Content ©2005 Patrick Matthews
Patrick Matthews takes a look at using gears to match speeds between frenetic electric motors and unhurried scale props.
Look at a Detroit hot rod, see the shiny wheels, open the hood, see the chrome bedazzled engine… but ask yourself how far it will go without it’s transmission? Much the same with many a scale model boat– a beautiful brass prop, perhaps even a shiny motor down in the engine room… but a gearbox? Yes, gearboxes can really make a difference. With the exception of fast scale boats– planing boats like PT’s and runabouts– most scale boats will benefit from a speed reducer between the electric motor and prop. The motor can run faster yet cooler, draw less current, and have extended run times, instead of lugging down to match speed with a big slow prop.
It’s not often that the R/C modeler is presented with an optimally matched kit of motor, prop, and where required, speed reduction unit for use in a given boat. Sometimes the kit supplier will give some general direction regarding possible motor and prop choices, sometimes very specific part numbers. Or perhaps the scratch builder will have a wonderful model but no inkling of how to power the beast. I can’t tell you all those answers here, as I’m still trying to discover them myself… but I can offer some general pointers about speed reduction:
- Motors like to spin quickly;
- Props, especially bigger scale props, like to spin slowly;
- Happiness can be achieved with a speed reducer– a gearbox.
It’s true that some modelers find happiness with a slow running motor to match their prop’s needs, but these tend to be big and expensive. The price also buys the quietest setup, with no distracting gear whine. And sometimes unhappiness can be found with a smaller motor that lugs down to slower speeds due to the load of the big prop. In this condition, the motor pulls excessive current and overheats.
The typical engineering solution is to mate a compact high speed power unit to a speed reduction unit, with the output driving the ship’s prop, or the truck’s wheels, or helicopter’s rotor, or the photocopier’s paper roller. The direct drive solution is more like having a big 15 liter, 1000 rpm engine in your car with no transmission!
An example of what can be achieved with suitably matched motors and gears: My test bed for this article was a converted Hobbico/Aquacraft Bristol Bay which included an additional six pounds of ballast and an new prop, upsized from 31 to 45mm. The original power came from a big 600 motor direct driving the small prop. The Bristol Bay was clearly marketed for those with limited patience, because it comes almost-ready-to-run, and it runs at a very unscale speed for stodgy old fishing boat, scooting across the pond with a tremendous bow wave. After loading the model down with all that ballast, I wasn’t expecting great results when trying out the smaller gearmotors, I really just wanted to judge the noise levels and ease of mounting. But I found that a little Speed 400 motor with 2.3 gearing reproduced the same hectic speed and big bow wave, even with the heavier loading from the ballast and bigger prop, albeit running the motor hotter than I liked (see Pat’s Number One Rule of Motors, below). But with 3:1 gearing, the speed was more subdued and a 400 size motor ran at a very acceptable temperature-– not a bad match at all.
When we start speaking of matching props and motors, we can’t escape numbers and a little bit of arithmetic. What sort of numbers are we talking about here? Raboesch is a maker of soldered-blade props, and is one of the few companies to earn Pat Points from me for actually publishing helpful engineering data on their website www.raboesch.com (thank you Raboesch!). They list maximum useful RPM for their scale props, which appear to be based on power or cavitation (rather than blades flying off!)– going any faster just won’t achieve any additional useful thrust. I’ve charted their data here– you can see for example, that a 50mm (2 inch) prop shouldn’t run any faster than 7000 rpm.
Now, what motor to use? A good rule of thumb is that a motor should never be lugged down to less than 75% of it’s free running speed. Doing so will pull excessive amps, generating lots of heat and shortening the motor’s life. (Pat’s Number One Rule of Motors: Water cooling a hot running motor is a crutch for those who don’t how to match a motor to a prop, or for those who need to squeeze the last drop of life out of a racing motor… NOT for long lived scale boats!).
So for this 2 inch prop, we’d want a motor that runs 9333 rpm on whatever full voltage we’re using, no load (75% of 9333 rpm = 7000 rpm). That’s unusually slow… typical low cost 400 and 600 size motors want to run in the 14,000 -21,000 rpm range on their design voltages. And this assumes that the motor is small enough to actually “feel” the load of the prop in order to be lugged down by the prop’s load. A motor with excess power will need to start even closer to 7000 rpm. Yes, you can find these slow runners, but when they have the required torque, they tend to be big, heavy, and expensive. If that’s what you like, fine, but I always go for the inexpensive and easy to find solution, especially for multiple motor installations. Then the only way to get the correct speed is with a speed reducer. For example, a 21,000 rpm motor and a 3:1 gearbox will put out a bit less than 7000 rpm due to gearbox friction.
