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Dr. Rx & rd??

Old 03-10-2021, 07:33 PM
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guzzijohn
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Default Dr. Rx & rd??

I'm at the point of shopping for my project's radio. I've been in evolved with motorcycles for 60 years and the Chinese scooter deluge has been interesting. I imagine the electronic industry there is like the scooter's where a few factories pump out a large number of models of differing quality each badged many ways. So is there a way to break through the confusion of the many models offered? I certainly don't need anything fancy with more than 4 channels . Thanx,GJ
Old 03-11-2021, 01:47 AM
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Do lots of shopping around comparing specs. Then be happy with the HK T4A V2. Cheap, simple, reliable. Replacement/spare/extra receivers among the lowest cost, readily and reliably available.
Back in the days of AM and FM radios, there was almost universal interchangeability between makes as long as you had the right crystals. With digital radios, every maker has his own protocols allowing his receivers to match exclusively to his own transmitters. Unless you go into open source, but for a beginner, that is likely a step up too far.
Old 03-12-2021, 11:40 AM
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guzzijohn
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Thanx, I ordered that one. Seems to have great reputation! Now let's see how many foot pounds of torque do I need to turn rudder??
Old 03-12-2021, 03:09 PM
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Originally Posted by guzzijohn View Post
Thanx, I ordered that one. Seems to have great reputation! Now let's see how many foot pounds of torque do I need to turn rudder??
Personally, I prefer Futaba radio gear but, as I said that's MY PREFERENCE.
As far as servos go, it really depends on the size of rudder and how fast your model will go. Since you're building a large freighter, speed won't be as much of an issue as size. With my 1/8th scale hydroplanes, I need 150 inch-ounces of torque or the boat won't turn while running at anything over half throttle. I actually have a 270 inch ounce servo in the boat right now, just to make sure it turns while running at over 45mph.
To get your 7 foot long ship to turn, you are going to need a larger rudder. You might be able to make it happen with a standard 50 inch-ounce servo, but I doubt it. It would probably be better to go with something in the 100 inch-ounce range. It will take a large rudder to overcome the inertia AND the length of the ship so a beefier servo will probably be required
Old 03-13-2021, 02:34 AM
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Standard size, but high torque servos are readily available usually marked as "sail arm servos". Not noticeably slower than regular servos, but they do pull more power. A metal geared one would probably be preferable, if available, Anything about or more than 6.5Kg/cm should be OK. Next step up is "1/4 scale", usually in the stores that flyboys go to.

Stainless bike wheel spokes make great linkages. Here in UK, we have 13A mains plugs which yield great tiller arms, being nice chunky brass that somebody else has done most of the engineering work like drilling and threading on.
Old 03-13-2021, 05:11 AM
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OK, first of all ask yourself where you're going with this boat business? Is this going to be a one of? Or might you go on to do more scale or even speed, sail or submarines? If you intend to go further, a little upgrade in radio might be called for. To save some money in the long run consider how many channels you might need for say a sail boat or submarine. I have a sailboat that uses four. A scale model can easily do more than that. It all depends on how many functions you end up putting on it.

The other thing to ask yourself is what features do you want to have? Break them down into must have and would like to have and write them down.
Things like transmitter form factor of twin joy sticks or pistol grip with steering wheel? I've never seen a pistol grip with more than 4 channels.
Receiver form factor too. I'd stick with the aircraft style twin external antennas vice the internal or single "surface use" radio antenna.
Do you want rechargeable batteries in the Tx or swap AA cells every now and then? How often will you actually use it? Maybe a fresh set of AA each time you go out would be best.
How about telemetry? High current = high heat and stress on electrical parts. Too much =........... Maybe some battery temp and current draw feedback might be useful.
Dual rates and exponential? Expo allows you to fine tune the amount of throw of a given channel so you're not over or under driving a servo or ESC.
Need a channel on a single throw switch?
How about a receiver that can run off 9 volts? Maybe even 12 by now, who knows!
Digital or analog servos? Basic use analog is good enough and less expensive. Though the price of digital is coming down.

