Hollow mast venting

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John Roomes
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Hollow mast venting

Post by John Roomes »

Having completed my mast at the mast camp at the Northwest Maritime Center I thought it would be interesting, while at the Wooden Boat Festival, to watch the spar making demonstration given by Bruce Tipton.

Not a lot of new information there but one thing he mentioned, and was quite firm on, was the need to have a vent hole at the top and bottom of any hollow wooden mast. His reason was to avoid vapor pressure build up inside when a cold mast was exposed to the hot sun.

I had never heard of this before. Has anyone out there heard of this or had any experience with vents? To vent or not to vent?
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simeoniii
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Re: Hollow mast venting

Post by simeoniii »

Good question!

Personally, I wouldn't be adverse to having a drainage channel at the base to take care of any issues caused by condensation. Then of course, that's a water ingress point in the event of a capsize if the mast base was submerged for any significant amount of time. Later, it would hopefully then become a water egress point.

I hadn't considered pressure build up from the sun (or vacuum issues when trailering over 12,000 foot high mountain passes 8-)

I'll be interested in following this thread.
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Re: Hollow mast venting

Post by JohnT »

Increased pressure in a hollow mast may bot be significant, but if it needs to be vented, it doesn't take a very big hole to do this. The chances of a significant amount of water entering a hollow mast through small hole (1/16 to 1/8") during a capsize are pretty remote. The volume of air and potential for expansion is probably greater in sealed flotation chambers. Might be a good idea to open access plates and hatches when the boat is not actually in use...
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Re: Hollow mast venting

Post by cdb9554 »

In John O'Neill' birdsmouth mast article on Duckworths site (http://www.duckworksmagazine.com/04/s/a ... /index.cfm), at the very end of the article, he states "However, and this is a big "however," don't seal off the inside of the spar from outside air. Go ahead and plug the top solid. But no matter how you plug the base make sure to leave a fat hole through it so that the moisture that will inevitably find its way in, can find a way out."

I also saw in another birdsmouth article that a builder did not, apparently, provide an air hole. And most other birdsmouth articles I read didn't address this at all.

When I built my mast, I decided to go with Mr. O'Neills advice, so after I built the base plug (from solid spruce), I ripped it in half lengthwise, cut a channel in the middle of the inside of each half, coated the groove with 2 or 3 coats of straight epoxy, then glued the halves back together. The result is a square hole through the length of the plug that is about 3/8" square and is epoxy coated.
Craig
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knasman
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Re: Hollow mast venting

Post by knasman »

When building my mast I did not put a vent hole in it. Maybe others from the "Mast Camp" class remember if Bruce had an opinion one way or the other. My question is, so what if pressure builds up when heated and drops when cooling? Unless you haven't done your gluing right, the epoxy will be stronger than the wood. Has anyone actually had a hollow mast rupture? It just doesn't seem likely. This forum will be the first to know if mine blows! :-)

I will say that if you plan on doing it, Craig's method does a good job of providing a nicely sealed hole that is big enough to let water actually come out, if it gets in. While a 1/8 or 1/6 hole is wonderful for pressure equilization, unless there is some pressure forcing the water out, water droplets just aren't coming out on their own. I think putting a vent at the top is asking for more trouble than it's worth.
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simeoniii
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Re: Hollow mast venting

Post by simeoniii »

Good points Keith.

The masts that Bruce Tipton typically builds are, I believe, fairly large and probably have more than one reinforcing plug along the length of the mast. These would be positioned at the spreaders, the gaff impact point etc. In that case, the wisdom of putting a vent at the top makes more sense, although he probably vents all the plugs within the mast..

A mast on a large keel boat not prone to getting its mast tip in the water could be thought of a bit differently than a capsizable dinghy mast
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Re: Hollow mast venting

Post by Anders Bjorklund »

A drain hole in the mast heel wouldn't hurt much. But I wouldn't want a lug yard to fill with water in a capsize.

There have been scores of articles featured in mainstream publications about building hollow spars. Has anybody read anything about this in any one of them?

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Re: Hollow mast venting

Post by boatsbabies »

The Universal Gas Law states that P*V/T = k. P is pressure, V is volume, T is temperature (in absolute degrees - either Kelvin or Rankine) and k is an arbitrary constant. So, we can consider the volume constant (i.e the inside volume of a finished hollow spar) and then P/T = k/V (or a new arbitrary constant K). Therefore the pressure and temperature vary together. A new arbitrary constant K can be calculated at the initial state (say for example 14.7 psi and 60 degrees F = 520 degrees R as 14.7/520 = .028). Elevate the temperature by something huge, say 50 degrees and you would find an internal pressure of .028*570 = 16.11 psi, which is an increase in internal pressure in the spar of 16.11 - 14.7 = 1.41 psi. Given the size/scantlings of spars for SCAMP, this pressure increase would not be likely to cause a rupture in either the wood or the epoxy holding it together. An increase of internal pressure of this magnitude will not increase the ring stress in the cross section of the spar to a level of the failure stress of the wood or epoxy. This analysis supports no need for vent holes. With no vent holes (and no other faults in the glue up so no moisture can enter), there is no moisture to collect and none to drain.
This same Universal Gas law can also be used to determine whether the stress caused by increasing altitude (i.e. lower external pressure on the spar) would cause rupture. Again, not at all likely.
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Re: Hollow mast venting

Post by fred4win »

Wow! Steve looks like you just answered my question of the pressure gas build up in the mast...venting or not venting that was the question. Now we know.
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Re: Hollow mast venting

Post by kenjamin »

It's not quite the open and closed case that one might think. First of all, the air we breathe usually has water in it so chances are there is moisture inside a mast that has been sealed up tight. Secondly, epoxy can be considered highly water resistant but technically, it is not waterproof and neither is wood. So if moisture can it will get into places that one would normally assume would stay dry. Wood itself has moisture in it and with a hot sun beating down on it this moisture can be made to condensate forming beads of water on the coolest parts inside the mast.

The safest thing to do is let the mast breathe a little with a very small vent hole at its base and through any solid blocks inside the mast but it's important to keep the mast 100% buoyant should it ever get dipped into the water in a capsize.

I kind of agree with Steve in that it is not a hugely critical issue or we would hear more about wooden mast exploding or imploding but to think moisture can't get inside a hollow epoxy sealed wooden mast is a bit overly optimist from everything I've read.
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