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Hi John,
I was not till I came to suggestion 13 that I realized how pertinent your article was to boat owners who do not have aluminum boats as there are a lot of aluminum masts and booms out there.
Alchemy was 2+ years old and a fresh water boat when I bought it and within a couple of years I had pulled all fixtures on the mast and had Duralac-ed or Never Seize-ed them. Some of them machine screws were already hard-ish to remove, even in that short period of with favorable conditions. I, of course, missed a few and at a later date (5-6 years later), they needed an impact driver activated with 3-pound sledge hammer to remove and most times the fastener was not re-usable.
I have since suggested to every new boat buyer I know to remove and TefGel (preferred now) all ss to aluminum connections ASAP. Interestingly, those owners who have done so and gotten back to me all report no dis-similar metals protection for the connections on these brand-new often high-end vessels.
Used boat owners should do the same. It would be an item I would check when buying a new boat. It would tell a number of things: the depth of maintenance for sure, but also, I would consider spars where the fixtures are un-removable with regular hand tools as damaged. I would direct a surveyor to check this and might try to parley this into a price reduction, especially if the spars are in need of painting or other immediate attention.
My best, Dick Stevenson, s/v Alchemy
It’s generally wise to assume that on anything aluminum that comes into your possession – whether new or used, and whether it’s a $10 or a $1,000,000 item – whomever last put it together will not have coated the fasteners properly. Even if it’s brand new, the guys who built it probably either don’t know why Tef-Gel is important, or it wasn’t in the spec, or they were in a rush and omitted it. Just budget the time to back them all out and coat them properly.
And I agree with Dick’s recommendation to try backing out fasteners that
thread into aluminum, and to consider the item damaged if they’re seized or corroding, before buying.
Aluminum is a great material, for many reasons. It just requires a different set of construction and maintenance rules than other materials do.
Hi Dick,
Very good suggestions, thank you. That said, I fear that the problem of dry threaded SS fastenings into aluminium is so common that anyone who takes on getting them all out on a boat more than a couple of years old would be taking on a task that could keep them at the wharf for a very long time, rather than cruising. Not sure what the answer is, except to keep beating on mast manufactures about this.
Hi John,
Agreed. I would not wish anyone to stay at the dock or the boatyard doing a non-essential job like that. That said, many PITA jobs lend themselves to being done out cruising as long as one borrows the head-set espoused by the Hippocratic Oath of “doing-no-harm”. There can be lots of “down time” where you can go after a few fasteners at a time. Another similar PITA job that can be done sporadically and pays big benefits down the road is doing wiring and plumbing schematics. For those who stop cruising for the winter, these projects can be excellent winter projects.
My best, Dick Stevenson, s/v Alchemy
Hi Dick,
That makes a lot of sense. I love the Hippocratic Oath parallel.
John
Having brought a 20 year old Ovni 2 years ago l am reading your articles with keen interest. You have confirmed one change l am about to do as the prop shaft bronze packing gland is earthed to the hull. Did not think that was correct so thanks for the very clear clarification.
Effendi did not have a leakage meter at time of purchased. After researching l ended up getting the SeaBis mainly because l could buy it locally in Australia. Yes the website is poor and vague. But unit works well and shows the slightest fault of which Effendi had several which l have solved. Still need to isolate the engine, windlass and bow thruster which all have negative leaks. Though these all have both postive and negative isolation switches when not in use so only a concern when in use.
In regards to isolating the engine. I have a Yanmar 4JH4-TE and the pressure and temp senders are single pin so negative is through the engine. I have contacted Yanmar and they don’t make 2 pin senders. As such any advise on isolating the engine with single pin senders
Thanks Chris
Hi Chris,
Good to hear that the SeaBis is a viable leak meter, thank you.
As to the isolation on the Yanmar, that’s a royal pain in the neck. The best course of action would be to replace the senders with two pin. I know that VDO makes them. Of course here the issue is going to be compatibility with your oil pressure and sea water temp gauges since there are no standards, at least that I have ever been able to find.
I think if I was faced with this I might take a big step and consider replacing the engine panel with one of the new NMEA electronic monitoring systems that will have a spec of what sensors to buy.
On the other hand, In theory, I think but am not certain, if you can totally isolate the engine from the hull the fact that the battery negative is connected to it should not matter since there would be no circuit. Still, if it were my boat, I would be a lot happier if I could get the engine truly isolated from the negative side.
