Gear Reduction or Direct Drive, that is the question...

[warpatroller]

There seems to be a mix of boat models running via direct drive propulsion and geared drive setups. The gear drive, as i understand it, is used to increase torque, decrease current draw and improve efficiency. In very small models, it is impractical to implement a gear drive do to lack of space. What do you sub building veterans have to say about this subject? Do most people just run a motor direct drive to the prop?

I notice that David Merriman appears to be running a speed 250 motor in his Kilo SubDriver and twin speed 280 motors in both his Seaview and Revell VII SubDrivers. In all three of these cases, the motors seem to be direct drive with no gear reduction going on. These two motors are specified to spin at around 2000 RPMs per Volt applied to them. Which would be around 12000 RPMs @ 6V, which seems to be a very high speed for the propellor to be turning in a model submarine. I am guessing they run off a 6Volt power source.. Wouldn't these particular SubDrivers be very inefficent with power consumption?

If you had a motor that turned 4000 or 5000 RPMs at 6Volts wouldn't that increase your run time substantially (and draw less current) over one that did 12000 RPM at 6Volts? If one could find a lower RPM motor like I described, would that make the use of a gear reduction unit unnecessary?

Steve

[hakkikt]

This is a question that

This is a question that interests me too.
I think it was somewhere on this forum that I have read that model sub propellers run efficiently at around 1000 rpm. That would call for slow-running motors, but they dont seem to be used widely (price?).

[mike dory]

Guy's, This is always a

Guy's, This is always a subject of much interest. One of the major things to look at is, to what prop or props are you connect the motor too. What a motor is turnning by it self is going to change greatly as soon as you add the props and put it into the water. The size of the props, number of blades even how there are made are all going to infulence the R.P.M. What I like to do is put an AMP. meter in line and see how many Amps. a given set up draws. ( I like to stay at no more then a half amp draw out of the water). As to motor, gear set ups one of my favorites is a 6 volt dumas motor running on 12 volts and geared down 3.5 to one. That seems to shake out to about 3,500 to 3,600 RPM at Just over 1/2 amp draw. Two of these motors works very well in my Type 7 1/32 scale. I have yet to change it and it's been in use for over 25 years. If your looking for something very small try looking at Modell-Uboot-spezialtaeten V11C (Revell) Best Wishes Mike Dory

[Guest]

A gearbox is a lever,

A gearbox is a lever, nothing more.

The best type of gearbox, is no gearbox at all. Electric motors, unlike internal combustion engines, develop maximum torque at zero RPM.

Direct drive is always the best option as gearboxes are only about 90% efficient at best, or in other words you're throwing away 10% of power if you have to use a box, not to mention the whine induced by the gears.

Selecting a motor/prop combination is tricky, and sometimes involves a bit of trial and error.

A couple of things to bear in mind. For direct drive a motors armature diameter should be about 70% that of the prop or greater. Specifying an RPM limit is tricky, as this depends on the torque rating of the motor and the pitch of the prop. However if you measure the current load of the motor free running, and then stalled, you should aim for a reading of about 50% inbetween these values. Free running RPM should be about twice that of when the prop is loaded.

An example. Experiments I've carried out with my Vanguard propulsor- a 15 blade, shrouded, high pitch prop and a Premotec 24 Volt motor, connected direct drive via a shaft.

Prop diameter 2.25"
Armature diameter. Unknown but somewhere between 1.5-2", so within 70% of the prop diameter.
Free running RPM- 2800-2900 at 24 volts
Free running current - 100mA
Stall current- 3A (70 watts)
Prop running in water current- 1.5A at 24 volts (35 watts)
Prop running in water RPM- 1700-1800

Motor gets mildly warm, indicating it's working for it's living, and the prop churns a bucket of water up into a mini maelstrom, indicating this combination should power the Vanguard well.

Unfortunately, it's not always possible to source a motor with the right RPM or torque requirements. Most model shops stock motors to cater to the flyboys or car brigade. These motors tend to be high revving low torque (will need a gearbox), or high revving high torque (thristy).

IMO the best place to look for motors for submarines is surplus stores on ebay etc. Buhler, Maxon, Pittman, Portescap, premotec- all good motors which cost a lot of money if purchased new, but can be picked up cheaply surplus stock.

