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Narrowboat with GRP roof: Leakproofing and attaching new handrails


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Hello Folks,

 

My partner, Isobel and I purchased our first narrowboat last July (2015)

 

Its a proper 'project' boat, which we spent a month fixing things like the engine, and fitting in insulation, new T&G cladding on the walls etc before embarking on an epic journey from London to Bath, where we are now moored in a marina and have just survived our first winter aboard.

 

Now thats its spring at last, we want to start repair works again, but could use some advice!

 

The boat is a 48' cruiser stern narrowboat, built in 1975. Steel hull and a GRP fibreglass roof. We have learnt now what a royal pain it is to have a fibreglass roof, and have spent a large part of the winter catching leaks which have been coming through under the gunnels :(

 

We applied sealant all along the joint outside the gunnel, but have found we probably missed making a full seal and water has been getting in. We will be scraping out and redoing this this spring.

 

Also, the handrails are snapped and broken. They were wooden and not looked after, and now need fully replacing. We were hoping for advice about doing this, and generally in ensuring the GRP roof is totally TOTALLY TOTALLY waterproof (please!) I've seen a few people covering their roof with products like flow coat and a product called rubbaroof. Does anyone have experience of applying this and can recommend whats best and how to do it?

 

Also, we're debating whether to reuse the existing holes drilled in the roof (which are currently sealed over with Sikaflex) and make wooden handrails, or if after sealing over the whole roof anew, we can just drill new holes, perhaps adding metal handrails instead such as these:

 

http://www.ebay.co.uk/itm/Pair-1800mm-316-Stainless-Steel-Boat-Grab-Rails-Handles-/290374973664

 

Any thoughts and advice would be much appreciated, we would love to get the boat waterproof, looking great and properly shipshape before next winter!

 

Thanks a lot for any help : )

 

Laurence and Isobel

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I have had similar problems over the years.

 

Leaks - I have used a polyurethane sealant/adhesive along the gunwhale/cabin top joint, after first running an angle grinder along the gap to make sure there is a reasonable amount of sealant in place. (Looks like a welded joint now!) Don't use silicone, you can't paint over it.

 

Many of my leaks came from the window to GRP sealant failing. This led me to removing the windows, cleaning off the old sealant and replacing it with a non-setting butyl sealant as used to be used in the caravan industry. It had been suggested that I used polyurethane sealant for this, but, if needing to remove the windows at any future date would have proved difficult, to say the least. The windows themselves created leaks due to the tumbleholme of the cabin sides and the twin sliding window tracks, so ensure the drain holes are clear, glue in place a little 'wick' of string or similar to encourage sitting rainwater to run out.

 

Handrails - I had considered using something similar to the posh rails from LWB Renault vans LIKE THESE or Vito or VW T5, but my rear handrails are 11ft long so I will be making my own in hardwood. They will be screwed in place with a suitable spacer between the top skins and seated on Sikaflex and treated with wood oil rather than varnish.

 

Hope some of that helps ......oh and welcome to the forum!!

Edited by OldGoldy
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It is very difficult (nay, almost impossible) to get a waterproof seal between a steel hull and a GRP / Wooded cabin top.

 

You may get a seal for a short time but the differential expansion on a hot / cold day means that the seal is soon broken.

The most successful way is to weld a 2" angle iron all along the gunwhale and tight up against the cabin side. Grind back the cabin side to get a rough surface, fibreglass over the angle-iron and cabin side creating an overlap, Paint and jobs a good-un.

 

Any movement means that there is still a 'skirt' of fibreglass overlapping the angle iron.

 

Not a 'small job' but one you only need to do once.

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There only a few products on the market that are a liquid rubber which would resolve to issues.I would recommend these two, first is Stirling Lloyds eliminator system which I have used before. and a product called C.I.M. both are a liquid rubber system,. you also need to find out if you can paint over them. Eliminator is a cream colour and C.I.M. is black. The most important thing when applying any waterproofing system is to apply a scrim over the problem area or cracks otherwise due to expansion it can re open. there are some other products that can be purchased at a good roofing suppliers but I have never used them buy it is worth making an enquiry.you must check that they are UV resistant.

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There was someone on here a year or two ago writing up the refit of a fibreglass cabined boat, but I can't now find the thread. As I recall they lifted the cabin up, derusted and painted the flange to which it is bolted, and then refitted it with suitable sealant in between, a steel strip along the outside of the cabin and much more closely spaced bolts to hold the lot together.

