Tankage · A buyer's reference

1. Materials

Tankage materials in current use

Material	Best for	Wall thickness, typical	Service life	Approximate cost / 40 gal
Cross-linked polyethylene (XLPE)	Holding tanks, freshwater on production boats, custom freshwater under 80 gal.	1/4" rotomolded	20–30 yr; longer than the boat in most cases.	$340
Marine aluminum (5052-H32)	Freshwater, diesel fuel; custom builds 40–200 gal.	3/16" walls, 1/8" top	25–40 yr with proper galvanic isolation; failure mode is corrosion at fitting penetrations.	$1,420
Stainless 316L	Diesel fuel above 100 gal; specialty freshwater on commercial vessels.	1/8" walls	40+ yr; failure mode is welding-zone corrosion at heat-affected zones.	$2,800
FRP (fiberglass, integral)	Freshwater on production sailboats (factory-installed only).	Varies; typically 3/16" hand-layup	Lifetime of the boat; failure mode is leaching of styrene into freshwater on older tanks.	N/A (factory)
Galvanized black iron	Historical (pre-1960). Do not specify on new builds.	3/16"–1/4"	40–60 yr; we are seeing service failures now on the 1947–1960 cohort.	N/A (replace)

The dominant pattern in the current market: XLPE for any holding tank, marine aluminum for any custom freshwater tank under approximately 80 gallons, stainless 316L for diesel fuel above 100 gallons. We have not recommended galvanized black iron since the shop opened; the only black-iron work we do is removal (see the M/V Wyman feature in this issue for the worked replacement of a 1947 original).

2. Fittings — top-mounted, every time

The single most consequential design decision on a custom tank is the placement of fittings. Our standing rule, derived from Practical Sailor's 2015 holding-tank comparison and from twenty years of seeing the alternative: every fitting on every tank we build is top-mounted. No exceptions on holding tanks. Discharge outlets, vent fittings, pumpout fittings, gauges — all top.

The reasons: (1) any side- or bottom-mounted fitting creates a static-sewage column in the connecting hose during heel, which permeates and smells; (2) any below-waterline-of-the-tank fitting is the failure point that floods the bilge when it eventually fails; (3) top-mounting allows the tank to be drained almost completely via a vertical pickup tube reaching to within an inch of the bottom, which is the practical equivalent of bottom-draining without the failure mode.

For freshwater tanks, the same rule applies, with the same reasoning. The pickup tube is what gets you to within an inch of the bottom; the top-mounted fitting is what survives twenty years.

3. Baffles

Internal baffles are required on any tank above approximately 20 gallons, to break up the slosh of contents at heel. The rule we follow: baffles on 9"–12" centers in the tank's longest dimension, with cutouts in the lower corners (about 25% of the baffle's depth) to allow drainage between cells and equalization during fill. The baffles are welded to the tank walls in aluminum and stainless construction, and integrated into the rotomold tooling in XLPE.

The slosh-load on an unbaffled 80-gallon freshwater tank at 20° of heel is on the order of 250 lb of dynamic force against the tank walls. The baffles convert that into static load distributed across the wall. The difference is felt in the boat's motion as much as in the tank's failure life.

4. Vents

The vent line is what allows a tank to fill without creating a vacuum at the deck fill (which manifests as a slowed fill flow and, on a holding tank, a backed-up head). The vent must be sized at least one pipe-size larger than the largest inflow line, must vent above any maximum-fill level, and must not be susceptible to dunking under the waterline at heel. The standard install runs the vent to a clamshell or mushroom vent on the deck or topsides.

Holding-tank vents require additional discipline: a vent-line filter (we use the Sealand HK-100 series, replaced every two years) keeps the sulfur smell out of the cabin; an anti-siphon loop in the vent line prevents the rare-but-real case of holding-tank contents siphoning out through the vent during a knockdown or heavy seas.

5. Gauges and senders

The cheapest reliable freshwater gauge is a sight tube — a clear PVC tube run vertically alongside the tank, equalized at the bottom, with graduations in gallons or liters. It has no electronics, no failure mode, and no power requirement. It also requires inspection through a locker; for that reason it has been steadily losing market share to electronic senders.

The electronic options are float-style (which fail at about year 5, silently), strap-on capacitive (which fail at about year 8, drifting), and ultrasonic (which fail at about year 12, and announce their failure with a fault state). We recommend SeaLevel ultrasonic senders on every new freshwater install and on every holding-tank install over 25 gallons. The cost is about $380 per sender, against approximately $80 for the float-style; the difference buys you a decade of reliable reading.

For diesel fuel tankage, the same recommendation: SeaLevel ultrasonic, NMEA 2000 output. For holding tanks where N2K is not available, the SeaLevel analog-out 0–5V version with a discrete gauge.

6. ABYC H-22 (fuel) and H-23 (potable water)

The two ABYC standards that control marine tankage are H-22 (Design and Construction of Fuel Systems for Inboard Gasoline, Diesel, and Hybrid Engines) and H-23 (Installation of Potable Water Systems for Use on Boats). Both are reviewed on a five-year cycle through ABYC's standards development process; both are referenced by the USCG, the EPA, and most state-level marine inspection authorities.¹

The operative provisions for the small-boat refit trade are:

• H-22 §22.5 — fuel-tank material and construction requirements: tank thickness, baffle spacing, fill-and-vent specifications, anti-static grounding.
• H-22 §22.7 — fuel-line fittings and routing.
• H-23 §23.5 — potable-water tank material and construction: NSF 61 certification for the tank material in contact with potable water, top-mounted fittings, vent requirements.
• H-23 §23.7 — potable-water plumbing: tubing certification, fittings, isolation from non-potable systems.

The full text of both standards is available through the ANSI store at a cost of approximately $80 per standard for non-members. ABYC members access the full library through the ABYC website.

For non-standards reading on tankage that bears on what we do: Nigel Calder's Boatowner's Mechanical and Electrical Manual (5th ed., McGraw-Hill, 2022) Chapters 7 and 9; Don Casey's Sailboat Maintenance Manual (2nd ed., McGraw-Hill, 2006) Chapter 6.

References