Do you have any pictures of this Bill? Very interesting solution!
I am not an engineer, but the question sounds like "how much pressure do you need to a lift a 1/8 water column the distance from the bottom to the top of the transom". If the answer is more than the hull can withstand, or the pressure finds another hole to vent trough, you have a problem <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
The diameter of the water column doesn't have any affect on the amount of pressure required (since, as the diameter of the tube increases, the amount of surface presented to the applied pressure also increases equaly).
To raise a column of 12" in a round tube requires a little less than 1/2 psi. (.34 bar for 30 cm) - that's not much pressure.
It does make me wonder though, if the water came in from somewhere other than the tube, there is a hole somewhere else in the hull. This hole would tend to bleed off the pressure before the tube could push out the water. I'm not sure what you've gained with the tube when it comes to getting water out of the hull.
