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An inrunner is the default answer for any RC boat fast enough to need a water jacket, because a jacket can only clamp around a can that does not spin, while an outrunner earns its place in scale hulls, tugs and the compact 18 to 26 inch deep-Vs that cool through the mount instead. If the brushed-versus-brushless question underneath is still open, RC boat motors explained settles that one first.
What separates the two types inside a hull is not torque and it is not efficiency. In an inrunner the wound stator is the fixed outer can, which is exactly why a jacket slides over it and why a single rudder or transom pickup can feed the motor and the ESC in the same loop. In an outrunner the windings sit in the middle and the magnet bell spins around them, so the water has to reach the stationary parts instead: the mount, the backplate, or an air path through a ventilated compartment.
Pro Boat and Traxxas between them show what that produces in practice. Outrunners go into the small and mid-size deep-Vs. Four-pole jacketed inrunners go into the big, fast ones. Everything else in the choice comes down to cooling path, Kv band, pole count, ESC commutation limits, and whether the motor physically fits between the hull rails.
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| Inrunner | Outrunner | |
|---|---|---|
| How it is cooled | Water jacket clamped on the fixed can | Water-cooled mount, backplate or airflow |
| Hull size it suits | Roughly 36 in and up, 6S to 8S | Up to roughly 26 in |
| Best use case | Sustained high load in a sealed hull | Slow, high-torque scale and displacement work |
| Kv band | About 740 to 5000 | About 300 to 3000 |
| Weak point | Needs clear water flow and a jacket that fits | Has a smaller heat escape path than a jacket gives |
How the Two Motors Are Built (and Why It Matters in a Boat)
In an inrunner the magnets are bonded to a rotor on the shaft and the wound stator is the fixed outer can. That can is smooth and stationary, so a jacket clamps over it or arrives pre-fitted. Hobbywing's SeaKing V2 manual lists the motor's parts as rotor, stator set, front and rear end sets and a water cooling jacket, so on that series the jacket is a factory component rather than something you go hunting for.
An outrunner reverses the arrangement: the stator stays in the centre and the magnet bell spins around it. The consequence is simple: an outrunner is hard to cool because its outer part rotates. Nothing gets clamped onto a bell turning at 30,000 rpm.
Size codes are where the two stop being directly comparable. Inrunners are quoted as can diameter by can length in millimetres, so a 3660 is 36.5 mm by 60 mm, a 3674 is 36.5 mm by 74 mm, a 4082 is 40 mm by 82 mm, a 4685 is 46 mm by 85 mm and a 2850 is 28 mm by 50 mm. Outrunner codes vary by brand, and most quote stator size instead, so a 22-20 is a 22 mm stator that is 20 mm long rather than a 22 mm can. Spektrum's 3542 marine outrunner is listed with a 42 mm length, and the 4335 outrunner measures 43 mm across and 66 mm long, which no single reading of the code explains.
The practical rule that follows: never size an outrunner by reading its code the way you read an inrunner's. Measure the can, check the mount bolt pattern, and confirm the shaft diameter before buying. The same discipline applies to the motor mount, drive shaft and flex shaft you are bolting it to.
Sizing by Hull Length: The Hobbywing Ladder
One manufacturer publishes a hull-length ladder that turns a vague question into a table. Hobbywing's SeaKing V2 inrunner series is rated against boat length, from a 2040 for hulls up to 45 cm to a 54113 for hulls up to 180 cm.
