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Member postings for Malcolm Frary

Here is a list of all the postings Malcolm Frary has made in our forums. Click on a thread name to jump to the thread.

Thread: How to measure what motor I need.
01/08/2020 12:24:57

Finding the right motor for a hull.

Back in the elder days there was a limited range of motors in the model trade. They were marked in the adverising of the time as suitable for boats of this length or that length, depending on the motor. Perhaps surprisingly, it worked well. Now we have lots of choice, which means, generally, a lot more ways to mess up.

A given displacement hull length has a top speed through water governed by a set formula (look up "hull speed". A freighter is unlikely to ever get anywhere near that, whatever the scale. Scale speed, according to people who use test tanks, is relative to the square root of the scale. One result of this is that rotation speed of a scale pro has to be multiplied by the square root of the scale. A full size prop turning at say 1000 rpm, on a 100 scale model will need to turn at 10000 rpm. (numbers picked for easy mentarithmetic)

Moving a hull through water requires that the hulls volume be shifted in the time it takes for the hull to pass a point. It also requires that the propeller move that water the length of the hull in that time (Newton mentions that sort of thing). In practical terms, more water needs to be shifted further, because there are losses. Moving enough of this water requires enough energy, This comes down to current, but the current is dependant on the way that the prop converts power from the motor into moving water. A big prop turning relatively slowly will move a large volume, but not far in a set time. A small prop rotating fast will shift much less volume, but will move it a long way. Big slow prop, pulling power, not much speed. Small fast prop, speed but little pulling power.

Stuff happens on a scale model a lot faster than full size real life. Time can be considered to scale the same as speed - it is divided by te square root of the scale. Slowing control servos makes no sense. Slowing servos working other functions does. Extra boxes like the ServoMorph that connect between the radio and servo do jobs like that.

Thread: My new boat: Ramsey tug
01/08/2020 11:54:19

The diodes are to allow the current to only go where it is needed. The original, un-dioded version required that the battery supply would be connected to either the forward or reverse terminal through a switch, either manually operated or controlled by another channel.

As originally supplied, no matter what the direction of the current, the motor would always spin the same way and only changed direction if the current was passed via the other terminal to use the other field coil. Thus it needed a controller for speed on one channel and anothe control channel to operate the switching device.

A modern ESC reverses the current according to the information on just one channel, which for this type of motor gives a choice of forward or forward. Each diode directs the current to the right terminal for the wanted direction of rotation. Instructions in the link that both I and redpmg pointed you to.

On most boats, suppression is done by capacitors. Modern motors with just two terminals, one component across the two terminals is usually enough, but some theoreticians insist that an extra two are needed between terminals and case. A field wound motor will need two capacitors, two on the common terminal with their other leg going to the other two terminals. A Taycol will benefit from a shot of oil on its brushgear and commutator, unlike any other motor.

I suspect that back when the "new" ESC was installed, the required diodes had yet to be invented. The MFA box might still work, but there is no saying what might have happened to it while the attempts to get it working with the wrong type of motor. A pair of Schottky diodes of at least the current capacity of the ESC that you are going to use are needed. No need to bother about their voltage -the available ones start well above the voltages that are found in model boats.

30/07/2020 15:12:03

Someting that you should never ever do with a brushed motor is lube the commutator. Unless you have a Taycol whose brushes really do need regular oiling.

The simplest way to make it compatible with a modern controller and get the benefits of modern control is to invest in a pair of Schottky Diodes, as suggested in the link I offered earlier.

As the motor works, it may be that the conroller is at fault. Since it was of its time, it could well be repairable with a screwdriver and file (I'm not familiar with that one), unless there is another problem. One such problem might be that the control just won't work with a modern radio - I would not reccomend using a radio that old anyway, technical sandards and legal requirements have changed a lot. Without going the diode route, a double switcher would give basic forward - off - reverse and work from a modern radio.

After writing that, I looked for the "MFA Electro boat throttle".  It looks like a considerably later, but still early, electronic box.  It could well be that it was dropped in as a modern replacement for whatever was there before, found to be incompatible with the motor, and the whole project consigned to the attic.  The ESC, provided its plug will fit a modern receiver, should be testable needing little more than a battery and a modern motor and, as a way of keeping life fairly simple, a servo tester.

Edited By Malcolm Frary on 30/07/2020 15:14:11

Edited By Malcolm Frary on 30/07/2020 15:25:16

30/07/2020 09:30:30

Thinking back to past conversations (something over ten years ago now) that I've had about Taycol motors, they were not particularly radio control friendly, being very good interference generators. The answer was a pair of Schottky diodes rated for about the same as the possible output power of the ESC, one for the "Forward" terminal, the other, connected facing the other way, in the lead to the "Reverse" terminal. I think most of the information ended up here - **LINK**

Add to that that the controllers were generally not up to the abilities of modern ones. It was generally thought wonderful that you could actually cause the motor to turn when you wanted it to. Being able to reverse it was a bonus. Getting various speeds was almost beyond wonderful.

