The volume of water that enters a boat is proportional to the square root of the depth of the hole and proportional to the square of the hole’s radius. That is, a hole twice as deep allows in 1.4 times more water, and a hole twice as large lets in four times more water. A deep but small hole is safer than a shallow large one!
A two-inch hole that is one foot below waterline can fill a 55-gallon drum in 42.4 seconds!
Remember that each gallon of salt water weighs 8.6 pounds. A one-inch hole that is two feet below the water line is forcing your boat’s weight to increase at 240 pounds per minute or four pounds per second. Remember, too, that as your boat fills with water, the effective hole depth increases, forcing faster water ingress!
A 1600 GPH (standard capacities for bilge pumps are “gallons per hour” or GPH) bilge pump in perfect conditions (no friction with the hose, no lift height) can remove 26.7 gallons per minute. A 2000 GPH bilge pump in perfect conditions can remove 33.3 gallons per minute. Using the table above it’s easy to see that a 1600 GPH bilge pump can, at best, keep your boat afloat if the hole diameter is no larger than one inch and the hole is no deeper than two feet. The largest capacity 12V bilge pump now available is a Johnson 4000 GPH or 66.7 gallons per minute. According to Johnson, the pump can provide this capacity only with no hose attached. Pumping water a more realistic 3-feet uphill, the capacity drops to 2600 GPH or 43.3 gallons per minute.
All of these numbers allow us to plan for the unthinkable.
1. Few bilge pumps are suited for saving a boat that has a sizable hole below the waterline.
2. Hole size is the major determinate for how long your boat will float after being holed.
3. Your primary effort should be aimed at reducing the flow of water into the boat. Plugs and collision mats are your best bets and deserve to be aboard and part of your primary hull-hole toolkit.
Resources for planning:
West Marine has a fine page on bilge pump selection.
Boat US has a fine article by "This Old Boat" author, Dan Casey.
Dan Casey provides specific information on installing bilge pumps.
SailNet has a good article on installing an improved bilge pump system.
This link takes you to a serious discussion on bilge pump issues.
This PDF discusses the proper installation procedure for bilge pumps.
A brochure from the Alaska Marine Safety Education Association concerning emergency flooding preparation.
[All numerical values calculated by William Ennis.]
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