| Motor | Kv | LiPo | Can Ø × L, no jacket | Shaft | Weight | Hull length |
|---|---|---|---|---|---|---|
| 2040SL V2 | 5000 / 4000 | 2S / 2–3S | 20 × 41.2 mm | 3 mm | 73 g | ≤ 45 cm |
| 2850SL V2 | 4500 / 3600 | 2S / 2–3S | 28 × 50 mm | 4 mm | 146 g | ≤ 60 cm |
| 3660SL V2 | 3400 / 2600 | 2–3S / 2–4S | 36.5 × 60 mm | 5 mm | 290 / 284 g | ≤ 80 cm |
| 3674SL V2 | 2300 / 1900 | 2–6S | 36.5 × 74 mm | 5 mm | 372 g | ≤ 100 cm |
| 4082SL V2 | 2100 / 1700 | 3–6S / 3–8S | 40 × 82 mm | 5 mm | 490 g | ≤ 120 cm |
| 4685SL V2 | 1500 / 900 | 6–8S / 6–14S | 46 × 85 mm | 5 mm | 642 / 610 g | ≤ 140 cm |
| 54113SL V2 | 800 | 8–12S | 54 × 113 mm | 8 mm | 1260 g | ≤ 180 cm |
Most of the range is 4-pole. The 3660SL V2 at 3400 Kv is confirmed as a 4-pole with a 7.6 A no-load draw and 290 g, recommended for hulls around 80 cm on a SeaKing 90A V4. The 2040SL appears as 2-pole in the manual table, which is typical of the smallest cans on the market.
Read the ladder as a Kv starting band, not a law. The same 36.5 mm can spans 2600 to 3400 Kv depending on winding and cell count, and the recommended ESC steps down with the hull: 120A for boats up to 100 cm, 90A up to 80 cm, 60A up to 60 cm and 30A up to 45 cm.
Outrunners do not come with a published ladder, but the Kv band tells a similar story in reverse. Scale and tug builds live well below 1000 Kv; scale builders commonly settle on outrunners around 300 Kv, and the usual advice is to stay under 1000 Kv and roughly 8,000 rpm for a 40 to 50 mm prop. Small fast electric deep-Vs sit far higher, at 2300 to 2950 Kv on 2S to 4S.
The scale method is worth knowing because it works backwards from the prop. Keep the propeller no larger than about twice the stator diameter, then work out the rpm you need. A 60 mm tug prop turning about 4,200 rpm calls for roughly 350 Kv at 12 V or 700 Kv at 6 V, and a 28 to 30 mm 730 Kv outrunner on 6 V works out at around 5 A and 86 percent efficiency. That is a load light enough that the motor does not need water cooling at all.
Cooling: The Real Dividing Line, Not Torque
In a boat the topology argument collapses into one question: how does the heat leave a sealed hull?
An inrunner makes that easy. The windings sit in the fixed outer can and the jacket wraps that can, fed from a rudder or transom pickup. Traxxas does exactly this on the Spartan SR, where the pickup in the aluminium rudder feeds the motor and the ESC in one circuit. Hobbywing's manual is blunt about the two rules that matter: secure the cooling hoses, and stop running above 100 °C / 212 °F because high temperature weakens the rotor.
An outrunner cannot be cooled that way, but it can still be cooled. Traxxas builds the Disruptor around a 14-pole outrunner and states that a machined aluminium water-cooled motor mount draws heat away from the motor. That is the general answer, and it is worth saying plainly: an outrunner is cooled through the stationary parts, and a direct jacket architecture simply does not apply because the outer can rotates. On kit builds a flat plate added to the motor mount does the job, and the water-cooled section of a bought mount will cool an outrunner where an inrunner in the same boat would need its own jacket.
Not every outrunner needs water at all. A low-power scale boat running a 250 W-capable motor at around 30 W will not build enough heat to bother with. The problem case is a fast, sealed hull pulling sustained current, and that is why fast electric racing settled on 4-pole jacketed inrunners in the 36 mm, 40 mm and 46 mm can sizes.
Whether an outrunner or an inrunner runs cooler has two opposite answers and both are true somewhere. Cooling a fixed can works well because the windings are in contact with it. Cooling the backplate of an outrunner works just as effectively because that is where its coils sit. The variable that actually decides it is whether the hull is sealed or ventilated, and how much sustained current the prop pulls.
