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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: Spektrum radio
22/06/2020 11:13:01

Always good to learn something new. Some swear by Spektrum, some swear at them.

A problem with failsafes on boats is that what is safe in one set of circumstances might not be in another. Probably "Safe for everybody else" should win, and slowing a power boat should be helpful. But I can't think of any setting that is universally safe for a sailboat. Planes and buggys that have been brought to a halt can generally be walked over to. Boats, less so. ESCs cuttng off to a signal loss was OK, no problem with that. But that was from the days of "dumb" radios that just passed information on. Some clever modern receivers think they know better, and do their best to retain normal service by remembering what they last heard and keeping that going. If it is just a few frames of information, OK, but any longer could create problems.

Dymo - a machine for permanently displaying mans inability to spell. I have fond memories of a row of part drawers where various indicators were kept in an office variously labelled "Inicators" and "idicators" and "idnicators". I have found that printing on a slip of paper and covering it with a length of Sellotape works and lasts well. Its what I did with the bind instructions for my Saturn transmitter. Very necessary due to the many unintuitive steps needed.

21/06/2020 15:22:08

And the usual rule as I understand it is that if you select the wrong model, you wind up using it with the wrong settings. Receivers don't bind with settings, they just bind, and it is up to the operator to ensure that the profile selected is the one for that model. Sometimes a big ask.

I try to stick with nice simple radio and adjust my model to fit what the transmitter is going to give me. This thinking was validated by one of my members have a sail arm punch its way through a bulkhead at switch-on.

Thread: Diesel fuel
20/06/2020 18:21:29

It was amazing what could be bought over the counter at the big chemist near the Grammar School back in the late '50's.

"Barlord! A gill of your finest draught ether, if you please!"

You had to go elsewhere for the cocktail sticks and cherries, and got funny looks in that shop back then.

The victim in that was my cousins ED Bee, which had not worked before, and didn't after, either.

Edited By Malcolm Frary on 20/06/2020 18:24:34

Thread: Tx modify from dry cell to rechargeable battery pack
18/06/2020 11:10:19

My preference is for rechargables and a transmitter with a charge socket, but not for battery life . More because I am perfectly capable of forgetting to turn the transmitter off or accidentally nudging it on. This tends to disappointment a few days later at the poolside, but at least I have the option to swap to the emergency dry cells hiding in the door pocket of my car, or, if I spot the goof at home, recharge.

My 8 cell transmitters have internal regulators allowing a wide range of battery voltage. 4 cell ones have less voltage headroom and very often no charge port, so there is a built-in bias to Alkalines. Using rechargables would involve a lot of removing and replacing cells for recharging, the high capacity and long life of Alkalines reduces this level of messing about. Choices, choices.

Thread: Fairmile D - help needed
17/06/2020 14:29:05

Having a constant resistive load and a constant supply voltage makes life simple for doing the required calculations. A motor running on pure DC is not quite as simple, because it does have an inductive component that varies with the speed, and further varies with the load, and both these variables are compounded by the design of the motor, which, while moveing, presents lots of varying conditions. Most of us don't bother doing precise calculations - we just derate generously to avoid letting the magic smoke out, because magic smoke is expensive.

Some sage advice about lead batteries was offered earlier. They are great for tugs, only good for demonstrating that the motors work on a fast boat. After that, they are good for providing weight when you need to glue a deck down on a later model. Or for powering a tyre inflator or, via an invertor, supplying standby power for small mains powered items.

15/06/2020 09:51:40

There is quite a lot of gear on the aft deck, which should be very amenable to hiding the join of a removable panel. Also good for hiding the screw heads holding it down on its gasket. Anything mechanical is, at some time, going to need access for attention.

Thread: sp610 getting warm/hot even
14/06/2020 09:49:05

The old Hitec 6/10 was state of the art when it first appeared, but used what were probably the most terrible power transistors ever let out into the world. The components that drove them used a lt of power to do so, and tended to heat up to their thermal limit. They had a tendency to grill both themselves and the PCB where they were mounted. It is very possible that over time something has cooked. I would have expected the "forward" circuit to be distressed first.

When the label said "8.4 Volts" maximum, it meant it. When it said 10 Amps current, it really meant it. Back in the day, 540 motors were a sort of standard, that ESC was designed with them and 7.2 volt battery packs in mind. But a lot depends on how the 540 is loaded. A heavily loaded one might want to pull more current than the ESC can supply.

Does the motor show that behaviour when not mechanically connected? i.e. no load. If so, its not the prop shaft misbehaving.

Does it behave like that without the ESC? i.e. direct connection to the battery. If so, its a broken motor.

