A few data points and procedures that may help close the gap between lore and measurements. These come from manufacturer specs, standard marine/aero materials practice, and what’s shown up consistently on inspection and repair benches in mixed fresh/brackish use.
Alloy/temper and thickness by era
- Skins and decks: 5052-H3x (H32/H34) sheet remains the norm for Grumman/Marathon-era boats. Extrusions (keel cap, stems, gunwales, some ribs) are typically 6061-T6.
- Thickness: Standard-weight 17′ double-enders are 0.050 in skins; lightweight variants are 0.040 in. The weight spread in current Marathon brochures (roughly 64-75 lb for 17′ LW vs standard) corresponds to that change. Older 1970-80s literature aligns with the same split (no credible evidence of a “thinner modern” standard hull; the difference is model, not era).
- Quick field verification: Ultrasonic thickness gauges with a thin-sheet delay line transducer will read 0.040/0.050 reliably if you get off the lap seams and away from stretch-formed radius. Leeb or UCI portable hardness tests on original paint-free interior typically return low-60s HB for 5052-H32/H34 and mid-90s HB on 6061-T6 extrusions.
Work hardening and fatigue
- 5052 does not have a true endurance limit; fatigue strength at 107 cycles is on the order of 9-14 ksi depending on surface condition. In canoes, stress amplitudes in flat panels are low; fatigue problems concentrate at stress raisers (rivet holes, keel seam steps, rib terminations, dent margins).
- Where it shows up: fine, radiating cracks from rivet holes at the stems and mid-keel laps; crescent cracks at the ends of ribs; occasional through cracks at the apex of a sharp dent that was hammered out cold. Dye penetrant will find these long before they leak.
- Practical inspection: Clean to bare metal around suspect rows, apply visible dye penetrant, dwell, wipe, developer, borescope the lap edges. Eddy current is effective for second-opinion screening around hole edges without full sealant removal.
Rivets and sealants
- Rivets: Solid, bucked aluminum alloy. For salt/brackish exposure with 5052 skins, 5056 solid rivets are preferred for galvanic compatibility. 2117 (AD) will work but corrodes faster in chloride environments. Avoid monel and stainless fasteners in the skin; they become galvanic cathodes.
- Blind rivets: If access forces a blind fastener, use closed-end aluminum-body/aluminum-mandrel structural blinds; seal the stem. Stainless-mandrel blinds are a known pitting starter in brine.
- Sealants: Polysulfide seam sealants (e.g., BoatLife Life-Calk) remain the most aluminum-friendly over long service, especially under laps. Polyurethane (3M 5200/4200) bonds well; 5200 is essentially permanent and makes later service harder; 4200 or Sikaflex 291 are workable compromises. Do not use acetic-cure silicones. Priming holes and faying surfaces with an anti-corrosion jointing compound (Duralac, Tef-Gel, Noxudol 3100) before wet-assembly measurably reduces crevice corrosion.
Galvanic traps (stainless retrofits)
- In brackish use, 316 fasteners in bare aluminum without isolation will pit the aluminum under the head and washer within 1-3 seasons if routinely wet. The small-anode/large-cathode geometry is the worst case.
- What holds up: 316 fasteners plus nylon shoulder washers and non-conductive bushings through the hole, a non-conductive washer under the nut, and Tef-Gel or Duralac in the threads. Better: use aluminum or titanium hardware for through-gunwale clamps; for seats/thwarts, keep stainless entirely isolated from bare aluminum with sleeves and coated brackets.
Foam flotation
- Older end-blocks vary from EPS to PU; many are water stained, friable at the surface, and 1-3 lb heavier per block after years of wetting/drying. If the canoe gains more than 4-6 lb after a week afloat and doesn’t drip at lap joints, suspect foam.
- Replacement: Cross-linked polyethylene minicell (2-4 lb/ft³) billet or contoured blocks are durable and essentially non-absorbent. Mechanical retention (perforated 5052 cradles with nylon straps or HDPE tabs) beats adhesive-and avoids crevice sites. Leave a drain/air gap at the low point; do not fully encapsulate with polyurethane foams.
Noise and damping
- Aluminum does transmit structure-borne taps efficiently. Constrained-layer damping (CLD) patches work. 80 mil butyl-aluminum car-audio sheets (Noico, Second Skin) covering 10-20% of the bay area between ribs on the inside reduce ring by roughly 6-10 dB in the 200-800 Hz band in similar-gauge marine panels; the penalty is 2-4 lb total if you’re selective.
- Placement: 4×6 in patches mid-bay and a strip along the inside of the keel flat are effective. Roll firmly; seal edges with thin polyurethane to discourage water creep. Avoid open-cell foams or carpet backings that trap moisture.
Keel effects
- The Grumman external keel is shallow; at touring speeds (Froude 0.2-0.3) its added wetted area penalty is small. Where it earns its keep is directional stability in quartering winds and against current seams.
