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Lena II
Lena II is a lighter, faster, easier to drive, and much more reliable version of recently posted Lena S/P. Play value has been enhanced considerably.
About this creation
Please feel free to look over the images and skip the verbiage.

Funny how spending quality time on the water with "finished" boats has a way of turning them back into WIPs. Studying the videos and photos on my own MOCpages does the same thing. Unfortunately, going over images prior to posting them is never as illuminating.

The all-LEGO® Lena S/P below is a typical case in point. I thought I had her dialed in fairly well when I posted her, but time on the water and online proved otherwise. Hence, Lena II, the all-LEGO® update featured here.



The video below shows Lena in transition from S/P to II, with 1:25 overdrive already in place but with most of the other changes detailed below yet to come.



Compared to Lena S/P, Lena II is lighter, faster, easier to drive, and a lot more reliable. In other words, Lena II's a lot more fun.

On this page:


Overview

Like Lena S/P, Lena II is a 4.9 W twin-screw inboard-outboard monohull powerboat with all-LEGO© ducted props mounted on steered outboard drive units and driven by twin XL motors. The next 3 photos show Lena II in overview.










Displacement and mass distribution

For starters, Lena S/P was overweight (0.82 kg to Lena II's 0.78). In a high-resistance hull driven by weak LEGO® props, that's a big problem.

Lena S/P's propulsion and steering equipment literally filled the 52x16x6 LU Family Yacht hull (FYH) -- the 2nd largest powerboat-compatible hull available.



My usual biases toward differential-drive steering and away from putting more than one XL on the same IR receiver terminal led me to overlook the fact that Lena S/P's propulsion motors didn't need to be controlled independently.

To my surprise, the XLs also proved able to get all the current they wanted from a single V1 terminal.1 Hence, Lena S/P's 2nd IR receiver turned out to be superfluous WRT both control and current delivery.



Looking over Lena's original MOCpage also revealed opportunities to pull mass out of her outboard drives and their mounts without weakening them.





Hence, the new versions below.







Unfortunately, all these weight-saving measures conspired to reverse Lena S/P's undesirable positive (bow-up) trim at rest a little too much. Hence, I had to move her battery aft by 2 LU to even her out.

That's way it is with boats. Every little thing affects everything else.




Controller

With Lena II down to just one IR receiver, I no longer needed Lena S/P's original dual handset.



The simple handset below would have sufficed, but every time I took Lena II for a spin with it, I had to figure out which lever did what all over again. (This is what you have to look forward to when you get to be a GFOL -- geezer fan of LEGO®.)



Hence, the best interface I've found by far is the one in the lower half of the next photo, with mnemonic gas pedal-like control for propulsion and steering wheel-like control for, well, steering.



The modified speed control in the upper half of the photo also works well with Lena II.




Gearing

When I finally got around to putting Lena S/P through our motor/gearing/prop (MGP) optimization process, it became clear that her 1:13.89 overdrive ratio was far too low for her hull, props, and XL motors.



In other words, the props needed to spin a lot faster to load the motors down to the point where their shaft speeds approximated the optimal 50% of no-load speed.

Lena S/P's overdrive ratio had already been boosted to the current 1:25 by the time I shot the video above. The effect on speed was pronounced and positive, but with progress came a new problem: The heavily loaded 1st stages of her new twin 4-stage transmissions had developed a nasty habit of popping out of mesh -- usually when Lena S/P was far from shore.2



Hence, I confined the first-stage 24- and 8-tooth gears on both sides to hold them positively in mesh.



And while I was at it, I threw in a cross-brace to keep the shaft supports from moving laterally.




Steering mechanism

Lena S/P also sorely needed a better way to connect her steering (M) motor to the linear actuator, as the black connector attached to the M tended to pop out if the actuator was driven past the drives' hard stop to port.



With the drives stuck hard to port, Lena S/P could then only go around in fairly tight circles -- again, usually far from shore.2 Indeed, such was her fate when the video ended.



After entertaining all manner of heavy, complicated solutions, the simple displacement-neutral way out finally dawned on me. All I had to do was to move the steering motor cross-bracing aft and alter it a bit so as to block the black connector from pulling out.




Bottom line

In the end, Lena II came out 35 g lighter, a lot more reliable, and much easier to drive than Lena S/P. The increase from 1:13.89 to 25:1 overdrive and the reduction in displacement made her a good bit faster as well.

Since all these improvements went straight to play value, Lena II promises to be a keeper, though she'll no doubt go through another WIP phase or two before she's done.




Specifications
NB: Specs refer to Lena II only.

Overall dimensions:416 x 116 x 84 mm (LxWxH)
Displacement:0.781 kg
Displacement volume:7.8 x 10 -4 m3
Depth:50 mm
Waterline length:364 mm
Waterline breadth:116 mm
Draft at keel:22 mm (midships)
Freeboard:28 mm (midships)
Wetted surface area:n/a
Midship section area:2.6 x 10 -3 m2
Waterplane area:3.6 x 10 -2 m2
Block coefficient:0.84
Prismatic coefficient:0.84
Midship coefficient:1.00
Waterplane area coefficient:0.85
Length-breadth ratio:3.1
Breadth-draft ratio:5.3
Length-displacement ratio:4.0




Performance measures
Hydrodynamic regime:Low-speed displacement
Installed power:4.9 W
Installed power to displacement ratio:6.3 W/kg
Top speed:Modest but improving
Critical speed:0.76 m/s
Froude number at top speed:n/a
Reynolds number at top speed:n/a
High-speed index:n/a