For the brand I'd stick with the better known manufacturers. Futaba, Spektrum, Hitec (do they still make radios?), JR (supposed to be coming back) and last, FRSKY. A newcomer showing a lot of promise. Part of the reason is after purchase support. You may never need it but......... Resale value could also play into it should you leave the hobby. I'd also stick with the same brand for all my receivers. You can get after market for some of them but the quality simply isn't always there. Some decent off brands can be found in Europe but without an FCC sticker (I have no idea if they have them or not) they are technically illegal here.

For servos pretty much any brand servo will work with any brand radio. I know of none that won't but you never know. For most applications the Hitec brand. of servo works great and generally cost less. For heavier tasks I currently lean towards a brand called Promodeler. Futaba also makes a good servo if you're so inclined. The one thing to watch for with servos is voltage requirements as many now will run off voltages that will fry the receiver. But then some receivers now have a built in regulator to protect the radio side of things while allowing higher voltages to go to the servos. Of course those are probably out of your intended price range. (I don't blame you!) And current too as most receivers can only handle 2 Amps through their internal bus that feeds the servos. Unless you use some kind of power distribution system.

I could probably say more but this should answer some of your questions and probably raise more.
Hope this helps though.
Old 03-13-2021, 05:19 AM
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Originally Posted by Hydro Junkie View Post
Personally, I prefer Futaba radio gear but, as I said that's MY PREFERENCE.
As far as servos go, it really depends on the size of rudder and how fast your model will go. Since you're building a large freighter, speed won't be as much of an issue as size. With my 1/8th scale hydroplanes, I need 150 inch-ounces of torque or the boat won't turn while running at anything over half throttle. I actually have a 270 inch ounce servo in the boat right now, just to make sure it turns while running at over 45mph.
To get your 7 foot long ship to turn, you are going to need a larger rudder. You might be able to make it happen with a standard 50 inch-ounce servo, but I doubt it. It would probably be better to go with something in the 100 inch-ounce range. It will take a large rudder to overcome the inertia AND the length of the ship so a beefier servo will probably be required
Here ya go, standard sized servo pushing 360 ounces for only $50. ProModeler
Old 03-13-2021, 10:39 AM
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Wink Cheep as a bird sings!

Well I do pinch pennies sometimes. The ebay ads muddy the water a bit. Spent $20-motor, $25-ESC, $30- radio so $50 servo seems a lot. I need to learn and perhaps destroy some bits and then maybe get tricker stuff. The least I'm hoping for is a functioning boat that will mayhap chase ducks in a pond. Thanx for all info guys! GJ
Old 03-13-2021, 06:29 PM
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I'm going to respond to two different posts in this one.
Retired, Futaba does make a 7 channel pistol grip transmitter. Here's a link to it on Futaba's website but, be advised, it's not cheap:
https://futabausa.com/product/7pxr/
Guzzijohn, there are some areas where pinching pennies is fine, and many that are not. Electronics will be the most expensive part of your project and the place I would stick with quality stuff. When it came to putting a radio in my scale 2000 Miss Elam Plus, there was no question that I was going to spend much more than $100 on the radio. I ended up spending the following:
$250 on a 4PLS transmitter and R304SB receiver as a package. The receiver alone runs $70 by itself which many consider to be both expensive or cheap, depending on what they run. The transmitter is now OOP, but was still available when I bought a second for $180
$130 on an S9156 high torque digital servo for the rudder
$14 on an S148 standard analog servo for the throttle though, truth be told, it's getting upgraded to an S3151($25) or S3152($41)digitals.
$90 on a 6.6 volt 2100mAH LiFe battery pack for my first transmitter
$25 per pack on a 4.8 volt 600mAH NiCad receiver battery pack, of which I have four or five.
By the time all is said and done, I'm in to it $520 for just what it takes to get the boat on the water. If you add spares to some of the parts, chargers, my 9CAP 9 channel aircraft radio and multiple receivers for it, I'm looking at someplace around $2000 in radio gear.

