"Sterndrives are tablets that dissolve in salt water"

[skipjack27]

A local expert in marine corrosion made this remark to me (except that he used the term "Disprin", rather than "tablet", because in Australia Disprin are what we toss in a glass of water and gulp down to get rid of that hangover headache.) Expensive tablet, though: my Volvo Penta Duoprop (1987)would cost about the same here as the average car to replace if I went for a brand-new unit.

This bloke has long experience as a metallurgical analyst in our local mining industry, and now runs a marine protection system company. His point was that sterndrives were originally designed for the freshwater Great Lakes of the USA, and have no business being translated to saltwater. His view is that it is difficult to combat galvanic corrosion in a sterndrive, and even harder to combat electrolytic/stray-current corrosion of the kind often encountered in marinas.

My call to him was motivated by the discovery that the prop anode on my drive was being chewed away in about 3 months. This strongly suggests stray currents from surrounding boats in my marina (I don't connect to shore power myself, but they mostly do around me).

There was no commercial interest in his advice - his company deals only in stainless steel protection systems, and he was unable to offer any service to me at all (my gear is aluminium alloy).

He was sceptical about zinc "fish" anodes being hung overboard to supplement the prop anodes, although I was not entirely clear why this was so.

The general impression I got from him - and from other boat owners - is to stay well away from stern drives in the future. But when I look around the waters, I see stern drives dashing everywhere as far as the eye can see.

What's the deal? IS there any way of reliably protecting your precious stern drive in a pernicious marina environment?

Jeff

 

[rodbolt]

Re: "Sterndrives are tablets that dissolve in salt water"

while there isnt much you can do about stray currents from faulty shore power connections on other hulls you can measure the currents your hull is seeing with a digital multimeter and a reference electrode.
mine is from quicksilver.
this will tell you if your anodic protection is over or under protecting.
you can also install an active corrosion supression system,mercathode is one,volvo makes one and they can be had aftermarket.

 

[Philster]

Re: "Sterndrives are tablets that dissolve in salt water"

Zincs getting eaten up rapidly? Just means you have to replace them more often. They are sacrificing themselves so that more desirable metals (say your drive unit, etc) are not getting eaten away.

For other electrical issues, there is the Mercathode system, and there are other ways to prevent metals from suffering corrosion due to electric currents. Someone will speak to those.

As long as you know the type of water, and the type of electrical issues on the boat and at the docks, you can fend of corrosion for a long, long, long time.

Unprotected, a drive unit is just ready to dissolve. We just don't leave things unprotected. Water always wins, but we just want to take it the distance before it wins by decision.

 

[achris]

Re: "Sterndrives are tablets that dissolve in salt water"

...He was sceptical about zinc "fish" anodes being hung overboard to supplement the prop anodes, although I was not entirely clear why this was so....

They are useless unless electrically connected to the drive... That can be done by running a wire from the zinc fish to the engine block....

Chris.....

 

[tkrfxr]

Re: "Sterndrives are tablets that dissolve in salt water"

He is right in some respects, but you are right to wonder why so many drives are out there in the salt...

If your anodes are eaten in three months, your local soup is the reason and you have to come up with more metal to protect your drives.

Proper bonding to the rest of the boat may help. You also have to look for stray currents.

If you connect to the electrical system, you may need a galvanic isolator...

Lots of variables means lots of different solutions to look at...

When it comes to salt water, "resistance is futile."

 

[Lou C]

Re: "Sterndrives are tablets that dissolve in salt water"

Well they do take extra work and care than a straight inboard or outboard would but it depends on what boat you want and what power packages are available...here in the LI Sound area there are about 1/3 inboards...1/3 ob and 1/3 I/Os...I have run one and kept it in the water for 6 months at a time for 7 seasons so far....the main problem where I am is marine growth since the paints do not work that well....here's mine at about 18 years old...

 

[Shamus O'toole]

Re: "Sterndrives are tablets that dissolve in salt water"

Skipjack...since your in Australia you should be familiar with this...give it a try.

http://www.inox-mx3.com/inox.htm

lemme know what you think.

