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Old 04-10-2016, 20:11   #31
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Re: thinkness of hull steel

With metal vessels one of the considerations is loss of plate thickness due to corrosion.

This is a long term issue, on the order of 20yr or so, but if you are going to build a steel boat, do you want it to last 30y or 60? Consequently plate thickness is often up sized for this reason.


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.
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A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
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Old 05-10-2016, 00:50   #32
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Re: thinkness of hull steel

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Originally Posted by Adelie View Post
With metal vessels one of the considerations is loss of plate thickness due to corrosion.

This is a long term issue, on the order of 20yr or so, but if you are going to build a steel boat, do you want it to last 30y or 60? Consequently plate thickness is often up sized for this reason.
.
And this is sound engineering practice if done as part of design.

A home builder second guessing the designer without doing new calculations can create unexpected buoyancy issues.
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Old 05-10-2016, 04:37   #33
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Re: thinkness of hull steel

Reacting on Tkeithlu, you are talking about a test electrode. What material is it because differennt materials show of course different voltages.
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Old 05-10-2016, 04:41   #34
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Re: thinkness of hull steel

I forgot Theithlu, are you iin fresh or in salt water?
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Old 05-10-2016, 05:13   #35
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Re: thinkness of hull steel

[QUOTE=Adelie;2227654]With metal vessels one of the considerations is loss of plate thickness due to corrosion.

This is a long term issue, on the order of 20yr or so, but if you are going to build a steel boat, do you want it to last 30y or 60? Consequently plate thickness is often up sized for this reason.


This used to be the case but with properly applied epoxy coatings on a thoroughly sandblasted hull INSIDE and out (inside being the real problem area) corrosion is only a problem if you damage the coating.I expect to get another 25 years from my recently blasted and repoxied hull. The first epoxy coating was beginning to break down so has been replaced. The hull has lost no steel to external corrosion.

The corrosion that I have had and have repaired/replaced has ALL been internal and only shows on a sonogauge if you hit the right spot. 95% or more of the hull is still its original thickness
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Old 05-10-2016, 06:08   #36
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Re: thinkness of hull steel

My Steely 42' built in 89, I had to make a new SW inlet, made a cutout the thickness was 5mm near the keel.
Further up near the turn of bilge, where I thought it would be nice to have some protection was thinner 3mm I think, reason being, that there is a lot of curve in the bilge and the thinner material was easier for the fabricator to bend to a nice curve.
Had some issues inside due to blocked limber holes, really felt like I had gone to half thickness before the rust dust stopped, I used a variety of rotary brushes. Other than that there has been no thinning of plate.
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Old 05-10-2016, 08:21   #37
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Re: thinkness of hull steel

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So how does one test for thickness.. wander round whacking it with a 10lb lump hammer.. "hole.. new plate"
During my last complete internal hull refit I used an air hammer with a chisel bit to remove rust scale and check for hull thickness. If it went through or dented I cut out the thin spots and replated. I figured that if the air hammer driven chisel would not go through neither would the water.

I have given up on epoxy coatings internally and am now using a combination of boiled linseed oil and cement render and wax oil in the bilges. The internal areas which drain properly still have the original epoxy coatings on them which will be 30 years old next year.

With the 29 years of experience I now have with building and maintaining a steel yacht I would not build anything less than 40' from steel or use plate thinner than 3/16" for the hull or 1/8" for the decks.
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Old 05-10-2016, 09:35   #38
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Re: thinkness of hull steel

I have a 1982, 40 foot steel sailboat, "one off" that has a design weight of 20,000 pounds. In real life, with full tanks and all the goodies on board, it's probably closer to 24,000. The topsides are 1/8" with excellent ribs and stringers behind (all coal tar epoxied so zero rust). As is common with most steel boats, the gauge of the steel increases as you get closer to the bottom of the keel. I couldn't imagine using anything heavier and then try to get any kind of performance out it.

