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Illustrated guide to powerboat-compatible unitary hulls
Seaworthiness and performance depend critically on hull selection. Here I share my experience with the 10 unitary hulls we consider to be powerboat-compatible.
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) boats for over 2 years now. To date, we've come up with dozens of seaworthy no-frills speedboats based on various LEGO® unitary hulls.



Shawn's Radio Flyer above even took 1st place in the boat drag races at BrickWorld 2015! We've also dabbled in motorized ship models like R/V Stormin' Norma II, the marine geology research vessel below.



As a result, we've had the opportunity to build, test, and in most cases either race or clock, LEGO® powerboats using all 10 of the powerboat-compatible hulls (PCHs) described below. The twin-screw propulsion systems and third-party props seen on this page are explained here.



The info below comes 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.

On this page:


LEGO® unitary hulls

Bare hull refers to what's left after removing everything from a LEGO® unitary hull (LUH) except for decking specifically made for it. The unitary part is the one-piece bottom.



Typical bare LUHs: The big blue 74x18x7 LU City Lines hull, the largest LUH available, dwarfs the DBG and white 28x8x3 LU Fishing Boat hull and the black and white 24x6x3 LU hull from the 1996 Riptide Racer set (4002). The CLH is powerboat compatible. The 2 smaller hulls are not.

The nominal LUH sizes quoted on BrickLink -- e.g., "74x18x7" for the City Lines hull shown above -- usually refer to bounding box dimensions in LEGO® units (LU; 1 LU ≡ 8 mm) in Length x Breadth x Depth format.

Naval architects would refer to such hull dimensions as overall or extreme. The 5 LU nominal depth (height) quoted for the Family Yacht hull (6 LU by my ruler) is only grossly inaccurate nominal size I've come across.

Nominal hull dimensions are useful only as rough guides WRT hull size and proportions. The only hull dimensions that count in open water are those of the finished boat's underbody -- i.e., the portion of the hull submerged at operating displacement.

Due to bow and stern overhangs of varying degree, load waterline lengths (LWLs) typically run 5-15% smaller than nominal and vary with displacement and attitude. Waterline breadths are usually a few millimeters under nominal.



∧ NB: All LUHs are effectively displacement hulls in that they push through the water at attainable speeds -- as in the video above -- rather than plane (skim) over it. Most look the part.

Displacement hulls are supported by buoyancy alone, with no help from hydrodynamic lift. The few LUHs with planing hullforms would all sink under the weight of a propulsion system capable of getting them to lift-generating speeds.

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LUH decks

The deck pieces made for specific LUHs are never watertight but some can be made so.

By far the easiest to seal are the weather decks, which completely cover the underlying unitary bottom and overhang it all the way around.

Only 4 of the 10 available PCHs have weather decks. They arePhotos of these boats appear below in the order just listed.









All of the weather decks above are sealed with electrician's tape. Clear silicone caulk also works well. Both are easily removed. Tape seals add more weight to the hull but are easier to repair in the field.

∨ NB: Sealing tape around bows and stern corners needs frequent maintenance to remain watertight. The tape stretched over Laverne's bow is a case in point.



I almost lost Trident to a slow leak in the tape around the bow of her center hull.

Like most PCHs, Laverne's Police Boat hull has an open-top deck well large enough to trap dangerous volumes of water above decks.



Covering deck wells with large plates effectively slows water accumulation, but the added weight is often prohibitive, as it would be in Laverne's case.

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Powerboat-compatible hulls (PCHs)

In our experience, only 10 of the dozens of LUHs made to date qualify as powerboat-compatible. These 10 hulls are listed and linked in the table of contents above and described in detail below.

We define a powerboat-compatible hull (PCH) as an LUH capable of supporting a 0.35 kg no-frills speedboat load in a seaworthy manner -- either by itself or joined to another copy of itself to form a twin-hull catamaran. 1

To be fit for multihull use, a PCH must also have an adequate number of secure cross-structure attachment points.

Most PCHs have flat, smooth, near-vertical sides and sterns like those below. The truncated transom sterns simplify stern drive and outboard attachement and may reduce total resistance at speed. Surprisingly, the rather broad entries (bow angles) on all PCHs may also reduce total resistance at speed.



Typical LUH bottoms: From left to right, FBH (white, planing hull form, not powerboat compatible), FFH (black), SBH (red), PBH (red, planing hull form), CCH (red), and CLH (blue).