What you check on the water is the same either way. Pickup placement, either behind the prop or in a rudder, matters more than the type of tube you buy; a decent silicone line rather than aquarium tubing will survive the vibration; and in some scoop systems you simply cannot see the flow at all, which is a reason to test on the bench first. A blocked inlet is a real failure mode, weeds included. If you are chasing temperatures in a sealed hull, the fixes in how to cool your motor and ESC on the water start with water flow and end with prop load.
Torque, Kv and Pole Count
Owners consistently report that outrunners pull harder. The engineering behind that feeling is Kv, not layout.
Torque per amp is set by the motor's Kv constant through Kt = 60 / (2π × Kv), with Kv in rpm per volt. Work it through and a 4-pole 3674 at 1900 Kv produces about 0.0050 N·m per amp, while a 2950 Kv outrunner produces about 0.0032 N·m per amp at the same current. The inrunner is the torquier motor in that comparison. It just happens to sit inside a higher-Kv can when people put the two side by side.
Outrunners do tend to run lower Kv for a given diameter, because they carry more poles and a larger air-gap radius. Outrunners as sold do spin slower than inrunners while producing more torque, which describes the market rather than a rule about what is possible. On a scale hull, a 2200 Kv inrunner on 18.5 V turns a submerged prop far too fast, and the usual advice is an outrunner under 1000 Kv. The winding choice was the problem, not the topology.
Pole count decides something else entirely: how fast the ESC has to commutate. An outrunner in a boat typically carries 8 to 14 poles. The Firma 2931 in the Recoil 2 18 is an 8-pole, the 3542 in the Sonicwake 24 is a 10-pole, and the Disruptor's is a 14-pole. Inrunners sit at 4-pole as standard on the boat versions from Hobbywing, Spektrum, Dynamite, Castle and TP Power, with 2-pole on the smallest and oldest cans and an occasional 6-pole legacy unit.
As a rough guide, owners report running 3000 to 3500 Kv 3660-size outrunners on 2S in hulls 30 inches and shorter, and 3800 Kv in boats up to 48 inches. That is useful evidence that the band is wider than a specification table suggests, and equally useful evidence that the same Kv that works in a 26 inch hull will cook a 40 inch one.
ESC Compatibility: eRPM, Timing and Reverse
Electrical rpm is pole pairs multiplied by mechanical rpm, and it is the number that quietly decides whether a motor and an ESC get along.
A 10-pole 2500 Kv outrunner on a full 3S pack at 12.6 V turns about 31,500 rpm at no load, which the ESC sees as roughly 157,500 eRPM. A 4-pole 3400 Kv inrunner on the same pack turns about 42,840 rpm but only about 85,700 eRPM. The inrunner spins faster at the shaft while the outrunner demands nearly twice the commutation rate.
Manufacturers publish that ceiling in eRPM terms without always saying so. Hobbywing's aircraft Skywalker ESCs list a 210,000 rpm limit for 2-pole motors, 70,000 rpm for 6-pole and 35,000 rpm for 12-pole, which resolves to a fixed 210,000 eRPM regardless of pole count. No pole-based figure is published for the marine SeaKing ESCs, so the habit that keeps you safe on a high-pole outrunner is headroom rather than running to the limiter.
Timing follows the pole count too. The one explicit rule that turns up is in an aircraft ESC manual rather than a marine one: low timing from 7 to 22 degrees for 2-pole inrunners, high timing from 22 to 30 degrees for 6-pole and up outrunners, with switching frequency moving from 8 kHz to 16 kHz. Most marine ESCs hide all of this, and the SeaKing V4's programmable list does not include timing at all. If you are pairing an ESC for the first time, the hull-length ladder in the SeaKing V3 vs V4 breakdown is a better starting point than a timing number.
The compatibility trap that actually sinks boats is reverse. Aircraft ESCs usually have none, and some car ESCs need a double-tap on the throttle to engage it. A boat needs reverse for retrieval, for self-righting in the models that use it, and for backing off a bank. That is one of the reasons a boat-rated ESC is worth the money, and it is covered in more depth in the best RC boat ESCs roundup.
