Boat Deck Plate Replacement: Checking the Cutout and Access

Quick Answer

The inch label on a deck plate is not the size of the hole it covers, and no Alastin deck-plate page publishes the hole size a plate needs. A replacement is ordered against an aperture that already exists in your deck, so the order begins with a measurement rather than a catalogue label. Four things decide whether it seats and stays usable: the aperture, the seating flange and its fasteners, the lid retention, and the room left underneath. The deck plate range is organised by nominal size, which is the one thing your deck is not.

Real-World Explanation

A deck plate is usually the last part of the repair, not the first. The hole was cut years ago, the ring around it is bedded into the deck, and what has failed is a cracked lid, a thread that no longer holds, or a lid that went over the side. That order matters, because the catalogue you are about to order from runs the other way: by nominal size, ascending, with millimetre figures beside the label as though they explained it.

They do not. Eight inches is 203.2 mm, and the ABS round deck plate prints 180 mm against that row. A 23 mm difference is wide enough to see on a bench, let alone against a hole in a deck. The 6 inch rows print 150 and 151 mm against a nominal 152.4, and the 4 inch rows print 100 and 101 mm against 101.6. Those two are close but not the same, and the gap is small enough to be waved through, which is the reason to write the numbers down rather than carry them in your head.

Then compare the pages with each other. Three Alastin tables publish a 4 inch part: the ABS series ALS-004 prints 143 and 101 mm, the white ABS series ALS01804A prints 145 and 100 mm, and the stainless plate ALS3104A prints 147 and 106 mm. One label, three pairs of figures, six millimetres apart at the widest. If the label were the fit, those three would be interchangeable into the same hole. They are not, and nothing on the pages says they are. The label sorts the catalogue; the deck needs a millimetre figure.

The direction of the second column also changes with the material. In the ABS tables it sits just below the inch nominal: 101 against 101.6, 151 against 152.4, 180 against 203.2. In the stainless table it sits above, from 81.8 mm at 3 inch to 161.8 mm at 6 inch, between 5.6 and 9.4 mm above nominal across the range. Neither table is wrong. The families were drawn to different conventions, which is the practical reason not to carry a figure from one series across to another.

Here is the part that decides the order. Every one of these pages prints two millimetre columns, and none says what either column measures. No deck-plate page on this site publishes a cutout dimension, which is the one figure a replacement buyer needs. The table gives you the size range, the material and the colours. It does not tell you which of the two numbers is the hole, and the letters used as column headings are drawing references rather than definitions.

The second half of the decision is access. The aperture you are covering is also the aperture you work through, and a plate can seat correctly and still be the wrong choice because the working opening is smaller than the hole in the deck: a seacock, a hose clip or a pump that used to be reachable at arm’s length becomes a job done by feel. Ask what the opening is, not only what the outside measures. Ask the same of the lid, since the general description says it is unscrewed or removed and no model page says whether that means threaded, bayonet or lift-out. The hatch hardware guide covers hinges, latches, handles and struts; a plate needs a hole measurement and a matching ring.

Practical Insights from Marine Hardware Supply

1. Measure the old part before it goes in the bin, and measure the ring rather than the lid

The lid is the part that failed, so it is the part in your hand, and it is the wrong reference. The ring is what sits in the deck, and the ring is what the new plate has to agree with. Measure the aperture across at two points at right angles, note whether the opening is round or has been opened up, and photograph the ring with a rule lying across it. A measurement taken after the old ring has been discarded is a measurement taken from a hole with no reference left in it.

2. A flange that covers the hole is not the same as a flange that seats on it

Buyers compare diameters, because diameters are what the tables print. What keeps water out in service is the condition of the gasket, the flatness of the seating face and how evenly the fasteners pull the flange down onto the deck. A flange wide enough to hide the aperture can still rock on a deck that is not flat, or land half on a liner and half on the skin. Look at the fastener spacing and at what the screws are going into before you look at the width.

3. Ask for the drawing with the column meanings written on it

The table alone cannot tell you which figure is the hole, and no amount of arithmetic recovers it: the label matches neither column on any row. A drawing with the columns named, or a written statement of the opening size for that code, turns a three-week argument into one email. Ask before you cut, and ask for the model code in the answer so the figure is tied to a part rather than to a family.

Buying Checklist

Start with the aperture. Measure the opening across at two points at right angles, because an aperture cut by hand or enlarged during an earlier repair is often slightly out of round, and a plate that agrees on one axis can sit proud on the other. Note whether the edge is clean, and whether the deck skin has a liner or a core behind it.

Then the flange. The seating face has to cover the aperture and land flat on the deck all the way round, which is a question about the deck around the hole as much as about the plate. Check the fastener pattern: how many screws, at what spacing, and what they are going into. A cored deck, a plywood pad and a thin liner all take a fastener differently, and a pattern that does not line up with the old holes means new holes or a new ring.

Then the lid. Confirm the retention type, because a lid that does not match the ring is a lid that does not stay down, and the pages describe the lid as unscrewed or removed without saying which mechanism applies to which code. Confirm as well that the lid is supplied with the plate rather than separately.

Then the space. Record the thickness of the deck or liner the plate has to span, and look under the aperture for anything that would foul the lid or narrow the opening. Say whether the plate will be stepped on, since a plate set into a walking surface is stepped on in service and that belongs in the conversation with the supplier. Finally, match the material and finish to the hardware around it.

Decision Guide

Replace Only the Lid

This applies when the ring is sound, still bedded, and the damage is confined to the lid: a crack, a lost lid, a torn gasket. The aperture does not change and no measurement of the deck is needed beyond confirming the ring is still where it was. The open question is supply, because the pages describe a plate as a lid and a ring without saying whether the lid is sold on its own for a given code. Ask for the lid as a separate part against the model code, and ask what the retention is.

Replace Plate and Ring

This is the common case. The ring is cracked, the thread has gone, the fastener holes have pulled, or the ring is bedded so badly that re-using it repeats the problem. Now the aperture is the constraint: the new ring has to cover the old screw holes or the deck has to take new ones, and the flange has to sit flat on a deck that already carries a ring-shaped mark. Re-bedding the new ring is part of the job, and the old bedding comes off before it goes down.

Change the Opening Size

This applies when nothing in the published range suits the aperture, or when the opening is too small to reach what is below it. It is a deck job rather than a parts job: the aperture is cut or filled, the cut edge has to be finished so the core does not take water, and the plate is chosen against the finished hole. Enlarging a hole to fit a plate that still leaves the seacock out of reach solves nothing, so settle access first. If the fitting in the deck is a filler rather than a plate, the deck filler fitment guide covers that case.

The rule underneath all three: measure the aperture first, and buy the plate that is drawn for it, not the plate whose label looks closest.

Need Help Choosing or Sourcing?

Send the aperture measurement, a photograph of the old ring with a rule across it, and the model code if it is still legible. We will tell you which published size is worth checking against a drawing, and where the drawing is the only honest answer. Yards and distributors can take the same route through the OEM and ODM enquiry page, which covers custom drawings, samples and bulk supply, including plates drawn to an existing aperture rather than to a catalogue label. For anything else, the contact page reaches the sales team directly.