Any thoughts, Matt?
John
I am thinking isolating the engine from the hull only works until you open the seacock as the salt water then completes a circuit. I have plastic seacocks which electrically isolate the engines raw water from the sea when closed. I see this as I have battery isolating switches from both positive and negative. When I close the engines negative switch I get a caution warning on the SeaBis. Then when I open the seacock the warning goes to critical. With the engine off, I can leave the seacock open and open the negative switch and the SeaBis returns to normal.
Thanks for the lead on the VDO senders. Looking in the Yanmar manual it appears they use VDO senders. So I am hoping if I can get the VDO PN off the senders then I should be able to get the 2 pin equivalent. I like the idea of having a negative bus. That would be the solution wiring the new negatives for the senders. Then instead of having a heavy-duty starter solenoid in the negative lead. I would put a battery isolating switch between the starter and the negative bus. I could then simply turn it off after starting. This is following the mantra of “keep it simple”. It seems a workable solution to me. Though what am I missing?
Chris
Hi Chris,
No, that’s not right. The fact that you are seeing a warning when you open the seacock shows that the engine is not in fact isolated from the hull. If it were there would be no circuit and no light on the SeaBis. The path through the water is only one side, it is not a circuit without something on the engine being connected to the hull. See #18 above.
One common place to for this to happen is the gear shift controls, but there are many other possibilities. The easiest way to trouble shoot is to do it with the boat on land and connect an ohm meter between the hull and the engine frame and then trouble shoot until you get infinity.
You need to get this sorted, otherwise any time the seacock is open you will be getting stray current corrosion.
Also, I would use a solenoid in the negative, not a switch: In an emergency when you need the engine right now, you don’t want to be going below to turn on a switch. Good quality solenoids are very reliable, so not a problem.
Hello Chris,
You might have there a mounting bracket where oil pressure switch and sender are installed, that is mounted on rubber pads and fed by a short hose – ie. pressure sensors are not necessarily directly screwed on engine block.
I was lucky to insulate one 6-cyl yanmar engine by just cutting the metal braid on oil hose, so that it didn’t earth the sensor mounting bracket anymore.
There was some reason why we had trouble sourcing suitable insulated pressure sensors, so that bracket insulation was necessary then. But 2-wire temp sensors were easy to find at the time.
Last thing to add was insulated alternator and earthing solenoid for starter motor, and Voilà – insulated Yanmarin.
We did use Bluesea ML 7701 heavy-duty thing for solenoid , but on hindsight, it was a bit overkill. Regular heavy-duty automotive solenoid would have been fine.
It seems that 2-wire pressure senders are readily available now. I cannot remember what was the problem on 2014, perhaps there was a temporary shortage.
Hi JCFlander,
Good suggestions on where to look for the hull short, thank you. And given that I regard being able to start the engine at a moments notice as one of the most important additions to safety we can have, I like your original choice of the Blue Sea solenoid over a cheaper automotive one.
John
I figured I was missing something, opening the seacock complete the other half of the circuit. I also see my logic of using a switch to isolate the starter is flawed. I was thinking that as I have to enter to cabin to turn on the negative switch I would also turn on the starter switch at the same time, then turn if off after starting. Though once I have isolated the engine from the hull I will be able to keep the engine positive and negative isolator switch on at all time while under way. So a big safety plus and a good reason to spend the time and money in isolating the engine. So thanks for helping me to clarify the logic. And thanks for the advice on trouble shooting on land. It is very timely as Effendi will be lifted out next week.
JCFlander
Thanks for the information on how you isolated and the source for senders. And I see they are in Australia so a bonus.
Chris
I’m a bit surprised that you got what I meant 🙂 I really need to wait before posting, I seem to have a tendency to encrypt myself with plain words… :d
Anyway, I managed to find two pictures of the thing:
– First: mounting bracket. And yes, this was Yanmar gray before blasting. From left to right, lo press switch, press sender, and starter solenoid (which is optional).
http://aijaa.com/pBLZdg
– Then, oil hose, on “as mutilated” state. Doesn’t win prices, but wins a day…
http://aijaa.com/aqjkCq
HTH, Cheers.
Our new Ovni 395 is currently being made at Alubat for launch in May 2018. Very useful, interesting and pertinent article. Going to have a busy summer refining the commissioning!
