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Earline's a reasonably fast 0.915 kg, 2.4W outboard powerboat based on the 74x18x7 LU City Lines hull (CLH). More a rough-water boat than a speedboat, she excels in seakeeping ability by adding exceptional maneuverability to the CLH's unrivaled stability and freeboard.
About this creation

Please feel free to look over the images and skip the verbiage.

Shawn Kelly and I have been avidly building, racing, and occasionally sinking Power Functions (PF) remote control (RC) speedboats for 2 years now. To date, we've come up with dozens of seaworthy no-frills LEGO® powerboats (mostly no-frills speedboats) based on various LEGO® unitary hulls (LUHs).

Shawn's boat Radio Flyer below even took 1st place at the boat drag races at BrickWorld 2015!

We've also dabbled in LUH-based motorized ship models like the R/V Stormin' Norma II, a marine geology research vessel.

The comments below come out of that experience and a very deep plunge into naval architecture -- the engineering discipline devoted to the design, testing, and construction of boats and ships of all kinds.

Above is a brief video of Earline's maiden voyage giving a sense of her speed. She's a little heavier and perhaps a little slower now. A longer video focused on her maneuvering ability is embedded below.

On this page:


Earline's a reasonably fast, seaworthy 0.915 kg, 2.4W monohull powerboat with a single-XL outboard motor based on the largest LUH available -- the 74x18x7 LU City Lines hull (CLH).

Her forte is seakeeping ability -- i.e., a mix of stability, controllability, maneuverability, and speed optimized for seaworthiness in very rough water. Though not quite the all-out speed-demon her sister Nadine is, she's still pretty quick by LEGO® standards.

Earline's highly optimized 2.4W powerplant consists of (i) a single XL motor powered by a PF AAA battery box with NiMH cells via a V2 IR receiver, (ii) an efficient outboard mount with 3-stage 1:8.33 overdrive gearing, and (iii) a very efficient third-party 55 mm 3-blade counter-rotating prop.

The rationale behind her third-party prop is discussed here, and the motor/gearing/prop optimization process, here.

I always use PF Li polymer rechargeable batteries in speedboats (i) to save weight, as installed power to displacement ratio has a huge effect on top speed, and (ii) because the rechargeable dishes out more current at a steady 7.4V with negligible voltage sag under load.

However, Earline's more a rough-water boat than a speedboat and needs a more or less level keel to maximize her dynamic stability on wave crests. Hence, she needed some forward ballast to balance the positive trimming moment of a heavy (0.071 kg) XL motor hanging over her stern.

Restoring fore-aft balance with the least possible added displacement meant putting the counterweight as far forward as possible. The transversely mounted PF AAA battery box seen at the bow on this page did the trick with only 0.045 kg of added displacement.

Only after taking the photos did I recall that the 7.2V output of the NiMH-powered AAA battery box translates directly into an installed power loss relative to the 7.4V Li polymer battery. Before voltage sag, the loss comes to at least (1 - 7.2/7.4) or ~3%. Since NiMH batteries sag more than Li polymers do (though not by much), the actual loss would be greater than that.

Hence, I've since replaced the AAA box with the Li polymer battery and installed boat weights up front to adjust the fore-aft balance.

A robust bow-tie (aka antiparallogram) four-bar linkage couples her PF servo steering motor to the outboard motor mount. The 5-photo sequence below shows the linkage in action.

The thrust vectoring provided by the outboard makes Earline the most maneuverable of our CLH monohulls by a good margin. She can probably out-maneuver Laverne and Trident as well, but head-to-head tests are still on the to-do list.

Return-to-center accuracy -- an absolute must for controllability and hence seaworthiness in single-screw boats -- is excellent.

Earline has yet to be properly clocked or raced, but I'm guessing that she makes close to 0.90 m/s (Froude number ~0.39).

She's probably in our top 6 boats or so speed-wise, but she's definitely slower and heavier than the top 3 shown above: Big blue Nadine (0.855 kg, 4.9W, 5.9 W/kg, 0.540 m, 0.99 m/s) in the distance, Laverne (0.545 kg, 4.3W, 7.9 W/kg, 0.345 m, ~0.97 m/s) in the middle, and Trident in the foreground (0.495 kg, 4.3W, 8.7 W/kg, 0.372 m, ~0.95 m/s).

She'd have to be slower than Nadine with similar total resistance and less than half the installed power to displacement ratio (2.8 W/kg to Nadine's 5.9 W/kg). That she's also slower than Laverne and Trident shows that the length advantage WRT wave-making resistance only goes so far.

Design goals

I built Earline to try my hand at an outboard as a way to add maneuverability to the unrivaled stability and freeboard of CLH-based monohulls. If easily controlled and reasonably fast, such a boat would also have unrivaled seakeeping ability among LEGO® powerboats.

The outboard's thrust vectoring ability was the main attraction here.

As shown in the maneuverability trial below, CLH maneuverability under differential-drive twin-screw steering is good but not great -- even with one prop full ahead, and the other full astern.

A brief glimpse of an early Earline appears at 0:15.

Seakeeping ability
Seakeeping ability, Earline's specialty, lumps seaworthiness and operability together in the context of very high sea states like the one recorded in a Gulf of Alaska storm below.

Equivalent LEGO®-scale sea states can and do arise in busy swimming pools with busy diving boards -- especially if the wind comes up.

Running in rough water adds excitement to LEGO® powerboating. I don't want to drown $100 worth of LEGO® electricals any more than the next guy, but I also don't want to have to turn tail and run every time waves come up.

What I do want is a boat that can hold her own in very rough swimming pool conditions, and Earline may be just the ticket. I should have a rough-water pool trial to share shortly.

The seaworthiness part of seakeeping focuses on mainly on capsize prevention in the face of potentially large wave-induced attitude changes. It generally requires ample freeboard to minimize deck wetness and exceptional dynamic stability -- especially on wave crests in heavy longitudinal seas.

That's a tall order for any boat, but CLH monohulls are up to the task -- at least when lightly loaded with low centers of gravity.

In the LEGO® realm, operability in high sea states demands, among other things, the ability to adjust course and speed to minimize capsize risk, evade steep breaking waves, and get to calmer water when the sea state becomes untenable, as it sometimes does in pools -- e.g., when the diving area suddenly heats up or when the water suddenly shallows.

Hence, rough-water operability hinges on reliable remote controllability, good maneuverability regardless of attitude, and a certain degree of quickness.

In real boats, operability also demands softened boat responses to wave impacts in order to keep equipment and load in place and allow the crew to perform their duties without being injured or incapacitated by sea sickness -- especially during sustained rolling and pitching in heavy seas.

I didn't bother softening boat responses because, frankly, minifigs aren't much help on boats to begin with.


Earline's best operated with a dual PF 3-state (full forward, full reverse, off) handset and speed control like the one above (more info on the speed control mod here).

The 3-state on the right is good for full-throttle starts, emergency braking, and abrupt hard to port and hard to starboard turns. The attached speed control on the left allows more nuanced operation but response lags at the receiving end are noticeably longer, as usual.

The speed control on the left provides 15-step adjustment of prop power from full forward (+7) to full reverse (-7) and, in Earline's case, 7-step adjustment of steering angle in ~13° increments.

The remaining 8 steps of potential electronic control and stepper motor response are blocked by the limited range of motion of the bow-tie steering linkage. Nevertheless, the resulting steering angle range of ±39° is more than enough for a substantial gain in maneuverability over that of a CLH monohull like Nadine with no steering mechanism other than differential-drive twin screws. Earline pays the price in speed.

Stepper and linkage return-to-center accuracy is generally excellent, but the stepper motor occasionally misses the mark by 2-3° -- especially after the outboard's been bumped. Cycling power several times re-centers it.

Having steering and propulsion on the same IR channel allows me to switch back and forth between handsets for either steering or propulsion while underway. I generally prefer this more flexible single-channel mode.

Problem is, the 3-state and speed control don't play nicely together on the same IR channel. Any use of the 3-state has the same effect as hitting the kill button on the speed control when the latter is already active on the same channel. Conversely, attempting to steer with the speed control while the 3-state is already active on the propulsion port puts the stepper motor into a tizzy marked by small, rapid oscillations about center and terminated with the kill button.

Hence, the only way to use the 3-state and speed controllers simultaneously -- e.g., to get stepped steering and bang-bang propulsion control at the same time -- is to put them on different channels, which of course means 2 IR controllers instead of one.

Since Earline needed the foreward ballast anyway, I decided not to choose and installed two receivers, one a V2, to be wired up as needed at launch. However, the XL propulsion motor aways goes on the V2, as the V2 is always happy to give the current-hungry XL all the juice it wants for as long as it wants without tripping the V2's internal thermal protection.

Prop walk
I was also curious to see how much prop walk I'd get with a single third-party prop. Course-keeping in rough water is hard enough without having to contend with prop walk.

