STEEL HULLS with Composite Superstructure / Topsides

[brian eiland]

Let me preface this discussion with this; I come primarily from the Multihull ranks, and steel boats have never really been a contender as a building material for these vessels,.....just too heavy. So my knowledge of steel boat construction is limited, and that might become apparent in some of my postings, particularly some out-of-the-box thinking I might suggest.

Steel boat construction has come to my mind recently as I review 3 older boat designs that I have identified for a 're-design' (modernization) possibility. In all 3 cases these are monohull displacement vessels, and therefore are candidates for steel hull construction.

Why steel? Its an inexpensive material, easily fabricated, and very durable for worldwide cruising. It's a material that inspires confidence in a boat's survivability from mishaps and collisions by both experienced boat owners and newly minted ones.

Why a new steel subject thread? As the thread title indicates I wish to explicitly explore the combination of a steel hull with composite superstructures. Naturally I want to review and cross-reference to those other good discussions on the forums dealing with these two construction materials,....steel & composites

 

[brian eiland]

There is just something about STEEL

....interesting quote

I've been aboard every kind of vessel that there is, made of: Wood, Fiberglass, Cement, Composits, Aluminum, and yes steel. And there is something about steel that just seems to say it all; strength, seaworthiness, a sense of security, and durability (Especially in foul weather.) that the other hulls just don't inspire. Steel, And Fun Everyone is an acronim for Safe, and I think that explains how we all feel on a steel hulled vessel.

True, Aluminum comes close, but I always think of aluminum foil, and how it crinkles, and not that the space shuttle's 2" thick hull is made of aluminum. LOL

And fiberglass, well just having glass in the word is enough to unsettle a real swab.

And didn't they use to put cement goulashes on people like Jimmy Hoffa for a reason?

Now wood is okay for someone like Noah who had no idea of what steel was, as it hadn't been invented yet, and his boat wasn't being built for long term usage anyway, it just needed to float for a while. And too, polished wood is wonderful for a nautical setting inside of a steel boat too, to remind us of our roots.

And finally, composits are fine light weight material when used for building air, and spacecraft, but they cost too much, and are as yet untested by time, and really are no more than a space age form of fiberglass to me. And there's that word glass all over again. Geesh!

Now truthfully, when you measure your boat against another out in the water which one would you want to have under you when they collide in the fog?

Capt John S. Keller
Great Lakes Pilot

 

[keysdisease]

I believe Lee Creekmore designed and had built a 40'+ steel hulled composite decked monohull racer named Desperado in the late 70's. I raced against her in 1977 and heard she was lost several years later. I can't find anything about her online, but she won line honors that race.

Steve

 

[brian eiland]

Composite Superstruction Considerations

There are any number of notable ship constructions where a lighter-weight superstructure has been sought out for a basic metal hull. Alum topsides over a steel hull comes to mind for a number of larger yachts and navy vessels. Creative bonding techniques have had to be invented for these cases. Likewise the joining of composite superstructures onto steel hulls requires creative thinking about both the mechanical and the 'chemical' bonding of these two very different materials.

I use the word 'chemical' in the sense of adhesion. There are any number of mechanical methods of bonding that are conventional in manner. It's the adhesive bonding that is continuously under development with ever-stronger, greater adhesion products. I think we are currently at a point that we have a number of very good adhesive products that can join our composite superstructures to our steel hulls with extreme confidence.

My thoughts are this 'transition' from steel to composite should take place at the hull-to-deck interface, NOT at the deck to cabin superstructure joint. In other words I favor a composite deck, ...more specifically a sandwich-cored composite deck onto which the rest of the cabin/superstructure is attached.

I've witnessed years and years of sandwich cored composite constructions. Basically it boils down to using 3 types of cores; 1) balsa, 2) various foams, and 3) honeycombs of either alum, Nomex, or polypropylene. Generally the balsa cores have won out over the foams for deck fabrications due to their greater stability under the extremes of tropical heating. But balsa cores have a significant history of susceptibility to rot from water penetration at various hardware attachments and migration of water along the core-to-skin bond line.

Alum & Nomex honeycombs are a little pricey, and extra care must be exercised to obtain a good bond with the very 'thin edges' of the honeycomb chambers. Polypropylene honeycomb however has a fiberglass cloth scrim thermo-fused to its cell structure,...a very consistent bonding that doesn't provide voids for water to migrate across. This 'scrim layer' in turn provides a 100% bonding surface for the fiberglass skins to be applied to. And the polypropylene material itself is totally rot proof in the case of any water penetration.

I've come to the conclusion that this 'poly-core' material is the best choice for my composite decks and superstructures. Poly-core is one name the Aussi's and NZ boat builders term it. They actually pre-fab sheet panels (akin to sheets of plywood) of these materials in a controlled environment, and subsequently computer cut those sheets into specific panels that will be joined together to form a structure, a bulkhead, a deck section, a cabin side, etc., Here is one example
http://www.polycore-australia.com.au/

NidaCore and Plascore are two polypropylene honeycombs better know in the American market. But neither of two produces have been reformulated into pre-fab sheets/panels for some reason?....perhaps not as great a demand for kit-boats here in the USA? It could be easily done though if there were sufficient demand

For reference here are a couple of other pre-fab panels utilized by these kit boat designers, Duflex, Durakore. But these panels utilize balsa or foam at their cores.
http://www.duflex.com.au/duflex2/
http://www.atlcomposites.com.au/atl_composites2/products/composite_panels/durakore


....and some of my confusion about the nomenclature
http://www.cruisersforum.com/forums/f48/are-cats-made-from-duflex-panel-kits-strong-22948-7.html#post806984

 

[Ad Hoc]

As the thread title indicates I wish to explicitly explore the combination of a steel hull with composite superstructures.

Like any combination/mixture of materials in design, one not involved with the design would ask, why?

There are many factors that come into this decisions making but the principal one is saving weight. Thus in order to establish whether a mixture of materials is beneficial, one needs to know what is the weight of an all steel build and what is the weight of a steel hull and composite superstructure?

Thus you need an SOR...without which, you can only discuss how it has been done before, in terms of its fabrication. Since any design decisions are based upon its own SOR and the conclusions one draws into....hmmm..too much weight, lets lighten the load. So, you need to establish a proper SOR and GA, and then you'll know.

Since in any design the choice of materials is usually dictated by the SOR and not because :- well, they did it why don't we!

 

[jehardiman]

FWIW, depending on the SOR and other requirements, steel is often lighter than "composite", as well as wood being heavier then steel, but this is a side issue.

In any hybrid (as opposed to clouding the issues by calling steel/anything a "composite" which is now a pejorative to mean something specific) construction you have to look at what advantages the material brins to the table, i.e. the original "composite" (the original use of the word) vessels were iron framed and wood planked circa 1855. To build a steel hull and a composite or aluminimum superstructure can be done for weight savings, but at a significant compromise to overall safety which is why you see so few of them now (see the SS Morro Castle loss, USS Belknap). Additionally, there is always the joint stiffness and flexiural issues. Often modern composites cannot support the point loading issues.

 

[brian eiland]

Like any combination/mixture of materials in design, one not involved with the design would ask, why?

There are many factors that come into this decisions making but the principal one is saving weight. Thus in order to establish whether a mixture of materials is beneficial, one needs to know what is the weight of an all steel build and what is the weight of a steel hull and composite superstructure?

Thus you need an SOR...without which, you can only discuss how it has been done before, in terms of its fabrication. Since any design decisions are based upon its own SOR and the conclusions one draws into.

Well I had a nice answer penciled up in draft form and lost 2 pages of it the other day....

Suffice it to say the 'principle reason was NOT weight'. There are a number of other considerations that I will try to post when I reconstruct that damn reply

 

[michaeljc]

You are probably aware that larger steel craft have an extra thickness incorporated into plates and sections to accommodate corrosion over time. On smaller craft where weight is important we don't have the liberty to do this. Steel hulls look great when freshly sandblasted and painted, but any abrasion through the coatings or seepage into joints results in ugly weeping. In short, steel hulls can have good endurance but require extraordinary annual maintenance. This includes all internal scantling. Over the long term steel can become expensive.

