As has been explained the mast step is the method of attaching the bottom of the mast to the boat.A keel stepped mast typically sits on a reinforced structure in the bilge of the boat. The mast passses through a hole in the cabin top or deck and is continuous from the bilge to the top of the mast. A deck stepped mast is stepped in the cabin top or deck, and there is a supporting structure below the deck that keeps it from piercing the deck. The following is the draft of an article that I wrote for a different purpose but which talks about the relative metits of each.
Deck vs. keel stepped masts
While I personally strongly prefer a deck-stepped mast over a keel stepped mast, I completely understand that once again this is an area where opinions can differ widely. There is no right answer here. There is a contingent that thinks that the only proper way to step a mast is on the keel. There is a logic to that opinion but it is a logic that can be engineered around and which comes out of a historical context that is less relevant with modern materials and design approaches.
I would like to start this discussion with the structural basics, the base of a mast has a vertical and horizontal thrust to it that tries to push it down through the bottom of the boat and also sideward off of the mast step. In normal conditions the down load is several times greater than the side load. Beyond the loads imparted to the boat, there is also the issue of the loads that happen internally in a mast. When you look at the structure of a mast it is really a truss standing on end but it does not completely act as truss because the components of a truss are not supposed to have bending loads on them. Ideally the loads in the mast are primarily axial (acting along the length of the mast) rather than in bending (acting perpendicular to the long axis of the mast). But because of the continuous attachment of mainsails and the point loadings of intermediate control lines like vangs, pole lifts and the like, masts do have fairly large bending loads imparted into them. The two most often cited reasons for keel stepped masts being considered stronger is the way that the bending loads (moments) are distributed within the mast itself and the way that the mast imparts its loads into the boat.
One of the goals in designing a mast is to create a configuration that, within reason, reduces the size of the bending moments within a mast. In a general sense, the greater the number of panels (segments between shrouds and other supports) the smaller the moments within the mast would tend to be. In the days when single spreader rigs were most common a keel-stepped mast added one extra panel, the segment between the mast partners at the deck and the keel. This has become less significant as bigger boats have routinely gone to multiple spreader rigs (adding more panels above the deck) and to a lesser extent as moment connections at the deck are being employed on deck mounted mast steps.
In terms of the way that the mast imparts its loads into the boat, masts are generally located in the area of the cabin trunk and because of the shape of the cabin (i.e. the deck folds up at the cabin side and horizontal again at the coach roof) this area, if the deck is not engineered for side loads it is more prone to lateral flexing than would be the keel. One idea behind a keel-stepped mast being stronger is that with a keel stepped the mast is not superimposing loads into the deck.
In reality, this ideal is rarely accomplished for a number of reasons. First of all, if the mast is not tied to the deck or the deck tied to the keel near the mast, either with a tie rod or with a tie from the mast to the deck and a connection from the mast to the keel, the downward force of the mast working in opposition to the upward loads of the shrouds can pull the hull together like a bow and arrow lifting the deck and separating the joint between bulkheads and the deck. You sometimes see this type of separated bulkheads on inexpensive or early fiberglass boats with keel stepped masts.
Not only do keel stepped masts impart vertical loads into the deck (through the ties mentioned above) but they also typically end up imparting side loads as well (they must if they are going to reduce the moments in the mast as mentioned above). This added side load when combined with the multiple panels above deck greatly reduces the structural advantages of a keel-stepped mast to next to zero assuming that a deck-stepped mast is properly engineered, and of course that is a big ‘if’!
There are several things that I consider critical to engineering a deck stepped mast properly. Primary is having a properly engineered jack post below the mast to take the vertical loads of the mast. (A jack post is a vertical member that carries the vertical loads of the mast to the keel.) My preference is to have an aluminum jack post rather than a wooden one but a wooden post can work as well. My preference would also be to design the jack post, mast foot and mast step to create a moment connection between the base of the mast and the top of the jack post, so that the jack post could still act as an extra mast panel. Then the deck and interior structure need to be designed to distribute side loads. Ideally, there should be a bulkhead or ring frame adjacent to the mast that can take the side loads and distribute them into the hull. These elements are obviously more complex to engineer and build properly than simply having a fat spot on the keel for the mast step to land on.
My biggest objections to keel stepped masts are to the purely practical. Keel stepped masts with internal halyards mean that there is always water in the bilge. This water comes in at halyard boxes and other openings in the mast and there really is nothing you can do will stop that. Second, it is way harder to step and un-step a keel-stepped mast making the boat more subject to damage in the process. Beyond that if you loose a mast (I have lost two in my life) it is better in my opinion to loose a deck stepped mast because a keel-stepped mast is more likely to damage the deck when it fails and a deck-stepped mast is easier to clear away. The keel stepped mast for offshore use advocates point out that if you lose a keel stepped mast you are more likely to end up with a bigger stump after the mast fails. I am not sure that that is always the case. For example with a deck stepped mast there are cases where you are able to tow the rig as a drogue until things quiet down enough to rig a jury rig. I am not sure what you do when the boat is being beaten to death by the upper portion of a mast that has buckled 20 feet off the deck at the spreaders. .
As mentioned above, my preferred set up is a deck stepped mast that has a welded flange on its bottom that is through bolted through the deck into the top flange of a structural aluminum jack post. If the mast buckles it can be unbolted and jettisoned, or kept up partially by the moment connection at its base. My current boat has a keel stepped mast. It is my intent to pull this mast and have it modified to that arrangement if I ever go offshore with her.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies