Vendée Globe Nail Biter

Clark January 18th, 2017

If you haven’t been following it, the leaders in Vendée Globe are within a day or two of finishing, to cap one of the greatest games of cat and mouse in sailing history. At the time of writing, after racing for 73 days and over 24,000 miles solo, Alex Thomson is only 34 miles behind Armel Le Cleac’h.

Alex Thompson has slashed the gap by two thirds in the last few days, in part by setting the Vendée single day record. Video here. You will also see in the video that Mr. Thompson is battling multiple equipment failures, but still fighting to the end.

Thompson acknowledges that it would be tough to win at this point (yet he keeps closing the gap!) Full story here. He’s been up for days, and is on the verge of collapse.

At any rate, in the next 48 hours we’re going to see either:

1. Thompson overtakes Le Cleac’h to be the first non-Frenchman to win the Vendée Globe. In this scenario, could the Vendée Globe get into close tactics? Le Cleac’h luffing Thompson up as they turn off the autopilots a go into full close battle mode, when neither have slept in days dodging fishing boats and freighters, hallucinating at the wheel? Unlikely, but would be something for the record books.


2. A very close finish, in which Le Cleac’h crosses the finish line, then just has to wait an hour or two to embrace the man he has battled all the way around the world. The two have thought about each other every moment for 75 days, yet haven’t see each other’s faces since the start. When they finally meet face to face, it will be a moment to remember.

3. Or some crazy thing we never could have foretold…

Ways to watch it are here.

Electrical Fire! (and some lessons learned about starters)

Clark January 2nd, 2017

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Voice Mail: “Hi Clark, it’s (name withheld). I was out sailing today with my daughter and we had an electrical fire on the new starter you installed. Because of the fire we lost the engine and hit the south tower on the Golden Gate Bridge, called the Coast Guard, and had to be towed back to our berth. When I opened the engine compartment there were six inch flames rising from the starter, but I was able to blow them out. I don’t know where that leaves us, but I’d sure like to speak with you.”

Not what a marine electrician wants to hear. After my initial panic, I reflected that this was a basic R&R (remove and replace) of an old starter for a new one. I’d tested it several times, by cranking and starting the engine, and all seemed well. Various scenarios flew through my mind – defective starter, defective solenoid, some sort of shorted wire, stuck solenoid or stuck starter button, or, eh gads, installer error. I called the owner, who was very understanding, and was back on his boat the next day. If you look at the photo above, all the insulation on all the wires leading to the starter is fried, and was burning until he blew it out.

After an initial check, I called the owner and told him that no matter whose fault it was, the damage was probably less than the deductible on his insurance, and that I might as well remove the starter and start the replacement process. He agreed. I pulled the starter and found it well-burnt, and the solenoid completely melted, with both of the studs loose. The main linkage between the solenoid and the starter motor had acted as a fuse, melted through, and ended the fireworks:

I took it back to the starter store, where they were very understanding and agreed to replace it under warranty, but also opined that something had probably got stuck, and that the starter probably wasn’t at fault. They noted some damage to the pinion gear, which I hadn’t noticed.

I installed the (second) new starter and continued my postmortem, finding very quickly that the cranking circuit was closed, as in, if I’d connected it the starter would have started cranking and wouldn’t have stopped. In this instance the boat had a starter button, separate from the key switch that energized the circuit, and the button was stuck:

Blessed sweet mother of God, it wasn’t my fault! I replaced the button, and the burnt wires, tested it all out, and all was well, for the second time. The owner was very understanding, ended up buying my wife and me a nice bottle of wine, and we decided we owed the guys at the starter store a case of beer.

There are some interesting things that happen with a stuck starter, one of which I didn’t know about. I knew about shorts, of course, and 98% of high amperage starter circuits aren’t protected with fuses, so these can be spectacular. And I knew about all kinds of unintended open circuits, as with bad motors, bad solenoids, etc. But I always thought that a stuck starter, as in, a starter that stays engaged after the engine starts, would just burn out its innards or strip its pinion gear.

Nay. A starter that stays engaged after an engine starts gets spun continually, much faster than its intended rotation speed, and actually becomes a generator, sending high current back into the electrical system. In most cases the batteries and cabling can handle the current, but the starter can’t. It gets very hot and finally burns up (from high current, rather than friction, overheated brushes, or whatever). Even in normal use a starter is an intermittent duty motor: With a recalcitrant engine you should only crank it for ten seconds or so at a time, then give it thirty seconds to cool off, and to allow the surface voltage to come back on the battery.