Fit and Mounting: Width, Cooled Mounts and Couplers
Width is the quiet failure. A bare 36.5 mm inrunner grows to roughly 42 to 44 mm once a jacket is fitted. Hobbywing's 36 mm jacket is 42.3 mm across, a Surpass 36-series jacket is 44 mm, and the Castle 15-series jacket is 50 mm. That is the same footprint as a 43 mm outrunner, which is what the Disruptor and the Recoil 2 26 use, with one difference: the outrunner needs clearance around a spinning bell for hoses and wires, and the inrunner does not.
Jacket inside diameters follow the can class. A 28 mm motor takes roughly a 28.6 mm jacket, a 36 mm motor takes about 36.6 mm, and a 40 mm motor takes about 40.4 mm. Get that wrong and the O-ring either will not seal or the jacket will not slide on. Lengths for the 3650, 3660 and 3674 family run from 40 to 60 mm, and a 3674 needs the 60 mm version to cover the can.
Mounting hardware differs by can size. Hobbywing's 3660 and 3674 use six M3 holes on the front end; the 2850 uses four. Water-cooled mounts exist for the common sizes, including a B36 with a T-bar clamp for 36 mm motors and a B28 for 2835 and 2850 cans with 19 to 21 mm hole spacing, 5 mm nipples and a 6 mm plate.
A 3660 is a 540-size can, and that matters more than the numbers suggest. A small kit whose mount only takes a 380-size motor will not swallow one without work, and in an 18 inch hull a 540 can fits tightly, adds weight and can throw the driveshaft out of line.
Shafts have to line up with everything downstream. The 3660SL V2 runs a 5 mm shaft, the 2850SL V2 a 4 mm, the 2040SL V2 a 3 mm and the 54113SL V2 an 8 mm. Flex cable, collet and stuffing tube must match all of that, and a mismatched driveline loads the bearings long before it shows up as a speed problem.
What Production Boats Actually Use (2024 to 2026)
Look at what ships in a box and the split is cleaner than any specification table.
| Boat | Hull | Motor | Type | Cooling |
|---|---|---|---|---|
| Pro Boat Recoil 2 18" | 454 mm deep-V | Firma 2931, 2950 Kv, 8-pole | Outrunner | Water-cooled heatsinks |
| Pro Boat Sonicwake 24" | 610 mm deep-V | Firma 3542, 2500 Kv, 10-pole | Outrunner | Mount and airflow |
| Pro Boat Recoil 2 26" | 660 mm deep-V | Firma 4335, 2300 Kv | Outrunner | Mount |
| Traxxas Disruptor | 666 mm deep-V | 14-pole, 43 × 68 mm, 228 g | Outrunner | Water-cooled aluminium mount |
| Traxxas Spartan SR | 927 mm deep-V | Velineon 540XL Marine, about 1800 Kv, 36 × 82 mm, 386 g | Inrunner | Rudder-fed water jacket |
| Pro Boat Sonicwake 36" V2 | 914 mm deep-V | 40 × 68 mm 4-pole, 1900 Kv | Inrunner | Water jacket |
| Pro Boat Super Sonicwake 48" | 1,219 mm deep-V | 4685, 950 Kv, 4-pole, 46 × 85 mm | Inrunner | Jacket installed |
The pattern holds: outrunners to about 26 inches, jacketed 4-pole inrunners above about 36. The Recoil 2 18 is worth noting for a different reason, in that its outrunner carries an integrated weight that replaces the weighted coupler used for self-righting. That is a boat-specific trick no generic motor comparison would tell you about.
The exceptions matter too. The 4335 2300 Kv outrunner is listed by one retailer as 4-pole, which is unusual for an outrunner, so confirm the pole count against the manual before you rely on it for eRPM maths. And the Disruptor's Kv is not published at all, which means an upgrade has to be chosen from the hull size and cell count rather than from matching a number. If you are working on one of these boats, the Pro Boat brand guide and the Traxxas model breakdown list the exact stock power systems.
