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On November 8, 2012, in Designer Articles, Glen-L Styles, WebLetters, by Glen L. Witt


Let’s set the record straight. Almost everyone would like to have a boat with a cockpit that would drain any rainwater or spray back to the ocean or lake the boat is floating in. Why aren’t ALL boats made with such a feature? Let’s analyze the subject.

A self-draining cockpit must be above the waterline, or water would come into the cockpit rather than flow out through the scuppers (cockpit drains). How much above the waterline must it be? Obviously, the higher the cockpit level above the water the better for drainage, but in a 14-foot to 16-foot boat the hull depth limits the cockpit level. Way back in memory a figure of 9 inches above the waterline for a self-draining cockpit was considered desirable for a boat in the 20-ft. range; obviously, any figure must take into account the size and shape of the boat.

Let’s look at a few facts.

A boat has a CB (Center of Buoyancy) that is essentially the same as the pivot point on a balanced teeter-totter. Moving a weight forward or aft of the CB makes the boat go down by the bow, or stern, respectively. Architects use a PPI (Pounds Per Inch immersion) to calculate out-of-trim. A 20-ft. boat may have a PPI (it’ll vary with the boat) of 250 lb.ft. In this example, if a 200 lb. passenger moves 1.25 feet aft of the CB, the boat would go down by the stern 1 inch when at rest. Now suppose our 200 lb. passenger goes to the transom, 8 feet aft of the CB, to pull up the outboard motor. That will bring the stern down more than 6 inches (200 x 8 divided by 250 = 6.4 inches) further in the water. Then if his buddy comes back to help him… well, you get the idea. Any self-draining cockpit level must be quite high above the waterline or water will come into the cockpit through the scuppers.

So what? If a cockpit level is only 4 inches above the water the passenger’s feet may get wet, but the water will flow back out when they come forward, you think. Very true, but the water in the cockpit weighs more than 60 lbs. per cubic foot and the boat could go down by the stern even more as long as the guys remain back there. Perhaps this example is a little extreme, but the idea is to illustrate that a small boat cannot have a practical self-draining cockpit.

Water can be prevented from flowing back into the cockpit by crossing the drains. The port drain exits on the starboard side and vice versa; a method common on sailboats that heel. Also, check valves can also be used at drains. These allow water to flow out but not in, however they can malfunction if debris gets in the valve.
Of course a typical small planing boat at speed is not that critical to passenger movement. In addition, the static waterline is not the same as the craft when planing and water backing up in the cockpit through scuppers would not occur.

Consider the above factors, the pros and cons, prior to insisting that the boat you build must have a self-draining cockpit.


Your Thoughts?


  1. Dave Solt says:

    Why are scuppers set up in pairs on a sailboat? It seems that when the boat heals the drain could be quite close to the waterline and no matter where the scuppers feed (is crossed hoses or uncrossed hoses) the leeward scupper will have no draining capability but will also be where all the water accumulates. So the water will always accumulate at the non functional drain. Why not have a single drain at the centerline that drains very low?

  2. Steve Koschella says:

    It is more practical in a small boat to have a sealed cockpit that is small enough that if it totally swamps it will not sink the boat. A decent hand bailer should handle all but the most severe water ingress, or a bilge pump that can be either permanently installed or placed in to handle such events should be enough. I have sailed a 16 ft boat for years and the only water I have ever had come in is wave splash and rain, which rarely accumulates faster than intermittent bailing can’t handle.

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