Why Are Saildrives Even A Thing?

OK, I admit it, that’s a bit of a clickbait title.

And, yes, I do understand that saildrives are:

  • Much easier to install than a conventional driveline.
  • Give the designer more flexibility in siting the engine.
  • Reduce vibration and noise, at least potentially.
  • Are more efficient than most drivelines since the prop shaft is horizontal.

But…you just knew there was going to be a but, didn’t you? Actually, there are two buts, and they are doozies…when we buy a boat with a saildrive:

#1 We just bought an aluminum boat:

Yup, if we want that aluminum saildrive housing under our boat to last any length of time, we should carefully observe at least half of my 27 Aluminum Boat Care Tips.

For example, spend a month in a marina plugged in to shore power without an isolation transformer—I’m not convinced about galvanic isolator reliability or efficacy in this application—and our saildrive may easily become Swiss cheese. Why? Because it’s been acting as a sacrificial anode for every boat in the marina with a screwed-up electrical system…which is a lot of ’em.

Also, to really take care of a big chunk of aluminum in the water properly, the boat’s DC system should be fully isolated, not a practical thing to do on a boat that was not originally set up that way.

What a bummer: We now have most all of the drawbacks of an aluminum boat, and none of the advantages.

#2 We just bought another battery:

Yup, again. Combine an aluminum housing with a stainless steel shaft and a bronze prop—sailors want folding or feathering props, which are made of bronze—in salt water, and what do you get? A battery.

Wait, it gets worse. On aluminum boats with conventional drivelines we deal with this problem by isolating the shaft from the rest of the boat (see tip #15), but there is no way, at least that I can see, to do that on a saildrive.

So this thing is going to munch zincs like a hungry gorilla munches bananas. And what happens when the zincs are gone? Yup, the aluminum housing is next on the electrical gorilla’s menu.

Wait, it gets even worse. If any of the seals in this thing fail and water gets in, things will really start to perk, since there are a bunch of parts in there made of stainless steel and bronze.

That Bad?

So, am I being overly dramatic here? (Who, me? When has that ever happened?)

Anyway, I think not in this case. As most of you know, I have been looking at boat listings lately and have seen at least two boats where the specification says “new saildrive in year x”, with x being a different year than the boat was built. And a broker I was chatting with mentioned two trashed saildrives in passing. Conclusive? No, but a disturbing indicator.

Always Bad?

So, being an even-handed kind of a guy—OK, not really, but I’m working on it—I tried to come up with a situation where a saildrive made sense on a cruising boat.

After much furrowing of my already wrinkled forehead, all I could come up with was, it might make sense on an aluminum boat, since at least the owner will be able to care for the thing properly, without any additional work or knowledge…that is, until there is a leak in the seal.

That said, it’s still not a great solution since I can’t see any way to isolate the saildrive from the hull, so we will be connecting a bronze prop to our aluminum boat—be prepared to go through zincs fast. And if we miss that all our hulls zincs are used up, the hull is next.

On the bright side, we do get rid of the shaft tunnel, which can be a vulnerable spot on aluminum boats.

Back to the dark side. What about the bolts that hold the saildrive down? How do we do that without passing a stainless bolt through our aluminum hull? I don’t know.

Summary

Oh dear, that was not a great attempt at being even handed. Might as well just go for it:

Over my 50+ years doing this stuff, I have seen some seriously bad ideas foisted on us poor boat owners by the marine industry as they make their own lives easier. At least to me, the saildrive comes close to the top of that list of infamy.

Wait…There’s More

So, after reading the above, it would be logical to assume that I would not even consider buying a boat with a saildrive. And that was certainly my first conclusion.

But, having thought about it some more, I realized that, since there are so few really good boats out there that come even close to meeting Phyllis’s and my needs going forward, rejecting all boats with saildrives, or twin rudders, or any of my other pet hates would be a bad idea and contradict my own fundamental thinking on buying a boat.

Why is this stuff never simple?

Update July 2019

If you are still considering a boat with a saildrive, or have one, I strongly recommend reading Ed Sherman’s excellent article on maintaining them over at Cruising World. Thanks to Steve D’Antonio for pointing us to it.

Key take away, saildrive owners need to plan to take the engine and saildrive out of the boat every six years to replace the “bellows” seal that keeps the boat from sinking.

Update July 2022

Well, Phyllis and I ended up with a boat with a saildrive. But that does not mean that I have changed my opinion of the damned things.

