Ok so I was told that it would be a good idea to get rid of the little metal sail feeder on my mast(Hobie 20). It really is a POS because it chews up the bolt rope in the foot of the sail. When I backed the screw out there was a 3-4 second pshhhhh of escaping air. Great! Previously when I tumped the mast floated well giving me ample time to right it before going turtle. With the comptip it seems like I need all the air I can use. Anyway, I took it all off, dabbed some silicone on the screws, put them back on and covered both screw heads with silicone. Does Hobie pressurize their masts when they send them off? How screwed am I?

you'll be fine

Warm day + black mast = air inside expands

seal it up on a cold day if you want to keep the pressure up - won't make any difference to its bouyancy tho'

Remove the screw and lube it up with silicon. Flush the mast out with nitrogen and reinsert screw. You shouldn't have any more problems with expanding air.

actually, scientifically speaking, more air pressure inside the mast will decrease the buoyancy.

When I did this air temp was prob in the upper 50s. So when it is summer and its 90+ does that mean it will repressurize ie cold air is more dense

actually, scientifically speaking, more air pressure inside the mast will decrease the buoyancy.

Hw does this work?

Be careful with sealing your mast up when its cold out, you might pressurize it to the point that it will explode on a hot day. Or if you fly with it, it will explode when the plane reaches 20,000 ft, especially if its raining when you take off as the air pressure is usually lower when its raining. I usually carry my mast on when I fly to avoid this problem. I have to fold it a couple times to make sure it fits into the overhead. And make sure you rotate the air in your trailer tires while you are at it. The old air gets denser over time and this leads to blow outs.

For an ideal gas (and for our purposes, air is close enough), PV=nRT (where P is pressure, V is volume, n is the number of moles of gas, R is a constant, and T is temperature in degrees Kelvin). If the mast is sealed airtight, then n will not change. If we assume that the mast does not balloon up (a good assumption, I think), then V does not change either. As T rises, P rises proportionately. That's why there's more pressure when it's hot.

Let's look at it differently, though. Instead of holding n constant, we hold T constant. That is, we put a valve on the mast and pressurize it. Remembering that PV=nRT, as we add molecules of air, the pressure goes up proportionately. The volume doesn't change. Because we've added molecules (which have mass) without increasing the volume, the density of the mast has increased. Denser means less bouyant.

So technically, if you seal your mast on a hot day, then you'll wind up with fewer moles of gas inside than if you did on a cold day. That's because in this scenerio, P and V are constant (14psi at sea level and the volume of the mast). As T varies, so does n in inverse proportion. Fewer moles of gas means less mass and therefore less dense and more bouyant. I doubt you could measure the difference in any meaningful way however.

As for nitrogen making a difference, well I don't believe that either. If the mast is sealed, then the first scenerio above holds: as temperature rises, pressure does too. It doesn't matter what the gas is.

When it comes right down to it, your mast doesn't need to be airtight, just watertight.

Regards,
Eric

You can only do that with Unobtainium masts.

OK...but how do you get the moles in and out????

One molecule at a time. It only takes about 600,000,000,000,000,000,000,000 of them.

You start at the masthead ... tapping on it w/ a claw hammer and drive the little furry critteres out the mastbase.

The toughest part is shinning up the mast and holding the hammer at the same time .....

Hee-hee, Ha-Ha

Harry

You did it perfectly. No worries! The only thing I do is use a rivet and a cap and pop it in the holes to seal. Just looks cleaner. Don't worry about it though. Sail it! You're stoked you have a mast that is that sealed.

The air pressure that you described is why when a boat capsizes and it's hot out, if the mast is leaky, when the hot mast hits the cold water it actually sucks the water in. Hulls do the same thing.

Hope that helps.

Buoyancy is a function of density (mass per unit volume). Air has mass. By increasing the pressure you are thus shoving more air into the same sized volume, increasing the density (more mass for same sized volume). As your density goes up, your buoancy will go down. This is such a small difference I don't think you would ever see a difference unless you are pumping 15,000psi into your mast.

so I guess this release of air was actually a good thing then.

The horribly made point made here is that that little bit of pressure really didn't matter - it wasn't enough to affect anything.

Thread Hyjack... (seems appropriate here)

What is the estimated Airspeed Velocity of an Unladen Swallow?


http://www.style.org/unladenswallow/

African or European?

Thanks for playing!

http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050197