water. In simple terms - I hope the glue and patch holds.

CapeAnn - former scientist

 

[NetDoc]

Re: A question for any scientists out there

Every 33 ft of salt water yields 14.7 psi. That's less than a half psi per foot. What is your boat's draft? Now figure just how much of the entire boat (%) is beneath the water line? Get the idea? The increase is incredibly minimal. Welcome to Scuba 101 as I like to tell my students: Its not rocket science, its submarine science!

 

[CaliforniaDreamin']

Re: A question for any scientists out there

Thanks, everyone, for your replies!

@GaBoater - A gauge is the obvious answer, but I won't have the Zodiac in the water until May.
@Dennis461 - Not sure I understand the analogy between sitting on a balloon on the floor and sitting on a Zodiac in the water
@CapeAnn - Archimedes may be my answer (my conclusion above - but didn't remember his name). But it sounds like NetDoc disagrees
@NetDoc - I never studied ship or submarine design, but it sounds like you may disagree with CapeAnn

 

[GA_Boater]

Re: A question for any scientists out there

Hey CD - By May the patches should be well cured. Something you can do to help prevent holes in the air floor is cut a piece of cheap carpet to fit the floor. Not a lot of weight but a fair amount of protection. And you can pull it out to dry easily.

 

[jacoboregon]

Re: A question for any scientists out there

If you patched the boat properly, using the correct materials and procedure then the patch should hold indefinitely. The material around the patch should give before the patch fails. And the other answer is "No" the pressure will not increase and cause undo stress on the patch or any other area. You are no way even close to the analogy of sitting on a balloon (unless you weigh several thousand pounds).

I also agree that you should have a gauge. The reality is that most people use their boats at pressures below the maximum recommended if the don't use a gauge to check. The only real danger is blowing it up to max and then let it sit in hot weather when not in the water. The water keeps the material cool, preventing the air in the tubes from expanding to over-pressure, which happens when an inflatable boat is left aired-up and not in the water.

 

[CapeAnn]

Re: A question for any scientists out there

Thanks, everyone, for your replies!

@GaBoater - A gauge is the obvious answer, but I won't have the Zodiac in the water until May.
@Dennis461 - Not sure I understand the analogy between sitting on a balloon on the floor and sitting on a Zodiac in the water
@CapeAnn - Archimedes may be my answer (my conclusion above - but didn't remember his name). But it sounds like NetDoc disagrees
@NetDoc - I never studied ship or submarine design, but it sounds like you may disagree with CapeAnn

Netdoc and I - we're on the same page. There are multiple forces being exerted on your boat. These forces affect the internal pressure on the flexible tubes. Think of your tubes as capped, empty soda bottles. If you put a load on top, bottom, or even the sides the pressure will minimally decrease the size of the tube/bottle increasing gas pressure within. It's a proportional effect.

That said - it's too bad patches cannot be glued from the inside vs. from the outside. The good thing about the internal pressure is that it isn't all that much by say car, or bicycle tire pressure standards. Also, having redundant chambers within a tube helps alleviate that nasty "I am going to Davey Jone's Locker feeling." If glued properly I wouldn't sweat it.

Cheers,

CapeAnn

 

[CaliforniaDreamin']

Re: A question for any scientists out there

CapeAnn + jacoboregon = I'm confused. Am I making too much of this? I know the correct answer isn't going to solve any of the world's problems. In May, my Zodiac will either still leak or it won't (I'll stay near the launch ramp the first time or two). But it is an interesting question.

 

[TOHATSU GURU]

Re: A question for any scientists out there

Pressure decreases due to the cooler water tempature. All inflatables require additional pressure to be added after they have been put into the water.

 

[Sea Rider]

Re: A question for any scientists out there

If wanting top performance you should check pressure with boat on water before passengers get on board, will need a minimum of 3.0 psi on each chamber, air floors works best with pressures between 10-12 psi. Finger presure testing is useless even for pro sib/rib people, get yourself a pressure gauge.

