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Radio Flyer, drag boat champ, BrickWorld 2015!
This twin-screw RF-controlled 1.2 kg, ~6 W no-frills monohull speedboat won the BrickWorld 2015 boat drag races handily but was only so-so in the slalom.
About this creation
Please feel free to look over the images and skip the verbiage.

Shawn Kelly, Annette Kirkpatrick, and I are proud -- no, dead bloody chuffed! -- to present Radio Flyer, winner of the LEGOŽ boat drag races at BrickWorld 2015!



The name "Radio Flyer" reflects the boat's main contributions to the drag race win -- namely, raw speed and reliable, long-range radio-frequency (RF) remote control. Annette's driving did most of the rest.



The video above, uploaded by James Fitzgerald, is the only 2015 boat race footage I've found. It shows Radio Flyer well ahead at the finish line in each of her recorded drag race heats. The winning heat is at 13:59-14:35.

Driver Annette took Radio Flyer to BrickWorld (BW) for owner Shawn, who couldn't attend. (I couldn't go, either.) Cool-headed and observant, Annette won 3 elimination rounds to compete against 3 other boats in the finals.

Shawn and I collaborated on the propulsion and steering systems and some of their mounts. Shawn did all the rest.

Above all, Shawn had the moxie to put a very expensive RC Race Buggy RF receiver and two XL motors on the line in the BW pool, where collisions ending in sinkings occur with some regularity.

Be sure to visit Shawn's Radio Flyer MOCpage.



Warning: Shameless abuse of bragging rights ahead.

This page discusses Radio Flyer and her competitors from a technical standpoint only. The solutions presented were carefully worked out over 2 long years of collaboration that included hundreds of hours of boat testing, rebuilding, and research into real-world naval architecture.

It was very gratifying to see our hard work finally pay off at BW 2015.

On this page:


Overview

Radio Flyer's a fast, seaworthy ~1.2 kg no-frills twin-screw monohull speedboat based on our favorite powerboat-compatible hull -- the long, blue 74x18x7 LU City Lines hull (CLH).



The one frill remaining in the photo above (the bowsprit) was removed for BW. The red objects in the forward deck well are Radio Flyer's BW-legal LEGOŽ props. The much faster black 55 mm third-party props mounted are the ones she runs at home.



The no-frills outfit keeps displacement down. This maximizes installed power to displacement ratio and minimizes viscous resistance at the same time. It also serves seaworthiness by maximizing freeboard, which reduces the risk of flooding in waves and collisions.



Winning speed
We haven't had a chance to clock Radio Flyer at home, but she averaged ~0.[] m s-1 in her championship heat. Top speed was perhaps 10% higher.





With an estimated waterline length of 0.55 m, that average speed corresponds to an average Froude number (dimensionless speed) of ~0.[].

This Froude number, at the transition between the low- and high-speed regimes for displacement hulls, indicates that hull-related viscous resistance and wave-making resistance play comparable roles in limiting top speed. Appendage drag also takes its toll, but not to the same degree.



Seaworthiness

Radio Flyer proved herself seaworthy in the moderate chop of the BW pool. Her initial stability (near 0° roll), flooding angle (taken at the upper edge of the hull at midships), and angle of vanishing stability (AVS) are all excellent -- even with the top-heavy RF receiver onboard.

Freeboard suffered a bit under her ~1.2 kg displacement but was more than sufficient, as she saw very little deck wetness during the races. 1



Radio Flyer had her red BW props on in this tub trial photo.



Directiononal stability, maneuverability and control

The only boat with more raw speed than Radio Flyer (by perhaps 5-10%) was a very erratic IR-controlled paddlewheeler that could turn on a dime -- usually much to its detriment in the drag races.

The paddlewheeler also made it to the drag race finals, but Radio Flyer beat it and the rest of the pack by a good quarter of a pool length on a combination of (i) speed, (ii) directional stability, (iii) flawless remote control via the RC Race Buggy RF system (below), and (iv) flawless driving by Annette.