Thanks for sharing your thoughts. Looking forward to Part-3.
Hi David,
Glad the timing was good. I have had a rough outline of this piece for a year and was hesitating to go any further with it because I feared it was too specialized, but judging from comments and members sign ups, it seems that it was worth the effort. It’s great to have comments like your to confirm that.
Not all aluminum is created equally.
https://www.proboat.com/2014/11/jade-resurrected/
Hi Chris! Like you I have Yanmar 4JH (whithout any Turbo). It has done 10 000 hours and is fine. Normally this engine will do 22 000 hours. The electric system is not good and made for cars. The starter and its panel is the main problem. I have tried to improve it, but that is difficult. Several oil engineers have given me their solution: They build a stainless steel sink anode box on the hot water pipe line system of the Yanmar. The hot water of the Yanmar is connected to a heating system with floor heating, hot water tapping tank, oil heater for water, electric heating and a 10 000 W truck heater. The sink anode is in this system. The sink must be checked regularly. This is not a perfect solution, but much better than nothing at all.
The perfect solution is of course to have no electricity on the engine and start it by compressed air. I have a Callesen built to expedition standard this way. It has made 130 000 hours and is fine.
To John: Using plywood as isolation material seems a bit original to me: Most ship yards I know use nylon plates. The do not lead electricity and if wet by salt water, they expande a little and their by prevent any leak. Their life time is endless.
Hi Svein,
I don’t really see how adding a “sink anode” to the engine is going to solve Chris’ problem. It’s not going to protect the hull.
And, sure, plastic is a better insulator than wood, but what I wrote was “In theory, damp wood will pass a bit of electricity, but I have never had a problem with this, probably because we paint the ply. If you are worried, you could substitute G10 for the ply.”
G10 is plastic, albeit glass reinforced.
That said, ply is easier to source and work with when building mounting brackets and we check for leakage down to a few thousandths of an amp and see no problems.
Hi John, I had a talk with Christian Jonsson on this matter sa few years ago. Christian is a boat builder in Denmark and has worked with many expedition ships with success. He showed me several 10 years old marine plywood parts used as insolation plates. The plywood was originally of the highest quality. After 10 years they where all soft in the corners. Christian said that humidity will attac the marine plywood and especially in the corners. After 10years use they should be changed. Christian advised me to use nylon instead of plywood. I use nylon. Nylon has no such problem and has an undefined life time. Checking the plates when out sailing is something I do not do. It is done when at home again. That is why a do not like amateur solutions. They can lead you into trouble. I also advocate starting a diesel without electricity and use the simple way by compressed air. Almost any commercial engine is started this way because of several good reasons.
Hi Svein,
I enjoy your contributions. I guess I knew about diesels being started by compressed air, but I never think about all the different variations of doing things and it is good to be reminded.
Thanks, Dick
I’m looking forward to Part 3, where we’ll get some ideas on why we should want aluminum boats. After reading Parts 1 and 2, I’m filled with more than a little panic — because I already have an aluminum boat. Of course, all of us with such boats should be mindful of the main ideas, and John gives us much practical detail that is hard to get elsewhere. While I let the Valium go to work to help me recover from John’s reminders, I have a question and an observation.
My question is this: Would any of this matter for a boat with a dry exhaust system? That’s what I’ve got: no seawater circulating into the hull to cool the engine (the coolant circulates in a “keel cooler,” which is actually the skeg). Now, if understand where electrons are flowing, this means that such a boat would not have the problem of dissimilar metals inside the engine posing a corrosion threat to the hull. Have I got that right? Now, there are other potential pathways between engine and the sea. If the engine (and every last wire attached to it directly or indirectly) isn’t completely isolated, I guess that having a dry exhaust/keel cooler is no panacea. What are the advantages of such a system (with respect to hull corrosion)? (I’ve mentioned some other advantages of such a system, unrelated to corrosion, in replies to other articles.)
My observation is this: I want perfection. John’s boat seems to be as near as one can get to that realistically. But it does make me wonder what the zincs are for. If I interpret John’s approach, the zincs are a tertiary backup system to rely on when all else fails. That’s one approach. But I wonder if that’s effectively missing out on the benefits of zincs. They are “sacrificial anodes” after all. The idea is that the zincs are part of the “circuit” of dissimilar metals. If the zincs are working as intended, they will protect more noble/cathodic metals in the circuit, including aluminum (the hull). If so, isn’t seeking perfection — which is very costly indeed if you start pricing up all the ways to do it, and very costly in time, too — bordering on extreme and, beyond a certain point, unnecessary IF one has properly installed zincs? I’m not at all suggesting that we should not aim for perfection — I lose sleep about this stuff, so I worry about it all the time. However, a zinc is a zinc. It’s there for boat’s that aren’t perfect (which I guess is nearly all boats). Most folks seem to replace zincs annually, with some noticeable-but-not-extreme erosion. That ought to be doing all of the work of protecting the hull, takes little time, and isn’t very costly in the big scheme of things (but good-quality zincs ain’t cheap; cheap zincs are, well, cheap). Naturally, if a zinc wastes away hugely in a year (and maybe if not at all), there’s a problem that needs to be sorted. But if the zinc erodes, say, by a quarter, should I be losing sleep and pricing an isolated ground alternator, etc.?
Hi Paul,
Wow lots to strain my brain first thing in the morning!
Let’s see…I had not really thought about keel coolers much, but my first take is that the engine should still be isolated, both from the hull and the electrical system. My thinking is that if the engine is connected to the hull then a battery will be formed by the more noble metals in the engine (steel, copper, etc) and the aluminium in the keel cooler, with the circuit being completed by the cooling water. That said, I don’t know how conductive fresh cooling water with anti-freeze is. But assuming that it is fairly conductive, then we definitely want the engine isolated to break the circuit, particularly since there are no anodes in this circuit to protect the aluminium.
As to perfection, I cover that a bit in part three, but the short version is that you are right, a lot of what anodes do is come into play if something goes wrong. So I think your approach is good: aim for perfection, but have good anodes as a backup. Over the years on at least a couple of occasions, we have had a stray current problem that took a while to fix and in these cases the anodes showed much more wastage than normal but no harm was done to the hull.
Many thanks John. What I don’t quite understand is to what extent engine coolant might act as an “electrolyte” of sorts. I think it might NOT do that, with nitrite in the coolant being an aluminum protectant of some sort. If so, even though the coolant is in contact with the hull, it ought not be a problem (but I could be totally wrong about this). The electrical system was built on top of the sprayed foam, so it’s isolated from the hull (assuming no shorts, of course). However, the engine is not isolated from the sea: it connects to the sea via the prop shaft, which itself is nominally isolated from the hull on the shaft bearing. I don’t know to what extent this might threaten the engine, but it has gone a decade like this. If there’s a problem, it’s a slow-acting one. Assuming the engine is still isolated (I’ll do checks when I get back to the boat in March), the hull should be OK. I guess all of this makes me moderately worried about this issue, leaving a small surplus of worry to apply to other things (there are so many things to worry about that I need to spread the worry rather thinly on each item!).
I guess one thing for aluminum-boat lovers to consider is whether a keel-cooler system might help out with the overall project of isolation. If my understanding is correct, it’s a positive in this respect.
Hi Paul,
I don’t think there’s any need to over complicate this. The fundamentals are that if the coolant running in the circuit conducts electricity and the engine is connected to the hull, current will flow and any aluminium that the coolant is in contact with will be damaged. So the bottom line is simply how conductive is the coolant? (Because it is made of the more nobble metals the the engine will not be damaged.)
Anyway, if it were me, I would make sure that the engine is isolate from the hull, and also isolate the shaft from the engine for the reasons I detail in the post above.
And yes, I know exactly what you mean about worry!
Thanks again John. Regarding isolating the shaft, now that’s a tricky one. I wonder if anyone else reading this has a system similar to mine. I have a direct connection between shaft and transmission. I’m concerned that if I were to install a Drivesaver there might be a disaster if the Drivesaver were to disintegrate, which is what it’s supposed to do (if I understand its function correctly). The consequence could be the prop hitting the rudder (because my boat’s single rudder is directly behind the prop).
Do you carry a spare Drivesaver in case it fails? I assume that you do.
Hi Paul & John
I put a zinc on the drive shaft inside the boat as it exits the boat (and the boat is fiberglass). This limits the loss of the shaft.
I like the idea of the drivesaver. Thanks!
