Space frames for main beams

Yes, truss space frame are the most efficient IN TERMS OF STRENGTH/STIFFNESS TO WEIGHT RATIOS. If you have other considerations than stressed skins such as monocoque or uni-body designs have advantages.

With structures like car bodies, aircraft, and boat hulls and even shipping containers, you need to enclose the space and reduce drag etc. If you have to put a skin/fairing or hull skin on for other reasons, than there is some advantage to have that skin carry some of the structural loads (which means the skin has to be thinker and heavier) A really light skin of course would not be very durable in a car body or a boat hull so you get double duty out of a heavy hull/body skin. In the case of shipping containers you need a secure and durable envelope for the cargo, so a tough heavy skin can serve duel purposes.

I can build you a the same strength carbon graphite space frame that is lighter than a carbon graphite monocoque, but it would not be very competitive since it would have much higher drag (and not be very attractive either).

An interesting effect of the sea kayaks is the "hard shell" fiberglass, Kevlar or carbon hulls have to have much heavier skins to prevent bucking failure of the hull, especially around the coaming/cockpit (consider it is a structural tube with a big hole in the top). They have to be stiff with thick skin so the skin will not buckle, making them heavier than an optimized structure and also prone to fatigue failure at stress concetrations (like around the coaming). I have seen this many times. But you can get any shape you want with a molded hull and it lends itself to automated production.

On the skin-on-frame design you have discrete frame members they can flex without bucking, with the flexible skin accommodating the changing shape of the flexing frame. The hull structure be lighter, and you can control the hull stiffness to allow some flex for rough conditions. This will actually improve the comfort makes the kayak more controllable in heavy seas.

 

Petros,

would it be possible to have some details about the catamaran you are planing to build with trusses?

What will be your concept of the hull skin? Similar to your kayaks with a thin skin over framing?

Longitudinals (or sandwichskin) and transverse frames maintain the box shape of the hull and the longitudinals together with the skin withstand the local surface pressure.
Todays scantlings mix the two concepts. What would be the ideal concept just for the structural loads without the surface pressure?


Simon

 

I have not worked out all the details yet, but for now it a kind of proof-of-concept all wood fun fast cat to try out a bunch of ideas I have for both the hull and a new concept sails/rigging.

It is 20 ft LOA, 12ft+/- beam, 23 ft high mast and for now have 10 sq meter sail (107 sf), though I may make a bigger rig for it later if it works out.

It would have light dougfir wood space frame hulls, that are structurally two full length and almost full depth trusses with either truss bulk heads (with plywood gussets) or thin plywood bulkheads about every foot or so. It will use 3 or 4 oz polyester skin and polyurethane sealant. It will use thin high aspect ratio dagger boards as far forward as practical that will protrude out from a kind of foil/lifting/planing surface attached to the forward hull. It will have two cross beams also made from laminated dougfir with web truss or box beam construction, that will eventually get a streamlined faring over them.

The rigging is a highly modified cat rigged kind of junk sail with no external stays except for two rigid and streamlined (laminated wood) struts to hold the mast up in a tripod arrangement (the struts have less drag than cables and spreaders). The pivoting mast will also be a laminated wood/truss assembly about a foot deep and 6 inches thick that is the front part of a foil shaped "D" section. The sail will be two layers of batten supported skins, rigged in such a way that I can control both the sail twist and camber over the whole surface from the streamlined cockpit.

I will have a trampoline deck for sorting it out and recreational use, but I am designing it to be controlled from being enclosed within the hulls without the trampoline in place, to reduce drag for high speed runs.

I expect the thing to weigh about 200 lbs empty, perhaps a little more or less.

It started out as a fun fast light play around sloop rigged catamaran that I would build in my garage with salvaged lumber and other junk fast and cheap. I just wanted to go out and blow away the million $ racing yatches around here with a $200 home built cat. Just for fun, tee hee.

