: Hello Paul,

: You know, I've always wanted to fill an inflatable dinghy with helium and
: park it at the dinghy dock floating above the water, then stand back and
: watch folks reactions. Better yet, at one of the more crowded dinghy docks
: you could park it hovering above the others. I think one of the small
: Zodiacs with the soft bottom without motor just might do it. These are the
: things that fill my feeble mind.

: -Malcolm

Sorry, but the FOrce (of gravity) is not with you on this.

My heavy inflatable dinghy weighs in at around 40 pounds, and my previous one (smaller, lighter and thinner / more fragile material) was about 25 pounds. They held enough air to displace over 900 pounds, so that would be about 15 cubic feet. That is assuming one cubic foot of air displaced 1 cubic foot of water which would weigh a little over 60 pounds. Because of overengineering, I suspect the things probably held two or three times that, or about 30 to 45 cubic feet of air.

I checked out the lifting strength of helium ( and hydrogen, too) and the results are already fairly well summarized in htis posting I found, so there is no sense in my repeating it all.

http://lists.contesting.com/archives/html/Topband/1996-12/msg00524.html

Basically, a cubic foot of helium or hydrogen will lift about an ounce. An inflatable dinghy with 45 cubic feet of capacity will not lift itself -- unless you can manage to make one out of mylar, cellophane or some other very thin material where the "boat" would weigh less than about 2.5 pounds.

You have another option: tie on some balloons. 160 baloons holding about 1 cubic foot each would lift about 10 pounds. You'ld need about 200 to lift a one man liferaft. I suggest you connect them as three groups and tie them to three points around the boat using just 3 strands of a nearly invisible 10 to 25 pound test fish line. One strand connects each bundle of balloons to a single point on the boat. Better "anchor" that boat with a dramatic anchor. Connect the boat to the anchor with a thick rope to give the impression that it NEEDS to be held down securely.

Let's see. Halloween and April Fool's Day are both good times for such spoofs. Put a "ghost" at the oars in the boat for Halloween.

And have a good time. Buy the weigh, did anyone happen to mention the COST of the gasses for this kind of a prank? Helium is not cheap, but if you can find a high tech supplier who would sell you a quart of liquid helium you could fill all your balloons from a modified thermos bottle -- assuming you don't have the Dewar flask for holding it. Some hospitals use this stuff (along with liquid nitrogen -- which you don't want) to cool their MRI machines, and they need to be refilled weekly or monthly. See who services these machines and you might be able to get all the gas you'll need from them.

Hope this helps

PGJ