Which is better, OR, what's the difference?

F

Fish Guts

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Hello all, Just got another boat after about 5 years boatless. This time it is big enough for a slip. I am dealing with growth and corrosion on my outdrive (Alpha 1 gen 1). I would have thought that a boat with shore power would have a Mercathode or a galvonic isolator. I know there is no Mercathode, haven't checked deep enough to see if there is an isolator. My question is... what is the difference, if any, or which is better? I am going to replace the transom/gimbal assembly when I pull her out this year and want to make sure my drive doesn't look this bad again (looks like it's been in the water for 3 years, not 3 months). Any and all input would be appreciated.

Many thanks,
Richard
 
Both. IMHO, an outdrive has no business being slipped in salt water--WAY too corrosive. Add a SS prop, and you've got twice the problem.
 
Put the boat on a lift for storage between uses. I used an Air Dock for 7 yrs. Hull stayed clean and outdrive lasted a lot longer.

www.airdock.com
 
First, they changed the anodes for salt water to aluminum. There's also an improved mercathode. GeeBee knows about it...or you can check online. Also drop a 'fish' in the water for more protection. You might get the marina to check for stray current if other boats are having problems.
 
1. Mercathode is an option on Alpha I drives - standard on BIII. It's a Mercruiser item that in simple terms introduces a small amount of current into the water around the drives to counteract the galvonism that occurs between the dissimilar metals on the dirve (ie aluminum outdrive and SS props). Past that I don't fully understand how it works. But it does - I had issues with my BIII, but after the Mercathode was replaced and the anodes were replaced (magnesium=freshwater, aluminum/zinc = saltwater) I had not more corrosion issues with my BIII.

2. Anodes, make sure you use the right ones and change them when needed, magnesium=freshwater, aluminum / zinc = salt water.

3. If you can live with an aluminum propeller then do it. The root cause of corrsion on outdrives is the reaction (galvonism) between dissimilar metals submersed in a conduite (water). The outdrive is made of aluminum, when you put on a SS propeller you now have a big issue with two types of metal. Get a nick in the paint to expose some alumnium and corrosion will start. First signes are bubbling of the paint along sharp edges. I'm not a chemist, but the effect is the less noble metal (aluminum) slowly but surely erodes away to the more noble metal (SS). Remember the periodic chart of elements in Chemistry? Alumnium is lower on the chart than the allowys that make up SS. Zinc and Magnesium are even lower than aluminum, that's why the anode erodes first and protects the alumnum.

4. A galvonic isolator is really a filter that prevents your boats bonding system from becoming the ground from the AC system. It will allow low voltage (ie the 12v system) to flow to the ground, but prevents higher voltage. If there is a problem then the reverse polarity indicator should light (meaning the AC system is trying to ground to the boat not the shore system). Without it, if the AC system grounds to the boat then you will have a major corrosion problem with all underwater metal. Your boat could also become the ground for other boats around you. This will corrode underwater metal in a matter of day/weeks not months. All boats with AC shorepower should have a galvonic isolator.

5. You didn't say if you have a SS prop - if you do that is probably the source of the corrosion, BUT I would expect the corrosion to be limited to the area around the propshaf, not the gimbal. Make sure you are using the right anodes, they are all in place and that the ground wires that connect the drive and gimbal houseing are in place. If you have an aluminum propeller and are seeing significant corrosion I would say you have a different issue - I no or non-functioning galvonic isolator and either the dock or a neighboring boat is the source of the issue. Especially if you are near a metal hulled boat, a sailboat or a metal seawall.

There are others on this board that know more than I do, but I think I got the basics.

Bottom line, with an Alpha I you should not have an issue with corrosion - it is much easier to control than on a duoprop BIII.
 
If you have Growth then you probably didn't paint it.
I had BIII twins.
Painted every year with Trilux.
Add second Mercathode so I have 2 per drive.
Added after market prop zinc (standard now on BIII)
Tested Hull potential monthly.
Never had a issue.