Does a different motor misbehave when connected to the ESC? If it does, the ESC is the guilty party.

What happens if you operate the "servo reverse" on the transmitter and connect the motor reversed so that forward on the stick is still forward on the motor?

Thread: Propshaft coming away from coupling
13/06/2020 10:59:00

Even with a reverse thread, locknuts will still be needed. On many boats that have reverse, reverse is the boats brakes, useful on occasion for avoiding expensive damage to concrete banks.

An unlocked connection between shaft and either coupler or prop will allow the inertia of one or the other to move whenever force is applied at one end with resistance at the other. Turning one way, something will try to unwind, turning the other way, things will tighten up. Both are undesirable, and it doesn't matter whether cheap standard threads are involved, or expensive special ones. The expensive ones just cost more to replace when they fall off.

As far as I know, angle grinders only turn one way, and I'm fairly sure that they turn to cause a standard thread to self tighten. Tooling to produce left hand threads is more expensive than for normal threads, the captains of industry are likely to go for the lower cost option.

I wonder if the OP got sorted, or whether it was a drive-by posting.

Edited By Malcolm Frary on 13/06/2020 11:05:41

Thread: Inherited model identification & help
13/06/2020 10:41:34

Actual Robbe spares are unlikely, but many of the parts are likely to have generic equivalents.

A 40 year old radio is probably not going to be a good option, times have moved on and standards have changed. Depending on what controls are needed, a modern radio can be had for quite modest cost, and will probably be more reliable than one from a long time ago. A complete 4 channel radio outfit on 2.4GHz can be had for about the same cost as a receiver and crystals for older systems, and will work without the probem of avoiding sharing channels.

Thread: Yacht Ardent
12/06/2020 10:14:10

Rule of thumb for mast height - about 1 1/2 times the boat length for a Bermuda rig will look about right and work. About the boat length for a gaff rig.

Having the spreader built into the joining splint works well. It means that the rigging not only does the job of keeping the mast vertical, it also braces it straight. An aluminium knitting needle about 3mm diameter makes a good spreader. Strong enough to do the job, wide enough to have a hole drilled each end to take the rigging lines.

Thread: Tx modify from dry cell to rechargeable battery pack
09/06/2020 09:53:39

Looking at the instructions, it is designed for a 6 volt battery pack, being 4 of 1.5 volt AA cells. Rechargeables generally start at a lower voltage, 4 of them giving 4.8 volts nominally, which is quite near an alkaline being empty. The transmitter might well warn of low voltage earlier than expected, but it just means recharging more often.

Thread: Thames Sailing Barge Stuff
06/06/2020 09:45:36

5 minute epoxy is good for fixing prop shaft tubes in place where they pass through a hull, but for actually holding in place anything that can try to unhitch itself, real epoxy (traditional, slow setting) is better. Thats assuming proper surface preparation.

The rivets should make up for the basic weakness of the 5 minute epoxy and any ageing of the superglue should the obechi try to regain its original shape.

Thread: Aerial positioning
04/06/2020 18:19:41

Very very shortly after my 1st year Penwortham exchange went auto. So there was nobody left in the country who had to "ring off" at the end of their call, although the phrase persisted. I never had cause to go there, it was the wrong side of the river.

Like yourself, I never used any of that area of theory, but I did have an older workmate who had done National Service as a radar tech, and had formulae dripping out of his earhole when he canted his head. He used to have meaningful discussions with the local CAA radar satation when he was sorting out interference on his video recorder. It seems that it broadcast on the default UHF channel that video recorders output on.

The trick on models using 2G4 is to just have the shiny bit as hgh and straight as possible to collect the maximum signal. Messing with a critical length of coax shielding does run the risk of an impedance missmatch, but it is very possible that any extra loss would only be noticed using the right test gear.

04/06/2020 10:13:44
Posted by Dave Milbourn on 03/06/2020 18:02:15:


Did they actually have microwave frequencies back then? I thought it was mostly cat's whiskers and valves, and aerials were about thirty feet tall and put up without planning permission by spotty geeks in garden sheds...

DM (I remember Hilversum on the dial, too)

We got taught about all sorts of stuff that I never went near after handing the exam paper in. First year CGLI insisted that we know about magneto exchanges, of which there was a total of one left in the country. Later years they got quite forward looking, especially the theory bit where actual experience on the part of the syllabus writers was not needed. I wandered away from Radio and Line Transmission and into the Telephony side of things, so the oddments of what happens to high frequency electric going up cabes sort of faded. I remember "Droitwitch" on the dial. Much more evocative than a row of numbers.

