Space frames for main beams

I would be interested to know the difference in weight / strength and advantages / disadvantages of using a space frame concept for the construction of multihull's structural bridgedeck beams.

One of the few references I have found is

www.john-shuttleworth.com/Articles/52AeroDesign.html

Does anyone have further knowledge or references?

Simon

 

1. The solution can be precisely engineered to suit the boat.
2. It should be as stiff as is achievable
3. You can allow for accommodation in the engineering of the frame.
4. There should be no excess weight.

Really if the thing is engineered correctly it should be the most unobtrusive way to gain strength where needed. I would think the concept would be hard to fault... but then again lets see what our resident rocket scientists come up with

 

Also, for the boat in question, you can built it to support the loads of a free standing rig.

 

A space frame is a three-dimensional truss ... or at least that is what I remember.

In space frame buildings, the parts are bolted together. This could be an advantage allowing easier construction, repairs, and use of steel tubing.

 

Truss space frames are the most structurally efficient type of structures ever devised in terms of overall strength and stiffness to weight ratio. As an engineering consultant I have designed many of them for many different applications. In fact I am intending to build an all wood cat in the next year or so and I am designing both primary hulls and beams structures as trusses. Even the mast can be designed as a truss structure with a streamlined fairing skin.

Space frames can be tailored to specific torsion or stiffness properties, are VERY materials efficient, and very light.

They do have some drawbacks however. They tend to be more labor intensive to build as compared to molded structures, they are generally not as compact in overall dimensions, and they are not very aerodynamically shaped (a lightweight faring or fabric "skin" solves this, but adds weight and more labor to build). The aerodynamic drag on a space frame is not insignificant in a fast boat.

They also are not very damage tolerant, a single element failure could cause you to lose the whole structure (as in the failure of the steel truss bridge in Minneapolis MN last August). This can be compensated for by some over-design or redundant load paths, but both add weight and cost.

To optimize the design they are also very labor intensive to design as well. The more and smaller each element or web member, the more weight efficient, but the more parts and connections you have to design and build.

But you do see production roof trusses that now completely dominate almost all types of building roof construction, multiple trusses are redundant and lend themselves to machine manufacturing. And it saves a lot of material cost, mostly in wood, but also in steel as well.

Trusses or frames can be done in wood with either metal fasteners or glued joints, steel or aluminum also with fasteners or welded joints, or composite with glued or molded joints. Metal can be either tube or angles or even flat strap for the tension members.

For building in a home workshop I like wood because it is inexpensive, nontoxic, available locally, and easy to work with simple hand tools, and corrosion resistant (especially with glued joints). If it will be stored in a garage on a trailer rot can be controlled, for moored or exposed storage wood of course is not a good choice, especially for large boats. But still good for a scaled proof-of-concept example.

Optimized structural design for either wood or metal space frames are very similar in terms of weight to strength ratio, fiberglass frames are actually heavier than wood or metal frames. You only get better strength to weight ratios with carbon graphite composites (at much more cost) and more demanding construction details.

Also to optimize a space-frame/truss structure you will need an experienced designer or engineer who knows all of the forces that need to be accommodated (not always obvious on a sail boat, especially a catamaran), or you have to teach yourself the design process, both in terms of hull forces and the frame analysis process. BTW, the computer programs that presume to do this for you are not a substitute for understanding the process, they only speed the calculations. If you feed in bad information/assumptions, you will get a bad design no matter how good the design software. I have seen this many times with "newbies" right out of collage.

But to give you an idea of the kind of weight savings that can be achieved allow me to share with you some typical numbers of sea kayaks that I have built using wood space frames with moderately durable synthetic skins (8 oz/sq yd with polyurethane sealant).

A typical production fiberglass or roto-molded polyethylene sea kayak (with typical productions bells and whistles) weights about 60 to 70 lbs before adding options. Retail costs from about $1200 (polyethylene) to $2500-2800 (quality fiberglass). Material costs are likely about $300-700.

A typical production "lightweight" carbon graphite hull similarly equipped would weight about 38-45 lbs and cost about $3500-6000+. Material costs could be about $1400-2000.