Head-to-Head: Outrunner vs Inrunner for Boats
| Point | Inrunner | Outrunner |
|---|---|---|
| What spins | Inner rotor and shaft, magnets on the shaft | Outer bell, magnets bonded inside |
| Size code | Can diameter × can length (3660 = 36.5 × 60 mm) | Varies by brand, often stator size |
| Typical poles | 4, with 2 or 6 on small and legacy units | 8 to 14 |
| Typical boat Kv | About 740 to 5000 | About 300 to 3000 |
| Torque per amp | Set by Kv, not layout | Set by Kv, not layout |
| eRPM at 10,000 shaft rpm | 20,000 | 50,000 at 10-pole, 70,000 at 14-pole |
| Cooling path | Water jacket on the fixed can | Water-cooled mount, backplate or airflow |
| Fitted width | 36.5 mm bare, about 42 to 44 mm jacketed | About 43 mm |
| RPM ceiling | High, with 75,000 rpm burst ratings on premium 40 mm cans | Lower, with a heavier rotating bell |
| Shipped in | Spartan SR, Sonicwake 36, Super Sonicwake 48 | Recoil 2 18 and 26, Sonicwake 24, Disruptor |
The row that catches people out is the eRPM one. A motor that looks slower on paper can put twice the commutation load on an ESC, which is why an outrunner swap sometimes needs an ESC change even when the shaft speed goes down.
Scale Boats, Tugs, Bait Boats and Sailboats
Scale boats and tugs are outrunner territory without much argument. Displacement hulls run slow, high-torque and cool, which is exactly what an outrunner is good at: a low-Kv, high-pole motor on a direct drive with no gearbox. The published scale method is to keep the prop no bigger than about twice the stator diameter and let the motor run far below its rating. A 28 to 30 mm 730 Kv outrunner on 6 V turning a 60 mm prop works out at roughly 5 A and needs no water cooling at all. If you are building in this corner of the hobby, the best RC tug boats roundup is the practical companion.
Bait boats use a third variant. They run long, slow missions and the makers publish very little about motor topology. What the examples show is a sealed or potted low-Kv brushless unit driving a prop, whether that is a 1000 Kv underwater bait-boat motor, 2216 potted thrusters, or a single brushless prop motor on 7.4 V delivering 90 to 120 minutes of running. These are outrunner-style thruster motors in practice, and none of them carries a water jacket, because there is nothing to clamp one to and no sustained current to shed.
Sailboats get the short answer: there is no outrunner auxiliary on a DF65 or an IOM. Racing Rules of Sailing 42.1 requires a boat to compete using only the wind and the water, so the only motors aboard are the rudder and sail servos and the winch. The AMYA class breakdown covers what is actually permitted.
Which One Should You Buy?
Work from the hull you already own rather than from the motor.
For an entry-level RTR up to about 26 inches, you do not get a choice. Pro Boat's Recoil 2 and Sonicwake 24 and Traxxas's Disruptor all ship outrunners at that size, cooled through the mount or through heatsinks. Replace like for like with a motor of the same can size and a similar Kv band.
For a mid-size deep-V between 26 and 36 inches, the decision is yours. A hull that came with an outrunner is simplest to keep on an outrunner, with a lower-Kv version of a similar can size and a marine ESC with reverse. A hull already running a jacketed inrunner should stay on an inrunner of the same can and a Kv matched to the cell count.