So, it is very important to make sure your starter disengages after your engine starts. In most cases this is obvious, as in your car, where if you held the key in the cranking position, or the starter got stuck this way, you’d hear it. But on many boats it’s not so obvious, since the engine panel might be some distance from the engine, and once started the engine noise can drown everything else out.

This boat happened to be a Catalina, and on Catalinas it’s standard to have a starter indicator light on the instrument panel. This is a good feature, and not common on other boats, but you’ve got to know it’s there:

On this boat it was there and still worked, but the owner didn’t know about it, plus it’s hard to see in daylight, and easy to miss in full combat mode (they were close to the south tower of the Golden Gate Bridge in an outgoing tide, after all). But also on Catalinas, and many other sailboats, the engine panel is exposed to the elements in the cockpit, sometimes gets kicked a lot, and generally takes a beating. In this case the starter button saw constant rain and spray, and eventually corroded and got stuck.

On my boat I’m standing right over the engine when I start it, so I’d hear it in a nanosecond if my starter got stuck, but not so on many boats. If it wouldn’t be obvious to you if your starter got stuck, you should consider an indicator light or buzzer. When you consider that it would result in not only a destroyed starter, but in not being able to start your engine again, and maybe an electrical fire, it’s worth some thought.

Shore Power Cord Economics

Clark December 13th, 2016

Unfortunately, as in the photo above, the connectors on shore power cords often get toasty. It always seems to happen on the neutral connector (white wire in the US system) and I don’t know why. Maybe the electrons get all gummed up and dirty from being on your filthy boat, then get stuck on the way off?

Sometimes it happens on the male side too, and the guts of the shore power inlet have to be replaced:

At any rate, a burned/melted shore power cord is bad, and should be repaired, but therein lies the rub. The new connector for the end runs about $35, but that’s not all. In order to make it like before you also need a new boot, which you can buy with or without the threaded ring, but call it another $15:

So now we’re up to $50 (prices vary, but you get the idea) in parts alone to repair a shore power cord. Fifty foot, 30 Amp shore power cords sell for as little as $80, if you shop around. It’s fairly straightforward to re-terminate a shore power cord, which a do-it-yourselfer can easily do. It takes me about ten minutes, but it’s easy to see that the cost of parts, plus the cost of a marine electrician quickly makes the cost about a wash.

To do it right you’ll want some good wire strippers, a cable stripper (judicious use of a box cutter will suffice), a cutter big enough to lop off the whole fried end cleanly, then it’s nice to have a multimeter or AC tester to check that you haven’t reversed something that will really make things burn. So the task becomes daunting without all the proper tools, and if you don’t have the right tools they’re not cheap.

So what are we to do? It’s terrible that we live in such a throw-away society. I once toured the second largest open pit copper mine in the world, in southern Peru, and it’s no small feat to get copper out of the ground and turn it into copper wire:

So alas, if you just need to replace one end of a 50-foot, 30 Amp cord, repair costs enough less than replacement that you should fight the good fight and do it, if you can do it yourself or your electrician happens to be around working on other things anyway. If you have to replace both ends it’s cheaper to just buy a new one. If the whole cord is looking fairly tired and sun-baked, then definitely replace the whole thing.

If it’s shorter than 50 feet, it’s probably not worth repairing it.

For 50-Amp cords the whole magilla gets much more expensive for either repair or replacement, but the economics are about the same.

Copper wire should always be recycled, but finding where to do this can be a pain. As a marine electrician I take a big box to be recycled every year. Back during the height of the economic crisis people were desperate and copper prices were at an all time high, so shore power theft was common.

New Chainplates

Clark November 30th, 2016

I had to remove a few chainplates for an unrelated project and one of them broke upon removal. I guess I can count myself lucky it happened this way, rather than in full combat mode. Only 49 years old, and it just fell apart in my hand! I plan to write a strongly-worded letter to these Alpha England people about the quality of their product:

I blame the dreaded crevice corrosion:

Years ago I read something about replacing stainless chainplates with titanium, which is stronger, doesn’t corrode, doesn’t crack, and yada yada. I looked into it, and Holy Halyard Slaps! For the price of two simple (meaning flat), small, titanium chainplates I could buy enough 316 stainless bar stock in AND A BRAND NEW DRILL PRESS, which was long overdue:

Upon further reflection, I might have made them out of silicon bronze, but if my 316 stainless chainplates last half as long as the old ones I’ll come out winning. Brion Toss has a good riff on the subject here.