By the way- some questions that often come up:
- How fast do I want to run my prop?
Well, the Raboesch chart gives a reasonable upper limit, but you may find that running a scale prop at it’s maximum speed will drive your boat much faster than “scale” (a whole discussion in itself). You’ll likely want to gear down, or “motor” down, to lower speeds… trials will be required!
- How fast will my motor spin?
Good question. If you don’t have access to the manufacturer’s specifications or some sort of tachometer, you can estimate the free running speed thusly:
- Chuck the motor shaft in a drill press that runs at a known speed (e.g., 3600 rpm);
- While running your motor as a generator, measure the voltage across the leads (maybe you measure 1.2 v);
- Divide rpm by volts- in this example, 3600 / 1.2 = 3000. This is the approximate motor constant K, = RPM/V. Multiply K by desired running voltage to get free running speed. Example, at 7.2v, free running speed = K x V = 3000 x 7.2 = 21,600 rpm.
- But how will I know if my prop isn’t lugging the motor down too slow?
Well, without a tach, it’s not easy… except run it hard, and then feel the motor. If you can touch the motor can comfortably, all is ok!
All gears make noise, but some are definitely better than others. I came into this study with the position that good gearboxes can be silent, and that only poor quality units have caused gearboxes to have an undeserved bad reputation for noise. I now realize that the application is critical too. My first gear driven model was a wood hulled tug with a fine gear box, and it truly is silent. But for this test, I used a converted Bristol Bay from Aquacraft/Hobbico, with a “lively” fiberglass hull, and no real isolation of the motor mount from the hull. I seem to have created a wonderful speaker box, because even gearboxes which were virtually silent when run on the bench were audible at sea in the Bristol Bay.
So, it’s beyond today’s article, but possibly there should be consideration given to isolating the motor and gearbox from the hull structure. But first off, you want to pick the quietest gears to minimize the whining, and that will be accomplished with:
- Fine gear tooth pitches;
- Precision gears;
- Precision gear spacing;
- Precision shaft alignment (parallelism);
- Rigid gearboxes;
- Fine bearings;
- Good lubrication
Cheat on any of these, and noise and shortened gear life will disappoint you. I don’t see any reason to build your own contraptions on flimsy breadboards when affordable quality gearboxes are available.
Another option for controlling gear noise may be as close as your trigger finger. Usually, I found the noise annoying when I ran my long suffering Bristol Bay around at full chat, definitely not at scale speed. At more appropriate speeds, the better gear boxes were subdued, even in this floating amplifier box.
What I’m presenting here is a selection of several reasonably priced speed reduction units in ratios that are useful for model boaters- from about 2:1 to 3:5:1 ratios, which accept common low cost 400, 500, and 600 size electric motors. A wide range of reduction units are available from several sources… some intended for boaters, many originally for electric aircraft. Not every available unit was tested, but often one from a family, enough to get a feel for the features. Some of the features to consider:
Reduction Type: Usually gears, occasionally toothed belts. In this review, the gears are all straight cut spur gears; helical cut gears would give a quieter run, but at much higher cost. One tested unit has a true planetary, or epicyclic, gear layout. Planetary gear sets have the potential to offer quiet running in a compact package.
Reduction Ratio: Some units have one available ratio, some can purchased in one of several ratios, some let you buy spare gears to change ratios, one comes with a full set of gears to build several ratios.
Gear Pitch: How fine are the teeth? A finer pitch usually runs more quietly.
Mounting: A variety of mounting options are offered. Some of the units with aircraft pedigrees have flat bottom flanges similar to some aircraft engines; these are convenient for mounting into blocks in your bilges with vertically oriented screws. Others simulate an electric motor and want a pair of screws through a bulkhead into their front face, or can accept some sort of strap around the cylindrical motor or gearbox.
Output shaft: A variety of sizes, in metric and English, with and without flats. We’ll need a whole new article about couplings between motors and prop shafts!