While researching how I need to care for this abomination that the marine industry foisted on us boat owners I discover this service bulletin from Yanmar on suggested care.

If reading it does not make you blood boil, as it did mine, you are a saint…or not paying attention.

Just one example,

When the boat is first placed in the water, is moved to a new marina or mooring or if new equipment is installed on the boat that is connected to the cathodic bonding system, it is recommended that the boat owner inspect the condition of the sacrificial anodes every 30 to 60 days to determine the rate of consumption of the anodes.

Yanmar Marine service Advisory

Seriously, we need to jump overboard to check our saildrive once a month? And how many boat owners will do that in the marina and thereby risk death from electrocution?

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Eric Klem

Hi John,

I see saildrives as one solution available to designers. All things being equal, I would not want one as the issues outweigh the benefits for me. However, because designing always involves compromises so things are not equal, I do think that they have their place. If you took them away as an option, you would also take away a lot of hull shapes and interior arrangements. The danger is that it is easy for the designer to include a saildrive potentially allowing other desirable traits without having to answer for all of its shortcomings down the line. Balancing all of these traits and thinking about the overall design is really important and if boat design is anything like the design world I work in, people miss the mark a lot. In the engineering world, it is well known that in-house engineers tend to think about the product lifecycle much better than contract engineering companies which don’t have the regular exposure to long-term issues and generally don’t have to answer for them. I don’t have a ton of experience with sail drives but it does seem that many are not installed correctly. To me, the solution probably lies somewhere in the saildrive maker designing to allow proper isolation, documenting requirements for the designer and builder, the designer doing a fully defined design that includes those provisions and then the builder following the design exactly (and a little testing once launched would be a great QC step too). As we discussed elsewhere, the QC is not there right now which makes used equipment a major liability.

On our boat, Ray Wall decided that the correct compromise was a V-drive. Of course, I would still prefer to have an inline drive when only considering the drive but when the boat is looked at overall, I think he made the right call.

Eric

Kenneth McCallum

John,

Instead of selling a boat that you both truly enjoy due to advancing age related issues, why not consider what we’ve seem many people doing over here in the Med. Hire a seasonal skipper, over here in the Med they cost hardly anything above room and board. Then you can kick back and enjoy doing what you can still do onboard well into your 90’s… yes a good friend is 92 and still sailing his 54ft wooden boat purchased in 1978 with the help of his companion. It’ll leave time for your online business and save all the trouble of selling and buying a boat.

Cheers

Ken

Richard Elder

Hi Ken
Thanks for the morning laugh. I can just see John and Phyllis, after spending 25 years refining MC to exactly suit their preferences—- sequentially firing (for cause!) every professional skipper I’ve ever come across!

Ernest E Vogelsinger

I just had the same thought when reading Kens suggestion but I didn’t dare to come out with it here *giggle*

Robert Newman

Good grief. I agree. Me no unnerstan. Why are sail drive housings not available in some mix of fibreglass. Or monel. Or silicon bronze. I think there was a British company doing them in bronze. Are they now not? Plus other frustrated words.

Christian Henry

I agree that saildrives are less than ideal… Combine that with the fact that we have two of them on our Lagoon 52 catamaran and I’m always worrying about them. However, my wife is much happier with all of the space we have as a result of the engine placement at the stern so I’m stuck. I’ve had the boat only a few years and I’ve already replaced one of them (Yanmar SD60) and I’ve already put the money aside to replace the other one as well. Completely agree that they are “expendable”, which means that we need to be within a 1,000 miles of somewhere at all times.

Rob Gill

Hi Christian, WRT the 1,000 NM – we cruised in company with a Lagoon 50+ in the Pacific which had the Saildrives. After dropping a spectra rope tail overboard that was swallowed by (and destroyed) one of the Saildrive units, they subsequently cruised with a complete spare. They also had an air bag made that they could secure under the stern of one hull, manually inflate with their dinghy pump, then once clear of the water, remove and replace the unit without hauling! Clever guy (engineer).
I’m with your wife on the space for cruising on Lagoons, so if she’s out there with you sailing, then Saildrives are a small price to pay – yes?
Br. Rob

Paul Browning

Goodness me Christian, how much room do you need if you can’t find enough on a Lagoon 52 without saildrives? You guys seriously need to declutter! LOL 🙂

Thanks for the article John, which takes my reservations about saildrives to a whole new level approaching paranoia. Previously my concern was largely limited to the whacking great hole they cut in the bottom of the boat to fit them, as well as their comparative flimsiness in light of modern dyneema and kevlar etc ropes, which are all strong enough to snap ‘em off. But now you’ve added batteries and sacrificial anodes for a whole marina to the mix!