Staying on course the entire heat made all the difference, and Radio Flyer and Annette did that repeatedly in the drag races.

Radio Flyer didn't fare as well in the salom, however, and Annette's driving wasn't to blame.

When the boat left for Chicago, the steering system was still a work in progress. Steering response was slow and turning radius large compared to our other CLH-based boats, and the rudder still had a lot of play near dead center.

As a result, Radio Flyer just couldn't turn quickly enough and tightly enough for the slalom course at hand -- at least, not going forward.

Annette noticed in practice that the rudder play actually quickened steering response and tightened the turning radius in reverse. She briefly considered running the slalom backwards but would have needed a lot more practice time than she had to pull that off.




Propulsion system

Radio Flyer uses a propulsion system with the following main components:
  • 9V (nominal) electrical power supply (6 alkaline AAs inside the receiver) 2
  • Twin XL motors
  • Efficient, low-drag twin inverted-V outdrives
  • LEGOŽ 5.5L (44 mm) 2-blade "twisted propellers" (4745) 3
  • Three-stage 1:15 overdrive gearing optimized to maximize top speed
The CLH is the only powerboat-compatible hull capable of supporting twin XLs in a monohull boat. The rationale behind the twin inverted-V outdrives is discussed here.

Motor placement and the onboard 1st stage of the 3-stage 1:15 gear train are shown below.



Radio Flyer's outdrives are almost identical to Nadine's below.





I estimate the installed mechanical power at ~6 W after alkaline battery voltage sag. 2

If correct, Radio Flyer's all-important installed power to displacement ratio is a respectable 5 W/kg or so. That's closer than expected to Nadine's (5.7 W/kg) considering that there's an RF receiver onboard but nothing like Trident's (8.7 W/kg).



Propellers
Propeller-driven entries haven't been competitive with the fastest paddlewheelers and air boats in recent years at the BW drag races, but we thought we might be able to change that.

After all, screw propellers are much more efficient at real-world marine propulsion than paddlewheels and air props. That's especially true in rough water, and the BW pool is always choppy.

In fairness, the BW rule forbidding non-LEGOŽ components other than sealing tape leaves few good options in the way of propellers.

Built-up LEGOŽ props like the one below are hopeless in water due to excessive size and weight, poor blade geometry, and surface roughness that would give a real prop designer nightmares. The result: Dismal lift/drag ratios, no thrust to speak of, and in many cases, very high torque requirements.



A few ready-made LEGOŽ props manage to perform somewhat better than that. (We've tried them all.) Compared to hobby-shop RC boat props, however, they're all extremely inefficient. Hereafter, all mentions of LEGOŽ props will refer to the ready-made kind.

In reviewing videos of past races, we noticed several opportunities for improvement among the prop-driven boats. For starters, most of the prop support structures we saw put way too much plastic in the water -- usually in very unhydrodynamic arrangements to boot. These boats lost significant speed to appendage drag alone.

Additionally, many entrants had been lured into using 3-blade 3L or 2-blade 9L props, presumably by their torque-friendly center axle holes. The 2-blade 9L props were often doubled up to form 9L 4-blade props (far left in the photo below).



Unfortunately, these axle-ready props produce very little thrust at any torque or speed. Mounting them coaxially at close intervals, as shown below, doesn't help much, as it disrupts flow around all blades involved.



We chose the red 2-blade 5.5L props below for BW for one simple reason: They're far more efficient than any other LEGOŽ prop.



That Radio Flyer was a good 25% slower at BW than she is with the black 55 mm third-party screws normally used at home (here on the floor) shows that LEGOŽ prop efficiency still has a long way to go.

These 5.5L props were simply the best we could do under the rules. There's a catch, though: Their center holes resemble Technic pin holes capped at one end but are actually a hair larger than standard pin holes.

