Zumo radial tube mainframe with fishbone subframe structure

Hard part of the holiday season seem to be how to keep the mind idle..

The idea is Zumo, a 20m boat with guesstimate 20 ton displacement fully loaded. Power would be maybe 1000hp or 1500hp.

What’s new about the idea? It’s the tube frame with the “fishbone” (steel made fishbone) subframe design. Well at least I have not seen something like this before.. Has anybody seen this kind of design?

Why? Well that inverted-table flat boat is the coolest s**t I’ve ever seen with boat design! lol

That’s just flat out heroic! Dam it! And it just works! But somebody already done it!

This idea is to use maybe 60mm tube x 4.5mm wall for the main tube frames. The keel & chines will be maybe 5mm or 4mm plates, and add a coat of Line X to. The upper hull will be maybe 3mm or 4mm plates. Does this sound credible for blue water? (I’ve read some arctic boat used 3/16” for hull and 1/4” for keel.) The V-chine will probably start about 21° stern and steepen rapidly at the bow. Also, those two are sponsons.

And I’m guessing the kind of precision needed for the tube bending and fitting the skin.. maybe will need to build a factory or two first then? Can’t imagine how to build this by hand right now.. But this is probably all the time I have for this this year – at least. But then why bother the time with this? Maybe it’ll save some time later if this ever get a go at all I guess.

Any comments welcome & appreciated. Thanks!

 

Interesting idea, sort of "skin on frame" but in steel. But sadly, not very efficient of material/mass. In most materials, wood, metal, or composite, the thickness of the skin necessary to resist water pressure and knocks makes it useful to form part of the structure (semi-monocoque), and then the rest of the structure can be useful for forming bulkheads and mountings. You get none of those benefits from a tube frame scheme.

 

James is right. Tube frame is not the most cost effective or structurally sound way to go.

Explore some of the sites that auto racers inhabit. You can buy a tube bender for a modest price from one of the suppliers of that line of business. You can also get equipment to make joint configurations in the tubing. A lot of careful welding will be needed as in a roll cage for a race car.

 

Thanks James & messabout.

So I get it now boats are probably more exoskeleton creatures like shellfish rather than meat carrying skeletons like land animals. Makes sense. Think about this again it's actually the hull bearing all the loads, frames included, and take the knocking & pressures.

So if taking a page from the arctic boat's design and use 3/16” for upper hull and 1/4” for the chines, the hull will weight about 15 tons. But if I did the numbers right, assuming all the main tube frames and other types of subframes together is equivalent to about 500m the tubes, it's only weight about 3.7 tons. Even if assuming 800m of it is about 5 tons. So actually don't seem that much compare to the hull. Tubes are rigid & high strength structure. So the gut feeling still says it can give quite good rigidity, the hull should be quite solid - with the diagonal force transfer of almost entire hull. I could just increase the fishbone density or add some mini T-beams if necessary. Can conventional bulkhead & frame weight less I don't know, but 20m is not a tiny boat but not a ship either, the benefit of the open space without bulkheads should be a plus. Also tubes are likely easier to fabricate.

One thing I still don't get is how to figure the thickness of the steel hull (suppose assuming 2205)? Is there design rules somewhere for this kind of thing?

Also I should revise the max displacement guesstimate of this to 30 tons now. Hopefully it's realistic this time..

 

An internal skeletal structure is there to allow flexibility, not rigidity nor keep water out which is what a boat hull needs. So unless your boat is going to swim like a fish, your biological analogy probably isn't a useful design methodology.

Boats are built the way they are because of thousands of years of trial and error development that has found the most effective and efficient way of making a hole in the water.

 

James, I respect your opinion and I believe you are a true pro. But there is people that proved a boat can be made out of inverted table. And designs have certainly changed many times over thousands of years. If people had not experiment with new designs and test new ideas, boats are certainly built the same as thousand years ago. Boats today do look different and work different. I also agree flexibility is also the strength of structure, but rigidity makes movement more efficient & waste less energy. Unless it's a fish that needs to swim flexing the body. Both rigidity & flexibility are inherent with steel structure anyway.

 

GLWT.

 

One could argue that Baidarkas use a skeleton framework and the skin is actually animal skin. Those boats are very flexible and they seem to work remarkably well. But those are human powered boats used for hunting seal or other prey. Check out some of Platt Montfords ultralight little boats. They are small SOFs and are not suited for use in a seaway.

The fact is that a well engineered tube frame can be exceptionally rigid if that is the aim. Race car cages survive 20G crashes although they may be bent up a bit.

I reckon that the joints between tube and skin would be a continuous source of oxidation that will require constant attention. If the tubing is closed, then we'd have to worry about deterioration on the uninspectable inside of the tubing. If the hull is to be steel then best to stick to conventional practice that uses flat bar frames and stringers. Flat bars can allow limber holes. How would you accomplish that with closed tubing?

Please accept my apologies for being a naysayer.

 

@James Thanks!

@messabout Thanks & no apology necessary. Actually quite appreciate your inputs, devil is in the details, the more tested the better. It's lots of good points to think about. The other reason that had me started thinking tube frame is if sealing off the tube, they can be additional buoyancy in case of flood, and basically indestructible. Though after crunching the number, it's probably just a ton. Also had the though tubes won't deteriorate easy once sealed. Also had the idea to blow dry nitro into it before sealing up. The tubes - for the main frames - are not welded together at the crossing points and just form one single plane; each the loop or box section is one single pipe, but where the mainframes need to cross, they're stacked and use sort of like tube clamp & weld together. The gaps under higher pipes and the hull will have lighter i-beam type flat bar / stringer welded between. And I think these can have limber holes, but not sure yet if can cover all places. Though just heavier flat bars are probably enough for the whole structure, but still pipes can be strong on all sides while flat bars not as strong with diagonal or twist. Will also try to fit part of the contours contacting the tubes or where tube on hull to make more surface contact, or square tube might be another option. Subframes will have flat surface to contact. Still searching better to weld the hull (to frame) or just use softer & more corrosion resistant solder material instead. Also what if coating the inside hull with Line X? Will be no longer inspectable, but if the coat is good as it can be, maybe corrosion can just be forgotten. It's good point on limber holes. Need to think how to pump water off where could be trapped. Thanks!

But with all that said, it's definitely mad loads of work the more thinking about it. Certainly easier just thinking for now than do..