Paul Browning

Goodness me Christian, how much room do you need if you can’t find enough on a Lagoon 52 without saildrives? You guys seriously need to declutter! LOL 🙂

Thanks for the article John, which takes my reservations about saildrives to a whole new level approaching paranoia. Previously my concern was largely limited to the whacking great hole they cut in the bottom of the boat to fit them as well as their comparative flimsiness in light of modern dyneema and kevlar etc ropes, which are all strong enough to snap em off. But now you’ve added batteries and sacrificial anodes for a whole marina to the mix!

Steve D

John:

Another worthy subject. I share your consternation with sail drives, and so do many others who have experienced first hand their susceptibility to both galvanic and stray current corrosion. This article https://stevedmarineconsulting.com/unraveling-the-corrosion-mystery/ has a photo of one I encountered after it lost a stray current battle. Not surprisingly, there have been a few lawsuits against manufactures…

You said, “That said, it’s still not a great solution since I can’t see any way to isolate the saildrive from the hull, so we will be connecting a bronze prop to our aluminum boat—be prepared to go through zincs fast. And if we miss that all our hulls zincs are used up, the hull is next.”
The speed at which the anode is consumed is a function of its size/current relative to the protected metal, the prop is pretty small compared to the hull, however, if only very small areas of aluminum hull are exposed, inside a shaft or rudder log for instance, then that pushes the equation back in favor of the prop winning and hull losing. It’s worth noting, aluminum is amphoteric, this is the only common boat metal that can be damaged by over-protection, i.e. too much anode (zinc, aluminum or magnesium) is as bad as too little. And, the prop is attached to the sail drive via an oil filled medium, which is actually a pretty poor conductor, so again while I am no fan of sail drives, the likelihood of the prop affecting an aluminum hull is not as high as you might expect, you need a really low resistance connection to make galvanic corrosion work, and it’s just not there. I believe most sail drives come to grief by lack of anode maintenance, lack of paint maintenance, and stray current corrosion.

You said, “Why? Because it’s been acting as a sacrificial anode for every boat in the marina with a screwed-up electrical system…which is a lot of ’em.”
While leaking current can be an issue, none of the surrounding boats need to have screwed up electrical systems to adversely affect a sail drive, they only need to have depleted anodes, and no galvanic isolators or transformers, whereupon the sail drive anodes, and then sail drive itself becomes the next least noble metal in the dock “circuit”. I just wrote a two part article for Cruising World, not yet published, on the subject of “Hot Marinas”.

All of these issues similarly affect power boat stern drives.

You did leave out one of a sail drives other un-attributes, the huge rubber boot that is supposed to keep the ocean on the right side of the hull. Where else on a boat do we willingly rely on a large hunk of rubber sheet to keep the water out?

Steve D

John:

Totally agreed on the approach, having a sail drive, just like owning an aluminum hull, means you need to be corrosion savvy, and most sail drive owners haven’t a clue. The same is true, once again, for stern drives.

“As to the zincs on the saildrive, and the connection to the prop, you are probably right. That said all the noble metals in the engine will be very well connected to the aluminium case on the saildrive so that will erode quickly just as soon as the zinc is gone.”

The engine’s cooling system metals have no affect on any metals outside the hull, sail drive and otherwise, because they are immersed in different “bodies of water”. In order for ions to flow through the water, between any two metals, they need to be in the same body of water, and essential be able to “see” each other. The circuit is completed by electron flow through a physical connection, i.e. the metals are directly actually connect to each other, or via a low resistance wire connection, a bonding system for instance. It is because of this phenomenon that heat exchanger pencil anodes don’t protect underwater metals, and vice versa.

Again, I applaud you for broaching the subject. Having dealt with this matter for decades, the view I’ve arrived at is as follows, the reason for using a sail drive, or a stern drive or pod for that matter, is a lot like front wheel drive cars, it’s not necessarily better (although there are some benefits, better in the snow), but it is easier to install, more compact, and less costly for the builder and buyer. Installing and aligning shafts, struts and shaft logs is time consuming and fought with opportunity for errors, and it takes up more room inside the vessel. Sail drives, and pods, offer less drag and better performance, pods offer enhanced maneuverability and significantly better fuel economy, at a price most users aren’t aware of, or taking into account because all of the manufacturer’s advertising is geared toward the advantages, and why wouldn’t it be? It’s up to the buyer to do his or her homework.