The largely flat bottoms on most PCHs favor roll stability, maneuverability, and top speed while penalizing directional stability. Some bottoms are dirtier than others -- hydrodynamically speaking, of course.

The largest LUH failing to make the PCH cut is the 28x8x3 LU Fishing Boat hull.



I gave it a good shot, but it proved to be totally inadequate WRT (i) below-deck volume for monohull use and (ii) cross-structure attachments for catamaran use. Pity, as it's the cleanest LUH of all from a hydrodynamic standpoint and the only one with any chance of getting up on plane under PF motor and battery power.

Below are the 10 PCHs we can recommend from experience, in order of decreasing nominal length. The names I've given them come mostly from their debut sets and are by no means official.2

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74x18x7 City Lines hull (CLH)









∧ "City Lines" hull (CLH, 57789c01): The long, blue tanker-like 0.43 kg, 74x18x7 LU hull used by Nadine above has been offered only once -- in the 2007 LEGO® City Harbor set (7994). My name for it comes from the big white "City Lines" stickers it usually bears.

The CLH is by far the most forgiving of all PCHs and therefore an ideal choice for a first LEGO® powerboat or a heavy ship model. Perhaps surprisingly, it's also ideal for speedboats.

One way or another, all of those advantages relate to the fact that the CLH is hands-down the largest of all LUHs in every dimension -- especially LWL.



The CLH is the only LUH capable of carrying either triple XL motors or the RC Race Buggy RF remote control receiver in a monohull (single-hull) boat.

Unequaled waterline breadth (~0.14 m) and breadth/draft (B/T) ratio (>7) also make the CLH the most stable LUH by a wide margin. In turn, the stability and great length and mass add up to unrivaled seakeeping ability despite its so-so maneuverability.

The videos below show Nadine at her 1st pool trial, where she turned in our fastest-ever top speed of ≥0.99 m/s.





The icing on the CLH cake is its speed: Unmatched LWL (~0.54 m), better than average slenderness (L/B ≈ 3.9), and an exceptionally clean bottom make for a very fast hull indeed -- primarily via much-reduced wave-making resistance. In fact, CHL-based Nadine and Earline both owe as much of their speed to their hulls as to their installed power.

The CLH lacks a weather deck, having instead an inset one-piece deck with 3 wells: 4x6x2.67 foreward, 6x8x2 LU aft, and 46x14x3 LU in between, with the last 2 adjoining. Together, these deck wells can trap a whopping 1.23 kg of water, but they seldom see a drop at speedboat loads due to the hull's unequaled freeboard, slightly flared bow, and excellent stability and seakeeping. Good thing, too, as the hull interior can trap several times that.

The boats below show how displacement, depth, breadth, draft, freeboard, and stability interrelate in the CLH.



Speedboat Nadine draws 20 mm of water at 0.855 kg displacement.

The boat below is a much heavier test platform for Voith-Schneider propellers (VSPs).3





The VSP test boat draws ~32 mm at 1.62 kg displacement. The video shows just how well the CLH handles such loads. A much higher center of mass and a lower B/T make this boat noticeably less stable than Nadine, but she's still more stable than most of our non-CLH boats.

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52x16x6 "Family Yacht" hull (FYH)





∧ Family Yacht hull (FYH, 30215): The very rare 0.21 kg, 52x16x6 LU hull used by Lucille above gets its name from its one and only set (Belleville 5848, 1998). It has 70% the nominal length, 89% the nominal breadth, and 86% the nominal depth of the CLH.



FYH-based Lucille's MOCpage alongside CLH-based Earline (blue) for comparison.







From a practical standpoint, the FYH's most notable features are (i) an open stern with an 8 LU-wide step-through and a 2 brick-high sill, (ii) the dirtiest of all PCH bottoms, and (iii) lack of a weather deck.

The pros and cons of the open stern and the dirty bottom are discussed on the MOCpages devoted to FYH-based Lucille and Lucille II -- especially here.

Once the open stern is dealt with, the FYH is more forgiving than the CCH, PBH, and CGH and almost as seaworthy as the CLH.

An FYH monohull will be more stable than one based on the CCH, PBH, and CGH by virtue of at least 33% more waterline breadth and much higher breadth-draft ratio under a speedboat load. It will also have more freeboard than the PBH and CGH at the bow and sides.

The video below shows Lucille in a tub trial.



