Measure the inside edges of your deck opening — not the old hatch's outer trim — to get the true cut-out size, then match that figure to the profile spec Lewmar publishes for each hatch line. MAURIPRO stocks Lewmar's Low Profile, Medium Profile, Ocean, Flush, Ultra Low, and Pilot hatches pre-sized to those cut-outs. Order by outer frame dimension instead of cut-out and the new hatch either won't drop into the hole or will leave gaps the old trim used to hide.
This is written for the cruiser (~30-45 ft) staring at a cracked, crazed, or leaking hatch lens who needs to order a correct replacement — not necessarily the same brand or profile that's there now, but one that will actually fit the hole already cut into the deck.
Two numbers decide the order: the cut-out opening in the deck, and the profile (how the frame and lid are built) that matches where the hatch sits. Get the first number wrong and nothing lines up with the fasteners; get the profile wrong and you'll fight fit, seal height, or hinge clearance even when the cut-out matches.
Lewmar hatch profiles at a glance
| Profile | Example (Size 10 unless noted) | Cut-out | Typical use |
|---|---|---|---|
| Low Profile | Lewmar Low Profile Hatch Size 10 | 26 x 26.0 cm (10 1/4 x 10 1/4 in) | Protected deck areas, low-snag surface |
| Ultra Low Profile | Lewmar Ultra Low Profile Hatch Size 10 | 26 x 26 cm (10 1/4 x 10 1/4 in) | Same footprint as Low Profile, flatter lid |
| Medium Profile | Lewmar Medium Profile Hatch Size 40 | 42.1 x 42.1 cm (16 9/16 x 16 9/16 in) | Deeper frame, exposed deck positions |
| Ocean | Lewmar Ocean Hatch Size 10, Flange Base | 26 x 26.0 cm (10 1/4 x 10 1/4 in) | Foredeck, heaviest-duty offshore build |
| Flush | Lewmar Flush Hatch Size 10 | 26 x 26.0 cm (10 1/4 x 10 1/4 in) | Sits flat with the deck, walk-on areas |
| Pilot | Lewmar Pilot Hatch Size 60 | 50.7 x 50.7 cm (19 15/16 x 19 15/16 in) | Large flat overheads, pilothouse/doghouse |
What measurements do you need before ordering a replacement Lewmar hatch?
You need the cut-out size — the finished opening in the deck, measured edge to edge on the inside of the hole, not the visible trim ring or the old hatch's outer frame. Lewmar (and every other maker) specifies each hatch model by this cut-out figure, since that's the dimension the frame's fastener pattern and flange are built to match. If your current hatch is still in place, remove it first: measuring around an installed frame's outer edge will read larger than the actual cut-out and lead you to order an oversized replacement.
How do you find the cut-out size of an existing hatch?
With the old hatch removed, measure the raw opening in the deck at its widest points, corner to corner and side to side, and note whether the corners are square or radiused. Lewmar hatches produced after 1997 also carry a code hot-stamped into the acrylic lens, which can identify the model, size, and lens color directly — worth checking before you unbolt anything, since it can save a remeasure entirely. Round measurements down to the nearest matching published cut-out rather than up; a hatch sized fractionally larger than the hole won't seat.
How do you tell a Low Profile hatch from a Medium Profile hatch?
Even when the cut-out matches, Low Profile and Medium Profile hatches aren't interchangeable, because the lid section and seal are different depths. A Low Profile lid section runs about 25 mm deep with a closing seal roughly 17 mm wide; a Medium Profile lid is deeper at about 32 mm with a wider seal near 22 mm. From the outside the two can look similar, so if you're unsure which you're replacing, measure the lid's thickness or the width of the rubber closing seal rather than guessing from the frame alone.
Which Lewmar profile should you choose — Low Profile, Medium, Ocean, Flush, Ultra Low, or Pilot?