Keep in mind than in warm weathers a sib inflated at morning on water at 3.0 psi will have 0.5 psi or slightly more at noon, anyway general working pressures for all sibs/ribs tubes are 3.5 psi. If seams or patches are well glued nothing will happen. Just don't leave sib out of water laying on hot sand, tubes must always rest on water. The same principle applies to cold environments, you will lose top pressure, so a reinflation will be a must. Test sib inflated to 3.0 psi at home/garage and perform a water/soappy test on seams and patches to look for small leaks for your peace of mind.

Happy Boating

 

[CapeAnn]

Re: A question for any scientists out there

CapeAnn + jacoboregon = I'm confused. Am I making too much of this? I know the correct answer isn't going to solve any of the world's problems. In May, my Zodiac will either still leak or it won't (I'll stay near the launch ramp the first time or two). But it is an interesting question.

I wouldn't worry about it because the factors involved are really small and the tubes designed to accommodate all the variables - including thermal variables. We patched boats in freezing water, at sea and all patches held well on PVC and Hypalon fabrics. As suggested - using a gauge helps.

 

[NetDoc]

Re: A question for any scientists out there

@NetDoc - I never studied ship or submarine design, but it sounds like you may disagree with CapeAnn

Negative. I'm combining both Archimedes' Principle and Boyle's Law. I work with buoyancy on a very personal level when I Scuba. I don't want to be a boat or anchor: I want to be a fish!

 

[NetDoc]

Re: A question for any scientists out there

Here's something to consider: what PSI are you putting in your boat? The more air you add, the denser the boat and actually the less it floats. A normal Scuba tank has about an 8 pound swing from full to empty. If we don't accommodate for that swing we find it hard to do our safety stop!

For Scuba divers the most important thing about Boyles law can be summed up like this: Breathe Or Your Lungs Explode Stupid.

 

[Sea Rider]

Re: A question for any scientists out there

Sorrry, don't know both reputed mentioned gentlemen, but you cannot compare 3 K psi for scuba tanks with 3.0 psi for inflatables, keep in mind that with 3.0 psi you will have near hard rock solid tubes and best water performance, if you inflate less will have a less ridgid & water performer sib with transom issues in the long run. Inflatables are highly buoyant and still float well with 1.0 or 3.0 psi of pressure.

Happy Boating

 

[jacoboregon]

Re: A question for any scientists out there

CapeAnn + jacoboregon = I'm confused. Am I making too much of this? I know the correct answer isn't going to solve any of the world's problems. In May, my Zodiac will either still leak or it won't (I'll stay near the launch ramp the first time or two). But it is an interesting question.

I apologize but you are making too much of this. There are no outside forces you need to be concerned about other than temperature, if you have your boat correctly inflated. Use a gauge if you are not experienced.

As I said earlier, if your patch was done correctly IT WILL NOT FAIL with normal usage of your boat. You don't have to worry about your weight, the force of your motor, or the force of it being in the water. The design of the boat takes all these things into account.

Your question has less to do with science than it has to do with common sense, proper use, and design application.

 

[idonthaveaname]

Re: A question for any scientists out there

"does the air pressure in the tubes increase"

Overall, the answer is yes...this is why big trucks have big tires. Still, the pressure will be noted at the patch...the weakest point.

 

[TOHATSU GURU]

Re: A question for any scientists out there

The pressure decreases when the boat is put into the water. That's it. No if's, and's or but's. That's why you have to top the boat off after 15 to 30 minutes to return it back to the correct pressure. Is it possible that the boat can gain pressure during the day as the tempature warms? Absolutely or it may further lose pressure if the tempature continues to drop. You can test this by inflating a ballon at ambient tempature and then emersing it in cold water. It shrinks. Or you can put it in hot water and it will expand. You guy's waste far too much time overthinking this crap

 

[NetDoc]

Re: A question for any scientists out there

"does the air pressure in the tubes increase"

Overall, the answer is yes...this is why big trucks have big tires. Still, the pressure will be noted at the patch...the weakest point.

Yes, air holds up a tire (not the sidewall), but tires are big to give a bigger area to support the weight. They are made THICKER to hold more pressure. If the tire repair is done correctly, it will outperform the tire as far as holding pressure. Most of the big rigs use a plug patch that fills in the hole as well as provide more ample thickness for the pressure.