Self-Draining Cockpit

Some considerations, from Glen-L Marine's Boatbuilders Notebook

Excerpted from Boatbuilders Notebook:

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 in the cockpit, not flow out through the scuppers (cockpit drains). How much above the waterline? Obviously, the higher the cockpit level above the water the better for drainage, but in a 14-16’ boat the hull depth limits the cockpit level. Way back in memory a figure of 9” above the waterline for a self-draining cockpit was desirable for a boat in the 20’ range. Obviously, any figure must take in 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’ boat may have a PPI (it’ll vary with the boat) of 250 lb. ft. If a 200 lb. passenger moves 1.25’ aft of the CB, the boat would go down by the stern 1” when at rest. Suppose our 200 lb. passenger goes to the transom, 8’ aft the CB to pull up the outboard motor. That will bring the stern down more than 6” (200 x 8 divided by 250= 6.4”) further in the water. Then 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.

The Glen-L Minuet design manages to incorporate a self-bailing cockpit.

So what, if a cockpit level is only 4” above the water the passenger’s feet may get wet, but the water will flow back out when they come forward. 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 in place. Perhaps the 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. 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. •SCA•