How To Preserve Hull Integrity In Fiberglass Boats
Fibreglass is viewed by many as a superior material when it comes to boats, but it needs to be carefully maintained.
Fibreglass is an amazing material. It can be moulded into far more complex shapes than can be achieved with aluminium. It’s also easy to repair, and correctly supported on a trailer and kept away from direct sunlight, a well-built fibreglass boat will last upwards of 40 years. My 1981 3.8m Fairlite Gull sailing dinghy, for example, is as structurally sound now as she was when built. There are plenty of Haines Hunter hulls (such as the brilliant 445R) from the eighties that are still in good condition — built by a company that knew how to build fibreglass trailer boats!
Gel coats (the thin cosmetic layer outside of the fibreglass matting that provides a smooth finish) can be affected by prolonged exposure to dampness, known as osmosis. Gel coats are not impervious to water entry, as can be seen in older yachts and cruisers kept permanently afloat — where insufficient curing of the fibreglass laminate in the hull can lead to a pockmarked finish needing sanding and re-filling.
Before I kept my Gull under cover, I attached a plastic tarpaulin over her to keep out leaves. However, the tarp was kept tightly pulled over the hull topsides and retained moisture after rain that the gel coat absorbed, causing it to begin to separate from the fibreglass laminate on either side. The damaged gel coat was above the waterline, but I never thought that a damp plastic tarp could do this.
Another point to consider when re-powering your fibreglass trailer boats is to check the transom integrity. Some older fibreglass boats had construction ply filling between the external and inner transom to handle the thrust of an outboard. Construction ply can last the life of a boat if fully encapsulated to prevent water entry — I've used it with great success in small boats — but the trouble begins when holes are drilled through the transom to bolt on an outboard. Most times, the holes aren’t lined with waterproof sealant or an anti-rot two-pot mix such as Everdure before bolting on the outboard. This means that over time, water gets past the washers and seeps into the ply which can't dry out and starts rotting, structurally weakening the transom. In the case of rot, the entire transom timber work must be removed and new timber (preferably marine plywood) installed before flow coating the inside of the transom. I recommend using sealant or Everdure in the bolt holes to ensure the vulnerable edge of plywood is sealed before bolting on the outboard.
The rot issue also applies to seats bolted down through the internal flow-coated plywood floor. Reputable manufacturers such as Lewis Ski Boats use rot-proof plywood for floors, transoms and even engine mounts that has a 40-year guarantee against de-laminating, but some older manufacturers used construction ply for flooring that was never designed for a marine environment. If you have to remove the flooring, I'd check the timber stringers that again when encapsulated should last the hull life. If this encapsulation is damaged, rot may occur from the small amount of water that always seems to get into underfloor buoyancy tanks in fibreglass boats.
Older fibreglass hulls should not be over-powered. Most boat manufacturers stipulated the maximum recommended power on a small plate affixed to the inside of the transom and this figure should never be exceeded. If you have a hull from the early eighties, remember it's likely this is flywheel rating and not prop rating, so if a hull such as the 445R states 90 HP maximum then no more than a prop-rated 80 should be fitted. In my opinion, however, a 70 is more than adequate and will give better than 35kts which is quick in a hull of this length.
Finally, if considering fitting a new four stroke outboard that's heavier than the carbie two stroke it replaces, then my suggestion is — don’t. Not only will the heavier engine stress the transom, but it will impose more load on the trailer frame aft of the axle which can, over time, bend the frame.
With a little TLC, a fibreglass hull can provide years of pleasure and a ride in seaway that a tinny can never equal!