Anodes, thongs and perfect bonding...

[Trident]

I have an alloy drive leg (Sillette Sonic Cat Drive) on my Prout which has suffered some corrosion. In part this is because Sillette in their wisdom chose to put anodes only on the transom plate which on a Prout is out of the water by 8 inches so does no good whatsoever.

So I have, after taking advice from Sillette, drilled through the aluminium cavitation plate, and bolted on an anode there so its always in the water when the leg is down. However, as its bolted on with a stainless bolt I was advised to use Duralac to stop corrosion from dissimilar metals. My question for the techies is, if the Duraclac stops the two metals interacting so they don't corrode together, won't it stop a direct connection with the zinc anode too, rendering it useless?

 

[boatmike]

It may be useless anyway unless you have cleaned the paint off the booger first! Ideally the anode needs to be in direct contact with the aluminium not connected through a bolt, which if you have put duralac on it will also be isolated as you suggested. Problem with most Prout installations is that the leg is mounted too high if the boat is lightly loaded which hopefully it is Best plan is to always lift the leg when leaving the boat. At sea it should only need to be down when motoring anyway so the opportunity for electrolytic corrosion should be minimal..... The quality of casting is also fairly poor on these agricultural lumps and often they corrode simply because they don't like salt water. The only cure I found was to clean thoroughly every year and repaint with hammerite.

 

[savageseadog]

You shouldn't really need Duralac as the anode is right there to stop corrosion. We've always used a touch of grease on our Yanmar saildrive anode securing screws to good effect. There would be no harm in using a bit of Duralac if you wanted to.

 

[coreng]

Of course anodes are useless out of water. Though it is not unusual to bolt anodes with stainless, on aluminium, you should better use (cheaper!) galvanized mild steel bolts. I don't know for Duralac but a paste like Tef-Gel is not an electrical isolator : it just prevents formation of a galvanic cell by inhibiting water coming where it should'nt, thus eliminating a corrosion necessary condition. According to aluminium quality, it might be wise to use a flexible mastic at the back of the anode (contact through the bolt must then be perfect). In any case and as it is not a guess, only a measurement of the electro-chemical potential of the protected item will tell if protection is (still) effective or not.

 

[Trident]

Thanks for the replies - I'll mask round the anode and sand off the paint directly beneath it for better contact then. Mike, I have indeed just repainted the leg completely - there was a fair bit of surface corrosion on the 6 year old leg despite it being out of the water most of the time (and on the dry for 2 years before I bought it) - not helped of course by the previous owner turning it in to a battery by greasing everything with copper grease!!

 

[boatmike]

Of course anodes are useless out of water. Though it is not unusual to bolt anodes with stainless, on aluminium, you should better use (cheaper!) galvanized mild steel bolts. I don't know for Duralac but a paste like Tef-Gel is not an electrical isolator : it just prevents formation of a galvanic cell by inhibiting water coming where it should'nt, thus eliminating a corrosion necessary condition. According to aluminium quality, it might be wise to use a flexible mastic at the back of the anode (contact through the bolt must then be perfect). In any case and as it is not a guess, only a measurement of the electro-chemical potential of the protected item will tell if protection is (still) effective or not.

I agree that using a galvanised bolt is preferable to stainless but there are clear reasons why the bolt itself is not ideal to rely on for good electrolytic contact. The first is that if all anodic current flows through the bolt then it will cause a greater depletion of material in the area around it which is the main reason why anodes become loose in time. Where it is unavoidable, like a through hull bolted anode on a GRP boat, it is often found that the bolts become loose over time. Thats why such anodes are moulded to steel plates rather than simply bolting to the anode itself. Secondly, an anode directly in contact with the item (like a shaft anode for instance) will always have a lower resistance than one relying on a bolted contact, especially through a stainless steel bolt. Using Duralac on the joint may well reduce cavity corrosion but while it wont isolate the two parts entirely, will certainly introduce a resistance that will reduce the effectiveness of the anode.
When using direct, through bolted anodes which I suspect the OP is using here, the best plan is to clean the area around the anode of all paint and bolt it direct to the aluminium. Then the bolt itself can be isolated completely from the anode if so wished.

 

[boatmike]

Thanks for the replies - I'll mask round the anode and sand off the paint directly beneath it for better contact then. Mike, I have indeed just repainted the leg completely - there was a fair bit of surface corrosion on the 6 year old leg despite it being out of the water most of the time (and on the dry for 2 years before I bought it) - not helped of course by the previous owner turning it in to a battery by greasing everything with copper grease!!

I think that is the right thing to do, but I still don't think you will avoid some corrosion as I don't believe it's entirely electrolytic. In fact when I built my Prout it was in build with the leg attached for three years in my back garden and the paint was falling off in places even before it saw the water! Matters improved when I abraded it all over and painted with Hammerite but I think they originally paint the things with an air drying paint that is about as much good as chocolate without an etching primer...... When you buy an outboard motor the castings are of better quality and prepared properly before stoving. Doesn't help using copper grease though.....Doh!

 

[Trident]

Yes I think you're right - the paint is a two pack from the factory - and I've just sanded it all off and given it two coats of etch primer and 3 of acrylic paint and then will lacquer on Monday when its hard. It's interesting that a friend who just bought a reconditioned unit from Sillette was told they'd repainted it with an epoxy paint when they rebuilt it... so why not use that on new ones if its better or why not use the original 2 pack if they think its fit for purpose. Then again after supply Prout for 30 years why not move the anodes. I have to say I do love my leg - its great for steering, easy to remove fouled lines, comes out of the water to sail reducing drag but it does seem like a good idea poorly executed by the factory.

 

[vyv_cox]

The anode on a Bruntons prop is held on with nylon bolts because the original stainless ones led to rapid wasting of the anode surrounding them, upon which the anode fell off even though barely consumed. This could be an option for you provided your anode is not too large and heavy. Good electrical conductivity needed between anode and saildrive, as already pointed out.