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Another fast, seaworthy twin-screw remote control speedboat -- my first to use the excellent 52x12x6.33 Cargo Carrier hull.
About this creation
Please feel free to look over the images and skip the verbiage.

Q: What do you get when you install the 4.3W twin-L, twin-screw propulsion system powering Laverne below in a boat that's 14% heavier but 16% longer and a little more slender at waterline?

A: A slightly faster red boat named Celine. And no, she's not named after a singer. I just liked the name.

My main purpose in posting Celine is to showcase her Cargo Carrier hull (CCH). This excellent hull has made cameo appearances in several of my boat MOCpages but never before in a starring role.

The next 3 shots show Celine and Laverne together.

Though only 6% longer than Laverne's 51x12x6 Police Boat hull overall, Celine's 52x12x6.33 CCH is a full 16% longer at load waterline (LWL), where it counts. She's also 11% more slender at LWL (slenderness ratio = 4.8 vs. 4.3).

If it weren't for Celine's advantages in waterline length and slenderness, Laverne would undoubtedly be the faster of the two.

Celine's disadvantages -- namely, lower installed power to displacement ratio (7.3 vs 8.3 W/kg), slightly greater wetted surface area, and a less hydrodynamic hull form -- are less important for 3 reasons: (i) Added waterline length and slenderness strongly reduce wave-making resistance, which vastly outweighs all other forms of water resistance near the top speeds of both boats. (ii) Wetted surface area affects only viscous resistance, a minor player at these speeds. (iii) Hull form counts for little in this particular hydrodynamic regime.

Hence, Celine's advantage in wave-making resistance more than offsets her 12% disadvantage in installed power to displacement ratio, though not by much.

On this page:


Celine is a fast, seaworthy 0.59 kg no-frills monohull speedboat based on a favorite powerboat-compatible hull (PCH) -- the rare 52x12x6.33 LU Cargo Carrier hull (CCH).1

Just to be clear, by "fast", I mean only that she's a good bit faster than most LEGO® boats I've seen. She's still no match for a hobby-shop RC boat.

Note the red electrician's tape used to fair the hull bottom.

Without the tape, the bottom looks like this.

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As mentioned, Celine and Laverne use identical 4.3W propulsion systems consisting of (i) twin L motors powered by a PF Li polymer rechargeable battery via SBrick or V2 IR receiver, (ii) efficient twin inverted-V outdrives with 2-stage 1:5 overdrive gearing, and (iii) very efficient third-party counter-rotating 55 mm 3-blade props.

The thinking behind our twin-screw propulsion systems and third-party props is discussed here.

Celine still had her V2 infrared receiver when these photos were taken. The Bluetooth-based SBrick installed now is a much better way to control a LEGO® powerboat -- especially one steered by differential power to the screws.

Laverne's "2L/5.0/55" propulsion system (twin L motors, 1:5.0 overdrives, and 55 mm props) was a reasonable first guess for Celine. Subsequent motor/gearing/prop (MGP) optimization showed it to be a good one.

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Top speed

Here I'll focus on a comparison between Celine and big blue Nadine below. As we'll see, these boats differ greatly in size but less than 1-2% in speed.

Celine (0.99 m/s, Froude number 0.50) is probably our 3rd fastest boat after Triton (1.09 m/s, Froude number ≥0.47) and Nadine (>0.99 m/s, Froude number >0.43). Laverne (0.97 m/s, Froude number 0.53) comes in a close 4th.2

These 4 monohulls are in turn at least 23% faster than Dubble°°Bubble, the recently posted catamaran below. Note that the same 2L/5.0/55 propulsion system powers Dubble°°Bubble, Celine, and Laverne.

The supercritical Froude numbers posted by all 4 monohulls can only mean one thing: Total resistance near top speed is overwhelmingly dominated by wave-making resistance in every case. That's especially true of Celine and Laverne.3

I very much doubt that Celine would be able to carry Nadine's much heavier XL motors safely but can't say that I've tried.

Relative to Nadine, Celine is at a 25% disadvantage WRT waterline length (540 vs. 400 mm), a 16% disadvantage in slenderness ratio (5.7 vs. 4.8) and a 12% disadvantage in installed power (4.3 vs. 4.7 W) due to her smaller motors.

