This article is derived from a posting I placed on the Paceship forum a few years back. I thought I would bring it to the forefront and expand a little on the subject “What’s the Big Keel” (or what's the big DEAL about KEELS!).
There is a lot of misconception about the purpose of the keel and even more about those so called “Keel/Centerboard” rigs common on sailboats both new and old.
As a start let’s examine the purpose of a keel in a sailboat. The most important purpose of a keel is to convert to forward motion the sideways motion of the wind and to counteract the leeward force of the wind. In ballasted boats a secondary purpose of the keel is to provide ballast to lower the center of gravity of the boat thereby providing a righting moment to stabilize the boat against capsize.
Without a good keel, sailboats would be relegated to sailing mainly with the wind as did the old square riggers. Even the earlier schooners had relatively poor performance sailing to windward. Trial and error ultimately developed the technology adding better keel designs resulting in the high performance sailboats of today.
Unballasted boats such as Lasers and other small planing boats utilize only centerboards (CBs) whether a dagger board or swing up cb arrangement. The dagger board slides straight up and down in a slot while other designs utilizes a board that swing back and up into a “trunk” built in the boats bottom/cockpit. Generally, vertical boards are more efficient (better lift-to-drag ratio) than boards that angle back. .Additionally, long and narrow width (fore to aft) boards (so called high aspect boards) are also more efficient than wider boards and keels.
There are many boats that incorporate both ballasted keels and centerboards. Many people believe they have a “Swing keel” boat if their centerboard swings up and back into the keel’s trunk. However, a swing keel is truly a different animal and one in which the entire ballasted keel itself is retracted into the hull. An example of this is the Catalina 22 “retractable keel” model.
Most of the boats that use both ballasted keels and a centerboard that swings back and up into a keel trunk are actually and technically a “Keel/centerboard” arrangement. there the board is lightly weighted and only enough to keep the board down by gravity under maximum boat speeds.
The keel/cb boat is a hybrid which although may have initially been developed to allow boats to be operated in shallower waters have additional advantages. As noted above keels and CBs that have a high aspect ratio are more efficient. Also vertical boards are more efficient. Ballasted keels are mostly long swept back relatively shallow and wide. Keel/cb boats have the additional advantage of providing both needed ballast and better lift.
Some think the CB should remain down to provide ballast. However, most CB's of a keel/CB arrangements are pretty inconsequential in adding additional righting moment. For example, the 50-60 or so pound Paceship PY23 CB with a moment arm lift center of effort of 2 ft longer (half of its 4 ft length) than the iron keel, will likely add only about 10% additional righting moment than the iron keel by itself) and although I don't mean to get boat specific here, it is an illustration of the point.
Now with regard to "righting moment" or "moment arm" it is possible those may be strange terms to some folks. In this discussion on the forum a member used the term "leverage" which is a good analogy. If you think of a "see-saw" and think of moment arm as being a combination of weight and position on the see-saw board extending beyond each end of the fulcrum (pivot point). A long see-saw length (arm) and a small weight can out-leverage a short arm and a big weight.
Back to the question, of what happens as the keel or CB goes through the water. Well, sideways motion in combination with forward motion causes LIFT. The lift pulls the boat towards windward and that is how leeway is reduced.
So although many folks (in fact some seasoned sailors) believe the force of a keel or CB moving through the water results in lower heeling, actually, boats heel MORE as a result of these forces!
Think of it, if the keel/CB reduces leeway (which it does), then the force developed by the keel/CB must be TOWARDS windward. Since all of this force is developed BELOW the center of gravity of the boat, then the keel is pulled TOWARD the wind making the MAST tip move AWAY from the wind causing more heel. So outside of the keels static righting ability as a result of its ballast, its "aerodynamic" force creates more heel.
However, there other and perhaps more important implications of these facts!
Anyone who has sailed Lasers and other dagger board boats (which have CB's that move straight up and down vs swing up) knows that when the wind is strong you can REDUCE heeling by pulling the dagger board up some. Pulling it up reduces the surface area (and therefore its lift and drag) and shifts the center of effort UP towards the center of gravity which reduces the heeling MOMENT ARM. So, when you run out of counter weighting (like hiking out on the rail or hanging out on the trapeze), pulling the board up a bit can allow you to sail without going over. It might be at the expense of a little leeway but it may also win you the race!
Now for you Keel/CB sailors also note that CB's that are more straight up and down are more efficient (a better lift-to-drag ratio ) than those at an angle. CB's pitched at an angle are less efficient and have more drag. So, as a swing type CB is pulled partially up, three things happen; wetted area decrease along with drag, the moment/lever arm is shortened and the center of effort (lift) of the board shifts AFT as the board angles backwards. The result is that it reduces heeling and shifts the center of effort of the cb aft which also reduces weatherhelm! And this is exactly what is needed when the wind really starts to blow.
Generally keel/cb boats have a distinct advantage over the fixed keel versions of their model. pulling your CB up when sailing with the wind allows you to reduce drag and eek out more than a few seconds per mile. The long narrow CB is also more efficient when deployed than the fixed keel model allowing it better pointing abilities and less leeway.
As you can see the centerboard is a consequential tool in your sailing toolkit! Imagine the concept of modulating your CB to help manage your wind horsepower and win you races!
In summary, the keel / CB reduces leeway but increases heel left alone. Using it tactically can reduce weatherhelm (swing type CB's only), reduce heel, reduce drag and help you more effectively use powerful winds.
Although the above might be a little over-simplified and a bit broad, it is very close in practical application out on the water.
Please feel free to contact me with your comments and questions.