29/07/2020 22:05:34

That sounds very much like a motor from a different age. Almost certainly not compatible without a collection of modifications. Any other labels, like make or model?

Present day motors just have two wires, direction of rotation is decided by the direction of the current working against the direction of the permanent magnet, which is fixed.  Older motors from the days when high power permanent magnets were not easily come by had a field coil to do the fixed magnets job which could have two windings, one for forward, the other for reverse.

With a well hidden motor, this might need some serious digging.

Edited By Malcolm Frary on 29/07/2020 22:12:55

Thread: water jet boat
29/07/2020 09:26:19

A read of the wiki about jet boats indicates that, however counter to intuition, having the outlet above water works better

One of the most significant breakthroughs, in the development of the waterjet, was to change the design so it expelled the jetstream above the water line, contrary to many people's intuition. Hamilton discovered early on that this greatly improved performance, compared to expelling below the waterline, while also providing a "clean" hull bottom (i.e. nothing protruding below the hull line) to allow the boat to skim through very shallow water. It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but having it above the waterline reduces hull resistance and draught.

Thread: Twin Brushless Set Up
28/07/2020 16:37:33

Model inflatables are not usually inflatable. They just built and finished to look that way. Cunningly cut plastic conduit or drainpipe is the usual choice.

An actual inflatable model, loaded with a lot of moneys worth of gear runs the risk of becoming deflated when "out there". If a real one starts to deflate while manned, it has a good chance of the problem being spotted by the crew and action taken in time to get it to shore, because they have a vested interest in not drowning. With a model, by the time a problem gets noticed, it might well be too late to recover it.

Thread: My new boat: Ramsey tug
26/07/2020 12:13:46

The great thing about modern electronic speed controls for brushed motors is that one that will handle a big motor will handle a small one just as well, so one that is rated for lots of amps will control a motor that doesn't need much current.

Wth early ESCs (going back to the previous century) a general limit was 10 or 15 Amps in quite a large package, because that was the limit of the transistors on the market. Times and tech have moved on, transistors capable of much more current are available in much smaller packages. It is still important to look at the ESC's rated voltage and make sure that it can handle what the battery is going to offer.

Thread: How to measure what motor I need.
26/07/2020 10:24:57

The original 4800KV motor "could" be used, but would need gearing down by about 6:1 to get the prop revs to a sensible level.

A 42mm prop seems small for a 7 foot hull.

To get an idea of how much power is needed, it helps to know the horsepower of the real thing. Converting to watts and dividing by the cube of the scale will put you in the right area. Guessing at a Welland laker, at 1:100, only about 20 watts output is appropriate. They were not designed for performance, rather carrying ability. Call it 30 watts input to give a bit of margin and allow for inefficiencies. On 7.2 volts it should be pulling about 4 Amps.

Thread: End point question
23/07/2020 10:54:37

A trick I did with a stick transmitter with mechanical slide trims and an ESC with a setting button and a bit too much motor was -

Set Neural, stick center, trim full down

Full forward, stick and trim full up

Reverse, stick and trim full down

A test of memory and manual dexterity, but it had some bonuses.

You had the option of setting a gentle cruise speed with the trim being used as an engine room telegraph and stick at center, normal full speed, stick full forward, get out of trouble spedd, everything forward. I wouldn't like to try this with a modern clever radio - even if they let you tweak things on the fly, you need to see the screen, and that means not looking at the boat.

Most electric motors run quietly provided that the bearings are not shot and that they are being operated in their comfort zone.

Gearboxes and poor alignment are the princilpal sources of noise. Direct drive and the right choice of coupler help keep things quiet.

There is usually some motor whistle from the ESC switching at anything other than either full stop or full speed, due to a waveform being presented to a set of coils that can move. Thats pretty much a description of the works inside a loudspeaker, so no real surprises there.

Thread: Boat decking
22/07/2020 10:11:33

Generally fore and aft.

On many, following the curve of the deck edge. For the brave and talented, meeting in joggles at the king plank in the middle.

Some launches had diagonal deck planking, but I've never seen pictures of deck planling going across. When you think about it, the beams holding the deck up run across, so the planks that they support won't.

Thread: End point question
22/07/2020 10:04:51

ESCs run coolest at their extremes - fully off, no current, no heat. Fully on, no switching, just creating heat by virtue of component resistance. Anything in between, they are switching on and off several thousand times a second, and it is during the transition time between on and off that they create heat. If they can't get rid of the heat fast enough, they get hotter. And hotter, over time.

Having the right battery, motor and drive train is important. Getting round bad choices by bodging electronically is always second best. Going for a drastic end point reduction will likey result in rough control. Digital stuff works in steps. Closing the end point makes for fewer, but bigger, steps somewhere along the line.

Thread: Engine mounting
16/07/2020 09:31:33

If it is a single joint, alignment needs to be, and stay, perfect. That means having the center lines of the motor and prop shaft intersect at the center of the universal's spider. Anywhere else, something will be trying to realign itself, giving a repeating clonk. A resilient mount might reduce that, but will leave the motor constantly trying to realign itself. Anybodys guess how long before you find out what the weakest point of the mounting is.

