Essential Guide for Hatch Cover Sizing for Dinghy Sailing
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The essential guide to sizing a hatch for a dinghy focuses on the trade-off between accessibility (can you fit your hand or a gear bag through it?) and structural integrity (will the hull flex or leak because the hole is too big?).
In the dinghy world, sizes are almost always quoted by the Internal Opening Diameter (ID), though the actual hole you cut (the cut-out) will be larger.
1. The Standard Size Chart
Most dinghy manufacturers (Allen, RWO, Seasure) stick to four primary sizes. Choose based on your specific need:
| Size (ID) | Primary Use Case | Hand/Arm Access |
|---|---|---|
| 4" (100mm) | Inspection only; airing out tanks between sails. | Fingers/tools only; cannot fit a hand. |
| 5" (125mm) | Standard Industry Size. Most common for Lasers/ILCAs. | Fits an average adult hand comfortably. |
| 6" (150mm) | Mechanical repairs (reaching nuts/bolts for deck gear). | Fits a hand and a small wrench/tool. |
| 8" (200mm) | Storage: using "Fat Bags" or "Cat Bags" for gear. | Fits a full arm and large dry bags. |
2. Measuring for a Replacement
If you are replacing a lost or broken cover, never measure the lid itself. Lids from different brands often have different thread pitches even if they are the same diameter.
- Measure the Frame: Measure the internal diameter of the plastic ring still attached to your boat.
- Identify the Brand: Look for a logo (e.g., a "shield" for Allen or "RWO"). If you can't match the brand, you will likely need to replace the entire unit (ring and lid), as threads are rarely cross-compatible.
- The Cut-Out Rule: If you are installing a new hatch, remember that the cut-out diameter is usually 10–15mm larger than the ID. For example, a 5-inch (125mm) hatch usually requires a 140mm hole.
3. Critical Sizing Factors
Surface Curvature
The larger the hatch, the flatter the deck must be.
- The Problem: If you put a 6" or 8" hatch on a curved deck, the plastic frame will "warp" as you tighten the screws. This prevents the lid from threading correctly and creates a permanent leak.
- The Solution: For curved side-tanks or narrow decks, stick to a 4" or 5" hatch.
Structural Integrity
Every hatch removes material from the hull.
- Keep Distance: Ensure there is at least 2 inches (50mm) of solid deck between the edge of the hatch flange and any high-load hardware (like a shroud plate or hiking strap attachment).
- Stress Points: Avoid placing large hatches (8") in areas where you walk or sit frequently, as the deck flex will eventually crack the waterproof seal around the screws.
4. Selection Tips: Screw-In vs. Clip-In vs. Storage Bag
-
Screw-In (Threaded): The safest choice for dinghies. They won't pop out during a capsize.
- Snap-In / Pry-Out: Faster to open but can "pop" if a crew member steps directly on them or if the hull undergoes high pressure during a "turtle" capsize. Use these only for internal bulkheads, not external decks.
- Storage Bag: If you require extra storage inside your hatch cover. Like keys or food, then choose either the Allen Range or RWO hatch cover.
5. Maintenance Checklist
If you are replacing a lost or broken o ring seal, never measure the o-ring itself. O-rings from different brands always have unique sizes, even if they are the same diameter.
- The O-Ring: This is the most common failure point. Rub a tiny amount of silicone grease or Teflon lubricant on the rubber ring once a season to keep it supple and prevent the lid from "salt-locking." If you need to buy a new O-ring, make sure you choose the correct manufacturer. As RWO, Allen and Seasure have unique sizes.
- Sealant: When installing, use a marine-grade silicone or butyl tape. Do not use permanent adhesives (like 3M 5200) because you will eventually need to replace a cracked hatch frame.
