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X-ray, a rubber band boat
This first attempt at a LEGO rubber band boat makes a great tub or wading pool toy. As with any other boat, the right combination of motor (rubber band), gearing, and prop makes all the difference.
About this creation
Please feel free to look over the images and skip the verbiage.

Is it possible to over-complicate something as simple as a rubber band boat? If I'm involved, you bet it is!



The brief tub trial below shows the winding process.



When favorite builder David Roberts mentioned the possibility of a rubber band-powered LEGO® boat, the Fishing Boat hulls (FBHs) sitting in my Bin of No Return came to mind.



My FBHs were retired after a catamaran made from them nearly sent $90 worth of LEGO® electricals to the bottom during a powerboat compatibility test last year. But perhaps I hadn't given these lightweight, low-resistance hulls a chance to shine.

Could rubber band power be their true calling?



Yes!

On this page:


Overview

The X-ray twins -- my first attempts at rubber band powered LEGO® boats -- make great tub and even better wading pool toys under adult supervision1 (needed only to insure that the structures resisting rubber band tension remain firmly seated).



The only reliable way to tell these 0.100 kg monohulls apart is by the location of their "X-ray" logos. Collectively, they're simply "X-ray".



A twin going incognito with her rubber band wound and locked.



A shorter band stretched between its attachments prior to winding might produce more torque.



However, the friction to be overcome (within the coiled band itself and between the stern drive and the gear on its input shaft) would also be greater, and band life would definitely suffer. My experiments with bands shorter than the 190 mm (nominally 7-inch) ones shown on this page haven't been encouraging.




Directional control, seaworthiness, speed, and endurance

The V-bottom and spray rails (ridges along the edges of the bottom) on the FBH give it a good bit of directional stability -- an valuable attribute in a boat without remote steering.



Among other things, the directional stability reduces the prop walk inherent in any single-screw design.

Any remaining prop walk can be eliminated with 1-2 small rudder-like "tabs" fitted to the stern as needed. The tabs can also be used to cause X-ray to go in circles.



The appendage drag induced by a single tab must be pretty small, because I can't pick up an associated loss in speed. Nor is there a visible list, as ABS plastic is only 4-5% denser than water.



Tab deflections are set by hand prior to launch. I angled the one above to correct a prop walk-induced veer to starboard.




Seaworthiness, speed, and endurance

Freeboard and stability are adequate for mild chop at best.



The one-piece weather deck keeps minor splashes out of the hull's interior, but its large 16x6x2 LU deck well holds enough water to submerge the stern. Since the deck-hull interface isn't water-tight anywhere (unless made so with tape or caulk), the boat would eventually sink after lingering at the surface for tens of minutes.

Speed and range depend strongly on (i) the nature, length, and freshness of the rubber band; (ii) choice of prop; and (iii) gearing, with higher overdrive ratios generally going farther.



X-ray's average speed across the 4.8 m wading pool in the 1st video above is ~0.4 m/s (Froude number ≈ 0.28) with a modified 5.5L 2-blade prop (see below) and 1:1 gearing. With an unmodified 5.5L 2-blade prop and 1:2.78 overdrive, speed drops to ~0.26 m/s (Froude number ≈ 0.19) but remains entertaining.




Props

Having twin X-rays allows various rubber band, gearing, and prop combinations to be tested head-to-head. The ongoing testing program amounts to a motor/gearing/prop (MGP) optimization with rubber bands substituting for electric motors.

I'm still working through rubber band options, but I think I have the gearing and props pretty well sorted out -- at least for the bands tested so far.

The props I consider suitable for X-ray appear in the next 2 photos.



Running from left to right above are modified and unmodified 5.5L 2-blade props (4745), the 5.5L 3-blade (4617), the 3L 3-blade (6041), and the 5L 3-blade (92842). The last produces so little thrust that I won't refer to it again.



