Below The Waterline Plumbing Checklist

  1. "Maximum heeled waterline" for powerboats, means a heel of 7 degrees. Any fitting below the waterline when the vessel is heeled in this manner is considered a below the waterline fitting and thus requires a seacock.
  2. The seacock installation must meet the following criteria. The installation must withstand 500 pounds of static force applied for 30 seconds to the inboard end of its connecting fitting, at any pointing its most vulnerable direction, without the system failing to perform as intended. What this means for a reviewer is, you should be able to stand and, depending on your weight, be able to jump and down on any seacock. I’m not recommending you do this; it’s simply a frame of reference. This means PVC and nylon (gray and white respectively) fittings and most other plastics, other than glass reinforced polycarbonate (which goes by the trade name Marelon and is usually black but sometimes white in color), are not acceptable. This also means plastic fitting should not be mated-up with bronze seacocks and strainers. BEWARE, many boat builder use through hull fittings and in-line ball valves in place of proper seacocks. This is not acceptable for several reasons, the most important of which is the incompatibility of thread types, straight vs. pipe. Also, be on the lookout for freshwater system strainers being used in the raw or seawater circuit. These usually very lightly built, plastic strainers have a screw off bowl. Again, if you don’t think they’d survive being stepped on, they’re probably not up to the task.
  3. If the seacock incorporates a flange, the flange should be securely mounted to the hull (SDA/PMM, with bronze lag or through bolts).
  4. Seacock backing blocks must NOT be solid timber. Rather, they must be fiberglass encapsulated (preferred) or epoxy encapsulated (also acceptable) marine-grade (no voids, checks or “footballs) plywood.
  5. All seacocks must be readily accessible and easy to operate. You should never have to use tools to unscrew an access panel or port in order to gain access to a seacock.
  6. If a bilge pump discharge is located below the heeled water line (not preferable but also not strictly prohibited), the discharge line must be equipped with a vented loop. Check valves are not permitted in bilge pump discharge lines except to prevent pump cycling in small bilge sumps. (SDA/PMM. Check valves are, however, notorious for seizing and should, therefore be avoided if possible).
  7. Bilge pump discharge lines should not be reduced in size (this is an ABYC guideline, however, their language includes the phrase, “as practicable as possible”) as they travel from the pump to the discharge seacock. In addition, diaphragm bilge pumps (non-submersible pumps) should not serve as the sole method of bilge dewatering, as their capacity is insufficient compared to centrifugal submersible pumps.
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