Boat Survey Guide — What a Marine Surveyor Checks and Why It Matters

A pre-purchase survey is the single most important step in buying a used boat. It is the marine equivalent of a building survey on a house — a professional, independent assessment of the boat's condition, seaworthiness, and value. Skip it, and you are buying blind. Get it right, and it can save you tens of thousands of euros and years of regret.

This guide explains what a marine surveyor checks, what a survey costs, and how to use the survey report to protect yourself.

1. What is a marine survey?

A marine survey is a comprehensive inspection of a boat by a qualified, independent surveyor. The surveyor examines the hull, structure, systems, and equipment, then produces a written report detailing the boat's condition, any deficiencies, and an opinion of its market value.

There are several types of survey, but the one that matters when you are buying is the pre-purchase survey — the most thorough type. Other types include condition and valuation surveys (for insurance), damage surveys (after an incident), and compliance surveys (for flag or charter). When buying, you want a pre-purchase survey.

2. Why you need one

• You do not know what you do not know. Even an experienced boater cannot see inside the hull, assess the moisture content of a laminate, or evaluate the structural integrity of a keel joint. A surveyor can.
• Negotiation leverage. The survey report is your most powerful negotiation tool. Every deficiency the surveyor finds is a reason to adjust the price or require a repair.
• Insurance requirement. Most insurers require a survey before they will insure a used boat, especially one over 10 years old. The survey report is what they base the policy on.
• Financing requirement. If you are financing the purchase, the lender will require a survey and valuation.
• Safety. A survey identifies safety issues — failed seacocks, corroded gas lines, inadequate fire protection — that could put you and your crew at risk.

3. What a survey includes

A full pre-purchase survey is typically a full-day inspection (or longer for larger boats), followed by a written report. The surveyor checks:

Hull and structure

• Hull condition. The surveyor sounds the hull with a hammer (tapping for changes in tone that indicate delamination or voids) and uses a moisture meter to detect elevated moisture levels in the laminate — a sign of osmosis or water ingress.
• Osmosis / blistering. Blisters on the underwater hull indicate water penetration into the gelcoat and laminate. Minor blistering is common on older boats; extensive blistering requires treatment (peeling, drying, epoxy barrier coat) that can cost €3,000–€10,000.
• Keel and keel bolts. The keel-to-hull joint is inspected for cracks, movement, or weeping. Keel bolts are checked for corrosion and tightness (where accessible). A grounding can damage the keel in ways invisible from inside.
• Rudder and steering. Rudder stock, bearings, and steering system are checked for play, corrosion, and proper operation.
• Hull-deck joint. The joint between hull and deck is inspected for separation or cracking — especially on boats that have been raced or hard-used.
• Bulkheads and structural members. Chainplates, bulkheads, and floors are checked for rot, delamination, or movement.
• Through-hulls and seacocks. Every underwater fitting is checked. Seacocks must operate freely and be free of corrosion. Bronze that has turned pink indicates dezincification — a failure waiting to happen.

Engine

• Visual inspection. Oil and coolant condition, belts, hoses, raw-water system, fuel system, exhaust.
• Operational check. Cold start, idle, running temperature, oil pressure, exhaust colour. The surveyor listens for unusual noises and checks for vibration.
• Engine hours. Verified against the hour meter and service records. Discrepancies are a red flag.
• Oil analysis (optional but recommended). A sample of engine oil is sent to a lab to check for metal particles, coolant contamination, or abnormal wear. This can reveal internal engine problems that a visual inspection cannot. Cost: €50–€150 per engine.

Note: most surveyors are not marine engineers. Their engine inspection is visual and operational, not a full mechanical teardown. For a high-value or high-hours engine, consider a separate engine survey by a qualified marine engineer (€300–€800).

Systems

• Electrical. AC and DC systems, battery condition, wiring integrity, shore-power installation, GFCI/RCD protection. Amateur wiring is endemic in used boats — the surveyor looks for automotive wire, inline fuses, and twisted-and-taped connections.
• Plumbing. Freshwater system, heads, bilge pumps, hoses and clamps. Perished hoses and missing hose clamps are common and a sinking risk.
• Fuel system. Tank condition, fuel lines, filters, shut-off valves. Diesel fuel that has been sitting for years may be contaminated with water or biological growth.
• Gas system (if fitted). LPG installations are checked for leaks, proper ventilation, and compliance with safety standards. Gas leaks on boats are lethal.