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It is very difficult (nay, almost impossible) to get a waterproof seal between a steel hull and a GRP / Wooded cabin top.

 

You may get a seal for a short time but the differential expansion on a hot / cold day means that the seal is soon broken.

The most successful way is to weld a 2" angle iron all along the gunwhale and tight up against the cabin side. Grind back the cabin side to get a rough surface, fibreglass over the angle-iron and cabin side creating an overlap, Paint and jobs a good-un.

 

Any movement means that there is still a 'skirt' of fibreglass overlapping the angle iron.

 

Not a 'small job' but one you only need to do once.

 

 

Very good advice here from Alan.

 

It would be a really big job , but you will only ever need to do it once.

 

Just a word about silicone sealant :

 

Once you contaminate a surface with silicone sealant you can never paint over it again , ever.

 

A better sealant for a moving joint with a large expansion differential is Butyl Mastic.

It never really cures as such , but skins over on the surface so you can paint it , but remains soft underneath , indefinately.

 

Stay away from silicone or Sikaflex / 3M 5200 type of permanent sealant / adhesives as you cannot remove them after , without much effort and grief.

I have been there and done that before , repairing a previous owner`s " efforts ".

 

Good luck .

Edited by boat fan
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Hello everyone

 

Thank you so much for all your advice! Its so nice to not be totally alone in our wonderings!

 

Welding plates to the side sounds like a good idea, but something we're unlikely to get done anytime soon. Will have a look at the suggested products and see if they could work for us.

 

Thanks again!

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  • 2 years later...

Hi there

 

i am helping a friend paint their GRP narrowboat and trying to choose sealant, especially for the GRP/steel join. I have found an Everbuild Butyl Mastic but the product details say not to be used on plastic. Does this mean dont use it on GRP? I can call them but last time i didnt feel that confident in the person i spoke to. I have also found a product called GEOCEL GEOBOND HM CARAVAN & MOTORHOMES 290ML WHITE - when i spoke to sealants online this is the product they recommended for the job - used for sealing windows, vents etc, usually on caravans, which i guess is GRP to metal.They do a Tremco Butyl sealant but the spec does not mention GRP. I wonder if anybody knows a specific product for the job if welding a new flange to the boat is not the preferred option. Many thanks

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11 hours ago, Janey72 said:

Hi there

 

i am helping a friend paint their GRP narrowboat and trying to choose sealant, especially for the GRP/steel join. I have found an Everbuild Butyl Mastic but the product details say not to be used on plastic. Does this mean dont use it on GRP? I can call them but last time i didnt feel that confident in the person i spoke to. I have also found a product called GEOCEL GEOBOND HM CARAVAN & MOTORHOMES 290ML WHITE - when i spoke to sealants online this is the product they recommended for the job - used for sealing windows, vents etc, usually on caravans, which i guess is GRP to metal.They do a Tremco Butyl sealant but the spec does not mention GRP. I wonder if anybody knows a specific product for the job if welding a new flange to the boat is not the preferred option. Many thanks

Sorry to be 'doom & gloom' but it really doesn't matter what you use, it will only seal for a very short time.

 

The thermal expansion of steel and GRP is very different and will just 'tear' the 'mastic' seal - particularly in the current weather where the temperatures are so extreme.

A boat roof could easily be over 40 C in the direct sun.

 

There will also some effect in cold weather with 'shrinkage'.

 

Pipes - thermal expansion diagram - celsius

 

 

See post #3 for a solution that will not need replacing every month or two.

Edited by Alan de Enfield
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7 minutes ago, Janey72 said:

Thank you - dont worry about doom and gloom - it is good to know - thank you, thanks for the advice

Just a thought - is this a GRP narrowboat, or a steel hull narrowboat with a GRP 'top' ?

Are you talking about sealing metal framed windows into a GRP 'top' ?

 

If the latter use a rubber channel (like on car windscreens)

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On 21/04/2016 at 16:02, Alan de Enfield said:

You may get a seal for a short time but the differential expansion on a hot / cold day means that the seal is soon broken.

The most successful way is to weld a 2" angle iron all along the gunwhale and tight up against the cabin side. Grind back the cabin side to get a rough surface, fibreglass over the angle-iron and cabin side creating an overlap, Paint and jobs a good-un.

