On mast-step structures
If I had a pencil and a piece of paper (or cardboard, or sheetrock, or plywood, or the top of the table saw) as I usually do this would be easier to explain. I'll consider drawing this and scanning it for posting here-- but with this old computer that may take some doing.
Dalliance, you raise some vital points and, in short, you are correct that the metal structure is not the way to go at all. Somewhere I posted some pics of my (unfinished) and I'll post them here again.
Simply stated, the mast/hull/chainplates structure resembles a bow and arrow. The mast is the arrow. The shrouds (all the stays) are the bow string(s). The hull is the bow. The compression always wants to push the mast through the bottom of the hull. The tighter or more loaded the stays are, the more this is true. You could make the shrouds and mast/compression post good and strong and then the hull (the bow) would bend upwards at the chainplates. All of it has to be sound in the same scale.
In about 1970 my dad designed an addition for our house. The rec room had/has a cathedral ceiling and through it he ran three crosswise joists, like for a second floor, at eight-foot centers. When I (the future architecture major) asked him what they were for, he said that they tied the tops of the walls together to keep the roof from sagging. Without them, the top edge of the walls, where the rafters sit, would bow outwards allowing the ridgepole to sag. Eventually (in a pure world) the walls would fall outwards and over, and the whole thing would collapse. The beams were there to triangulate the structure. (Buttresses on the outside of cathedrals support the walls against the same thing, so that is why cathedrals don't have tie-bar joists running through them.)
The hull and bulkheads serve the same purpose. In an all-open boat, the chainplates and shrouds would be pulling upwards enough to cave in the sides of the boat, allowing the bowstrings (shrouds) to relax. The 'bow' has to be kept in place as well. This leads to-- you guessed it-- crosswise beams.
In most of our boats these 'beams' are the main bulkhead. Kept adequately stiff, the plywood separates the sides of the boat against the shrouds. So the whole hull structure has to be tied together with material that will not compress or collapse, from chainplates to mast step (and I mean the base of the mast column at the keel, including the compression post) to chainplates the other side. In a wooden boat, this is done with laminated crossmembers, reinforced ribs that hold their shape in a curve, support the mast base (for it is never on the keel itself) and providing anchor places for (at least) the upper shrouds. Fiberglass boats rely on a stressed skin-- there are no ribs, so the whole hull is a structural member. This is why fiberglass, not paper-mache, is used, because the fibers laid-up in their proper directions provide structural rigidity. So hulls are beefed-up at this location (and others) using more and different kinds of 'glass.
For Cherubini 44s, though we already rely in a phenomenally strong hull-to-deck flange, we lay up a heavy web of fiberglass in this area, a triangular box section knitted into the hull with layup extending almost all the way down to the keel. The boat is incredibly strong in this area-- many cases have been of impact with boats and bulkheads during storms in which the C44 did more damage to the other stuff than the other stuff did to it. Only one C44 in my knowledge ever failed here; and that was because the owner had tensioned his shrouds so unbelievably tight that he had drawn the flange up, actually delaminating his teak deck. I'd never known that was even possible. For our older Hunters the metal toerail bent along the gunwale serves the same purpose as a flange and the best thing, for structural integrity if not for sailing, is to mount the chainplates (or u-bolts) to it.
The fix for our cheaper, lighter, simpler boats is to ensure that as much mast load as possible is transmitted not to one point but across the whole skin of the boat. The main bulkhead (where the mast step is) should be sound, with no softness and no separation from the hull. Check all visible or accessible bulkhead edges (of the plywood) for voids. At Cherubini we bead the edges of plywood in 5200 before 'glassing them in. This fills the space between the edge of the plywood and the hull with something that will not allow air (which becomes water) and enables a fat fillet of material over which a smooth lay-up can go. If you have any exposed plywood edge showing, or know of any gap between the ply and the hull, fill it. This is number one.
Next ensure that the plywood is soundly fastened to the hull. I would not go overboard but adding well-laid-up 'glass to the bulkhead-hull joint and to the bulkhead-deck joint as well will help a lot. Most boats don't have this 'glassed to the deck. Why? --I have no idea. It's vital. Again exposed plywood edges are your Achilles heel.
The other thing, of course, is the soundness of the deck structure itself. All permeable core should be replaced, or just re-permeated, with epoxy, preferably with thickener like silica (which doesn't crush) and fair, freshly laid-up 'glass. Do not replace only the bad core with some sort of block, as I have admonished earlier. The step structure, like the plywood-to-hull bond, has to be transmitted throughout as wide an area as possible to lend as much stiffness and strength as possible. 'Micro' fixes without an eye to the 'macro' requirements are always bad business. Never overlook that on a sailboat, like a motorcycle, everything has a purpose and all the purposes are interrelated. Considering one part of the structure in a vacuum almost always leads to shortsighting one of the other structures. Ideally the whole deck is part of the mast-step structure. Give it the best chance you can.