The only BW-legal way to mount a 5.5L prop on a cross-axle is to stick the pin end of an axle pin or bush pin in the hole and pray that it doesn't slip too much under load. 5

Problem is, a high-torque XL motor pushing a boat as heavy as Radio Flyer qualifies as a substantial prop load -- especially in chop.

Shawn gambled that the higher efficiency of the 5.5L prop relative to other props and paddlewheels would make up for any slippage over the course of a single heat (one pool length of ~20 m), and it paid royally. How much slippage actually occurred during races is anyone's guess, but it clearly wasn't enough to keep Radio Flyer out of the winner's circle.

Besides, Shawn had 3 more aces up his sleeve: (i) Solid RF remote control, (ii) Radio Flyer's excellent directional stability , and (iii) Annette's well-practiced driving would keep Radio Flyer moving dead ahead all the way to the finish line heat after heat. No other boat included in the BW 2015 video accomplished that. Most veered drastically off course at least once a heat.

With identical left-handed 5.5L props on both sides of the boat, one might have expected more prop walk, but Radio Flyer's large rudder apparently suppressed most of it.



Optimal gearing
Radio Flyer's 3-stage 1:15 overdrive gear train was arrived at through our usual motor/gearing/prop (MGP) optimization. Two of the overdrive stages reside on the outdrives themselves.



Having settled on twin XL motors based on previous CLH-based boats, and having little choice WRT props due to the BW prohibition against non-LEGOŽ components, Radio Flyer's MGP optimization boiled down to final overdrive ratio selection.

We knew (a) from experience with Nadine (1:8.33 overdrive) and (b) from the greatly reduced efficiency of the 5.5L props Radio Flyer would be running at BW that her final overdrive ratio would be greater than 1:8.33.

The only remaining questions: (i) How much greater than 1:8.33? (ii) How close can we get with available LEGOŽ gears?

The goal of MGP optimization is to maximize top speed by arranging, through gearing in this case, for the motors to reach peak mechanical power output just as the boat tops out in forward speed.

For DC electric motors like XLs, maximum mechanical power output occurs when motor shaft speed is ~50% of the motor's no-load speed at operating voltage. For an XL motor powered by 6 alkaline AAs, the target shaft speed is ~100 RPM after voltage sag.



Since we have no way of measuring motor shaft speeds at top boat speed short of resorting to Mindstorms-based propulsion systems (which would far too heavy for speedboats), we measure them instead with an inexpensive handheld laser tachometer (below) during static thrust tests.

In a static thrust test, the boat is held stationary with the fully immersed props running at full power. Motor shaft speeds will be a little higher than the values obtained by this when the boat is moving freely at top speed.

This discrepancy is quite acceptable for 2 reasons: (i) The target peaks in DC motor power-speed curves are broad enough that shaft speeds within 10% of the mark are usually good enough. (ii) It's better to err on the high side.

The images below show a static thrust test prior to motor shaft speed measurement.



The height of the propwash above ambient water level with the props held a standardized distance from a wall provided a rough indication of the thrust produced by Radio Flyer's BW-legal 5.5L props.




Steering system

The good news about the RC Race Buggy RF remote control system: The RF receiver provides a dedicated steering output shaft connected to what appears to be an internal stepper motor.



Fore-aft balance and trim control considerations dictated the receiver position just forward of midships. The long, black reinforced shaft in the centerline transfers torque from the receiver's steering output to the rudder linkage.

The bad news: The control layout on the RF transmitter is ill-suited to differential-drive steering. We generally prefer to steer our twin-screw boats with their throttles alone, as it eliminates the need for a steering system separate from the propulsion system. Unlike rudders, it also works well at low speeds, in reverse, and in rough water.

Since Shawn was after reliable control above all else, however, he opted to use the easier user interface, run both props at the same speed at all times, and steer with a rudder driven by the receiver's steering shaft.

The result is the relatively simple steering system below.





The red Technic ball on the rudder crank arm helps the driver assess rudder position at a distance.

