Guide
What Do ISO 9093 and ABYC H-27 Mean for Thru-Hull Fitting Materials on a Drawing?
ISO 9093 is the international standard for seacocks and through-hull fittings on small craft with a hull length up to 24 m. Since the 2020 edition it is one document that sets material requirements for the fittings below the maximum load waterline, and its text is sold, so a part maker works from the fitting brand’s drawing and purchase order rather than from the standard itself. The useful question for a machined body, flange or nut is therefore what the PO must name: alloy family, zinc limit or dezincification test, and certificate.
By Saola Manufacturing engineering team · Published · Updated
General information, not legal advice. This guide is for product and compliance engineers at seacock and thru-hull makers, and for the sourcing teams who buy their machined bodies, flanges and nuts. It states what can be verified from public pages about ISO 9093, the European directive that points to it and ABYC H-27, and it is explicit about what it cannot quote. It does not list approved alloys, because no public source read for this guide lists them. It gives the alloy families that practitioners use below the waterline, and the wording that moves the decision onto the page where it can be audited.
On this page
What does ISO 9093 cover, and which edition is current?
ISO 9093 covers seacocks, through-hull fittings, hose connections and their fittings on small craft up to 24 m hull length, including installation. The current edition is ISO 9093:2020, published in Europe as EN ISO 9093:2021, which merged the earlier metallic and non-metallic parts.
The BSI Knowledge summary of BS EN ISO 9093:2021, published on 31 May 2021, says it sets requirements for seacocks, through-hull fittings and hose connections in small craft, covers their installation with the aim of preventing parts from loosening under normal operating conditions, and excludes engine and heater exhaust fittings and sail drive through-hull fittings (BSI Knowledge, 2021). It sets material requirements for fittings located below the maximum load waterline, and it requires fittings and seacocks to be compatible with each other and corrosion resistant.
The same page says the standard replaces BS EN ISO 9093-1:2018 and BS EN ISO 9093-2:2018, combining the two parts, and lists these technical changes: a revised definition of corrosion resistance, and new tests for installed strength, corrosion resistance and UV stabilization. It describes the standard as identical to ISO 9093:2020. If a customer’s drawing still cites “ISO 9093-1” or “-2”, ask which edition they are working to.
What the public summary does not give is the clause text: the permitted alloys, the definition of corrosion resistance, or any test values. Those live in the purchased standard, and this guide does not quote them.
How does the EU Recreational Craft Directive connect to it?
Annex I of Directive 2013/53/EU requires that thru-hull fittings below the waterline at maximum load have a readily accessible shutoff, and the European Commission’s 2018 application guide lists the 9093 standards as relevant harmonized standards. That listing predates the 2021 merger, so check the current list.
The guide’s section 3.4, openings in hull, deck and superstructure, says openings must not impair the structural integrity of the watercraft or its weather-tight integrity, and that through-hull fittings allowing water into or out of the hull below the waterline corresponding to the manufacturer’s maximum recommended load must be fitted with a means of shutoff that is readily accessible (European Commission, RCD Application Guide, 2018). It lists EN ISO 9093-1 (metallic) and EN ISO 9093-2 (non-metallic) as the relevant harmonized standards for that requirement.
Two points follow for a part maker. The directive sets a function, shutoff and structure, not an alloy. And harmonized-standard status is what gives a boat builder a presumption of conformity; it is the fitting brand’s declaration, not the machine shop’s. A machined body, flange or nut should never carry a claim of directive conformity on its own drawing.
The current harmonized-standard list is published in the Official Journal of the EU, and that list, not the 2018 guide, is where to confirm that EN ISO 9093:2021 has replaced the two parts. This guide has not checked it.
What about ABYC H-27, and why is it not quoted here?
ABYC H-27 is the American Boat and Yacht Council standard that addresses seacocks, through-hull fittings and drain plugs. Its text is held in ABYC’s members’ library, so this guide does not quote it or claim which alloys it accepts.
ABYC’s own technical toolkit page says its online library includes ABYC standards, CFRs, Coast Guard compliance guidelines and ISO standards, and the library link requires a login (ABYC technical toolkit, 2026). No public page read for this guide shows H-27’s alloy language, so no approved-alloy list is given here.
In practice the customer tells you. A U.S. seacock brand works from its own copy of H-27 and its own drawing; the part maker’s job is to ask which edition and which clause the drawing relies on, and to quote exactly that. If a drawing says only “per ABYC H-27”, ask for the clause or a statement of the alloy and the zinc or dezincification requirement. That is not a request to read the standard; it is a request to put the requirement on the page that will be audited.
Which alloy families belong on a below-waterline drawing?
Practitioner sources support low-zinc bronzes and silicon bronze for parts below the waterline, warn off high-zinc brasses sold under bronze names, and flag 316 for crevices and stagnant water. Which of them a given standard accepts is the customer’s decision, read from the standard text.
Dave Gerr’s 2009 marine metals article calls silicon bronze the top pick for most marine screws and bolts, calls cast leaded red brass C83600 (85% copper, 5% tin, 5% lead, 5% zinc) effectively a bronze and a good casting alloy for many marine fittings, and warns that manganese bronze, Tobin bronze, commercial bronze and naval brass are really brasses that are potential problems below the waterline (Gerr, 2009). HG Houston adds that brasses with 15% or less zinc are practically immune to dezincification, that alpha-beta brasses are susceptible, and that a dezincification-resistant duplex brass, CZ132, is heat treated after forging (HG Houston, dezincification).
