A Lewmar Ocean hatch's flange base has a 1 in. lip that seats down into the cut-out and is the standard pick for a new installation; the flat base sits directly on the deck and suits a retrofit over an existing or irregular opening -- but only the flange base accepts a trim kit or roller shade. MAURIPRO stocks both base styles across the full Ocean Series size range, ready to match your existing cut-out.
Every Lewmar Ocean hatch ships in two base styles that look nearly identical from on deck but mount very differently underneath: flat base and flange base. Get the wrong one and either the hatch won't seat against your existing cut-out, or you'll find out later that the trim kit and screen you wanted won't fit.
On a 30-45 ft cruiser replacing a hatch that's fogged, cracked, or simply worn out, the base style is usually the first decision -- before size, before frame finish. This guide covers what actually separates the two, which one a new install should default to, why the trim kit question matters, and how to confirm which base is already on your boat before you order.
Flat base vs. flange base at a glance (Size 40 example)
| Base style | Best for | Cut-out (Size 40) | Trim kit / roller shade | MAURIPRO SKU |
|---|---|---|---|---|
| Flange base | New installations on a flat deck | 42.1 x 42.1 cm (16 9/16 x 16 9/16 in) | Compatible | LEW39640070 |
| Flat base | Retrofits over an existing or irregular cut-out | 41.6 x 41.6 cm (16 3/8 x 16 3/8 in) | Not compatible | LEW39640050 |
Both base styles are available across the full Ocean Series range (sizes 00 to 77) at MAURIPRO -- cut-out dimensions shift slightly by size, so always confirm the exact figure for your size before cutting the deck.
What's the actual difference between a flat base and a flange base Ocean hatch?
A flange base hatch has a 1 in. flange that extends down into the hatch opening, seating inside the cut-out for a clean, finished look both on deck and from below. It's the standard choice for a new installation on a flat deck surface. A flat base hatch has no downward lip -- its lower frame sits flush against the deck surface itself, which makes it the better fit over an existing or irregularly shaped opening where a flange wouldn't seat cleanly.
Both styles use the same lid, acrylic lens, and friction-lever hinge -- the difference is entirely in the lower frame and how it meets your deck.
How do both base styles actually mount to the deck?
Per Lewmar's own mounting instructions (confirmed on MAURIPRO's live product data for both styles), every Ocean hatch -- flat or flange -- fits to a flat surface with a maximum tolerance of ±1 mm. The hinge section fastens with M6 (1/4 in.) hardware, and the lower frame fastens with a 5 mm CSK screw, No.10 UNC or 2 BA. Neither base style tolerates a deck surface that isn't genuinely flat within that tolerance -- crown or camber under the hatch opening needs to be faired flat first, regardless of which base you choose.
Why does the trim kit and roller shade question matter so much?
Because it's the one functional difference that can't be worked around after the fact: flat base hatches cannot be used with trim kits or roller shades, per Lewmar's own specification for every size in the Ocean range. The interior trim kit -- the plastic surround with an integrated flyscreen that finishes the hatch from inside the cabin -- and the roller shade both need the flange's downward lip to mount to. A flat base simply doesn't have anywhere for that hardware to attach.
If there's any chance you'll want an interior flyscreen, a blackout roller shade, or a finished trim surround later, buy the flange base now. Retrofitting a flange base after installing a flat base means re-cutting the opening.
Do the two base styles share the same hatch specs?
Yes -- the lid, lens, and hinge are identical across both base styles at a given size; only the lower frame changes. Across independent Lewmar dealers' published specifications for the Ocean Series: sizes 10 through 40 use a 10 mm thick smoked grey acrylic lens in a clear anodized aluminum frame, sizes 44 through 77 step up to a 12 mm thick lens, every size is CE certified for Area 2 use (deck, coachroof, cockpit, and transom installations), and the friction-lever hinge holds the lid open at any position up to 95°. Sizes 30 and larger add lockable internal and external handles.
What does change between base styles, size for size, is the cut-out itself -- the flange's lip takes up a small amount of the opening, so a flange base's cut-out at a given size number can run a few millimeters larger than the same size number's flat base cut-out. Always pull the exact cut-out figure for your size and base style before marking the deck; don't assume the two are interchangeable dimension for dimension.
How do you tell which base style is already on your boat?
Look at the underside of the existing hatch frame from inside the cabin. If there's a metal lip extending down into the deck opening beyond the visible top frame, that's a flange base. If the frame sits flush with the deck core and you can see the raw cut-out edge right at the frame's edge with no lip extending downward, that's a flat base. Lewmar hatches and portlights made after October 1997 also carry a model, size, and date code stamped into the acrylic lens near the right-hand handle -- quote that code when ordering to confirm the exact match rather than guessing from the base style alone.
From the maintenance bench
We check the deck for camber before we ever discuss base style. Both flat and flange base hatches need a surface flat within ±1 mm -- if the existing opening sits on a crowned coachroof, that gets faired flat first, or neither base style will seal properly.
We default new installations to flange base unless there's a reason not to. It's Lewmar's standard configuration, it keeps the trim kit and roller shade option open, and it gives a cleaner finished edge from below.
We match the existing cut-out on a replacement rather than assuming. A boat built with a flat base hatch from the factory usually keeps a flat base on replacement, since re-cutting the deck to switch styles is real fiberglass work most owners don't want to take on for a hatch swap.
Common mistakes to avoid
Ordering a flat base and then wanting a trim kit or roller shade. Neither accessory mounts to a flat base frame -- decide on interior finishing before you order the hatch, not after.
Assuming both base styles share one cut-out size. The flange's lip changes the opening dimension slightly at a given size number -- pull the exact figure for your size and base style rather than reusing a number from the other style.
Skipping the ±1 mm flatness check. A crowned or cambered mounting surface will prevent either base style from sealing correctly, regardless of which one you choose.
Frequently asked questions
Can I put a trim kit on a flat base Ocean hatch?
No. Lewmar's trim kits and roller shades mount to the flange base's downward lip -- a flat base frame has no equivalent mounting point, for any size in the Ocean range.
Which base style should I choose for a brand-new installation?
Flange base, unless your deck opening is irregular or doesn't suit a flange lip. It's Lewmar's standard configuration for new installs and keeps the trim kit and roller shade option available later.
Do flat and flange base hatches use the same cut-out dimensions?
Not always. At Size 40, for example, the flange base cut-out is 42.1 x 42.1 cm versus 41.6 x 41.6 cm for the flat base -- always confirm the exact figure for your specific size before cutting the deck.
How flat does the deck need to be for either base style?
Within ±1 mm, per Lewmar's mounting instructions for both base styles. A crowned or cambered surface needs to be faired flat first.
Is one base style cheaper than the other?
The difference is minor and inconsistent across sizes -- a few dollars either way at MAURIPRO, not a meaningful factor. Choose based on your installation and interior-finish needs, not price.