After getting my bar stock from and my new benchtop drill press from good old Sears, I set to work. Cutting was straightforward with an angle grinder with a stainless cutting disc, followed by a grinding wheel.

Drilling holes was also straightforward, using the slowest speed on the drill press, and lots of cutting oil. The old chainplates were countersunk for flat head screws. This would not only be a lot of work, but it seems to me it reduces strength by removing material, and provides a lot of hidey holes for crevice corrosion. My new chainplates will have non-countersunk holes and round head screws, with heads standing proud.

Then comes the hard part: They say the way to prevent corrosion is to polish the chainplates to a mirror finish. I was more or less successful at this, but I started my sanding with a 50-grit disc on an angle grinder, which left some irregular gouges. In the real metal polishing world they have all kinds of wheel sanders for the heavy stuff. I moved up into my higher grits with an orbital sander, then stainless polishing compound on a polishing wheel. All went well, but in the final result I could still see the gouges from my angle grinding. I’d say it’s good enough, and still qualifies as a “mirror finish,” but a mirror finish on some ridges and valleys left by my aggressive angle grinding. I think a belt sander would do a better job, but my belt sander broke.

While I was at it I replaced the backing blocks on the interior with G10. The old backing blocks were teak:

I’m on a kick lately of using butyl rubber, instead of polysulfide, for bedding deck hardware. Which is better? Ask me in twenty years. With butyl rubber it’s a long process of gradually tightening the fasteners over days or a week, as it is stiffer stuff, and takes a long time to squeeze out and find its place:

What Is It?

Clark November 8th, 2016

I was working on a boat in a dry storage area in San Leandro, California, when I saw the boat above. What this strange aperture in its side? On closer inspection the outside of the aperture has fixed vents, made out of plywood:

This outside part does not rotate. In the middle is a galvanized steel pipe, which is designed to rotate, as it is supported by several carrier bearings athwartships:

But on the other side of the boat this axle just sticks out an inch through another hole in the topside, with nothing like the contraption on the starboard side:

I’ve been noodling on it for a few weeks and have absolutely no idea. It wasn’t some thoughtless lark, because the vent thing on the starboard side is very symmetrical, and took a lot of work.

A sideways jet engine? Some sort of revolutionary propulsion system? Something not even meant for water; an unrelated project for which a fiberglass runabout just seemed like the right raw material? I’m stumped.

Advanced Electrical: Galvanic Isolator Case Study

Clark November 4th, 2016

Once I was buying a galvanic isolator in a West Marine store, when a West Marine employee, of all people, was really insistent that I shouldn’t buy it: “Those things are a scam! They don’t do anything. I have it on good authority that they’re a big waste of money!” He had that look in his eye, so rather than get all marine electrician on his ass, I just said, “Well, my customer wants it, so I think I’ll buy it all the same.” There are a lot of misconceptions about the purpose of a galvanic isolator, and what it can and can’t do.

Flash forward to this week when another friend/customer calls me down to his boat, says his zinc anodes are being eaten too quickly and he’s thinking about a galvanic isolator. When I get there he shows me a propeller shaft anode that a diver had replaced just over a week before, and it’s already over a third eaten. To get to that stage should take a few months.

A galvanic isolator will sometimes, but by no means always, solve the anode-eating problem. A galvanic isolator is installed in-line along the safety ground (grounding conductor, green wire) in the shore power connection, so of course if you’re not plugged into shore power at a dock, then you’re barking up the wrong tree.

My customer was plugged into shore power, but being a thorough guy I unplugged him from shore power and did a galvanic survey of all the underwater metal items on his boat using a reference cell, dangling in the water, connected to a digital multi-meter. All the numbers came up about right for a standard fiberglass boat, his bonding system was all intact, and he had the right amount of zinc anodes to protect his underwater metals.

But if we plugged in his shore power cord, his hull potential changed by about 200 millivolts. Aha, problem found, but people want to point to some AC problem, since this is coming from the AC shore power cord. Nay. Galvanic corrosion and stray current corrosion are caused by DC currents.

In fact, I could measure half an Amp of DC current by putting my Amp clamp, on the DC setting, around his shore power cord.

If your boat is bonded properly then all of the major metal parts, above and below water, are bonded together, usually using big green wire. It will also have an AC safety ground, also usually green wire, connecting all the AC outlets and appliances, just like at home. These two systems cross-connect at one, and only one, point. So, when you plug into shore power, your boat is connected, via the safety ground, to the safety ground system on every other boat on the dock, and thus to the bonding system and all the underwater metals, on every other boat on the dock. What could possibly go wrong?