- If you want data on your boat: lay out a 500 m reach, do GPS-timed round trips at a fixed cadence/power (metronome, same crew, same load, alternating with/without a temporary low-profile keel cap); take five runs each, toss outliers, compare median speeds and cross-track error in gusts. Expect speed deltas in the noise (<1%) and measurable tracking improvement with the keel. Snag risk is use-case specific.
Stiffness/oil-canning test you can repeat at home
- Supports: Two padded trestles 1.2 m apart centered on midships. Dial indicator or digital gauge on the interior center panel.
- Load: 50 lb sandbag at midspan, centered.
- Reading: Measure unloaded baseline and loaded deflection at center. Record temperature.
- Typical numbers: A healthy 0.050 skin panel between ribs will show a few millimeters of elastic deflection and fully recover. Growth in residual “set” after a season of rocky use (increase in unloaded sag) is the meaningful trend to track, not just elastic deflection. Photograph a straightedge across the panel at each session.
Salt and storage
- Rinse within 24 hours of brackish or road-salt exposure. Focus on lap seams: a low-pressure, high-volume fresh water flush along the keel seam and rib laps until runoff TDS matches tap water.
- Annual: Remove seats/thwarts, lift one gunwale at a time enough to wash out salt/silt, dry thoroughly. Re-wet lap seams with warm fresh water to dissolve chlorides; allow to drain nose-down and tail-down in turn.
- Corrosion inhibitor: A light-film aviation-grade anti-corrosion fluid (ACF-50, CorrosionX) wicked into lap edges after a dry-out can arrest nascent crevice corrosion; wipe off excess.
- Sacrificial anodes: Not effective for canoes that live out of the water and see intermittent wetting; without continuous electrolyte and intentional bonding, an anode won’t protect the broad, intermittently wet skin. Put effort into rinse/dry and joint isolation instead.
Finish
- Polishing removes the oxide temporarily; a new oxide forms in minutes. If you’re removing chloride-laden grime and then sealing, you can reduce corrosion risk, not increase it.
- Best-in-class: Clean/deoxidize (alkaline cleaner, then non-etch deoxidizer), chromate conversion coat (Alodine 1201 or a trivalent equivalent), then a clear, flexible urethane like Nyalic. This stack is widely used on bare-aluminum small craft; in brackish service it holds up for years with spot touch-ups. If you skip conversion, expect more underfilm corrosion over time.
- If you prefer patina: Leave as-mill finish, wash and dry religiously, and wax with a non-silicone synthetic to slow chloride ingress. Avoid aggressive compounds that thin the sheet at ribs and along the keel crown.
Structural red flags seen repeatedly
- Hairline cracks at the forward and aft three rivets adjacent to the stem castings; also in the first bay aft of the stems where panel curvature changes.
- Fretting corrosion and weeping at keel-lap rivets near midships; look for black oxide traces.
- Rib termination cracks at the “toes” of the ribs where they meet the flat; these are small at first and easy to stop-drill and strap.
- Sportboat transoms: cracking at the lower transom brackets and around motor-mount bolt holes if run heavy; inspect for elongation.
- Gunnel rail corrosion under stainless carriage bolts at seat/thwart retrofits lacking isolation.
Economics (10-year view, livery/camp duty in mixed fresh/brackish)
- Aluminum (5052 riveted): Upfront used cost is low; predictable maintenance is seam reseal and occasional re-buck/replace rivets. If you standardize on 5056 solids, polysulfide at seams, and isolation bushings on any stainless retrofits, the service interval is commonly 4-7 years between leak-chasing in brackish fleets that rinse. Hulls remain straight enough for service into decades unless badly dented.
- T-formex/composites: Lower noise, warmer, often better hull shapes; maintenance tends toward skid-plate work and fixing oil-canning or cracks from pinning. In brackish, no corrosion, but UV and thermal distortion are real. Replacement cost is the big line item.
- A simple way to compare: Annualize total cost = (purchase - resale + maintenance + downtime) / service years. Well-kept aluminum often wins on annualized cost in hard-use fleets because resale never goes to zero and downtime for repairs is short and field-serviceable. If you do not have rinse/dry control and hardware isolation discipline, aluminum’s corrosion penalty can erase that advantage.
Five-year tasks that actually move the needle
- Full fresh-water flush/dry after every brackish exposure; annual deep flush under laps and gunwales.
- Inspect with dye penetrant around stems, mid-keel, and rib ends; stop-drill and strap small cracks.
- Replace any non-isolated stainless touching bare aluminum; add isolation sleeves/washers and a jointing compound.
- Renew lap seam sealant where weeping is visible; prioritize keel seam.
- Replace degraded foam with XLPE blocks on mechanical cradles; ensure drainage and airflow.
If you want to crowd-source numbers, propose a common test sheet for contributors:
- Ultrasonic thickness at three bays per side (forward/mid/aft), and a Leeb hardness reading on bare interior skin and on a gunwale extrusion.
- Midship deflection test as above with load, temperature noted.
- Dye-penetrant result photos with locations marked.
- Weight dry vs after a 7-day float with daily air temp/water temp logged.
- Hardware inventory and isolation details, plus time-in-service in brackish.
That kind of data will separate “forever” from “how to keep it that way.”