Have any builders dabbled Galvanized Iron?

[HeelBoy]

Hey all, I'm a manufacturing engineer in the aerospace industry, who can't shut that part of my brain off.
In hunting for a Trailer Sailer, Iron swing keels occasionally come up, and I like the idea of that.
But, many of these things are nasty looking, and a grind/repaint kinda kicks the rusty can down the road.

Here in town we have a Thermal Spray shop that can blast molten zinc (or aluminum, or Inconel$$, etc) on iron. Definitely more expensive than paint, but seemingly more permanent once berried under a epoxy paint.

But, I'm no naval architect, so I thought I'd ask. I know of many tales of "New tech seemed like a great idea, but then everyone died.", and there's probably a reason the Navy likes HY alloys with epoxy rather than plating.... and it can't be cost. lol

Thanks for humoring me!

 

[FastOlson]

I recall reading about dinghies with galvanized steel centerboards but I have no idea if the protection worked out over time and if so how long a time. Since smaller boats are often stored parts of the year on trailers it might have been a great idea. Painted steel can rust rather fast when immersed full time.

 

[rgranger]

I like these kind of thought experiments.... here are my ponderings....

Epoxy will certainly stick to zinc... but I wonder if coating the entire keel adds any real protection in the long run. In a salt environment, it will not last long... and then you have a 600lb+ keel to remove and do it again... PITA.

The purpose of a galvanized piece of sheet metal (or wire fence) is for the zinc to act as a sacraficial anode ... that is why we put zincs on our prop shafts etc....when the sheet metal on a car (or roof) gets scratched, the exposed metal starts to oxidize but instead of the iron giving up the electrons, the zinc does. Zinc forms a hard ZnO oxide that acts like paint... on the other hand, rust flakes off and exposes fresh iron. So the zinc slows down the oxidation process but it does not stop it. On land it can take decades for a galvanized piece of metal to oxidize... in salt water.... much faster....

My guess is that keels get scratched up too easily... as soon as you knick the keel and expose any iron the zinc will start to oxidize... and as it turns into ZnO, (my guess is) it will expand and push off the epoxy coating... exposing more keel and now you are back to where you started without the galvanization of your keel.... and there is no easy way to galvanize a keel ...

Circling back to your manufacturing engineering background... I can imagine adding a welded iron bolt on the leading edge (or even better the bottom edge) of the keel and bolting on a sacraficial anode. If you shaped the anode to a foil shape, it would not detract from performance. Even better, if there is room in the keel trunk, bolt a flat bar of zink to the bottom of the keel. The electrical connection will be maintained and the iron keel won't start to rust until the zinc is consumed.... just replace the zinc every few seasons.

If you have the ability to put your boat on a lift, you could even place the zinc anode on the part of the keel that remains in the keel trunk. This would keep the anode from detracting from the keels proper shape.

 

[HeelBoy]

I Iike the idea of a zink anode, same prinipals as a metal hull.
I can make one of those for about $100, far less than thermal spray. Placing it on the leading edge also has it serve as a bumper to protect the paint on the keel... the trick is fastening it without the threads being a new vector for corrosion.

 

[dLj]

Hot dipped zinc is used in the marine environment. But not zinc coatings. Why? Zinc coatings are fairly soft and wear off easily. Hot dipped galvanized produces a coating that has 4 distinct layers and the inner layers are of increasing hardness and abrasion resistance. The overall thickness also tends to be thicker than typical zinc coatings. I don't know how big the piece is you are talking about but might be big enough to be difficult to find a hot dipped galvanizer - however if you can, it would be an excellent choice.

Here's a quick image of a hot dipped galvanized profile.

dj

 

[HeelBoy]

Hot dipped zinc is used in the marine environment. But not zinc coatings. Why? Zinc coatings are fairly soft and wear off easily.

As it's advertised, the thermal spray supposedly rides closer to hot dipped than any kind of coating or cold-galvanizer. It's shot out as molten material and deposited thick.

The inspiration for the question are swing keels like that of the old Macgregor 25. 600+ lb, so hot dipping would be a challenge. Thermal spray is much more economical... however, I could see issues arising from that localized heat application, such as warping ant embrittlement.

 

[capta]

Before I bought my first big boat I was low on the totem pole in a boat yard. We were preparing a steel boat for a circumnavigation using "tar-epoxy". It made a great isolator and metal preservative.They had a special antifouling to use with it.
A few people have used Corten steel to build sailing boats, which is not supposed to rust. It's pretty expensive. Though softer than steel, I've always liked the idea of iron boats. I've been on a few very old (around 100) ones that had little rust.
I doubt that any of these would be useful on a trailer sailer.

 

[dLj]

As it's advertised, the thermal spray supposedly rides closer to hot dipped than any kind of coating or cold-galvanizer. It's shot out as molten material and deposited thick.

The inspiration for the question are swing keels like that of the old Macgregor 25. 600+ lb, so hot dipping would be a challenge. Thermal spray is much more economical... however, I could see issues arising from that localized heat application, such as warping ant embrittlement.

The only zinc coating that produces the layers mentioned as hot dipped. The layers are formed through a metallurgical reaction with the base ferrous alloy.

Ask the supplier to provide you a metallurgical cross section like the one shown above including the hardness values. If they can’t they are just blowing smoke out their a$$

No worries for embrittlements and warpage…

dj

 

[rgranger]

I Iike the idea of a zink anode, same prinipals as a metal hull.
I can make one of those for about $100, far less than thermal spray. Placing it on the leading edge also has it serve as a bumper to protect the paint on the keel... the trick is fastening it without the threads being a new vector for corrosion.

Thinking on this some more.... I believe you would have to put the zinc in a place that is submerged when the boat is in the water AND is electrically bonded to the rest of the keel iron. Not as much of a problem sailing but if you keep the boat at a slip, you then need to decide if you are one of those who believe the keel should be swung up or left down in the slip (I'm an up kind of guy)

I found this schematic of a Macgregor 25 keel trunk

... and I drew in blue my ideas for two places where zincs could be attached ....

#1 position has the advantage of always being submerged and you could probably even change this zinc out with the boat on the trailer. The disadvantage of position #1 would be that you would have to expose the trailing edge of the keel's steel to weld on a bolt and then you have to close it all up again.

#2 position has the advantage of being easy. A split egg zinc could be attached in about 5 minutes.

The stud for the keel cable is already welded to the keel so no extra work involved. There are some disadvantages though. It is unclear if the zinc would be in the water when the keel is retracted. I guess it would depend on the boat you find. But if you are aware, you could raise the keel to a point where the zinc is just at the water surface, then mark the cable inside the boat. This would be your "slip" level when raising the keel.

 

[HeelBoy]

I have a decently capable shop, my ide was to order some Zink flat bar stock, and drill a couple thru holes in it.
On the keel, I'd grind a flat and drill/tap 2 corresponding holes to mount it as the leading edge, at the lowest point.

I then mount the bar up, shape it to match the profile of the leading edge with a sanding wheel... then disassemble it all to re paint the keel. $20 will get me a few Monel cap screws big enough for the job.

 

[rgranger]

I have a decently capable shop, my ide was to order some Zink flat bar stock, and drill a couple thru holes in it.
On the keel, I'd grind a flat and drill/tap 2 corresponding holes to mount it as the leading edge, at the lowest point.

I then mount the bar up, shape it to match the profile of the leading edge with a sanding wheel... then disassemble it all to re paint the keel. $20 will get me a few Monel cap screws big enough for the job.

That seems like a winning design to me.