Select Page

It is quite common to find cracks in the gelcoat when inspecting the deck and superstructure of a GRP boat. A trained eye will be able to inspect these and know whether they are purely cosmetic or of structural concern.

It is important to differentiate between a gelcoat crack and a scratch. Scratches in a gelcoat may be unsightly but these are surface blemishes only. Cracks on the other hand go right through the gelcoat and are an indicator that there has been some level of structural failure beneath. How do you tell the difference if the answer is not obvious? See this table:

Cracks Scratches
Are always narrow Variable width
Go through the gelcoat Are shallow, do not penetrate the gelcoat
Bottom of crack can’t be seen Bottom of the scratch can be seen
Sharp edges Ragged edges
Can run in parallel lines Hardly ever run in parallel lines
Can form starburst patterns Never form starburst patterns
Can branch and spread out Do not branch or spread out
Do not crisscross Can crisscross
Seldom single Can be single

Stress crack GRP boat 1    Stress crack GRP boat    Stress crack GRP boat 1

Causes of stress cracks

The main causes of gelcoat crazing are due to stress and movement. Gelcoat is hard and brittle while by comparison the glass fibre laminate beneath is softer and more flexible. So if the laminate bends or moves, the gelcoat won’t bend with it and cracks as it is brittle.

There are two main types of stress crack – linear cracks and star cracks. Linear cracks tend to run in parallel lines and star cracks spread out in a starburst type pattern. The patterns of cracks are a clue to what caused them:

  • Star cracks – star cracks are caused by external impact, with the central area of the crack being the point of impact and concentric circles spreading outwards, like a spider’s web. They are also caused by flexing around a point, for example as seen around a stanchion base which has been leant on or pulled heavily by crew members getting aboard.
  • Linear cracks – linear cracks indicate damage caused by bending or flexing. These occur for example along deck edges, where the deck meets the cabin sides, where the moulding curves upwards away from the cockpit area to the deck, or around the edges of cockpit lockers.

mast deck mount deck crack

Stress crack repairs

The underlying cause of stress cracks need to be identified before a repair is carried out, because if there is a structural problem beneath the deck this will need to be repaired first. This may be a question of fitting backing plates or stiffening an area with additional glass fibre laminates. Check the integrity of the laminate beneath the affected area and see if it is watertight.

Taking advice on how best to proceed at this stage would be wise, as it will depend on your specific circumstances.

Thankfully, most deck stress cracks can be repaired fairly easily, providing the underlying laminate is intact. Here are the steps involved:

  1. Remove any deck hardware necessary to gain access to the whole length of the crack (or cracks).
  2. Grind out the cracks using a small, high speed rotary tool. An alternative is to use the corner of a good quality scraper to open the cracks up, chamfering both edges in the process.
  3. Clean the cracks and repair area with acetone. Then mask the repair area off prior to filling. This will prevent the surrounding gelcoat and fittings from being damaged when sanding the repair.
  4. Decide whether you need to tint the filler to match your gelcoat, using gelcoat pigments. This will require some practice to get a good result. See page xx: How to colour match gelcoat.
  5. Fill the exposed cracks with gelcoat filler using a plastic spreader, making sure the cracks are completely filled.
  6. Leave the filler to harden off. When fully cured, sand the filler with a 240 grit wet and dry paper, used wet.
  7. Check the filler repair is flush and refill if necessary. Then rub back and smooth off with a finer 600 grit wet and dry paper.
  8. When the gelcoat surface is flush, remove the masking tape and apply rubbing compound to give a gloss finish to the repair, which can then be polished with a wax polish.

Boat gas system maintenance

There are correct types of hose for marine plumbing, sewerage, exhaust, cooling and gas and all hoses should be checked regularly for wear and deterioration.

Understanding your boat’s compass

Article submitted by Mike Rossiter, Certificated Compass Adjuster. Since the magnetic compass was first used by the Chinese...

Seasickness – how can you prevent it?

Seasickness is a common problem at sea and affects both seasoned sailors and novices. What are the causes and symptoms of seasickness?...

Antifouling for leisure boats – Part 4

Applying antifouling. Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.

Boat maintenance below decks

While most interior maintenance work can be done when a boat is afloat, some jobs such as servicing the seacocks have to be done ashore. It makes sense to do any major interior repairs and improvements with the boat hauled out in the boatyard.