The good news about this solution: The large rudder gives Radio Flyer excellent directional stability, and the play in its linkage makes it strongly self-centering.

The bad news: The play in the rudder linkage and torsion in the long steering shaft make for a slow steering response and a turning radius that turned out to be way too large for the BW slaloms. The large rudder also adds appendage drag.

The upshot: These properties -- excellent for drag racing, bad for slaloms -- were reflected in Radio Flyer's victory in the drag races and so-so showing in the slalom event.

Tightening up the steering mechanismn -- or eliminating it altogether -- will be Job #1 when Annette stops hugging the boat and gives it back to Shawn.




Comparison with other entries

Shawn made his boat drag racing debut at BW 2014. His DNF due to an all-too-typical IR remote control lapse convinced him that a boat with both speed and control would be hard to beat.

But that's exactly what we'd been working on for the last 2 years, and that's what he sent to BW 2015.



Speed
Radio Flyer's speed comes from a combination of high installed power, efficient props, and low total resistance.

Her twin XL motors pump out gobs of mechanical power with good torque at all speeds. Most of her competitors had less installed power, and many suffered from inefficient drivetrains as well.

More importantly, Radio Flyer's installed power to displacement ratio remained relatively high despite the RF receiver onboard. The relative merits of her chosen props have already been discussed above.

The biggest difference, however, was probably the low total resistance.

You can't just wish resistance away. To build a truly fast boat at any scale, you must understand the relative importance of the many sources of resistance in that particular boat and attack those most responsible for holding it back first. The Froude number (dimensionless speed) provides much of the necessary guidance.

For starters, I spotted only one other monohull in the running. Nearly all the boats were catamarans with broad demihulls at very narrow separations. Multihulls always have more wetted surface area than monohulls of the same displacement, and closely spaced hulls are a recipe for high interference drag due to wave generation between hulls. Obviously, monohulls aren't subject to this special form of wave-making resistance.

Most of the other boats were seriously overloaded with little remaining freeboard, and the associated deep drafts only added to their wetted surface areas. Many of them had much bulkier appendages as well. In contrast, Radio Flyer drew only 25 mm. As a result, she had less plastic in the water than any other boat I got a good look at.

These factors gave her a substantial edge WRT both viscous resistance and appendage drag.

A long waterline is the best countermeasure against wave-making resistance, and Radio Flyer's CLH provided the longest available. That alone gave her a substantial advantage over shorter entries. The other CLH-based boats (2 at most) were multihulls.

Slenderness at waterline reduces both wave-making resistance and viscous resistance, and Radio Flyer (length-breadth ratio 3.9) had most of the field beat on that score as well.



Control
Shawn recognized the BW pool as a difficult environment for IR remote control. Boats in the center lanes are almost out of IR range, and the overhead lighting may produce IR interference as well.

Given that most BW entries use IR remote control, the near-Brownian motion prevailing in most of the heats in videos from 2015 and previous years confirms that assessment.

He also knew from our testing that Radio Flyer's CLH had the stability and freeboard to keep his heavy and expensive RF receiver dry in pool chop and minor collisions.

The rest, as they say, is history.






Specifications



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

Overall dimensions:~600 x 88 x 54 mm (LxWxH, including rudder)
Displacement:~1.2 kg
Displacement volume:~1.2 x 10 -3 m3
Bare hull depth:58 mm (midships)
Waterline length:~550 mm
Waterline breadth:~142 mm
Draft at keel:~25 mm (midships)
Freeboard:33 mm (midships)
Wetted surface area:n/a, but less than other entries
Midship section area:~3.4 x 10 -3 m2
Waterplane area:~7.1 x 10 -2 m2
Block coefficient:~0.62
Prismatic coefficient:0.62
Midship coefficient:~0.99
Waterplane area coefficient:~0.90
Length-breadth ratio:3.9
Breadth-draft ratio:5.7
Length-displacement ratio:5.4
Form factor:0.48