Motor Size: Most of these gear boxes are sized to run with particular motor frame sizes- those known as 380/400; and 500/600.
Motor included? Some units allow you to purchase the gearbox separately, some only with motor. If you need to swap a motor, you’ll want a good pinion puller tool for those units with pressed on pinions. Installing a press fit pinion onto the new motor needs to be done carefully too to avoid damaging the motor.
The reviewed units were assembled per manufacturer’s instructions and run on the bench to assess general noise levels– pretty subjective, I’ll admit. Most were then adapted into my converted Bristol Bay for sea trials. Again, not so much to see if they were good matches for the Bristol Bay, but to judge general noise levels and ease of mounting. Being brief tests, I can’t comment on long term durability.
Master Airscrew MA3030G: Nicely molded plastic spur gear housing, available in ratios 2.5, 3.0, 3.5:1 for 500/600 motor, and 2.5 & 3:1 for 400 motors. Two ball bearings, 3/16” output shaft. Right angle mounting bracket available. Brass pinion is press fit onto motor shaft, requires skill and some precision. Motor shaft will need to be cut (use Dremel cutoff wheel, and keep debris out of motor), no cutoff dimensions are provided, and will vary depending on mounting bracket thickness; careful measurements required. Ran quietly on bench and in boat, though tested sample had one rough ball bearing. Tested MA3030G 3:1; $22.95 w/o motor; retail source- Master Airscrew.
Graupner GR179550: Similar to the Master Airscrew unit above, available from Hobby Lobby in 3:1 ratio for 500/600 motors. Has one ball bearing and one sleeve (which is fine for these applications), 4mm output shaft, you’ll need to make your own mounting. Same notes as above for mounting the pinion, or purchase with Speed 600 motor included from Hobby Lobby. I’ve been running one of these for years in a wooden tugboat with no complaints, and virtually no noise. Tested GR179550 3:1; $36.50 w/o motor; retail source- Hobby Lobby.
MFA Como Olympus 1092/4: A cogged belt unit, 2.3:1 ratio for 500/600 motors. MFA Como have produced some newer products with their Torpedo line of direct drive and geared motors, but for those who would rather listen to the pleasant rubbery whirring of belts instead of gear whine, this is an option. The unit is intended for airplane props with 6mm bores, and something must be clamped in the prop’s place to keep the lower shaft from walking out of it’s bearings. A marine version of the unit is available direct from MFA Como which has a special adapter for this purpose, but Hobby Lobby carries the aircraft version only. Therefore I was obliged to make up a spacer, and to machine the shaft to accept my Dumas u-joint cup. I also took care to make sure that the hub of the pinion was inboard towards the motor, while the driven pulley’s flange was outboard, effectively trapping the belt, as I had heard rumors that the belt liked to “walk” off the pulleys and bind the unit. Once adapted into the boat, it ran nicely and quietly. Tested Olympus 1092/4 2.3:1; $27.90 w/o motor; retail source- Hobby Lobby, p/n HLH712.
Graupner Universal Marine Drive 1114: Plastic cased spur gear unit for 500/600 sized motor. Over/under arrangement, with flanged foot mounting. Has bushings to accept various stuffing tube sizes (6/8/10mm), but shaft must be 4mm. Two plastic gears with course pitch teeth have brass hub rings and grub screws. Available in three ratios- 2, 2.5, 3:1.
This one breaks several of the anti-noise ordnances. The course pitch gears don’t help. The pinion and motor mountings are solid enough, but the bushing that mounts the lower stuffing tube and driven gear is just too imprecise, so that shaft alignment and gear mesh aren’t guaranteed. Next, the case itself is “tinny”, and serves as a sounding box for all the vibration coming off the gears. I ran this on the bench, but couldn’t bring myself to put it in the boat due to all the noise. Tested Graupner 1114.25 2.5:1 Universal Marine Drive; $25.50 w/o motor; retail source- Hobby Lobby, p/n GR1114.
Graupner “1700” Series: Small spur gear units available in 1.85 and 2.33:1 ratios, with Speed 400 6v or 7.2v motors included. Ball head bearing, bronze tail bearing; 4mm flatted output shaft. Convenient flat flange mounting. Nice quiet running, but the tall ratios are better suited to smaller props; the 2.33:1 unit ran rather hot with the 45mm prop in my test boat. Tested GR170323 2.33:1 7.2v; $39.50 w/motor; retail source- Hobby Lobby.
