Join Date: Feb 2012
Location: New Orleans Louisiana
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Re: Cockpit drains
Properly sized cockpit drains should be able to empty a swamped cockpit within one minute or less when the cockpit is full. Almost all production boats fail this, with the exception of open transom boats.
Having 2 drains feed four pipes is really pretty useless, the amount of water that can be moved is determined by the size of the opening, and the restrictions. In this case, the four pipes really just act to ensure the choke point is at the scuppers not the pipes, but so long as the scuppers are flow restricted, the system is as well.
And any reliance on draining the cockpit by way of the bilge pumps is really poor planning. In bad weather there is likely enough water coming into the hull from other things, that cockpit overflow very well may swamp the boat.
The real question is how large do the drains need to be. Without knowing the size of the cockpit, I can't begin to guess, but assume that a 1/2" scupper can move a max of around 6 gallons/minute assuming it is 20" underwater.
Keeping the same depth (20 inches under water the following are ideals)
1" = 25 gallons per minute
1.5"= 56 gallons per minute
2" = 100 gallons per minute
From this, just figure out the volume of the cockpit in cubic feet. One gallon is equal to .133 cubic foot, and figure out how many, and of what size drains you need to match that size.
Realistically most boats don't meet this requirement, but if you are doing the work, you might as well do it right. And normally there isn't a major problem increasing size by a good bit, once the drain ways have been identified.