Torque vs RPM

[David Santistevan]

So I have a general question about motors. Is it better to run a bigger prop with a lower amp motor or a higher amp motor with a smaller prop to get the RPMs. What are the determining factors?

Dave

 

[Steven Bryant]

What application are you looking at?

 

[David Santistevan]

This is more of a general model boating question. Lets say a hydro or tunnel hull. Would it be better to run a higher amp motor with a smaller prop and go for RPMs or a lower amp motor and a bigger prop?

 

[lohring]

The question really is is a prop more efficient at a larger diameter and lower rpm. If you are after SAW records the maximum available pitch comes into play. Electric motors just draw more current and develop more torque until something melts. The P spec classes seem to have settled on the 2030 Kv motors over the 1800 Kv motors. However, I've seen Brian Buaas win with the lower Kv motor and the right prop. Electrics typically run higher rpm than glow engines. Their torque characteristics allow electrics to pull as large or larger diameter props at these rpms.

In a different electric area, we had a motor that was limited to around 5,000 rpm and props that were designed for around 10,000 rpm. The solution was a quick change gear box to experimentally match the two. In models there is a huge range of motor rpms available. It would be interesting to run a series of tests to help resolve the question. So far the P spec classes are the only place I've seen anything like this done. The maximum current the power plant can take will always be the limiting factor. Serious testing would need to hold the input watts (volts times amps) constant

Lohring Miller

 

[RaceMechaniX]

It is highly dependent on the motor and prop selection. Like Lohring mentioned I have seen competitive boats running lower kv motors swinging bigger props. One very good reason is a larger prop will typically hook up better in rough water compared to a small prop. There are practical limits, but hulls will generally prefer certain props and speeds. Too small of a prop and the tails blows or you have poor efficiency. Too large of a prop and it affects hull attitude. The same goes for pitch and lift.

I set a 2-lap record using a lower kv motor swinging a larger prop because the prop hooked up better than spinning a small prop to the moon.

From the motor perspective there is an ideal load and RPM range where the motor is efficient. Typically this depends on type of motor, # of poles, type of winding, air gap, etc........

TG

 

[raptor347]

What they said. There is no one best answer. Mostly setup preferences.

 

[David Santistevan]

Great feedback and thought processes. I have some ideas that I am playing around with and I was really looking to see what kinds of general wisdom is out there on this subject. More to come as I work through some of this.

 

[Christian Lucas]

Hi,

some thought about . The modelairplane speedrecord have a history that show that to lower rev and rise propellerdia and pitch will be successful. The first time when they hit 250 mph the motor was a small 3525 outrunner with a 7x10 propeller turning 30000 rpm flowen by Stefan Penz the Hyabusa . To rise rev was not successful as tipspeed will be to high . So we lower rev and rise dia and pitch . Currently the record was set with much lower turning motor only doing around 14800 rpm or with an other set up 17400 rpm for near 300 mph somtimes higher .

OK with boatpropellers we have not the tipspeed problem to reach subsonic speed but to rise dia will give moore trust .

It allthough show that the lower rev airplane have a better accleration as the reach top speed faster and need not so high skydive to find the messuring aera . And you know the pilot are limited by what they can see far away it dos not belong to the rev of the driveline.

Read moore about speedflying at rc-groups , http://www.rcgroups.com/forums/showthread.php?t=2032381 .

Corrently best electric motor for low rev speed are XNova , Kontronik Pyro and Scorpion motors .

For high rev setup Neu motors and Lehner motors are the best way to go .

 

[David Santistevan]

Here is where I am going with this. I have been thinking that that with increased prop efficiency and higher pitch the net would actually be better performance from a larger prop at a lower RPM. The balancing act here is that as we increase prop size it is going to require MORE torque to accelerate so we will have to weigh that against the torque characteristics of the motor. At some point there will be diminished return for heat racing. After all what good does it do you if you can hit the high on the straight but it takes you forever to get there out of the corners. This is what Lohring was referring to as far as the bigger prop being better for SAW. I found this from the Formula Marine website on selecting the right prop for your boat and it generally encapsulates the thinking here;

[SIZE=small]"First think of a propeller as a gear, whether it be on a car or even a bicycle. The lower the gear, faster hole shot, more power, but revolutions are much faster so at a point you can only turn that gear so fast until you over rev the engine (or you legs). Now think of your car stuck in only 5th or 6th gear, very slow out of the hole, little torque, but eventually it will catch up with itself so you can cruise to a good speed."[/SIZE]

[SIZE=small]So with electric motors we can generate incredible torque and RPM. Once again like Lorhing said we can basically go until we melt something. What I am thinking is that running the lower amp motor will allow us to swing a bigger prop with less amp draw and these electric motors will be able to deliver the torque needed for the hole shot with only momentary high amp demands vs continuous high amp demands of running a bigger prop on a high amp motor.[/SIZE]

Now here is the thing. Power to weight is everything in any kind of vehicle (**** gravity). But consider that the with the higher gear (lower amp motor and bigger prop) it becomes very important. To really be successful in taking this approach you have to pay particular attention to how light you can get the boat. Now unfortunately there is not much we can do about the weight of the battery or even the motor. But there is a lot we can do with the hull and supporting hardware. The amount of torque and therefore amperage for the hole shot will be directly proportionate the overall weight of the boat.