The results on this one are already in: No significant prop walk with this particular prop.

Appendage drag vs. prop inflow
The outboards I'd seen on YouTube left considerable room for improvement WRT propeller choice and appendage drag, and their speeds suffered accordingly. I also hoped to bring naval architecture to bear on the situation, as Earline would need to be quick enough end-run breaking waves with a single screw.

In that regard, I was particularly interested to see how the following trade-off would play out: A single outboard prop support strut would clearly generate less appendage drag than our usual twin inverted-V stern drives like those in the group shot above.

On the other hand, a single centerline prop would generate less thrust, as it would then be fed water maximally retarded by interaction with the hull bottom dead ahead. The connection is that thrust is proportional to water inflow speed.

I'd be able to tune the trade-off to some extent by adjusting the length of the prop support strut. A very long strut would move the hub below the boundary layer (BL) of water retarded by the hull, at least in part, but only at the expense of added appendage drag. (Our inverted-V stern drives evade this trade-off by moving the props out of the BL laterally with the least possible wetted strut length.)

Minimum strut length would in all cases be dictated by the hub depth needed to keep the prop from ventilating (i.e., sucking in air from above) while in forward motion, as ventilation causes an abrupt and marked loss of thrust in props designed, like ours, for full-immersion operation.

As seen in Trident's hand-held static thrust test video above, the props readily entrained air when I failed to keep them sufficiently submerged. I knew from experience that the prop would still ventilate in reverse at minimum strut length, but that's pretty much a non-issue in single-screw outboard speedboat.

Drivetrain optimization

As with all our boats, Earline's "XL/8.33/55" drivetrain (XL motors, 1:8.33 overdrive, and 55 mm prop) came out of a methodical and rather tedious motor/gearing/prop (MGP) optimization process.

The single-XL outboard configuration emerged from a large testing matrix that also explored single-L, twin-L, and twin-M options. (By "twin" here, I mean two motors in the same outboard mount.) Each was tested with 1:5 vs. 1:8.33 overdrive ratios, and 52 vs. 55 mm props.

More photos

Earline's no-frills outfit keeps her displacement down and installed power to displacement ratio as high as possible. It also increases freeboard, but CLH-based boats remain seaworthy at much higher displacements than Earline's.

Steering angle is the outboard's only degree of freedom. The steering angle bearing is a small Technic turntable -- excellent for horizontal bearings like this -- at the base of the motor mount. A trim adjustment is on the "maybe someday" list.

When centered, the 4 corner pins of the bow-tie steering linkage form a 7x9 LU rectangle with the shorter sides parallel to the hull's centerline plane. Diagonals exactly 11 LU in length divide the rectangle into 4 isoceles triangles consisting of back-to-back 3:4:5 LU triangles.

When the steering servo turns, the rectangle deforms into an antiparallelogram. This geometry makes for a smooth, robust steering linkage and guarantees that the outboard will return to center if the steering servo does.

The importance of return-to-center accuracy in single-screw boats, whether steered like Earline or with a rudder, can't be overstated.

I first learned of the bow-tie from the legendary Stilzkin Indrik artic vehicle by Technic magician and master videographer Peer Kreuger, aka "Mahjqa", who kindly published instructions for the Indrik's chassis.

The photo of my Indrik-inspired Polar Research Vehicle (PRV) above provides a glimpse of Kreuger's bow-tie steering linkage. The Indrik and PRV use it to insure that when the steering motor turns the front track set, say, 30°, the rear track set turns exactly the same angle in the opposite direction, thereby keeping both track sets on the same turning circle. Earline's use of the bow-tie is kinematically equivalent.

Fairing the submersible motor/weight mount recess on the bottom of the CLH by covering it with electrician's tape turns the CLH underbody (submerged part of the hull) into the cleanest powerboat-compatible underbody possible.

In the upper photo, Earline's about to accelerate to the right. The lower photo shows the surface disturbance kicked up by her propwash. If Earline had been held in place, the steady-state height of the surface disturbance would have been a rough measure of her static thrust. In this case, however, Earline carried some of the thrust power away in the form of kinetic energy.



Both videos are of an early Earline's maiden voyage. The upper video was embedded at the top of the page.

The lower, much longer one focuses on maneuverability and stability in turns, but there are brief bursts of speed at 0:27 and 1:50. The dazzling displays of bad driving on my part reflect the fact that I was used to differential-drive twin-screw steering at the time and still hadn't gotten the hang of her dedicated controller and outboard steering.

