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mast bob volumes
http://www.hobie.com/au/en/forums/viewtopic.php?f=17&t=5322		 Page 1 of 1
Author:  Muzzer [ Mon Nov 13, 2006 12:59 am ]
Post subject:  mast bob volumes
Hi
I see the Hobie mast bob for the H14T is around 13 liters in volume. I was thinking of strapping a plastic bottle (as described in these forums) to the top of the mast to prevent turtling. What volume bottle would be needed assuming the mast is sealed?

Also, I presume a gas which is lighter than air (helium?) could be used in the bob, thus reducing the volume needed to produce the same buoyancy? Anyone had any experience with this approach? Or is this a wild stupid idea?!
Author:  mmiller [ Mon Nov 13, 2006 10:21 am ]
Post subject:  Volume
Similar volume to the small "Baby Bob", but can be smaller due to the extra length of the H14 mast which is a longer "lever arm" requiring less effort at the end to hold it up. Not sure just how much.

The biggest concern is the rigidity of the float, so it does not collapse under load. The type of gas or air inside does not change the buoyancy. The volume of the float displaces water... which is buoyancy. Air filled is fine.
Author:  Muzzer [ Mon Nov 13, 2006 10:18 pm ]
Post subject: 
Thanks Matt

Re: buoyancy, if buoyancy is simply water displacement (volume of bob) then technically I should be able to fill the bob with lead and the buoyancy would be the same as with air or helium as the volume (water displacement) is the same. That does not seem right though! (I'm not gonna try that BTW :wink: )

To me, the practical difference between a bob filled with air and one filled with lead is that one will sink and one will not. The physical difference is the substance within, which surely means the amount of buoyancy is relative to the the substance within.
Now I've confused myself!
Author:  worldinchaos [ Wed Nov 22, 2006 10:27 pm ]
Post subject: 
buoyancy of an object is related to objects density (actually specific molar volume, which is density and molecular weight, or based upon the inherent properties of the component) and the density of the fluid, as well as weight or volume of object afloat( or sinking). The specific molar volume of any normal gas, including air or helium, related to that of water, is much less. I.e. most gases in water will float about the same based upon volume, which is why it doesnt really matter what you fill it with for our purposes, but between different gases, there is a difference, which is why a helium or hydrogen balloon rises in air.

A much bigger role is making sure the container is rigid, like he said. I am so scared of the mast falling right on the milk jug and making the entire purpose of it worthless by popping the top off, no matter how secure it is.

thats my 1 1/2 cents.
Author:  mmiller [ Mon Nov 27, 2006 10:58 am ]
Post subject:  Float
Displacement volume will be identical whether filled with air, gas or lead. The displacement is about 64 lbs per cubic foot in salt water I think. The air being virtually weightless... gas being lighter than air would each offer positive floatation less the weight of the float materials. The lead fill, if more than 64 lbs of weight, would offset any gain in flotation from the displacement provided by the float. The lighter the materials in the float, higher the volume... the more benefit in flotation.
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