Last edited by Hydro Junkie; 03-13-2021 at 06:43 PM.
Old 03-13-2021, 09:05 PM
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Default I'll never have a C8- drive a '99 Camry

Ok, top shelf for some items. Rode an Aprilia Futura for 15 years until it got too heavy for aging body. Might get really taken by rc and go nutsy on upgrades or a second boat. Only time will tell.
Old 03-14-2021, 09:57 AM
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The best bang for the buck right now with high-torque servos seems to be in the RC crawler market. They're using kg's instead of oz./in.. I just bought 3 of them off eBay for something like $27.00 each, metal-geared and waterproof, 35 kg of torque, more than my $70.00 Savox servo.
Old 03-15-2021, 12:49 AM
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Originally Posted by Ron Olson View Post
The best bang for the buck right now with high-torque servos seems to be in the RC crawler market. They're using kg's instead of oz./in.. I just bought 3 of them off eBay for something like $27.00 each, metal-geared and waterproof, 35 kg of torque, more than my $70.00 Savox servo.
Usually Kg/cm. The last bit is important.
It hasn't been stated yet, but what type and size of rudder? Is it an end pivoted barn door, or a semi balanced type? Semi balanced needs a lot less force.
Old 03-15-2021, 01:03 AM
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Actually, I did mention rudders back in post 4, just didn't differentiate between the two types as I didn't want to confuse the issue. Regardless of which type he goes with, he's looking at needing a large surface area to overcome the length and inertia of a 2.15 meter long model
Old 03-15-2021, 04:31 AM
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Originally Posted by Hydro Junkie View Post
Actually, I did mention rudders back in post 4, just didn't differentiate between the two types as I didn't want to confuse the issue. Regardless of which type he goes with, he's looking at needing a large surface area to overcome the length and inertia of a 2.15 meter long model
Rudder design plays into it as well. Put some of the rudder in front of the shaft and it will act as a boost which helps the servo thus reducing the load on the servo. But no more than 25% of the rudder goes in front.
Old 03-17-2021, 03:35 PM
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Wink Rudder flubble- manic physics??

Ok, someone inferred 100oz/inch might work. That appears to equal approximately 23.4kg/cm. So I should be looking at a servo rated 25kg/cm?? Another bit I don't get is servo actuation rates. Planes, cars, and fast boats probably need fast reaction but my beasty doesn't. That said how does the reaction time translate to my situation? Thanx GJ
Old 03-17-2021, 04:36 PM
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It doesn't hurt to go with more torque in anything. I ran the Hi-tec 645-MG's for years in my gas boats which isn't much more them 100 oz./in. I'm just happy now that I can get big torque for not the money I used to spend.
A lot of servos run at the same speeds so you really don't need to worry about that. You'll see in the spec's where it might say that it takes for instance .16 seconds to travel 60 degrees.
Old 03-17-2021, 07:40 PM
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Some transmitters have a setting for servo speed as well. My Elam transmitter is set to turn the rudder at 50% of the servo's maximum speed. You can also, if the transmitter has the capability, control how far your servo will turn. Again, I have the rudder on my boat set to turn less than full since, to do so, makes my boat harder to handle. Then again, I'm also running at well over the speed your boat will be and having to navigate much sharper turns so....................
Old 03-17-2021, 08:22 PM
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Default Rudder area not locale!

