STEEL HULLS with Composite Superstructure / Topsides

Yes AH, agree. Class rules are designed to be simple and easily understood (and may I add, conservative). Step a little out of line, with an improved laminate design and you will have a hard time explaining to them your design.

To be fair, LR covers all bases such as using aramid, carbon fiber, S glass, epoxy, VE resins, cored laminate and the limiting strains (placing it in advanced composite category). With careful calculations, following the rules, it is possible to design a lighter structure but stay within the rules.

I was referring to generalization that "composite isn't lighter than aluminum" when all the data/publications indicate otherwise. In practical ship design composite is referred to as an alternative leading to lighter structure.

 

Rx

This is the problem with a lot of these books, they are a little out of date. The theory and often generalisations does sound very plausible, however the real data, that which is performed by NA on actual designs today, is somewhat different. This data is rarely published too as most yards/designers wish to keep this for themselves, since any high speed craft is always weight driven...any savings in weight is an "edge" over the competitor!

As I noted above, on designs we have done over many years, general Glass composite is no better than ally, in terms of weight. (I am referring to Class approved designs). Only going to 'advanced' or higher modulus can one realise any weight savings.

 

RX, you are quite correct, the hi tech composites you listed can be used to create a lighter than aluminium structure but for any one off project does the marginal weight gain difference justify the expense - moulds, materials, processing and mating joint issues.

Where as fabricating an aluminium superstructure is a single procedure, much more straight forward and simple to mate to the steel deck.

 

Some time way back I came across and article reviewing how ISO went about establishing its scantling rules for craft under 24 m. It started by studying existing craft that had not been built to rules and comparing them with classification society rules. It found that the vast majority of craft exceeded the CS rules by a considerable amount. It then left a great deal of responsibility of applying its rules in the hands of builders and designers i.e. flexibility.

The word around here is that within planing craft composite is heavier than alloy. This is probably due to accepted and applied local norms. On average New Zealand hulls are built heavier than those in Australia. Our seas are on average heavier. On looking at LR rules for scantlings on alloy hulls I found them to be significantly lighter than what I see industry. Maybe the heavier composite hulls are due to a higher inbuilt factor of safety i.e. allowing for variations in construction quality. Aside for welding, an aluminium structure’s strength is in the hands of the designer. Not so with composite.

M

 

It is an economic consideration based on;
Fuel savings over time vs cost of construction
Where speed over weight is the determining factor
Stability of craft due to modifications leading to higher VCG.

One off can be built using a wooden “bird cage” and foam construction. No need for molds.

Mating joints? I think they have solved it. Here is one where it is composite over steel. The composite parts were fabricated in the US and shipped over to Abu Dhabi. Two renowned NA's, a Dutch and a British were in the lead. Cost is no object when it is a flagship. http://www.yachtandcrew.com/index.p...dm-2011&catid=61:motor-yacht--100-m&Itemid=94

 

So true. The Class Rules for special service crafts under 24 meters is still too much for say 5 meters.

The ISO rules are simplified for the general builders to follow which leads to overbuilding with so many details left out. Better err on the safe side.
IF LR rules and computational procedures are followed disregarding the minimum constraint imposed, you can have a boat lighter than one built to ISO standard.

Steel and Aluminum has been around for hundreds of years and the materials, fabrication techniques, and rules on computation has been standardized. Materials have standardized codes, joining (welding) has symbols. In the hands of an expert designer, it can be built light.

In contrast, composite technology is relatively young, having been declassified by the US defense in the mid 80’s. Materials still uses the manufacturers’ code, Math is still not standardized with so many authors proposing their own method (with the exception of some who have established their names in these fields). Class rules are still gathering data based on the number of ships built. Lastly, the engineers (composite) hasn’t “meld” with shop practices. There is always conflict with the engineers and builders. It is a two way street because it was the builders who developed the fabrication technique and the engineers have to re learn by using standardized structural method analysis based on the strength of materials. Long way to go but we will get there.

 

…. a recent private note I received from an experienced steel boat builder

Dear Sir,
First off I appreciate any and all input I receive from experienced steel builders

I must take issue with this portion of your reply, “GRP in all its forms are much cheaper and easier and faster to produce.”

This might be true in a large volume production situation, but for a low volume quantities (might be termed semi-production) the cost of plugs and molds for the GRP boat must be factored into each vessel's cost of building for at least the first 10-15 units built. The cost of those plugs and molds can be considerable in both time and materials, and if you seek to hold to a price point on your finished vessel, it could take the builder 2-4 years to recover their 'tooling' investments.

Furthermore if a firm anticipated offering a particular vessel design in a 'kit form' for some private clients, or a remote builder in another part of the world, the traditional 'GRP mold method' does not make sense.

Fiberglass building materials have also increased dramatically in the last several years,...along with oil prices. Couple this with the skill of labor you really need to properly build a composite vessel, particularly a hi-tech sandwich-cored one, and you begin to see a competitive edge for alternatives.