My customer may have been doing everything right, but since his bonding system is connected to his AC safety ground system, and since his neighbors’ boats are the same, and since they’re all connected together via the AC safety ground wires in their shore power cords, his zinc anode was protecting his boat plus the boat next door, or maybe down the way, or maybe all the boats down the row. But even though it’s via the AC shore power cord, it’s galvanic corrosion, which means a DC current.

It could also be stray current current corrosion, that is, corrosion caused by an electrical current, but stray current is still a DC thing. AC current just doesn’t cause or accelerate corrosion in normal circumstances.

Whether his half an Amp of DC current came from pure galvanic action or stray current corrosion is beyond my pay grade. I tend to think the latter, because half an Amp is quite a bit. Maybe there’s no way to tell, but the solution, in any case, is a galvanic isolator.

A galvanic isolator blocks low level DC currents from traveling down the AC safety ground.

It doesn’t do anything about galvanic corrosion within the confines of the boat it’s on, thus the corrosion survey before zeroing in on the galvanic isolator. As a marine electrician, I think I had to go this route to rule out other causes beforehand. If I just slapped a galvanic isolator on there without poking around, I’d have to advise him to have his diver come back in another two weeks to check the anodes, and this would cost more than my additional poking around.

Before the galvanic isolator was installed:

And after:

Voila! But two days before I’d read .5 Amps, instead of .3 Amps. What changed? I don’t know, but .3 is still bad.

As a boat owner who is constantly/often plugged into shore power, should you just slap one on as a matter of course?: Yes

Will it solve your problem with fast anode depletion?: Probably maybe.

But even if you don’t have a problem now, you may in the future, as your fate is tied to all those other boats on the dock, so it’s good insurance. It’s especially good insurance when you compare the couple hundred bucks for the galvanic isolator to the financial ruin of a devoured prop and shaft, keel/keel bolts, outdrives, et al. It’s not just about saving anodes; it’s about saving what gets eaten after the anode is gone.

Likewise, it will not solve a stray current problem within your boat. If you’ve got nasty old exposed bilge pump wires sitting in the bilge water, you might just stray current your keel off.

And a galvanic islolator only protects against low level DC, up to 1.2 or 1.4 Volts, so if there’s some banzai stray current issue in your marina, which creates a voltage higher than that, the isolator won’t do anything, but this would be unusual.

It used to be that a galvanic isolator could fail, not only negating its anti-corrosion purpose, but creating a potentially deadly break in the AC safety ground. So after that they made it so you had to have a monitoring system that would warn you if its function was compromised. Now, galvanic isolators are fail safe, meaning that if they lose their anti-corrosion function they still maintain their AC safety ground function. They are still supposed to be tested annually. If you’ve got an older galvanic isolator, you should replace it with a fail safe.

There is a magic box called an isolation transformer that makes all of this go away, but isolation transformers are big, expensive, and heavy, so not practical for the average sailboat…unless your sailboat is steel or aluminum, in which case you should pony up and make the space.

Teaching Marine Electrical Seminar In Sausalito This Saturday

Clark October 25th, 2016

How many wrong things can you find in this picture?

If you happen to be in the Bay Area this Saturday, October 29th, I’m giving a marine electrical seminar at Spaulding Marine Center in Sausalito, where I will teach electrical excellence, simplicity, and how not to get electrocuted. They suggest a $50 donation and always provide a great lunch. Starts at 10:00AM; goes to about 2:30. Please RSVP. Link here for registration

Electrical Basics: Bus Bars

Clark October 17th, 2016

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Just twenty or thirty years ago the electrical system on the average sailboat was very simple. It had two batteries connected to an OFF-1-2-BOTH battery switch, and all the loads were fed from there:
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On the back of the battery switch were three studs: one for each battery, and one for the common, that is, the terminal that connects to the alternator and all the loads:

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The battery switch for this Catalina 30 is this way. In addition to the connection to each battery, the battery studs on the back of the switch are good places to connect the outputs for the shore power charger, the voltmeter, and the bilge pump, all things we want permanently connected to a battery, and never turned off.

On the common terminal of course is the big cable connected to the engine’s starter, and the feed wire to the main distribution panel, which in this case is just a 10 gauge wire: ah, the days of such simplicity. The back of this switch might be a little crowded, but all of these wires fit.