I think part of your issue is the growth. When growth starts what looks like corrosion is just growth.
You may need to sand to bare, do the whole alumiprep thing and paint, then cover with trilux.
 
My alpha 1 drive gets 2 coats of Mercury phantom black, then trilux twice a year. Once before it goes into the slip in the Hudson in the spring and again in the fall before we trailer it to Florida. There it is in the canal behind our house. It's been in the florida salt for 13 winters and no corrosion. You need more paint than just Trilux. That alone isn't enough.
 
Galvanic isolators are not standard on most boats unless they are fairly new. There are hardly any boats in my marina that have them. Most use sacrafical anodes and bonding. I added a dual galvanic isolator and monitor about 3 years ago to my boat.
Make sure the boat has the correct Anodes for the water type and all bonding wires are in place.
Bill
 
Thanks for all the input everyone. A couple of things... The drive was painted, with what, I don't know. It seemed to be the same thing they slopped on the bottom. I have an aluminum prop. I added all zinc anodes before I dropped her in. It may be growth... not sure, there is a lot of "white" stuff along with the normal brown/green that I see on other boats in the marina.
 
Thanks for all the input everyone. A couple of things... The drive was painted, with what, I don't know. It seemed to be the same thing they slopped on the bottom. I have an aluminum prop. I added all zinc anodes before I dropped her in. It may be growth... not sure, there is a lot of "white" stuff along with the normal brown/green that I see on other boats in the marina.
 
If they used regular bottom paint on an outdrive (paint with copper) thats your problem right there. That is a major NO NO with aluminum drives. You have to use a paint designed for aluminum and follow the directions, including using the special primer to isolate the alu from the paint. I have an alu prop on my Cobra which is 25 years old and moored in salt (not a marina so no shore power or stray currents) and I have not had a lot of corrosion. Some is inevitable but no deep pitting. Proper paint+prep+primer and anodes make all the difference and use an alu prop if you can.
 
Agree, if they used copper bottom paint, you are screwed.
You need to strip it down and redo it.
Also there should always be a 1 inch gap between the drive and the bottom paint on the bottom of the boat. This is around the housing on the transom.
Copper and aluminum don't mix well!
 
Do they still use Zinc Chromate for prepping Aluminum for paint?
Bill
 
+1 to the copper bottom paint comments. You really should not have a corrosion issues at all with an Alpha that has an aluminum prop.
 
Thanks for the continued responses all. I figure they did use the same paint for the drive as the hull. I knew about the paint gap around the housing. I was hoping to get the year out of it at least. I have already purchased a new drive, gimbal assembly, bellows kit, lower cable, anode kit and a mecathode kit to install for next year. Do you think I would still need to paint the new stuff? I am in some pretty dirty water behind the hurricane barrier in Providence.

Richard
 
If you got growth this year it will be the same next year. Yes you will have to paint it. If you are starting new then you can prep it by the book and do a much better job. Keep in mind that if you use Trilux you must use the Primocon primer to isolate the drive from the paint. Trilux does have copper it it but it is not the same as regular bottom paint copper. Regular bottom paint has cuprous oxide, Trilux uses cuprous thianocide. It works much better if you brush on multiple coats, the spray on version is too thin. I am in a high fouling area, mine does not last the season, I have to pull it mid season, clean it and touch up the paint. You can also get barnacles growing in the water intakes and that can cause overheating. I have a piece of stiff wire I use to rod it out with the drive tilted up on the mooring. I remove the drive each season and check the bellows, gimble, ujoints and input shaft seal. I spilt the drive to clean the barnacles out of the water intake area every 2 seasons. Also paint that area with Trilux. You can make an I/O last in salt this is what it takes. Or get a new but very expensive outboard. The amount of fouling you get makes a huge difference. In a low fouling area it would be much easier. Also using the boat a lot helps but even then if I put the boat in during May, by August it has a lot of growth on the drive and in the pivot housing. I wish they could come up with a paint that worked better than this.
 
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