STEEL HULLS with Composite Superstructure / Topsides

Non-Steel Decks

And you realize that I am considering NOT utilizing steel decks.

So what if I am considering plywood decks, or some composite sandwich deck material construction. Then I assume I am back to providing some sort of landing/sheer clamp at the deck-to-hull joint. Do you have any experience with such a construction?

Thanks for your help, by the way.

 

Hi Brian,

Yes the series of pics shows the fabrication of a Kit, the steel for the main is pre coated & in this case also has scribes detailed onto the plating for positioning of intersecting structural members, the build method stated for this vessel was to tack entirety before weld out, the kit relies on fit & scribes to make shape, often easiear & better to stand frames to a strongback, install stringers & hang plating as level & fair are always definable rather than "massaged" with this style of kit build. As you have seen some parts of the kit are in "mill scale", this kit was shipped to Australia from Holland with the wrong keel option & wrong cockpit option as ordered by the owner, also there was discrepancies in plating thickness from port to starboard, these Problems where sorted & recut in Aus, a warning should be included on buying expensive kits from overseas from marketers that so talented Shipwright & Boilermaker can make them good . The pre primed steel is of little advantage to my mind, in some repair actions it simplifies a clean on location work method but for construction in a yard with blasting facitities it's probably better to build quick in black steel & blast out interior & exterior for painting any way. The zone where framing meets plating, prepainting is a moot point, the heat of the weld destroys the coating, what should be avoided is a "lap joint" as you seemed to indicate although these are allowed by some rule if not too wide, seal welding should make ok. When coatings are applied they should end up sufficiently thick to include a "sealing miniscus/radius" of paint between the structural members, this is especially important in bilges & some brush work along the intersections is a good practice.
Jeff.

 

Polypropylene Honeycomb Core Superstructure

Just happened across this trawler construction site. Some interesting discussions here on some of their building philosophies including heavy hard-chine hulls and lt-weight superstructures.

Looks like I'm not the only person sold on this idea for the superstructure of trawlers. Have a look here at what Great Harbor Trawlers has to say:

http://www.greatharbourtrawlers.com/design_spaceage.html

 

Completely meaningless because nothing but 2 anecdotes with no detail, no controls, no nothing.

I don't think that a deck & superstructure made out of materials other than steel on a steel hull is a bad idea, FWIW, but any time I see people quoting stuff like the above as though it actually means anything, I come to the conclusion that either they have no training in critical thinking/analysis, or they have a barrow to push.

Sorry but Brent Swain used to use this sort of 'logic' all the time and it got really old...

PDW

 

Any overlaps can be troublesome particulalry in the lower parts of a boat unless they are fully sealed around, that can be done best by fully welding eg frame to floor connection overlaps and brackets.

With steel try to never use any flats against the hull plate, only edges.

The deck shelf that the composite deck sits over is a structural member too and it's simply Teed into the topside plate or terminated overlapping the deck edge longitudinal the composite deck should never be taken to the gunnel but leave a gutter.

Laps are usual on the deck to deck-stringer interface and they don't tend to give any problems but the deck can be cut to butt to the stringer instead and I design both ways depending on the yard.

The chain or staggered welds of longitudinals to the plate is not an issue and the resulting gap is not a concern if you get some sealant in there and it doesn't take much.
They won't cause much significant distortion even if they are fully stagger welded. It's the transverses that will give the distortion if over welded as plate that's under some stress from bending will stress relieve 'relax' where it's welded and the main curvature will be fore n aft.

Some designs strongly weld the stringers but only weld the transverses at the bulkheads and below the waterline. that works better with L section transverses and gives fairer topsides.

 

Brian,
I have some experience in that, the vessel I quoted at the thread start with the fiberglass cockpit insert, the other I did some fitout work to, on this one the glass cabinwork/superstructure was attached to a steel upstand(the side decks were steel.... this seems to be the commonest method of dissimilar superstructure materials, wood/ply & especially alu, I've worked on plenty of Naval vessels from patrol boats to ships were this is the case(involves fiber washers & insulation of alu/steel surfaces, also expansion joints on the bigger stuff), once I also built a removable timber deckhouse for a self propelled barge to lower air draft to access some low bridges, I also framed up in staino box & channel a wheel house on a mooring service lighter that was clad with some sheet plastic material(I wouldn't undertake that part as I thought the material to be cheap $hit, but it's holding up well after around 12 years of inshore service, saw it yesterday), in this case the material was backscrewed to the staino framework onto a Sika bead, the deck was staino too but continuous except for hatches etc. So what your proposing is possible although as mentioned side decks often in steel, the thing is that with the polyprop core some method of "closing out" & tieing the perimeter skins would be important.
Jeff

 

This tends to be a tradespersons fantasy. It's not sensible to discriminate between academics and tradesmen, and it's far more likely that you'll find a better understanding of weld shrinkage with an experienced designer than an experienced welder in any shipyard.

Weld shrinkage and distortion are well considered, and sometimes in great in detail in the weld schedule. In problematic areas the instructions to the tradesmen have to be very detailed wrt specs sequences and procedures. Even to the extent of prior detailed studies of the weld shrinkage for complex welded assemblies.

For a start it pays to consider just what the imagined benefits are !

As I posted prior, it's going to be either significantly weaker or significantly heavier. Also for both cases significantly more expensive and time consuming.
So before you start testing anything it's very sensible to actually stop and determine what it is that you are trying to achieve.

 

Thanks very much for your analysis of my critical thinking/analysis capabilities. :!:
BTW, Great Harbor Trawlers have built quite a number of their vessels with these NidaCore superstructures and ALL have turned out to be good boats as far as I can determine.
http://www.trawlerforum.com/forums/s3/great-harbor-trawlers-still-business-11097.html

....more Great Harbor photos etc
http://www.yachtforums.com/forums/general-yachting-discussion/15706-new-yachting;-13-million-spend-6.html

 

Thanls Mike and Jeff for your help with my poor knowledge of steel building technics

Understood

don't know that I fully understand this, particularly the gutter part??