Remember that, given that the plywood is sound, the compression post is really only there to ensure the plywood does not bow out of column from the mast's load. On my small boat the plywood holds almost the full load-- the compression post (which I replaced) is really just a stiffener. Without it, the plywood would split or bow and thus break. A freestanding compression post is hardly as stiff as one attached to a bulkhead. (I always recommend that a freestanding compression post be made out of a section of the very same mast extrusion-- why should it be anything else?) And a freestanding compression post point-loads the hull, or sole, or bilge, or keel, at its base, which negates the value of a stressed-skin structure. So ensure that the compression post is well-fastened to the plywood, all the way along its height.
At the base of the mast, again, the load should be transmitted to the skin of the boat, not to one point, same as the plywood is attached all the hull along its edges. On my boat I made sole joists of 5/4 mahogany, thoroughly saturated in epoxy, their ends fitted to the hull, on like 10-12-inch centers. This replaced the single sheet of 1/2 plywood that had been 'glassed to the hull (and had rotted from bilge water). At the mast step I made a doubled joist, fastened to the bulkhead along the forward edge, on which stands the compression post (which is fastened to the bulkhead). This 2-1/2"-wide joist is fitted to the hull at its ends (it's wider not for strength but to spread the load where it meets the hull). All of these joists are bonded in with 5200 and the outer sections of the new plywood sole itself (like 12"-wide floorboards) are bonded to them with 5200 and epoxy, making this an integral structure. I did not 'glass this to the hull all along (but could and probably should have). It's held down by stringers on the bunk fronts.
Shortly after I had rebedded the keel in new 5200, and before the keel-bolt nuts had been all tightened down (some were well off the washers), the yard moved the boat. The keel did not fall off and the hull remained stiff. 'Nuff said there.
Without replacing a sound plywood sole, you can get under it (cutting new access holes, which should have been there anyway, to get to it), and fashion a crossmember under the floor to support the mast base or compression-post base. This does not have to touch the actual bilge of the boat (indeed it should allow gooky water to pass freely under it). My joists have limber holes just like in an old wooden boat. Most importantly it should be bonded to the hull and the structure above it that is-- the cabin sole (subfloor)-- and to the bulkhead. Given that your main bulkhead is adequately bonded to the hull, the deck, the sole (and your crossmember) and to the compression post, this should provide sufficient stiffness to keep the bulkhead from bowing, the compression post in column, the keel from falling off and the mast from denting the deck.
I will reiterate what those who have read plenty of my incessant posts have heard often before-- avoid materials for the carpentry and homebuilding trades. Oak is strong but loves to rot. So do ash and most other furnituremaking hardwoods. Most are also impervious to adequate epoxy saturation. Plastics are insufficient (the word 'plastic' means 'pliable'). Metals are both unnecessary and prone to corrosion; also they are hard to work with and vulnerable when tooled. Your best solutions for structural members in small boats are always the traditional boatbuilding woods-- mahogany, spruce, cedar, and so on. Ideally sitka spruce is best for compression, but it has become hard to get. I used white spruce for mine, laminated out of two 2x4s of the stuff, using epoxy. It's strong, light, stiff and pretty. I put a corner bead on it like on a old stairway. That was the hardest part of making it (and not hard at all).
The crossmember can be of good Philippine mahogany (aka lauan). Laminate it in two pieces for stiffness and strength. Avoid the more recently-popular African varieties; they are oily and ugly and take epoxy less well. Honduras/Brazilian is too expensive and unnecessary for looks. WEST epoxy (or MAS or Smith's or anything primarily meant for laminating wood) is your friend. Use it liberally and properly. Avoid any glues you can buy at Lowe's. Via the Web you can locate sources for such things, even wood, that are only a Fed Ex shipment away. Learn to use boatbuilding methods and materials correctly, ask questions, learn more, take advantage of what you learn. There is always a brain trust in every boat shop and marina. You just have to be judicious about whose brains to pick and whose to avoid. Some people hold opinions that hold no water (and nor do their boats!).
I have 40 years in this business, I learned from and still work with some of the very best people ever in it, and I don't mind talking or writing, so pick my brain while I am here.
Here are some older pics of the structure I made. This weekend I will take some more photos of the boat and see if my meagre handiwork can illustrate what I am talking about with the compression post and crossmembers.