But when I started working out the loads, forces and speeds it appeared I was close to making something, with the right wind conditions, that could set some speed records, or get close anyway. So then it occurred to me to use it to try out ideas I had been thinking about for years from some professional aerodynamics experience I've had designing high speed aircraft, and apply it to a sail/wing thing on this fast cat. And see if I can pinch out just a few more knots, reduce the drag, and then see where else I can save weight, in the beams, in the mast, in the hulls (with trusses). So now it will be more work to build, but much lighter than my original plan, and faster too. And an interesting experiment.

So I have been refining the design to have this elliptical, two surface variable camber, controllable twist foil shaped sail, that can be controlled from within the hull. The details may be patentable as a sail, but I want to build one first to see if it is even practical to use (a working example is better than a patent for protecting a design idea anyway). And we will see if it is even controllable at 50+ knots. Preliminary calcs show it should get close anyway.

Realistically setting a record with such a low budget boat is a long shot, but if I can prove a concept, and attract even partial sponsorship, than maybe a carbon graphite version of the same idea, enlarged and refined a bit more, might actually do it. If anything I will train my petite 18 year old daughter how to use it, and go set some women's closed coarse speed records. They are much lower that the men's in most categories (she is game, and she weighs 100 pound less than me!). That might also help attract attention for full sponsorship of a faster boat.

But even if that does not happen, I will still have my fun fast play around yatch beater cat. That did not cost much to build.

 

If the 200 lbs you mentioned is for the 2 hulls and beams, you could do the same weight with cedar strip with carbon skins. Probably much stiffer (locally) and a bit more damage tolerant.

But if it is really to go that fast maybe you should think of 2-3mm plywood, skinned in carbon, with some ring frames. chines aft would likely help with speed. Maybe violates the ethos but functional....

Trusses are great, so is skin on frame, but the surface loading at 20, 30, 40kts is really different from 5-10kts as you surely know. Drag from skin deformation could be a speed limiter that might set in way before skin friction. And consider frequency response of the drum you are sailing. damping is nice but bone-jarring rides are often fast because they are bone-jarring.

On the other hand if you do it just right maybe you can get it to just track better and better as the panel loading increases with speed, making yr longitudinals into mini-keels.... Flotsam would loom large though. Maybe throw in some float bags?

It would be awesome if you could make something ultralight that would go like an Acat in light wind, at 15-25% of the cost. For that a top speed under 20 could definitely be forgiven.
Intriguing concept, overall. Puzzling though. Lots of interesting questions. Even more interested in the wing, actually. Good Luck!

 

Well yes, there are a lot of ways to build something similar but they all cost a lot more. And usually are much more labor intensive. I know a similar sized cedar strip and fiberglass sea kayak cost about $700 in materials and about 500 hours, a skin on frame is about $100-150 in materials (even if you pay retail for them), and about 100 hours to build. If the skin was also part of my structure, and I needed it to carry hydrodynamic loads what you describe might be be better, but I think I have worked around it.

The skin deformation issue did concern me, I was going to use plywood on the bottom side with a planing hull design (kind of like a float plane sponson), But later developed a foil based "shoe" idea I want to try out (that I can remove when I am not going for all out speed). The winged shoe should bring the entire fabric part of the hull out of the water at about 25 knots or so. So the fabric covering becomes only a streamlining fairing, and it is not structural. Below that speed closely placed bilge stringers will support the fabric with little deformation just fine.

200 mph aircraft have used fabric covering for many decades. There is a design process taking into account the fabric strength, the pressure on it, the area distribution, support member spacing, etc. It is just like any other material you would work with in a structural design. Not difficult to design at all if you know the fabric strength, which can be determined by testing or by use of the aircraft type fabric, for which they have specs.

It may not work out as I planned, I am concerned about torsional stiffness, which can be dealt with in a space frame but it too adds weight. I might end up with something as heavy as anything else, with a lot more parts and labor than if I made a hollow monocoque shell hull. But it would still be less material cost, so I learned something without investing too much recreational money into it.

BTW, from the outside it will look like any hard chine hull, such as a stitch and glue plywood or even a welded steel hull. Seeing the inside of the hull will raise a lot of eyebrows I think. Only sea kayak builders would recognize it.

 

Ahh, foils. Could have some potential! (and/or increase your budget a lot )
speaking of which, Reynolds numbers....
They use skin on frame for wings, but not for supersonic flight. They have used low speed water tunnels to model supersonic aerodynamics though.... staying up on the foil would be key.

