SD-20 saildrive zincs thread repair

[surf_km55]

I dove my boat to relieve the prop of a mass of purple nylon line [lobster pot?]. While there, I found the upper saildrive zinc mounting screw loose, so I decided to swap out the zincs while I was at it. Low and behold, both fasteners barely snugged the zincs down when I felt them both spin too freely in that "oh crap, something's not right" kind of way. My torque-sensing skills told me I did not surpass 5 N-M of force. FWIW: I use a torque wrench on my carbon bike parts, so I've developed a keen sense of force.

Can I simply haul out the boat and install 304 SS helicoils? Would that create a dissimilar metal problem? Suggestions?

The zincs are snug, but not tight, so I write this plea for input in a highly concerned state of mind. Any input welcome!

[ggray]

Well, are your bolts SS ? Mine are. I also have an SD20

To a small degree, I've fretted about what thread compound to use on the bolts securing "zincs". Grease can electrically insulate the point of contact, but would also reduce corrosion which can also insulate the point of contact.

So I found a electrical conductive grease mentioned in Practical Sailor to use in such cases. I have no idea how effective it is for this, but figure it can't hurt.

Many years ago I installed a galvanic isolator. "Zincs" last much longer.

[surf_km55]

Originally Posted by ggray

Well, are your bolts SS ? Mine are. I also have an SD20

To a small degree, I've fretted about what thread compound to use on the bolts securing "zincs". Grease can electrically insulate the point of contact, but would also reduce corrosion which can also insulate the point of contact.

So I found a electrical conductive grease mentioned in Practical Sailor to use in such cases. I have no idea how effective it is for this, but figure it can't hurt.

Many years ago I installed a galvanic isolator. "Zincs" last much longer.

The bolts removed were 304 SS, A2-70 marked. The threads are clean, no corrosion.
Haulout scheduled for next Thursday and will convert the holes to 304 SS M6 1.0 x 20mm, as I could not find M6 1.25 helicoil. I'm doing Cu/Al hybrid bottom paint, flexo-fold propcoat refresh, saildrive seal, & watermaker thruhull.

Re: the grease, I also found on Practical Sailor:
"Nearly all of the conductive greases we tested actually made corrosion worse than no coating. Our testers concluded that the metals in these greases actually increased galvanic corrosion if there is seawater present."

Link: https://www.practical-sailor.com/boa...s-vs-corrosion

[tkeithlu]

It doesn't sound as though you have a problem. I paid too much attention to avoiding dissimilar metals and not enough to "the anodes will take care of it" in my boat. Helicoil, snug it down, check that you have continuity with the bonding, and move on.

[thinwater]

Originally Posted by ggray

Well, are your bolts SS ? Mine are. I also have an SD20

To a small degree, I've fretted about what thread compound to use on the bolts securing "zincs". Grease can electrically insulate the point of contact, but would also reduce corrosion which can also insulate the point of contact.

So I found a electrical conductive grease mentioned in Practical Sailor to use in such cases. I have no idea how effective it is for this, but figure it can't hurt.

Many years ago I installed a galvanic isolator. "Zincs" last much longer.

No grease does NOT insulate when there is active clamping pressure, such as a bolt. This is myth, supported by the existence of conductive grease, which is also not what it sounds like.


You do NOT want to use conductive grease anywhere on a boat. It is really only for specific high current applications in industry, where it reduces scaring on large breakers. But nowhere on a boat. What you want is either Tefgel or Locktite Marine (marine, because non-marine anti-seize can cause galvanic corrosion. There are other good products.



(I've done a lot of corrosion testing)

[slug]

Stainless is not the best choice ...normal steel is best

Unfortunatly the world is not perfect

Try to make the bolt joint as waterproof as possible

Telgel is very effective when combining dissimilar metals and the stuff is super resistant to washout

[Jim Cate]

Stainless is not the best choice ...normal steel is best

Why is this so?

Jim

[slug]

The galvanic scale

Steel and aluminum are friends


SS and aluminum are enemies

Many times compromise must be made and stainless is used

[Jim Cate]

The galvanic scale

Steel and aluminum are friends


SS and aluminum are enemies

Many times compromise must be made and stainless is used

OK, so you don't use s/s because it can lead to galvanic corrosion of the aluminium alloy housing... but this can be largely corrected by use of isolating ompounds like Tef-gel or anti-seize.

Instead you use mild steel... less chance of galvanic corrosion, but the screw itself rusts away.

It's a hard world we live in, but to me t he s/s option is the better one.

Jim

[Scubaseas]

https://www.olander.com/ has cadmium plated 6 X1.0



304 stainless will last as long as the rest of the metal on the sail drive if it's anything like a Yamaha outboard.

[slug]

Originally Posted by Jim Cate

OK, so you don't use s/s because it can lead to galvanic corrosion of the aluminium alloy housing... but this can be largely corrected by use of isolating ompounds like Tef-gel or anti-seize.

Instead you use mild steel... less chance of galvanic corrosion, but the screw itself rusts away.

It's a hard world we live in, but to me t he s/s option is the better one.

Jim

I’ve never seen steel rust away under water

Steel it troublesome in the wet dry cycle

If you think that you application is wet dry then SS is the choice

If you review technical information concerning anode installation a mild steel star washer is always used between the bolt and the anode

This is because SS to zinc is galvanically active . Galvanic Corrosion will occur between a stainless washer and the anode creating a poor electric connection and degrading the anodes effectiveness

You will also note that high quality anodes have steel molded into the zinc anode to facilitate proper grounding

[Jim Cate]

This is because SS to zinc is galvanically active . Galvanic Corrosion will occur between a stainless washer and the anode creating a poor electric connection and degrading the anodes effectiveness

Umm, Slug, my anodes include zinc on my (gasp) stainless steel prop shaft. The electrical connection seems to survive pretty well... I think that is what it is supposed to do.

I don't believe that non-s/s star washers have been standard on anodes that I have used for some few years now, and I'm quite sure that s/s screws are used to clamp the halves of the anodes together. Dang if they haven't worked pretty well... at least none of my props have gone pink and fallen apart.

I don't dispute the disparity of the electromotive series differences between s/s and zinc, but I do think that the use of s/s screws is pretty standard in this application.

Jim

[slug]

To overcome the stainless to zinc issue high quality shaft anodes have a copper or steel contact molded into the shaft anode to improve conductivity with the SS shaft

https://www.solentanodes.co.uk/produ...0mm-ball-heavy

[ggray]

Well, I've learned more here about conductive grease!


Actually, I think the only place I've used it is some electrical connections. I believe I used Lanocote on the "zinc" bolts as it was close at hand.


But it sounds like I'm wrong in both cases!

[Disailor]

For shaft zinc's, I have found that coating a minimal area around the clamping screw with nail polish helps to keep enough zinc from disappearing & the screws hold tight.