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Dubble같Bubble
This fast, ultra-stable 0.71 kg, 4.1 W twin-screw catamaran is a ton of fun in the water -- especially with Bluetooth remote control via SBrick.
About this creation
Please feel free to look over the images and skip the verbiage.

Dubble같Bubble (DB) evolved from one of my earliest LEGO powerboats. She's still one of my favorites for 2 reasons...

(i) She's a lot of fun in the water.



(ii) I like the way she looks.





The original DB was built in August, 2013 in collaboration with long-time boat-building buddy Shawn Kelly. We later discovered a similar catamaran by favorite builder Walter Lee. The 2 designs represent a clear case of convergent evolution in response to a particular set of challenges -- in this case, the difficulties associated with extracting a seaworthy powerboat from a pair of inherently unstable, leaky, low-volume LEGO hulls with all their attachment points located aft.



Here, DB's lined up behind Trident, a less stable but considerably faster power trimaran started at about the same time.

Over the last 2 years, DB has steadily lost weight and picked up propulsive efficiency and speed. Her play value's always been high, but her greatest advance on that front came just this week from Budapest: An SBrick allowing a long-awaited switch to Bluetooth-based remote control. Hallelujah!



DB was still using TLG's exasperating IR-based PF remote control system when I shot the video. Her speed and maneuverability still come through, but the demo is marred by repeated shutdowns due solely to the PF system's dismal range and susceptibity to IR interference from sunlight.

Thankfully, the new SBrick put an end to all of that. The new smartphone-based control interface is also a vast improvement over any conceivable PF handset.



This Android interface is the best I've found so far. Vertical movements of the left and right joysticks continuously vary power to the port and starboard motors with no detectable latency. The rest of the controls are unassigned.

NB: DB appears with her new SBrick only in the bathtub shots. The video and the rest of the boat photos are otherwise current except for the lights, which DB later lost to another MOC.

On this page:


Design notes

DB is a fast, seaworthy 0.71 kg, 4.1 W twin-screw power catamaran based on the long, narrow 48x6x5 LU Speedboat hull (SBH, set 7244, 2003).



A best speed of 0.79 m/s puts her in our 2nd tier of LEGO speedboats1, but I should put that in perspective: Though unable to keep up with Triton, Nadine, Laverne, and Trident, she's still plenty fast -- faster, in fact, than most of the LEGO boats you'll find on YouTube and at least 30% faster any boat raced at BrickWorld 2015.2

The main components of DB's powerful twin "L/5.0/55" (L motor, 1:5.0 overdrive, 55 mm prop) propulsion system are easier to see in this early version. Trident and Laverne also use the L/5.0/55.



DB's motors and final drive ratios are set now, but she might be a hair faster with 52 mm props. An upcoming time trial will tell, at which point her motor/gearing/prop (MGP) optimization will be complete.



The rationale behind our angled outdrives and third-party props is discussed here.

With only 6 mm of freeboard, DB has no capacity to take on water gracefully.



The overhanging cambered black weather decks quickly shed deck wetness, but they don't keep water from leaking into the hull bottoms from the sides in rough water -- hence, the red electrician's tape sealing the hulls.3



With the tape, DB's like a submarine cruising at the surface in that her seaworthiness doesn't depend on freeboard.

Plate and beam construction nicely stiffens the all-important cross-structure holding the SBHs together. Should the cross-structure or its attachments fail, all of the electricals would surely end up in the drink.





The bubbles were originally meant as splash guards. Testing proved them unnecessary, but I kept them anyway. With the lights now on another MOC, everything else aboard is 100% business.

Wave-making resistance accounts for most of DB's total resistance near top speed. In catamarans, the power lost to wave-making between hulls can be reduced by increasing hull separation, but there's a catch: The greater the separation, the greater the danger of cross-structure failure in collisions and wave impacts.



DB's current 176 mm hull separation seems to play that trade-off reasonably well.4

The lines would be a lot cleaner without a battery and receiver (here the old IR) cantilevered way out front, but this arrangement minimizes the mass that had to be added just to balance the motors and outdrives. Walter Lee arrived at a similar solution.



You can tell from this early DB's excessive positive (bow up) trim that the battery+receiver "diving board" was still too short. You told me so, didn't you, Pops?



My initial battery choice was the heavier 7.2V AAA box seen here, but only because was I focused on the wrong thing -- namely, battery as counterweight instead of battery as battery.



Thankfully, favorite builder jds 7777 reminded me in an early comment that I could use the lighter, higher-draw 7.4V Li polymer battery, bring its mass up to that of the AAA with a little dead weight, and come out way ahead performance-wise. Done! I love comments like that.

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Specifications (AAA version)
All measurements taken at rest in fresh water (density 1,000 kg m-3).

Overall dimensions and design features
Hull configuration:Catamaran
Construction:Studded except for outdrives
Overall dimensions:420 x 224 x 92 mm (LxWxH) excl props
Displacement:0.712 kg
Displacement volume:7.16e-4 m3
Wetted surface area:n/a
Hull separation:176 mm on center
Demihulls:48x6x5 LU Speedboat hulls (set 7244, 2003)
Propulsion:Twin inverted-V outdrives
Motors:2, 1 L on each prop
Propellers:Custom 55 mm 3-blade counter-rotating pair
Gearing:2-stage 1:5 overdrive
Propeller separation:238 mm on center
Steering:Differential power to props
Electrical power supply:7.2V from 6 NiMH AAAs
Bluetooth receiver:SBrick
SBrick control interface:"Jurgen's Excavator" profile on Android phone
Modified LEGO parts:Prop hubs
Non-LEGO parts:SBrick, props, electrician's tape
Credits:Original MOC in collaboration with Shawn Kelly


Demihulls
Displacement share:50% each
Displacement volume:3.58 x 10-4 m3 each
Waterline length:372 mm each
Waterline breadth:44 mm each
Freeboard:6 mm each
Draft:32 mm each
Length/breadth ratio:8.5 each
Length/displacement ratio:5.2 each
Breadth/draft ratio:1.4 each
Block coefficient:0.68 each
Prismatic coefficient:0.73 each
Midship coefficient:0.93 each
Waterplane area coefficient:0.82 each


Performance measures
Installed power:4.1 W at 7.2V
Installed power to displacement ratio:5.7 W/kg
Critical speed:0.76 m/s based on hull separation
Best speed:~0.79 m/s1
Froude number on hull length:0.41
Froude number on hull separation:0.60
Reynolds number at top speed:2.9 x 105
High-speed index:0.71


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Footnotes

1 This average speed was clocked over a relatively short 13.3 m course from a standing start. Top speed is probably 5-10% higher.

2 I know this because I collaborated with Shawn Kelly on Radio Flyer, the boat that won the drag races at BrickWorld 2015. DB's at least 30% faster.

3 We seal all our unitary hulls with weather decks with either electrician's tape or silicone caulk. Both both easily removed.

Such seals should be inspected before every use -- especially near sharp corners. A very close call with Trident taught us to check for hidden water accumulations due to slow leaks by weighing sealed boats once when known to be dry and then again before each use.

4 It's quite possible to build LEGO powerboats with 3 or more Speedboat hulls (SBHs), but I don't recommend it, as acceleration, top speed, and maneuverability all go down with each SBH added.

I've seen multihulls consisting of 4-6 SBHs on the internet. They're all very slow and hard to turn.

For example, the all-SBH trimaran airboat below started out as speedboat with a twin L/5.0/55 propulsion system just like DB's. DB was at least 15% faster.



And Trident below is 20% faster than DB.



Problem is, the total resistance of a multi-SBH boat equals the sum of the total resistances due to each SBH in isolation plus a penalty for wave-making between each pair of adjacent SBHs. And since safe hull separations tend to decrease as the number of hulls increases, the total interhull wave-making penalty actually grows faster than the number of interhull spaces.

Granted, as the number of SBHs goes up, so does the boat's total carrying capacity, and that allows one to add installed power in various forms. However, the total resistance generally grows faster than the thrust one can add to overcome it.

These burdens become especially onerous at speeds where most of the total resistance is due to wave-making. Such speeds are readily attainable by well-designed LEGO speedboats.

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Selected references

Blount, D.L., 2014, Performance by Design (self-published book)

Molland, A.F., Turnock, S.R., and Hudson, D.A., 2011, Ship Resistance and Propulsion: Practical Estimation of Ship Propulsive Power, Cambridge University Press

Noblesse, F., He, J., Zhu, Y., et al., 2014, Why can ship wakes appear narrower than Kelvin뭩 angle? European Journal of Mechanics B/Fluids, v.46, p.164171

Please click here to see the complete list.

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Comments

 I made it 
  June 8, 2016
Quoting Oran Cruzen Very nice! You know that some of your creations are all wet, till you take them out of the water and dry them off! LOL
Thanks, Oran! My mother taught me never to put LEGO away wet, but now that I'm a GFOL (geezer fan of LEGO) like you, I put my LEGO away any darned way I please. ;^}
 I like it 
  June 8, 2016
Very nice! You know that some of your creations are all wet, till you take them out of the water and dry them off! LOL
 I made it 
  June 8, 2016
Quoting Clayton Marchetti Wow she is sleek! I love the cell phone remote idea.
Thanks, Clayton! She's not my fastest boat, but she's definitely the coolest-looking, and one of the very best in terms of fun in the water. I'm pretty much done with LEGO infrared remote control at this point. If only SBrick prices would come down!
 I like it 
  June 8, 2016
Wow she is sleek! I love the cell phone remote idea.
 I made it 
  December 23, 2015
Quoting Nerds forprez Its fun to tinker with past models and make them even better and more functional. Great job tinkering with this baby and making 'er better and better....
Thanks, NFP! Yes, that's another aspect of the "lives of MOCs" you were writing about recently. Some of my hulls have gone through many different boats, but these 2 started out in DB snd stayed there.
 I made it 
  December 23, 2015
Quoting Elk Guard Great work, nice write up too!
Many thanks, EG! Have a great Holiday.
 I like it 
  December 23, 2015
Great work, nice write up too!
 I like it 
  December 22, 2015
Its fun to tinker with past models and make them even better and more functional. Great job tinkering with this baby and making 'er better and better....
 I made it 
  December 22, 2015
Quoting Navy Person Wow, this is way cooler than any toy boat I used to own! I took a brief overview of your page, and I absolutely love how you manage to combine LEGO's, playability, math and engineering in all your creations. I'll definitely be taking a look at the rest of your stuff. Cant wait for more!
Much too kind, NP! For lack of a better term, I think of that mix of LEGO, playability, math and engineering as "technical LEGO". It's not for everybody, but I sure have a lot of fun with it and am always glad to hear from others who enjoy it, too.
 I like it 
  December 22, 2015
Wow, this is way cooler than any toy boat I used to own! I took a brief overview of your page, and I absolutely love how you manage to combine LEGO's, playability, math and engineering in all your creations. I'll definitely be taking a look at the rest of your stuff. Cant wait for more!
 I made it 
  December 22, 2015
Quoting Didier B Nice moc ! Does not seem very easy to control on the video. Maybe the shape does not allow to turn more easily. Didier
Thanks, Didier! Yes, control was definitely an issue in the video, but as I pointed out above, most of the blame for that lies squarely with PF remote control and its many shortcomings -- especially WRT boats. The SBrick installed after the video finally gave the boat what any differential-drive twin-screw needs -- fine, reliable, independent control of the power going to the motors with no latency. As for the boat's inherent maneuverability, a catamaran with widely separated narrow hulls will always need large steering moments to get it to deviate from a straight line. The steering moments generated by DB's differential drive propulsion system are more than adequate for high maneuverability. However, applying them properly takes practice and a power control system up to the task. Real twin-screw drivers also need these things, but at video time, my driver had none of them.
 I made it 
  December 22, 2015
Quoting Chris Maxter Definitely had to add you as a favourite builder.
Thank you very much, Chris!
 I like it 
  December 22, 2015
Definitely had to add you as a favourite builder.
 I like it 
  December 22, 2015
Although this probably isn't available to you in this situation, I like to make some sort of small function for my models when I need extra weight. That way it technically isn't dead weight, and I can rest easy. :) Merry Christmas!
 I like it 
  December 22, 2015
Nice moc ! Does not seem very easy to control on the video. Maybe the shape does not allow to turn more easily. Didier
 I made it 
  December 22, 2015
Quoting Turbo Charger Very nice!
Thanks, TC!
 I made it 
  December 21, 2015
Quoting Walter Lee Just Love it! I've been doin' the Lego Train Display stuff ... but this makes me wanna go back to the speed boat stuff...oooh yeah!
Thank you very much, Walter! Hope you do. Pools open again in 6 months!
 I made it 
  December 21, 2015
Quoting jds 7777 Awesome! Another RC post! Love all the tech info. I didn't fully understand why you went with the AAA battery box. For ballast, perhaps? If so, I would suggest you use a rechargeable, then add (shudder) dead weight to hold the nose down. Reason being the AAA's horrible power curve. Again, truly a well built boat!
Yes, for ballast, JDS, and geez, why didn't I think of that tack??? It will be just as you describe by tonight. We have the same feelings about (gasp shudder) dead weight. (I can barely bring myself to say it. I even have nightmares about it.) But I wasn't able to break through my (omg, I'm gonna say it, I'm gonna say it!) dead weight phobia, and you were. I will conquer this aversion one day and will have you to thank for it, my friend. Have a wonderful Holiday! ;^}
 I like it 
  December 21, 2015
Just Love it! I've been doin' the Lego Train Display stuff ... but this makes me wanna go back to the speed boat stuff...oooh yeah!
 I made it 
  December 21, 2015
Quoting Oliver Becker Suggested for the "Blue Riband" making the fastest and finest crossing of your pool, Jeremy! Great cat and perfect performance! Btw, is this the actual weather in Colorado? Enviable... We've storm and rain here in Northern Germany! ;)
Thanks, Oliver! Man, I wish it were like that here now! We visited my daughter in Oldenburg 3 years ago in March. Guess I'll have to torture you now with the weather around here. Video is from August, when it's often just as shown. We do cool off in the winter and have our winter storms, but they're generally short and well separated by gorgeous bright winter skies with almost no humidity. On any given winter day here in Denver, the temperature can be anywhere in the 0-75캟 range. And whatever it is, it won't last long. Highly entertaining. If it weren't for the cold air masses dumped on us repeatedly -- and I'm sure quite intentionally -- by the Canadians, who aren't the "goody-two-shoes" they make out to be, our winters would be rather mild. ;^}
 I made it 
  December 21, 2015
Quoting Gabor Pauler It has quite a styling, just put in the middle the Nasty Big Nuclear Missile Launcher to hide the battery box somewhere...
Splendid suggestion, Gabor! Will start outfitting it immediately. Nothing says Christmas like an NBNML, you know, and I have only 4 days now to weaponize it for you.
 I like it 
  December 21, 2015
Suggested for the "Blue Riband" making the fastest and finest crossing of your pool, Jeremy! Great cat and perfect performance! Btw, is this the actual weather in Colorado? Enviable... We've storm and rain here in Northern Germany! ;)
 I like it 
  December 21, 2015
Very nice!
 I like it 
  December 21, 2015
Awesome! Another RC post! Love all the tech info. I didn't fully understand why you went with the AAA battery box. For ballast, perhaps? If so, I would suggest you use a rechargeable, then add (shudder) dead weight to hold the nose down. Reason being the AAA's horrible power curve. Again, truly a well built boat!
 I like it 
  December 21, 2015
It has quite a styling, just put in the middle the Nasty Big Nuclear Missile Launcher to hide the battery box somewhere...
 I made it 
  December 21, 2015
Quoting Desert752 Kirill Very cool design!
Thank you very much, DK!
 I made it 
  December 21, 2015
Quoting David Roberts Possibly the most precariously poised battery box in the history of your Technic boat building! Daring design!
Thanks, David! Glad to see you back here. Thankfully, that battery and receiver "diving board" has never lived up to its name. Gave me the willies at first, too, but the strength and stiffness of plate-and-beam construction is pretty phenomenal, as I'm sure you know. Here, the Technic beams are 3 extra-strong 4x6 frames joined end-to-end and then sandwiched between long 4-wide plates.
 I made it 
  December 21, 2015
Quoting Seaman SPb Excellent boat!
Thanks, Seaman!
 I like it 
  December 21, 2015
Very cool design!
 I like it 
  December 21, 2015
Possibly the most precariously poised battery box in the history of your Technic boat building! Daring design!
 I like it 
  December 21, 2015
Excellent boat!
 
By Jeremy McCreary
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