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Old 02-17-2015, 08:48 AM   #1
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Riveted alloy on top of GRP hull.

Hi all,

I love trawlers and workboats, but I find myself put off by modern GRP plastic reproductions. However lovely and accurate a GRP replica boat may look the question always remains 'Is that fibreglass?' Wood trim, SS fittings and period gear still do not answer the dilemma.

So I was thinking......

Why not trim a GRP hull in riveted alloy plate; it's a corrosion proof authentic material that's been used on smaller hulls for decades , so why not as a finish on bigger trawlers?

How would you attach it to the outside of glass fibre hull?


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Old 02-17-2015, 09:19 AM   #2
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Some boats do the opposite with an aluminium hull and a GRP deck or roof.

Look at Allures and Garcia.

They join the GRP to the aluminium hull (welded) mainly with adhesive, but it is one material or the other not the two together (apart from the join)

Riveted aluminium is tough to get completely watertight. If you you want to put this over GRP there is a risk of crevice corrosion underneath the aluminium.

Marine aluminium is very corrosion resistant, but it is reasonably susceptible to crevice corrosion. It would be very tough to completely prevent water between the aluminium and the GRP and any spots where this occurred would almost certainly get some crevice corrosion.

So aluminium on its own is fine, but over GRP not so good.
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Old 02-17-2015, 09:31 AM   #3
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Thanks for those links; I hadn't heard of those makes before.

Pretty...



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Old 02-17-2015, 09:39 AM   #4
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Originally Posted by Noelex View Post
Some boats do the opposite with an aluminium hull and a GRP deck or roof.

Look at Allures and Garcia.

They join the GRP to the aluminium hull (welded) mainly with adhesive, but it is one material or the other not the two together (apart from the join)

Riveted aluminium is tough to get completely watertight. If you you want to put this over GRP there is a risk of crevice corrosion underneath the aluminium.

Marine aluminium is very corrosion resistant, but it is reasonably susceptible to crevice corrosion. It would be very tough to completely prevent water between the aluminium and the GRP and any spots where this occurred would almost certainly get some crevice corrosion.

So aluminium on its own is fine, but over GRP not so good.

The problem with an alloy hull is the cost; more than twice the cost of steel or GRP. welding alloy is only a job for a professional coded welder, and repairing it is a nightmare because of the problem of getting the area to be welded clean enough to stop stress cracking.

My thinking goes like this:
wooden boats are beautiful, but impossible to maintain.
Steel is too heavy for a s/d hull.
GRP is perfect but makes a horrid finish; would you live in a plastic house?

So my search is for a authentic finish for GRP boat that does not make it into a replica......
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Old 02-17-2015, 09:50 AM   #5
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Yes Garcia especially make some nice boats including a nice trawler.

We visited the factory recently and this is a photo I took.

This is one of the Allures yachts being built. These boats have an aluminium hull and all the deck structure is all GRP.

Here is the aluminum hull and it shows some of the deck / hull join. They glue and bolt plywood to aluminium then glue bolt and screw the deck on top.

I prefer an all aluminium structure, which is how some of the Garcia boats are constructed.
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Old 02-17-2015, 09:56 AM   #6
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Originally Posted by Rustybarge View Post

My thinking goes like this:
wooden boats are beautiful, but impossible to maintain.
Steel is too heavy for a s/d hull.
GRP is perfect but makes a horrid finish; would you live in a plastic house?


Cold molded timber with a clear finish on the hull looks nice with almost no maintence. It is really an expoxy fiberglass boat, but it is the timber that is seen.

Just a thought.

Stress cracking on displacment aluminium boats does not occur. It is only high speed ferries and the like that have this problem. They need to be built with a very thin skin.
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Old 02-17-2015, 11:09 AM   #7
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Yes Garcia especially make some nice boats including a nice trawler.

We visited the factory recently and this is a photo I took.

This is one of the Allures yachts being built. These boats have an aluminium hull and all the deck structure is all GRP.

Here is the aluminum hull and it shows some of the deck / hull join. They glue and bolt plywood to aluminium then glue bolt and screw the deck on top.

I prefer an all aluminium structure, which is how some of the Garcia boats are constructed.
To my eye it's a bit top heavy; reminds me a bit of the Swift trawler 55...
Nice nonetheless.


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Old 02-17-2015, 11:15 AM   #8
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Cold molded timber with a clear finish on the hull looks nice with almost no maintence. It is really an expoxy fiberglass boat, but it is the timber that is seen.

Just a thought.

Stress cracking on displacment aluminium boats does not occur. It is only high speed ferries and the like that have this problem. They need to be built with a very thin skin.
Spiritboats in the uk makes a strip planked boat with epoxy/mat finish, they are truly beatiful boats.

But...
With a 3-4mm of GRP on top of the plywood/strip plank are they a wooden boat or a plastic boat?

This is the dilemma of materials choice: Which is nicer concrete or stone
Wood or GRP!

Some of the finest French furniture is made from veneered timber; so maybe the outside finish is the most important bit; not the structure.
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Old 02-17-2015, 11:26 AM   #9
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It would be difficult to keep polished, but on a boat a painted finish can look good with the rivet heads and joints adding to the overall patina...

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Old 02-17-2015, 01:42 PM   #10
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You call that shiny?
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Old 02-17-2015, 01:47 PM   #11
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Aluminum forms that grey patina to protect itself. It corrodes to a uniform (not on my boat, sadly) grey, then the corrosion stops. If you polish off the grey, it will just keep deteriorating. On riveted things like airplane's that are already thin, the skin gets thinner and the rivet heads do too until they fail. I polished a DC3 once, when I was a copilot. A polisher, Neverdull and bronze wool. Worse than cleaning toilets.
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Old 02-17-2015, 01:56 PM   #12
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The problem with an alloy hull is the cost; more than twice the cost of steel or GRP. welding alloy is only a job for a professional coded welder, and repairing it is a nightmare because of the problem of getting the area to be welded clean enough to stop stress cracking.