Best wishes,
Charles
Charles L Starke MD
s/v Dawnpiper
Thanks Charles. Indeed, on my previous boat (a Pacific Seacraft) I put a couple of beefy hose clamps on the shaft forward of the shaft seal to keep the shaft inside the boat in extremis. I don’t know why I didn’t remember that when replying to John previously.
I have been digging around online (for what it’s worth) to learn more about Drivesaver-type devices. There’s at least one other on the market that is designed to keep the shaft connected to the gearbox if the plastic bit fails as designed. Just going on this (and not having any experience at all with the quality of these things), I would be inclined to go with something like that instead of a “Drivesaver” brand per se. (Paradoxically, all of the instructions for these devices describe how to ADD a grounding strap between engine and shaft. Of course the metal boat owner wouldn’t need or want to do that.)
Hi Paul,
Yes, we carry a spare Drivesaver and we also, Like Charles, have a collar on the shaft inside the boat and just forward of the dripless seal. We use this one:http://www.pyiinc.com/shaft-retention-collar.html
Keep in mind that it would take a hell of a prop impact to shear the Drivesaver and in that case we are going to be a lot happier if it shears than if the drive line is irrevocably damaged.
Thanks John. Regarding the shaft collar and PYI, they also sell the alternative to the Drivesaver I mentioned previously.
I will install one of the flexible couplers unless there’s an obstacle. It’s just a matter of finding one that will fit (my gearbox is commercial grade so less common). It’ll put the prop closer to the rudder, but I doubt the effect will be significantly adverse. One thing I cannot work out (revealing my ignorance) is what the shear key on the shaft is meant to do. I always thought that the key was there to shear away, thus protecting the engine-gearbox from a stopped shaft due to prop strike, etc. But I think I have this all wrong.
What I like about these couplers is that they (in principle) provide some vibration absorption in addition to isolating the shaft — that is, they do three things: isolate, absorb a bit of vibration and disintegrate if needed. (I say “if needed” because I once met a sailor who had one of these things fail in the Med, possibly due to incorrect bolt tension. He needed to get a tow because there was no wind.)
Hi John
Is it easy and worthwhile to install a flexible shaft coupling on my engine without the need for electrical isolation (Fibreglass Trintella 47)? Is it a job I can do myself?
PYI and DriveSaver both have models. Is one better or recommended over the other? PYI looks much more substantial than Drivesaver. Thanks.
http://www.pyiinc.com/flexible-shaft-couplings.html
Bruntons has one that serves as a drive thrust bearing and coupling:
http://www.bruntonspropellers.com/sigmadrive/
Best wishes,
Charles
Charles L Starke MD
s/v Dawnpiper
Hi Charles. I have nearly the same questions — which means I cannot answer them from any experience, but I can share my thoughts. Like you, I came to the conclusion (from reading online only) that the PYI-marketed coupling appears to be more substantial. It seems to also have the advantage of (in principle) hanging on to the shaft if it fails, whereas the Drivesaver would require another method to hang on to the shaft. As for isolation, if you don’t want that, the PYI device has an optional connection.
I didn’t know about the Brunton’s device. Now that looks great in principle (and very costly?). Depending on space behind your prop, it might require shortening the shaft (not a do-it-yourself job, unless you have a lathe et al.). From the look of it on their website, it does not appear to provide any isolation. Otherwise it looks cool — but maybe too cool? All I can think about is how long it is likely to last, and what happens when it fails, or whether it’s designed to fail (like the polymer flexible couplings).
I would like to have a way to reconnect the shaft directly to the engine if (when?) the coupling fails. With the polymer couplings, I think this could be done “easily” while the boat is in the water (but it would require fiddling with the shaft seal so that water doesn’t flood in when the shaft is moved forward for recoupling to the transmission). Carrying a spare is an obvious option, but it’d be nice to be able to do a direct connection in extreme circumstances.
That’s my thinking so far. Right now, I’m inclined toward the PYI first, Drivesaver second. I’d like to hear more about actual experience with these couplings.
Best,
Paul
Hi Charles and Paul,
We have both a Drivesaver and an Aquadrive (equivalent of the Bruntons), but I think I would say that installing either just for the sake of it is covered under that old piece of wisdom: “if it’s not broken, don’t fix it”.
(We need the Drivesaver for isolation and installed the Aquadrive as part of a repower. Not sure if I would bother with the latter if doing it again.)
As to which product is best, since I have only used a Drivesaver, I can’t tell you.
What I would say is that I have used Drivesavers for 30 years and never had a failure. Also, there’s a simple answer to the issue of what to do if it does fail: carry a spare, as we do.
If I understand the Bruntons product that Charles is referring to, it can be used like a Drivesaver (one unit) or like an Aquadrive (more than one unit plus a thrust bearing). If I understand John’s system, the Drivesaver isn’t doing nearly as much work as it would in a system without an Aquadrive. It might not be experiencing any stress at all apart from that arising from rotation — none fore-aft or (what’s the word?) wobbling due to slight misalignment. If I’m right, John’s Drivesaver ought to last forever because it has a very easy job to do — it’s not being asked to do nearly what it was designed to do. If I’m right, its longevity may not be indicative of the potential longevity of an installation in which the Drivesaver connects the transmission directly to a prop shaft and thus gets all the stresses in all different directions. It’s that taking of multiple stresses, in addition to isolation, that is the potential value of such a device. And I guess the inclination of the device to fail so as to protect the drive train is useful, too.
I would wager that John’s Drivesaver has helped to protect his Aquadrive, and vice-versa, from stresses. But going this way has its own challenges, as John implies. (I think I mentioned in another reply a Drivesaver disaster on an aluminum sailboat. Getting the installation just right, and the bolts torqued precisely, seems to be vital to success.)
Hi Paul,
Drivesavers aren’t that fragile. We have only had an Aquadrive since the last re-power in 2010. Prior to that I have had Drivesavers for some 25 years on two boats without a failure. That includes a period of four years where we had a slightly bent shaft and several prop fouling events, together with the shock loads from a Maxprop. Also, I have to admit that I have never been that great about perfectly torquing the bolts either. Bottom line, it’s a rugged piece of kit.
Not trying to sell anyone on Drivesavers, but if you decide you want one, go ahead, buy a spare, and don’t worry about it.
Hi John,
Concerning your #15 and #16; why should the voltage of the shaft be at least 0.03V lower than that of the hull? Thank you for your help!
Kind regards,
Wouter
Hi Wouter,
The lower natural voltage of the bronze and stainless steel of the shaft and prop, drags the zinc’s potential down a bit more than an aluminum hull does, so we get a difference in voltage. If the voltage is the same, that indicates that the shaft has somehow become connected to the hull and that needs to be fixed.
Hi John,
I always had between -1.11 and -1.04 voltage on the hull and between -0.93 and -0.99 on the propshaft. The last time I measured I got -0.51 on the propshaft (the hull was at -1.11). I dried out the boat to check the zinc anode on the prop. Took it of and made sure it was properly back on again and making good surface contact. But I am still getting the -0.51V reading. Any thoughts? Regards, Wouter
Hi Wouter,
Clearly something is wrong, but it is perplexing.
Have you checked the metre and silver chloride anode? My anode went bad a couple of years ago and I got all kinds of screwy readings before I figured it out. Try it against a piece of zinc over the side and see if you get a sensible reading. That said, since you are still getting the same reading on the boat that is probably not it.
Also, did you by any chance move to fresh water?
Another thought is stray current from your batteries screwing things up. So disconnect cables from all batteries and try again, to see if that changes anything.
Also, I think that it’s just theoretically possible that there is a strong current running from another source back to the ground on the wharf, so try changing locations and see if that helps.
If none of that works, please leave another comment and I will think again.
Hi John,
My bad! I was measuring from the metal ring of the pss shaft seal instead of the shaft! Got -1.00V on the shaft, 0.10 less than the hull. Apologies for wasting your time!
Hi John, I was told that aluminum boats cannot be docked near to steel boats for a long time and if that happens you need to find another spot. It’s ok for a short time, but for a full season, it will be a problem. How true is this?
Also I have a marina with corrugated metal walls all around it, below boardwalks, rusting away. Do you think it is safe to keep aluminum boat in such an environment?
Hi Rusian,
As long as the aluminium boat remains isolated from the steel (wharf or boat) there should not be a problem (see part 1). The key point is that corrosion requires a circuit to be completed. (See #18 above).
That said, I think one may need to be careful of rafting with steel boats that are running an impressment system (see part 1). In this case wet dock lines attached to the steel boat could complete a circuit, particularly if they are salty.
