It is a big deal. All other things being relatively equal, there is a major impact on stability vs drag. Iron is roughly 2/3 the density of lead (490 vs 708 lbs per cu ft) If you have two keels of equal external shape the center of gravity of the iron will be roughly 1/6th of the height ballast casting above the cg of the lead. That is very significant.
So much so, that what normally happens when a boat goes from a lead to iron keel is that the designers will often increase the weight of the iron ballast 20% or so above the weight of the lead, or else increase the length and width of the keel so that the cg of the iron is roughly the same as the lead, or some combination of a larger keel external dimension and increased weight with iron. Either can really damage performance and harm seaworthiness. The increase in ballast weight means additional structural loads and decreased carrying capacity. The extra keel area means greater drag and so brings the risk that comes from the need to carry more sail longer in heavy conditions.
Also it is generally considered very bad practice to encapsulate an iron keel. If moisture enters the encapsulation envelope, as happens more often than not, it is able to cause small amounts of corrosion which progressively destroys the bond between the ballast casting and the encapsulation envelope. Since that bond is an important part of the keel structure in most production boats, this is the beginning of the end for that boat. Iron works better for a bolt on keel where it can be maintained with a sacrificial barrier coat.
Last edited by Jeff_H; 05-18-2006 at 04:53 PM.