Returning to original question and the last post - if the end bit of the antenna is taped to the underside of the deck near to the center line, or pushed into a plastic tube taped there, it should never be below water unless something terrible has happened, when I suspect that a loss of signal is the least of your problems.

03/06/2020 17:42:53

The shiny bit at the end is a tuned length, shortening it will reduce its ability to receive signal.

It is a small coax cable, the shiny bit is just where the outer sleeve and the layer of wire that forms the shield removed, leaving the inner wire inside its insulation. Restoring the original length of stripped outer should restore most of the original range. Most, because the length of the shield is also a tuned length. Its almost 60 years since I had to do the theory about such things for an exam, and its been gathering dust ever since I handed the paper in.

Thread: Duplex 575
02/06/2020 09:57:49

For a boat this size, white bin liner bags are a good source for sail material. "Magic Tape" from the stationery shop is excellent strengthening for the parts that need it, just the leading edges that are subject to stretching and bracing patches at the corners and possibly battens on the main if required. A pointy soldering iron makes excellent rigging holes in plastic, forming a plastic eylet as it melts the hole. The only downside to the plastic/tape method is that you learn a lot about static electricity that you never wanted to know, but the items are cheap and plentiful.

The common arrangement for running rigging lines is to go from the servo arm, backwards to emerge over the deck and turn round something near the stern. This line then splits to work the two sails. As the end of the arm moves forward, the line and the attached sails pull in, as it moves toward the back of the boat, it allows the wind on the sails to pull the line out.

On its way to the main that line passes through a bridle ring sited where you think best, then up to a point on the boom.

The fore sail line goes through a deck eye at a preferred spot in front of the mast and onward the the jib boom. An elastic line from the splitting point helps avoid tangles in light airs, but with an arm servo isn't really vital. It is extra work for the servo to pull against when you really want all of the effort to be on pulling in the sails.

Some kit models (and this might be one) run the lines all below deck until they pop out of deck holes, or in some cases, out of the cabin roof.

Thread: aeronaut Pilot boat
30/05/2020 09:30:09

Motor current draw is governed not only by the battery voltage, but also by the load from the propeller. The simplest way to check this is by using a test tank and measuring under a realistic load. Connect the motors direct to the battery with a meter in circuit, read the current.

If a meter is not available, a bag of assorted fuses and a fuse holder can be got from most car accessory shops for a little less than a cheap meter from the internet. Just try the various values of fuse starting with the lowest until you get one that doesn't blow. Next value up from that is the ESC rating needed. Two motors, twice the current, probably a higher rated ESC.

Most DC motors work the same rotating both ways. They usually have a red dot by one terminal. If the dots are connected to the same wire from the ESC (say, the blue one), both motors will turn the same way. If one motors red dot terminal is connected to blue, the other to yellow, they will spin in opposite directions for any movement of the ccontrol. Any differences will be down to manufacturing tolerances and different levels of drag introduced in the two drive lines.

Thread: Motor /Controller /Prop upgrade
28/05/2020 10:10:08

Mention was made earlier about the receiver having a BEC. The ones that I have seen were shy about actual information saying just what the BEC capabilities were. I suspect (and am open to correction) that the RX BEC just regulates the internal voltage within the RX to something that the RX is happy with, and does nothing for the rest of the system. Originally intended to simplify things in the days when 6 volt SLA and 6 cell 1.2AH NiCads were the state of the art.

This might well leave anything else plugged into the receiver running on full battery voltage, which might not be a good thing. Certainly not with things like Arduinos and the like. Many modern ESCs have a built-in BEC that supplies a regulated 5 volts dropped from the main battery voltage.

27/05/2020 08:51:30

Apart from the space taken, there are two main downsides to Bobs boards. One is that they need a very good match to the motor to give control. Either next to nothing until at full power, or everything all at once as soon as the wiper is off-normal. The other is that at everything but full speed, they waste battery power.

Electonic ones take up less space and offer good control with a wide range of motors with good efficiency at all speeds. A "maybe" downside is that most cause the motor to whine oe whistle to a greater or lesser degree.

When needed, a Bobs board can be serviced. The downside of that is that it will need servicing from time to time.

Electronic one don't, which is just as well because most are sealed for life.

Thread: Propshaft coming away from coupling
26/05/2020 09:43:54

An unlocked coupler has a motor at one end turning it, and a prop that doesn't want to turn at the other. If it is just sitting on the thread unlocked, it will move on the thread. A correctly applied locknut and/or thread locking compound will prevent this, as will following Ashley's suggestion of locking it in place using the grub screw on a flat filed on the shaft. The flat is a must - grubscrewing on to a thread might well render the thing unmovable later.

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