If it is going to work, I bet you would be foil borne in the realm of 8-10kts. Maybe with a special, small foil for top end speed you would need higher speed to lift off, but I suspect you would also need rigid skinned hulls to reach 20 kts.

With lots of little wood members though you might indeed survive a crash better than a light carbon/foam shell... I see your point on that.

Since aero drag is also a factor you might want to make it more of a Moth or proa than a cat.
As far as "cat" rig I guess you mean "single sail" not "mast up front."
I would recommend keeping the sail rather far aft.
Forgive me if I am speaking way beneath you.... not exactly an engineer myself.

Any way, I like it when folks post on new concepts. For fun, I drew a crummy picture, like to see what you come up with designwise or especially if built!

 

Not bad BWD! I like the shoe. This looks close along the lines of the hull, I would have the rudder under the aft end of the hull (for end plate effect) without a foil, and the dagger board would move forward to within 3-4 feet from the bow.

Yes a single sail/mast that would be all the way forward on the first cross beam (I have my reasons for wanting it forward having to do with the camber/twist control).

What I meant by "shoe" (not my choice of words btw) is a short pedestal about 6" high, with a wing or foil about 2 square feet (2 ft beam, 1 foot lenght) on the bottom, with the dagger board protruding out from the bottom of that. So if you move the foil up on the dagger board to just below the hull it would be about right.

BTW, the Germans during WW2 built experimental jet aircraft with wood wings and fabric covered control surfaces that flew at high speeds. Of course a sailboat hull will not be anywhere near that speed, and above 12-20 kts, my design will have all of the fabric hull out of the water anyway.

Here is my thinking for the foil surface on the bottom of a short pedestal, (and I welcome comments): The most efficient planing surfaces are higher aspect ratio or broad beamed bottoms, but with narrow catamaran hulls (for low drag in displacement mode) this is difficult to achieve.

So my next thought was inspired by powered race boats that only have two stepped sponsons and the transom trim flap in contact with the water when up on plane, something similar is also true with float equipped aircraft. The current record holder, Spirit of Australia, is an example that could be adapted to a catamaran hull (though designed for lower speeds of course). But the step has way too much drag when below planing speeds for a general use sailing craft, in fact the drag could be so high it might not even reach planing speed. Then I thought of a retractable step that should work, but that would add weight and complexity. Perhaps I could first build a fixed step just to test it out, and make it retractable later? Too much rework.

I examined foil quipped sailboats, but they appear to have severe disadvantages when not at foiling speeds and few sailing records have been set with foil equipped sail boats, and no all-out records (l'Hydroptere is the one notable exception). So how can I take advantage of this without the disadvantage? It also seems to me that with a small planing surface of a stepped hull (that lifts the rest of the hull out of the water) you should have less wetted area and less drag than with a pure lifting foil, which depends on both sides of the foil to be in the water. So I was back reconsidering the stepped hull again.

How do I get a high aspect ratio planing surface on a catamaran hull without a step?. I examined various ways to stream line a step for when in displacement mode, but none would work I am afraid. So it occurred to be to put a foil surface on a step, to get more beam, but if the foil surface brings me up on step, I do really need a step at all? Instead I can use a stream line short pedestal. It would look something like the famous winged keel of America's cup fame, but much shorter. A hybrid design occurred to me.

My current idea is this: as the hull speed increases out of displacement speed, it would reach foiling speed and the foil would lift the forward hull up, reducing hull wetted area. When speeds increase further the top/front of the foil breaks the surface and all of the pedestal is up out of the water, along with all of the hull except for the very aft end where the rudder is. At full planing speed, only the bottom of the foil is actually in contact with the water surface, it in-effect would surf like a water ski (or 'shoe') on the surface. As the speeds increase, the aspect ratio of what is actually touching the surface would also increase since it would lift higher and higher out of the water with the increased speed, leaving only the trailing edge of the surface in contact with the bottom of the foil.

