thinkness of hull steel

[Steadman Uhlich]

The question of plate thickness on recreational sailboats comes up, especially when someone is looking at older boats (ca 1980s).

I don't own a metal boat and it has been some years since I last read a book about building a steel boat, so my recollection is "rusty" (pun). But, my understanding is that over time the original 4mm plate may have been significantly reduced in thickness, and that this can be a very significant issue with older steel boats.

Assuming the 1980 year, 40 foot hull is given an ultrasound survey to determine thickness of the remaining plate (which may have lost thickness over the last 30-40 years of blasting the bottom), what would you consider "too thin" or "risky" for hull thickness?

Under the waterline:

2.5mm?
3mm?
4mm?
5mm?
_______

Also, does the ultrasound testing reveal weakspots due to rust scaling or flaking INSIDE the hull, in areas where the hull INSIDE cannot be seen? Does the measurement include the rust scale or does it somehow NOT include the thickness of the rust scale?

Put another way, if the ultrasound measurement shows 4mm on the bottom of the hull, could this possibly be 2mm of good steel and 2mm of rust scale (unseen)?

[hpeer]

Uncivilized,
That is about correct.
They do get heavy, but in remote places the sheer toughness adds a safety margin.
All depends you where and how you sail, no pat answers.

[qwert]

There is a very common habit amongs steel boatbuilders of increasing the hull thickness, thus a boat designed for 4mm will be built in 6mm "to make her really strong" or to cope with anticipated corrosion.

The big problem with this approach is that it completely stuffs the ballast ratio, particularly if the builder then tries to maintain the designed waterline by reducing the ballast further exacerbating the problem.

[qwert]

"Also, does the ultrasound testing reveal weakspots due to rust scaling or flaking INSIDE the hull, in areas where the hull INSIDE cannot be seen? Does the measurement include the rust scale or does it somehow NOT include the thickness of the rust scale?"

I had an ultrasound done.The standard method is to mark out a grid pattern on the hull and measure thickness at regular intervals. This is worse than useless. I got a clean bill of health, very reassuring.

When I came to do a major refit we found rust in inaccessible (and invisible) spots where the limber holes were blocked etc. If you are getting one done you would probably be better checking the most likely weak spots than relying on a grid.

[hpeer]

Qwert,

100% spot on. I agree ultra sound is almost dangerous.

[boatman61]

Originally Posted by hpeer

Qwert,

100% spot on. I agree ultra sound is almost dangerous.

So how does one test for thickness.. wander round whacking it with a 10lb lump hammer.. "hole.. new plate"

[Celestialsailor]

I love it...the op gets his answer on the 4th post, thanx everyone on the 5th post, then the debate starts and the book readers come out for 2 pages...so far.

[hpeer]

Originally Posted by boatman61

So how does one test for thickness.. wander round whacking it with a 10lb lump hammer.. "hole.. new plate"

Pretty much. Outta the water.

Mark 1 eyeballs, great flashlight, big screw stick, lotsa time and ambition.

Crawl and touch EVERYWHERE. if you can't touch it no one else did either. Poke any new paint. I have found small bad spots, think dime size, no bigger.

[jag39]

I have seen 1/4" on 35' boats, but usually 1/4" hulls start at around 40' with 3/16 deck and cabin plates. The 1/4" on a 35' hull is considered heavy duty, and heavier than necessary. However, it does depend a lot on frame and longitudinal spacing and material. There is a good website for this at Metal Boats For Blue Water - Kasten Marine Design

[hpeer]

Ted Brewer wrote one also. Can't recall the exact title or find it on amazon right now.

[captlloyd]

I built a 64ft. steel sailboat. 17-1/2 foot beam. Keel bottom-1/2''- keel sides1/4'' hull bottom 1/4'' hull sides 3/16''. deck 1/8''. People said the boat would not float, ha !! Without ballast that thing floated around like a cork ! I added 7 yards of concrete AND steel punchings in the box keel, still not down to the waterline. 5.9 Cummins engine. You could have hauled a small dozer on that boat. And yes, they do corrode mostly from the inside-out. Proper coating is the key. And anode protection.

[jag39]

Surveyor will test hull thickness with sonogauge.

[boatman61]

Originally Posted by jag39

Surveyor will test hull thickness with sonogauge.

That's what my insurer wants to upgrade me from liability only..

[hpeer]

Fine, give the money man what he needs.

Just don't be lulled into thinking it is meaningful to the integrity of your hull in any way. If it finds something good. But not finding a bad spot is meaningless. That's why I said it's dangerous, it gives a false sense of security.

Been there, done that, made the repairs the gage didn't find.

Twice.

[leftbrainstuff]

Originally Posted by kingofcary

Hi Folks .. first time question on this forum. Appreciate any and all input. My question is related to the thickness of steel hull. I know there is no true answer but what is an optimum gauge. I see a 60' sailboat with 1/4" for the hull. It's a blue water vessel. I have also see 3/8" on other boats. Certainly seems thicker is better up to a point of diminishing returns. Seems 1/4 is small number. Thanks for feedback ... Dennis

Thicker is heavier. Weight itself is a load on the structure. Higher strength materials can have substantially thinner sections and still exceed design requirements.

The design criteria go like this:

1) min thickness to avoid static stress failure and thresholds
2) min thickness to avoid strain failure and thresholds
3) avoid large changes of section and localized stress concentration and other fatigue concerns
4) loss of thickness due to corrosion and erosion over the design life
5) availability of standard thicknesses
6) codes, standards and mandated material selection
7) fabrication skills and processes

The design process is iterative. There is no right universal thickness.

The above are traded off for:

1) cost
2) weight
3) stiffness
4) formability
5) toughness
6) survivability
7) maintainability
8 ) repairability
Etc


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