I've chosen to consider this alternative of steel hulls with prefabricated panels of cored composite panels for the decks and superstructure. With the advent of computer-cut steel panels and computer-cut composite panels, the
1) time required to build the basic shell of the vessel might be significantly reduced
2) the skill of the labor to perform this portion of the built might be less
3) a kit form of the basic vessel could be shipped to a private buyer, or another sub-production facility anywhere in the world.

Some have asked, 'why not an all steel vessel rather than mix in composite structures'?? As you pointed out in your note above, the steel decks can be more problematic than the hulls. Neither the steel decks, nor the steel cabin sides/structures are by nature insulated. Also one must add in a substructure (battens, etc) to these metal skin panels to both get a decent attachment for the interior finishing panels, and to get a non-moisture condensing, insulated interior. The cored composite panel decks and superstructure are already self-insulating and non-condensing by their nature. As a bare necessity the inner surfaces of these composite panels could be simply painted a pleasing color, or a decorative wall covering fabric glued on.

So lets summarize at this point. I have 3 different vessel designs in the 40-60 foot range that I feel could have moderate to good success in today's yacht market. But of course in the interest of being cautious the plan is to NOT over-extend ones self with a BIG investment in plugs, molds, tooling for any one of these designs.

A steel hull shell fashioned from computer cut panels that would be assembled in a external jig/frame (reference 'frameless steel construction') appears to be fairly 'quick and dirty'.

This steel shell could then be reinforced with the traditional bulkheads, frames and stringers all welded in,

…....OR, it might well be 'framed up' with composite sandwich members that would be glued in! These 'internals' might well eliminate a considerable amount of welding that is both time consuming, and that distorts the hull plating in many metal boats. Those distortions subsequently result in a lot of 'fairing work' being required for the finished hull.


…..Sorry for this rather dis-jointed posting, but I thought but I thought I had better go ahead and make this additional posting to encourage the traditionalist (and naysayers) to take a little more time to reconsider some 'out-of-the-box thinking on steel hull construction. I have some more surprises.

 

So, is your enquiry for an actual design/build, or is it a purely hypothetical review?

 

Brian: can you describe this craft's dimensions, cabin configuration, and displacement? I would like to discuss further topsides weight.

 

I didn't really want to bring the specific designs up at this time as I do not want the discussions here to drift off onto these particular designs themselves. Lets say two of them are world capable motorsailers, and one is a canal capable trawler....or it could even be a canal barge that might be built in this manner.

I'll start specific subject threads on them at a later date.

To satisfy your curiosity here is one of the vessels specifically....a 'revitalization' of this old Rhodes 44 design
http://www.boatdesign.net/forums/boat-design/motor-sailers-philip-rhodes-john-alden-16721-5.html#post472327

http://www.boatdesign.net/forums/boat-design/motor-sailers-philip-rhodes-john-alden-16721-4.html#post472208

 

In that case I'll let you debate away then. Since it is totally driven by the SOR a real one along with budgets and time scales. Otherwise it'll just be a case of my dog is bigger than yours as their is no "target" to aim at and thus "anything" is possible until something is firmed/fixed to act as a criterion.

 

I know of a vessel built with a steel & fiberglass mix, unfortunately it came to grief on some rocks & was only salvaged for parts.
The fiberglass section was a full cockpit insert, the owner builder was on about his fourth or fifth cruising yacht & wanted the cockpit area to be more comfortable & kinder than a fabricated steel cockpit. A very neat job was contact molded with a gelcoat finish mostly off a melamine sheet mold with generous radius etc, so far as I remember it was bolted to a surrounding flange(possibly staino) with Sika or similar in the join. May have had teak overlay to sole/seats but too long ago the remember. Jeff

 

Steel Hull, GRP deck and cabin

....successful example from another forum

 

I can't see any really compelling reason to add the expense and complication of a composite deck to a steel hull.

To me one of the very greatest features of a steel boat (hull, deck, and house) is the welded watertight integrity of the thing. It's truly great to be able to attach all deck fittings and hardware with a weld, no bolts and leaky holes.

Using prefab composite panels will mean very little or no shape like curved house sides or deck camber. It will require endless hours in cutting away core and re-building solid edges to accept backing plates for leaky hardware joints, hatches, windows, doors....all problems with short life span gaskets.

These old motorsailers are not particularly weight sensitive designs, and they don't have huge deckhouses AFAIK. The main reason for a composite deck and house would be to reduce VCG, but that seems to not be an issue here.

And market for motorsailers? Roughly half the Nordhavn motorsailers produced so far (9?) are for sale or unsold (and price reduced).......

 

Spot on. So far - and I'm about done with the hull & fittings - the *only* bolt holes are for the deck fills, and I seriously considered welding them to the deck as well, just to avoid the bolt holes. As it is, I welded down 6mm standoff plates to keep small amounts of water from the bolt holes anyway.

IME - which is limited to building a single hull - a steel deck and cabin sides is not a difficult or time consuming task. Templating the pieces using strips of door skin then cutting with a plasma cutter gives very accurate fits quite quickly.

Consider that if you weld a 316 bonding bar all around your deck edge to bolt the composite deck to, you've already done nearly as much welding as you would have done to weld down the entire steel deck.

PDW