When I have clients who are interested in buying a vessel with pods, or a sail drive, I explain to them those pros and cons, so they can make an informed decision. Too often, however, buyers consider only the advertising, or the propulsion system is only an afterthought because the vessel has everything else they want.

Ernest E Vogelsinger

Hi Steve,
you say “The engine’s cooling system metals have no affect on any metals outside the hull, sail drive and otherwise, because they are immersed in different ‘bodies of water’.”. Is this also true for the most usual seawater cooling components? At least at the intake side they are open to seawater, and in my understanding the sewater part of such a cooling system should actually be considered part of the same ‘body of water’. As such the seawater part pf the heat exchanger would very well be affected by electrolysis corrosion – or am I missing something here?
Thanks for any insights, Ernest

Steve D

Ernest:

“Is this also true for the most usual seawater cooling components?”

SDA: Yes.

“At least at the intake side they are open to seawater, and in my understanding the sewater part of such a cooling system should actually be considered part of the same ‘body of water’. As such the seawater part pf the heat exchanger would very well be affected by electrolysis corrosion – or am I missing something here?”

SDA: The raw water components of the engine, the water pump, heat exchanger and exhaust system, are inside the boat, for corrosion analysis purposes the water inside them is considered a different body of water, and too far from the components outside the boat, through hulls, anodes, and even a sail drive, to have any interaction with them. Ions will not travel though raw water hoses for any great distance, usually just a few hose diameters.

I realize it’s commonly used, so I don’t fault you, however, “Electrolysis” is a word that the corrosion community, and ABYC, frowns upon as it does not accurately describe this process. While there are many kind of corrosion on boats, crevice, poultice, dezincification, dealuminification, and erosion corrosion etc., for the most part, and in this case, we are talking about two kinds of corrosion, galvanic, also called dissimilar metal, and stray current. If no external battery power source is involved, then the corrosion is typically galvanic. This article describes both in detail https://stevedmarineconsulting.com/unraveling-the-corrosion-mystery/

Steve D

That’s an interesting test result with the seacock, I’ve honestly never tried it. However, if this were true, if gear outside the hull affected engines, then pencil anodes in heat exchangers would waste away very quickly trying to protect struts, through hulls etc. and that doesn’t occur, and if that were true we wouldn’t need anodes in heat exchangers as they would be protected by hull anodes.

We may agree to disagree, but this isn’t my opinion, the ABYC Corrosion Certification program spends significant time on the ‘different bodies of water’ subject, making it clear engines and generators are isolated from hulls where cathodic protection is concerned.

From the ABYC Corrosion Exam Study Guide…

“Common Electrolyte
The term common electrolyte means shared electrolyte. An ocean,
or any body of water, has unrestricted ionic flow. Now think of
how many bodies of water we have in and around our vessels.
Certainly the lake, river, or ocean on the outside of the hull is
relevant. But how about the electrolytes in the bilges, potablewater
tanks, engine-cooling system, fuel tanks, and so on? They
can also act as their own separate electrolyte. To be totally clear on this point, the water on the outside of the hull and the bilge
water are not the same electrolyte. They should be considered as
“separate oceans.” Other considerations are that the atmosphere,
with humidity as a factor, or saltwater-soaked wood can also act
as electrolytes.”

Ernest E Vogelsinger

Steve, the ABYC article you cite mentions the bilge water and tanks, water bodies that are (and hopefully stay) separated. Even the _inner_ part of an engine cooling system is separated – but the seawater part is not, and I don’t see the ABYC article contradict this.
Maybe this is mainly academic and has no practical relevance, but IMHO the “water body” would be the same if not interrupted by a closed seacock, as John mentions.

Steve D

I’m in the midst of a corrosion analysis and while preparing my findings I came across this citation from ABYC E-2 Cathodic Protection Systems. See note 5 in particular.