Bottom line stray current corrosion (like the example above) is way more of an issue to worry about than galvanic (voltage difference between steel and aluminium). Also the latter can be managed with good zincs and checking with a meter (tips in the series) for years, but the former can do damage in weeks.
Got it. Thank you John.
Hi John and Rusian,
Am I correct in saying that once a boat is plugged in to shore power, the boat are no longer isolated?
My best, Dick
Hi Dick,
Depends on whether or not she has an isolation transformer, and all metal boats should. See tips in part 1.
When I was checking out Boreal, it had big isolation transformer to prevent that from happening. There are also small ones that ppl sell for GRP boats.
Ahh, yes, forgot about those. Thanks, Dick
My 6 month old Ovni395 was fitted with a MasterVolt isolation transformer by the boat yard Alubat.
Out of curiosity, may I ask which model? We have a 40 foot steel yacht and the relatively compact Mastervolt GI 3.5 model seems well-matched to a 30 A/120 VAC boat accepting a 30 A service (although I can take 60 A, we never do). It’s this model: https://www.fisheriessupply.com/mastervolt-isolation-transformer-88000355
Thanks.
Marc
Alubat fitted the GI 3.5 along with a MasterVolt combi inverter/charger and other interfaces. My only input was to specificy MasterVolt and Alubat decided on the detailed spec. Alubat probably have more accumulated knowledge of wiring metal boats than anyone else. We have only had the boat 6 months and anode wear seems “normal” although we are not heavy users of shore power (D400 and solar keeps up with requirements). I periodically test the hull potential with a test silver-carbide anode and it reads -1000mV (in the non-fizz zone) in the marina but drops to -800mV (fizz!) at anchor or on a mooring chain which I have not got my head around yet; anyway, that is nothing to do with transformer. Hope that helps.
Very much so! Thanks for the field-testing information. We will have largely a similar setup. I concur the fizz zone reading is a bit puzzling, unless there’s some impresssive underwater power cable near your mooring. The silver test rig is on my purchase list as I’ve confirmed to my own satisfaction that the end of the dock finger where we are at is not a particular problem. I need to go from freshwater magnesium anodes to zinc for the trip to Nova Scotia next summer. Good luck with your Ovni…they are beautiful boats.
Hi Marc,
If you instal aluminium anodes, you will not need to change when you get to salt, they work fine in both salt and fresh.
Hi David,
800 mv sounds a bit low and indicates either that you are under zinced a bit, or the zincs are not properly connected or scaled up. Either way, you should get to the bottom of that.
Also, there is no good reason that the marina reading should change from the at anchor reading. Given that, I would check that your isolation transformer is in fact installed properly. Or to put it another way, if it’s installed properly, there should be no “fizz zone”. (Hot docks are largely a myth.) See tips #9 and #10. I would also check for a stray current leak on your own boat.
As to Alubat’s experience, while it’s true they have a lot of it, I’m seeing and hearing about quite a few installation mistakes on their boats. I have also seen two Ovnis with bad plate pitting indicating a screw up somewhere.
In summary: trust, but verify.
Thanks John and all points noted. I have been putting the -800mV readings down to the proximity of a mooring/anchor chaim or local conditions; currently they are -1000mV in our long term marina. I will check the isolation transformer as you describe and continue monitoring readings; still on a learning curve. BTW, would aluminum anodes work on an aluminium hull?
David
Hi David,
As a general rule, the proximity, or otherwise, of ferrous will not make a difference to the readings you get so you really do need to figure out why you are getting this lower reading. One worth while experiment is to get one of the “fish” type anodes that you hang over the side and see what effect that has. My guess is that the voltage will go up when you attach it to the hull.
And yes, aluminium anodes work on an aluminium hull. Counter intuitive I know, but the secret is that the anodes are made from a much more active alloy than the hull.
Hello John,
This is my first comment on your site. Great site! These three articles on care of aluminum boats are worth much more than a year’s subscription.
I would just like to make a couple comments about the SeaBis leak detector. I recently purchased one and athough recommended to connect permanently and to be left on at all times, I installed a two-pole switch to enable me to turn it off/on at my discretion as it leaks almost 0.1 mA on both + and – when turned on. I have solar panels and am using a good PWM charge controller but whenever the batteries reach the preset “float” setting of the charge controller, the SeaBis unit starts flashing it’s red AC alarm light as well as the green + and green – normal lights. I have done a lot of testing with another controller, changing float settings and even without a controller (direct to batteries) to pin down the cause. The seller doesn’t fully agree but doesn’t dispute my conclusion either that the SeaBis unit is recognizing a slight pulsating DC current as AC. Not a big issue but false warnings can be concerning if one is not aware of the reason. Just wanted to pass that on as I see someone else in these comments using one as well.