Rudders and steering systems – Part 3

In the third of our three blog articles on rudders and steering systems, we look at how to replace rudder bearings and repair a water-saturated core.

Keel maintenance and Repair – Part 2

If you have ever witnessed a boat colliding with a rock or other submerged obstacle you will know that there is an almighty thump and the whole boat shakes and judders. While such hard groundings seldom result in catastrophic keel failure, something has to give and even the sturdiest keels can easily be damaged by such an impact.

Boating Etiquette: Do’s and Don’ts on the Water

Boating etiquette is essential for ensuring the safety, enjoyment, and harmony of everyone on the water. Whether you’re a seasoned boater or new to the boating community, it’s important to be aware of the do’s and don’ts of boating etiquette. Take a look at our guidelines:

Boat decks and superstructure

The deck of a boat is constantly exposed to the elements and should be inspected on an annual basis. Particular attention needs to be given to the overall condition of deck fittings such as the stanchions, cleats and chainplates.

First aid at sea – four common emergencies

In this blog we look at what to do in the event of a crew member choking, drowning, or suffering from hypothermia or fatigue. Knowing how to cope with them could well save a life, while not knowing could result in an avoidable tragedy.

Light characteristics – how do navigators identify lights at night?

How do navigators identify the different types of light around our coasts at night and what are their characteristics?Navigating at...

Fire safety advice at sea from the Marine & Coastguard Agency

Fire safety advice for boaters Top fire safety advice at sea: 1. Fit smoke alarms, carbon monoxide and gas detectors 2. Turn...

ColRegs – avoiding collisions at sea

ColRegs - avoiding collisions at sea ColRegs Rule 8: Action to avoid collision (a) Any action taken to avoid collision shall...

First Aid Afloat – fish spine injury

First Aid Afloat - Here is what to do if somebody stands on a fish spine: • Check for dangers. Is it safe for you to enter...

Boat interior varnishing

Most boat interiors have a combination of varnished and painted surfaces including solid wooden joinery, plywood laminates with thin hardwood veneers and glass reinforced plastic. When making your assessment of what you are going to do, bear in mind that the varnishing process consumes a lot of time, especially if the existing surfaces are in poor shape.

Navigation safety: a quick-reference mobile app to learn the ColRegs NavLights and Shapes

 Safety at SeaSafety at sea will always remains a topical and important subject that will no doubt dominate the syllabuses of nautical...

The Boatyard Book – a boat owner’s guide to yacht maintenance, repair and refitting

The Boatyard Book is a fully illustrated 224 page practical reference manual that provides advice for boat owners on planning and carrying out annual maintenance, repairs, upgrades and refits of sailing yachts and motorboats, up to 20 metres in length.

Tacking a sailing boat

Tacking is the sailing manoeuvre used to change a boat's direction through an oncoming wind. Tacking a sailing boat calls...

The Round The Island Race 2023

This year’s Round the Island Race turned out to be a really thrilling and competitive event. 1100 boats entered the race, which took place on Saturday 1st July. The race started from the world famous Royal Yacht Squadron line in Cowes UK, with the largest and fastest boats starting first, leaving the rest of us to wait our turn.

How to operate a winch

Winches are drum shaped mechanical devices used to handle halyards, sheets and control lines. One of the important crew...

Boat Improvements

My Boat - practical improvements Author - Mike Rossiter Most boat owners who have had their craft for any length of time will have made what they...

Sailing at the touch of a button

Easier and more controlled sail handling can also be achieved by powering up a furling mast. I came across some interesting solutions at the Southampton Boat Show this week on the Selden Mast stand, where they were running demos of their E40i electric winch and SMF furling system.

Boat Security: Protecting your vessel from theft & vandalism

Boats are susceptible to theft and vandalism. Protecting your vessel from these risks requires a proactive approach to boat security. In this comprehensive guide, we’ll explore the various threats to boat security and provide practical strategies and technologies to safeguard your investment and enjoy peace of mind on the water.

Dripless shaft seals

Dripless shaft seals are designed to completely stop water from entering a boat’s hull via the stern tube. There are two main types of dripless seals known as face seals and lip seals which many boat manufacturers now fit to production boats.

Seized fixings and fastenings

Maintaining a boat can be a rewarding experience but at times it can also be frustrating. A prime example of this is when you come across a seized fixing or fastening that refuses to budge. Read our tips on how to release and fix them: