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Platform supply vessel "Stella"
This RC platform supply vessel comes in 2 flavors: "Stella L" with twin L motors, and "Stella R" with twin RC Race Buggy motors. Both have great play value -- especially the faster, more seaworthy, and =much= less costly Stella L.
About this creation
Please feel free to look over the images and skip the verbiage.

Stella is a motorized model platform supply vessel based on TLG's latest powerboat-compatible hull -- the workboat-like 40x20x7 Exploration Vessel hull.



The far-forward deckhouse and the ability to span the hull's large stern cutout securely with a 16x16 plate opened up a new possibility for my LEGO® powerboats -- easily swapped modular propulsion systems.





On this page, I present two Stellas differing only in "propulsion module" (PM) and associated forward ballast: A well-optimized Stella L fitted with a twin-outdrive PM driven by 2 L motors...



... and an experimental Stella R fitted with a twin-outdrive PM driven by 2 "RC Race Buggy" (RCRB) motors (5292), or "R motors" for short.



Stella L will be our reference case. The video at the top of the page is from her first pool trial.

On this page:


Common features

All Stellas share a common core weighing only 0.57 kg.



This core consists of
  • A unitary 40x20x7 Exploration Vessel hull
  • A forward deck house loosely modeled after that of a typical platform supply vessel (PSV)
  • A PF rechargeable LiPo battery box hidden in the deck house
  • US Coast Guard-compliant running lights (not all lit)
  • A Bluetooth-based SBrick RC receiver with a maximum range of ~15 m
  • A very precise single-joystick control interface on my Android phone
  • A twin-outdrive "propulsion module" (PM) nestled in the hull's stern cutout
  • Steering by differential power to the motors
  • Some forward ballast to counterbalance the PM
Much of the deck house exterior came from the hull's parent set (Fire Boat 60109, 2016).



To remain true to the PSV look and balance out the propulsion modules, I had to move the deck house far forward. I also had to hollow out the interior to make room for the battery box and forward running light LEDs.



A hinged aft windscreen provides access to the battery's power toggle and charging port.

To get the speed, stability, seaworthiness, and reserve buoyancy I wanted out of this boat, I had to keep displacement down and the center of mass low to the keel. That meant choosing my frills carefully.

Since platform supply vessels tend to have very uncluttered decks anyway, I put a basic mast and antenna package on top of the deck house, added a full set of running lights, and left the cosmetics at that.



Only the port and starboard running lights have LEDs.





Stella's value as a propulsion system test platform rests on several key features. For starters, she has the stability, reserve buoyancy, and freeboard to handle fairly heavy PM and ballast loads, as evidenced by Stella R.



Another key feature is the unique layout of the Exploration Vessel hull -- basically a monohull forward and catamaran aft.

The hull's large 12x14 stern cutout can accommodate a variety of propulsion system types, including the single-screw inboard-outboard below.



Many powerboat-compatible hulls have built-in stern bulwarks that limit the propulsion system's access to the water, but not so here.

More to the point, the cutout also accommodates self-contained PMs mounted on 16-wide plates.





Here we have Stella R's PM on the left and Stella L's on the right. Steering is by differential power to the props in both cases.

Finally, PM test results using this platform are probably reasonably representative, as hull resistance is not too far above average as powerboat-compatible hulls go.



Water comes over the bow in moderate to heavy swimming pool chop in both Stellas, especially Stella R. This water accumulates deep in the hull and must be drained periodically to maintain seaworthiness. A few degrees of positive (bow-up) trim slows the flooding but doesn't eliminate it entirely.

Swapping out a PM can alter the boat's forward-aft balance drastically, but the desired bow-up trim can be restored in a number of ways, including (i) adding or removing forward ballast or frills, (ii) shifting the SBrick receiver or deck house and battery forward or aft, and (ii) choosing a lighter (74 g) LiPo or heavier (110 g) AAA battery box as needed.



Pushing the ballast as far forward as possible minimizes its impact on displacement. With the deck house in the position shown, a single boat weight (73090b) balanced out Stella L's PM, but Stella R's heavier PM needed two. Each boat weight added ~55 g.



After the photos on this page were taken, I discovered that I could get the trim I wanted with 1 less boat weight each for Stella L and Stella R by (i) moving the deck house and battery forward one more stud, and (ii) adding or removing some more frills forward.







The final forward layout, here modeled by Stella L, reduces total displacement by 30 g, increases freeboard by a millimeter, and looks a bit more realistic to boot.

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Stella L

Since Stella L will be serving as our reference case, I'll document her in some detail. Here's an overview...













Stella L is a seaworthy 0.79 kg, 4.3 W, 5.5 W/kg twin-screw powerboat. Top speed, conservatively estimated at ~0.60 m/s, is quite good considering the short workboat-like hull. Stability, control, and maneuverability are excellent. The only freeboard issues are at the bow in moderate to heavy swimming pool chop.

Each of Stella L's outdrives is powered by an independently controlled L motor via a 1:5 overdrive transmission. This twin "L/5.0/52" PM came out of a rigorous hull/motor/gearing/prop optimization process. (The "L" here refers to motor type; the "5.0", to final overdrive ratio; and the "52", to propeller diameter in millimeters.)





The highly efficient counter-rotating 52 mm 3-blade props are made for hobby shop RC boats. The thinking behind our twin-screw propulsion systems and third-party props is discussed here.



This 208 g twin L PM is 60 g lighter than Stella R's twin R PM. Neutral trim was achieved by placing a single boat weight at the bow and shifting the SBrick to the location as shown.

Aside from the slightly smaller props, Stella L's propulsion system is identical to the twin L/5.0/55 systems installed in Celine, Laverne, and Trident below.




Stella L gets less top speed from the same 4.3 W of installed power for 2 main reasons: (i) Lower installed power to displacement ratio, and (ii) greater total resistance, primarily from the hull. Appendage drag is about the same in all cases.

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Stella R

NB: Unless otherwise noted, all comments here refer to the slower of the R motor's power take-offs (PTOs), as that's the one actually used. The faster PTO is quite hopeless in boats with efficient props for lack of torque.

Relative to Stella L, Stella R is 14% heavier and ~20% slower. Roll stability is adequate but also reduced. The lesser freeboard looks adequate at rest but becomes marginal underway due to the high standing waves along the hull at speed.









I worried that Stella R might get stranded far from shore, as her R motors are quite prone to overcurrent shutdown during continuous full-power operation. But with the gearing+prop combination shown here -- and with very few others -- her run times became charge-limited and quite acceptable.1

Importantly, Stella R has one and only one advantage over Stella L: The sound made by her R motors is more realistic.

Beyond that, there's nothing to be gained and potentially a great deal to be lost by using R motors to drive underwater propellers. (Airboats are a different story, but they're not under discussion here.)

Consider 3 facts:
  • The PF motors of most use in powerboats -- the L and XL -- are cheap and readily available.
  • R motors are very expensive (typically $50 or more used) and very hard to find.
  • Unexpected things happen on the water, and LEGO® powerboats sometimes sink -- even in the best of hands.
In light of these facts, one should have a compelling reason to put an R motor on the water in lieu of an L or XL.

I'm now convinced that for boats other than airboats, no such reason exists.

The 268 g twin R/1.08/52 PM below is the best I've found for Stella R. With it, she displaces 0.93 kg and carries 8.4 W of installed power. The resulting peak power to displacement ratio (a whopping 9.1 W/kg) is the highest of any of my boats, but lack of torque at the prop shafts leaves much of that power inaccessible.









This PM is about as compact and lightweight as a twin-R PM can be. Available LEGO® gears limit the possible final ratios to 1:3.00, 1:1.80, and 1:1.08, all overdrives.



The preferred 1:1.08 overdrive shown here maximizes speed while keeping run times charge-limited. At 1:1.80 and 1:300, run times wer consistently protection-limited and frustratingly short. The timing disk on the port outdrive is used in hull/motor/gearing/prop optimization.

For kicks, I also tried the twin R/5.0/44 PM seen below on an earlier Stella R. The "44" here refers to the yellow LEGO® 44 mm 2-blade "twisted propeller" (4745).



This prop is TLG's most efficient by a long shot. Hand-sanding the slab-like blades into proper airfoil profiles improves efficiency quite a bit, but this prop is still no match for the non-LEGO® prop just shown.



In testing, this R/5.0/44 PM proved to be comfortably charge-limited but much slower than Stella R's preferred R/1.08/52 propulsion unit. A geared-down R/3.0/44 PM (not shown) was even slower.



So why are there so many (slow) LEGO® powerboats driven by R motors on YouTube? The only reason I can think of is the (undue) allure of the R motor's unmatched peak mechanical power: At 7.4V, the R, L, and XL generate 4.21, 2.15, and 2.45 W of peak power at 510, 156, and 90 RPM, respectively.

Quite a difference on paper, to be sure, but in my experience, the R motor's power advantage just doesn't materialize on the water for lack of torque. At 7.4V and peak power, the R, L, and XL respectively produce 0.08, 0.13, and 0.26 N m of torque.

Twin-XL Nadine here owes her great speed to the XL's unbeatable combination of high peak power and high torque at peak power.



The R-driven LEGO® powerboats I've run across on YouTube appear to be a good bit slower than either of the Stellas, but especially Stella L. The highly inefficient LEGO® props they all use are largely to blame, but I also have to wonder if they've undergone any kind of rigorous hull/motor/gearing/prop optimization, as both Stellas have.

In my experience, optimization can make a huge difference in top speed.

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Specifications (Stella L only)


Dimensions and hull form coefficients
All measurements taken at rest in fresh water (nominal density 1,000 kg m-3).

Overall dimensions:382x154x188 mm (LxWxH, including outdrives)
Scale:~1:220 relative to 70 m PSV
Displacement:0.786 kg
Forward ballast:None
Displacement volume:7.9 x 10 -4 m3
Depth:66, 46 mm (bow, midships)
Waterline hull length:296 mm
Waterline hull breadth:154 mm
Draft at keel:27 mm (midships)
Freeboard:39, 19 mm (bow, midships)
Midship section area:4.2 x 10 -3 m (monohull section)2
Waterplane area:2.9 x 10 -2 m2
Block coefficient:n/a
Prismatic coefficient:n/a
Midship coefficient:~1.00
Waterplane area coefficient:0.64
Length-breadth ratio:1.9
Breadth-draft ratio:5.7
Length-displacement ratio:3.2


Performance measures

Hydrodynamic regime:High-speed displacement
Total installed power (propulsion):4.3 W at 7.4V
Installed power to displacement ratio:5.5 W/kg
Critical speed:0.68 m/s
Course length:n/a
Average forward speed:Conservatively, ~0.60 m/s
Froude number:~0.35
Reynolds number:1.8 x 10 5


Design features

Construction:Studded except for outdrives
Hull:Exploration Vessel hull
Propulsion type:Twin outdrives
Propulsion motors:1 L on each outdrive
Propellers:Third-party counter-rotating 52 mm 3-blade
Propulsion gearing:2-stage 1:5.0 overdrive
Prop separation:160 mm
Prop depth:30 mm at hub
Steering:Differential power to props
Electrical power supply:7.4V from PF rechargeable LiPo battery
Lighting:PF LEDs in port and starboard running lights
Remote control receiver:Bluetooth-based SBrick
Control interface:Single joystick on Android phone
Modified LEGO® parts:Prop hubs
Non-LEGO® parts:Props, SBrick
Credits:Deck house modified from the hull's parent set; otherwise original MOC
See also:Rotortug RT Vanna, twin-VSP workboat Miss Vickie

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Footnotes

1 By "charge-limited" here, I mean that the only thing limiting run time is remaining battery charge. In contrast, a "protection-limited" run time is cut short by the tripping of the overcurrent protection built into the battery, RC receiver, or motors.

R motors guzzle more current than any other LEGO® motor by a wide margin. When fed by PF LiPo batteries and SBrick RC receivers, the R motors themselves are usually the first to go in my experience -- especially when lugged or stalled on hot pavement on a hot day.

When an R motor quits, recovery can take minutes to over an hour depending on how fast the built-in thermistor can cool. Coming off the throttle right away and taking the disabled R motor to a cool place out of the sun and off any hot pavement can improve recovery time considerably.

Frankly, I was surprised that Stella R could be made charge-limited with any gearing+prop combo, but I'd forgotten a key factor -- external temperature.

This is no RC car on hot pavement, after all. Stella R's motors operate in the evaporation-chilled air just above the equally cool water surface. And that would go a long way toward keeping her R motor thermistors below critical temperature -- even at the high, sustained current draws involved in pushing her through the water lap after lap around the pool.

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Comments

 I made it 
  June 1, 2016
Quoting Henrik Jensen Very nice post Jeremy! I like your little PSV. It`s a great feature that you can quickly swap the power-drive. Have you considered using the cutout in the hull for a voith-schneider drive or maybe a paddle-steamer whell?
Thanks, Henrik! Excellent idea regarding the Voith-Schneider propeller (VSP) option! I happen to have several VSP units laying around waiting for boats. A single VSP would fit quite nicely in that space. It would also be very interesting to see how controllable the boat would be with just one VSP, as the real boats with VSPs that I'm familiar with generally have at least 2. Stay tuned! Paddle wheels are so darn inefficient that I'd have a hard time bringing myself to build a boat with that kind of propulsion. I'd also have to come up with a separate steering system. (As you know, one of the beauties of the VSP is that the same unit provides both propulsion and steering.)
 I like it 
  June 1, 2016
Very nice post Jeremy! I like your little PSV. It`s a great feature that you can quickly swap the power-drive. Have you considered using the cutout in the hull for a voith-schneider drive or maybe a paddle-steamer whell?
 I made it 
  May 31, 2016
Quoting jds 7777 I'm surprised the motors are the first to trip their thermal sensors. Usually two R motors one output is considered overloading the battery box and the receiver. I've never had a problem with the thermal protection on my motor, but I use a custom battery box which can handle a much higher current. I'm not sure that has anything to do with the motor's thermal sensor, though. Here's a link to that blimp I told you about: https://www.youtube.com/watch?v=FkfPeKyFG4I
Thanks for the link! The R motors are definitely the first to go in my twin-R racer (coming soon). (For starters, the steering motor continues to operate normally when the R motors stop working.) That also appears to be true in improperly geared versions of Stella R. In both cases, changing the battery right after a shut down has no effect on recovery time, and the currents involved are insufficient to shut down the SBricks in these MOCs.
 I like it 
  May 31, 2016
I'm surprised the motors are the first to trip their thermal sensors. Usually two R motors one output is considered overloading the battery box and the receiver. I've never had a problem with the thermal protection on my motor, but I use a custom battery box which can handle a much higher current. I'm not sure that has anything to do with the motor's thermal sensor, though. Here's a link to that blimp I told you about: https://www.youtube.com/watch?v=FkfPeKyFG4I
 I made it 
  May 29, 2016
Quoting Oliver Becker "Stella"-r performance,Jeremy! Even I haven't understand everything in special in the description I really have understood that you are playing the first fiddle on this concert platform (maybe this platform also supplied by Stella...?) ! ;)
Too kind, Oliver! Normally, Stella services oil platforms, but in your case, we'll make an exception.
 I made it 
  May 29, 2016
Quoting Nerds forprez When in question, test, test, test! I love your post, my favorite being the optimization process of prop speed. Great stuff. Not surprised by the findings either. The youtube reference IMO is a classic example of behavior driven by ideology and not proven, or at least objective, information.
Thanks, NFP! Yes, "testing, testing, testing" is the necessary mantra -- especially with boats, where everything depends on everything else to an extreme degree, and conventional wisdom often fails as a result. But that's part of the fun.
 I like it 
  May 29, 2016
"Stella"-r performance,Jeremy! Even I haven't understand everything in special in the description I really have understood that you are playing the first fiddle on this concert platform (maybe this platform also supplied by Stella...?) ! ;)
 I made it 
  May 29, 2016
