Sacrificial Anodes on Tinnies

[DaveSue_Fishos_Two]

G'day all

I have just noticed (after all these years.........must be getting slow! ), that my 4.4 mtr tinnie does not have an anode below the water line. There is one up high under the motor leg, but after 16 years there is no sign of corrosion on that anode. So what's it's purpose? I have a few electric devices on the boat and so the battery is on every time I am on the water. Shouldn't there be an anode BELOW the waterline??

Cheers and thanks

Dave

[seatime]

Well spotted, does the tinnie need the anode or the motor leg?
It'll be interesting to read replies, it's not something I've thought about before. If your anode hasn't corroded in 16yrs, I'd dare to say it must be insulated from other metals and isn't working. Might be only the leg needs cathodic protection as there is a mix of metals in the casing?
better not say any more, it's not my field :)

[joey_1987]

Aluminium is an extremely reactive metal, it reacts with the air rapidly and even more rapidly in a salt water environment. It is chosen for boat manufacturer because it is light and quite strong, and when it reacts, the Aluminium Oxide forms a layer which protects the aluminium from further attack by oxygen so it can last a long time because this layer of Aluminium Oxide is extremely hard and protects the Aluminium. You could use a sacrificial anode on an aluminium boat, but there's not really any need for it. Motors are not pure aluminium, they are made of many alloys, and sacrificial anodes are used here to protect the motor from corrosion, the anode has to be more reactive than the metals in the outboard so that it reacts first in preference to the metals in the outboard, thus sacrificing itself. The motor leg is what needs the anode, and a lot of outbaords have a Zinc anode on the bottom of the cavitation plate. Hope this helps you understand it better. Cheers, Joey.

[DaveSue_Fishos_Two]

Thanks fellas

The counter directional? fin under my cav plate is plastic. On other boats I have had this has always been metal and self sacrifices. I have noticed a few little bubbles appearing on the paint of the leg below the waterline and have begun thinking that I should replace the plastic fin with metal?

Cheers and thanks

Dave

[DaveSue_Fishos_Two]

Please has anyone else got any comments on this??

Thanks

Dave

[Rogues]

Cant believe ya dont have an anode below the cav plate on your leg mate,
Ya sure it aint just gone missin' over the years..........you dont have this nice lil' neat cut-out (hole thru leg) that doesnt seem to do anything do you......
I agree, you've gotta have one down there or ya motor's gonna rot :(
What sorta motor is it anyway........with the torque adjuster thingi in plastic, its gotta be a 'Poly'....... ;D ;)
Cheers, Rogues

[drb]

If you intend putting a sacrificial anode on an alloy boat may sure it is of the correct type.

Using a anode designed for a steel boat will not work as the "Nobility" of these anodes can be higher than the alloy thus turning the alloy boat into the anode and the sacrifical block into the cathode.

A good quick article on it can be fouund here
http://boatpoint.ninemsn.com.au/portal/alias__boatpointau/tabID__200856/ArticleID__119752/Default.aspx

[DaveSue_Fishos_Two]

Thanks for the replys.

Surprisingly, there is not an anode on the leg of the motor. I think I will change the plastic counter directional thingo for a sacrificial model.

Cheers all and thanks again

Dave

[Blackened]

G'day
Dave, what make and model is your engine? My little merc has a sacrificial anode under the cav plate, after the water intake.
Dave

[DaveSue_Fishos_Two]

I have a 1990 40hp Evinrude. Electric start and ptt.

Cheers and thanks
Dave