For stainless, BSSA says stagnant seawater must be avoided for 316 and that any form of crevice must be avoided (BSSA, 2026). A seacock body can hold stagnant water, which is a reason to take the alloy question seriously and why the dezincification guide matters for any brass proposal. The table lists the options without ranking them.
| Alloy family | What the sources say | Drawing action |
|---|---|---|
| Silicon bronze (C65500 bar) | Gerr: top pick for marine screws and bolts; low zinc | Name UNS number, form and temper |
| Cast low-zinc red brass or bronze (C83600 type) | Gerr: 5% zinc, effectively a bronze, good for casting marine fittings | Name the casting alloy by UNS number and cap zinc; casting is managed by a partner foundry |
| High-zinc brasses with bronze names (manganese, Tobin, commercial bronze, naval brass) | Gerr: brasses; potential problems below the waterline | State “not allowed” unless the customer’s standard says otherwise |
| Dezincification-resistant brass (for example CZ132) | HG Houston: arsenic and a post-forging heat treatment prevent dezincification | Only if the customer accepts it; call the dezincification test they name |
| 316 stainless | BSSA: avoid stagnant seawater and crevices | Customer decides; design for flow and no crevices |
| Non-metallic body | The 2021 edition merged the former metallic (-1) and non-metallic (-2) parts (BSI; RCD guide list) | Molded, outside CNC; machine only inserts or mating metal parts |
What should the drawing and PO say for a machined body, flange or nut?
Name the alloy by UNS number or specification, the product form, the temper or condition, the certificate type, any zinc limit or dezincification test, and the standard edition the fitting brand relies on. Leave all claims of ISO 9093 or H-27 conformity to the fitting brand.
A workable material line reads: “Material: UNS C65500 round bar, hard temper, mill certificate with chemistry; no brass substitutes.” For a casting: “Cast bronze UNS [number], zinc maximum as customer specification, foundry certificate with heat lot.” Add the line the customer controls: “Fitting designed to ISO 9093:2020 or EN ISO 9093:2021 by [brand]; part maker makes no conformity claim.”
If the alloy is a brass, require the dezincification test the customer names; HG Houston describes a cupric chloride exposure test followed by sectioning and notes that BS EN ISO 6509 covers the European version, and the dezincification guide explains the call-out. For tapered pipe threads on seacock bodies, the NPT thread chart and the NPS vs NPT guide cover the thread callout.
Certificates matter here more than usual because the alloy is the compliance argument. Ask for the type the customer needs; the 3.1 vs 2.2 guide explains the difference. For galvanic pairing with the hull and any zinc anodes, the galvanic corrosion guide sets out the logic.
What does machining a bronze thru-hull body actually involve?
Mostly boring, facing, threading and a good datum strategy on a cast blank. The casting is placed with a partner foundry and the machining runs at the lead CNC plant, with both sites named on the quote.
A seacock body is typically a cast bronze blank with a flange, a through bore, a tapered or parallel thread, and a seat for a ball or plug. The machining is where the sealing surfaces are made, and the datums must come from features the casting can hold. The CNC milling page describes brass and bronze valve bodies sand cast by a partner foundry and then bored and faced at the lead plant, and the guide on datums on cast parts covers how to choose them.
Gerr notes that true silicon bronze is harder to cast cleanly and harder to machine after casting, which is one reason cast fittings are often made in the low-zinc red brass types he names. Nuts, hose barbs and drain plugs are different: they are often turned from bar, and CNC turning is the natural route for silicon bronze bar. Leak and pressure tests on the finished fitting belong to the fitting brand’s test plan; the machine shop reports dimensions and surface condition, not a standards test.
Frequently asked
Which edition of ISO 9093 is current?
BSI lists BS EN ISO 9093:2021, identical to ISO 9093:2020, as current; it replaced the two earlier parts for metallic and non-metallic fittings.
Does ISO 9093 forbid brass thru-hulls?
The public BSI summary does not say. It says the standard sets material requirements and requires corrosion resistance. Read the clause in the purchased standard, or ask the fitting brand for it.
What does ABYC H-27 say about materials?
This guide does not quote it: ABYC standards sit in a members’ library. Ask the customer for the clause or the alloy requirement and the edition.
Is 316 acceptable below the waterline?
BSSA warns against stagnant seawater and crevices for 316, so it needs flow and a crevice-free design; whether a standard accepts it is the customer’s reading of the text.
Can the machine shop claim the fitting meets ISO 9093?
No. Conformity is declared by the fitting brand for the finished fitting, not for a machined body, flange or nut.
Sources
- BS EN ISO 9093:2021, Small craft - Seacocks and through-hull fittings, summary, BSI Knowledge, published 31 May 2021
- Recreational Craft Directive 2013/53/EU, Application Guide, section 3.4, European Commission, 2018 edition
- ABYC Technical Toolkit, American Boat and Yacht Council, accessed October 2026 (page copyright 2026)
- High Strength with High Corrosion Resistance (marine metals, part 2), Dave Gerr, Gerr Marine, 2009
- Dezincification, material property data, H&G Houston (undated; accessed October 2026)
- Selection of 316, 304 and 303 types of stainless steels for seawater applications, British Stainless Steel Association, accessed October 2026 (page copyright 2026)