Match the profile to where the hatch sits and what it takes on deck, not just to the cut-out. Low Profile and Ultra Low Profile suit protected areas — coachroofs, aft decks — where a clean, low-snag surface matters more than heavy boarding seas. Medium Profile steps up frame depth and lid strength for more exposed positions without going to full offshore spec. Ocean is Lewmar's heaviest-duty line, built for the foredeck and any spot that regularly takes spray or green water. Flush hatches sit level with the deck for walk-on areas where any raised frame would be underfoot. Pilot hatches are the largest format in the range — MAURIPRO's Pilot listings run to a 50.7 x 50.7 cm and a 72 x 77.0 cm cut-out — sized for large flat overheads like a pilothouse or doghouse roof rather than a standard foredeck opening.
What if you can't find the etched code on the old hatch?
Older or pre-1997 hatches, and units where the code has worn off the acrylic, still come down to the same two numbers: your measured cut-out and the lid/seal thickness described above. If both the cut-out and lid depth match more than one profile, go with the more conservative (deeper-frame) option for any position that sees spray or crew traffic — it's easier to accept a slightly heavier hatch than to discover the lighter one flexes or leaks in a seaway.
Which should I choose?
Start with the cut-out you measured; it's the one dimension that isn't negotiable. From there, keep the same profile family the hole was designed for whenever the lid depth and seal match, since that's what the surrounding deck and any trim were built around. Only step up a profile — Low Profile to Medium, or either to Ocean — if the position is more exposed than the boat's original builder anticipated, such as a foredeck hatch on a boat that now does more offshore miles.
Expert notes
From our refit bench: the single most common return we see is a hatch ordered from the outer frame dimension instead of the cut-out — it arrives correct by spec and still doesn't drop into the hole. Always measure the raw opening with the old unit out, and if you're between two adjacent sizes, cut-out first, lid depth second.
Common mistakes
- Measuring the old trim instead of the deck opening. The visible frame or trim ring reads larger than the actual cut-out and leads to an oversized order.
- Assuming Low Profile and Medium Profile are interchangeable at the same cut-out. The lid depth and seal width differ enough that a swap without checking will bind the dogging handles or leave the seal proud.
- Not checking the etched acrylic code before removing the old hatch. On units made after 1997 it can identify the model directly and save a full remeasure.
- Rounding a cut-out measurement up instead of down. A replacement hatch has to be equal to or smaller than the measured opening, never larger.
Frequently asked questions
How do I measure a Lewmar hatch for a replacement?
Remove the old hatch and measure the raw deck cut-out — the finished opening, edge to edge on the inside — rather than the outer trim or frame. Match that figure to the cut-out spec published for each Lewmar profile before ordering.
What's the difference between a Low Profile and a Medium Profile Lewmar hatch?
A Low Profile lid section is about 25 mm deep with a roughly 17 mm closing seal; a Medium Profile lid is deeper at about 32 mm with a wider, roughly 22 mm seal. They can share a cut-out but aren't interchangeable once you factor in lid and seal depth.
How do I identify my Lewmar hatch model without the manual?
Check the acrylic lens for a hot-stamped code — Lewmar hatches made after 1997 carry one identifying the model, size, and color. If it's worn off, measure the cut-out and lid/seal depth to narrow it down.
Can I replace an Ocean hatch with a Low Profile hatch if the cut-out matches?
Not for the same location. Ocean is Lewmar's heaviest-duty line for foredeck and spray-exposed positions; a Low Profile unit in that spot trades away the frame strength and sealing margin the original position was designed around.
What size is a Lewmar Size 10 hatch cut-out?
26 x 26.0 cm (10 1/4 x 10 1/4 in) across the Low Profile, Ultra Low Profile, Ocean, and Flush Size 10 lines — the number stays consistent across profiles at that size code, though lid depth and frame still vary by profile.
Recommended gear
Lewmar Low Profile Hatch Size 10 — the standard replacement for protected deck areas at the 26 x 26 cm cut-out.
Lewmar Ocean Hatch Size 10, Flange Base — same cut-out, built for foredeck and spray-exposed positions.
Lewmar Medium Profile Hatch Size 40 — deeper frame for larger, more exposed openings.