Steel, Aluminum, and Fiberglass

Then there are numerous designs that mix the materials. Anybody know of examples with fiberglass superstructure on a steel hull?

Kelly Cook

As Lars says, the materials for a hull can become a Chevy vs. Ford kind of thing, i.e., pick a side and argue for it.

Being currently engrossed in Expedition steel hull/aluminum superstructure-type boats, here are some arguments for that side, keeping in mind that we are discussing displacement hulls here:

Sudden impact-- fiberglass can crack, aluminum can tear, and steel, with it's higher modulus of elasticity and greater yield strength, can undergo quite a bit of deformation prior to failure.

Fire--though rare, it always is a nasty possibility. FRP burns at 500F, aluminum melts at 1220F, steel @ 2800F. I could be wrong, but steel and aluminum, unlike FRP, more easily meet IMO ( SOLAS ) fire containment standards.

Mechanical-- welded hull/deck joints ought to be at least a tad stronger and more leakproof than a screwed/bonded FRP joint. Chocks, cleats, and towing bits are welded on, not just bolted on. FRP composites have the possibility of core delamination, unlike a single metal skin.

Over the years-- a hull surveyor can utilize audiogage + X-ray to precisely determine structural integrity.

Rust-- steel still gets an unfair rap here. No longer called 'paint jobs', the Coating Systems have pretty much done away with that. Steel that is pre-primed from the mill and is coated with more layers of inorganic zinc primers, fairing, then epoxies, then the final LP coats tend to look like FRP finishes and last as long.

Tankage-- fuel/water/waste are typically integral resulting in more capacity for a given size boat, plus a side benefit of having a double bottom over a good portion of the submerged surface.

Interior volume--metal boats just seem to have a greater useable interior compared to a FRP boat.

Stability at rest-- by keeping the weight aloft with an aluminum house atop a steel hull, it's relatively easy to achieve a proper metacentric height, i.e., a GM number resulting in a short roll period and good stability.

Like Burger King, have it your way-- metal boats, unlike FRP which requires tooling costs/constraints, are easily custom built to suit an owner's needs.

Again, I have my biases, and Lars makes an example of Royal Denship who will build in any of the three materials as well as in various combinations.

Thanks guys

You mentioned that the paper will focus on "cost/beneift and risk analysis". What do you mean exactly by risk analysis? Are you talking the material science aspect (clearly each have their pros and cons) or the financial aspect of things (how likely will the hull crap out on you e.g. rust, crack, and the repair cost if repair is possible)?

I think the purpose of the boat and the type of hull will dramatically affect the cost/benefit. If we are talking about full displacement hull, then I will also strongly favor steel. But if we are talking about a semi-displacement yacht, then I have good arguments for the lighter aluminum and fiber glass.

If money is not an issue, then I would go with carbon fiber. If the hull is cracked and somehow I am still alive, then I will build another boat.

What about climate? If a yacht is not a world traveler and say spends most of its time in the warm latitudes compared to those that stay or venture to the colder ones,what impact would that have? Heat transfer through a steel & aluminum hull would be much different than composite hull.I sure climate affects Them in other ways Also.

How would the total environment a yacht is in affect your hull choice,say from the cold waters of Alaska down to B.C . to a yacht that will spend most its life in south Florida and the Caribbean? As to how water temperature ,sunlight, what lives in the water ,to the water itself(wave frequency, average wave conditions,etc. ) would affect a yachts hull choice.

Every answer brings another question for me. In the End, how much consideration does a buyer give to these parameters? Or does it just usually come down to a given "look" a buyer wants when it comes to hull choice?

Interesting ... hard to see where the steel ends and the fiberglass begins ...

http://www.yachtforums.com/forums/14564-post5.html

Kelly

I agree with Loren about the rust factor on steel hulls, if the right coatings are applied rust is not a big issue, my hull is of corten steel and was galvinized inside and out and two part primer and paint applied and have no rust problems, the downside of the galvinizing is when welding has to be done the area has to be well cleaned (grind) to obtain a good weld.
C

Hey Bill,

Here is an excellent design summary of aluminum vs.steel.

http://www.kastenmarine.com/alumVSsteel.htm

Evan

Excellent overview!

MK's -ultra clear explanation about the properties of steel and aluminum play well into this discussion about hull design criteria. Overall, I'm far more interested in using Aluminum as a result. Floating up here in the hills, I've seen more planing, lighter, -service-duty vehicles, like canoes. (The ones that didn't float, often had been displaced by flying through a tornado.)

For expediency, one assumes we're talking about the use of homogenous materials at both the skin and the ribs. What about applications where these differ? Like what happens as we use an aluminum frame within, and particularly to strengthen a fiberglass, or multi-core hull?

What about castings and extrusions, alternating these weights to meet the local stress-positions? ...best of both worlds.

Finally, we've skimmed past the topic of the galvanic properties of the aluminum hull, and particularly in relation to any secondary steel structural elements, such as those that may receive constant exposure to salt water spray. Paints, anodizing, anodes...?

Steel Hull discussions

I was doing a little research on the subject of steel hulls, and particularly looking for the most modern methods one might employ to protect them internally. I ran across this extended discussion that had a lot to offer on the subject and related issues.

It starts out with a relatively simply question about insulating a steel hull, but gets into some galvanic questions, mixtures of steel types, insulation and condensation issues, etc.....a number of intelligent participants as well.

Steel hull isolation: alternatives to foam? - Boat Design Forums