Boat Portholes: Plastic vs Aluminium and Sizing the Opening

Quick Answer

Start with the hull, not the catalogue. A boat porthole is a hole in the structure with a flange laid over it, so the opening that already exists decides both shape and size. Material follows the structure and the frame already in place: the plastic models are listed for cabin and hull sides, the aluminium one for hull sides, bulkheads and hatch windows. The figure that settles the order is the porthole cutout, and no page here publishes it, so it has to be measured on the boat.

Real-World Explanation

A porthole is a hole with a lid on it. Cut the structure, and the frame covers the cut edge, gives the glazing something to sit in and gives the fasteners something to bite. Every dimension on a porthole page describes one of those three jobs.

The outer size is the footprint of the frame on the outside of the boat. The inner figure is the smaller pair and reads as the clear aperture you look through, though no page defines it as the cutout — treat that as a reading, not a specification. The difference between them, halved, is the width of the flange — the band that overlaps the hull and takes the screws. On the plastic oval porthole that band is about 43.5 mm on each side of the width and 45 mm on each side of the height, from an outer of 410 by 220 mm against an inner of 323 by 130 mm. On the plastic rectangular porthole it is 40 mm and 36 mm, from 400 by 200 mm against 320 by 128 mm. The two models look similar and are not interchangeable on one opening.

The fit is arithmetic. The hole has to be smaller than the outer size, leaving hull material all round for the flange to press on and fasteners to pass through. It also has to be larger than the inner size, or the frame masks part of the aperture you paid for. That band is narrower than most people expect on a small porthole.

Closing is a separate mechanism from holding open. The plastic models use two locking knobs that draw the glazing onto a rubber gasket, and how far the knobs are turned sets how hard the gasket is pressed. The aluminium oval porthole works the other way: a friction hinge holds the pane at an adjustable opening position, in an anodised silver frame around acrylic glazing.

Ventilation and light are two different requirements arriving through one hole. An opening porthole gives both, but a pane held open at an angle does more for air than for reaching out, and one set high in a deckhouse lights a cabin while doing little for a seated helm.

The most common error is buying from the outer size. That figure is the larger of the two and it is printed first, so it reads like the size of the part. Cut to it and there is no flange left to land on; the frame drops through. The second is assuming the inner figure is the cutout. It is close, but no page defines Inner as the opening, and the round porthole table does not name its columns at all — only Type, A, B and mm, with ALSPP1-01 at 215 and 147 and ALSPP1-02 at 265 and 197, and nothing to say which figure measures what. A page names the family, colour and locking method. It cannot tell you where to put the saw.

Practical Insights from Marine Hardware Supply

1. Two plastic models do not share one opening

Oval and rectangular differ by more than shape: flange bands of roughly 43.5 and 45 mm against 40 and 36 mm, and footprints of 410 by 220 mm against 400 by 200 mm. A swap on a hole cut for the first leaves either a gap at the flange or a smaller aperture. Treat the model code as the unit of ordering.

2. The aluminium figures cannot be carried into a quotation

The oval aluminium page publishes a size block that contradicts itself, with an inner length of 358 mm sitting against an overall length of 305 mm, which cannot both describe one part. No number from that table is usable; what remains is the description — anodised silver frame, acrylic glazing, rubber gasket, friction hinge, adjustable opening position, for hull sides, bulkheads and hatch windows. Two further aluminium titles in the catalogue lead to products described as deck hatches with telescopic stays and a turning handle, so a title is not proof of the part either.

3. Nothing in the catalogue is fixed, and no stainless porthole page exists

All six products on the porthole and hatch range are opening models. There is no fixed porthole page and no stainless steel porthole product page, though one has been promoted in news copy. A job calling for a fixed window is a drawing request, not a catalogue selection.

Buying Checklist

Measure the cutout, not the frame. With the old porthole out, take length and width at three points each plus the diagonal, and note whether corners are square or radiused and at what radius. An opening a previous owner has enlarged is rarely a clean rectangle.

Check the band between cutout and lining. Note the flat structure between the opening and the cabin lining, inner moulding or nearest stringer. The flange lands there, and a thin cored side may need a backing pad.

Ask for glazing type and thickness. No page publishes glass or acrylic thickness, and the pane material appears only in descriptive copy. Both come from the drawing, and both matter most on a hull side where the pane is exposed.

Decide the opening direction and the angle. A hinge along the top lifts the pane clear of spray; a side hinge gives a hand out and a view along the hull. The friction hinge holds a set position, the two-knob arrangement holds a closed one.

Settle the locking method against the user. Two knobs give a visible closure that can be checked at a glance, in a sequence of two. A friction hinge is one hand and no sequence. Neither is better in the abstract; the difference is who closes it and how often.

Buy the gasket as a spare. The gasket is the part that ages, and it is compressed every time the pane shuts. Confirm a replacement is available and at what section before the frame is bedded into sealant, because removing it later is a different job.

Confirm the fastener pattern and countersink. The plastic flanges arrive with pre-drilled countersunk holes. No page states how many, at what pitch, or what screw fits, so take count and pitch from the drawing or a sample and match the countersink to the head.

Match colour, symmetry and quantity in one pass. Black or white frames read differently against teak and against a white moulding, and a pair on opposite sides of a cabin should be one batch. Count openings on both sides before ordering.

Decision Guide

Choose a Plastic Opening Porthole If

The cabin side is comparatively thin, weight and cost lead, and the moulded frame in black or white suits the interior. The moulded frame in black or white covers a wide range of interiors, and the two-knob closure suits portholes opened and shut deliberately rather than constantly adjusted. Both shapes publish outer and inner dimensions you can plan from.

Choose an Aluminium Opening Porthole If

You want a frame that reads as metal, an anodised silver finish, acrylic glazing and a pane that stays where it is pushed. It is the family for a hull side, bulkhead or hatch window adjusted often through the day. Order it from a drawing rather than the table, because the published size block does not hold together.

Choose an Opening Porthole If

The cabin needs air as well as light, which is the way to get both through one aperture and is what every product here is. Accept the consequences: a hinge, a gasket, a closure to check, and a pane to be dogged down before weather arrives.

Choose a Fixed Porthole If

Light alone is the requirement, or the opening sits where nobody can reach it — high in a deckhouse, behind a berth, over a galley counter. A fixed pane carries no hinge, knobs or compressed gasket, which removes the parts needing attention. This catalogue has no fixed porthole page, so that route runs through a drawing and a quotation rather than a cart.

Choose Round, Oval or Rectangular If

The existing cutout decides it. Round suits a hole already drilled or moulded round and reads traditional on a wheelhouse; oval gives more light for the same height and follows a hull side; rectangular gives the most glazing for a given width on a flat panel. Where the opening is cut fresh, pane shape follows the space inside.

One rule holds all of that together: measure the opening first, then choose the frame. Material, hinge and colour are reversible choices on a drawing. The cutout is not.

Need Help Choosing or Sourcing?

Send the opening measurements — length, width, corner radius and the flat band around the edge — with a photo of the cabin side and the model being replaced. With those we can point to the plastic oval, rectangular or round model that fits, or take the aluminium oval route and issue a drawing with the cutout, fastener pattern and glazing called out. Custom cutout drawings, samples and bulk supply run through our OEM and ODM programme; for a single replacement or a mixed order, contact the team. If the fitting in question is round and in the deck rather than the cabin side, the deck plate cutout guide covers that different part, and the plastic porthole overview is the shorter read on the moulded family.