Looking for accelerometer recommendations

[Lomax]

I'm working on a remote monitoring/surveillance system for my boat, and would like to add a three axis accelerometer inside the enclosure - partly to keep track of her trim and list, partly to detect events such as large waves, collisions, break-in attempts, even people stepping aboard. Seems like a logical and relatively simple metric to add to the system. But having had a look around, I am a little overwhelmed by the options - from cheap $4 10-bit chips to advanced "IMUs" costing hundreds of dollars. I guess an I2C device is the way to go, but it would be great if I didn't have to read it at a very high frequency just to detect short lasting events - are there any such chips with an "alarm" function? For list / trim it would be fine to read the data once a second! Keeping these requirements in mind, and the fact that she's a steel boat, so having a magnetometer would be a waste of time (I already have an external VDO fluxgate sensor for compass data anyway), which chip would you recommend?

[Lomax]

I actually woke up having a dream about this forum. Someone had replied to this post, telling me how I was an idiot for looking for an accelerometer which could generate interrupts, and that everyone knew the only way to do it was to sample at high Hz and apply lots of complicated maths to the signal in Python in real time. That it didn't matter if the data was flowing continuously even though an interesting event might not occur for days, since the Pi was always on anyway, and that I was silly to worry about CPU usage. Now that I'm awake I'm kind of surprised that hasn't happened yet. After all, isn't that what always seems to be the outcome when you post an honest question? First you get told how stupid the idea is, that it will never work, that there is no such thing, or that you've completely misunderstood the purpose of X. Then you get all sorts of irrelevant suggestions for other things you should be doing (use a microphone instead! no a camera pointed at the stars!), and the conversation gradually descends into an angry argument - while studiously avoiding anything that resembles a reply to the actual question posted. And then, after getting all worked up and frustrated, you go off in a huff and do your own research, only to find that not only was your question valid and true, but that it has already been addressed by manufacturers, for precisely the same reasons that you raised in your question.

Beginning to think I should give this project - and/or Internet forums - a rest.

[Heater]

I think you do the good folks who answer questions and suggest solutions to problems a great disservice. There are lots of useful replies passing through here everyday.

In this case it is a very short time, 12 hours, between your asking the question and going off "in a huff" then coming back to slag of any possible responders. That is not a good way to elicit help.

Also, one can't expect that those people who have read your question have actually used an accerometer in the manner you would like. Perhaps those with such experience were sleeping, like you.

Anyway, glad you found a device that looks like it may do what you want with it's "tap detection".

Thanks for the link, that is an idea worth bearing in mind in case the need arises.

It would be interesting to hear how you get on with it.

There are of course other devices that provide interrupts on motion detection, like the BOSCH, https://cdn-shop.adafruit.com/datasheet ... 000_12.pdf Although that particular device may be overkill for your application.

Still, if you can mount the thing above deck and make use of the magnetometer it would be a great solution.

[Lomax]

I think you do the good folks who answer questions and suggest solutions to problems a great disservice. There are lots of useful replies passing through here everyday.

I was talking about a dream I had. And forums in general.

[davidcoton]

Consider this: often in writing up a question, you learn more about the problem you are trying to solve. This can lead you to do some more research, perhaps with a better idea about what to look for. You have more motivation than anyone else to do that research, so it's quite likely you can solve your own problem before anyone else does. Especially when your situation is slightly unusual, probably no-one has done exactly what you want, so anything they post may seem to you "irrelevant". Don't give up just because no-one has replied in a few hours -- it may indicate that what you are doing is novel, it says nothing about it being "stupid".

It is wise to wait at least 24 hours after your post before commenting on the lack of good answers. It is unwise, even rude, to complain about unhelpful answers when you have not received any.

Incidentally when you receive unhelpful answers you may need to look at your own problem description to see why someone misunderstood it enough to think it was "stupid". Communication is a two-stage process, the "transmitter" and the "receiver" need to be compatible ("on the same wavelength") for the results to be useful. Then decide whether to explain why it is unhelpful, or just ignore it.

