Steel Hull Construction

[sailingdog]

Valiente-

Just curious, how is the aluminum connected to the steel?

[Johnrb]

"The great lakes is not exactly 'fresh water' it is brackish."

With all due respect, the Great Lakes are not brackish by the accepted definition of brackish.

"Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (ppt or ‰)."

http://en.wikipedia.org/wiki/Brackish

"Fresh water is defined as water with less than 0.5 parts per ten-thousand dissolved salts"

"Freshwater lakes, most notably Lake Baikal in Russia and the Great Lakes in North America,"
http://en.wikipedia.org/wiki/Fresh_water

"however, Lake Ontario has a rather high salt content
(about 185 ppm: Beeton 1965; about
235 ppm: Dobson 1967). The average
salt content is roughly 0.209% in salinity,"

http://www.aslo.org/lo/toc/vol_22/issue_1/0158.pdf

I expect Lake Ontario has a somewhat higher salt content it's downstream location from Erie, Huron, et al.

For what it's worth, I've kayaked Georgian Bay and, to a lesser extent, Lake Superior and drink the water from the lake. It is definitely fresh.

[artbyjody]

You proved my point - salt is what makes water brackish...

You seeing it is fresh is just because you drink tap water...

Thank you

[Valiente]

Originally Posted by sailingdog

Valiente-

Just curious, how is the aluminum connected to the steel?

With about 28 1/4" bolts, some tapped into the aluminum cross-frames, others secured with Nylock nuts where accessible. Lucky me, I got the bolts off last weekend, only to find that a thick bead of 5200 is also holding the roof down firmly. 5200 is a glue, not a gasket or a bedding compound, to my mind, so now I have to get a couple of Chinatown bread knives and get sawing.

Yes, I am buying a few tubes of dielectrical goo, and I need to seal over some previously drilled and inadequately capped holes that allowed water to intrude and which has rusted a bit of the flange. It's not bad, but I can reprime and topcoat the whole thing on any day above freezing.

I suspect I will use a continuous strip of rubber gasketing outside of the boltholes when I replace the roof.

[sailaway21]

Jody,
It probably would come as a surprise to those communities that get their drinking water from the Great Lakes to hear it categorized as brackish. Potable water standards do not allow for salt in drinking water. Since the average fresh water allowance on a ship is 10" it's important that one know the specific gravity of the waters one will be transitting. I've always measured a SG of 1.000 on the Lakes and my employers would have been sorely plexed had I not loaded deeper and been able to. (g)

The longevity of the newer ships on the Lakes will probably equal their predecessors even though they are now constructed of thinner, lighter, stronger high-tensile steel. Most ocean-going ships are not done in by corrosion as they are by stress, increased maintenance costs, and the advances of technology. Great Lakes ships experience far less stress than ocean-going ships, they only sail nine months a year, and there is far less maintenance to be done on them. The advent of catodic protection systems has allowed ocean-going ships to reduce their shipyard visits from annually to bi-annually. The ABS and USCG have seen the results and only require dry-docking every two years instead of annually as required on those older ships you refer to. They certainly were built heavier, but not better in most respects.

[steelboat]

Sailing Dog, steel hulls with aluminum superstructures are a routine way to go with big stuff, from super yachts to war ships as sailaway 21 has noted. It's usually done with a dupont product (in this country) called a deltastrip or deltacouple, which is a bi-metallic strip, explosively formed, which joins the 2 materials together (don't ask me exactly how). You then simply weld the steel to the steel and the aluminum to the aluminum. In europe it's called tri-clad. I believe there may be a third element (composite or other?) between them to allow for the different coefficients of expansion in big stuff. I'm told the stuff is quite expensive, but in mega yachts the weight savings up high negates the cost of the strip. In smaller boats it's pretty simply done with bolts and flanges, as long as everything is properly isolated, as Valiente has apparently done. Hope this helps. Bob www.sv-restless.com

[sailingdog]

Bob—

The material I'm familiar with is called Triclad, and is IIRC a laminated strip with aluminum on one side and steel on the other.

[steelboat]

sd: yeah that's the stuff. Bob www.sv-restless.com

[sailingdog]

I only know about it since my friend built a horse trailer with the stuff.

Originally Posted by steelboat

sd: yeah that's the stuff. Bob www.sv-restless.com

[Valiente]

Originally Posted by steelboat

In smaller boats it's pretty simply done with bolts and flanges, as long as everything is properly isolated, as Valiente has apparently done. Hope this helps. Bob www.sv-restless.com

Well, I will do it, because I'm going into salt for the first time next year, and I notice that some of the steel bolts didn't come out without a lot of persuasion this time. So a proper coating on the bolts and tapped threads, plus a gasket of some description to keep the aluminum roof from directly contacting the steel flange (which itself will be ground, primed and painted entirely).