Is 4mm steel plate too thin?

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NealB

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A pal is considering making an offer on a steel boat built for inland waterways use.

She's made of 4mm plate.

There are obviously other factors that can affect boat longevity, but 4mm seems very thin to my very non-technical mind.

Any words of wisdom, please?
 
Most of the inland craft I have to cut holes in for heater exhausts are about 4mm, otherwise i would go bust buying hole cutters, mind you I refer to narrow boats and wide beam cruisers. I did do a couple of Stevens and they weren't any thicker from memory, all topsides of course so not an idea of below waterline thickness.
 
My 30 year old Dutch cruiser has 4mm plate. In a recent survey the surveyor could only find one area where the plate was down to 3.9mm and I reckon that is where I ground it down when cleaning off ready for some epoxy patching. 4mm is standard up to 35-40ft.
 
What kind of boat?

Many narrowboats have a bottom of 8-10mm, sides of 6-8mm, and a top cabin of 4mm, so 4mm for the hull seems thin. Some boats were built 20 years ago, or so, (Springers?), with thinner hulls, 3mm or 6mm??, and many survive.

Steel hulls can rust and pit and, over the years, this can cause holes, so they need a good survey, or a willingness to take a risk.
 
Depends entirely on the boat
Thinnest narrowboats I know of are springers with 3/16 plate or 4.8mm
Dutch barges are quite often 4mm but there is a huge amount of framing to keep them rigid
My NB is an exception 15mm base plate 6mm sides.
The real concern however is pitting a 1mm pit on 4mm is far more serious than a 1mm pit on 15mm
 
I think the important thing is how thick was the plating originally? how thick is it now? and how old is the boat. Then you can judge the rate of thinning.

I think narrowboats are so much thicker because there's usually very little framing. Plus my experience of the canals, they are so shallow in places, most narrowboats regularly go aground so the bottom paint doesn't last long so they are more likely to corrode.
 
ProDave said:
I think the important thing is how thick was the plating originally? how thick is it now? and how old is the boat. Then you can judge the rate of thinning.

I think narrowboats are so much thicker because there's usually very little framing. Plus my experience of the canals, they are so shallow in places, most narrowboats regularly go aground so the bottom paint doesn't last long so they are more likely to corrode.
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I think you've nailed it Dave!

Despite all the nonsense of peoples opinions of what is thin, the fact that 4mm is standard for most boats below 40ft is what the OP needs to be looking at.
If the boat is properly epoxy protected there won't be any pitting.
So it's all down to age, maintenance and condition.
 
Trinity Marinas first time buyers guide says
3) Steel plate:
Steel plate figures you will see will be for example 10/6/4 and relate to 10mm bottom plate, 6mm hull sides and 4mm cabin.
Nowadays standard specification is 10/6/4 but steel thickness of 8/6/4mm and 6/5/3mm – if well looked after – will provide you with many years of service, so they should not be ruled out.
Click to expand...
 
She might be made of 4mm but get her hauled out and have a ultrasound test done. Be careful it has to be an ultrasound machine that doesn't measure the piant/antifouling and the steel (which most do), just the steel. Find a surveyor who specialises in steel boats. Our Dutch built yacht was 4mm.
 
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Magnette said:
Depends entirely on the boat
Thinnest narrowboats I know of are springers with 3/16 plate or 4.8mm
Dutch barges are quite often 4mm but there is a huge amount of framing to keep them rigid
My NB is an exception 15mm base plate 6mm sides.
The real concern however is pitting a 1mm pit on 4mm is far more serious than a 1mm pit on 15mm
Click to expand...

No it's not, a 4mm pit on 4mm plate is far more serious than a 4mm pit on 15mm plate. Although people seem to have kittens about pitting, basically as long as the framing is adequate, isolated pittting is not a real problem. More problematical is pitting caused by electrolysis, and often along the water line/splash zones
 
Considering the OP's question and the fact that 10mm is recommended these days, the rate of pitting will be the same for two boats in the same location being used in the same way (all else being equal). So the point at which re-plating becomes essential will be much closer for the thinner-plated hull.
Why buy a secondhand boat that is definitely going to need some expensive work done in the near future when looking for a thicker hulled boat would put that moment off by 5 / 10 or more years?
I can't see what relevance the framing has. The frames aren't going to either corrode or let water in. They are only to do with structure.
 
4mm above 6mm below

I was told as a general rule of thumb when surveying steel sailing boats, that 6mm below the water line and 4mm above is normal.
 
The thinnest I've ever seen was a 1970s Springer narrowboat made from 1/8th of an inch (3.2mm) plate. It was still 1/8th of an inch thick nearly 40 years later when I surveyed it, with only very minor pitting at the wind and water line. If the new owner maintains the anodes and blacking properly, and keeps the bilges dry, then I see no reason why it should not continue to be 1/8th of an inch thick for many years to come.

4mm is quite a standard plate thickness for properly constructed steel boats (as opposed to narrowboats, which tend to use thicker hull plating as a structural element, with fewer frames and stringers than you'd find on a steel sailing yacht or motor cruiser).
 
KellysEye said:
She might be made of 4mm but get her hauled out and have a ultrasound test done. Be careful it has to be an ultrasound machine that doesn't measure the piant/antifouling and the steel (which most do), just the steel. Find a surveyor who specialises in steel boats. Our Dutch built yacht was 4mm.
Click to expand...

An ultrasound tester will work inside the boat while in the water, there will be lots of places that the OP won't be able to reach but a good idea of thickness can be had without lifting.
 
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