I don't have enough knowledge about accelerometers to help there, but dream on, it sounds a fun project.

[Heater]

Lomax,

I was talking about a dream I had. And forums in general.

Ha, OK!

It's hard to tell fact from fiction on the internet

Good luck with your project. Let's hear how you get on.

[Lomax]

I'm just so tired. I'm doing this all on my own, and I've been working on it since the beginning of the year. The cost in time and money has been astronomical. I just want to get to a point where the system can be deployed, but there's always something that doesn't work the way it should. In fact, virtually every step along the way has been problematic. And yes, it has involved a lot of needless arguing on forums - as illustrated by the fact that I now even dream about them at night

[davidcoton]

I'm just so tired. I'm doing this all on my own, and I've been working on it since the beginning of the year. The cost in time and money has been astronomical. I just want to get to a point where the system can be deployed, but there's always something that doesn't work the way it should. In fact, virtually every step along the way has been problematic. And yes, it has involved a lot of needless arguing on forums - as illustrated by the fact that I now even dream about them at night

You've taken on a complex project, and you're doing it to a high standard. I wouldn't be surprised if it has been six months full time to get where you are with hardware and software. Sit down, take a cup of tea/coffee, and just look at what you have achieved. Then prioritize what's left. Decide what must be done before deployment, what is a nice to have, and what is definitely not necessary now (though it might be something for next year). Do the essentials, cherry pick the easy bits of the middle group, and sideline the rest. Test and deploy! (Sorry, that is too simple. Test, debug and deploy!)

I can say more about the tiredness vs. project completion battle , but a public forum isn't the place. PM me if you like.

[Heater]

Lomax,

I'm just so tired. I'm doing this all on my own, and I've been working on it since the beginning of the year. The cost in time and money has been astronomical.

I just took a look at your project from the link above. Sorry I did not before, I have no interest in security systems as such..

From there I read:

My situation is quite different in a number of ways; I have limited power (solar panels), limited bandwidth (mobile network), limited space and limited CPU cycles. And the boat is made from steel, so WiFi, Z-Wave and other wireless technologies may not work well. Everything has to be small, power efficient, CPU efficient, low bandwidth and connected by wires. The whole system needs to idle at very low power consumption (way under 10W). Etc etc etc.

Wow, no wonder you are tired.

Juggling all those conflicting requirements: power consumption, size, bandwidth, CPU power, reliability etc is a challenge. It's what separates engineers from tinkerers and "makers". Judging by the photo it looks like you are making a quality job of it. It requires, skill, judgement, experiment, tenacity and yes, money. It causes frustration and grief. Yep. I know, I have been in much the same place these last few months.

Sometimes it's effective to take time out. Drink some beer. Hopefully find someone to discuss it with, even if they have no useful ideas in particular. Or think about something else entirely. It's odd how solutions to problems pop into mind when you are busy not thinking about them!

May I ask what kind of boat this is? What does it look like? Any pictures?

Good luck with your project.

[Lomax]

You've taken on a complex project, and you're doing it to a high standard. I wouldn't be surprised if it has been six months full time to get where you are with hardware and software. Sit down, take a cup of tea/coffee, and just look at what you have achieved. Then prioritize what's left. Decide what must be done before deployment, what is a nice to have, and what is definitely not necessary now (though it might be something for next year). Do the essentials, cherry pick the easy bits of the middle group, and sideline the rest. Test and deploy! (Sorry, that is too simple. Test, debug and deploy!)

Thank you for the kind words - I did take a break and went for a cold beer outside (glorious weather here!), which helped clear my thoughts and I came up with exactly the kind of priority list you suggested.