Performance measures

Installed power:~6 W after voltage sag
Installed power to displacement ratio:~5 W/kg
Critical speed:0.93 m s -1
Top speed:≥0.[] m s-1
Froude number at top speed:≥0.[]
Reynolds number at top speed:≥4.0 x 10 5
High speed index:≥0.82




Design features

Construction:Studded and studless
Hull:74x18x7 LU City Lines hull (set 7994)
Propulsion:Twin inverted-V outdrives
Motors:3, 1 XL on each prop and internal RF receiver steering motor
Propellers:Twin BW-legal 5.5L (44 mm) 2-blade LEGOŽ "twisted propellers" (4745)
Gearing:3-stage 1:15 overdrive (not optimized for BW-legal props)
Propeller separation:158 mm on center
Steering:Rudder driven by RF receiver steering motor
Electrical power supply:9V (nominal) from 6 alkaline AAs in the RF receiver
RF receiver:RC Race Buggy
External RF receiver connections:1 shared by both XL motors
Modified LEGOŽ parts:None
Non-LEGOŽ parts:None
Credits:Shawn Kelly's original MOC with my collaboration; Annette Kirkpatrick's excellent driving at BW





Foototes

1 Radio Flyer draws ~25 mm of water at a displacement of ~1.2 kg. In contrast, lightweight Nadine draws 16 mm at 0.86 kg. Radio Flyer's RF receiver, alkaline AA batteries, and rudder account for most of the difference in displacement and draft.

2 If Radio Flyer's 6 alkaline AAs maintained their no-load voltage (9V total) at full speed, her installed mechanical power would be ~7.3 W.

However, alkaline batteries are notorious for voltage sag under high current draws, and a pair of current-hungry XL motors in parallel at full power would certainly do the trick. Hence, actual installed power during races was probably more like 6 W and installed power to displacement ratio was ~5 W/kg.

3 The so-called LEGOŽ "twisted propeller" isn't really twisted. It just has the largest blade angle to be found among ready-made LEGOŽ props.

Its unmatched efficiency among LEGOŽ props reflects a favorable combination of diameter, blade angle, and total blade area. The much less efficient 3-blade 3L prop has the same blade angle but is otherwise far from optimal.

Blade angle is the acute angle between the plane of the propeller and the chord of a blade at a given radius from the spin axis. In a true twisted blade, blade angle decreases steadily from root to tip so as to maintain the same favorable angle of attack along the entire length of the blade.

In contrast, all LEGOŽ props have the same blade angle at all radii. This lack of twist reduces lift and increases drag in the outer portions of the blades, where propellers normally produce most of their thrust. (This is true in both air and water.)

Inadequate twist may even cause the outer portions of the blades to stall, at which point the lift/drag ratio and thrust there both go to zero.

4 Much lighter Nadine is ~15% faster than Radio Flyer with the same hull, outdrives, and 55 mm third-party screws.

If Radio Flyer's alkaline battery voltage sag2 at full tilt isn't too bad, Nadine may even outrun her with less absolute installed mechanical power.

Added appendage drag due to Radio Flyer's rudder (Nadine has none) accounts for some of the difference in top speed. Much of it, however, has to do with the wetted surface area added by Radio Flyer's greater draft.

This example clearly illustrates the importance of increasing installed power to displacement ratio and reducing total resistance by keeping displacement down.

5 Shawn chose to mount Radio Flyer's 5.5L props on 2L axle pins and inserted their axle ends into 2L axle joiners to present axle holes to her prop shafts.



I opted to mount Stormin' Norma's 5.5L props on bush pins instead but didn't take any chances with slippage. I (gasp) shortened the bush pins to 2L and (double-gasp) glued them into the oversized prop holes. Then I (triple-gasp) hand-sanded the blades into proper airfoil profiles.

The result was a highly modified 44 mm 2-blade LEGOŽ prop that, in Stormin' Norma's case, performed almost as well as the 40 mm 2-blade third-party RC boat props at upper right.