Earline's outboard motor mount is a little lighter now, but she displaces about 0.050 kg more than she did in the videos -- mainly due to a change from the PF Li polymer rechargeable battery to the AAA battery box. The 1x11 liftarms used as crosslinks in her bow-tie (aka antiparallelogram) four-bar steering linkage were later swapped out for the lighter 1x11 Technic links seen in the photos. Nothing else of importance has changed.

Comparison with Nadine

The photos below shows Earline with sister Nadine alongside. Nadine's our fastest boat ever at ≥0.99 m/s. I haven't had a chance to time Earline yet, I can bracket her top speed: She's a good bit slower than Nadine, but she was clearly faster than Trident when Trident was still topping out at 0.84 m/s. I'm guessing that Earline's top speed is around 0.90 m/s now.

Their displacements -- 0.915 kg for Earline, 0.855 kg for Nadine -- differ by <8%. Earline's draft is 1 mm more than Nadine's and her freeboard that much less, but there are no measurable differences in waterline length and breadth. Hence, slenderness and other hull form coefficients are effectively equal.

If both were run at, say, Earline's top speed, I'd expect Earline to encounter a little more viscous resistance at the hull due to slightly greater wetted surface area and somewhat less appendage drag than Nadine, the latter by virtue of having one prop support in the water to Nadine's two.

However, their total resistances are probably be quite similar, as the fraction of total resistance due to wave-making resistance is very high at such speeds, and the determinants of wave-making resistance would identical.

It follows than any difference in top speed must come down to the difference in installed power, and that difference is substantial.

Nadine's big advantage is her 4.9W of twin-XL installed power (PI) to Earline's 2.4W. That gives Nadine an installed power to displacement (PI / Δ) ratio of 7.9 to Earline's 2.8. These ratios differ by a factor of 2.8.

So why isn't Nadine 2-3 times faster? To answer that question, note first that identical waterline lengths of 0.540 m mean identical critical speeds -- i.e., Ucrit = 0.92 m/s for both.

Hence, Nadine's top speed of 0.99 m/s is supercritical, but Earline's is very close. The corresponding Froude numbers (Fr) -- 0.43 for Nadine, 0.39 for Earline -- tell the same story: Nadine tops out while climbing their mutual wave wall, whereas Earline gives up at its foot, which marks the onset of explosive growth of wave-making resistance with speed.

Let the subscripts "N" and "E" to point to Nadine and Earline, respectively. Since their propulsive efficiencies are probably comparable, their installed power ratio can be written

PE / PN = (UE / UN) (RN / RE),

where U is top speed, P is installed power, and R is total resistance at top speed.

Up there on the wave wall, Nadine's total resistance is roughly proportional to the 4th power of speed like so:

RNk UN 4

where k is an unknown constant.

Given Earline's close proximity to the wave wall and the uncertainty in her top speed estimate, we'll assume the same resistance-speed relationship for her. A little algebra then yeilds an expression for Nadine's top speed in terms of known quantities.

UEUN (PE / PN) 1/5

Plugging the knowns into the right-hand side of the last equation gives

UE ≈ 0.85 m/s,

which isn't too far off the mark.

Though by no means precise, this "back of the envelope" exercise illustrates the huge increases in installed power needed to make even a little headway up the wave wall.

Given the strong positive correlation between installed power, propulsion system mass, and fuel or battery consumption, the need to maintain appropriate margins of seaworthiness generally requires real-world boats to be designed around relatively small ranges of potential top speeds and powerplants. Hence, once on the wave wall, substantial gains in top speed generally require not just a change in propulsion system but a change in boat.


Since all our other speedboats top out higher up their respective wave walls than Nadine does (i.e., at Froude numbers higher than 0.43), I have doubts as to how much farther up their walls they can be safely pushed -- e.g., with non-LEGO® motors and batteries -- in their current hulls.


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

Overall dimensions:592 x 106 x 58 mm (LxWxH, excluding outboard)
Displacement:0.915 kg (1.85 lb)
Displacement volume:9.2 x 10 -4 m3
Depth:68, 58 mm (bow, midships)
Waterline length:540 mm
Waterline breadth:142 mm
Draft at keel:16 mm (midships)

42 mm (midships)
Wetted surface area:8.1 x 10 -2 m2
Midship section area:2.3 x 10 -3 m2
Waterplane area:6.3 x 10 -2 m2
Block coefficient:0.75
Prismatic coefficient:0.75
Midship coefficient:~0.99
Waterplane area coefficient:0.82
Length-breadth ratio:3.8
Breadth-draft ratio:8.9
Length-displacement ratio:5.5
Form factor:0.75