Why don't the likes of D&E use them you might ask? Simple, if you're making a commerical product, you need a motor that is easily available. Surplus motors may come in batches of 10 or less, then you may never see them again- fine for a one off, but not for a commercial product.

Easier to source motors that will do the job can be found in the Graupner range. The 500E is a low revving 540 type motor (11000- 12000 RPM) and will direct drive most props upto about 1.25"., The 720BB is a high torque low revving, large can motor and suits larger models with prop sizes 1.5-2". Bigger props thatn this may need a gearbox or a special motor. Car breakers yards are a good source for larger model motors. Heater fan motors, windcreen wiper motors etc. Cheap and usually high quality.

Andy

[warpatroller]

Thanks for the input, gentlemen.

I've

Thanks for the input, gentlemen.

I've been eyeing this forum long enough to know that Mr. Dory is an RC submarine legend along with Skip Asay. Therefore, Mike, I will especially take note of the information you posted. Was your figure of 3400 to 3500 RPM @ 1/2 AMP draw, for the motor under no load while prop is out of the water? If so, the Promax 180 motor below should give me similar RPMs, as my system power will be 4.8Volts. I have a multi-meter, so I could measure the current draw and see if I can get to about that 1/2 Amp mark that you suggest.

Andy, I am not as familiar with your background, but have seen your name on this forum quite a bit. You also seem to personally be more interested in the LARGER models. I am working on a little boat, that will be spinning a single 1" diameter propeller. Will be building my own custom 1.5" OD 1.375" ID WTC for it. Hull will only be about 19" long.

Think I will end up making a decision between the direct drive (SWM 300 motor 4500 RPM @ 5V no load) and a geared drive (Promax 180 motor w/ 2.9:1 reduction gear driving a 3/16" shaft - around 3400 RPM @ 5V no load). Or I might actually try them both..

Steve

[mike dory]

You Sir, are most kind,

You Sir, are most kind, Yes the information I posted is correct. But remenber, A stock Pittman or Dumas motor is 5200 RPM. by running a 6 volt Pittman or Dumas motor at 12 volts you get 10,400 RPM. By using a three to one reduction unit you end up with 3,466 RPM with alot of torque My own personal choice in reduction units is a belt drive, this dose away with all gear or electrical noise. As small as the unit is that your building, I'd give Dave Manley of Small World models a call. Dave as several small motors that might be just what your looking for. See his business ad on this web site..... Best Wishes Mike Dory

[cstranc]

Ok so curious as ever.

Ok so curious as ever. We are talking about the size of the electric motors, but then there is the number of turns.

A couple of weeks ago I bought a nice small (1" diameter) electric motor and build a housing and impeller for pumping water in/out of my ballast tank. Draw on the unloaded motor was under .2 amps. When I put the little impeller on it and submersed: 15amps! Good thing it was water cooled.

So immediately I retested the bow thruster I had created and at full power it was only drawing 2amps and moving a lot more water.

The thruster pumps are using 75 turn "lathe" motors: high torque, low rpm, low current motors. The little motor must be a low turn, high RPM variety.

When I have a choice I always go for high turn motors, but sometimes I cannot find out this information for a motor. Then I gues you have to look at the application and guess if the motor is designed for high RPM, or high torque.

I am guessing the "turns" refer to how many times the wire is wound around the armature. Is this correct? And the higher the turn count the stronger thei field generated, but the lower the RPM?

What would the dumas motors be in terms of turns?

[Guest]

In a nutchell, yes. Turns=torque,

In a nutchell, yes. Turns=torque, although you are limited in the number of turns you can get onto an armature.

When purchasing surplus motors, there is an element of chance. For instance with the Premotec motor I purchased for the Vanguard, I had no idea of it's torque or RPM rating. I did know it was a 24 volt motor, cause that was on a sticker on the motor. I also looked at the size and shape of motor and guesstimated this motor was low revving high torque. In this case I hit the jackpot, but I still have a few motors sitting in drawers that I've picked up over the years waiting for the 'right' model to put them in.

Andy

[warpatroller]

Thanks Mike!

One of the motors

Thanks Mike!