Someone wrote in an ancient tome that one should have 1.5 square inches of rudder per foot of boat length. 7x1.5=10.5 square". It would stick out beyond the stern and or hang down below the keel line. That would look something like what the Nile sailing scores have hanging out the back. HMM? Maybe think more about building a bow thruster.
Old 03-17-2021, 09:34 PM
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And that is one of the trade offs we make as model boaters. Something has to give, be it details, draft or running gear being not to scale. That being said, if you look at the Titanic, it had a very small rudder for the length of the ship. When you consider that the Titanic was only 6 feet shorter than an Iowa class battleship, at 882 feet, and just as heavy at over 50,000 tons loaded, it's almost astounding that she was able to turn as much as she did when trying to avoid the iceberg that damaged the hull. Another interesting fact is that the Titanic turning radius was 3850 feet for a complete circle at 20.5 knots while the Missouri only needed 2050.5 feet at 27 knots. That kind of shows how undersized the rudder on the Titanic really was, considering the two ships were designed only 25 or so years apart
Old 03-18-2021, 01:54 AM
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Originally Posted by guzzijohn View Post
Ok, someone inferred 100oz/inch might work. That appears to equal approximately 23.4kg/cm. So I should be looking at a servo rated 25kg/cm?? Another bit I don't get is servo actuation rates. Planes, cars, and fast boats probably need fast reaction but my beasty doesn't. That said how does the reaction time translate to my situation? Thanx GJ
Servo speed is more for advertising than anything else, The airplane crowd has always been convinced the faster the servo the better the flying. Bottom line is, servo speed is controlled by how fast you move the sticks. Move them slowly and your servo will move slowly. After all, you're the one doing the sailing. It's up to you whether the boat looks smooth and scale like or jerky and unrefined. No, your boat won't need any kind of blinding speed on the rudder but you can control that simply by not slamming the stick from side to side. Focus on a servo that can handle the load.

And to that end, do you have the rudder figured out yet? SIze, design, etc?
Old 03-18-2021, 02:08 AM
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Originally Posted by guzzijohn View Post
Ok, someone inferred 100oz/inch might work. That appears to equal approximately 23.4kg/cm. So I should be looking at a servo rated 25kg/cm?? Another bit I don't get is servo actuation rates. Planes, cars, and fast boats probably need fast reaction but my beasty doesn't. That said how does the reaction time translate to my situation? Thanx GJ
One thing to remember about the required torque. The formula for determining the torque required includes not only the forward speed of the control surface but the degrees of deflection. If all you do is swing it 5 degrees to either side your torque requirements will be a lot less than it would be for 15 degrees.

And along those lines, the racing sailboat crowd generally limits their rudder throw to 30-35 degrees side to side. It seems the drag imposed by the rudder when above 30 degrees hinders the boat more than the turning force benefits it. Maybe Hydro can add his experience from the power boat side of things.

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Old 03-18-2021, 05:49 AM
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Originally Posted by Hydro Junkie View Post
Some transmitters have a setting for servo speed as well. My Elam transmitter is set to turn the rudder at 50% of the servo's maximum speed. You can also, if the transmitter has the capability, control how far your servo will turn. Again, I have the rudder on my boat set to turn less than full since, to do so, makes my boat harder to handle. Then again, I'm also running at well over the speed your boat will be and having to navigate much sharper turns so....................
What you're doing is adjusting the end points to limit throw, I do this with rudder and throttle.Some rudders don't need any more than 50-65%.
Old 03-18-2021, 07:38 AM
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Good Morning Stranger,
I know, just letting someone else know what might be available to him. Many new boaters don't know they can adjust end points and servo speed with their transmitter. I didn't catch the speed setting until I stumbled across it by accident
Old 03-19-2021, 12:21 PM
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Smile Each Question Raises MORE!!

I've wondered about internal servo limits and angle adjustments or do they just follow tx commands. It seems a 90 degree unit would be plenty for a rudder. I'm curious about internal servo differences-different gearing or motor size to change speed/power. Learning this stuff is great fun!
Old 03-19-2021, 05:17 PM
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Servos are pretty much standard in that they turn 60 degrees. IF they turned a full 90, it's possible they would jam and not be able to return to their center position due to loading.
To find out about internal differences, you would almost need to talk to someone that does repairs on servos as I just use them, don't know the differences inside of them

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