1) Connectivity. The biggest problem I'm having is that the OpenVPN client on the AirLink RV50 is severely out of date. In fact it is five years out of date, at version 2.1. This is outrageously bad considering that this is a current model of a $700 professional device, which is widely used in mission critical applications such as first respondent vehicles. And it's not like I'm running an outdated firmware; I'm on version 4.8.1.006, released on July 16th this year. Yet despite spending the best part of two weeks I have been unable to get it to connect to my v2.3 OpenVPN server. Repeated inquiries in the Sierra Wireless support forums about this issue, going back to August 8th, have gone completely unanswered, and no other support options are offered by them unless you pay them loadsamoney. I need to make some decision on how to deal with this, and see a few different options: either downgrade the server to a compatible version (i.e. 2.1), with all that means in terms of manually building it, lack of updates, and known and serious vulnerabilities. Alternatively, give up on the idea of the router being the VPN endpoint and install OpenVPN on the Pi instead, with all that means in terms of increased CPU usage (presumably the RV50 has encryption hardware to deal with TLS etc) and difficulty getting other machines behind the router to use the VPN. A third option might be to look at other VPN technologies - the RV50 also claims to support IPSec and GRE, though if its OpenVPN "support" is anything to go by I might just be opening another can of worms. A fourth option is to cut my losses on the Sierra Wireless router alltogether, and replace it with something else, though this is not appealing considering the money spent (£400) and the time and effort invested in the physical mounting of it, as well as writing the software to communicate with it (for example I make use both of its SMS gateway and its GPS -> UDP service). Regardless of how I deal with this, it really must be dealt with before the system can be deployed; without a functioning VPN I will have no way "in" from outside.

2) Time. Or rather the lack of it. In a very real sense. One nice thing about the RV50 is the built in GPS service, which can easily be configured to send NMEA GPS data over UPD to any listening clients. I have successfully set up gpsd to listen to this, and I get a good fix etc in gpsmon - but, I am unable to get ntpd to read the shared memory provided by gpsd (or maybe it is that gpsd doesn't populate the shm correctly). I have spent weeks on this issue. I have posted questions and detailed descriptions of my problems and the symptoms to the gpsd and ntpd mailinglist, which in typical fashion have received responses which completely disregard the information I have provided, in terms of configuration files and log output. I have also spent considerable time trying to get the new ntpd driver type 46 (JSON) to work without luck. It is a curious shortcoming that the RV50 does not provide an NTP server - most routers I've worked with in the past have included this functionality. But that's beside the point; it is capable of providing highly accurate time data via its GPS -> UDP interface, which should be possible to use to set the clock on the connected Raspberry Pi. I really do not think what I'm trying to do here is too far fetched; the correct time is right there in the NMEA data stream, and it is picked up correctly by gpsd. Why this obsession with GPS time? Well, it is the only reliable [insert mad laugh here] time source available to this system; anything can happen to the mobile network, and the internet connection, and an on-board RTC, while provided by the Monarco Hat, relies on its own button cell battery which will die at some point, and it will drift over time. An RTC is great and useful, but only if you can periodically re-sync it with a reliable and accurate time source. This being for a boat, I cannot imagine any scenario short of a catastrophic failure (or WWIII) that would make a GPS fix unattainable. Without accurate time, automatic updates will fail, the VPN will fail, and the data logging (which is 75% of what this project is about) will fail. This needs to be fixed before the system can be deployed. I am now working on writing my own parser for the NMEA UDP datagrams, and my own function for updating the system time from this data. Yes, if someone handed me a gun I'd put it to my own forehead and pull the trigger quite happily.