Understood



BTW, what do you think of this little discussion and photos I posted over here?....... Steel hull with steel stringers and bulkheads.....but composite-cored deck still in the plan
http://www.trawlerforum.com/forums/showpost.php?p=172896&postcount=18

 

Rather than taking that conversation over to the other forum just let me post it here:

Why Steel ?

Why steel for the hull,...why not fiberglass like most other production boats these days? One of the key words here is 'production boat'. Sure making up the plugs, then the molds for a production run of boats makes sense. But what if you don't really know how many copies you may build,...what if its a limited run geared for a specific market?? Then you are trapped with an up-front, expensive bit of tooling that you can not amortize over a goodly number of vessels.

But if we still consider a fiberglass hull, we certainly know by now we don't really want, nor need, sandwich core construction in the hull structure below the water line,...for that matter we might well leave it out of the hull structure altogether. That leaves us with solid fiberglass construction utilizing some decent resins, some decent fiberglass, and some good gel coats. Great Harbor Trawlers brags that their hull bottoms are solidly built with “laminates of more than 1-inch thick”.

When I start thinking about the labor hours to lay-up the laminates of that thick solid glass bottom, and their cost of quality resins in today's new oil price market, I just have to come back to the reality that just as tough a hull can be fabricated from a single, much thinner thickness of sheet plate steel at a fraction of that cost. And the steel's ductility makes it all the more appealing.

Why steel? It's an inexpensive material, easily fabricated, and very durable. 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.

Can we build the steel hull shape we might want, and can we build it at a reasonable price? I certainly believe so. I believe we could build an almost identical hull to that existing one in steel. I also believe it could be made even easier by modifying the hull slightly to a single chine, or maybe even a double chine if so desired.

I would propose that this steel hull could be built in a 'frameless fashion'.
http://5psi.net/index.php?q=node/11

As noted the computer cut steel panels are welded-up together while supported by this external jig-frame. Then the internal framing members (stringers, frames, bulkheads) can be added as deemed necessary. I've attached another photo example of a bulkhead with stringers. I think the Pilgrim design could get along fine with 5 of these major bulkhead types tying the hull sides together, and supporting the thick sandwich-cored deck I wish to place on top of their upper edges.




Note that the welded-up hull, with the bulkheads all installed, could remain in this fixture while the engine and other equips are being installed (no deck in the way yet). The deck piece, and then major cabin superstructure, could actually be assembled on another part of the shop floor and then brought over and placed onto the assembled hull.

There are several other advantages to this steel hull idea. You will note that I mention 'computer cut panels' of steel. This not only shortens the time of construction of the steel hull, it also makes it a potential kit-boat candidate.

It has yet another potential benefit. Unlike a fiberglass hull where I am married to a single bottom design, I can change this hull's bottom design readily if something new looks feasible.

 

Brian
Here's a drawing to make the "gutter" a bit clearer.

A composite GRP core decks would be solid at the deck shelf interface and it would need to be designed with a thick enough sealant bed to allow it to work as an expansion joint and the bolting also allowing expansion.

The deck shelf is also a structural part of the hull , it can be stainless steel galvanised painted steel or just well painted steel. It needs to be cambered a bit to keep water running away from the join.

Resist the temptation to take a composite deck all the way to the topsides if you do you'll get a water trap and endless corrosion problems down the track.

 

Brian,

I wouldn't get too hung up on the kit thing, whilst there is some streamlining there is also some risk & lead time. The Van De Stadt "frameless fairing"/dressmaker system is pretty quick so far as SVP(Strong Visual Progress).... as a 20 year old I put together one of the first in Aus, in a laid back fashion I'd go for a morning surf, then go to the yard & weld, mark & cut out a hull strake for the 34, week & 1/2 later with my dad helping started the day with no hull & by the end had 4 strakes a side up, instant hull! is what everyone said, but of course the stem still needed fitting properly, transom, sub sole/keel framing, bulkhead landings, chainplate knees etc. A great system that gives a clean nicely limbered structural interior but in reality not much quicker than building a set of frames, fitting stringers & plating up. Also the jig used a fair bit of material with 5/6ths of the jig location also sporting a matching internal frame, seemed a bit like doubling up at least on a one off basis although I did use the jig structure with 4 chain blocks to lift the hull to accept the keel.
Regards from Jeff

Regards from Jeff.

 

Computer cut is called NC cut ( NC stands for Numerical Control). It pays for itself rather than paying the shipwright to loft the patterns and it's so accurate. These days the NC plasma cutters accept a variety of CAD formats directly which makes it a very accessible service.

For larger vessels absolutely everything is NC cut in a modern facility.

Jig then Plate then framing suits itself better to transversely framed chine designs and floors are usualy tacked in prior to welding plate seams. But it doesn't suit longitudinal framing very well. You also have to build an accurate Jig.

If you were just using minimal transverses anyway then they make the 'jig' and you only need a base to set them up on and the 'jig' remains as part of your boat as Jeff points out above.

If it's being built by a yard they will be better off going frame then plate.

 

Jeff

Did you finish her to sailing trim ?

 

In hindsight I kinda wish I did....
That vessel I launched with Yanmah engine & partial fitout, but not to sailing, the rig I built from an Alspar kit(they were just down the road), had heaps of fun for a summer fishing & cruising around with mates etc then got offered a good price, bought land(south coast) & built house with the funds,plus some other earnings ..... & started another boat... & repeat with variations.... wish I had the same energy & impetuous nature now...... at 48.
Jeff