My thinking goes like this:
wooden boats are beautiful, but impossible to maintain.
Steel is too heavy for a s/d hull.
GRP is perfect but makes a horrid finish; would you live in a plastic house?

So my search is for a authentic finish for GRP boat that does not make it into a replica......
Wood epoxy glass composite in strip or ply is the answer. Not hard to build strong and can be light. My boat is built in ply glass composite and there are many thousands out on the water. If the technique is used and built poorly the results can be poor, but properly done it has great merit. the external surfaces are all glass epoxy with two part paint very low maintenance. There is very little maintenance needed other than proper care in sealing holes made in structure.
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Old 02-17-2015, 02:03 PM   #13
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Aluminum forms that grey patina to protect itself. It corrodes to a uniform (not on my boat, sadly) grey, then the corrosion stops. If you polish off the grey, it will just keep deteriorating. On riveted things like airplane's that are already thin, the skin gets thinner and the rivet heads do too until they fail. I polished a DC3 once, when I was a copilot. A polisher, Neverdull and bronze wool. Worse than cleaning toilets.
Grey alloy workboats don't really hit the spot but painted riveted alloy can look beautiful.

If you've ever seen a newly painted light aircraft they look amazing: the rivets give structure to the 'look' and make a much more exciting finish than a smooth sleek structure.

I suppose it's a bit like 'high tech' buildings that show off the structure as a feature.
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Old 02-17-2015, 02:07 PM   #14
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Wood epoxy glass composite in strip or ply is the answer. Not hard to build strong and can be light. My boat is built in ply glass composite and there are many thousands out on the water. If the technique is used and built poorly the results can be poor, but properly done it has great merit. the external surfaces are all glass epoxy with two part paint very low maintenance. There is very little maintenance needed other than proper care in sealing holes made in structure.
I've built a ply dinghy, it looked good but the epoxy scraped off easily when dragging it up the ramp.

On a big boat like your's how does the build cost of the hull compare to GRP or alloy?
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Old 02-17-2015, 02:10 PM   #15
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I've built a ply dinghy, it looked good but the epoxy scraped off easily when dragging it up the ramp.

On a big boat like your's how does the build cost of the hull compare to GRP or alloy?
My guess is same a glass when done one off. Production glass would be cheaper but that is apples to oranges. If your epoxy glass came off then not done right. There are now many 30-40 year old boats out there that have worn quite well from dinghy to cruising boats.
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Old 02-17-2015, 02:42 PM   #16
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My guess is same a glass when done one off. Production glass would be cheaper but that is apples to oranges. If your epoxy glass came off then not done right. There are now many 30-40 year old boats out there that have worn quite well from dinghy to cruising boats.
I heard tales of problems with 'ply and balsa sandwich' in the decks of some 70's and 80's trawlers.

Does epoxy solve the problem of water absorption into the ply?
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Old 02-17-2015, 04:27 PM   #17
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I heard tales of problems with 'ply and balsa sandwich' in the decks of some 70's and 80's trawlers.

Does epoxy solve the problem of water absorption into the ply?
It solves the problem if done right if done wrong bad news. The same bad news can be said for steel or aluminum or glass that is poorly done. There is no material that can make up for bad application. Steel can rust and welds fail panels can warp. Aluminum welding is even more critical than steel. Glass done poorly can delaminate or blister. For each material there are horror stories. Wood particularly ply got a bad rap because many were DIY with bad products no experience or little knowledge. Someone who knows what they are doing gets good results. The small shop that did my boat has been doing it for 30-40 years with hundreds of boats out there and 30 year old boats have held up very well and look great.
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Old 02-17-2015, 06:57 PM   #18
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It solves the problem if done right if done wrong bad news. The same bad news can be said for steel or aluminum or glass that is poorly done. There is no material that can make up for bad application. Steel can rust and welds fail panels can warp. Aluminum welding is even more critical than steel. Glass done poorly can delaminate or blister. For each material there are horror stories. Wood particularly ply got a bad rap because many were DIY with bad products no experience or little knowledge. Someone who knows what they are doing gets good results. The small shop that did my boat has been doing it for 30-40 years with hundreds of boats out there and 30 year old boats have held up very well and look great.
I'm totally amazed that your hull is stitch and glue; I've had two boats constructed with this method, a Mirror sailing dinghy 10' and a Cowes punt of 12'!

I have to say it's a very efficient way of construction that is surprisingly quick and gives very accurate results; it's just that I've never considered it relevant to big boats.

Some drawbacks that I see is the high cost of marine ply, and the expense of epoxy; how many tons were used to glue your boat?

Plus of course the labour element in producing one offs; but that's a bonus for home builds.

There is little doubt that epoxy is far superior to polyester resins, even if it does cost many times more; it should last for many many decades!
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Old 02-17-2015, 07:27 PM   #19
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Greetings,
Mr. R. What boat is that a picture of in your initial post and is there a shot of the whole boat?
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Old 02-17-2015, 08:29 PM   #20
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Greetings,
Mr. R. What boat is that a picture of in your initial post and is there a shot of the whole boat?
Not sure if the whole boat is on here; but some other beautiful clasdic boats....
https://www.pinterest.com/kustombuil...s-boats-boats/
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