 

[Ralf Moser]

Correct, this is why you drive a big prop on bigger or heavy boats. The lighter the boat the smaller the prop can be. And the point is you need a smaller prop, a big prop on a super light and small boat can cause handling issues.

So basically all comes down to the boat type, weight, aim of performance level.

Motor is seventh in the check list after

Boat, weight, prop, cellcount, aimed speed, necessary controller

 

[lohring]

Propeller efficiency increases until pitch to diameter ratios get much above 2. The SAW props I know about have pitch to diameter ratios of 2.1 to 2.5. This isn't to be confused with cup. The power absorbed goes up as the diameter to the 5th power and the rpm cubed. It increases approximately as the pitch and blade area to the first power. That all means that it always pays to increase the pitch for more speed. A little diameter trim or bigger blade area trim can keep the current draw down.

Model boaters have been playing with these things for years. Texas cuts, Barr cuts, cupping, progressive pitching, and all the other modifications are even more easy to test with data logging in electric motors. Measure the radar gun, GPS or SAW trap speed, look at the rpm and current draw, and try another prop mod. Write the results down and tell us what you find.

Lohring Miller

 

[Christian Lucas]

Hi,

yes, that's the way . Your first look at what boatdesign do you run . Monohull or outrigger .Count all parts waight and see what power to waight ratio do you reach . For an outrigger propdia can not be big enough for my opinion. Do you know the steam powered hydroplane ? This modelboat do 129 mph with a 13ccm steam motor . A similar fast ic engine boat with 15 ccm ic motor has about 7 horses ,(some told) .A 10ccm ic motor has about 4 kilowatt or 5,4 horsepower. The steam motor only turn up to 10000 rpm at a very low efficancy level.

But at this low rpm torque is high and the propeller is realy big with 4" dia and 14" pitch . Boat waight is 13 lb 9 oz or 6,15 kg .

Here a vid of the current record steamboat ,

.I have a copy of a artikel from Bob Kirtley that he wrote 1990 - 1991 in the Model Engineer . If someone is irnterested i make a pdf and can send it . Photo is from this artikel showing the plan of Pisces II . Would be intresting to run this boat with a electric motor or a ic engine .

 

[KenOlvis]

I believer torque gets it moving horsepower keeps it moving.

If lacking torque maybe us a catapult system to launch

Ken

 

[lohring]

Torque and power are two ways of looking at performance. Torque is a measure of the amount of work and power is a measure of how fast that work gets done. You can change the torque up or down with gearing but the power won't change except for small losses in the gearing.

Lohring Miller

 

[RaceMechaniX]

There is a significant difference though between straight-line SAW props and tetherboat props. In RC SAW boats we try to minimize the prop walk effects and maximize forward efficiency. In tetherboats regardless whether they are steam, gas or nitro you have a tetherline to counteract the prop walk, also the prop walk does not hurt you as much as you are constantly going in a circle.

Although I will say the most recent gas SAW record props do resemble tether props with double digit trailing edge pitch. FE's have much more freedom due to RPM availability and torque profile compared to gas and nitro motors. As such we tend to spin more RPM with reasonable pitch/cup ratio's and try to reduce the losses as much as possible. Most FE SAW competitors will tell you 50% of the set-up and tuning is in the prop. I am 99% sure I could not have achieved my speeds using larger diameter props with less pitch due to their affects on the hull compared to a smaller prop.

Makes me wonder if I brought my SAW boat to a tether course and let her rip I bet she would be close to 200mph.........

 

[Christian Lucas]

Hi Tyler ,

Highest torque effect occer during start when the boat speed is zero and the motor accelerate with full power . I had a FE rigger with a smal ruder that when i start the propwalk let the rigger turn on a dime . For show i made some propwalkcircle for fun . After a short accelerating way when the ruder has enough waterspeedflow it counter propwalk easy . Best way to counter propwalk is a counter rotating Propsystem like dragboats use. I wonder that no SAW boater use such driveline as it will give highest accelerating possibility and lowest drag as ruder can be much smaler and no need of a wedge profile ,than with a singelpropdriveline. And for tru i have run tethered line propeller on SAW boat and get absolutly no trouble with this propellers . A tetherd boat has no flyforce during start and the stabilisation effect is not counter propwalk at this time . No they have much trouble with the tethercable as it is partly underr water and is braking the boat on the left side not easy to overcome this force . You can try your boat on a tethered line . NAVIGA has since some years a electric tethered line class ,most Bulgarien boater run this class .