This single modified part took Stormin' Norma from a floating slug to a model ship that could easily make its target scale speed of 12 knots.

Selective impurism may be totally illegal at the BW boat races, but it pays royally at home.




References
Most of the titles below are free online for the digging.

Abramovitch, D., 2005, The Outrigger: A Prehistoric Feedback Mechanism, IEEE Control Systems Magazine, August, 2005

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 like it 
  July 6, 2017
Gotcha. I do run them in a lot of water, a small pond/fountain which is about 20 feet wide and 10 feet tall. My Fire boat does this as well. Here's a video of my Tugboat running, and I think you can see stuff going through the propeller: http://www.moc-pages.com/moc.php/439862
 I made it 
  July 5, 2017
Quoting Angelo Filipelli What a beast! Quick question about propellers though, is there any way to prevent debris in the water from getting sucked up through the propeller on my Police boat? (I don't have any cleaner water to run it in, I run it in a pond/fountain)
Thanks, Angelo! I think your props would foul less often if they had more open water around them. Those underwater "pods" combining battery, motor, prop, and rudder are easy to mount, and they keep the propulsion system out of sight. But they don't produce much thrust and really crowd their props.
 I like it 
  June 22, 2017
What a beast! Quick question about propellers though, is there any way to prevent debris in the water from getting sucked up through the propeller on my Police boat? (I don't have any cleaner water to run it in, I run it in a pond/fountain)
 I made it 
  December 21, 2015
Quoting Vibor Cavor Wow, I never knew there were LEGO boat races. Congrats on the win! Those custom blades you made look very interesting. A shame that they don't work as you wanted them to. All in all great job!
Thanks, Vibor! Yes, the lack of an effective all-LEGO propeller is a great frustration for boat-builders. Good thing I'm not shy about using non-LEGO parts when there's no viable purist alternative.
  December 21, 2015
Wow, I never knew there were LEGO boat races. Congrats on the win! Those custom blades you made look very interesting. A shame that they don't work as you wanted them to. All in all great job!
 I made it 
  November 7, 2015
Quoting Clayton Marchetti Congratulations on your win at BrickWorld! Your work on these Lego built speed boats is really brilliant. I've watched the videos you've posted and it looks like a lot of fun.
Many thanks, Clayton! Our boats get a lot more play time than any of our RC land vehicles. Highly recommended!
 I like it 
  November 6, 2015
Congratulations on your win at BrickWorld! Your work on these Lego built speed boats is really brilliant. I've watched the videos you've posted and it looks like a lot of fun.
 I made it 
  October 31, 2015
Quoting Franciscus van der Maat That was also a lovely read, thanks for that Jeremy.
Thanks, Franciscus! If you read it all, you should get a medal.
 I like it 
  October 31, 2015
That was also a lovely read, thanks for that Jeremy.
 I made it 
  August 2, 2015
Quoting Oran Cruzen Oh yes, and congratulations on your award!
I'd start with hard plastic front wheels to let the slot do the steering without interference from front wheel traction.
 I made it 
  August 2, 2015
Quoting Oran Cruzen Oh yes, and congratulations on your award!
Thanks, Oran. Not familiar with that chassis, but I don't see anything standing in the way of a Power Functions slot car built from the ground up with LEGO. An M motor and a lightweight PF rechargeable battery would be good 1st guesses.
  August 2, 2015
Oh yes, and congratulations on your award!
 I like it 
  August 1, 2015
Another great work of Lego "I Can Built It!" creationism from you! Hey Jeremy, if you have some time could you check this out, I am trying to find more information on a non-Lego slot car chassis, do you know anything about it or something similar? Thanks! http://mocpages.com/moc.php/416998
 I made it 
  July 29, 2015
Quoting Topsy Creatori Congratulations on the win! :)
Thanks, Topsy. Your comments are always appreciated.
 I like it 
  July 29, 2015
Congratulations on the win! :)
 I made it 
  July 26, 2015
Quoting Nerds forprez Congrats Jeremy!
Thanks, Nerds
 I made it 
  July 26, 2015
