Is a dinghy heavier when inflated?

R

Ronald_M

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Usually the biggest struggle when I go sailing is heaving my rubber dinghy the hundred or so yards from its shoreside storage rack to the water's edge.

Feeling virtuous now - I've given the thing a good soaping down, rinsed it off, dried it in the bathroom (SWMBO was out), folded it up neatly and tidied it away in the garage. Funny thing - it doesn't seem nearly so heavy when fully deflated.

Is this my imagination..............?

Have a nice Christmas,

Ron.

/forums/images/graemlins/smirk.gif
 
Ours would be heavier in our bathroom because you'd be lifting the ceiling with it.
Yes, the air inside the tubes is slightly pressurised, so weighs more than the atmosphere it is displacing.
To quantify this, volume of dinghy is something like piD^2/4 x (2L+(B-D))
Where D=tube dia,0.4m, L=length2.5m, B=Beam,1.2m
gives 0.73m^3 or 730 litres.
Air density about 1.2g/litre say 876g at atmospheric which is about 15psi. So when inflated to 3psi, which is too much, there will be an extra 175grammes of air in there!
So i think its probably just more awkward when inflated.
Cheers,
Chris.
 
I think it weighs more for the same reason that a bucket weighs more the right way up than it does upside down. It's full of air.

Mind you if you fil it full of hot air (plenty spare on these forums) it will weight less!
 
"piD^2/4 x (2L+(B-D))"
pi r squared for the x-section area of the tube r=D/2
2 lengths and a bit across the bow of Beam less half a tube width at each side. My Ax3 has a wood transom.
It's a rough order of magnitude based on a crude approximate model of the shape.
It could be wrong I've got flu.
Volume looks vaguely believable.
What do you reckon?
Cheers.
 
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Is this my imagination..............?


Ron.

/forums/images/graemlins/smirk.gif

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Yes - probably caused by the dinghy being more unwieldy when blown up.
 
Covered recently HERE
My "Fag packet" calc. gave a typical increase in MASS of about 3.5 kg and an increase in WEIGHT of 1.2 kg.

Which means it wouldn't be much more difficult to pick up but considerably more effort to fling around.
Not only because the increased mass would mean increased inertia, but also considerably increased moment of inertia.
 
Sorry MikeMonty, I didn't spot that thread.

I've not been well these last couple of days - I was trying to suck the air out of the dinghy by mouth and I've got the valve stuck in my gullett.

I've got to blow up some balloons for Christmas - do you think this might reverse the situation?

Ron.
 
I think your fag packet calculations are probably a bit off, 2 cubic metres is a lot and 1.5 bar would probably do much the same as the semtex mentioned in that thread, slightly takes the precision away from the 273.15+20 temperature.
Must admit my figure for air density was just the first thing google spat out so maybe no better.
Is it beer o'clock yet?
 
So presumably you can now only breath in through your mouth and out through your nose! /forums/images/graemlins/blush.gif

Should be good for blowing up balloons as long as you can fill a balloon through your nose /forums/images/graemlins/tongue.gif

I guess the other advantage is that you can blow up two at the same time. /forums/images/graemlins/grin.gif
 
Well I DID say "fag packet" and I DID say "substitute you own assumptions". /forums/images/graemlins/ooo.gif
I was doing this in my 1/2 hour lunchbreak! /forums/images/graemlins/blush.gif

pi r 2 *2.75 meters with approx 400 mm tubes times two is about 0.6 m3.

1.5 bar is Absolute which equates to 7 psi guage in yer old money - I've never measured how much I pressurise my dinghy but I guess it must be a bit less than my weight acting over the area of the footpump and I bet I'd get at least 7 psi.

And finally, the two decimal places precision on Kelvin was pure laziness, I work with compressors and gas laws in my daily grind, its easier to work with 273.15 than bother about rounding.
 
Points taken, the old total vs xs pressure trick!
Just a different volume.
Don't have any data on what the pressure should actually be.
Was having this issue with scuba bottles a while back, they can vary by over 4kg! from full to empty.
Cheers.
 
Weight difference would be un-noticable, the reason it felt lighter was because it was folded up, hence less awkward to manouver. When inflated it's harder to move due to it's bulkyness, and wind would catch it much easier etc
 
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