Galvanising a cast iron keel?

[Ric]

I've never come across a yacht with a galvanised cast iron keel - why not? Admittedly there would be a cost - but surely a lot less than a lead keel and plenty of boats have lead keels. So why are galvanised cast-iron keels not seemingly common?

 

[sailorman]

http://www.galvanizeit.org/design-and-fabrication/design-considerations/dissimilar-steel-chemistries

 

[VicS]

I've never come across a yacht with a galvanised cast iron keel - why not? Admittedly there would be a cost - but surely a lot less than a lead keel and plenty of boats have lead keels. So why are galvanised cast-iron keels not seemingly common?

One reason would probably be the short life of galvanising in salt water.
Imagine having to remove several tons of keel and carting it off for re-galvanising every 2 or 3 years , quite apart from the fact that the cost of galvanising is usually based on the weight of the object

 

[TimBennet]

Cast iron isn't an ideal material for a keel: It's not that heavy, it rusts, has problems being bolted on with long lasting bolts, and is too hard to provide energy dissipation when hitting a rock.

So there's a limit to the expense you're going to lavish on something with so many short comings when a much better lead keel is not that much more.

It's only really in the UK that cast iron keels are the 'norm'. In the total scheme of a new boat's price, a lead keel wouldn't add that much to the cost. And if lead was the norm (like in the USA), the scrap value of the keel would cover the cost of scrapping all the old wrecks that litter our boat parks.

 

[Seajet]

2 points there.

1, My mild steel keel plate is galvanised - hot dipped which is far preferable to spray on - and the galvanising lasts much, much longer than 2-3 years, my replacement keel is probably 8 years old now and the galvanising is good as new, despite being a lift keel which naturally scrapes in the keel casing going up & down.

2, Cast iron doesn't corrode, my cast iron ballast bulb is 36 years old and as new; never heard of anyone galvanising cast iron, it isn't necessary for the material.

 

[TSB240]

Cast iron doesn't corrode,

Cast iron isn't an ideal materiall: it rusts.

Must be some special cast iron on an Anderson or possibly the galvanised plate is acting as a sacreficial anode?

 

[Ric]

My cast iron keel definitely corrodes!

I realise that cast iron keels are the choice of low-end manufacturers, but even some fairly high end manufacturers use cast iron now.

 

[Sailfree]

Can you get lead replacement keels made to match existing. I appreciate that they will be heavier.

 

[ganter]

I'm going to be dropping the cast iron keels off my Westerly in the next few weeks when she's ashore. I certainly wouldn't bother galvanising. Instead, I'll take a grinder to get back to bare metal, as much as I can without being silly about it.
Then, I'll prime with an epoxy primer, then an undercoat of lead, over that will be a paint and over that will be the anti-foul.

Reckon after that lot she'll be good to below the anti-foul for another ten years or so. (Anti-foul once a year obviously)

The only other option is to do all of the above but skip the anti-foul and copper coat but I don't think I can afford that.

Galvanising would be a royal waste of time in salt water. It'll eat through that in minutes.

 

[VicS]

Can you get lead replacement keels made to match existing. I appreciate that they will be heavier.

Instead of hot dip galvanising how about the equivalent hot dip coating in lead. That wont corrode away like the zinc applied by galvanising.

 

[ganter]

Instead of hot dip galvanising how about the equivalent hot dip coating in lead. That wont corrode away like the zinc applied by galvanising.

That would be very good indeed. Imagine the glow from the boatyard shed with the molten lead lighting the faces of the workers as they dip your pride and joy's keels.


N.B. Zinc has no place on a boat except in a sacrificial capacity.

 

[MM5AHO]

Galvanizing cast ireon can be problematic. Its highly dependent on the analysis of the CI. In the best cases it galvanizes like mild steel, in the worst it dissolves!
We have often refuse to galvanize CI, especially old material, suspicious that it will dissolve. I have seen a case where what went into molten zinc didn't come out at all, dissolved completely. Big problem for customer, also big problem for galvanizer.

