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Why not Fibre glass?


Brian 65

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Hi, as yet I have not bought a boat, but I am looking. It may seem a strange question, but why doesn't anyone put a couple of layer of fibreglass over the complete hull instead of Blacking all the time? I use Chopped strand Matting for lots of applications and it adheres to metal very well. Yes you would have to take it back to bare metal, but wouldn't this stop any rusting Etc? Would you still need Anodes?

Yes I get that it would get damaged from time to time from scrapes etc, but so would blacking and surely you would have to only repair the damaged area and not have to recoat all the whole hull as with Blacking every so many years. Just something that I was thinking, but the more experienced of you will know. Marine standard fibre glass matting would surely work out cheaper in the long run. Just a thought.

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The fibreglass would get damaged in multiple places just from normal boating. This would give not only a break down to the metal, but delamination between the GRP and steel. Corrosion in a crevice tends to be worse than on a larger surface for weird electrochemistry reasons that I vaguely remember being taught about at Uni. When you have the boat out, are you sure you could find and fix every tiny bit of cracking and delamination? Easier to black. If you don't like blacking, then epoxy coatings are supposed to be good.

You would also get differential thermal expansion. Imagine the hull above water in direct sunshine on a summers day and the hull below the water. Fibreglass expands by around 18ppm/degree. Steel at about 12. These would be trying to shear the fibreglass from the steel.

 

Jen

Edited by Jen-in-Wellies
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Yes I agree with Jen. The problem is that if or when you crack the fibreglass you'd get water held between the fibreglass and the bare steel which you wouldn't be able to do anything about. That could happen below or above the waterline. 

 

With paint you can scrape it off below the waterline and water will be against a very small area of the steel where the paint has been removed. Above the waterline you can rub any scrapes back and touch them up. With a GRP coating you wouldn't even know which bit of the fibreglass has cracked and let water through.

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1 minute ago, Sir Nibble said:

You wouldn't need the glass, only the resin to achieve a barrier between water and steel. There are drawbacks to a rigid coating subject to damage as described above and a more flexible, malleable coating is better. Over the years, bitumen has been found to be a good choice. 

Sounds a bit like 2 pack epoxy 

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48 minutes ago, Alan de Enfield said:

 

Yes.

 

My boat is GRP and has several anodes.

I thought that anodes where to help with the metal hull deteriorating. Why would you need them on a fibreglass hull? You can learn something new every day.

 

49 minutes ago, Jen-in-Wellies said:

The fibreglass would get damaged in multiple places just from normal boating. This would give not only a break down to the metal, but delamination between the GRP and steel. Corrosion in a crevice tends to be worse than on a larger surface for weird electrochemistry reasons that I vaguely remember being taught about at Uni. When you have the boat out, are you sure you could find and fix every tiny bit of cracking and delamination? Easier to black. If you don't like blacking, then epoxy coatings are supposed to be good.

You would also get differential thermal expansion. Imagine the hull above water in direct sunshine on a summers day and the hull below the water. Fibreglass expands by around 18ppm/degree. Steel at about 12. These would be trying to shear the fibreglass from the steel.

 

Jen

Interesting. 

40 minutes ago, blackrose said:

Yes I agree with Jen. The problem is that if or when you crack the fibreglass you'd get water held between the fibreglass and the bare steel which you wouldn't be able to do anything about. That could happen below or above the waterline. 

 

With paint you can scrape it off below the waterline and water will be against a very small area of the steel where the paint has been removed. Above the waterline you can rub any scrapes back and touch them up. With a GRP coating you wouldn't even know which bit of the fibreglass has cracked and let water through.

I see.

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3 minutes ago, Brian 65 said:

I thought that anodes where to help with the metal hull deteriorating. Why would you need them on a fibreglass hull? You can learn something new every day.

There are still metal bits in the water. Propeller, drive shaft, rudder, bearings. These need protecting.

Just now, Brian 65 said:

Why is this not used as much?

Cost, hassle. The hull needs to be taken to base metal and the first coat is expensive to do.

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18 minutes ago, Brian 65 said:

I thought that anodes where to help with the metal hull deteriorating. Why would you need them on a fibreglass hull? You can learn something new every day.

 

Metal prop shafts, metal props, metalk thru-hull fittings etc etc. All need bonding and having anodic protection.

 

3 anodes protecting the drive and prop, & one on the other side of the rudder, anodes further forward protecting the engine cooling water inlet and exhaust hull fittings, the toilet water inlet and outlet, and near the bow, the No2 toilet water inlet and underwater pump out hull fittings.

 

 

20191017-134944.jpg

 

 

 

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Sheaving knackered old wooden seagoing wooden boats in GRP is quite common. It is a bit of a bodge. On steel canal boats its even worse.

If you must do it then epoxy is better than GRP, but that's very close to what we do.

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15 hours ago, Jen-in-Wellies said:

You beat me to it. Take out the glass from GRP and you end up with an epoxy coat, which is a known good choice.

Take out the glass and you're left with polyester, not epoxy resin.  Not the same thing at all!

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4 minutes ago, Chris G said:

Take out the glass and you're left with polyester, not epoxy resin.  Not the same thing at all!

<pedant>You are right. Boat building GRP is polyester. Some other uses of fibreglass do use epoxy as the matrix for the glass fibres. The ones I'm most familiar with are circuit boards.</pedant>

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10 hours ago, dmr said:

Sheaving knackered old wooden seagoing wooden boats in GRP is quite common. It is a bit of a bodge. On steel canal boats its even worse.

If you must do it then epoxy is better than GRP, but that's very close to what we do.

 

Sea going boats have a much easier live with considerably less 'impacts' than canal boats.

Canal boats will be 'bumping and scraping along banksides / steel armco / concrete walls several times a day (mooring up, waiting at locks, entering and leaving locks etc etc), whilst a GRP boat will (maybe) only be 'touching something' (pontoon ? via big soft fenders) once a day, and many times may even spend the night at anchor, or, on a swinging mooring.

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33 minutes ago, Chris G said:

Take out the glass and you're left with polyester, not epoxy resin.  Not the same thing at all!

The most commonly used fibreglass resin is polyester,used in the layup of most grp boats.It is not the best to use for repairs because it doesn't stick very well.

Some of the more expensive sailing boats are built using epoxy resin and aircraft too.

Epoxy sticks much better than polyester so is better to use for repairs.

It is though,rather more expensive.

Both resins,as far as I am aware,are subject to degredation due to sunlight,so need either gelcoat or paint for protection.

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