2.5.6 All metals that are to receive cathodic protection from the cathodic protection system shall have a maximum resistance of one ohm to the cathodic bonding system anode.
NOTES:
1. An electrical resistance greater than one ohm will degrade cathodic protection system performance.
2. Propeller shafts do not provide reliable electrical continuity to the boat’s cathodic bonding system.
3. Resistance measurements should only be taken when the boat is out of the water.
4. Hull potential measurements using a reference electrode should be used to determine cathodic bonding system integrity when the boat is in the water.
5. Externally mounted hull anodes do not provide effective cathodic protection to the interior surfaces of seawater piping systems, seachests, and heat exchangers due to the restricted electrolyte path through the seawater in the narrow piping channels. Other means of protection from galvanic corrosion may be employed to protect these metals.

Stein Varjord

Hi John,
I really agree in the general criticism towards builders for quite often choosing solutions for the wrong reason. Often it looks like the added price means added functionality or more comfort, while my opinion is that frequently the benefits are not really true, and the real motivation seems to be that the added item or added complexity justifies a more than proportional price increase. I’m a speed fanatic and KISS lover, so for me a lot of the present day standard equipment falls into this category of poor choices. Most cruisers will disagree with some of my opinions. I’ll take a hit on the baby that is probably the hardest one to kill:

Furling sails are used on near 100% of modern boats. They have two good benefits: Fast deployment and easy stowage. The long list of disadvantages, compared to the alternative, should ring the alarm bells of serious ocean sailors. For the boat builder, the increased sales price makes the choice a no brainer. In my serious opinion, the sailing community has ended up partly brain washed. (Pun intended.) Furling sails are great for light wind sails where the “sausage” can be lowered, and some more. Otherwise, the disadvantages of furling sails are vastly more important than their advantages! I think the number of people who agree with me on this is extremely low, but I enjoy feeling like the only enlightened one. 😀

So why this rant in the context of sail drives? Well, I think sail drives are much the same story as furlers. We also have a furling head sail and two sail drives. I can observe as a fact that my rant on head sails isn’t really that important. Our furling self tacking jib works very well, most of the time. Same goes for sail drives. 5 days ago I actually took them apart to inspect them. They’re 24 years old and in just like new state. Zero wear and zero corrosion. Even the factory powder coating paint looks like new. I only changed the rubber seals, which were also apparently like new. In general they tend to last longer than the engine that runs them. Engine companies actually make adaptors for the sail drives of the market leaders Volvo Penta and Yanmar because they want to sell engines to those owners who want to keep a perfectly good sail drive when the engine is dead.

So am I suggesting that there’s nothing wrong with sail drives? Nope. All the points in the article are valid, in my opinion. I think sail drives work well on our cat, because the engines can be in a separate room at the stern, with zero connection to the boat interior. For me that’s a must on a boat, due to the otherwise inevitable “boat smell”. The reason I’d like some other solution, is that a sail drive is more complicated and thus more vulnerable than a straight axle. Sail drives are also much more expensive than a straight axle….

I think sail drives and furling sails are two elements of a general trend in the boat market, and probably all markets. Make products more complicated so they seem better, but in reality are mainly more expensive to buy, need much more service, the service needs to be “professional” and the whole product gets a shorter and more definitive usable life. All in the interest of the dealer side of the equation, not the user.

I think the sail drives and furlers are among the least harmful elements of this development, and that boats are not as far lost in this game as say cars, but they are getting there. Mercedes has several years ago claimed that their aim is a 10 year usable life for their cars. I think most consumer boat companies would love to achieve the same. No matter which market, I think this attitude sucks! I think those advocating this attitude are the main reason our planet is being destroyed. I think it’s high time to kick some ass!

Richard Elder

Hi Stein
I’ve driven two Mercedes diesels for a total of 500,000 miles over three decades. Total repair cost less than $3,000. Total purchase cost $6,000. The secret— buy used after the new car depreciation has all been blown away, and buy cars from the era before they became mobile electronic nightmares. So Mercedes now hopes to deliver a ten year life expectancy for their current model Hundia-replicas-with-a-Star? It’s been 30 years since they they built that kind of car!

Steve D

Ernest:
It does, it says, “Certainly the lake, river, or ocean on the outside of the hull is
relevant. But how about the electrolytes in the bilges, potable water
tanks, engine-cooling system, fuel tanks, and so on? They
can also act as their own separate electrolyte.” This clearly draws a distinction between lake, river, ocean and engine cooling system, they are all considered separate.

It is widely if not universally accepted in the corrosion analysis community, that the water in which the vessel floats, and the water in the cooling system, are two different electrolytes. I’m working on a more detailed response to John’s comments.