Design features
Construction:
Studded and studless
Hull:
Unitary 52x16x5 LU Family Yacht hull (Set 5848)
Propulsion:Lockable pivoting twin inboard-outboard drives
Motors:3 inboard -- twin XLs for propulsion; M for steering
Propellers:Ducted LEGO® 24 mm 3-blade
Gearing:3-stage 1:25 overdrive
Steering:Linked steered outboard drives
Steering angle range:±40°
Electrical power supply:
7.4V from a Power Functions Li polymer rechargeable battery
IR receivers:One V1
IR receiver connections:2, 1 for both propulsioins motors and 1 for the steering motor
Modified LEGO® parts:
Tips of stopped 3L axles holding ducts to outboard drives shortened 2 mm to prevent contact with prop tips
Non-LEGO® parts:None
Credits:
Original MOC aside from linear actuator steering (adapted from Sariel's RC Fire Boat)





Footnotes

1 If the ducted props were able to absorb the torque of the XL motors like our non-LEGO props do, they'd draw more current than a V1 receiver would be inclined to allow from one terminal. Thermal protection would trip early and often, and power to the XLs would become highly intermittent.

Instead, I can run both of Lena II's XLs from a single V1 terminal all day long in static thrust tests without a hitch. Just one more reason to say no LEGO® props and yes to running heavily loaded XLs on separate V2 terminals, if not on separate V2s.

2 Gumperson's Law states that the least likely contingency will always occur at the most inopportune time. Murphy's Law is much less predictive.

Gumperson's Corollary states that attempts to circumvent Gumperson's Law are futile and are themselves subject to Gumperson's Law.

Nothing I've run across in the LEGO® realm better illustrates Gumperson's Law than building and operating powerboats.




Selected references
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

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

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




Comments

 I made it 
  September 3, 2018
Quoting Seaman SPb Wow! Excellent boay, Jetemy!
Thanks, my friend! It was fun to look over this page again. Lena II looks very mechanical in a muscular kind of way. If only she had the speed to match!
 I like it 
  September 3, 2018
Wow! Excellent boay, Jetemy!
 I made it 
  December 22, 2016
Quoting The Actionfigure Great! I realy like functions on lego models! (Unfortunately, I am not an expert on the subject :( ) And that´s very elaborated! Did you ever think about a relistic model of a ship with this technique?
Thanks! I've posted at least 2 working model ships that go for some degree of realism -- though not with the propulsion and steering arrangements used here. Steerable outdrives like these are generally found on small boats, not ships.
 I like it 
  December 21, 2016
Great! I realy like functions on lego models! (Unfortunately, I am not an expert on the subject :( ) And that´s very elaborated! Did you ever think about a relistic model of a ship with this technique?
 I made it 
  August 17, 2016
Quoting Jason M I'm currently making my own motorized Lego boat with the family yacht hull. Could you please tell me if I should use 2 Xl or 2 L Motors.Thanks. I really like the steering system on your boat. I'm using the servo to steer.
Thanks, Jason! Since the Family Yacht hull generates more water resistance than most, you'll need as much installed power as possible, and that means 2 XLs, as in Lena and Lena II. How much you'll need to gear them up will depend on the props you use. See the page on the original Lena for details. Looking forward to seeing your boat. Did you mean to leave a like?
  August 14, 2016
I'm currently making my own motorized Lego boat with the family yacht hull. Could you please tell me if I should use 2 Xl or 2 L Motors.Thanks. I really like the steering system on your boat. I'm using the servo to steer.
 I made it 
  August 26, 2015
Quoting Johan van der Pluijm when there is the possibillity to put bricks on the bottom of the hull, would it be a good idea to make kind of 'fins'(like surfboards have) underneath the boat? it should make the boat more stable.
Thanks for the like, Johan. Yes, real boats use such roll stabilizing fins (e.g., bilge keels), so they ought to have the same effect at LEGO scale if properly sized, and a centerline fin would definitely add directional stability to a flat-bottomed boat like this one. (Note how she side-slips in turns in the video.) Most of the boats in this folder are stable enough in roll without them, but Laverne and the original Stormin' Norma might have benefitted, though their speed would have suffered.
 I made it 
  August 26, 2015
Quoting Topsy Creatori For someone like me who can kinda understand, yet who does not really have the intuitive feel for mechanics, the comparison between the two Lenas was most helpful. :)
Thanks, Topsy. Glad the comparison helped. I gained some valuable insights about both boats by working through the comparison as well.
 I like it 
  August 26, 2015
when there is the possibillity to put bricks on the bottom of the hull, would it be a good idea to make kind of 'fins'(like surfboards have) underneath the boat? it should make the boat more stable.
 I like it 
  August 25, 2015
For someone like me who can kinda understand, yet who does not really have the intuitive feel for mechanics, the comparison between the two Lenas was most helpful. :)
 I made it 
  August 23, 2015
Quoting David Roberts Some very nicely thought out mechanicals there. I like how you've spread the electrics and motors along the boat, to get balanced fore & aft trim.
Thanks, David. The heavy outboard drives pretty much forced the rest of the boat's load to the bow, but that turned out to be a good mass distribution for this particular hull. The broad flat bottom and relative short length make the hull too responsive to wave impacts resulting in pitching motions. Concentrating loads at the ends of the hull suppresses pitching in response to heavy swimming pool chop. Since the energy associated with excessive pitching can bleed into rolling motions, damping out pitching makes the boat less likely to capsize in roll. (If you enjoy counter-intuitive connections, boats are hard to beat.)
 I like it 
  August 23, 2015
Some very nicely thought out mechanicals there. I like how you've spread the electrics and motors along the boat, to get balanced fore & aft trim.
 
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