So it could sail fairly efficiently in three different modes: displacement, foiling and planing. Without any moving parts. It is also possible that at even higher speeds, only one planning surface and one rudder need be in the water (reducing hull drag further) when flying a hull at extreme speeds. I am in the process of determining the size of each of the surfaces are necessary for each of these four different conditions of sail.

I chose the catamaran configuration because I wanted something that can sail in both directions as a conventional sail boats does, but have less structure than a tri- and less weight than a monohull. Besides, it would be possible to have a relief crew contained in the other hull for longer records runs, or to have company when zipping around the sound.

Any thoughts?

 

I see, cool.
The Hobie trifoiler thing did 30+ with skis on a somewhat similar platform.....

If you use skis and make them retractable you could either fair them into the hull or not. If not, you would be able to remove them and only have a couple of strut holes to close, for a light boat, in light wind.
A foiling moth looks more elegant, but the trifoiler went faster. Have not read about the trifoiler for a while but it is worth looking into.

I still would put the rig aft a good ways. With a foil at the front and the mast also at the front you might pitchpole too easily. Maybe the hull could take that but I bet the rig could not.

Maybe some tiny trim tabs at the back? ?

 

If that were true, why aren't spaceframes used in at least general aviation? They would be entirely suitable for fuselage rears for instance, where a fabric covering would be adequate, but monocoque is almost universally preferred, and has been since the 1930's. Surely if spaceframes were lighter, stiffer and cheaper, companies like Cessna or Beechcraft would have tumbled to it by now?

 

Cargo/passenger space.

 

nope. The rear of the fuselage in planes like cessna's can't be used for cargo or passengers. Moves the CG too far back, and makes the plane unstable. Also, a spaceframe needn't intrude into this volume. Spaceframes used to be used here, but they have been replaced by monocoques. If spaceframes were lighter, cheaper and stronger that wouldn't have happened.

 

So you are going to change from monocoque to space frame at the point that U can no longer support weight? I don't think so! Most of the light aircraft I have flown in actually do have storage quite far aft, its up to the pilot to balance load... not nope. Partner is a pilot Ultralights make use of space frames where no load carrying is possible/expected. This is not black and white --- horse's for courses --- the older planes where space frame and canvas for a reason, for the given materials it was most effective. Now we have better material so in certain circumstances where the application requires it monocoque makes more sense but not always. It all depends on the application its not blanket one is better than the other.

Space frames do tend to impinge on internal volume, again depending on the application.

 

BDW,

Why would putting the mast forward increase the chance of pitch-poling? Pitch-poling is a function of the height of the aerodynamic center (or as sailer's call it, the center of effort) above the water, and volume of the bows, and your counterbalancing load (size and location). The location of the mast does not enter into it unless you have an especially heavy rig (which I do not). I intend to the have crew compartments in the hulls as far aft as possible to counter balance the pitch-pole tendency, and with the foil planes up front the bows should not dive.

Freenacin,

Space frames may not be cheaper, in fact they could be one of the most costly way to build a structure, what they do is save material costs. Many production aircraft abandon it because of the very high labor costs, and as you know many production compromises are made to save manufacturing cost (though there are many sub-assemblies where they are used, like in engine mounts, landing gear, etc.) But you still see it used in in ultralight and other homebuilt aircraft, and in many modern aerobatic aircraft (hand built), and in building structures such as roof and floor trusses (where assembly jigs save labor), and also you saw them in the Gossamer Albatross and Condor, where extreme weight savings was critical. You will see them used in tower structures and bridges (though in highway bridges recent regulations have made them no longer economically viable). You also see them used in modern folding kayaks since a frame will fold down to a much smaller package.

Besides, in boats, cars and aircraft you need a smooth outer skin of a certain shape, and if the shape carries loads as in a hull, the weight savings become somewhat less, you may be as well off with a rigid structural skin.

 

Absolutely!

 

So why did they move to monocoques for military aircraft around the 1930's? Cost is rarely an issue for the military,(and the monocoques were certainly not cheaper) neither is passenger/load space (esp in fighter aircraft) - what IS important is performance - weight and strength.

The fact is, if a carbon fibre spaceframe were stronger and lighter than a carbon monocoque, they would be using them in F1, with just a thin skin covering.