One of the motors I am considering is a Small World Models motor. I assume that if your sending 12 Volts to a 6 Volt Dumas motor, the motor's operational range goes as high as 12 Volts, but perhaps its nominal voltage is 6.

Toothed belt drives are nice, but I don't think I'll have room for something like that.

Steve

[Guest]

I have had extensive experiance

I have had extensive experiance in both large sub subs as the 5ft Alfa that displaces 45 lbs dry and small subs as well.
I like to run my subs for as long as posible and always use a gear reduction unit of some kind.For most small subs a 3.5 to 2.0 reduction depending on the props pitch is imployed and this along with the diamiter is the most important conditions to use when decieding what reduction ratio to use.On small subs either dual or single screw an inexpensive 540 SH can motors with Master Air Screw ball bearing and nylon gears in a sealed gear box have been very successfull.They run efeciently and very quite with no RF.
As for the larger 5.5 diamiter prop as on the U-32 I just finished and as the Alfa a Makita ball bearing 12 volt and planetary geardrive unit is used since the pitch is so high on these props ,a high torque medium speed drive is required and they are cheap if you know where to get them,new about 100$ for the motor and tranmition or cheaper at the robotic store .com
Be sure to get the units that are bolted together as some drill units are not and wold be dificult to mount.As for the shaft adaptation I simply remove the chuck and use the revrsed threaded screw that holds the chuck ,turn or grind down the head and there is a 3/16 ths male stud ready for the U joint.

Then the unit is coiled for water coolling and a pump driven from the main shaft pumps antyfreez throgh another external coil,a simple heat exchanger. Though the heat is not realy that bad but it helps in the long run times and the motor will last longer.

All the powerplants I install are watercoolled and has been very succesful for many years with very few motor changes,and I have put miles on my subs it seams!

In my experiance the directdrive aproch is not very cost efective as the motors no mater what you use will overheat ,be unscalelike and is only realy efective in very small props, as used in speed boats.
Another great aspect of drill drives is the adjustable clutch and the built in thrust bearings present in all quality drills.If you are building a large sub with a large prop or props a cordless drill drive is the way to go.

Best regards Dave Amur Ship Yard

[Guest]

In my experiance the directdrive

In my experiance the directdrive aproch is not very cost efective as the motors no mater what you use will overheat ,be unscalelike and is only realy efective in very small props, as used in speed boats.

Ohh that's not true at all. A small can motor will power a small prop, a large can motor will power a large prop. Motors will only overheat if they're overloaded.

[Guest]

As I mentioned the size

As I mentioned the size and pitch and ultimate end results determine the motor drive setup ,and that in my opinion and experiance a directdrive motor will eventualy run hotter than a geared down unit.That is why most speedboats have watercooled directdrive motors,so imagine what would happen if the same setup would be placed in a sub without watercooling as most of you dont use the benifits of cooling the motors with water instead of just the air inside of the wtc.
Sure any motor can turn nicely a prop of any size and yes if overloaded it will overheat but I mentioned that when done properly a geared down motor will run cooler and last longer and use less power.
Try it you wont go wrong.

[Guest]

You are comparing a speed

You are comparing a speed boat with a scale boat.

A speed boat motor needs watercooling because it is operating at high power. To get equivalent speed at lower RPM you would need a larger prop- torque versus RPM. We all know that a large prop produces a large moment of torque, and this can prove troublesome at higher RPM's on single screw boats i.e. most speedboats. Therefore we run a smaller prop at higher RPM.

Generally speaking most model motors are not well matched to driving large screws, hence they require some form of reduction. However where the motor can be accurately matched to the prop, direct drive is preferable.

Andy

[Guest]

No I am compairing drive

No I am compairing drive types to there intendid end use and results.And in my opinion no matter what drive you use in a submarine no mater the size watercooling is a must and especialy true or beter for directdrives.

I have a mini sub one of those 4 inch dual screw jobs and after a short run the thing burst its wtc seal from the heat produced inside,why put your sub threw that torcher,just water cool it.Please this is my opinion bassed on 22 plus years of hands on experiance and the extra work and money is well worth it,you take it from there.

In the 3.5 wtc the air inside will expand and cause a leak very possibly,keep them cool.

Dave Amur Ship Yard