As to galvanizing not lasting in salt water, that's misleading. Many items are galvanized for total permanent immersion, partial even intermittent immersion in sewater, and the galvanizing can give 20 years service. But this isn't definitive, some seawater is more aggressive than other. For example seawater in the Gulf states area is about the most aggressive. Not all seawater is equal!

But forget galvanizing a keel, many reasons not to.

 

[Rigger Mortice]

Wouldn't a darn great lump of cast iron tend to locally solidify the zinc if it were put into a bath? Or would it be preheated"

 

[Andrew G]

Two quick points. 1) Lead is used because it is denser than iron and hence a greater righting moment can be applied for the same volume (or for less draft) (Gold has been used - not by me). 2) perhaps the non-corroding lift keel is spending a fair bit of time lifted out of the water? Cheers, Andrew

 

[Seajet]

How would one securely attach the lead to a cast iron keel ? Just coating in lead like chocolate doesn't strike me as secure enough with two very different materials.

A new replacement all lead keel would work on traditional one piece fixed keels - it's easy stuff to pour into say a sand mould - but care would have to be taken over one's sums re the new weight, and on the attachment.

Decent galvanising on steel lasts well, and I'm still mystified by peoples' cast iron keels corroding; I wonder if cathodic corrosion is to blame, it's a very inert material but no harm fitting an anode.

Galvanised steel keel plate, 10 yrs old, & cast iron ballast bulb, 36 yrs old;

View attachment 39662

 

[Kelpie]

As I own a boat with an encapsulated keel, I'd always assumed that bolt ons or lift keels required an anode. Guess not?

 

[Seajet]

I use an anode on my keel, pictured above; it's a disc anode cut in half then faired into the leading edge of the steel plate, at the front of the cast iron ballast bulb so it doesn't interfere with raising the keel and stays immersed in at least wet mud when the boat is ' dried out ', as does the lower part of the keel plate ( wet, not mud ).

 

[vyv_cox]

How would one securely attach the lead to a cast iron keel ? Just coating in lead like chocolate doesn't strike me as secure enough with two very different materials.

I'm not at all sure how it could be done with a keel, but on sheet steel it is called Terne Plate. The alloy used is mostly lead with a small amount of tin. At one time it was widely used for automotive fuel tanks.

 

[JumbleDuck]

Galvanizing cast ireon can be problematic. Its highly dependent on the analysis of the CI. In the best cases it galvanizes like mild steel, in the worst it dissolves!

When I was rebuilding my wee Hunter, which has a keel almost identical to Seajet's, I talked to one of your competitors in Glasgow about galvanising the bulb. They said that they were willing to consider it, but would want to inspect the thing closely first. In general they said that modern cast iron was terrible for galvanizing (cracks and void, from memory, being the issue) but that older things were often better done and could take it. Does that chime with your experience?

In the end I went with shot-blasting and Zynga, which seems to be working OK. At some point I'll have the plate replaced, and like Seajet I'll have the new one galvanized. I'll probably leave the bulb alone - it would take an awful long time for it to rust enough to matter ...

 

[knuterikt]

Can you get lead replacement keels made to match existing. I appreciate that they will be heavier.

We did this on a small wooden sailboat keel weight is/was 250 kg.
The goal was to keep the shape of the underwater body and same weight.
On this boat the ballast keel is bolted on with a keel stub (deadwood) between the hull and the ballast keel, the deadwood needed replacing.

We had access to the old plug (keel pattern) for the cast iron keel, calculated how much less volume necessary to get same weight and marked this on the plug.
We also calculated the changed CG on the keel (vertical & horizontal), due to keel shape we had to accept that the CG moved a little bit aft and it also moved a little bit down.

The keel was cast in Denmark by Ørkild Foundry

Link to their web site (in English) http://www.oerkild.com/html/engelsk.html
Have a look at the bottom of this page http://www.oerkild.com/html/specialjobs.html