Rig and sails (sailboats)

• Standing rigging. Visual inspection of swage fittings, turnbuckles, and wire for rust, cracks, or distortion. Rigging over 10–15 years old is flagged for replacement.
• Mast and boom. Checked at deck level for corrosion, cracks, and proper step condition. A full aloft inspection is usually extra.
• Sails. Inspected for UV damage, torn stitching, worn hardware, and shape (a blown-out sail has lost its aerodynamic shape). The surveyor may recommend a sailmaker's assessment.
• Running rigging. Halyards, sheets, and control lines checked for wear and chafe.

Safety equipment

• Life raft (present, in date, capacity adequate), life jackets, fire extinguishers, flares, EPIRB, first aid kit, bilge pumps, and CE/design category compliance.

Valuation

The surveyor provides an opinion of the boat's fair market value based on its condition, age, equipment, and comparable sales. This is used for insurance and as a reference point for negotiation.

4. The sea trial

A sea trial is usually conducted as part of the survey (or immediately after). The boat is taken out on the water and operated under real conditions. The surveyor and the buyer both attend.

What is checked during a sea trial:

• Engine under load. Cold start, acceleration, cruising speed, maximum speed, running temperature, oil pressure, exhaust smoke. Issues that do not appear at idle often show up under load.
• Steering and handling. Does the boat track straight? Is there excessive play in the steering? Does the rudder feel balanced or heavy?
• Under sail (sailboats). Upwind and downwind performance, rigging tension, sail handling, helm balance.
• Vibration and noise. Unusual sounds at specific RPM ranges can indicate propeller, shaft, bearing, or engine mount issues.
• Systems under way. Autopilot, instruments, bilge pumps, and other systems are tested while the boat is moving.

5. The survey report

After the inspection and sea trial, the surveyor produces a written report — typically 15–40 pages. It includes:

• Vessel particulars. Make, model, year, HIN, dimensions, registration.
• Condition assessment. Section-by-section findings for hull, structure, engine, systems, rig, safety.
• Deficiencies list. Every issue found, categorised by severity:
  • Critical: Safety issues that must be fixed before the boat is used (e.g. failed seacock, gas leak, structurally compromised keel).
  • Major: Significant defects that require expensive repair (e.g. osmosis, worn rigging, engine overheating).
  • Minor: Cosmetic or routine maintenance items (e.g. faded gelcoat, worn upholstery, expired flares).
• Valuation. The surveyor's opinion of fair market value.
• Recommendations. Specific actions the surveyor recommends, with urgency.
• Photographs. Documentation of key findings.

6. What a survey costs

Survey costs are based on boat size and the scope of the survey. Typical ranges:

Add €50–€150 per engine for oil analysis, and €300–€800 for a separate engine survey by a marine engineer if the boat has high hours or a high-value engine. For a superyacht, the survey is a multi-day event with a team of specialists and can cost €20,000–€80,000.

7. How to choose a surveyor

• Independent. Do not use a surveyor recommended by the seller or the broker. Find your own. Independence is the whole point.
• Accredited. Look for membership in a professional body: IIMS (International Institute of Marine Surveyors), SAMS (Society of Accredited Marine Surveyors), or a national equivalent.
• Experienced on this type of boat. A surveyor who specialises in sailboats may not be the best choice for a motorboat, and vice versa. Ask about their experience with the specific type.
• Sample report. Ask to see a sample survey report so you know what level of detail to expect. A good report is thorough, specific, and uses photographs.
• Insurance. The surveyor should carry professional indemnity insurance.

8. How to use the survey report

Once you have the report, you have three options:

• Accept the boat as-is. If the survey is clean or the issues are minor, proceed with the purchase.
• Renegotiate. Use the deficiencies to negotiate a lower price. Be specific: "The surveyor found osmosis on the port quarter and recommends €4,000 of treatment — I'd like a €4,000 price reduction."
• Require repairs. Make the purchase conditional on the seller fixing critical or major deficiencies before completion. Get the repairs verified by the surveyor before you sign.
• Walk away. If the survey reveals serious structural or safety issues, walk. Your deposit should be refundable subject to survey — make sure this is in the purchase agreement.

The survey is not a pass/fail test. It is information. Use it to make an informed decision and to negotiate from a position of knowledge.

Ready to buy?

A survey is the best money you will spend in the boat-buying process. It turns a gamble into a decision. Find a boat you like, make an offer subject to survey, and hire an independent, accredited surveyor.

Start by browsing boats for sale on Owning and when you find the right one, make your offer conditional on a satisfactory survey.