Hull material for arctic cruising

Is it fair to compare different materials without having them dimensioned according their scantlings

 

Lazeyjack, because I'm using fairly thin plate (5mm) and thinking of home building I'm planning to go with stringers over frames.

Thanks very much for the offer of help, I hope to have finished a draft structural design by the end of the year, and if I could send you some drawings at this stage and get your input it would be great. I'll send you an email when I get a bit closer to the mark.

I would like to do my own fabrication but I've never welded alloy before. I've enrolled in a TAFE course and I'll see what my welds look like after a few months of practice before I make that decision.

Mike, I agree with you on steel. I calculated rough scantlings for this design with a variety of materials and the only way to get a stiffer boat than steel was carbon, which is a bit expensive and brittle for most of us.

I see that boat design is not to be taken lightly, but for me the challenge of doing my own design is as important (almost) as the boat at the end of it.

 

Teddy, sorry maybe I was not very clear

That is just illustrating equivalent stiffness. I am not implying equivalent scantlings, quite the opposite, I'm saying that alloy vessels often require more material than is suggested from comparing equivalent yield figures.

People get enthusiastic comparing alloys yield and ultimate strengths with steel, but weight for weight they have virtually the same stiffness. This lower stiffness of alloy is a problem when considering elastic instability (buckling). Stiffness counts for a lot in structural design . A quick comparison of materials and the design of a structure in that material is to consider the following:

Stiffness
Strength
Toughness
Weldability
Fatigue Resistance .

A good strong boat in alloy can end up with less saving in weight that you might have thought initially in the design process; 50% saving is a good achievement , 40% saving is a safe bet for the designer of say a rough tough coastal workboat or small cargo carrier . A lightweight sailing cat might save 60% over steel construction for minimum scantlings, but it will not in layman’s terms ‘be as strong’.

Alloy is still an excellent material and if it were cheaper-still it would be the material for the majority of boats. Given it’s production process and energy cost it’s currently very cheap as it’s production intensive and the final product is made with heavily Gov’t subsidized electricity around the world.

But as Wynand pointed out it is the cost/benefit that really drives the choice.

 

Carbon-Epoxy is very strong and stiff but it is not tough. Now if you used aermet................

Good Alloy welders come and go around here if you contact me closer to the event I will recommend someone unless you can get Stu (Lazyjack).

cheers

 

Thanks Mike

 

Do you mean Lignum Vitae? I think greenheart has a few different meanings.

 

One thing to consider is the fracture toughness when working in cold/icy conditions. If you're set on steel you need an EH grade, whereas with ally, no problems. The fracture toughness of ally doesn't really change with temp even down to -200C.

Personally I'd go for ally.

 

ops..forgot to add this in my post for info.

 

Temps where steel fracture becomes an issue aren't anymore just cold/icy. It is plain solid ice

 

TeddyDiver

Dont follow, care to explain?

 

Yes good point, the ductility of steels suffers with cold but probably not that much of an issue for a smaller vessel.
It is an issue for ships in the arctic and antarctic in the winter months. The hogging stresses broke several early steel ships in half from the deck down from rapid fatigue of the cold embrittled steel.

AS standard plate will pass a Charpy notch standard for ductility to -15 deg C which should be adequate for a yacht or a summer supply vesel. Stainless steel has excellent ductility at extremely low temperatures and stainless arrest bars or bulwarks or even timber or alloy decks could offer an interesting extreme low temp solution .

i wonder about the matrix embrittlement of FRP with low temp?

 

Sorry didn't notice earlier.. Mike pretty much said what I meant..
"AS standard plate will pass a Charpy notch standard for ductility to -15 deg C which should be adequate"

 

I'm reminded of a test back in the '90s when some researchers tried a Charpy notch test on a sample of the type of steel the Titanic was built from, chilled to some typical North Atlantic temperature (I can't remember the exact value). The pendulum barely slowed down as it went right through the sample, sending the broken pieces flying across the lab! They tried the same thing with a modern shipbuilding steel- probably the AS standard plate mentioned above- which passed the test easily.

Regarding Mike's comment about FRP matrix embrittlement at low temperatures. With aerospace-grade resins this should not be a problem; the temperature at 35,000 feet is a good minus 50 celsius and the aerospace composite standards reflect that.

Production boat polyesters and vinylesters, though, might be a different story. I doubt you'd be able to find a manufacturer of these resins who even knows how they behave at -30 C. Yachts are supposed to be tarped up on cradles in that kind of weather, so why design (or even test) the resin for it?

That could be an interesting test.... make a few dozen identical test coupons of e-glass/polyester, put a few in the freezer, soak a few more in liquid nitrogen, a few at room temperature and a few at 60 C or so. I'd be interested to see how the mechanical properties hold up at the extremes.

 

Without going to icebreakers, boating in the arctic waters is anyway generally boating only with some floating ice here & there. And so long as the water isn't ice covered the temps never go down as much as in one could expect. Let's take a typical excample from Varanger fjord in January. Air temp 50 km from the sea might be -35C, some kilometres to sea -25C, and just over the water only -10C. If it were colder closest to water it freezes in 24h..

 

MarshMat, I routinely fish in weather almost that cold with FRP and steel with no apparent problem other than a little icing if it's choppy. TeddyDriver, Our water here (southern Kenai Peninsula) is very well mixed and warm. It will freeze quickly in still areas but, for the most part, stays completely ice free (as it did forty years ago, to the consternation of the...oh, nevermind) even with a few weeks of -20, tho I have not held a thermometer right on the water...When a cold spell hits the Bearing Sea side, upper Cook Inlet or Valdez harbor (colder water) we see slush almost immediately or ice on still water, as you said. We work and play in this stuff all the time here (No play in the winter Bearing, albeit). - Mark