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m_cogs

Mercathode

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My 2002 248LS runs great. It has a 5.7L that pushes the bravo3 dual prop. My problem is that a previous owner has removed the Mercathode module from the engine. The outdrive doesn't leak, but does have that swiss-cheese look. The anodes are quite pitted and are being replaced this winter. How important is the module? I currently trailer the boat but will plan on having a slip somewhere. Should I replace the module? Is there a way to test the circuits to make sure they are not faulty before I install the new one?

 

I have included a picture of the drive unit.

post-16783-128378874062_thumb.jpg

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Hi m_cogs

 

If you don't plan on leaving the boat in the water, then the mercathode system is of no use. It only offers protection whilst the boat is stationary in the water. There is some discussion that the effectiveness of the mercathode system is reduced due to water movement in tidal zones and swing moorings (something to do with ion envelopes). If you pull your boat out of the water after each outing, make sure it is washed throughly and you shouldn't have too many problems.

 

I don't know enough about the different types of corrosion to comment on your particular issues however te general consensus seemed to be that the change to the 2 x stainless props in the B3 led to a lot of problems with "galvanic corrosion" or "electrolysis" between the larger area of stainless steel and the aluminium casings. Looking around the internet, you will find a lot of bad stories about "early" B3's that seem to disappear from 2005 / 2006 onwards.

(CAUTION - Not everything you read on the internet is true)

 

I can't see your prop nut in the pic, but apparently in later B3's, additional anodes where placed on the drive as part of a series of fixes. All B3's now have a large anode fitted to the end to the prop nut (it's a cone shape). I read somewhere this is available as an "aftermarket kit".

Other "fixes" included using magnesium alloy anodes as it is "more sacrificial" than zinc alloys although this means they need to be replaced more often depending on the water chemistry

 

You may need to check with Mercruiser but I am not sure the mercathode system was fitted as standard in 2002 series motor/drives.

 

I have found some articles about the mercathode system (you can find a lot more on the internet). See links below

 

You say the motor has had the engine module removed. You don't say if the "reference anode" plastic assembly is still fixed to the leg (see pics in first link)

 

Another issue, it looks like the trim cylinder anode pictured is half missing!. I know zinc is "soft" but I don't like the idea of the other half coming off and hitting your props under power. I would replace it ASAP

 

Hope some of this helps

Ian

 

 

http://www.mercstuff.com/bravo3corrosion.htm

this gives a good overview

 

http://www.marinemechanic.com/site/page200.html

and the following page

http://www.marinemechanic.com/site/page201.html

these 2 pages get a lot more technical about causes of corrosion with and without a mercathode system and how to test the mercathode system.

 

 

 

P.S. Seems Volvo have had isses along the same lines.

They did introduce a plastic composite drive casing and props to try to reduce corrosion issues.

Looks like the composite drive casing experiment has been dropped and they have done back to aluminium castings with titanium dioxide coatings

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Thanks for the info Ian!

 

Since I am going to be trailer-ing the boat, I am not too worried about the Mercathode. The module has been removed, but the wires are taped up and lying on the intake manifold. The reference anode is still mounted on the transom bracket.

 

During the winter storage I am going to replace the anodes and have the props reconditioned. I will then check on the prop anode too. I heard of stories about the prop shaft rotting withing the housing.

 

The trim anodes are not half missing, but rather rotated so the cutout is facing outwards.

 

Being a first time boat owner, I really appreciate this forum and all the great advice from you and everyone!

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