Ask a surveyor what they look for and the honest answer depends entirely on what the boat is made of. These four materials share almost nothing in how they deteriorate, and the habits that catch problems on one will walk straight past problems on another.
GRP and fibreglass
The most common leisure hull material since the 1960s, and the most forgiving. GRP does not rot and does not corrode. What it does is hide things.
What matters most
- Wet and rotten deck core. Most GRP decks are cored with balsa, plywood or foam. Water enters through unsealed fastenings, stanchion bases, cleats and window frames, and a saturated core loses strength and rots while the deck still looks immaculate. Sounding finds it; looking does not. Frequently the single most expensive finding on a GRP boat — £6,000 to £15,000 or more to do properly.
- Osmosis and hydrolysis. Widely misunderstood, and covered in detail in the osmosis article. The short version: blisters are usually cosmetic, laminate softening is not, and a single moisture reading tells you very little.
- Delamination. Layers separating, or skin lifting from core. Sounded for rather than seen.
- Hull-to-deck joint. Bonded, bolted, riveted or some combination. Leaks and movement here are common on older production boats.
- Keel joint and keel bolts on fin-keeled yachts. Smiles, weeping, movement and corroded bolts. Serious findings.
- Structural tabbing. Bulkheads and stringers bonded to the hull. Cracked or detached tabbing, particularly around chainplates, mast step and engine bearers.
- Previous repairs. Grounding and impact repairs of very variable quality, often disguised by antifoul.
- Gelcoat crazing. Usually cosmetic; occasionally stress cracking that indicates flexing or an underlying problem.
Where a GRP hull is undemanding
No rot, no corrosion of the structure itself, no annual paint cycle, tolerant of neglect over long laid-up periods. A well-built 1985 GRP hull with a dry deck can be structurally excellent today.
Wooden hulls
The most rewarding to survey and by a clear margin the most time-consuming. Wood tells you a great deal if you know where to press, and the areas that matter are seldom the areas that look worst.
What matters most
- Rot at fastenings and in end grain. Rarely in the middle of a plank. It starts where water sits and cannot dry: plank ends, hood ends at the stem, around fastenings, under hardware, in the top of frames and floors, and where deck meets hull.
- Fastening sickness. Iron or steel fastenings in oak corrode and chemically break down the surrounding timber, leaving black, soft, acidic wood — nail sickness. A hull can look fair while the fastenings holding it together are finished. Sample fastenings are sometimes drawn, with the owner's agreement, because there is no other reliable way to know.
- Electrolytic and galvanic problems around bronze, copper and dissimilar metals, degrading adjacent timber.
- Floors and frames. Steel floors in older craft corrode and split; wooden floors rot where bilge water stands. Both are structurally central and both are inconvenient to reach.
- Caulking and seam condition on carvel hulls, and the state of stopwaters, garboards and the hull-to-keel rebate.
- Sheathing. Glass or epoxy over timber is a common repair, and a mixed blessing. Where water gets underneath, it cannot dry out and rot proceeds invisibly. Sheathed wooden hulls need particular care.
- Deck and coachroof leaks, because in a wooden boat every leak eventually becomes rot somewhere below it.
- Previous repairs, which on older wooden craft can span a century of varying skill and philosophy.
The honest position on wooden boats
A well-maintained wooden hull can outlast several GRP boats, and rot is repairable — frequently more repairable than a hydrolysed laminate. The catch is that wood punishes neglect quickly and continuously. A wooden boat left alone for three years is in a materially worse condition; a GRP boat often is not. Surveys take longer, cost more, and insurers ask for them earlier and more often. Buy one with your eyes open and a maintenance budget, not as a bargain.
Steel hulls
Common on narrowboats, Dutch-built motor cruisers, expedition yachts and older workboats. Immensely strong, easily repaired by anyone with a welder, and engaged in a permanent argument with oxygen.
What matters most
- Plate thickness. The central question. Ultrasonic readings are taken on a grid and compared with the original scantlings, with particular attention to the waterline, bilges, chines and anywhere water sits. Many insurers require these readings on steel craft.
- Corrosion from the inside out. The failure mode people miss. External paint can be immaculate while the bilge under a lining or a concrete ballast bed is quietly eating the plate. Where bilges are inaccessible or filled, that is a serious limitation and the report should say so plainly.
- Pitting. Deep, localised pits matter far more than uniform surface rust — a small pit can be most of the way through the plate.
- Waterline and boot-top corrosion, which is where wetting and drying cycles concentrate.