For anything 36 inches and up on 6S to 8S, this is jacketed inrunner country, and the whole class does it for one reason: the jacket is the cooling path.
| Need | Pick |
|---|---|
| 4-pole inrunner for a 6S to 8S hull | Check the Spektrum 4685 1350 Kv on Amazon |
| Large 4685 inrunner, 6S to 8S | Check the Spektrum 4685 950 Kv on Amazon |
| Racing-legal 3660 can | Check the Spektrum Firma 3660 2400 Kv on Amazon |
| Budget 36 mm jacketed inrunner | Check the Surpass Hobby 3660 3250 Kv on Amazon |
| Low-Kv inrunner for a 3674-size hull | Check the Dynamite 3674 1900 Kv on Amazon |
| Hobbywing 3674SL V2 for hulls to 100 cm | Check the SeaKing 3674SL V2 on Amazon |
| Premium 40 mm inrunner for a race build | Check the TP Power 4070CM on Amazon |
| Compact outrunner replacement | Check the Spektrum Firma 4335 2300 Kv on Amazon |
| Scale and tug outrunner | Check the DAUERHAFT 4023 350 Kv on Amazon |
| Water-cooled mount, 36 mm cans | Check the B36 water-cooled mount on Amazon |
| Water-cooled mount, 28 mm cans | Check the B28 water-cooled mount on Amazon |
| 36 mm aluminium jacket | Check the 36 mm water cooling jacket on Amazon |
| Replacement cooling tubing | Check the Traxxas 5759 cooling tubing on Amazon |
Frequently Asked Questions
Q: Can I run a car inrunner in a boat?
Yes, and it is a recognised route with a proper part for it. Castle sells a bolt-on 15-series water jacket that fits its 4-pole 1515 and 1520 car motors of at least 75 mm length, with two O-rings, two 10-32 fittings and a 25 psi pressure rating. You also need a Kv low enough for a boat prop. A 1/16-scale car inrunner at 7200 Kv is built to run through roughly a 9:1 gearbox and is not a direct-drive boat motor.
Q: Can you water-cool an outrunner?
Not with a jacket, because the bell spins. You cool the stationary parts instead: a commercial water-cooled mount, or a flat plate added to the mount in a kit build. The water-cooled section of a boat mount is there specifically for the outrunner, while an inrunner in the same hull needs its own jacket. If the motor still runs hot with a cooled mount, the limit is usually prop load rather than cooling area.
Q: Will an outrunner's magnets come loose?
Not for the reason people usually give. In an outrunner the bell itself holds the magnets against centrifugal force, so they are not flung outward. The real failure is the glue bond degrading with heat and vibration, which is why filling the magnet gaps with thickened epoxy is a known precaution. In an inrunner the magnets sit on the spinning shaft and do need containment, which is why high-speed boat inrunners advertise Kevlar-wrapped rotors. Either way, check the bearings and clean the motor regularly, and stop running above 100 °C.
Q: Why does my ESC cut out under load?
Four causes turn up in roughly this order: a battery whose C rating cannot deliver the current, a prop too large for the Kv and voltage, a low-voltage cutoff set too early, and a motor running past its temperature limit. The first is easy to blame on the wrong part. Owners who swapped to a higher-Kv inrunner and then hit repeated cut-outs often find the pack, not the motor, is the limit.
Conclusion
Outrunner or inrunner is not a question about which motor is better. It is a question about whether your hull can get water to a spinning can or only to the parts that hold still. Above about 36 inches and into 6S and 8S, that means a jacketed 4-pole inrunner in a 36, 40 or 46 mm can. Below about 26 inches, it usually means a 2300 to 2950 Kv outrunner cooled through the mount, which is exactly what Pro Boat and Traxxas ship.
The middle ground is where you actually choose, and the tie-breakers are boring on purpose: match the Kv to the hull length and the cell count, check the fitted width with the jacket on, confirm the shaft diameter against your coupler, and buy an ESC with reverse configured out of the box. If your boat came with an outrunner, the cheapest correct answer is usually another outrunner.
Whichever way you go, give the motor a cooling path you can verify on the water. That starts with the cooling jacket sizing guide, runs through overheating fixes, and ends with a motor that lasts more than a season.
If you want the shortest route to a known-good pairing, check the Spektrum Firma 3660 2400 Kv on Amazon, a 4-pole jacketed can built to the racing rulebook that sits right in the middle of the sizes most 26 to 36 inch hulls use.