Steve D

John:

“…how can the water in the salt water side of the engine be separate given that with the sea cock open the body of water is contiguous with the water around the hull? Bottom line, it’s all about circuits and unless there is something to break the circuit (closed Marlon seacock) current will flow through that water with the chassis of the sail drive and engine completing the circuit.”

It’s a reasonable question, and while it is a circuit, unlike circuits that are connected by wires, which are low resistance, ions traveling through a seawater-filled hose are far different than electrons traveling through a low resistance wire. With galvanic current the effective resistance gets so high once inside hoses that 99.99% of the anode current will “short circuit” back through metals that are much closer to it. It is electrically possible to see a minute current flow (the highest resistive path still sees current but it can be very very tiny).

Many of the anodes I remove from heat exchangers on sailing vessels are below the resting waterline. Depending on the hull design, i.e. those with deeper bilges, engines are at or mostly below the WL, so heat exchangers are submerged.

The water pump is conductive, so are you saying electricity (ions in this case) are blocked by the rubber impeller, but the electricity cannot flow through the conductive pump? They, the ions in water or electrons in metal, will have an opportunity long before the impeller, to get back to the block and anode via the bonding system, so the impeller will have no effect.

I suppose the best way to prove, or disprove, this theory would be to measure the protection level of a drive or exterior underwater metals, and then remove heat exchanger anodes to see if it changes.

As an aside, Volvo sail drives claim isolation from the block, while Yanmar does not.

Devon Rutz-Coveney

Excellent yarn…. just get Cupro-Nickel heat exchangers… pretty inexpensive these days. We have four of them: the oil cooler, the tranny cooler, the water cooler and the frig condenser…..
Get spares too!
No more worries!
Saildrives: I’ll wait until there casting them in bronze. I would love that. I really do not understand why this is not happening… unless it is the worldwide rampant consumerism driving aluminium for this task. How much more expensive can it be to cast in bronze instead of aluminium?

Steve D

How many sail drives are made each year? How many sailing vessels were built last year and what percentage had sail drives? It’s a pretty small market that is very cost-conscious. Some of the same reasons stern drives are not bronze, however, for a while Volvo did make a composite stern drive (now discontinued, not a good sign and not a great reputation, and a class action suit-but for bellows failures not the drive housing), that’s a far more realistic possibility for a sail drive.

Bronze is very roughly $1.50/lb, aluminum $.40. Beyond that the cost of casting bronze is higher, different tooling, higher melting point (1700F vs. 1200F)…. IPS drives are bronze, but they are also much more common, geared to power vs. sail and an ever growing market.

jan rytenberg

Hi,
You are right about the sealing effect of the impeller blades.
“A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the dischargepipe”
https://en.m.wikipedia.org/wiki/Flexible_impeller

Colin Post

I think that there may be some confusion here. Cooling pumps with impellers are “non positive” displacement pumps. That is, the pump impeller can continue to rotate even though fluid is not moving. If the valve (seacock) is closed the pump can still run. However you won’t be doing it any favours. Whereas
, positive displacement pumps require a pressure relief mechanism typically employed in hydraulic systems to prevent catastrophic failure of system components.
All this means that cooling pump impellers do not effectively provide an insulator from one side of the pump to the other. It may be very small, but the electrical path still exists.

I have a relatively intimate knowledge of these systems from an industrial perspective in that my background as an industrial millwright.

Colin Post
Fresh water sailor
SV Post Haste

Ernest E Vogelsinger

This reply goes to Colin (thread limiter): please see the post of Steve below (https://www.morganscloud.com/2019/07/03/why-are-saildrives-even-a-thing/comment-page-1/#comment-282263) – while it is –theoretically– the same “body of water” the fact that there is a relatively narrow hose effectively isolates the engine from the outside water body. I suspect that the impeller pump, having even narrower gaps for for any current to pass, would provide the last “resistor” to the isolating effect.

Steve D

John:

Agreed, I don’t believe the answer is simple, and I continue to believe the relationship between engine and hull metals is limited at best.

Never the less it’s piqued my curiosity, I’m doing some research on this now, as well as polling corrosion community colleagues, I’ll share my findings when complete.

Marc Dacey

Coming late to this as I have limited access, but this discussion fascinates me and how I run my steel boat with an aluminum rudder!