3) Power. When I set out on this project, I tried to plan from the bottom up, and made it the first priority to ensure that the system would have its own reliable power source, which could be monitored and would notify me of any power related issues well ahead of them becoming a problem. I spent a lot of time researching my options here, but eventually settled on the OpenUPS from mini-box as the best - albeit pricey - alternative. This is a very clever battery management system, which can operate on a wide range of input voltages, and provides full UPS-style monitoring over USB, compatible with NUT and upsd. It really is the bee's knees when it comes to computer attached power management systems for anyone who builds bespoke "always on" systems and need UPS functionality. I also chose to go with a hefty, 17Ah, sealed lead acid battery as the failover power source, so that I would have plenty of time to "talk" to the system (and it to me!) in the event of some kind of power failure; no less than 24 hours availability when all other power has been lost. But. The damn thing wouldn't work properly. I had managed to find a second-hand board on eBay for "only" £50, but it would go into a curious pulsating mode, continuously switching between "on line", "charging" and "on battery" (and I do mean like every ten seconds or so). I battled for a long time with the many configuration parameters but could not get it to stop doing this. Some months later, having spent yet another few days researching alternatives, and having had no response from mini-box to my detailed and polite support requests, I came to the same conclusion I had previously; if you want to build a custom "always on" system backed by its own UPS there is precisely one product on the market which ticks all the boxes, and it is the OpenUPS board from mini-box. Maybe there was just a fault with the second-hand board I had purchased? I swallowed my pride and ordered another board, this time brand new, at a cost of £100. Guess what? I'm having precisely the same issues with the new board. It goes into some kind of mental break-down mode where it refuses to float charge the SLA, while constantly switching status as explained above, spamming the syslog, and spamming me with emails (unless I turn off the notifications), until the battery has been drained to the point where it will start a "bulk" charge. It then charges the battery up quite happily, after which it does apply a float charge, and the system appears to be stable. But if I remove external power for even the briefest moment, this fluctuating behaviour returns, and the cycle repeats. I cannot deploy this system until I have confidence in its power supply. I have just spent another two days trying to get to the bottom of this, re-reading the manual very carefully (I'm using the defaults for an PBSO4 SLA anyway), and logging and looking at days' worth of system metrics. Yet anything resembling a solution still eludes me, and mini-box are as silent as ever.

These are the three "show stopper" issues I am facing, and all three have some important properties in common:

• They are all issues relating to "off the shelf" technologies which should just work - I'm not re-inventing the wheel here!
• The equipment involved is expensive and has been purchased only after careful research, precisely because I hoped it would function well
• No constructive assistance of any kind has been offered by any of the people or companies behind any of the hardware and software involved, despite my repeated requests

It has taken another hour of my time to write up this post - an hour during which I managed to burn one of the frozen pizzas I have been forced to live on lately, due to not having time to cook - so please be gentle with your criticism.

[Heater]

Lomax,

Wow, a lot to digest there. After a quick first read here are my thoughts.

1) VPN.

My approach to VPNs now a days is to ignore them. In the past I have spent hundreds of hours fighting with configuring VPN's and banging my head against buggy VPNs. Generally they are overkill for a one off system. Recently I had to work with a cellular modem from Digi which has a built in VPN. I balked at getting that working.

What do I do instead of VPN?

a) One can use a service like dataplicity https://www.dataplicity.com/ with that their little Python client connects from Pi to dataplicity server. Then when you visit the dataplicity web dashboard you can log into the command line on your remote Pi. BINGO, secure remote access set up in five minutes. I have had dataplicity in use on a bunch of Pi for a long time now, it's pretty reliable. You can even access a web server on your Pi through a dataplicity tunnel.

The resin.io system is another approach to getting remote access: https://resin.io/ Although that takes a bit more effort. It's great if you want to deploy the same software to many remote systems.

Are you happy with a "cloud" solution supplied by a thrird party? Perhaps not but it gets the job done now.

If not one another approach is to use SSH tunnels. http://www.revsys.com/writings/quicktip ... unnel.html What I do here is get a Linux instance on Google Cloud. With the SSH tunnel the Pi connects securely to my cloud instance. Then I can ssh from the cloud instance back to the Pi. For a one of system this is a quick and easy secure connection.

