The keel has been an area of great innovation with yacht designers. In sailing craft, it has extra duties asked of it. We'll take a look at the tasks it is required to muster in this regard.
There have been many exceptions, but generally the keel is part of a larger assembly which usually has the stem, and stern timbers attached as well as any appendages, like fin(s) or skeg, plus supporting knees and braces. This assembly, for ease, we'll call the deadwood, though this isn't exactly the correct use of the term.
In traditional boat construction, "laying of the keel" was the mark beginning the project, the boat's birth as a structure. This is no different for Weekender/Vacationer even though the build for these boats is not very traditional.
As one would expect, the keel has to hold the ends of the boat up, preventing them from drooping from the weight of the framing, planking and equipment. A lot of keels have the planking directly attached; others have the frames bear the planking in areas, with planking attached at the ends. A number of other structural members may be fastened to the keel, such as; stringers, floors, mast steps, mooring posts and engine beds. Most of these pieces tend to carry local loads to the keel.
This, usually massive piece, also must tolerate the tremendous compression loads placed on it by the heel of the mast. The tension in the rigging tries to shove the mast out of the bottom of the boat. The keel and deadwood assembly must resist this without losing shape. These loads can be very high indeed, as an America's Cub hopeful found in the early 1990's, when the compression loads coupled with sea conditions, literally broke the boat in two and sank.
Understandably, you want this thing to be strong. The backbone, as it's called sometimes, also has to withstand lateral and twisting loads, placed on it by its underwater areas fighting the press on the sails and rig. This could be a fin, rudder, skeg, daggerboard, centerboard or anything attached to the keel and under the waterline. If that wasn't enough, the sea can toss conditions at a vessel that may cause it to be supported by a small area, as the boat passes over waves and troughs, in some cases, becoming airborne for a moment, before it crashes back to the sea.
We're not done yet, because the ultimate function of the keel/deadwood assembly, in a sailing craft, is to provide resistance to leeway and an efficient lift toward windward. As it applies to Weekender/Vacationer, we can skip the fin keel, center/daggerboard, leeboard and bilge keel arrangements seen on other types of boats.
In modern sailboats this is usually accomplished by shaping the underwater area of the keel so that it's shallow at the ends and deeper in the middle. This centralized, deeper area causes resistance to skidding sideways (leeway) and creates lift when heading into the wind. Without these features, it would be very difficult to get the boat to sail if the wind was coming from 90 degrees to the centerline (a beam reach) or farther forward. This wasn't always the case. 100 years ago, this centralized lateral plane was radical thinking, most designs opted for a drawn out, long keel with drag well aft to permit the boat to track in a straight line, hove to well and simplify the build. The complex shapes of the midship keel were technically difficult. The construction issues, plus the need for most boats to work for a living (carry substantial load) kept keel designs long, deep bellied and hefty.
Now that we've gotten to the underwater area of the keel, Let’s take a look at a few profiles of old and new shallow draft, flat bottom, small sailboats.
In the first drawing, we see how it was done (this should look familiar to Weekender/Vacationer owners) before rocker was introduced into the keel profile. The keel is a big hunk of lumber, running the full length of the boat. It's straight and has a bit of drag (fore to aft slope) to help her pivot during turns. You can see there's a lot of boat in the water, compared to current design trends. This was necessary to provide lateral plane, plus the abilities of the builders were limited with the materials they had. They would have killed for water proof glue and plywood. Instead, they relied on moisture swelling the wood to tighten the joints, which required thicker planking and stronger construction to support it.
The next drawing is a turn of the twentieth century model, where the ends
of the boat are much lighter. Rocker (sweeping curve) has been introduced
into the bottom, which lifts the ends of the boat, closer to the waterline,
permits less boat exposed to the friction of the water. This also means
the boat can weigh less, which translates into better speed and maneuverability.
The third sketch is another late 1800's craft. Note the deep forefoot and deeper deadwood aft. Most small boats like these three shown could have and likely did carry a centerboard and barn door type rudder to help get to windward and pivot on while turning.
Figure 4 was designed in the mid 1950's and shows the trend of moving
the center of lateral plane to a more centralized position. It also has
a well cut back forefoot. These two features help improve the maneuverability
of the vessel, lighten the structure and decrease wetted area (drag).
These same features also complicate the construction and increase the
"liveliness" of the helm.
A mid 1960's design (fig. 5), this shallow draft, flat bottomed sailer has divided appendages. The keel is a short, drawn out laterally, fin and the separated, balanced rudder operates in clean water, rather then the disturbed flow, found behind a deadwood assembly. The advantages of this arrangement can be dramatic over the long keel, with its transom or deadwood hung rudder. The rudder is more efficient in this configuration and the reduction in drag helps tremendously.
Refinement in appendage sectional shape and wetted surface reduction is basically all that can be done to advance the abilities of a shoal draft boat. To show the full evolution of flat bottom craft we must increase the draft as is shown in figure 6. This boat is typical of modern design. The fin is well shaped in profile and section, providing very good lift properties with minimal drag. The rudder is well separated from the fin and equally finely shaped. The underbody of the hull has very little in it to hold the boat back, while underway. Gone is the heavy deadwood assembly with blunt entrance and exit.
New construction methods, materials and engineering have gone a long way to reducing the structure weight, while increasing the strength and rigidity of the boat. Many newer designed craft have no proper keel at all, just a fin attached to the bottom of the boat. These new techniques permit this homogenous hull construction to bear the loads, which an older style keel/deadwood assembly would have carried. This opens up all sorts of new engineering possibilities for the designer. Frameless hulls, light weight stay and shroud free rigs, much higher ballast to displacement ratios, massive sail plans on long skinny hulls with the stability to support them, etc. are but a few examples of the benefits of these new materials and methods.
The future is bright for sail performance enhancements. In the last Americas Cup tune up races, a kite sail was tried, but put back in the bag for more testing. Canting, ballasted keels and twin rudders are common place on the racing circuits around the world. Seamless sails of perfect, computer shaped camber are being manufactured. Wing structures, hard, soft and a combination of both will replace fabric sails in the not so distant future. Push button sailing, guided by the requested destination and GPS technology will be common place, removing navigation and sail handling from the enjoyment of a cruise. Imagine requesting a leisurely sail to you favorite dockside restaurant from the onboard system and punching "Go". Instantly, the sails are hoisted and trimmed, the boat heads out through several way points, shutting down when alongside the wharf you asked for. This isn't crazy, just expensive, at the moment.
When our grandkids have grandchildren and buy a new Hunter 27, they may
remark of manually handling a mainsail halyard, as the manly things that
were done on the clipper ships. Their centerboard may be always adjusted
to optimum depth, automatically of course. It's a pretty exciting era
in yacht design, with the experimenting in new materials, concepts and
techniques. A large berth needs be provided for your imagination.
Back Yard Yacht Builders
A non-commercial association of amateur boat-builder enthusiasts.
All our wooden boats are Stevenson designs.