heckmotor

...well, why wouldn't they be in a DC circuit? Anyway, after learning a lot yesterday, largely from Catweasel and Gibbo, and being quite disappointed by the level of obstinacy and even petulance from elsewhere, I'm a bit reluctant to add anything to a useful thread, but here's a thing... I'm just off to wire up a new alternator on my boat, which is the sort of task that, rightly or wrongly, amateur boat owners will sometimes take on. When they do, they might be forgiven for thinking that they can isolate the work area and wiring by switching off what they perceive, without getting into semantics, to be an isolator key switch, particularly if their BSS-certificated boat has a plaque stating 'battery cut-off' adjacent to it? But my boat has its single isolation switch on the negative side, as it seems many do, and as soon as I touch any positive wire or cable against anything conductive I can expect an almighty flash at the very least. If the isolator was on the positive, I really would have isolated my workplace which, along with all the other reasons presented by Catweasel's Smartgauge link and by the very patient Gibbo, must surely be a powerful argument for abandoning negative isolation? Now, anyone keen to have the last word will say 'but you should physically disconnect the batteries before starting work', but these are private craft operated by amateurs, and often maintained by untrained tradesmen passing themselves off as professionals ...negative isolation is just an accident waiting to happen. By tonight, my boat won't just have a new alternator, but independent positive isolation of the cranking and leisure batteries, and I'll be a lot happier. For anyone who still thinks negative isolation is adequate, I can only say 'best of luck'! IAN

Hear! Hear! Pay attention, everyone! (Thanks, Gibbo!) IAN Bl***y right! IAN

Certainly, Chris! An analogy (for anyone who's not too sure of the difference between AC and DC!): Imagine those demonstrations of stationary engines that you see at steam fairs, where water is piped from a cistern to a pump that returns the water back to the top of the cistern (stick with this - it is relevant ...just!). If you close a tap on the cistern outlet, you can drill a hole in the plumbing or smash the pump and the tap will prevent any further loss from the cistern (think of the water as direct current). If you move your tap (or isolator) to the end of the hose that returns water to the cistern, any breach of the plumbing or pump (a short?) will cause unstoppable loss of all the water (or current, until the battery's flat). Yes, the water's acting like DC current, d'you see? Of course it should but, if it's a negative/return isolator, the boat's only shut down until water ingress, heat, physical damage, or some other event, leads to an unintended grounding to the hull anywhere inside the boat - you can complete the circuit anywhere, without going anywhere near the isolator. I know! ...I am probably labouring this, but I'm not sure the penny's dropped yet! ...or is it just me? LoL IAN

No, no, no it won't, Tim - that's the whole point! If you isolate the positive and then short out anything at all against the hull, due to physical damage, or a fire, or anything, then no problem because the whole boat's 'dead'. Isolate the neutral 'return' and do the same thing and you've got an open circuit, because the boat's always potentially 'live' - re-read the Smartgauge link ...it's definitive and unequivocal! It really worries me that this area seems to be so misunderstood and, indeed, that negative/neutral isolation hasn't been outlawed. IAN

Sorry to be picky again, Chris, and labouring the point, but these DC circuits are a 'one-way street' and the naming of +ive and -ive is anything but just a convention ...hence, no need for 2-pole isolators, surely? BTW, one thing that occurred to me is that our Bukh engine's cut-off is a powered solenoid, and there might be a case, on safety grounds, for bypassing the isolator to that? ...if someone's overboard, you want the engine off pronto, and if the isolator's been thrown, I'd have to raise the whole deck to decompress the engine. Suppose I should consider a cable decompressor stop, really, shouldn't I! IAN

Excellent link, Catweasel! Not sure Chris Pink has thought through the concept of 'upstream' of the batteries on the neutral side - we're talking DIRECT current, after all, so a neutral isolator next to the battery box is about as downstream as you can get, surely (IMHO!)? IAN

Yes, Ian, I'm beginning to think it's probably quite common ...but surely not very satisfactory? The whole point of easy-access isolation, as close as possible to the batteries, is to be able to break all circuits instantly in the event of an emergency, even when unfamiliar with the boat ...and a convenient consequence of positive isolation is the reduction of piggy-back terminals on the battery posts. Another IAN

Thanks for those thoughts, everyone ...haven't been able to catch up for the last few days. Andy's pic is interesting. Lark looks virtually as she did when we had her, with the exception of a shoreline power socket and well-polished mushrooms, but the photo is dated 30 August 2005, so a lot could have happened since then, I guess.

