- #1
Miss Gradenko
- 10
- 0
Picture a 15' sailboat with a hull made of a sandwich (from the outside in) of gelcoat (relatively impermeable), resin-impregnated fiberglass, stiff closed-cell foam, then more resin-impregnated fiberglass (which you see looking into the boat).
The foam is not one piece, but a grid of 2" x 4" pieces of foam on a flexible backing, like bathroom tiles. When flat and particularly when shaped around corners, there are voids between the foam pieces.
Rain falls on the boat. Small voids in the fiberglass layer allow water to penetrate the sandwich. Osmosis draws the water throughout the sandwich, leaving water in the voids.
Voila. A hull that weighed 240 pounds when built weighs 350 after just a few years.
We're going to try pulling a (deep) vacuum on a small part of the inside of the hull, after drilling some holes through the glass. If we can maintain a solid vacuum the water should flash to vapor and be collected in our cold trap (so we don't go through gallons of pump oil).
But is there a better/easier way?
THE QUESTION
What if we vacuum bagged the entire hull, which wouldn't be hard, and then pulled a partial vacuum? We wouldn't seek to reach the boiling point of the water (or, well, crush the boat), but to increase the rate of evaporation.
What is the relationship between (just) pressure and evaporation rate, and then among those and airflow? There will be some airflow, of course, through the bag material and our (presumably) crappy sealing job. We could regulate higher airflow into the bag.
And temperature...the boat will be in a shack where we could probably maintain the temperature at 16oC or so (this is Toronto, in November), and we could put a small heater inside the bagged boat.
In short, what conditions would maximize the evap rate, as a practical matter?
I've picked through a dozen threads on evaporation v pressure, but honestly they're beyond my basic physics. I'm pretty sure I never advanced through Poynting's Correction or any of the higher math.
Any help would be much appreciated. FWIW you'll be helping a non-profit community club that just wants our boats to be lighter, faster and to last longer.
Cheers...Ralph
Edited to add: For years we've tried putting a dehumidifier and space heater in a bagged boat, for months on end, to little or no avail. It may be that the attractive force between the water and the sandwich demands a flash to liberate the water reasonably quickly?
The foam is not one piece, but a grid of 2" x 4" pieces of foam on a flexible backing, like bathroom tiles. When flat and particularly when shaped around corners, there are voids between the foam pieces.
Rain falls on the boat. Small voids in the fiberglass layer allow water to penetrate the sandwich. Osmosis draws the water throughout the sandwich, leaving water in the voids.
Voila. A hull that weighed 240 pounds when built weighs 350 after just a few years.
We're going to try pulling a (deep) vacuum on a small part of the inside of the hull, after drilling some holes through the glass. If we can maintain a solid vacuum the water should flash to vapor and be collected in our cold trap (so we don't go through gallons of pump oil).
But is there a better/easier way?
THE QUESTION
What if we vacuum bagged the entire hull, which wouldn't be hard, and then pulled a partial vacuum? We wouldn't seek to reach the boiling point of the water (or, well, crush the boat), but to increase the rate of evaporation.
What is the relationship between (just) pressure and evaporation rate, and then among those and airflow? There will be some airflow, of course, through the bag material and our (presumably) crappy sealing job. We could regulate higher airflow into the bag.
And temperature...the boat will be in a shack where we could probably maintain the temperature at 16oC or so (this is Toronto, in November), and we could put a small heater inside the bagged boat.
In short, what conditions would maximize the evap rate, as a practical matter?
I've picked through a dozen threads on evaporation v pressure, but honestly they're beyond my basic physics. I'm pretty sure I never advanced through Poynting's Correction or any of the higher math.
Any help would be much appreciated. FWIW you'll be helping a non-profit community club that just wants our boats to be lighter, faster and to last longer.
Cheers...Ralph
Edited to add: For years we've tried putting a dehumidifier and space heater in a bagged boat, for months on end, to little or no avail. It may be that the attractive force between the water and the sandwich demands a flash to liberate the water reasonably quickly?
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