The Importance of Control
We have a unique perspective when it comes to keel design (philosophy) by having had many years of practical experience with a full keel, a modified full keel with a "Brewer Bite" and our current configuration; a modified fin keel with fully skeg protected rudder (and yes, we even have experience with full blown racing fin keels as well). There is no question each type of keel design has some advantages but, at the same time, each has its own set of disadvantages and understanding what best fits your needs is the real question. The following are some of our thoughts (and facts) in this regard...
Why do we like a Modified Fin over a Full or a true Fin Keel?
A typical fin keel used on your average club racer/cruiser has a significant performance advantage over your typical full keel - of this there is no question. A fin keel sails far better (especially to weather) and affords superior maneuverability both under sail and power (especially in reverse).
There is also no question that full keels have some advantages over a narrow fin and they include better tracking (especially in quartering seas), being obviously more durable structurally by simply having far more contact area with the bottom of the hull and allowing lower CG (center of gravity) storage capacity (which would make the boat more stable).
For the modern cruiser concerned with safety and comfort, obviously the requirements should more closely favor the full keel characteristics at the expense of the higher performance fin... right? Well, not so fast (actually slow) ... we think the answer is both yes and no. Here are a few facts based on our years of experience with full keels...
- Full keel designs have very limited directional stability in reverse as they have a tendency to either go straight or walk in the direction of the prop rotation. They will not steer in reverse unless you have enough speed over the rudder to counter the massive lateral plain of the keel. This means you have to have water flow (and a good deal of it) over the rudder before you will gain any ability to control direction... this often means you are going way too fast for the situation. Any one who tells you any differently is either trying to sell you a full keel boat or has never experienced anything else. This means that a skilled captain has to maneuver with a heavy hand on the throttle while favoring (trying not to get in a situation against) the boat's natural tendencies. An intimate knowledge of the boat may allow you to carry this off (and a bow thruster really helps) but you can imagine that sooner or later you will find yourself in a situation where you have to maneuver against the boat's natural tendencies and this usually happens in very tight quarters. For many this fear is real and far worse than getting caught out in any bad storm. You will often find these owners are very reluctant to take their boats into unknown situations which limits their freedom and ability to use the boat.
- Full keels do not turn very well. Again... it is all that lateral resistance. Some will tell you they can turn in their own length...and they can, but in only one direction and only while using the prop walk in reverse. Trying the same maneuver in the opposite direction is --well difficult, if not impossible.
- Leeway - while full keels do have the advantage of a lot of lateral resistance on some points of sail, they generally sail very poorly to weather. It is not that the boat can't point to weather; it is the fact that the leeway is so bad you will find it is much faster (Velocity Made Good or VMG) to crack off and build speed to allow the keel to work with its lateral surface. Unlike a full keel, a fin keel can create lift to offset the leeway (to a degree). It does this by utilizing an airfoil shape similar to that of a jet's wing which is designed to work at high speeds and, as it turns out, water has very similar hydrodynamic properties at low speed to air at high speed. All the various foil sections have been developed and tested and the simple fact is that there is a relationship between the shape of the foil and the length of the cord compared to the width. To work correctly the keel has to have foil selected from the NACA 62 to 64 series which sets a defined relationship of the width to the cord (length). To try and claim a full keel can create lift is misleading at best as any lift it can create is offset negatively (if not completely) by the increased wetted surface of the keel itself.
- Wetted Surface - As a vessel passes through the water it physically moves the water around the boat and this creates friction. The more wetted surface the water has contact with, the more effort is required to move the boat. Obviously a full keel has far more wetted surface than a smaller keel.
What about structural concerns?
We know that boats run aground all the time and, indeed, a good cruising boat should be designed and intended to withstand this kind of punishment. This is the one reason most do not consider using a higher performance fin keel on a cruising boat. As you reduce the interface area between hull and the keel, you increase the loading. The idea with the longer keel (including full keels) is that the keel is more like part of the hull itself as opposed to an appendage... it disperses the load over a larger area. While modern laminates can be designed to be strong enough to handle a given load often the overkill required to obtain the kinds of safety factors we are looking for also means that the structure would be too heavy to fit inside the displacement parameters (you would never find a high performance fin keel boat designed into the moderate displacement range). A modified fin keel (which is essentially a long fin keel or a short full keel if you prefer) has enough surface contact area to allow the structure to be robust enough to meet our requirements. We design our keels for the absolute worst case scenario with safety factors high enough to meet and cover any eventuality.
The Inevitable Conclusion
Considering you are going to have to maneuver the boat in a marina (to and from a dock) nearly every time you use it, we feel that the diminished direction control issues (especially in reverse) are reason enough to avoid the use of a full keel design. We feel you cannot underscore enough the importance of being able to properly control the boat in tight confines, whether under power or sail, and the high loading and skittish nature of the high performance fin keel detracts from the obvious performance advantages. For these reasons we design our boats with a modified fin... strength and durability with ability to maneuver and sail.
More about our Design History
Prior to 1980 all Ted's designs were based on the traditional full keel. By the mid 80's our keels evolved into a modified full keel by employing the Brewer Bit (first designed by Ted Brewer). Essentially it was a full keel design with a large section cut away in front of the rudder. This still provided complete protection for the rudder and prop with a full skeg (which also acts like a tail feather to improve tracking) but also dramatically reduced the overall size of the keel. The results allowed the boat to turn better, back up with an improved degree of control, sail with less leeway (although not much) and be more easily driven (sailed faster in light air).
In 1999 we modified the keel again (along with a complete redress of the hull structure). This time we further decreased the wetted surface and used a real 64 Series NACA foil section. The new keel was a little shorter (fore and aft), taller and far more defined. Still too large to be called a fin keel but at the same time it could not really be called a modified full keel either. The results were very positive. The biggest improvement is in performance without any noticeable loss of sea keeping ability. In fact the new design is easier to control, lighter on the helm and obviously faster in all points of sail.