Originally Posted by Faster
One of the largest factors will be keel material. Lead absorbs an amazing amount of energy as it deforms under impact. Iron keels do not and transfer the full forces to the hull - therefore internal damage is likely to be more significant with iron keels.
The designer Bill Garden said the opposite in one of his yacht design books. His opinion was that lead would crush and "form" around the obstruction and as a result, the boat would absorb the full energy of the impact. Iron, on the other hand would tend to bounce off the obstruction and not absorb the same amount of energy. He preferred iron on boats that would sometimes have to "feel" their way into anchorages.
I have seen or experienced both and I tend to agree with Garden. I hit a rock at full speed with a lead fin keel - the boat stopped dead, crew were thrown off their feet, minor injuries etc. The boat survived intact but I had an 8" square area of the toe of the keel to fill and fair on the next haulout.
On the other hand, I was looking out over English Bay one evening from a bluff in Kitsilano, watching a 33'ish wooden motorsailor motoring by fairly near shore. All of a sudden there was a very audible CLANG and the boat jumped about a foot. The skipper merely turned a few degrees offshore and just kept on truckin' - completely unruffled - no crew flying about, no drama, just an almighty CLANG, clearly audible from at least 300 yards.
I saw that long before I read Garden's book and immediately thought of it when I did - his words completely agreed with what I witnessed (and later experienced).
Also, when I was redoing the iron keel on my current boat, on the toe of the keel there were a number of 1/4" deep gouges in the toe area - the boat had been through El Caribe from New York to L.A. and had apparently hit some coral or something - 1/4" deep gouges in SOLID IRON - how much force did that take?
It left a lot less damage than the lead incurred on my old boat.
As to the OP's question, I am confident my iron keel can withstand a hard hit. It has a 1' wide, 10' long flange along the top with a double row of studs spaced 10" apart laterally. For those familiar with the Thunderbird, it is not unlike how their keels are mounted. The whole thing fits up into a 1 1/2" deep rabbet moulded into the hull. The bottom above it at that point is 2" thick. On the inside, each pair of studs passes through a piece of 2" X 4" X 24" stainless channel with thick fender washers and double nuts on each stud. There are also 8" timber "floors" glassed into the bottom between every other stainless channel.
I think it can take a hit.