How to choose a raised watertight hatch: dogs, shape, size and hardware
A practical spec guide for barge and vessel designers — from 2-dog ovals to 8-dog hatches with ullages.
Short answer
Pick the clear opening first (15" × 23", 18" × 24" or 19" round), then the number of dogs for the clamping you need, then the hardware package: standard (bronze tumble bolts, steel dogs, zinc-plated lugs) or premium all-stainless. Add an ullage if the tank needs venting or cargo measurement without opening the main cover.
1. Start with the clear opening
Sinooutput raised watertight hatches are built around three proven openings: 15" × 23" oval (UMC-174 / UMC-774), 18" × 24" oval (UMC-464) and 19" round (UMC-343). Smaller 12" round hatches (UMC-456) suit tight spaces. Choose the smallest opening that lets a crew member and tools pass safely — larger covers mean more weight to lift.
2. Decide how many dogs
Dogs clamp the cover onto the gasket. Two dogs (UMC-774) suit light-duty access, four dogs are the industry workhorse, and eight dogs (UMC-464-5-8D) give the most even clamping for large ovals in heavy weather. More dogs mean a more uniform seal but more time to open.
3. Choose a hardware package
- Standard: bronze tumble bolts, steel dog handles, zinc-plated 5/8-11 bolts and nuts, nitrile gasket.
- Premium: stainless tumble bolts, dog handles, bolts and nuts on a steel cover and coaming.
- All-stainless: 304 stainless cover and coaming with stainless hardware for caustic environments.
Lug bolts and nuts in dissimilar metals prevent galling and corrosion — a detail that keeps hatches opening smoothly years later.
4. Add an ullage when the tank needs it
An 8" or 10" ullage welded to the cover lets crews vent the tank or measure and verify cargo through a single-bolt opening. Sinooutput offers 4-dog and 8-dog ovals with ullage, with or without hasp.
5. Check the certification
Sinooutput's 18" × 24" multi-dog hatches are covered by ABS Product Design Assessment 25-0092892-2-PDA, and the 15" × 23" UMC-174 and UMC-774 families by 25-0387137-PDA. Ask for the certificate with your quotation.
Frequently asked questions
What is the difference between a 2-dog and a 4-dog hatch?
The number of dogs clamping the cover. Four dogs give a more even seal and are the standard for most barge applications; two dogs open faster for light-duty access.
Which gasket do Sinooutput raised watertight hatches use?
Nitrile rubber as standard; other gasket materials are available on request.
Can coaming height be customized?
Yes. Several standard coaming heights are available and custom heights can be made on request.