A typical "custom built" composite sea kayak might weigh 32-38 lbs and cost even more (mostly in labor). And I know of one custom built racing kayak build of carbon graphite that weighed about 18 lbs and was very expensive, and too fragile for general use (in fact it failed half way thorough a trial just aft of the coaming-it was too light)

The wood "skin-on-frame" kayaks I have built weigh 16 to 19 lbs complete (before accessories) and cost about $100 in materials and about 100 hours to hand build. These are not "optimized" designs since they are recreational sea kayaks that have many hundreds of hours of use on them (I have built seven of them of various designs). I was going to try to build an optimized hull structure this spring using lighter fabric and expect to get the weight down to 12-15 lbs, though I expect at some cost to durability. This is just a design exercise I am doing to gain some experience for building the cat hulls.

The high performance "fun" cat I was planning on building using wood space frame hulls and deck beams with fabric skins would be 20' LOA, 12' beam, 23 ft mast. I would expect it to weigh about 150 lbs (or less) complete empty weight (though I have not done a detailed weight analysis yet). That weight I think would be tough to beat unless you got real expensive and exotic materials.

Below is a pic of my wife paddling her 18 lb skin-on-frame sea kayak I built for her, as you can see it a perfectly normal full sized sea kayak. It has red cedar stringers, steam bent Alaska yellow cedar ribs, and a sitka spruce coaming, 8 oz raw nylon skin with about 5 coats of polyurethane paint finish (a fairly tough skin).

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Thank you Petros for your extensive reply.

I am following with interest the concept of larger aluminium catamarans (+/- 50' )
I really like the low maintenance aspect of the material, but think that weight or/and being prone to fatigue failure are a big drawback.
Bringing down the weight, with lighter scantlings, makes the structure more flexible and fatigue will set in earlier, or making the structure thicker and therefore more rigid will have a negative effect on weight.

I would like to find out if there are concepts, like the space frames, that could lower the weight, improve rigidity and fatigue resistance.

Maybe the load-paths can be distributed further over the hull-structures.
I am also wondering if, by using space frames for the hull connecting beams, the plating in low stress areas can be reduced and save further weight.

A good start into the new year to everyone

Simon

 

Big cats seem to be epoxy/foam composites in the main. Epoxy/marine ply has much to recommend it. Aluminium is rarer. However, use what you are comfortable with.

Space frame thinking and design can be incorporated into the structure as insurance. Belt and braces really.

Pericles

 

I'm interested in Polycore, it looks like a reasonable alternate, very light and stiff.

 

What are the shear numbers for Polycore compared with these foams?

http://www.boatdesign.net/articles/foam-core/index.htm

Pericles

 

I have just got on to it, all the info I have is hearsay but you can get the spec's here as I am doing...

http://www.polycore-australia.com.au/

 

Not monocoque? You should pass this information on to the people who build Formula 1 racecars. They moved away from spaceframes and on to monocoque construction decades ago.

 

Surely it depends on the application. It seems to me that the specific example given by the OP (the big Shuttleworth cat) suits a space frame.

Also F1 is a great example, it illustrates exactly what Petros is saying. He is getting close to performance of exotics at a fraction of the cost... that is efficient. Can exotic monocoque construction of a kayak exceed his homebuilds, yeah sure but at what cost and by how much?

Also F1 cars employ space frame type construction where specific loads are encountered .... suspension construction being an obvious.

Hold the sarcasm, please.

 

Sure, Formula 1 and mass car builder have moved a long time ago towards monocoque. But why would Audi, Mercedes and Fiat start to use them again?

It is easier and cheaper to produce space frames for small series and they are also lighter than their counterpart. High-tech monocoques are still out of reach for the mass production.

Of course there is no "the one and only truth", but I think it would be interesting to explore the path. As with modern software the loads and their directions can be calculated and space frames optimized.

Bulkheads and box beams are simplified solutions, that of course work perfectly. But aren't they just space frames with a lot of extra weight?

Especialy in the area of aluminium boats, I believe that there are better solutions, that can avoid local peak loads that can provoke fatigue failure.

Simon

 

Amen!

 

Which was my point exactly.