What kind of metal should you use for keel bolts?
Keeping your keel securely fastened to the vessel should obviously be a primary concern. Loose keels cause further damage, but having a keel fall off is a complete disaster. The last thing you want is your masthead light under water.
When it comes to building a boat and choosing the material for your keel bolts, or replacing your corroded or otherwise untrustworthy keel bolts your choice in material is VERY important! The wrong material can cause a reaction in the metals, leading to a very wet, very upside down disaster.
Keel Bolt Materials
If you have silicon bronze keel bolts, you're in luck. Silicon bronze is to many builders and designers the "proper" alloy to make keel bolts out of. Why? Because Silicon bronze doesn't corrode when in seawater. Keel bolts made of silicon bronze will last the lifetime of the boat.
Just be sure that your keel is made of lead. Silicon bronze keel bolts married to a cast-iron keel will create a galvanic reaction, as bronze bolts paired with a cast-iron keel will create a galvanic reaction and corrode the bolts. The other thing to keep in mind is that they are susceptible to over-tightening, so don't exceed the torque spec for the size of the bolt.
Galvanized Mild Steel:
Galvanized steel is the cheap option, and the one-time cost savings is negated later on down the road when it comes time to replace the keel bolts.
Boats were built with galvanized steel keel bolts either to save money on the fastener. While bronze would have been a better choice, it's a more expensive alloy and cannot be used with cast-iron keels. So if you have a cast iron keel and a production boat from the likes of Jeanneau, Hunter, or Beneteau, you'll probably find that the keel bolts are made of galvanized (or even worse - zinc coated) mild steel. If that's the case, you should be checking your keel bolts regularly and with the largest cost of inspection being the labor, it's worth it to just replace them during inspection. Keep in mind that even pristine looking keel bolts can have unseen hairline fractures.
One of the most common alloys used for keel bolts is stainless steel, and if you do a bit more research you'll find anecdotes all over the internet about why stainless steel keel bolts are great or why they are the worst choice. One sailor will write that he pulled his hundred year old stainless keel bolts out and they look just like they came from the factory that very day, when others will tell you that theirs corroded through seemingly overnight. The internet's vast supply of know-it-alls haven't yet come to an agreement on the efficacy of stainless steel keel bolts.
Here are a few of the facts; Stainless steel simply stains less than mild steel, it's not stain free! Particularly, when immersed in saltwater stainless steel becomes "active" and corrodes faster than a newly polished piece of brass! However, if they do stay dry then stainless keel bolts can last as long as anything. Because of stainless steel's need to stay dry, the tops of the bolts are often encapsulated in resin to help preserve them.
What the hell is Monel? Monel is a series of alloys that is made up of mostly nickel and copper. It's expensive, it's a bit more rare, but damn is it a good material for keel bolts. One of the best aspects of monel is how high up the nobility chart it is. Since it's such a noble metal it is extremely resistant to corrosion in a saltwater environment and will match well with your lead keel.
Another item to add to the "pro" list for monel is it's strength. Monel is stronger than both stainless steel and silicon bronze, allowing a builder to get away with smaller diameter keel bolts, or tack on a bit of extra strength for peace of mind.
The drawbacks to monel keel bolts of course include the price of the alloy, but otherwise you can treat it like a slightly more noble version of silicon bronze and respect the torque specs while not threading it in to a cast iron keel.
The most noble. The king. Titanium is the absolute best material to use for many kinds of keel bolts. For a performance yacht trying to keep weight down, titanium is extremely light and strong. They built the SR-71 Blackbird out of it, it's in high-end medical implants, and they're blasting the stuff off to space on a regular basis.
Titanium isn't used very often simply because of it's perceived high cost. While it's true that titanium is a high end material with higher costs, you may be surprised to find out that the price of titanium has been dropping rapidly over the years and it's similar in cost to monel or silicon bronze.