Quoting Seaman SPb Excellent work, Jeremy!
Many thanks, Seaman!
 I made it 
  May 29, 2016
Quoting jds 7777 Another great post, Jeremy! I'm not really surprised at all by the R motors not working well. I have one, and torque suffers quickly under heavy strain, which draws quite a bit of power. Most of the Lego boats I've seen with these motors use 2 battery boxes, which (usually) takes care of the thermal shutdown issue. The R motors usually get used in these builds because people typically use inefficient Lego propellers which only work if spun really fast - thus the R motor. Have you seen TLG's flying lego blimp? It uses 3 R motors to power its 3 propellers. R motors it seems, are best used on the ground and in the air, which actually seems to be the ideal place for them (low torque, high rpm).
Thanks, JDS! Wow, 3 R motors in one set!! Totally agree -- the R motor is at its best when pushing air, not water or ground. In my experience, the R motor itself is usually the first to trip overcurrent protection -- at least with PF LiPo batteries and SBricks.
 I like it 
  May 29, 2016
Excellent work, Jeremy!
 I like it 
  May 29, 2016
When in question, test, test, test! I love your post, my favorite being the optimization process of prop speed. Great stuff. Not surprised by the findings either. The youtube reference IMO is a classic example of behavior driven by ideology and not proven, or at least objective, information.
 I like it 
  May 29, 2016
Another great post, Jeremy! I'm not really surprised at all by the R motors not working well. I have one, and torque suffers quickly under heavy strain, which draws quite a bit of power. Most of the Lego boats I've seen with these motors use 2 battery boxes, which (usually) takes care of the thermal shutdown issue. The R motors usually get used in these builds because people typically use inefficient Lego propellers which only work if spun really fast - thus the R motor. Have you seen TLG's flying lego blimp? It uses 3 R motors to power its 3 propellers. R motors it seems, are best used on the ground and in the air, which actually seems to be the ideal place for them (low torque, high rpm).
 I made it 
  May 29, 2016
Quoting Gabor Pauler Serious attempt to unify minifig aesthetics and RC functionality!
Thanks, Gabor! This hull isn't the fastest (that would be the big blue City Lines hull), but it's very versatile and a lot of fun to work and play with.
 I made it 
  May 29, 2016
Quoting David Roberts A great boat, which looks just like one of those oil rig serving ships, with some equipmemt on its deck. As always, some interesting Technic food for thought. I'm glad that it's not me floating all of those pounds/dollars of PF kit in experimemts! I'll stick to strange forms of land locomotion. I'm surprised about the buggy motors too. I've never used one but I always understood that they were fast with good torque: interesting.
Thanks, David! I was too chicken to put the buggy motor version on open water in the big pool, so I confined it to the kiddie pool, where I could snatch it out the water quickly. Those buggy motors are really frustrating to build with and operate. I'm about to post a lightweight racer with 2 buggy drive motors -- dazzlingly fast, but only on cool, flat, low-resistance surfaces. The motors are quick to shut down everywhere else.
 I like it 
  May 29, 2016
Serious attempt to unify minifig aesthetics and RC functionality!
 I like it 
  May 29, 2016
A great boat, which looks just like one of those oil rig serving ships, with some equipmemt on its deck. As always, some interesting Technic food for thought. I'm glad that it's not me floating all of those pounds/dollars of PF kit in experimemts! I'll stick to strange forms of land locomotion. I'm surprised about the buggy motors too. I've never used one but I always understood that they were fast with good torque: interesting.
 I made it 
  May 29, 2016
Quoting Nick Barrett It's surprising the buggy motors didn't do better but then it's torque you want here, rather than power. And like you say, if an £8 XL sinks to the bottom it's a lot easier to take...
Thanks, Nick! Spot-on about the torque. Have to say, the R motors did better than I thought they would -- especially WRT not shutting down. In my experience, XLs are the way to go in boats with hulls that can carry their weight gracefully. This hull can handle one XL but not two.
 I like it 
  May 29, 2016
It's surprising the buggy motors didn't do better but then it's torque you want here, rather than power. And like you say, if an £8 XL sinks to the bottom it's a lot easier to take...
 
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LEGO models my own creation MOCpages toys shop Platform supply vessel "Stella"


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