Perfectly reasonable question, Chris! To start with, we weren't able to tell what the adhesion of the several unidentified layers was, so we began to scrape and then, being dogged Capricorns (if that's not too mixed a metaphor), we just carried on. The benefit, now, will be that instead of looking like black tree bark, the hull will once again look like a smooth metal thing with a fresh coating. A good thing, I think, because the layers were not all blacking, and included both gloss paint, of more than one colour, and the red lead primer that's already been flagged as unsuitable for underwater applications. Thanks, meanwhile, to Tim and to N.b. Alnwick for a recommendation that I'll take note of, and funny to think that it was posted probably no more than a mile from me, on the other side of the A361 ...I'm in Wardington!

Unless altered recently, Lark has its full length cabin in early FMC black & white, with replica signwriting by Ron Hough.

Ha! Thanks Timleech! My wife still blames me for selling her (the boat, that is) among other things, and I must say there's still a bit of a hole in our life. Had some adventures in Lark, over five or six years, and it would be nice to have sight of her and to know what her subsequent career has been.

I got to thinking, while posting on another topic, that it would be nice to know where our old 1913 Josher was now, and who had her. We bought Lark from Peter Topping, before he sold his Cowroast and Fenny marinas. He'd had her radically altered at Norton Canes, with a full-length steel conversion, reduced draft, steel baseplate and (shudder!) a new counter. Power was from a poorly installed and troublesome Gardner 2L2. Not all Gardners are the Holy Grail, because a lot can happen to them after leaving Patricroft. We loved our boat, and travelled most of the accessible system before selling her to novelist Jasper Fforde in order to buy a Dutch barge ...and then discovered that we were priced out of a market that everyone else had discovered at the same time. Just getting back to the water with a tatty old noddy boat appropriately named Fubar by its previous owner, but what of Lark? Has anyone seen her? She was back on the market a few years ago, through the excellent Virginia Currer's brokerage, but haven't seen hide nor hair!

Thanks for everyone’s thoughts, some more useful than others! Windjammer kicked off with ‘…work out what blacking product you are prepared to pay for’, and later added ‘some blacking products are more durable than others’ without letting on which, so I obviously hadn’t made it clear enough that these were the very questions that I was hoping for a helpful steer on? Spencer Coatings describe their Comastic, which I applied to FMC ‘Lark’ some years ago, as ‘a special selected vinyl resin formulated with coal tar pitch and a range of additives’, so I was interested to see the respected Phil Speight corrected for calling it ‘bitumen-based’ …I must confess to have been unaware of the distinction, too, but I’m obviously concerned about the compatibility issue flagged by Timleech. Kingfisher points out that I’d have to get right back to bare metal to be able to use epoxy primer but, without the £2000-worth of grit blasting that I can’t justify, I can’t achieve this …believe it or not, my wife and I have scraped most of the hull with 1” and ½” carpenters’ chisels (three tough days), in order to avoid the clogging of every other sort of mechanical medium, before moving on to an angle grinder and £21 heavy-duty wire cup brush from B&Q, but nothing short of blasting will ever get to bright metal. Does that rule out Comastic altogether, or just the epoxy primer? ...and how about the xylene International Primocon primer mentioned by blackrose? I like kingfisher’s idea of a poll to reveal what others do, how often, and with what result – I suppose that’s just the kind of experience I was looking to tap into, and I’d expect that others would find it very useful. Anyway, thanks for the feedback – I’m still not quite sure which way to go, so if anyone’s got any further ideas, I’ll be most grateful. Hopefully a Boat Safety Certificate will be in my sweaty palm by tomorrow afternoon, at the second attempt, so I need to make my mind up!

Couldn't justify my home marina's quote for a quality grit blast and 2-pack application, as it would double the value of our old boat, and it would end up looking better upside down, so I spent last weekend scraping several layers of old paint and blacking off the hull with a 1" carpenter's chisel before giving it a wizz with an abrasive disc, and am quite pleased with the result - couldn't have got the same back-to-metal result with just the rotary power disc or wire brush, because it would just have clogged. Has anyone got any thoughts on how to get a good-looking durable coating to take maximum advantage of the effort we've put it, and on what priming might be necessary? My plan was to Fertan the steel and red lead prime before applying Comastic, but Spencer Coatings suggest an epoxy primer, or direct application onto unprimed metal. I wondered about 2-pack, as I'd like to use the best materials, even though I'm not wanting to spend more than I need to. The main thing is that I don't want to do the wrong thing after putting so much effort into the stripping.