Quoting Nick Barrett Fascinating stuff, and I now know how to make a Lego boat fast and reliable. Congrats on the win, well deserved!
Thanks, Nick. I hope you give powerboats a try. I think you'd enjoy the challenge, and I'd love to see what you come up with.
 I made it 
  July 26, 2015
Quoting jds 7777 Glad I could help! From my experience with XL-motors and subtractors, it should withstand the torque. Especially when they are geared up and only spinning props. Unless you and Shawn can engineer a better design, (which you can probably can) I would suggest using the steering output to apply friction to either axle causing the diff to vector torque to either axle. I'm sure I can come up with an idea on how to keep the props from sliding if I think about it long enough. (I suppose gluing them is illegal in the competition)
We're definitely going to pursue the subtractor idea. As for slipping props, yes, glue is forbidden. We've be wracking our brains for over a year to find a legal, slip-free way to mount those "twisted props" on a cross-axle. If you find one, please drop me a line at mocpages@cliffshade.com.
 I like it 
  July 26, 2015
Congrats Jeremy!
  July 26, 2015
Glad I could help! From my experience with XL-motors and subtractors, it should withstand the torque. Especially when they are geared up and only spinning props. Unless you and Shawn can engineer a better design, (which you can probably can) I would suggest using the steering output to apply friction to either axle causing the diff to vector torque to either axle. I'm sure I can come up with an idea on how to keep the props from sliding if I think about it long enough. (I suppose gluing them is illegal in the competition)
 I like it 
  July 26, 2015
Fascinating stuff, and I now know how to make a Lego boat fast and reliable. Congrats on the win, well deserved!
 I made it 
  July 26, 2015
Quoting Walter Lee congrats!
Thanks, Walter. Wish I could have been there to watch the races. Have you been able to swing any pool time with your boats this summer?
 I made it 
  July 26, 2015
You can learn the darnedest things by looking at photos of your own MOCs! Turns out that we did optimize the final overdrive ratio to 1:15 to match the motors to the BW-legal props prior to sending the boat to Chicago, so no further speed gains available there.
 I like it 
  July 25, 2015
congrats!
 I made it 
  July 25, 2015
Quoting jds 7777 Great job! It just occurred to me that you could use a lego gear diff and use the RF receiver's steering output to adjust the speed to each prop. It's a lot of mechanical complexity, but it would get rid of the rudder, and give you a better turning radius while keeping the steering basic and easy to operate.
Excellent idea, JDS! If the subtractor can handle the torque of twin XLs on the power input, that might just work! Shawn's almost mastered our usual throttle steering with the RF transmitter, so if he can go next year, we might be able to ditch the rudder that way, too.
 I made it 
  July 25, 2015
Quoting matt rowntRee Congrats! I can't believe you were there and I missed the chance to meet you in person. I always figured your boats would destroy any competition at this event, guess I was right. ;) Hopefully you'll be able to get out there again next year and defend your title. Awesome!
Thanks for that big vote of confidence, Matt! Would have loved to meet you, too. Should've been clearer that driver Annette was the only team member there. We weren't so sure of the drag racing outcome, though, as slipping props could easily have done us in. Once they start slipping, they tend not to stop.
 I like it 
  July 25, 2015
Great job! It just occurred to me that you could use a lego gear diff and use the RF receiver's steering output to adjust the speed to each prop. It's a lot of mechanical complexity, but it would get rid of the rudder, and give you a better turning radius while keeping the steering basic and easy to operate.
 I like it 
  July 25, 2015
Congrats! I can't believe you were there and I missed the chance to meet you in person. I always figured your boats would destroy any competition at this event, guess I was right. ;) Hopefully you'll be able to get out there again next year and defend your title. Awesome!
 
By Jeremy McCreary
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Added July 25, 2015
 


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