An FYH monohull can easily carry a twin-XL speedboat load with stability to burn in rough water but gets much less top speed out of it. This slowness is mainly due to (i) the worst case of bottom roughness of any LUH, (ii) its great breadth, and (ii) the 3rd lowest waterline slenderness (L/B ≈ 3.1) to be found among PCHs.

All in all, the FYH is a good motorized model ship platform but leaves much to be desired as a speedboat hull.

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52x12x6.33 "Cargo Carrier" hull (CCH)





∧ Cargo Carrier hull (CCH, bfloat4c01): This rare, sleek 0.28 kg, 52x12x6.33 LU hull really shines in monohull speedboats. I named it after its 1987 debut set.

The CCH has 90% of the nominal depth of the CLH but only ~70% of the nominal length and breadth. The CCH and CLH hullforms are similar, but the CCH lacks a stern skeg and is slightly more slender at waterline under a speedboat load (L/B ≈ 4.1 vs. 3.9).

All that adds up to a fast, seaworthy hull with good stability and greater than average freeboard. Though somewhat less stable than the CLH and FYH, the CCH is much more forgiving than the PBH and CGH described below.

The CCH lacks a weather deck. Instead, it has short decks forward and aft with a large expanse of studded hull bottom exposed in between. Best to drain CCH-based boats early and often, as that large expanse makes them susceptible to the free-surface effect.

A CCH monohull can easily carry a twin-L speedboat load with freeboard and stability to spare in rough water.



If the many stud dimples fouling its bottom are faired with tape, it can produce a monohull speedboat at least as fast as PBH-based Laverne but a good bit more seaworthy.

The CCH-based boat shown above was an early version of Celine, the speedboat in the video below.



Celine shares our 2nd fastest top-speed of 0.97 m/s with PBH-based Laverne.

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51x12x6 "Police Boat" hull (PBH) and "Coast Guard" hull (CGH)

The 2 hulls belonging to the 0.22 kg 51x12x6 LU family have the volume and freeboard to support a no-frills monohull speedboat, but they lack the buoyancy and stability needed for a seaworthy monohull ship model the likes of Stormin' Norma without assistance.

Both hulls have the same easily sealed weather deck. Its large 37x8x2 LU deck well can trap 0.36 kg of water -- more than enough to a sink even a no-frills PBH monohull like Laverne.







Stormin' Norma above uses the 51x12x6 LU variant with side bulges. I call it the Coast Guard hull (CGH, 62791c01, 54100c02, 54100c01) after its 2008 debut set.

The CGH's unique open-bottomed side bulges add weight and resistance but, alas, nothing in the way of buoyancy or stability.







Much better for speedboats is the 51x12x6 LU variant without side bulges used by Laverne above. I call it the Police Boat hull (PBH, 54100c02, 54100c01) after the 2006 debut set.

The PBH is the only PCH with a planing hull form. Though much too small to carry a propulsion system potent enough to put it on plane, its unique combination of hull form and above-average LWL and slenderness add up to excellent directional stability and very low total resistance at all speeds.



Laverne's ~0.35 kg speedboat load is about as much as her PBH can safely handle. Her limited freeboard and reserve stability make her a bit hectic to watch in the water at times, but she's proven herself reliable in heavy swimming pool chop. Her deck well needs to be drained fairly frequently.

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51x22x10 "Pirate Ship" hull (PSH)







∧ Pirate Ship hull (PSH, 47980c01+47983c01+47986c01): The 0.43 kg, 51x22x10 LU hull used by "Maybellene" above gets its name from its 2004 debut set (Captain Redbeard's Pirate Ship, 7075).

The PSH is the only PCH to come in sections. The 3 sections join securely and are LUHs unto themselves.







The minimum calm-water freeboard at midships needed to keep water from entering the sections via the submillimeter gaps between them is ~25 mm.





Taping over the gaps is unnecessary when the hull's not overloaded but may reduce viscous resistance a bit.



Deck wetness can enter the sections via the three 8x8 deck wells.



The PSH ties the long blue CLH below as the heaviest LUH. It has the greatest waterline breadth, 2nd lowest waterline length-breadth ratio (L/B), and the 3rd greatest breadth-draft ratio (B/T) of any LUH. Large overhangs at the bow and stern make for a middling LWL of only 348 mm at Maybellene's displacement of 0.88 g.





Here we have Maybellene alongside the CLH-based Triton, our fastest boat ever. Note the differences in freeboard, waterline length and breadth, and waterline slenderness. Triton displaces 0.82 kg to Maybellene's 0.88 kg.