Finally the DIY approach. I create a little client using Javascript and node.js that opens a web socket connection over HTTPS to a node.js server on the cloud instance. That little client does nothing but pass data from the websocket connection to the command line shell and from the command line shell back to the server. The server forwards that data to an open socket on the cloud instance (not open to the public internet). I can now log in to my cloud instance and from there log in to my Pi. This of course takes a bit of programming effort.

2) Time.

I agree. GPS is the way to go to get time.

It's not clear to me that you need NTPD though. NTP is all about distributing time over the network. Hardly required for a single node. I would not bother.

Why not just get the time from GPS, in Python or whatever your language of choice, and set the time on the Pi from that? Do that once a day and on boot up. Sure your Pi may see some odd jumps in time here and there which NTP would fix. But does that matter?

3) Power

Sorry I don't have much to say there. It's a problem I have just been presented with myself. Surely there are marine solar systems that take care of all this already. At a price...

Now, get back to that pizza and beer for a while and mull it over

[Lomax]

Wow, no wonder you are tired.

Juggling all those conflicting requirements: power consumption, size, bandwidth, CPU power, reliability etc is a challenge. It's what separates engineers from tinkerers and "makers". Judging by the photo it looks like you are making a quality job of it. It requires, skill, judgement, experiment, tenacity and yes, money. It causes frustration and grief. Yep. I know, I have been in much the same place these last few months.

Sometimes it's effective to take time out. Drink some beer. Hopefully find someone to discuss it with, even if they have no useful ideas in particular. Or think about something else entirely. It's odd how solutions to problems pop into mind when you are busy not thinking about them!

May I ask what kind of boat this is? What does it look like? Any pictures?

Good luck with your project.

Thank you, you are too kind I am aware of the very real possibility that I've "gone in too deep", though I do hope this won't literally be the case. Perhaps I allowed myself to be seduced by the promise of the "IoT", which perhaps would be better described by adding another "i" and a "d". The thing is though, I am experienced and knowledgeable enough to see that what I'm trying to do is eminently doable; there's no smoke and mirrors here, in fact most of the technologies involved are decades old. How hard can it be? The answer seems to be: very, very hard indeed.

Here she is, "Eris", my pride and joy, still on the hard as as you can see, but slowly becoming ready for a return to the sea:

eris.jpg (232.34 KiB) Viewed 2159 times

[Lomax]

Wow, a lot to digest there.

Tell me about it.

My approach to VPNs now a days is to ignore them. In the past I have spent hundreds of hours fighting with configuring VPN's and banging my head against buggy VPNs. Generally they are overkill for a one off system. Recently I had to work with a cellular modem from Digi which has a built in VPN. I balked at getting that working.

I've done pretty much the exact opposite journey; I have ignored VPNs as being too complicated for many years, only to recently decide to give it a go - not least because I never had much need for one before, but the double whammy of a NATed dynamic IP on the mobile network necessitates some work-around, for which a VPN appears to be the obvious choice. Don't get me wrong, I understand why mobile networks do this, and I think it is a valid and sensible approach - I just need to get around it.

Are you happy with a "cloud" solution supplied by a thrird party? Perhaps not but it gets the job done now.

In a word: "no". I need to control and understand the systems I depend on. I am also highly hesitant to share any of my (meta) data with third parties. Trust is a very precious commodity these days. Also; if they can make it work, I can too.

If not one another approach is to use SSH tunnels.

Yes, an SSH tunnel is a nice and (comparatively) simple solution. I may well opt to use this, at least for now, as it's something I should be able to get up and running in a matter of hours. It should also be a negligible load on the poor Pi on-board, which has so much other work to do. But I hope you understand that I'm a little reluctant to give up on the RV50 here, considering the time and money already spent. This device was supposedly built to solve exactly this problem and it makes me really angry that it doesn't seem to do any such thing.

I agree. GPS is the way to go to get time.

