Bolts for Steel Keel - Bronze? Hot Dipped Galvanized?

I'm replacing a cast iron keel with a short steel keel as part of my conversion of a Pearson 26 One Design to a slow solar cruiser. The keel with be 3" or 4" thick plate with bottom tapped bolt holes. The closed tapped holes will allow me to remove / inspect / replace bolts one by one while the boat is in the water.

I know I should avoid stainless below the waterline. That seems to leave me with bronze or hot dipped galvanized bolts (5/8" -11) I'm thinking the cost of the bronze will roughly equal the cost of an oversize tap for the HDG bolts.

The strength of the bronze is certainly sufficient.

Does any one have any suggestions on which I pick? I'm thinking bronze.

 

Er.... Reading more Calder I'm thinking mixing steel and bronze is a bad idea. Better off with the galvanized or even regular steel heavily greased and replacing every few years?

 

Use Monel and be done with it forever. I have about 35 years experience of bolting big pieces of steel with Monel. As long as you keep up the coating on the steel, especially at any weld heat effected zone, it is definitely longer than your life, and most likely for several hundred years. Use Monel for the bolts (unless you want higher strength then use K-Monel) and use a blue moly grease (Bostik or Never-Seez).

 

I'm avoiding any welding due to the HAZ for sure.

Do you have any suggestions for a vendor for monel bolts?

 

Uhhh.... Monel Nuts | Monel Bolts (deltafastener.com) "in shipbuilding monel must be carefully insulated from other metals such as steel."

 

What size? Any good fastener vendor should have Monel (nickel 400) rod or bolts. A quick glance at McMaster Carr shows 3" 5/8-11 for $39 each or 1 ft 5/8-11 threaded rod for $76.

As far as that goes warning goes, it is incorrect in context. What you are working on here is the difference in area between anode and cathode. You are blind bolting a keel to the hull. The Monel is sealed inside the thread hole and insulated with the blue moly (which if you get on you will never come off, by the way), and the heads of the bolts are inside the hull. There is no electrical path, and even if some seawater does manage to make it into the bolt hole, the tiny area of cathode compared to the area of steel (and its associated zincs), means that only a few microns of anodic material are lost each year. Yes, if two exposed equal areas of steel and Monel are electrically connected in sea water, the Monel will eat up the steel fairly quickly. Bolting a 2 ton piece of steel with a 3 oz Monel bolt is not going to appreciably degrade the steel faster than normal, even in the bolt hole due to oxygen availability. The only place where this fails is in the case of coated steel bolted with Monel that have sea water exposed bolt heads and no zincs. In this case, any pinhole in the coating causes accelerated pitting but the fix is simple; paint the Monel bolt heads also.

 

If your existing keel bolts are in serviceable condition they can be reused. Instead of a new keel you could cut the existing one to make it shallower.

 

@jehardiman - I had looked at McMaster before I asked, but they didn't list them under monel. I looked again and found them listed as nickel. Thanks for that and for the explanation. At that price point though and how easy it will be for me to replace them, I am going to go with plain steel for now.

@gonzo - The bolts were rusted almost away and the keel has already been removed. I need enough weight to get close to the original waterline, and I need a flat enough keel bottom to rest on the trailer bunk safely. I am replacing only 1/3rd of the weight so I can tune the metacentric height as needed.

 

Does it mean you plan on adding weight after to trim the boat?

 

Yup. I figure with the exiting waterline stripe and roll period test I should be able to trim the boat level and appropriately stable.

 

I will have to tune it over time as I add hundreds of pounds of batteries and a solar panel canopy. And a swim platform!

 

Why do you need a keel at all? Just dump some ballast into the bilges and be done with it.

 

A keel helps to dampen roll.

 

Yup. I want roll damped and I want a solid and abrasion resistant base for the boat to rest on as that is how the hull was designed to rest out of the water. I hope to end up with something like a lobster boat keel.

 

A steel long keel/skeg combination? The hull was never designed for that, if you want it just glass a piece of wood or G10 there. If you want roll dampening it's better to have bilgeboards.

Anyway, if you want to use plain steel bolts, paint them with an epoxy primer and paint, and keep a strict replacement schedule. Replacing the bolts with the keel in the water is unnecessary (and not really smart), you do that at one of the regular haulouts when antifouling.
Just for your info, in the last 50 years the industry has been fastening cast iron keels with stainless bolts.