Today the electrical system on the average sailboat is more robust and complex. With just the aforementioned connections to the battery terminals – voltmeter, charger, bilge pump, maybe the memory wires from a stereo or other electronics – the studs are already too crowded. On the common terminal, forget it. You might have the big cable to the starter, a big cable to an inverter or inverter/charger, big cable to the windlass, and a good-sized cable to feed the main distribution panel, which now supplies a radar, a refrigerator, and a range of modern comforts.

All these cables simply won’t fit, and according to the ABYC standard, you shouldn’t stack more than four ring terminals on a stud anyway.

Enter the bus bar. Give yourself some breathing room!

A bus bar simply expands your single stud into four or more. A large gauge cable, and nothing else, connects to the common stud on the battery switch. The other end connects to the bus bar, where you’ve got a row of big studs for all the other connections. The same could be done for one of the battery connections if you find you’ve got too many cables and wires that need to be connected directly to a battery, without a switch in between. Generally speaking, we want to keep our battery terminals clean. Manufacturers sometimes dictate otherwise, as with some electrical system monitors and chargers, but we should endeavor to have nothing but the supply cables connected to our batteries.
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The bus bar is even more necessary on the negative side, where the negative cables from the batteries, negative ground from the engine, inverter, windlass, corrosion ground (green wire), and feed to main distribution panel, all must connect. Might also note here that bus bar covers are equally important, as they make for a lot of exposed, live metal:
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Many older boats foresaw this scenario, but it was before off-the-shelf bus bars, so they just added distribution studs, or what Blue Sea Systems now calls a Power Post, but one stud just isn’t enough. These are overcrowded and a bus bar would create more room, make circuits easier to trace, and ahem, that thing about no more than four ring terminals to a stud?:
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Now Blue Sea Systems has gone plum crazy with the PowerBar 1000. It’s the Super Jumbo Extra SuperMax GT version of the bus bar. I have yet to find use for one, but when I do I’ll know I’m serious:
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Remember, good wiring is not only electrically sound, but easy to follow. Wherever you find yourself running out of room and trying to cram too many terminals in a tight space, even if it’s electrically sound, it will be difficult to service and trace in the future. A relatively cheap and simple bus bar is often the solution.

Boat Command CONNECT! Meets Smoke Alarm

Clark August 1st, 2016


The manufacturers of Boat Command, the boat monitoring platform, didn’t build the system with a smoke alarm option. It’s got all manner of sensors, inputs, and alerts, but no smoke alarm, and if you read Boat US’s statistics on boat losses and insurance claims, fire is number five. Stats aside, the main things I worry about while I’m away from my boat are flooding (that’s a big number 1), fire, break-ins, theft, and dead batteries. These are the main reasons I installed the Boat Command system, but it does lots of other neat stuff too. Full disclosure: as a marine electrician I install a lot of these systems, and I’m a dealer for the product.

While neither Boat Command nor the smoke alarm manufacturers make it obvious, it’s a very straightforward to add a smoke alarm to the system. As you can see from the photo above, the right place for a smoke alarm on my boat is about a foot from the Boat Command CONNECT! base station, so running the wires was a snap.

Within the Boat Command platform are several functions that can be renamed and repurposed. One of these is the High Water Alarm, which is just a normally open (NO) relay to ground, designed for the connection of any old float switch: Float is down, circuit is open and no alarm; float goes up, circuit closes and ALARM! via text and/or email, and on the Boat Command dash board. Since Boat Command has “belt and braces” coverage for flooding, via detailed activity monitoring on two bilge pumps AND the high water alarm, I could repurpose my high water alarm as a smoke alarm. On my boat the second bilge pump essentially IS a high water alarm, as in, if it triggers then something is seriously wrong.

Repurposing is very simple: On the drop-down menu on the Boat Command dashboard select Boat Settings/Inputs, where you will see various inputs, all of which can be easily renamed. Just change High Water Alarm to Smoke Alarm and click Save Input Settings:
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Simple enough so far. Now, onto the smoke alarm. The smoke alarm manufacturers don’t advertise it either, but on what is commonly called a “Four-wire smoke alarm,” two of the wires are, guess what?, a normally open (NO) relay, which closes when the alarm is triggered. I think the reason they don’t advertise it is that these smoke alarms are usually part of large networks that connect to a sophisticated central monitoring computer. Think 90 smoke alarms on six floors, all connected to a central panel. They’re not used to selling just one to some dude for a single-unit installation.