Stability and overall seaworthiness are excellent, but freeboard (34 mm at the bow) is only so-so.

Four thin skegs run the length of the hull bottom, with another 2 short skegs at the stem and stern. The keel is dirtied hydrodynamically by the latter skegs and a motor mount. The bottom is otherwise smooth and flat. The result is a hull with better course-keeping ability than one might expect from good maneuverability.

Bottom line: The PSH is a seaworthy but heavy and slowish hull much better suited to workboats and ship models than to speedboats.

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48x6x5 "Speedboat" hull (SBH)









∧ Speedboat hull (SBH, 50821c01): The lightest and narrowest of all PCHs is the 0.13 kg, 48x6x5 LU hull from the 2003 Speedboat set. Some call it the "catamaran hull". Its waterline slenderness under a speedboat load (L/B approx; 8.0) is at least double that of any other LUH.

The photos above are of Trident, an outside-the-box trimaran with an SBH center hull.

The SBH sheds water like no other LUH by virtue of an easily sealed weather deck with 3 unique features:
  • A single 10x2x1 LU deck well able to trap a mere 0.012 kg of water
  • 12 Technic pin holes around the well serving as freeing ports (drains) when unused
  • Along most of its length, steeply cambered (sloping) studless surfaces to hurry water overboard.
Better yet, the pin holes are by far the strongest hull attachments on any LUH.



The SBH shines brightly in multihulls of all kinds, but very low overall volume, breadth, and breadth/draft ratio preclude monohull powerboat use. Trident takes full advantage of the structural strength and immunity to wetness afforded by her sealed SBH center hull but would go straight to the bottom without her side hulls.

In our experience, all-SBH multihulls like the trimaran and catamaran below excel in stability and structural stiffness but are slower than expected for reasons I'm still sorting out.



The raft of 3 staggered SBHs supporting the 0.92 kg trimaran airboat above ably resists wave- and thrust-induced flexure and attitude changes. When re-outfitted as a twin-screw speedboat, however, the same raft turned out to be ~10% slower than Dubble°°Bubble, the 0.71 kg all-SBH catamaran below.



Dubble°°Bubble, in turn, is at least 10% slower than Trident.

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40x20x7 "Exploration Vessel" hull (EVH)









∧ Exploration Vessel hull (EVH, 18913c01): This 0.24 kg, 40x20x7 LU workboat hull first appeared in the 2015 Deep Sea Exploration Vessel (60095) in dark blue. The red one comes in the less expensive 2016 Fire Boat set (60109). The red EVH boat above is "Darlene" (0.627 kg); the dark blue, "Della" (0.588 kg).

The EVH, TLG's latest LUH, is a unique hybrid -- monohull forward, catamaran aft. It has the 2nd greatest waterline breadth, the 2nd lowest waterline length-breadth ratio (L/B), and the 2nd greatest breadth-draft ratio (B/T). However, the middling LWL is only 50-60% that of the CLH. In fact, the EVH is the shortest PCH capable of supporting a no-frills monohull.



At Della's displacement of 0.588 kg, LWL and waterline breadth are 305 and 154 mm, respectively, and L/B ≈ 2.0. Draft, freeboard, and B/T are respectively 20 mm, 26 mm, and 7.7 at midships. (A CLH-based boat with twice the displacement would have a similar draft.) Freeboard at the blunt bow is a whopping 42 mm. These features make for a very forgiving boat.



The EVH looks like a workboat hull and functions best in that capacity. Maneuvability is critical in workboats, especially at low speeds, and the EVH really shines there by virtue of its smooth, flat bottom, short LWL, and very low L/B. The usual trade-off between maneuverability and course-keeping ability applies.



Stability and overall seaworthiness are also critical in workboats, and the EVH delivers there as well. Forward freeboard approaches that of the CLH, and initial roll stability (near 0° roll) is comparable. The heavier and much longer CLH pitches much less in waves, but the EVH could conceivably have greater dynamic stability in high, steep waves.

The large stern cut-out expands the EVH's propulsion, steering, and deck equipment options in exciting ways but can also be completely covered over with a single 16x16 plate.



A large flat deck surface results when the large forward deck well is also covered. Add the EVH's solid roll stability to the large deck area, and you have a great platform for a heavily outfitted workboat -- provided most of the load is forward.