However, Celine has 31% less displacement (0.590 vs. 0.855 kg), 27% more installed power per unit displacement (7.3 vs. 5.7 W/kg), much less wetted surface area from which to generate viscous resistance, and a more hydrodynamic hull form.

That these advantages and disadvantages barely balance out in Nadine's favor WRT top speed shows just how complicated and nonlinear boat performance can be.

Celine, however, is quicker out of the hole than Nadine, and Laverne may be a bit quicker still. You can get a sense of this by comparing the video above with Celine's.

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Seaworthiness and trim

Celine, Laverne, and Nadine are all quite safe in moderate to heavy swimming pool chop, but Nadine is way out front WRT freeboard, roll stability, and overall seaworthiness.

Celine is next in line WRT freeboard (at midships, 31 vs. only 22 mm for Laverne). The same is true of flooding angle (the roll angle at which water first comes over the upper edge of the hull at midships). These differences make deck wetness a much less common occurrence on Celine.

WRT roll stability, Celine and Laverne appear to similar angles of vanishing stability, but Celine is a little tippier near 0° roll due to a higher center of mass (CM) above the keel. Which boat has more dynamic stability is hard to say.

Celine's greatest rough-water vulnerability may well be destabilization via the free surface effect (FSE). Though she's less likely to take on water than Laverne, her lack of a weather deck means that any water that does come aboard will end up on the exposed inner surface of her hull bottom.

There it can spread out to form a free surface with a good bit more area than would be available in Laverne's deck well, and with fewer studs to retard sloshing as well. Hence, a layer of water only a few studs deep could seriously compromise Celine's roll stability via the FSE.

The first photo in this section shows the positive trim that results when no attempt is made to bring the bow down.

Adding the red boat weight seen in the next to last photo got the keel closer to horizontal, as seen here, but it was just too much dead weight for my taste.

Another trim correction that ended up adding too much weight is shown below. This approach also amplified pitching in waves by reducing the boat's transverse moment of inertia.

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

Overall dimensions:412 x 88 x 54 mm (LxWxH excluding outdrives)
Displacement:0.59 kg
Displacement volume:5.9 x 10 -4 m3
Depth:51 mm (midships)
Waterline length:400 mm
Waterline breadth:98 mm
Draft at keel:22 mm (midships)
Freeboard:30 mm (midships)
Wetted surface area:n/a
Midship section area:~2.1 x 10 -3 m2
Waterplane area:~3.1 x 10 -2 m2
Block coefficient:0.70
Prismatic coefficient:0.71
Midship coefficient:~0.99
Waterplane area coefficient:0.79
Length-breadth ratio:4.1
Breadth-draft ratio:4.6
Length-displacement ratio:4.8
Form factor:0.56

Performance measures

Hydrodynamic regime:High-speed displacement
Installed power:4.3W
Installed power to displacement ratio:7.3 W/kg
Critical speed:0.79 m/s
Top speed:~0.99 m/s
Froude number at top speed:~0.50
Reynolds number at top speed:4.0 x 10 5
High speed index:~0.93

Design features

Studded and studless
Hull:Unitary 52x12x6.33 LU Cargo Carrier hull
Propulsion:Twin inverted-V outdrives
Motors:2, 1 L on each prop
Propellers:55 mm 3-blade counter-rotating pair (non-LEGO®)
Gearing:2-stage 1:5 overdrive
Propeller separation:158 mm on center
Steering:Differential power to props (no rudder)
Electrical power supply:Power Functions 7.4V rechargeable Li polymer battery box
IR receiver:V2
IR receiver connections:2, 1 for each motor
Modified LEGO® parts:Prop hubs
Non-LEGO® parts:Props and electrician's tape (to fair hull bottom)
Credits:Original MOC

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1 Celine's Cargo Carrier hull (CCH) provides the 2nd longest waterline to be had among powerboat-compatible hulls (PCHs) after the City Lines hull (CLH).

Though Laverne's Police Boat hull (PBH) is almost as long as the CCH overall, its bow overhangs the water quite a bit -- hence the PBH's substantially shorter waterline length.

2 These speeds above were averaged over a 23 m course from a standing start. True top speeds are perhaps 5% higher.

3 The most informative resistance-speed curve in the LEGO® realm plots specific resistance (rTtotal resistance per unit displacement) against Froude number (Fr). The latter is given by

FrU / √(g LWL),

where g is absolute speed in m/s, g is the local acceleration of gravity (~9.81 m s-2), and LWL is waterline length at operating displacement.