Thank you. I will show this to my doctor next time so I can tell him "look, see, I'm not crazy!"

Why not just get the time from GPS, in Python or whatever your language of choice, and set the time on the Pi from that? Do that once a day and on boot up. Sure your Pi may see some odd jumps in time here and there which NTP would fix. But does that matter?

Yes. Agreed. Reluctantly this is the conclusion I have come to as well. But remember that ntpd is both a client and a server; it has been designed both to share the time and to set the time. I just prefer to use the established methods whenever possible, that's all.

Sorry I don't have much to say there. It's a problem I have just been presented with myself. Surely there are marine solar systems that take care of all this already. At a price...

Oh god yes, I also have a Mastervolt "Mass Combi" on board, which is a very sophisticated and expensive battery charger/inverter (made in Holland, specifically for house boats). This will take care of everything to do with keeping the ~1000Ah main battery bank "ship shape", and I will be talking to it over RS-485 to see how it's doing. Later, I plan to add two 300W solar panels on the roof, with two Morningstar Tristar MPPT-30 chargers, also connected to the monitoring system via Modbus. The separate battery back-up in the monitoring system is only there as a fall-back, should anything go wrong with the main supply. How else would I be able to receive notifications should the main supply fail?

Now, get back to that pizza and beer for a while and mull it over

Good advice, thank you!

[Heater]

Gosh, "Eris" is beautiful. She deserves a quality effort. Where is she?

You are right. Should be easy. Problem is most so called IoT stuff is marketed at consumer users in the comfortable homes. Serious off grid, off net systems get a bit more tricky.

My quest for solar power solutions led be here a few months back:
https://www.mrsolar.com/rv-marine-solar-power-kits/
https://www.mrsolar.com/telecom-solar-power-kits/

I'm sure there are others.

Currently I started getting into the new Cat M system for mobile connectivity. Which promises to be low power and much cheaper to use. At reduced bandwidth.
https://www.rcrwireless.com/20160216/in ... rizon-tag4
https://www.digi.com/products/xbee-rf-s ... e-cellular

This is all new stuff so that may be ahead of the curve a bit.

Another interesting thing is LoRa wireless. Very low power, very small, a range in the order of 10Km. Very low bandwidth, not much good for network connectivity but great for reliable health/status monitoring and alarm messaging from remote systems.
https://www.seeedstudio.com/category/Lo ... nKEALw_wcB

Again, quite a new thing. Cellular providers around Europe are offering it now I believe. Or if you live nearby you can set up your own LoRa base station for Eris to talk to.

[Heater]

Lomax,

...the double whammy of a NATed dynamic IP on the mobile network necessitates some work-around, for which a VPN appears to be the obvious choice.

That is true.

That work around for me is now HTTPS and or SSH. Whether to a third party cloud service or my own concoction on my own servers.

In a word: "no". I need to control and understand the systems I depend on. I am also highly hesitant to share any of my (meta) data with third parties.

Understood. I do sympathize with that view.

Also; if they can make it work, I can too.

That's the spirit.

Heck. If I can make it work so can you

I'm a little reluctant to give up on the RV50 here,

By all means keep the RV50. Looks like a great device. In fact I'm going to get hold of one for evaluation. Thanks for the heads up on that.

I'm only suggesting skipping the VPN functionality.

[Lomax]

Gosh, "Eris" is beautiful. She deserves a quality effort. Where is she?

Thank you Your request for a photo made me look for a recent one, and just looking at her felt like a lightning bolt through my heart. "What the **** are you doing sitting in front of your computer in a dingy old flat trying to sort out incompatibilities between OpenVPN versions when you have a beautiful boat just sitting there waiting for you to give her some TLC!?" What can I say, prioritisation is not my forté. I make my mind up that there's something I want to do, and I just have to make it so. Pathologically stubborn. Sometimes an asset, often a liability. And to answer your question, we are both in the UK, and well, close to the water...