Maxx Products (MPI) Offset Gearbox ACC347: Spur gear unit for 400 size motors. All metal frame, brass pinion and nylon driven gear, comes with gears to make three ratios- 2, 2.5, 3:1. Pinion has set screw, no press required, no cutting of motor shaft required. Two ball bearings. 4mm flatted output shaft. Open frame provides no protection to gears or containment for grease. Has two bulkhead mounting holes on front face.
This one puzzled me a bit. After some careful adjustment of the motor position, I obtained a good pinion gear mesh and quiet running on the bench. But in the boat, this was the loudest unit of all! May have been that the boat’s lively hull was simply “tuned” to the meshing frequency of the fairly course teeth used in this unit. But this looks like a robust unit, and the multiple gear ratios are an attractive option if you want to try tuning motor/prop matches. Tested MPI ACC4036 3:1 with Speed 400; $22.90 w/o motor; retail source- Hobby Lobby (p/n MPY345).
Maxx Products (MPI) Planetary ACC4036: Planetary gearbox for 400 size motors. All metal case, front face mount, 3.33:1 ratio, true epicyclic (planetary) gearing. Dual ball bearings, 5mm output shaft. The fine gear tooth pitch, precision machining, and shared loading across the three planet gears all contribute to quiet running of a good planetary gear set. This was easily the quietest running gear set in the whole review. MPI doesn’t have a planetary for 500/600 size motors, but MP Jet does (don’t be confused by the similar sounding names- MP Jet is in the Czech Republic, MPI is in Illinois). I didn’t get a chance to review MP Jet’s planetaries, but they are carried by Hobby Lobby. Tested MPI ACC4036, 3.33:1 planetary; $34.95 w/o motor; retail source- Maxx Products.
MP Jet Gearbox 400 BB: Plastic spur & ring gear unit for 400 motors. Available with sleeve or ball bearings, and in four ratios- 2.33, 3.0, 3.46, and 4.09:1. Three mounting options- has tabs on the case for bulkhead mounting, or you can get a “can” to mount on the nose to face mount like a motor, or straps are available to clamp onto the motor. Output shaft is 3mm, unflatted. Brass pinion gear is a light fit onto the motor shaft and is set with Loctite; the motor shaft will need to be cut to fit in the housing. The pinion drives the inside of a large diameter ring gear. Motor is press fit into the plastic housing, no other means of securing. I was a bit wary of this, both for coming loose, and for accuracy of pinion alignment, but all held together and the unit ran quietly on bench and in the boat. You won’t be able to use a motor with an extra field plate around the outside of the can, it won’t fit. Perhaps better suited for lighter duty applications? Tested MP Jet Gearbox 400 BB 8018 3:1; $19.90 w/o motor; retail source- Hobby Lobby (p/n MJ8018).
MFA Como Mini Olympus: An all plastic 2.3:1 spur gear unit for 400 size motor. One ball, one sleeve bearing, 5mm output shaft; flat flange mounting. My first warning was the plastic pinion. Because of the stresses that a small pinion gear sees, they are almost always metal, even when the driven gear can be plastic. This one is molded nylon, with a pair of set screws in an attempt to hold on to the motor shaft. Next, the plastic frame both face mounts the motor and has a saddle seat for the can as well, and this area seems to have too much material on it. This cocks the motor, forcing the pinion to mate poorly with the driven gear. I spent some time carving away the excess plastic in order to set the motor squarely, and the unit then ran reasonably quietly on the bench, until the pinion started walking off the motor shaft. Seems the grub screws had worked loose… went to tighten them down, and oops! Stripped the plastic threads. This unit didn’t go into the test boat. Note again that MFA Como have a fine new line of direct drive and geared motors, see the Torpedo 500 below. Tested MFA Como Mini Olympus, 2.3:1; $21.50; retail source- Hobby Lobby.
MFA Como Torpedo 500 2.5:1 : An all metal spur gear unit with low amp 500 motor included. A plastic cover encloses the gearhead to keep grease in and dirt out. Fine pitch gears help keep the noise down. Includes a bracket for convenient flat mounting. Output is a flatted 6mm shaft. The low current 500 motor is quite suitable for scale running- with other motors, I found it necessary to back off the throttle to avoid excessively silly speeds, while this motor, rated up to 15v, kept things sedate but still allowed for some emergency speed in the Bristol Bay on 7.2v. MFA Como has a few other ratios available too- the next one is a 6:1 though, so the steps are large. They also offer the “Stingray” series with higher current draw motors, so between choices of motors, gear ratios, and operating voltage, one should be able to find a suitable operating speed. A couple data points- the Torpedo 500 2.5:1 runs 6300 rpm on 12v, while the 6:1 version runs 2633 rpm.
Operation on bench and in boat was quiet. Side note- as you add gears in a reduction unit, you can expect more noise. I’ve been using a highly geared MFA Como 50:1 unit modified with a “hot” Speed 400 motor to power an R/C amphibious DUKW, and continue to be pleased with it’s quiet operation.
As of this writing, the Torpedo series doesn’t seem to have any US retail outlets; contact MFA Como directly. Price listed is my estimate of UK price less VAT, in US dollars. Tested MFA Como Torpedo 500 2.5:1 #1105/16; ~$32.60; retail source- MFA Como, UK.
MFA Como Torpedo 800: All metal belt drive for 800 size motor. This is a BIG unit, the motor is 2 inches in diameter and 2-3/4 inches long. The 2.1:1 ratio provides 2460 rpm on 12v, which is about right for a 5 inch prop on a big model tug. The motor isn’t particularly greedy, pulling only about 5 amps under load, but it’s putting out nearly twice the power of the Torpedo 500 6:1, which runs at about the same speed on 12v. Output is through a flatted 6mm shaft running in twin ball bearings. There are a pair of lock collars which set the lower shaft’s position; I strayed from the instructions by placing one of these to the outside of each ball bearing, and avoided a condition that would cause metal to metal squeaking. Another squeak is still with me- the steel bracket isn’t quite up to maintaining the belt tension, or rather the belt can pull the pulleys out of alignment just enough to cause a belt squeak. A stronger bracket would address this, or perhaps a tensioning strap around the tail end of the motor could counteract the belt tension, if the belt squeak is an issue for you. I wasn’t able to test this unit in the boat, not having space or a suitable prop! Tested MFA Como Torpedo 800 1114/5 2.1:1 Belt Drive; ~$95 w/motor; retail source- MFA Como, UK.
MACK Products 2041 Motor-Gearbox Unit: This is a heavy duty unit from long time supplier MACK Products. The anodized aluminum housing carries machined nylon gears for quiet operation in sintered bronze sleeve bearings. MACK offers the same basic gearbox with motor, as tested here, and without. The motorless version doesn’t accept a motor for direct attachment, but rather has a stub shaft allowing for remote mounting of the motor through a universal joint. Either way, you can order the gearbox in one of four ratios- 1.5, 2, 3, or 4:1, and you can also purchase gearsets separately to change the ratio of an existing unit. The tested unit had a 3:1 ratio, and the big 600 size motor spun the unit at a quiet 4500 rpm on 12v– about right for a scale prop near 3 inch diameter. The flat flanged base has a pair of holes for easy mounting, but make sure you have room if you have a smaller model– the base is just over 2-3/4 inches wide. Output is through an unflatted 3/16 inch shaft; not a bad idea to put a flat on any shaft where you’ll be using a grub screw for retention.
The MACK gearboxes are nothing if not beefy– the 1541 is suggested for “any electric motor, and gas motors up to 1.5 cubic inches”. In addition to these single shaft gearboxes, MACK also offers twin-output units with 3, 4, or 5 inch spacing, all on the same basic plan.
Tested MACK 2041 Motor-Gearbox 3:1 ratio; $91.75; retail source– MACK Products; see also gearbox 1541 w/o motor, $55.50
Well, that’s a lot to chew on. If you’re doubting whether gear reduction is worthwhile, don’t- the first evidence should be that so many companies have entered the market to make these units! Can I make recommendations? Most of these units have their strong points, but I’m partial to the smooth quiet running of planetary gear sets like MPI’s and MP Jet’s, and the simple solid set up of the Graupner 179550 and Master Airscrew units that bolt on to the front of 500/600 motors. The MFA Como Torpedoes are great little units that sell widely in the UK… they offer friendly mail order service to the US, and I’m sure that any US retailer would do well carrying this line.
Hey! Know about other gearboxes suitable for scale model boats? Leave a note in the Comments section, below!