I would not agree that steel's working life is longer than a wooden hull built from the right timbers.

One-off composite topsides is going to expensive right? There may not be any weight saving either. Naval ships have been built alloy topsides, steel hull. Here insulation between the 2 materials is crucial. My felling is if you want steel then go all-steel.

PS: I have seen well finished alloy topsides at a boat show that everyone (except the real pros) believed the material to be composite.

M

 

[Titirangi]

The expense of producing a one off composite superstructure with mould costs added to the on going cost of steel hull maintenance doesn't justify such a construction.

Fabricating an aluminium superstructure for a one off project has far more practical & cost benefits - permanent trilock welded joint, stiffer structure and more load bearing capability.

That steel may be of lower cost for fabricating a hull than using alloy is accepted but its not easier to work with, it's heavy, rusts while you work, just requires slightly less skill to weld (if not built to survey) plus steel is not cheaper in the long run.

http://www.superyachtnews.com/fleet/19042/biggest_yacht_in_china_joins_fraser_fleet.html

 

[rxcomposite]

The decision to use composite on the superstructure is made by the NA. Reason, to lessen weight on the topside. Reducing weight on the topside is a decision based on passenger comfort. Too low a VCG and the boat rides stiff. Too high, stability is compromised.

Steel is good as it is cheap and adds weight to the bottom. With composite superstructure of lesser weight, the designer can add more deck level, or increase height of the superstructure.

 

[Titirangi]

Problem is composite isn't lighter than aluminium, then added to the weight of a steel hull you have a heavier than ideal craft. Whether leisure or commercial weight reduces speed performance, reduced endurance so to compensate for weight engine size/bhp is increased so is the fuel burn.

Low cost steel fabrication may seem attractive but when all the affect/issues are properly analysed its not cheap at all.

 

[michaeljc]

I don't build in composite by I suspect that unless you can find moulds that come close this is the most expensive option. Furthermore, as well as being lower in cost aluminium topsides will be lighter.

It seems odd that the NA goes for a low cost hull and the most expensive topsides. Has he done a cost comparison?

Aluminium goes together vey quickly. Get the design drawn up by a pro and CNC cut. 2 good builders can assemble a 7m bare hull runabout in under 3 weeks. If you take the time to use filler on the welds, round external corners and paint, alloy topsides can look superb. Because the alloy is above the waterline galvanic reaction will not be such as issue. Very important though that all earth circuitry is isolated. i.e. nothing through the body.

Just my opinion of course.

M

 

[rxcomposite]

Problem is composite isn't lighter than aluminium, then added to the weight of a steel hull you have a heavier than ideal craft.

I hear that fallacy again. Advanced composites (carbon, aramid, honeycomb. autoclaved) wins against Aluminum alloy hands down in terms of weight v.s. strength/modulus. It is also superior in fatigue resistance.

Aluminum may win on some mechanical properties such as heat resistance, shear, or compression strength so it is used in conjuction with advanced composites. A good engineering design will use composites and aluminum.

 

[Ad Hoc]

I hear that fallacy again. Advanced composites (carbon, aramid, honeycomb. autoclaved) wins against Aluminum alloy hands down in terms of weight v.s. strength/modulus.

For advanced composites, yes I would agree with you. But general Glass and its variants, no. And it isn't a fallacy either. We have done many many tests/designs over the years and Glass structure is approximately the same weight as an ally structure.

Infused WR can be a bit lighter, but this assumes you're not worried about Class rules,, which tends to "normalise" this aspect a bit by the "minimum" values required to pass Class rules

 

[michaeljc]

We are talking over a practical situation here. What do you think the OP is referring to as regards design and materials of composites? If his solution is that clear and cost effective put in a quote for the guy. I could talk of super-thin high tensile alloy but that would be silly.

M