Performance measures

Installed power:2.4W
Installed power to displacement ratio:2.8 W/kg
Top speed:~0.90 m/s (est.)
Critical speed:0.92 m/s
Froude number at top speed:~0.39
Reynolds number at top speed:~5.3 x 105

Design features

Construction:Mostly studless
Hull:Unitary 74x18x7 LU City Lines hull (Set 7994)
Motors:2 -- an XL on the prop, and a PF servo on the steering linkage
Propeller:55 mm 3-blade (non-LEGO®)
Gearing:3-stage 1:8.33 overdrive
Steering:Outboard thrust vectoring via PF servo motor
Electrical power supply:Power Functions 7.2V AAA battery box (doubles as forward ballast) with NiMH rechargeables
IR receivers:V1 and V2 (to expand control options)
IR receiver connections:2, 1 for each motor

Modified LEGO® parts:
Prop hub, fairing tape over recess on hull bottom
Non-LEGO® parts:Prop
Credits:Entirely original MOC

All of the titles below are free online for the digging.

Anonymous, 2011, Basic Principles of Ship Propulsion, MAN Diesel & Turbo, Copenhagen, Denmark

Barrass, C.B., 2004, Ship Design and Performance for Masters and Mates, Elsevier Butterworth-Heinemann

Barrass, C.B., and Derrett, D.R., 2006, Ship Stability for Masters and Mates, 6th ed., Butterworth-Heinemann

Bertram, V., 2000, Practical Ship Hydrodynamics, Butterworth-Heinemann

Biran, A.B., 2003, Ship Hydrostatics and Stability, 1st ed., Butterworth-Heinemann

Carlton, J.S., 2007, Marine Propellers and Propulsion, 2nd ed., Butterworth-Heinemann

Faltinsen, O.M., 2005, Hydrodynamics of High-speed Vehicles, Cambridge University Press

Moisy, F., and Rabaud, M., 2014, Mach-like capillary-gravity wakes, Physical Review E, v.90, 023009, p.1-12

Moisy, F., and Rabaud, M., 2014, Scaling of far-field wake angle of non-axisymmetric pressure disturbance, arXiv: 1404.2049v2 [physics.flu-dyn] 6 Jun 2014

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 Kelvin’s angle? European Journal of Mechanics B/Fluids, v.46, p.164–171

Rawson, K.J., and Tupper, E.C., 2001, Basic Ship Theory, vol. 2: Ship Dynamics and Design, 5th ed., Butterworth-Heinemann

Schneekluth, H., and Bertram, V., 1998, Ship Design for Efficiency and Economy, 2nd ed., Butterworth-Heinemann

Tupper, E.C., 1996, Introduction to Naval Architecture, 3rd ed., Butterworth-Heinemann


 I made it 
  August 12, 2015
Quoting Rabbitdesign MB Great work! I am on holidays so time enough to read. I learned a lot about boats. Thanks. What I didn't find was details about the props. What brand or where did you buy them. I visited some model shops but non had the ones you use. Could you tell me where to buy these props?
Thanks! The props come from ebay. The sellers and brand names change, so just look for 52 or 55 mm 3-blade nylon RC boat props with 3/16" or 4.7-4.8 mm shaft holes. If the props are for a twin-screw boat, be sure to get a counter-rotating pair -- i.e., one right-handed and the other left. We've been paying $8-9 a pair.
 I like it 
  August 11, 2015
Great work! I am on holidays so time enough to read. I learned a lot about boats. Thanks. What I didn't find was details about the props. What brand or where did you buy them. I visited some model shops but non had the ones you use. Could you tell me where to buy these props?
Jeremy McCreary
 I like it 
matt rowntRee
  December 20, 2014
Cool true outboard with a wicked trick steering. Clever!
 I made it 
  December 10, 2014
Many thanks to all for stopping by and leaving the kind words below. They mean a lot coming from such accomplished builders. I prefer publishing dissertations on MOCpages because it's a heck of a lot easier than actually earning the degree, and that, of course, leaves more time for LEGO.
 I like it 
  December 9, 2014
Dissertation anyone? :)
 I like it 
  December 9, 2014
amazing ... this is really a great job with a lot of details...
 I like it 
  December 9, 2014
It is becoming every time better! Keep building these superb pieces of (lego) engineering perfection! 5/5
 I like it 
  December 9, 2014
cool :)
By Jeremy McCreary
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LEGO models my own creation MOCpages toys shop EarlineTechnic

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