This is a warning to other readers. I am new to this forum but have been referencing information here for my JJ Taylor Contessa 26 rehab for many months. Lately, I have wanted to increase the cockpit
scupper draining capacities to meet ABYC standards as mine are too small, just having 2 three-quarter inch and 2 one-and-a-half inch drains now. Using the formulas provided here, I’d need 4 five inch drains, which is simply incorrect. You will have noticed that the units in the formulas are not equal, some are in square feet and some in square inches, which as a rule
is incorrect from the get-go. All should be in one common unit.
I tried to use the formulas listed here by Gord May with no success, trying several unit variants and also just as written, and having a bit of maths capability, I suspect the formulas are incorrect, with something lost
in translation. Some of you here already suspect that, as the results we get are counter intuitive. Indeed, the ABYC regulations
are copyrighted, and since we don’t know the version number or where Gord got his information, I prefer NOT to go by these formulas. I am also cheap
and don’t want to spend Fifty bucks to look at the published formulas.
A simple, cheap
and effective way to determine a hole size for a drain is to use the plumbing
pipe flow rates, knowing how much water you need to get out of the cockpit in what time parameters. The results are much more logical.
First, figure out how many US gallons of water your cockpit holds, the cockpit being the height of the combing at its lowest point, since many cockpits are sloped. Mine holds 282 US gallons.
Second, multiply the gallons by 0.75, which is 75% of the water referenced by ABYC per Gord’s information. That piece is sensible, since the boat is in no danger
with a cockpit a quarter full of water. I have 211.5 gallons to get rid of in just 90 seconds.
Third, determine how much water you need to vacate in one minute, since the plumbing
industry uses “gallons-per-minute” rates. I need to get rid of 141 US gallons in one minute.
Now you are ready to determine how many drains you need and how big they should be. Here are the Flow rates for common boat drains in gallons-per-minute (GPM) (and for through-hull sizes using pipe interior
dimensions), assuming the water is gravity fed –not pressurized:
¾ inch: 11 gpm; 1 inch: 16 gpm; 1.5 inch: 35 gpm; 2 inch: 55 gpm
The existing drains on my boat allow 92 gallons to be vacated in one minute, a deficit of 49 gallons per the ABYC standards. Using the flow rate information, I can simply add one 2 inch drain and have done with it. But that means I have another hole in the hull
, so I decided to replace the three-quarter inch drains with 2 inch drains. That gives me a total of 180 gallons per minute, which is plenty. If I use only 1.5 inch drains all round, I have a drainage capacity of 140 gallons, which meets the minimum, at least on paper, but is a wee bit close for comfort.
I hope this helps readers.