You are right. Should be easy. Problem is most so called IoT stuff is marketed at consumer users in the comfortable homes. Serious off grid, off net systems get a bit more tricky.

Agreed. And I don't think that's a bad thing. I cannot imagine many more depressing future scenarios than the one where everyone is a computer geek - by necessity. It should be, and is, perfectly possible to design pretty advanced systems where most people won't even think about what they are doing as having anything to do with a "computer". I was talking to the guy in my local shop about this the other day, and our consensus was that you learn what you need to learn. Not a touch typist? Well, probably you don't need to type very much! I certainly didn't take any classes, beyond electronics and telecoms at A-levels - everything I know about coding and computers and the Internet (and typing) I have learned because I wanted/needed to know. Long may this remain the case. Do I understand how his till-system works? NO! I don't want to! I just want to get my four cans of European lager @ £4.99 so I can go home and drink them - in front of my own computer.

My quest for solar power solutions led be here a few months back

I think I've already embarrassed myself enough by complaining about the way you get the "wrong" answers in public forums, so I'm quite hesitant to counter your suggestions on PV systems, but I would say you might be better served looking at each component separately. LG make some really fabulous panels these days - take a look at the LG315N1C-G4 for example; 315W in a standard module size, with great partial shading performance and a 19.2% efficiency, with a 12 year warranty to boot. Although not intended for marine use, these are the panels I've been looking to get - though in my case roof space is severely limited.

Currently I started getting into the new Cat M system for mobile connectivity. Which promises to be low power and much cheaper to use. At reduced bandwidth.

Very interesting, thank you; I had never even heard about this, or "LoRa". I will be checking it out more in detail for sure. Ultimately though, I want to get as close as possible to Eris' "on-board" system being just like any other of the many systems I administer, and that does require bandwidth approaching at least a dial-up connection.

[Heater]

Lomax,

Yes, what are you doing in doors. You could be drinking that beer on the boat. If the weather is half decent.

We can expect everyone to a be computer geek to make use of IoT gadgets. Sadly the current crop of IoT gadgets are badly conceived and over hyped. And have no effort at security put into them. Which has been causing no end of problems.

I would not say my solar suggestion was "wrong". Just a suggestion based on what is written here. Perhaps not suitable for what you have in mind.

Those LG panels look great. Where are you buying them? I see no clues on the link you gave.

Cat M should be good for command line access to devices. If and when it rolls out. LoRa, not so much. Might be a alternative route for status and alarms though.

[Lomax]

Those LG panels look great. Where are you buying them? I see no clues on the link you gave.

Erm. Where I buy pretty much everything else I cannot get from Mouser/Rapid/Digikey/Farnell/TME at a reasonable price: eBay! (and I do hate eBay, make no mistake - only slightly less than I hate Amazon). Haven't bought them yet though - and it will probably be some time before I do. Many, many other things need to happen first, and by then there may be even better and/or cheaper panels available. So I wait. But I recognise my situation is a little different in that I only have room for two full size panels - it is worth it for me to spend a little bit extra for better efficiency; a bigger boat would be far more expensive! If you're not pushed for space, 250W panels are probably more cost effective.

[davidcoton]

Sorry I can't help much with any of the specific technical issues -- although I'm a software engineer and project manager by background, I've been doing "other things" for the last ten years or so. I would just say that the detailed "bottom up" technical planning needs to be balanced by the "top down" approach to see how the functional requirements pan out, and to track the problems of specific parts so that the implications are understood. That way the appropriate effort can be put into resolving issues, finding alternatives or putting things on the back burner (not the pizza, though ), and prioritisation becomes much easier.

Like you, I hate it when things that should "just work" don't. But sometimes I find bits that I expect to ber difficult, do "just work". Not sure which side wins in the long run.

I'm also in the UK, but far from the sea -- in any case I'd rather fly than float. If time and money permitted .