The EVH is neither slow nor exceptionally fast. Della and Darlene have yet to be clocked, but their top speeds probably fall in the high-speed regime (Froude number > 0.30), where wave-making resistance dominates the total resistance budget.

If so, the EVH's short LWL, great waterline breadth, and lack of overall slenderness (rock-bottom L/B) all add substantially to total resistance.



The net effect of the EVH's large stern cut-out on total resistance is hard to predict. On the one hand, it substantially reduces overall wetted surface area relative to a monohull of the same overall "footprint". On the other, it forces premature flow separation and extra wave-making between the stern extensions, and the last can be significant in the high-speed regime.

In Miss Vickie, the 1.1 kg workboat-in-progress below, I joined a pair of EVHs stern-to-stern to form a much longer compound hull with a fully enclosed moon pool.





The 14 LU-long hull join is sturdy enough to withstand at least moderate wave impacts and collisions.



The moon pools in real workboats provide protected water access for drilling rigs, deck cranes, submersibles, and divers, but I used Miss Vickie's to mount a pair of working Voith-Schneider propellers (VSPs) through the hull.



Unlike Della and Darlene, Miss Vickie operates in the low-speed regime (Froude number ≤ 0.25), where viscous resistance accounts for most of the total resistance. The doubled wetted surface area, the enclosed moon pool, and the small interhull gap remaining at waterline definitely add to total resistance, and the roughly doubled LWL and L/B do little to offset that.

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38x10x5.67 "Fire Fighting" hull (FFH)



∧ Fire Fighting hull (FFH, bfloat3c01): The twin 0.18 kg, 38x10x5.67 LU hulls used by Double^^Trouble above get their name from their 1987 debut set. The FFH hull form resembles that of a slighly less slender CCH with less freeboard and a flat-bottomed stern.

A pair of FFHs can carry a twin-L speedboat load with stability to spare but tend to take on water over the bow in moderate to heavy chop due to limited freeboard forward. Such water should be drained frequently, as the FFH shares the CCH's susceptibility to the free-surface effect.

Cross-structure attachment issues limit FFH catamarans to narrow hull separations, which increase interhull wave-making resistance. Adding below-average LWL and slenderness and marked bottom roughness to the equation yields a stable but very slow catamaran with freeboard-limited rough-water capability.



That said, I had a lot of fun turning a pair of FFHs into Double^^Trouble -- a very red twin-hull, twin-screw runabout custom-built for the fabulously wealthy Scarlotti twins, the (very) bad girls of the international offshore powerboat racing scene.

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32x10x1.67 "Coast Watch" hull (CWH)







∧ Coast Watch hull (CWH, 71610c01, 71610c02): This 0.17 kg, 32x10x1.67 LU hull used by Tramontana above is the smallest possible PCH (see below). The name is that of its 1999 debut set.

The CWH is also the shortest and most maneuverable PCH. It features (i) an easily sealed weather deck with a 20x6x1 LU deck well capable of trapping only 0.074 kg of water, and (ii) built-in sealed flotation found in no other LUH.

The low-volume deck well doesn't hold enough water to sink a sealed CWH, and the internal flotation provides some insurance against deck seal leaks.

Though far too small for any conceivable monohull powerboat, a pair of CWHs can safely carry a twin-L speedboat load if the meager freeboard is supplemented with bulwarks (wave barriers), as I did with Tramontana above.

Tramontana was our top-speed champ for several months early on and has always been our most maneuverable boat. An exceptionally clean bottom compensates somewhat for the performance hits associated with the CWH's short waterline and low slenderness (L/B ≈ 3.1). Lightweight construction and optimally separated hulls were essential to Tramontana's speed.

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A cautionary tale: The powerboat-incompatible 28x8x3 "Fishing Boat" hull (FBH)





∧ Fishing Boat hull (FBH, 92710c01pb01): I was attracted to the 0.063 kg, 28x8x3 LU Fishing Boat Hull (from the 2011 set of that name) for its easily sealed weather deck and exceptionally clean hard-chined planing hull form.

Experience with the CWH suggested that the FBH, the next smaller LUH, would not be powerboat-compatible, but I ordered a pair to confirm.

Once sealed with silicone caulk (tape would have been too heavy), I joined the FBHs with the sturdiest cross-structure I could muster given their paucity of deck studs. I then fitted the resulting catamaran with a twin-M speedboat load, saving weight wherever I could.