As far as strength goes, it doesn't matter what I rub against, the boat's fine. Steel is pretty good stuff but don't use any more that you have to or you'll quickly begin to feel the effects of "diminishing returns". You could have the toughest boat afloat but it ain't gonna be much fun if you need a hurricane to get her up to hull speed. So, it seems to me that 1/4" plate in a 60 footer would make for a solid boat.
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Old 05-10-2016, 10:14   #39
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Re: thinkness of hull steel

I'm in salt water, jakkum, it's a silver/silver chloride electrode, and the test voltage gives a good indication of anode effectiveness. It actually gives negative voltage, using the hull, hence the bonded anodes, as the ground, and in my case should stay above one volt. If it dropped I would pull the boat and clean/replace the aluminum anodes. So far, I've pulled the boat for other reasons before that happened. I've had a rough time with paint adhesion and rust above the water line, and am getting that professionally done this time, but below the waterline not a pit has occurred.
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Old 05-10-2016, 14:46   #40
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Re: thinkness of hull steel

I see. I am in fresh water and protect steel ships with Magnesium rods with an inner steel cable. This is ment for the period that the ship is stationary.
The rods are hung well below the hull, so they can "see"more of the ship. The end of the steel wire is connected electrically to the ship.
I measure the voltage with a red copper wire and also maintain a voltage around 1 volt. No rust at all.
When sailing the rods are pulled out of course.

For salt water Mg is far too powerfull. These rods are used to clean insides of ballast tanks of big ships/ All rust and rubbish and paint will come off if enough Mg is used and the tanks are filled with salt water.
Perhaps your Aluminium anodes are too powerfull also, which makes the paint come off.
I raise the voltage of an unprotected steel object, 0,6 -0,65 volt with about 0,25 volt but take care not to go over 1.1 volt against copper.
I have to look up what difference there is with silver silverchloride.
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Old 05-10-2016, 14:50   #41
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Re: thinkness of hull steel

that difference is only 0,050 volt.
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Old 05-10-2016, 15:08   #42
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Re: thinkness of hull steel

Jakkum

Thanks for that response. I'd like to read up on this if you know of a decent web site or other reference. How to use a half cell, how to make one, what voltage to expect, etc.

Thanks
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Old 05-10-2016, 15:46   #43
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Re: thinkness of hull steel

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Jakkum

Thanks for that response. I'd like to read up on this if you know of a decent web site or other reference. How to use a half cell, how to make one, what voltage to expect, etc.

Thanks
I'd second that, difficult to find concise decent information.

Does your voltage difference have an effect on nearby boats ? A friend with a fifty ft steel boat was fine for years till a similar sized aluminium boat took a pen three boats away. He started having problems on the side facing the aluminium vessel. Now he has a magnesium anode on a wire which he attaches as soon as he ties up and his problems seem to be over.Water is predominantly salt but in an estuary.
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Old 05-10-2016, 17:39   #44
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Re: thinkness of hull steel

Thanks for the informative responses. I really only know my own situation, and learned from the comments. My boat is isolated from others (our own dock) so interaction with another metal boat hadn't occurred to me. I think I got my understanding of my own situation from Nigel Calder's books.
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Old 07-10-2016, 14:17   #45
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Re: thinkness of hull steel

Hpeer, any two different metals in a conducting environment, make a battery.

Bare iron/steel, not covered by of attached to another metal will form a battery against red copper of about .6/.65 volt
Free rusting iron. You can measure this with a digital multimeter. Not an analog one. That will use too much power from your battery and will give a false reading.

To stop your free rusting iron from rusting, you have to raise that voltage with at least .25 volt.
any metal on the downside of iron can do that but the further away, the stronger. So Al, ZN and Mg will work well. Mg the best.

If the voltage is higher, things on the surface of the iron will be pushed off. Paint but also rust.
And here appears a problem with older very rusty ships. They might be still floating while there are hard rustpits all through.
That will eventually be removed as well. Not the iron, but the rust.
And you will find water in your ship.

So, if you use any protective metal, the stronger must be meters away. i would not recommend Mg in salt water. Use Zn.
Check your protection by measuring a bare spot on the ships hull or something that is connected well to it like some welded stainless against a red copperwire an inch bare, that is submersed next to the ship.

About other metals nearby.... if you use shore power, your earthwire can be the reason for bad protection if it is directly connected from shore to your ships hull.
Or, if your neighbour is connected that way too, you can have trouble because of his bronze propellor...the earth loop.

Some ships have an active protection system, a voltage pressed on an electrode that is slowly sacrificed, and I can imagine that this can cause trouble for a neighbouring ship.
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