 

[achris]

Re: "Sterndrives are tablets that dissolve in salt water"

... IS there any way of reliably protecting your precious stern drive in a pernicious marina environment?

Find the boat/s with faulty shore power systems and get them to fix thew problem. You should be able to instigate that through the marina owner.

 

[triman]

Re: "Sterndrives are tablets that dissolve in salt water"

Metallurgy has a lot to do with it. My Yamahahohoho sterndrive is 19 years old and as good as new, with anode changes every 4 years whether it needs it or not.
And as for Inox products, very good stuff!!!

 

[skipjack27]

Re: "Sterndrives are tablets that dissolve in salt water"

Thanks for the interesting and informed replies from all. A couple of comments:

Shamus O'Toole recommends Inox. On the advice of my mechanics, I regularly spray Inox over everything in the engine compartment to prevent rust. But I'm very puzzled as to which of their products is applicable to protecting a stern drive in the water. Can anyone enlighten me?

And here's an interesting idea: the marine protection bloke I mentioned in my original post had a very firm recommendation for reducing galvanic corrosion: get rid of the raw water intake through the drive, seal it off, and install an inlet in the floor of the engine compartment and suck the sea water directly into the system. He reckoned that I'd find my engine ran cooler with this more direct input to the heat exchanger. But more: he said I should flush out all the salt from inside the drive with detergent, let it dry for a few days or blow it dry with a hair dryer or similar, plug the stern drive intakes with epoxy, and then fill the whole raw water intake system in the drive with oil and replace the plug.

His argument is that this significantly reduces the area of metal in the drive that is exposed to sea water, and thus significantly reduces the rate at which the zincs are burnt up. I was initially a bit doubtful that this would make much difference, until I recalled that while the external surfaces can be protected with appropriate regular coatings, the internal raw water passages cannot be similarly protected. I presume that the internal surfaces in contact with sea water are thus effectively relatively large in relation to the external surfaces.

Those who viewed my thread on a serious engine overheating problem will recall that this problem was completely resolved by disconnecting the raw water intake from the drive and installing a through-hull. So, I am rather more than halfway toward implementing the marine protection bloke's advice. Unless someone has any persuasive arguments to the contrary, I plan to fill the drive with oil at the next slip.

Regards

Jeff

 

[skipjack27]

Re: "Sterndrives are tablets that dissolve in salt water"

Thanks for the interesting and informed replies from all. A couple of comments:

Shamus O'Toole recommends Inox. On the advice of my mechanics, I regularly spray Inox over everything in the engine compartment to prevent rust. But I'm very puzzled as to which of their products is applicable to protecting a stern drive in the water. Can anyone enlighten me?

And here's an interesting idea: the marine protection bloke I mentioned in my original post had a very firm recommendation for reducing galvanic corrosion: get rid of the raw water intake through the drive, seal it off, and install an inlet in the floor of the engine compartment and suck the sea water directly into the system. He reckoned that I'd find my engine ran cooler with this more direct input to the heat exchanger. But more: he said I should flush out all the salt from inside the drive with detergent, let it dry for a few days or blow it dry with a hair dryer or similar, plug the stern drive intakes with epoxy, and then fill the whole raw water intake system in the drive with oil and replace the plug.

His argument is that this significantly reduces the area of metal in the drive that is exposed to sea water, and thus significantly reduces the rate at which the zincs are burnt up. I was initially a bit doubtful that this would make much difference, until I recalled that while the external surfaces can be protected with appropriate regular coatings, the internal raw water passages cannot be similarly protected. I presume that the internal surfaces in contact with sea water are thus effectively relatively large in relation to the external surfaces.

Those who viewed my thread on a serious engine overheating problem will recall that this problem was completely resolved by disconnecting the raw water intake from the drive and installing a through-hull. So, I am rather more than halfway toward implementing the marine protection bloke's advice. Unless someone has any persuasive arguments to the contrary, I plan to fill the drive with oil at the next slip.

Regards

Jeff