Stern freeboard proved inadequate in tub trials, but a few strategically placed bulwarks kept water onboard to a manageable level -- in the tub, at least. Since top speed looked promising, I decided to attempt a pool trial.

The pool was a good bit calmer than that in the videos of Nadine, but we kept the FBH cat within arm's reach from poolside, just in case.

All was going well several minutes into the pool trial, when a beam wave flooded the port deck well and partially detached the port hull from the cross-structure. Shawn snatched the boat from the water just as the motors were going under.

NB: The offending wave here was larger than average for the pool sea state prevailing at the time but by no means a rogue. IMO, a properly engineered boat based on the PCHs listed above would have foundered under such a wave only if already endangered by water onboard. The FBH cat was dry when she sank.

The FBH therefore fails as a PCH on the following grounds:
  • Insufficient freeboard in a catamaran under the lightest possible speedboat load.
  • A disproportionately large 18x6x2 LU deck well capable of trapping 0.133 kg of water -- more than enough to sink any conceivable FBH powerboat.
  • Inadequate cross-structure attachments -- mainly due to the single row of studs on either side of the deck well.
In short, the FBH's clear lack of seaworthiness in calmer than usual water confirms the CWH as the smallest possible PCH.

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Underbody roughness

The main factors controlling a LEGO® powerboat's speed are (i) the mechanical power delivered to its prop shafts, (ii) the efficiency of the props in converting that power into thrust, and (iii) the water resistance encountered by the hull as a function of speed through the water. A fast boat needs to excel on all 3 counts.

This section focuses on a potentially significant but often overlooked contributor to viscous resistance: The roughness of the submerged portion of the bare hull. This part of the hull is imporant enough to warrant its own name -- the underbody.

The photo below shows some typical LUH bottoms: From left to right, FBH (white, planing hull form), FFH (black), SBH (red), PBH (red, planing hull form), CCH (red), and CLH (blue).



As noted earlier, LUHs are effectively displacement hulls. None are capable of carrying a propulsion system potent enough to propel it to lift-generating speeds, regardless of hull form.

Resistance-wise, the ideal displacement underbody would be long, slender, carefully streamlined, and polished to a mirror finish. Changes in hull curvature encountered by water flowing past would be as gradual as possible.

Unfortunately, the underbody best approximating that ideal belongs to an LUH that's not powerboat-compatible -- namely, the FBH below.



From a hydrodynamic perspective, the cleanest unmodified PCH underbodies belong to the CWH and SBH (below). Their bottoms are marred only by a thin longitudinal skegs 1-2 mm high. Similar skegs found on PBH, CCH, FFH, and FYH bottoms are the least of their problems.



The keels of the CLH (below), CCH, CGH, PBH, and FFH are marred by recessed mounts for submersible motors and weights.



The PBH is unique in having a long keel tunnel with 2 mounting brackets inside. The small stern recesses on the CCH and FFH are probably insignificant hydrodynamically.

The good news: All of these recesses are easily faired with electrician's tape, as shown in the next 2 photos. The particularly dirty recesses on the CLH, PBH, and CGH are prime candidates.



Fairing the motor mount recess on Earline's CLH with tape yields the cleanest PCH underbody available.



Fairing the keel tunnel on Laverne's PBH produced another very clean bottom. The open-bottomed side bulges on the CGH also benefit from fairing, but their abrupt lateral curvatures have no cure short of amputation.

Take a close look now at the hull bottoms at the centers of the next 2 photos. The black one belongs to the FFH; the red one, to the CCH. The many dimples are the undersides of studs. A large, sharp-edged circular mold scar also occurs near the center of each of these bottoms, but the scars aren't well seen here.





These dimples and scars are hydrodynamic disasters that should be faired with tape, at least in speedboats.





CCH bottom before and after fairing.

The dirtiest PCH bottom of all goes to the FYH. The stud dimples and mold scars found on the CCH and FFH bottoms are all here, but there are many more dimples, and they roughen a much larger wetted surface area.





To make matters much worse, the bottom of the FYH stern includes a large, complex recess sure to trigger premature flow separation across the entire breadth of the hull.

Do these textures, recesses, mounting brackets, side bulges, stud dimples, and molding scars really make a difference? You bet!

The total resistance of a bare hull is the sum of its wave-making resistance and viscous resistance. Abrupt underbody curvatures like those associated with the side bulges on the CGH are bad for both.