Steve D

John:

I was not able to find a scientific study regarding the distance ions will travel in water, through a hose. However, I contacted Ed McClave http://www.mcclavemarine.com/about/default.html Ed is an icon in the corrosion world, he was responsible for much of the work that was required to start the ABYC Corrosion Certification program. He’s pretty clear that the engine is in a different body of water, primarily because ions cannot readily travel through a water-filled hose, so it seems your most recent instincts were right, at least according to Ed.

Here’s his response.

“Electric current flowing in water, including the protective current flowing in the water from anodes to the surfaces they protect by cathodic protection, is the movement of ions dissolved in the water, as opposed to electric current metals, which is the movement of electrons. Electrons are zillions of times smaller than ions and they move through essentially empty space in a metallic crystal lattice. Ions have to bump their way past water molecules of similar size. I once calculated the difference. The conductivity (due to electron movement) of a #10 AWG copper conductor (which, I think, is 0.109” in diameter) is about the same as the conductivity (due to ion movement) of normal seawater in a pipe 30 feet in diameter. That cathodic protection works at all is due to the fact that we often do have very large cross-sections of water available.

Second: I don’t know of any references that directly address your question or that could provide “proof”. What I have taught and written about over the years is simply my application of basic corrosion science to the particular situation of boats in the water. The basic science of cathodic protection is clear that the protection provided by sacrificial anodes (or by impressed-current anodes, for that matter) is a result of ion-current flow in the electrolyte (seawater in our case) between the anodes and the surface to be protected. Thus, it is limited to metal surfaces immersed in the same body of electrolyte as the anodes, and even then it is further limited by distance, by constricted bodies of electrolyte, and even, in open water, by intervening objects.

Cathodic protection engineers are very conservative about this, and they want their anodes to be within “25 feet, and line-of-sight” of the protected surface. I have certainly seen references to the cathodic protection provided, for example, by anodes located near the entrances of tubes or pipes. The consensus seems to be that when there is water flowing in the pipe, even ten diameters into the tube the level of protection is greatly diminished, and twenty diameters into the tube the protection is essentially nonexistent. When there is no flow in the tube or pipe, the protection eventually reaches a lot a farther into the tube, but it make take many days, or even months, to become effective.

There is no effective, continuous, ion-conducting path between the outside water and the sea water on the raw-water side of a cooling system of a marine engine.

In fact, because of the severe limitations on the effectiveness of cathodic protection in constricted channels, I think that the anodes in a marine engine cooling system are mostly there for show and to make people feel good. In many cases, when the engine is shut off, the anodes are not even fully-immersed. The main protection against corrosion in a heat-exchanger is using the right material in the first place. Given the requirements for that material to be very resistant to corrosion in sea water, and often at elevated temperatures, and also that it be inherently anti-fouling (which requires an alloy with at least 70% copper), copper-nickel is pretty much the only available suitable choice, and it doesn’t need cathodic protection to be corrosion-resistant. (Maybe the cathodic protection is for the brazing or soldering material between parts of the heat-exchanger?) (In addition, cathodic protection of any surface exposed to sea water causes calcareous deposits on that surface, which in many cases are beneficial, because they cut down the amount of protective current needed, making the anodes last longer. However, in a heat exchanger, these deposits interfere with heat-conduction).

If you want to go to the true sources of information about corrosion science, without getting too mathematical, they are, in my opinion, the following (not necessarily listed in order of importance):

“Corrosion and Corrosion Control” by Henry H. Uhlig

“Marine Corrosion, Causes and Prevention” by Francis L. LaQue

“Corrosion Engineering” by Mars G. Fontana

“Cathodic Protection” by John Morgan

I’m afraid that most of the popular books written for boatowners about corrosion and cathodic protection contain at least some serious factual errors. They may be useful for their discussions about various metals, but not as references about corrosion science.”

Steve D

John:

Agreed, Ed was very generous with his time and knowledge.

The ion flow may never be zero, but it’s negligible due to hose diameter and length. From my perspective, and field experience, I have not found it to be a significant factor.

On the subject of bronze strainers on aluminum boats, I would not use one even if it was isolated, and it has nothing to do with the water in the hose. I’ve seen several examples of run off (including condensation) from copper alloy plumbing and hardware, onto aluminum hulls or hardware, which carries salts of copper, cause corrosion in the aluminum. IMO, all copper alloy plumbing should be avoided where alternatives exist, on AL boats. Where it can’t be avoids it must be encapsulated. There’s an example photo shown in this article https://stevedmarineconsulting.com/unraveling-the-corrosion-mystery/ third photo down.

