|
|
|
Fin Keel vs Full Keel - In order to properly understand various response and handling characteristics of a sailboat, it is important to first understand some of the inherent characteristics associated with a particular hull design.
There are a multitude of factors that contribute to the overall performance, stability and handling of a sailboat that are all incorporated into the initial design. Hull shape, keel and rudder shape and design, ballast displacement ratio, sail area, weight ratio, sail plan, etc.
The following explanations are somewhat simplified, but give you some of the basic considerations and flindamentals of keel shape and performance to be expected.
The Fin Keel and Spade Rudder
Note the shape of the fin keel boat. Here the keel itself is relatively short in comparison to the length of the waterline and the rudder is positioned well aft of the keel. Some characteristics of this particular design are:
1. Faster - Faster in terms of acceleration (all displacement hulls are governed by a theoretical hull speed of which they cannot exceed in flat sea conditions). Since the amount of surface area under the boat is minimal, there is less friction being produced, less drag, and therefore it reaches hull speed quicker.
2. Ability to Turn Faster - Because the rudder is positioned well aft of the keel it can get a clean movement of water across it; so as soon as it is turned off center the deflection properties are almost instant; and therefore the boat starts turning quicker. Also, because there is a smaller lateral surface area, a fin keel boat can spin a circle in about 2/3 the distance of a full keel boat.
3. Points Higher - Due to hydrodynamic properties of the fluid movement across the keel, a lifting and driving force are being created, which in a sense pulls the boat closer to the wind. (See diagram explaining hydrody- namic lift later on.)
4. More Demanding in Rougher Weather - Due to the sensitivity of the boat regarding speed and quickness, and because the keel has less projected surface area, the boat can have a tendency to jump around a little more in rougher weather, making it more demanding on the helmsman.
Because of the above mentioned characteristics, the fin keel boat is a sportier design and is preferred over full keel boats for performance racing.
Full Keel
The full keel boat, has a slightly different underbody. Here the keel runs almost the total length of the watefline, with the rudder connected to the keel (keel hung rudder). Some of the characteristics of this particular design are:
1. Usually it is a bit slower - There is more friction being produced due to added surface area moving through the water. Friction is the greatest handicap in light wind conditions. When winds are strong enough to achieve hull speed the Full Keel boat can sail as fast as other boats.
2. Slower to turn - Deflection of water off the rudder is not as clear. It takes a little more room to turn a 360 degree circle. Full keel boats are also more difficult to steer when operating in reverse.
3. Directional Stability - With a large projected vertical surface area, the full keel is said to keep the boat sailing in a straight line. On some boats the good tracking ability gained by a full keel may be offset by impaired ability to get the boat back on course after it has been offset. A full keel boat is best suited for sailing long distances and sailing ocean passages.
4. Strength - Modern full keel boats have internal ballast and therefore the keel is part of the same structural unit as the rest of the hull. This makes a stronger overall unit. Also the rudder is hung from the back of the keel giving the rudder a stronger attachment to the boat.
Hydro Dynamic Lift
A fin keel boat separates the water flow across the keel creating differences in hydrodynamic pressure resulting in beter performance and efficiency to windward.
As you can see in the illustration above, the boat is sailing to windward on a close reach or close hauled. Notice how the boat does not sail perfectly straight, but rather at a slight angle (yaw). The water that hits the downwind side of the keel creates a pushing force called drive. The water bending around the windward side of the keel is forced to follow more of a curved path. The curving of the flow of water across the windward side creates an area of less pressure (hydrodynamic lift), and has a tendency to pull the boat up closer to the wind. This same effect also occurs at the rudder resulting in a lifting force off of the rudder and better efficiency to windward.
The water flow under a full keel boat is forced to follow more of a straight line and is therefore unable to produce hydrodynamic lift The great area of the full keel simply resists sideways slippage while allowing the boat to proceed in a forward direction.