 

Any movement means that there is still a 'skirt' of fibreglass overlapping the angle iron.

 

You have confused me.

 

The usual original construction of boats like this already has something like a 2" upstand at the back of the gunwales that the GRP cabin is secured to.  That GRP cabin already forms a big overlap over that angle.

Why will replicating the whole thing again in the way you suggest make it leak free, if the current arrangement is not?

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2 hours ago, Alan de Enfield said:

The thermal expansion of steel and GRP is very different and will just 'tear' the 'mastic' seal...

Looking at that graph you posted it appears that the difference in thermal expansion of steel vs grp is about 25mm/100m. So isn’t that about 4mm on the length of a 57’ Boat?  Hardly enough to cause huge problems, surely?

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13 minutes ago, alan_fincher said:

 

You have confused me.

 

The usual original construction of boats like this already has something like a 2" upstand at the back of the gunwales that the GRP cabin is secured to.  That GRP cabin already forms a big overlap over that angle.

Why will replicating the whole thing again in the way you suggest make it leak free, if the current arrangement is not?

The ones I have been involved in did not have any upstand, the fibreglass was simply bonded onto the top of plywood and folded onto the gunwhale. This allowed water to creep under the fibreglass and into the ply. Adding an upstand will not stop the water creeping under the fibreglass but will stop it reaching the ply - similar to a door-lip.

 

10 minutes ago, WotEver said:

Looking at that graph you posted it appears that the difference in thermal expansion of steel vs grp is about 25mm/100m. So isn’t that about 4mm on the length of a 57’ Boat?  Hardly enough to cause huge problems, surely?

If the steel Hull is in the water at (say) 15c and the top (in the Sun) is at 40c then the expansion is 275mm over 100m, or 68mm over 20 metres.

 

2 1/2 inches is quite a difference.

Edited by Alan de Enfield
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Hi this boat has a steel flange - yet to be explored in depth - the gap is packed full of several types of gunk and the flange appears deteriorated in places, hopefully not too badly - i was wondering about that myself - if we repair the grp overlap where it is deteriorated it may not need to be sealed? 

Thanks

Overall, for sesling around windows (wooden ones), hand rails, ariels etc woukd folks go sikaflex or marine flex or either? Thanks

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20 minutes ago, Alan de Enfield said:

If the steel Hull is in the water at (say) 15c and the top (in the Sun) is at 40c then the expansion is 275mm over 100m, or 68mm over 20 metres.

 

2 1/2 inches is quite a difference.

 

Do you really think the cabin is going to extend by 2 1/2 inches more than the hull??

 

I think you've got your numbers wrong!

 

According to that graph, a difference in temperature of 25 C results in a difference in length of about 25mm over a 100m length. This is a 48 ft cruiser stern boat , so the cabin length is probably not much more than 30 ft - say 10m, and so the difference in length will be 2.5mm.

 

If the cabin is sealed to the hull with a temperature difference of less than your 25 C, but non zero, then in hot daytime weather the cabin will expand more than the hull, but at night it will contract more than the hull, so the total amount of 'stretch' on the sealant will be between say plus 1.5mm and minus 1.5mm. And if the middle of the cabin stays put relative to the hull, this will manifest itself as a +/- 0.75mm movement at each end of the cabin.  A decent flexible sealant should be able to cope with that.

 

16 minutes ago, Janey72 said:

Hi this boat has a steel flange - yet to be explored in depth - the gap is packed full of several types of gunk and the flange appears deteriorated in places, hopefully not too badly - i was wondering about that myself - if we repair the grp overlap where it is deteriorated it may not need to be sealed? 

Thanks

Overall, for sesling around windows (wooden ones), hand rails, ariels etc woukd folks go sikaflex or marine flex or either? Thanks

 

There was a post on here a year or two back with pictures of where someone had completely unbolted the fibreglass cabin, derusted and repainted the gunwales and flanges to which the cabin was attached, and then reassembled with new sealant.   This is only really practical if you are completely refitting the interior.

  • Greenie 1
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3 hours ago, Alan de Enfield said:

If the steel Hull is in the water at (say) 15c and the top (in the Sun) is at 40c then the expansion is 275mm over 100m, or 68mm over 20 metres.

 

2 1/2 inches is quite a difference.

You’ve misplaced a decimal point. Your numbers are out by a factor of 10. 

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