A better answer would be to use a double universal or a rubber coupler or a dogbone type.

Thread: 3 Motors RC Boat
16/07/2020 09:23:10

Speed requires power. Any motor will give its power as a combination of speed and torque. More speed, less torque.

Gearing down, much favoured by tug modellers, allows for a relatively small fast spinning motor to turn a large prop with lots of torque.

Gearing up, I have not heard of anybody bothering. Motors are easily available that can spin a direct drive train beyond any rpm where it might survive. Any gearing has losses,. Gearing down, nobody really notices, gearing up, the losses mount and become very noticeable very rapidly. Putting a any motor in a situation where it tries to deliver more torque than it can deliver results in smoke coming out of the motor as the insulation on the windings melts.

Boat speed. A model boat needs to travel at the speed of its real counterpart divided by the square rot of the scale. A 1/4 scale model of a 100mph real thing would need to go at 50 mph in the real world. A 1/6 scale model of a 200mph boat would need to be doing 81mph. That figure was worked out a long time ago for somebody wondering about doing a K7.

On a model, that, just as in the full size, means a large, well engineered motor. Similar standard drive train. A power supply that can supply the very large current involved for long enough without vapourising itself.

Thread: Self or non-centering rudder?
16/07/2020 08:59:25

Almost everybody with an RC yacht has the rudder worked by a servo which is plugged into a radio channel that has as its information source a sprung stick, Effectively, self centering.

Most try to set the sails so that the boat will balance and sail straight without controlling the rudder. Then, the next day, we can try to do the same again because the wind strength and direction patterns will be different. One of the things that ensures that sailing a model yacht is always interesting.

Thread: Returning modeller
08/07/2020 18:22:02

Servo connectors use the industry standard spacing for such things, i.e. 0.1". That standard made life relatively easy when designing circuits for stripboard because you could draw on 0.1" graph paper.

While the pins always have the same spacing, and usually the same wire order, some manufacturers like to be a bit different. To that end, they add a bit of extra plastic to the plug. But we are modellers and we are mostly allowed access to sharp knives, and a plastic tab is easily removed.

The difficult part follows - first, remembering that the black wire goes to the outside edge, and then inserting the thing on the pins rather than between them.

Thread: twin screw control
07/07/2020 10:34:06

When trying to do something that the manufacturers never thought of, you will need a "magic box" between the receiver and the ESCs. The transmitter is set as simply as possible, the "magic box" takes its instructions from the receiver, considers them, and gives outputs to the various things plugged into it according to the instructions that you have written into it.

One of the boxes is called an "Arduino". Its a computer on a chip that you program. Rather a steep learning curve.

A unit like the P94 will only mix according to what was designed in, you get one stick for speed, another for differential and rudder.

Using another channel with a switcher with a change-over relay in the signal, rather than power, wires could perform the link/not link function without interupting the power to the ESC control circuits, so the need for a magic box is eliminated. Rather than a computer providing the logic, the operator does it manually. I'm fairly sure that I saw somebody doing something like this with a rather fancy Futaba awhile back, but what extras were in the boat I have no idea.

Thread: Vospers Steam Launch?
30/06/2020 10:19:24

Interesting and very different. Freeoard shouldn't be a problem if it is sailed in wave conditions that scale to what a real one would sail in - I would expect flat calm conditions. When used as a Rescue Launch, they will handle a lot of weather, but then, they have a lid on to keep water out and the heavy bits are low down.

When doing the stability test, how full was the boiler? Weight of water high up won't help the ability to recover from being rocked.

Thread: Auxiliary Drive for Pride of Baltimore
25/06/2020 22:32:11

On a tops'l schooner I would expect that an auxiliary motor would be a good help tacking. A square sail does nothing helpful there, being a very effective air brake just when you don't want one,

From earlier reading elsewhere, the options were either to gybe round, to do a lot of very sharp work swinging the yards so that you got the wind on the right side at the right time, or to go into irons, fall backwards and hope that the rudder would do the job in reverse.

A fore and aft rig shouldn't need help tacking, having a prop hanging there might reduce performance enough so that it becomes needed, and learning to rely on auxiliary power on a boat that doesn't need it will not help learning.

25/06/2020 08:50:33

Possibly rather a lot of motor for an auxiliary on a 1.39Kg sail boat. A 385, possibly geared down, would probably be more appropriate. An ESC that can work a big motor can just as easily work a small one. When you buy a brushed motor, you have all that is needed, he ESC just controls speed. When you get a brushless motor, until you get its correct ESC and get them working together, you have a paperweight.

It sounds like an awful lot of big heavy servos/winches working the sails (not seen the sail plan, but there surely can't be a lot of sail area on a boat that light and shallow) Something a lot less bulky and heavy might be needed to ensure that the boat can have the weight where it is needed to ensure that it floats and sails upright.

Just found a reference to the variable pitch prop - with its 2" diameter prop, it will need to have a small motor geared down by about 6:1.  It is intended for use with a steam plant.

Edited By Malcolm Frary on 25/06/2020 08:56:23

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