The viscous resistance grows with underbody surface area and roughness and the square of speed. Flow separations occuring forward of the actual stern also increase viscous resistance.

The CGH, FFH, and FYH boats we've built have all been slower than expected from length and hull form alone. The most likely suspect in all 3 cases is increased viscous resistance due to bottom roughness. Short LWL in the FFH case only adds to the misery.

The CCH, on the other hand, has the length and slenderness to be a very fast hull. However, that potential is realized only when its major hydrodynamic flaws -- the mold scar and the many stud dimples on its bottom -- are faired with tape.

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About our propulsion systems

One must have priorities, and our top priority--LEGO® fun in rough water--mandates non-LEGO® props.

To get play time, an RC boat must be seaworthy, well-behaved, and fast enough to provide some excitement. Even the cheapest hobby shop RC boats meet these requirements. Our fastest boats can't keep up with them, but they're fast enough to draw more play time than our land-based MOCs ever did. With LEGO® props, they'd gather dust instead.

Problem is, it takes a lot of thrust to push even a no-frills LEGO® speedboat to target speeds of 0.7 m/s and beyond. LEGO® motors and rechargeable batteries are up to the task, but LEGO® props can't turn the shaft power delivered to them into useful thrust at anywhere near the required efficiency.

Nor do LEGO® props come in counter-rotating pairs. The incessant course corrections needed to counteract the resulting propwalk in twin-screw boats further penalize speed and cut deeply into seaworthiness and play value as well.



Hence, our speedboats run highly efficient 52 or 55 mm 3-blade counter-rotating third-party props at either 1:8.33 or 1:5 overdrive.

Top speed is maximized by bringing the motors to peak mechanical power output (near 50% of their no-load shaft speed) just as each boat tops out. Unfortunately, that means that prop sizes and overdrive ratios must be matched to motors and total resistance on a boat-by-boat basis.





The beauty of differential-drive twin-screw propulsion is that it obviates the need for a separate steering system and the weight and mechanical complexity that would entail. Unlike rudders, it also delivers effective steering at low speeds, in reverse, and in rough water.

Splaying the stern drive struts outboard as shown above improves both steering response and thrust -- the latter by feeding the props water less retarded by interaction with the hull(s). Longer vertical struts would accomplish the same thing, but with more appendage drag.

Aside from the electrician's tape or silicone caulk we use to seal hulls with weather decks, our boats are otherwise 100% LEGO®.

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Footnotes

1 The decision to disqualify trimaran-only LUHs as PCHs was somewhat arbitrary, as some of the washouts (pun intended) could no doubt be turned into seaworthy trimarans or used as trimaran side hulls.

However, trimaran-only hulls present several major problems: (i) Most of them have serious volume and cross-structure attachment issues. (ii) The few prototypes I've tried to make with them were too slow for my taste. (iii) Experience suggests that the untried possibilities would either be too slow or too prone to sinking with or without cross-structure break-up.

2 I initially came up with these names for my own use. If you know of more widely accepted ones, please leave a comment.

3 The VSPs seen here are refinements of Michael "Efferman" Wirth's pioneering Technic adaptation. Swapping out his blades for the white Technic #5 and #6 long, smooth fairings seen here added quite a bit of thrust.

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References

Complete list here.

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Overflow from my tutorial on powerboat seaworthiness

This important final section from my tutorial on powerboat seaworthiness wouldn't fit there, so it's hosted here instead.



Avoiding capsize in busy pools
First, keep an eye on the diving area, nearby swimmers, and the wind, as all can send swell your way. Be extra cautious if the swell is likely to become a stern sea or set up a confused or cross sea on arrival.

Also watch for (i) shallows like broad steps or shelves, (ii) pool corners where reflecting waves can interfere constructively, and (iii) side drains where converging currents might arise. These settings are very effective at generating high, steep, and even breaking waves. Like a harbor bar, a shoal need not be shallow enough to cause a bottom strike to be dangerous.

If a steep or breaking sea develops for any reason, get your boat out of the water.

Drain water taken on early and often -- especially if it has the potential to create a free-surface effect or a list.

Finally, keep common capsize modes and their predisposing conditions in mind.

Forewarned is forearmed.