- Weld condition, especially at chines, stem, transom and around repair patches, plus doublers welded over thin plate — a common bodge that hides what is underneath.
- Anodes and bonding. Wasted anodes, missing anodes, or an unbonded installation. Also shore-power-related galvanic activity in marinas, which can be aggressive.
- Internal coatings and insulation. Spray foam is excellent insulation and a superb place for hidden corrosion, because it holds moisture against the plate and cannot be seen through.
- Ballast and bilge access, since loose or concreted ballast over bare steel is a recurring source of trouble.
The trade-off
A steel hull that has been kept painted and had its anodes replaced can be in excellent order at fifty years old, and repairs are comparatively cheap — cut out a plate, weld in a new one. Left to itself with a leaking deck and no anodes, the same hull can become uninsurable in a decade.
Aluminium hulls
Less common in UK leisure craft, more usual in high-performance, expedition and commercial vessels. Light, strong and corrosion-resistant in isolation — and quite unforgiving of electrical carelessness.
What matters most
- Galvanic corrosion. Aluminium is anodic to most other marine metals. Any stainless, bronze or copper fitting in contact with the hull without proper isolation becomes a corrosion cell. Fastenings, skin fittings, stern gear and deck hardware all need looking at individually.
- Stray current corrosion. The one that does real damage fast. A wiring fault, a badly installed shore power system or a neighbouring boat's fault can remove significant metal in weeks. Bonding, isolation transformers and galvanic isolators get close attention.
- Crevice and poultice corrosion under fittings, gaskets, sealant, insulation and non-slip — anywhere water is trapped against the metal without oxygen.
- Pitting, particularly in bilges and under any standing water.
- Weld and heat-affected zones, and the alloy grades used in any repair. The wrong alloy or filler in a marine application causes problems later.
- Anode condition and the bonding system, which on an aluminium boat is not optional maintenance.
- Paint and coating condition, and whether antifoul is a type safe for aluminium — copper-based antifoul on a bare alloy hull is a serious error and still turns up.
The trade-off
A properly built and correctly wired aluminium hull is superb: strong, light, repairable and long-lived, with no rot and no osmosis. Its weakness is that it needs the electrical installation kept right for its whole life, and a single bad shore power connection can be expensive.
Side by side
| GRP | Wood | Steel | Aluminium | |
|---|---|---|---|---|
| Main enemy | Trapped water in core and laminate | Rot and fastening decay | Corrosion, often internal | Galvanic and stray current corrosion |
| Key tool | Hammer and moisture meter | Spike, hammer, drawn fastenings | Ultrasonic thickness gauge | Visual, plus electrical testing |
| Fails visibly? | Often not | Usually, if you know where to look | Rarely from outside | Often not until advanced |
| Survey time | Baseline | Longest | Longer, plus readings | Longer, plus electrical work |
| Tolerates neglect | Reasonably well | Poorly | Poorly without paint and anodes | Poorly if wiring is wrong |
| Repair cost | Moderate to high | Moderate, labour-heavy | Low if caught early | High, needs a specialist |
| Insurer attitude | Most relaxed | Strictest | Thickness readings expected | Varies widely |
Why the material shapes the survey
It shapes everything: how long the job takes, which tools come out of the van, where the time is spent, and what the report is capable of telling you. Sound a wooden hull as though it were GRP and you will learn very little. Meter a steel hull for moisture and you learn nothing at all. Survey an aluminium boat without looking hard at the shore power installation and you have missed the thing most likely to cost the owner money.
This is where thirty years of building, repairing and engineering across all four materials earns its keep. Having repaired a rotten hood end, welded a plate into a corroded bilge, ground out a hydrolysed laminate and dealt with an alloy hull ruined by a bad shore connection changes what you look for — and, just as importantly, what you are prepared to say about what you find.
When you are choosing a surveyor, ask what the last boat of your type and material threw up. The answer will tell you quickly whether they know your kind of boat, or only boats in general.
Whatever she is built from
Thirty years of building, repairing and engineering GRP, wood, steel and aluminium craft. Send the vessel's details and hull material for a guide price.
Related reading
- Osmosis in GRP boats: what it is and what it costs
- Pre-purchase survey: what to expect on the day
- Boat insurance survey requirements explained
- Frequently asked questions
This article is general guidance, not advice on a specific vessel, and does not replace a survey. Cost figures are indicative UK market ranges as at August 2026 and are not quotations.