The Froude number is a dimensionless speed that takes the effect of LWL on wave-making resistance into account.

All boats with displacement hulls have similar rT-Fr curves, regardless of LWL. Since all powerboat-compatible LEGO® hulls are effectively displacement hulls, we can use this fact and the Froude number to figure out the mix of resistance types a LEGO® powerboat faces when it tops out. That in turn allows us decide which potential modifications have the best chance of increasing speed.

Speeds corresponding to Fr > 0.40 are said to be "supercritical". At such speeds, wave-making resistance makes up the lion's share of total resistance, regardless of hull form. At speeds approaching Fr > 0.50, nearly all the resistance is due to wave-making.

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Please click here to see the complete list.

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 I made it 
  September 4, 2018
Quoting jim mcdonough have you tried putting a plate extended over the top of the propellor (idea is to keep the power from the prop in the water longer)once its on the suface power is lost also build a proper keel perhaps a lower wing for the boat to plane on,and move as much weight to the stern as possible the weight higher up on each side will make it a better seaboat than a weight low down in the centreline
Some very interesting suggestions. No time for a proper reply right now, but check back in a few days, as I'd love to hear your thoughts on some of the practical challenges involved in implementing them with Celine's hull. As you know, everything in functional boat design is a trade-off -- and no less so at LEGO scale. My 2 main goals for Celine were (i) maximum top speed and (ii) seaworthiness in moderate to severe swimming pool chop. From experience, I can tell you that any potential gain in one goal comes with inevitable setbacks in one or both goals. In working LEGO powerboats, the trick is play the trade-offs just so. I got reasonably close to that in Celine, but there's always room for improvement.
 I made it 
  September 4, 2018
Quoting Angelo Filipelli Nice looking boat! ....
Thanks, Angelo! Sorry I missed your comment. If still interested in this issue, email me at Email is a much better medium for things this complicated.
 I like it 
  September 4, 2018
have you tried putting a plate extended over the top of the propellor (idea is to keep the power from the prop in the water longer)once its on the suface power is lost also build a proper keel perhaps a lower wing for the boat to plane on,and move as much weight to the stern as possible the weight higher up on each side will make it a better seaboat than a weight low down in the centreline
 I like it 
  September 5, 2017
Nice looking boat! I have to inquire a new design question though. Just recently I built another motorized scale boat, a WWII PT-109 Torpedo boat. It uses the same orange motor and 2-blade cutter high-speed propeller as on almost all of my boats. Although on this one, I added a small scale feature in which an axle is mounted on the bottom of the motor which runs an axle underneath and past the main propeller and drives a scale 3-blade silver propeller. I have had some issues with it though, since it won't exactly drive right. The boat's motor runs fast and always has a fresh battery. It pushes out tons of thrust. The problem is that when the axle mount on the dummy propeller is mounted straight, it drives the dummy prop barely. It does move, but it only moves a little bit. I tilted the axle mount up towards the propeller and it seemed to work better. I don't want to do this all the time though since I want it to look scale. What I am thinking is just making the shaft connect to a Technic axle bushing and drive a friction-less pin, since my hypothesis is the the propeller thrust is moving in downward and the shaft is creating friction. I tried to solve this using WD-40 and it didn't work. If you could help me out on this, that would be excellent! Here is a picture of the motor and dummy propeller so you can get an idea: And here it is on the boat:
Jeremy McCreary
 I like it 
Tirrell Brown
  January 5, 2016
You have a new fan. As I look tgrough your builds, Im enjoying the functionality of them very much.
 I made it 
  January 5, 2016
Quoting Tirrell Brown You have a new fan. As I look tgrough your builds, Im enjoying the functionality of them very much.
Thank you very much, Tirrell! Playability seems to be my main goal these days.
 I made it 
  January 1, 2016
Quoting Gabor Pauler It is called Celine because Celine Dion will stand on its bow singing Titanic song? Then where is the iceberg?
Very funny, Gabor. I'll have you know that I take great pains to keep my boats as far as possible from both Celine Dion and icebergs. Celine's a perfectly good name -- or at least it used to be.
 I like it 
  January 1, 2016
It is called Celine because Celine Dion will stand on its bow singing Titanic song? Then where is the iceberg?
By Jeremy McCreary
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LEGO models my own creation MOCpages toys shop CelineTechnic

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