<< Return to the tutorial on powerboat seaworthiness



Comments

 I made it 
  January 24, 2018
Quoting Janss Net Just wanted to say thanks for this very useful post. Helped me creating a fast 2.4GHz LEGO RC Boat. Great speed, great fun. In case you're interested: https://youtu.be/qQd6RyZqyWY Thanks again.
My pleasure, Janns, and sorry for the delayed reply. Your excellent hybridization achieved something probably impossible with LEGO motors and batteries -- getting a LEGO unitary up on plane. I think you picked the right hull.
 I like it 
  January 2, 2018
Just wanted to say thanks for this very useful post. Helped me creating a fast 2.4GHz LEGO RC Boat. Great speed, great fun. In case you're interested: https://youtu.be/qQd6RyZqyWY Thanks again.
 I like it 
  June 13, 2017
I'm completely amazed that you are one of the VERY FEW people who actually know what the submersible motors and motor clips are. Those motors are very, very hard to find, but on my page I have a few on a few boats. Nice explanation! :)
 I made it 
  February 12, 2017
Quoting Nils O. That's just what I was looking for and I knew I would find it on your page :-)) I've just found my two old "swimmable" boat hulls in the basement. My son loves to play with them in the water. So my idea is to get a proper boat hull and add an RC propulsion system. Now I'm inspired... Thanks for sharing :-))
Thanks, Nils! My pleasure. You and your son are going to have lots of fun. For stability, speed, and control, you can't go wrong with a big blue City Lines hull and twin-screw propulsion without a rudder, as in my boat Nadine (still one of my favorites). For remote control, I strongly recommend the SBrick, as nothing brings out the PF system's weaknesses like a boat.
  February 10, 2017
That's just what I was looking for and I knew I would find it on your page :-)) I've just found my two old "swimmable" boat hulls in the basement. My son loves to play with them in the water. So my idea is to get a proper boat hull and add an RC propulsion system. Now I'm inspired... Thanks for sharing :-))
 I made it 
  November 25, 2015
Quoting Jarrid Mountford Very interesting. I've never heard of Lego power boats. Not sure how big a group or following there is for it. Unfortunately I don't have any of the bigger hulls. Not sure there are too many available now a-days. It is rare TLG releases floating boats. Two things: 1. Have you thought if using the lightweight plastic wrap that is used for RC airplane building to wrap your hulls? 2. Could you please indicate where I could acquire the aftermarket props that you use? Thanks. I enjoyed the primer.
Thanks, Jarrid! LEGO powerboats are fun and challenging -- at least if you want them to go fast. They're not the most popular genre, to be sure, but popular enough for BrickWorld to hold powerboat races every year. (One of our boats won the 2015 drag races.) You can get all of the hulls discussed here on BrickLink if you're patient and set the proper e-mail notifications. They're also on eBay, though generally at higher prices. Q1: Great idea! I'm familiar with the material but never thought to fair hulls with it. (The 52x16x6 hull would be the perfect candidate.) Q2: All the black nylon 3rd-party props came from China via eBay. Search on "RC boat props" and the like. In our experience, 52 and 55 mm diameters are the most useful. Look for 3/16 inch or 4.8 mm shaft holes to simplify adaptation to LEGO cross-axles. If you intend to use them on a twin-screw boat, be sure to get a counter-rotating pair (one left-handed, the other right). Otherwise, you'll have a lot of prop walk to contend with.
 I like it 
  November 25, 2015
Very interesting. I've never heard of Lego power boats. Not sure how big a group or following there is for it. Unfortunately I don't have any of the bigger hulls. Not sure there are too many available now a-days. It is rare TLG releases floating boats. Two things: 1. Have you thought if using the lightweight plastic wrap that is used for RC airplane building to wrap your hulls? 2. Could you please indicate where I could acquire the aftermarket props that you use? Thanks. I enjoyed the primer.
 I made it 
  February 17, 2015
Quoting Giorgio Ferrannini Your creations and your presentations always amaze me. Are you some kind of engineer?
Thank you very much, Giorgio. Not an engineer by formal training, but a life-long amateur. My background is in physics, medicine, and geology. All 3 have much in common with engineering. However, much of what I know about engineering proper comes from boning up on real-world design problems in order to bring known solutions to the LEGO realm. If you have the time, as I do now, this strategy always pays. As you probably noticed, I really got hooked on naval architecture, the engineer field devoted to boat design. If you like to wrestle with design problems where every little thing depends on everything else, and there's a big wild card like water in play to boot, boats are hard to beat -- even "simple" LEGO boats.
 I like it 
  February 17, 2015
Your creations and your presentations always amaze me. Are you some kind of engineer?
 
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