The two sides of the relay are two out of the four wires. The other two are for 12 or 24-Volt DC power. This is another great feature: it means that the smoke alarm can be powered from the same terminal strip as the Boat Command base station, the smoke alarm is powered from ship’s power, and you will never have to change batteries or deal with annoying chirping noises.

Here is the base of the smoke alarm, with the + and – terminals being ship’s positive and negative, and the two A terminals being the two sides of the relay. It doesn’t matter which way you connect to the two A’s. I happened to use 4-conductor cable with red, black, yellow, and green wires:

I used this smoke alarm, but again, I’m 98% sure that any four wire smoke alarm will do the same thing. It cost about fifty bucks.

Then I just had to connect the two power wires from the smoke alarm (red and black) to the + and – connections on my Boat Command terminal strip. One side of the relay circuit (the green wire, in my case) also connects to ship’s negative; the other side of the relay (the yellow wire) connects to the orange wire from the Boat Command wiring harness.


I tested it, with real smoke, and yes, a deafening alarm blasted out of the smoke alarm, which was especially blood-curdling within the enclosed confines of my boat, and fourteen seconds later I got a text from Boat Command saying “Smoke Alarm triggered for Condesa.” Now, given that it takes me fifteen minutes to get to my boat from home, it’s hard to say how much good this would do with a real fire, but I’d rather know than not know, and there are people I could reach by phone who are closer than I am.

The Truth About Watermaker Membranes…

Clark June 24th, 2016

…is that they’ve gotten pretty cheap.

In not-too-distant past replacing a single membrane on a small watermaker was a $600-$800 hit. Now, as with so many other things, you can go online and buy a membrane for $150-$220. And it doesn’t matter whether you’re replacing a 20-inch membrane or a 40-inch – the two most common sizes – the price is the same.

Before I go any farther, when a watermaker is performing poorly it is seldom the membrane, but the membrane is the first thing people want to blame. You must first ensure that everything else is within spec before you condemn a membrane. This means that the system must be doing exactly what it’s supposed to do with regard to flows and pressures, and still making crappy water (low quantity or high salinity). Pumps must be pumping the quantity of water they’re supposed to pump, at the right pressures, or water quality and quantity will suffer, even with a perfect membrane.

Membranes don’t just up and fail, or rather, when they do it’s a one in a thousand thing. When they fail they usually decline slowly, over a period of 5-10 years, sometimes longer, or they fail because they were abused (chemical damage, lack of flushing, or lack of pickling…tsk tsk).

Cautionary tale over. $150-$220 for a membrane still isn’t free, especially if you’ve got a system with multiple membranes, but it changes the game somewhat. Say you left your boat in a hurry last season in the Caribbean, and you’re not 100% exactly, positively sure you stored the watermaker properly. You could fly back to the Caribbean armed with various cleaning chemicals, your fingers crossed, and the prospect of buying a membrane anyway, at Caribbean prices, or you could just buy a membrane online, stick it in your baggage, and replace it as a matter of course. Guess work averted.

Likewise with the long term view: At this price you might just replace your membrane(s) after 4-5 years when you suspect they’re fading, and be done with it. An older or fouled membrane can often be brought back among the living by chemical cleaning, but the chemicals can be expensive and the cleaning process can take hours of hands-on time, and soaking overnight, with various buckets and hoses strung about in awkward places.

I don’t mean to encourage gratuitous membrane replacement, filling the worlds landfills with used membranes, but you get the idea. And it’s no sure bet a new membrane will make better water than an older one. There is a lot of variation in membranes, even the exact same part number from the exact same production run, so if you’ve got an older membrane that is still performing well, stick with it. I’ve seen them perform within spec for up to 15 years.

Final caution: new membranes are shipped stored in nasty storage chemicals. The membrane must be flushed for at least 20 minutes to remove the chemicals, or it will be damaged when the system is pressurized. With a new membrane installed, run the system unpressurized for at least 20 minutes before making water.

Final final caution: Membranes don’t have long shelf lives in their packaging. If you’re thinking you’ll just buy a spare membrane to have on hand for a few months or years down the road, this is a bad idea. The membrane will undoubtedly be dead after, say, six months.

Replacing a membrane is quick and straightforward, as long as you’ve got access to the pressure vessel end cap, and room to slide the membrane out. Here is a video on how to do it on a Spectra. The process is similar or identical on other types of pressure vessels. The only thing you really have to remember is to keep the brine seal on the correct side:

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