The 9L 2-blade (2952) and 3-blade (30332 or 15790) props above are both right-handed, as is the 5.5L 3-blade. The rest are left-handed.2

Unfortunately, the only props with central axle holes are the 3L 3-blade and the 9L 2-blade. Impure mounting methods are required to guarantee that the others won't slip on their shafts.

With the notable exception of the modified 5.5L 2-blade, however, wrapping ~1.5 turns of Scotch Magic tape around the pin end of an axle pin and jammming the wrapped end into the prop's central pin hole was as impure as it got.

As with any other powerboat, prop performance depends critically on gearing. The best-performing prop -- the modified 5.5L 2-blade -- requires the 1:1 final drive ratio shown below for maximum speed, but a 1:1.67 overdrive strikes a nice compromise between speed and range.



All other props in this series work best with a 1:2.78 overdrive. Their rank order WRT performance is 5.5L 2-blade > 9L 2-blade > 5.5L 3-blade > 9L 3-blade > 3L 3-blade.



NB: A rubber band substantially more powerful than those tested so far could easily change the optimum gearings cited above, but it probably wouldn't change the performance rankings.

Below is the highly impure but highly efficient modified 5.5L 2-blade LEGO® "twisted"2 propeller mentioned earlier. A 3L bush pin cut down to 2L and (gasp) glued into the hub serves as cross-axle adapter.



Hand-sanding the blades into proper hydrodynamic profiles resulted in a substantial gain in thrust and a 50% gain in speed relative to the original slab-like blades.






Design highlights

A low-power propulsion system demands a lightweight, low-resistance hull. The 28x8x3.6 LU Fishing Boat hull (FBH) fills the bill admirably.



Hydrodynamically, this single-chine planing hull is the cleanest of all LEGO® unitary hulls.

Attaching the rubber band to the stern drive input shaft was the trickiest part of the drivetrain. For maximum efficiency, the band and shaft should be exactly coaxial where they join.





The winder below is simple but effective. The red lock makes X-ray easier to handle when wound up.



I think of the black band segment adjacent to the forward band attachment as being "double-coiled". The next segment aft is only "single-coiled". The black band will tolerate double-coiling along its entire length but will break if wound much tighter.



The black band's latex-free EPDM material probably isn't the best choice for a torsion-powered boat, but it loses elasticity fairly slowly and is UV-resistant for outdoor use.

Competition rubber band airplanes use tan-colored latex bands exclusively. The tan-colored bands I found in the office may or may not be latex, but they seem to stay in one piece longer than the black ones. The jury's still out on which color performs better.






Bottom line

In summary,
  • X-ray's size and speed are well-suited to tub and wading pool use, and she has none of the worry associated with putting expensive LEGO® electricals on the water.

  • She's not seaworthy enough for anything more than mild chop, but calm water stability is more than adequate. Since she lingers at the surface for many minutes after swamping, retrieval in deep water shouldn't be an issue for anyone who can swim.

  • Speed is at least mildly entertaining and might well improve with the right rubber band (stay tuned). With a shaped 5.5L 2-blade prop, 1:1 gearing, and a fresh rubber band, X-ray can almost keep up with my slowest electric boat, Lena II, but my fastest electric boat is nearly 3 times faster.

  • Purists will want to use a single 9L 2-blade prop. (Doubling this prop to make a 9L 4-blade will be counterproductive WRT speed.) The more adventurous should head straight for a 5.5L 2-blade with or without hand-shaped blades.




Specifications



Dimensions and hull form coefficients

Overall dimensions:264 x 64 x 72 mm (LxWxH) excluding prop
Displacement:0.100 kg
Displacement volume:1.0 x 10 -4 m3
Depth:29 mm
Waterline length:200 mm
Waterline breadth:60 mm
Draft at keel:15 mm (midships)
Freeboard:14 mm (midships)
Wetted surface area:n/a
Midship section area:8.9 x 10 -4 m2
Waterplane area:9.6 x 10 -3 m2
Block coefficient:0.56
Prismatic coefficient:0.56
Midship coefficient:0.98
Waterplane area coefficient:0.80
Length-breadth ratio:3.3
Breadth-draft ratio:4.0
Length-displacement ratio:4.3




Performance measures
NB: Speed averaged over a 2.4 m course from a standing start with modified 5.5L 2-blade prop and 1:1 gearing.
Hydrodynamic regime:Displacement
Installed power:n/a
Installed power to displacement ratio:n/a
Speed:0.40 m/s
Critical speed:0.56 m/s
Froude number:0.28




Design features
Construction:
Mostly studded
Hull:
Unitary 28x8x3.6 LU Fishing Boat hull (92710c01pb01)
Propulsion:Elastic band motor
Propeller:Highly modified 5.5L 2-blade LEGO® "twisted" propeller (4745)
Gearing:2-stage direct (1:1)
Steering:None
Power supply:
190x3.2 mm (nominally 7x1/8 inch) elastic band
Modified LEGO® parts:
Propeller
Non-LEGO® parts:Elastic band
Credits:
Original MOC based on a suggestion by favorite builder David Roberts





Footnotes

1 Adults can play with X-ray in the tub with or without another adult present at their own (in)discretion.

2 The handedness of the prop determines the direction that the rubber band must be wound to produce forward thrust when it unwinds.

To determine handedness, view the boat from the stern and rotate the prop in the direction producing forward thrust. If blades sweep to the left as they pass through top dead center, the prop is left-handed.

Alternatively, lay the prop on a table forward face down with one blade at 12 o'clock. If the right edge of that blade is closer to the table, the prop is right-handed.

Each distinct ready-made LEGO® prop design comes in just one handedness, but some (like the 9L props and 5.5L 3-blade) are right-handed, while others (like the 5.5L 2-blade and 3L 3-blade) are left-handed. Real-world twin-screw boats use counter-rotating pairs (one right-handed, the other left) to eliminate prop walk, but TLG has never offered counter-rotating props.

3 The blades of this so-called "twisted" propeller really aren't twisted at all. Rather, their uniform blade angle is just greater than those found on most LEGO® props.





Comments

 I made it 
  September 3, 2015
Quoting jds 7777 Great job! I tried this once with an all-Lego submarine I built out of technic, but ran into some issues with the sub trying to rotate underwater. If you use 2 rubber bands, you'll have more torque to use a higher overdrive ratio. (3 is the max amount of standard rubber bands I've been able to fit through a technic hole)
Thanks, JDS! I can imagine that it would be very difficult to suppress torque reaction in a sub. Theoretically, you're right about adding rubber bands. In practice, however, it created 2 interrelated issues in this particular design. First, friction among the individual strands limited available torque during unwinding and decreased the extent of unwinding as well. Second, the added tension greatly increased the friction between the gear on the stern drive input shaft and the stern drive strut acting as its thrust bearing -- especially right after release. Net result: Speed and endurance actually suffered at all overdrive ratios tested. I'll be posting an updated X-ray with solutions to both limitations shortly.
 I like it 
  September 2, 2015
Great job! I tried this once with an all-Lego submarine I built out of technic, but ran into some issues with the sub trying to rotate underwater. If you use 2 rubber bands, you'll have more torque to use a higher overdrive ratio. (3 is the max amount of standard rubber bands I've been able to fit through a technic hole)
 I like it 
  August 31, 2015
Thanks for the answers! I understand the handedness aspect now.
 I made it 
  August 28, 2015
Quoting VAkkron ™ Reading such a summary when MOCpages' pictures are down proved difficult. But I found the experiment interesting, though I have a few questions. First, are all Lego prefab propellers left-handed? You mentioned them all being of one-handedness. The handedness is a dynamic I hadn't considered in the winding process. Your winding solution is quite clever. Now I want to try one of these as well! Also, why would there be an ugly gap attaching two prefab boat hulls stern-to-stern? I can't quite tell which piece you use; perhaps 4597140? Finally, exploring some of your old works, I found a comment mentioning pairing electronic motors in series, and it made me wonder what the possibilities are of pairing rubber bands in series? Thanks again for your work; I love the thoughts they produce.
Thanks for those very kind words, Vakkron. Each distinct prefab design (e.g., 9L 2-blade vs. 5.5L 2-blade) comes in just one handedness, but some designs (like the 9L) are right-handed, while others (like the 5.5L) are left-handed. Unfortunately, prefab contra-rotating pairs (i.e., right- and left-handed props of the same design) don't exist. The gap David and I were discussing arises because the deck overhangs the hull slightly in the Fishing boat hull and several other useful unitary hulls as well. Hence, only the decks would be butting up against each other. The hulls below wouldn't be touching. Some hulls don't have overhanging decks, but the actual bond would still be deck-to-deck, not hull-to-hull, and would have to be very strong to resist the wave-related forces trying to pull and twist the hulls apart. All multihull LEGO boats face that challenge, but you can recruit a lot more studs to the cause when joining hulls side-to-side. Since I'm still learning my way around rubber bands, I don't have a good answer for your last question. Stringing rubber bands end-to-end might be a way to reach the perfect length for a particular boat, but the knots might also interfere with unwinding. You'd just have to experiment.
 I like it 
  August 27, 2015
Reading such a summary when MOCpages' pictures are down proved difficult. But I found the experiment interesting, though I have a few questions. First, are all Lego prefab propellers left-handed? You mentioned them all being of one-handedness. The handedness is a dynamic I hadn't considered in the winding process. Your winding solution is quite clever. Now I want to try one of these as well! Also, why would there be an ugly gap attaching two prefab boat hulls stern-to-stern? I can't quite tell which piece you use; perhaps 4597140? Finally, exploring some of your old works, I found a comment mentioning pairing electronic motors in series, and it made me wonder what the possibilities are of pairing rubber bands in series? Thanks again for your work; I love the thoughts they produce.
 I made it 
  August 27, 2015
Quoting Nick Barrett Surprisingly effective, that's what you get with careful optimization! Great stuff :-)
Thanks, Nick!
 I like it 
  August 27, 2015
Surprisingly effective, that's what you get with careful optimization! Great stuff :-)
 I made it 
  August 26, 2015
Quoting David Roberts Yes, brilliant! That's exactly what I had in mind. I've been busy with work and the DA2 group on MOCpages and haven't even started to try elastic band boat ideas. Glad those hulls have found a role in life. Could joining them stern to stern give a higher aspect ratio hull and space for an even longer elastic band? I suppose that there'd be a nasty gap at the join of the two hulls. I like the neat little trim tabs that you added. They're simple, effective engineering solution. I think that I'll have to do an elastic band boat for my next MOC.
Thanks, David. We'll have to put together a rubber regatta when you're done! Next time I have these boats apart, I'll try them stern-to-stern. Agreed, the gap will be problematic -- especially with a highly tensioned band across it -- but my gut tells me there's a solution. FWIW, the elastics tried so far have been less than ideal WRT length, loss of elasticity, and breakage. However, I've ordered some latex "sport rubber" strip (single length, not a loop) from a model airplane dealer. If it works for hard-core rubber band airplane enthusiasts, it's gotta be worth a try here.
 I like it 
  August 26, 2015
Yes, brilliant! That's exactly what I had in mind. I've been busy with work and the DA2 group on MOCpages and haven't even started to try elastic band boat ideas. Glad those hulls have found a role in life. Could joining them stern to stern give a higher aspect ratio hull and space for an even longer elastic band? I suppose that there'd be a nasty gap at the join of the two hulls. I like the neat little trim tabs that you added. They're simple, effective engineering solution. I think that I'll have to do an elastic band boat for my next MOC.
 
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LEGO models my own creation MOCpages toys shop X-ray, a rubber band boatTechnic


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