Ernest E Vogelsinger

Hi Steve and John, and Ed as well,
thank you for fascinating insights and very interesting conclusions.
If I understood correctly:
1) The external cooling part, from a more scientific approach, cannot be seen as a “separate body of water”, but
2) the intake hose somehow acts similar to a resistor in an electric circuit, gaining resistance with length and diminishing cross section, plus water movement, so
3) during “regular use” the galvanic effects are too small to be considered

And there was another number in Eds text that I will try to remmber – the zincs not farther away than 25′ from metals to be protected – I always wondered how one would calculate zinc placement on a metal hull. This number for me would translate to
40′ hull: 2 zincs on each side, plus extra zincs for prop and rudder
50′ hull: 3 zincs on each side, plus (…)

Charles Starke MD

Hi John
I don’t have a pod drive but my friend had to replace the whole pod drive at a cost of $25,000 apiece since the dealers would not or could not replace the seals. The seals tend to leak after a year or two!
Pod drives are also designed to “supposedly” break off on hitting an object without causing a water leak into the hull. They may have to be included in the same category as sail drives.
Best wishes,
Charles
Charles L Starke MD FACP
s/v Dawnpiper

Steve D

Pod drives, IPS and Zeus are the most popular, have their own issues, of which there are many. However, their drive legs are at least made of a naturally corrosion-resistant metal, bronze, so corrosion is one few challenges with which they do not have to contend, and thus they are notably different from sail drives in that respect.

Richard Elder

Hi John

When I first caught the sailboat disease years ago I looked at a Crealock 37 owned by a boat salesman who had just relocated from Southern Cal to the Pacific NW. He’d arrived via a class victory in the SF-Hawaii Single Handed Transpac, so the boat had good credentials. As I was looking over the bottom I noticed several large blisters on the saildrive leg. Took my car keys out of my pocket and poked at one. Damn near drowned from the gusher of water that came out! A few weeks later a new Baltic sank when the bellows failed while trying to make the passage 10 miles across Puget Sound .

Ever since then I’ve always said I’d never build a boat with a sail drive without a full watertight enclosure around the engine/drive unit. It might actually be safer, and certainly quieter than a V drive or straight shaft installation, but that wouldn’t eliminate the reliability and maintenance issues.

Stephen Meginniss

John,

Been a member for awhile now and first comment (I think..). I have 2 13-yr old SD-40s on our catamaran and they have done just fine. I’m careful with the anodes. We have been in lots of marinas, all types of water and I haven’t seen any issue with the SD housing.

I guess this is ancedotal, but I see plenty of other boats out here with saildrives, without issue. My aluminum shaft anodes last about 6 months.

Respectfully,

Bill

Steve D

Stephen:

Glad to hear you’ve had a good experience. I’m curious, do you have a galvanic isolator or transformer?

Denis Foster

Hi

Amel had a proprietary drive gear mechanism that would go through the rear of the keel. Don t know if the new 50 has this also?
The draw back is that the propeller is far away from the rudder but well protected by the keel.

Regards

Denis

Victor Plavner

John,

This is my first comment after having been a subscriber for a few months. Now that you have given all of us notice about your views on saildrives, how about a follow up article on the maintenance. There was one sometime last year I believe in Cruising World magazine but more information, especially from your perspective, is always better.

Thanks,
Victor

Steve D

For the benefit of others, I believe this is the article Victor referred to, https://www.cruisingworld.com/saildrive-maintenance/

I have known the author for 20 years, ABYC’s VP of Education and very active in the corrosion education and prevention community.

I don’t necessarily agree with every observation or recommendation in this case, however, the concept is right.

Steve D

Completely agreed, and I routinely call them out on such crazy suggestions. Most stern drive manufacturers call for bellows replacement every two years, which no one does. When they fail, the dealer will say, “You know you are supposed to change these every other year”. Clearly much of this is a CYA for the manufacturers.

Jordan Burdey

Can the bolts running through the saildrive/hull have a plastic sleeve of some sort to separate the materials?

Wayne Berge

FYI, may be of interest to someone. I used to own a Prout catamaran that had a liftable and steerable stern drive made by Sillette in the UK. They also build the ordinary type sail drives with bronze housings as an option and directly boltable to the hull eliminating the rubber gasket. https://www.sillette.co.uk/sonic_Mk_1_saildrive.html. I was always quite impressed with Sillette’s engineering techniques.