Hi Wayne,
Just the kind of real world report that can save someone else endless heartache, thank you.
I’m in the process of repowering my aluminium yacht, and have found your material very helpful, many thanks!
I’m planning on using a Python Drive which will enable me to achieve a slightly better alignment with existing engine bed and hopefully have the side benefit of reduced vibrations.
The conundrum I’m facing at the moment is achieving electrical isolation of the shaft from the engine (I have an alu yacht). The Drivesaver suggestion you’ve made is neat — but very expensive, about $400USD including shipping for our situation! I was wondering if you have any other ideas of achieving this isolation at lower cost? R&D flex couplings are much cheaper, but have been warned that this may provide too much flex in the system. I imagine the coating systems used to achieve isolation in flanged piping systems would be compromised by the strong forces on the bolts in this application. Although, ultimately, I might try that (at near zero cost) and if it fails rely on carefully keeping the engine isolated from DC neg.
Any thoughts?
Hi Klaas,
Sorry, I really can’t see any other way to maintain reliable isolation. I guess I’m kind of surprised that you would spend the money on a Python Drive and then baulk at the DriveSaver if for no other reason that the latter could save the former from damage in a fouling situation.
Hi John,
Thanks for your quick reply. I thought as much. In 3000 hours of motoring we haven’t fouled anything dramatically so I consider it fairly low risk (except now that I’ve said that it’s bound to happen). For our setup, and with shipping, the python drive is $1030USD and the drive saver $410USD… So it’s a substantial extra cost. An R&D flex coupling would be $165USD, but I’ve been warned against that…
I enjoy reading all the advice on your website and have obtained lots of good tips. However for many of your articles I have to see your advice through the lens of my own budget and prioritise the most important aspects!
Cheers,
klaas
John,
I read somewhere that there is an ABYC requirement that the shaft must be grounded. Can you comment on this. I have recently checked with the ohm meter and see that the shaft on my OVNI 435 is not isolated from the hull. These boats normally come with MAXPROP propellors and are notorious for eating up anodes.
I’m considering fitting a drivesaver or RandD coupling
Hi Garry,
I’m not an expert on ABYC, but I can say that connecting the shaft to the hull on a metal boat is a not a good idea. Basically by so doing, we are creating a battery with bronze and stainless steel on one side and aluminium (or zinc for as long as the anodes last) on the other.
Also, did you check the reading out of the water? If you were in the water, you will read the resistance of salt water.
Anyway, as stated in the above articles, I strongly recommend isolating the shaft with a drivesaver, or similar. I also cover how to check that in the water.
John,
I agree totally with your analysis, I hadn’t done the test with a half cell, just the ohm meter, but I’d expected to get a few ohms showing up if it was just seawater. I’ll investigate further when I get my new half cell. I’ve been using a bit of silver wire dipped in bleach, not great.. How’s that for a cheapskate.
Re the ABYC ruling about connecting the shaft to the hull, I can’t find the link to where I’d read it, and was wondering if this was one of those situations like floating DC where you and Steve agree to disagre. Also I’ve since found more specific Abyc regulation saying that metal hulled boats should indeed be shaft isolated, presumably the original post was talking about non metal hulls.. I’ll investigate more on how my Ovni was built, but I’m thinking about a Drivesaver install would be a start to help with my excessive (to my limited mind) anode consumption.
Am I correct in thinking that on a aluminium hull the prop anode is to protect the hull/shaft tube. But on a non metal hull the anode is to protect the prop itself?
Hi Garry,
Yes, I’m pretty sure the drive saver will reduce your zinc use.
On the purpose of the zinc on the prop or shaft. No, it does not protect the shaft tunnel, just the prop. For a zinc to protect something it must be connected to it with as close to zero resistance as can be done. Therefore you still need hull zincs.
I have seen situations where the shaft is connected to the boat’s bond/ground system with brushes. The idea here is to let the shaft/prop zinc protect all the submersed metal that is bonded. It’s not a technique that I favour.