Sea-Dog 520500-1
Nylon scupper flapper valve
Surface-mounted nylon flapper with four #6 flat-head fastener holes. Fasteners are not included. Also used as a replacement valve in the Sea-Dog 520510 thru-hull scupper assembly.
Sea-Dog Marine Scupper Valves & Thru-Hulls: 3/4–1-1/2 in Hose Guide
Sea-Dog supplies above-waterline thru-hull fittings in several hose sizes, plus a separate flap-style scupper valve for self-bailing cockpits. This comparison helps distinguish the 520500-1 flap component from 520750-1, 520770-1 and 520790-1 white polypropylene outlets before measuring or drilling the hull.
Scupper flap or thru-hull outlet?
Scupper valve only
520500-1 uses a soft black flap to resist backwash. This is the replacement valve for Sea-Dog's 520510 scupper thru-hull, not a complete assembly.
3/4-inch hose outlet
520750-1 is white polypropylene with a 1-13/16-inch A dimension and 3-1/2-inch overall D length.
1-1/8 to 1-1/4-inch outlet
520770-1 has a 2-3/16-inch A dimension and 3-9/16-inch overall D length.
1-1/2-inch outlet
520790-1 is the larger white thru-hull, with 2-7/8-inch A and 4-3/16-inch overall D.
Sea-Dog scupper and thru-hull size comparison
The A, B and D labels come from Sea-Dog's individual dimensioned product drawings. A describes the outward flange dimension for these products; verify the drawing before treating any measurement as a hole-cutout dimension.
| Sea-Dog MPN | Component | Hose size | Drawing dimension A | Drawing B or D |
|---|---|---|---|---|
| 520500-1 | Nylon scupper flapper valve | Not a hose fitting | A: 2-13/16 in | B: 1-7/8 in |
| 520750-1 | White polypropylene thru-hull | 3/4 in | A: 1-13/16 in | D: 3-1/2 in |
| 520770-1 | White polypropylene thru-hull | 1-1/8 to 1-1/4 in | A: 2-3/16 in | D: 3-9/16 in |
| 520790-1 | White polypropylene thru-hull | 1-1/2 in | A: 2-7/8 in | D: 4-3/16 in |
Above-waterline use
Sea-Dog lists these polypropylene thru-hulls for drainage above the water line, and the scupper flap for self-bailing cockpits. Do not use them in place of an approved below-waterline seacock or shutoff assembly. The 520500-1 flap is a standalone valve component and replacement part for 520510.
Measure before replacing the hull fitting
- Locate the drain relative to the waterline and confirm that an above-waterline polypropylene thru-hull is appropriate.
- Measure the drain hose and match the exact Sea-Dog F hose size rather than substituting a similar-looking white outlet.
- Check the manufacturer drawing for mounting opening, flange A, body D and installation clearance before cutting the transom.
- If replacing the scupper flap only, confirm the matching Sea-Dog 520510 assembly. The 520500-1 flap does not include a thru-hull housing.
Find the corresponding Sea-Dog products
The four linked listings below correspond to the specific MPNs in the comparison. Verify the actual drain configuration before ordering.
Sea-Dog 520750-1
White polypropylene thru-hull
White injection-molded polypropylene thru-hull for above-waterline hose-connected drainage. The listed F dimension is 3/4 inch.
Sea-Dog 520770-1
White polypropylene thru-hull
Larger white molded thru-hull with a manufacturer-listed F hose range of 1-1/8 to 1-1/4 inches, for drainage above the waterline.
Sea-Dog 520790-1
White polypropylene thru-hull
White polypropylene 1-1/2-inch hose thru-hull. The listed overall D length and flange A dimension are greater than the smaller models.
More marine plumbing guides
Sea-Dog drain plugs and garboard fitting sizes • Sea-Dog tank vent selection by material
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