Ventilation by mast

sarabande

sarabande

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Doing some work on the "stack effect" for the unpowered transport of contaminated hot air from low Point A to high Point B, so I wondered...

There's lots of concerns about keeping fresh air circulating in the boat, so if you have a keel stepped mast, has anyone tried cutting a hole in the mast near the cabin floor, and seeing if the various holes for sheaves, etc, at the top of the mast act as extraction sources ?

It's not so much a true stack effect, which relies on a pressure differential (won't be much even over a 40ft mast) but the air in the top section gets heated, rises, escapes through sheave boxes etc, and sucks up air from the base. Hypothesis !
 
Yes, thanks, Bilbo. I served 9 years in the Emirates in the 70s, and lived in the old part of town, before all (nearly) were demolished. leaving the then ruler, Sheikh Rashid, to preserve just a few.

For most of the year, the winds are regular diurnal sea breeze functions, and the towers scooped the incoming breeze on the windward side, and pushed it down into the house. The leeward side was a zone of negative pressure, so sucked up the air from the interior. All very effective, and much appreciated before we had the luxury of wallmounted A/C.

This is not a stack effect, though, and such a model is, unfortunately, not applicable in the environment under research.

I've had a pm from someone trying to sell me a windscoop, which relies on the same principle as wind towers. My enquiry was to see if anyone had utilised the true stack effect with a mast.
 
Oh it rains and rains, horizontally sometimes, but mainly in the cooler winter ! On wind towers there are some poles set horizontally at the top of the tower, and cloth coverings are hung from these across the scoop areas to reduce ingress of rain.

The old buildings were made of limestone and "rag", a form of coral block. These absorb moisture and later evaporate it, so producing a cooling effect. Very environmentally conscious and effective.
 
Convention in the ventilation business is that temp rises in a building by 1 deg F (you work out the C conversion!) per foot of height and that is what natural vent systems are designed on -using theoretical roof temp and differential to outside to work out a theoretical velocity through the vent given adequate replecement air volume at low level, and indeed helpful prevailing winds.In a boat, as with single story building the stack effect is negligable.....so then you rely on prevailing winds or if you want something that works you put a fan in.Which is why natural vents in buildings are less favoured by some, because performance is not predictable. My opinion with 25 years of vent system design? If you ca'nt rely on helpful prevailing wind you need a fan.Stack effect negligable.All of which I hope compliments your thoughts on vent practice in hot countries
 
If the (metal) mast gets hot enough, the air inside will rise and the air that will be exited at the masthead will be replaced by air entering at the foot.
Not really a 'stack effect'; it is more simply 'convection'.
I don't know what temperatures you have there but down here there is a noticeable draft of air entering near the base of the mast. It is a common occurrence on racing boats that need to have keel-mounted masts secured to the mast step such that they cannot 'jump out'.
They are commonly lashed through the mast itself and, if you place a lighted match near the holes, the flame shows that there is a definite draft.
As regards rain, a hollow mast should always have a draining hole just above the heel casting so that any water that finds its way in does not form a puddle inside :- "easy-in, easy-out".
 
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