.

 

[lohring]

Makes me wonder if I brought my SAW boat to a tether course and let her rip I bet she would be close to 200mph.........

The two biggest drag producers in a SAW boat are the propeller and the rudder. We gained 2 mph at 108 mph by cutting 1/8" off the rudder. We found before that that a thin, steel blade was good for a 6 mph increase over the standard aluminum rudder at around 95 mph. Tether boats don't need a rudder, but the power required still goes up as the cube of the speed. Maybe they could flood the tether car track near Legg Lake so you could see. LOL

Lohring Miller

 

[RaceMechaniX]

Thanks for posting Christian,

I have recently been in contact with Norman Lara who holds a NAVIGA A3 record at 141.70 mph. He mentioned that the world champs were just over and an FE boat set a new record in the A2E class close to 140MPH I believe.

For an FE boat I would not be worried about the wire during the starts, I am sure with a predicable acceleration you could get the wire out of the water and accelerate steadily to a top speed.

Lohring, I agree that the power goes up with the cube of speed, but it is the acceleration that currently strains my system. I have to go 0-140mph in 1/16 of a mile and do it twice. With a tether boat you could ramp up against a maximum current draw over several laps and hold for the timed laps.

At the last SAW event, I had a small sidebar with Steve O'Donnell about building a tethercar with a FE system. I got the impression with was still a little taboo for him being a die hard nitro (or methanol) veteran.

TG

 

[Christian Lucas]

Hello Lohring , Tyler ,

I agree with you that acceleration is currently the most limiting factor as the boat become very small if it is so far away .

Tyler , Hans Lehner is offering the 22xx motorline in a high rev version that can easy do 120000 rpm . The rotor has a bandage from stainless steel or titanium and get higher power magnet to reach the same powerdata as without the bandage. A twin 2230 can so spin two counterrotating x440 Octuras up to 100000 rpm ,good for 175 mph . And yes ,i know ,it is not easy to get a smooth runing system with 100000 rpm . But it can be done with a pianowire runing in a brass tube with some thoroughly placed plain bearing . The X440 is only a example as at this size are moore counterrotating option to get . I think that such drive can boost acceleration and will be able for a topspeed we will look for . With a good friend ,we are designing a new speedhelicopter that use a 2240 Lehner that spins at 80-90000 rpm . Have a thought on that . I meet some tethered boat race in Bulgaria and Netherlands where i come in contact with Stuart Robinson and Subotin the Ruschian Record boater . I have the plan of his 6,5 and 10 ccm recordboat . I have them digital so i can send them to you if you like . I think it is good to see how other do .

Lohring , Jim Faulkerson wrote that when the ruder broke on the counterrotating Propeller Dragboat the boat is traveling straight the way and end with no big disaster . A singel propeller dragboat has no chance to slow down if a part of the driveline break at the boat breack .

 

[RaceMechaniX]

Hi Christian,

I was not aware of a special "sleeved" version of the 22xx series motors. I noticed of the LMT motors I have all the 2260 and 2280 motors are sleeved and the smaller ones are not "sleeved". I have spoken to Andi before about the sleeving, it is a thin stainless steel component. For my professional work I develop very high speed motors that go into turbochargers. So 150k to 320kRPM and operate at high temperatures about 100C. These all have titanium or Inconel sleeves, but there is quite a bit more engineering than goes into the rotor design for these speeds and temperatures.

I am sure you are familiar with Camile's twin counter rotating SAW rigger. This was a very creative design, but I don't believe it is as successful as a well designed single motor SAW rigger. I have a couple reasons why I don't like twins. The greatest reason is the availability of small high strength counter rotating props. Yes, Octura and others make symmetric left and right versions, but for serious SAW use they would need to be recast from a better material. I know Andi is doing this for some of his cats. I saw the props and they were very nice. Second, two power systems are twice the complication as a single power system. With electric power it is much easier compared to a nitro or twin gas rigger, but it is still complex. I have watched a friend struggle with his twin powered cat where one side was not quite as powerful or balanced as the other side and it would burn up the loaded side in seconds. Lastly, a single motor will likely be more efficient and more powerful compared to two smaller motors. For example a 2260 versus two 2230's. Maybe someday I will build a twin 2230 or even twin 1950 boat, but for now I will continue to try and improve upon my single motor rigger to see where the limit is.

When I mentioned "wire" I was referring to the tether wire, not the driveline wire. I have used wire drives in several or my SAW riggers, but they have limited power capability and are very susceptible to failure from a prop unloading.

Yes, I would love to see the plans you have. send me a PM.

Tyler