SailNet Community - View Single Post - Full Keel Vs. Fin or Modified Full Keel
View Single Post
post #8 of Old 11-13-2007
Senior Member
Join Date: May 2002
Posts: 3,233
Thanks: 9
Thanked 56 Times in 54 Posts
Rep Power: 16
Fred, re-read Jeff's article. It appears to answer most of your questions, except perhaps the specific characteristics of a full keel, backing down. It doesn't answer all your questions in the specific terms that you're asking them, but it describes the principles that lead you to most of the answers you're seeking.

For example you ask about "Full Vs Fins beating to windward." Jeff says: "[Full keels] have some advantages; they theoretically form a long straight plane, which keeps a boat on course better (greater directional or longitudinal stability)." Later, he explains that : "[Full keels] have some disadvantages; a larger portion of the keel operates near the surface and near the intersection of the hull and keel, which are both turbulent zones. They also have comparatively small leading edges, and the leading edge is the primary generator of lift preventing sideslip. Because of that they need a lot more surface area to generate the same lift. Surface area equates to drag so they need more sail area to achieve the same speed. Long keels tend to be less efficient in terms of lift to drag for other reasons as well...."

You also ask about the "Practical effect on speed." Jeff says: Surface area [of the keel] equates to drag so [full keels] need more sail area to achieve the same speed.

In discussing fin keels, Jeff says: "Fin keels...have less drag as explained above so they typically make less leeway and go faster."

There's a lot of "meat" in Jeff's article. If you'll re-read it carefully, thinking about each of your questions as you do so, you'll find that he answers most of your questions.

With regard to the above two questions, let me take a stab at explaining the general principles in a little different manner. A boat with a large amount of "wetted surface" has more drag, and is therefore slower, than a boat with a smaller amount of wetted surface. Wetted surface is the amount of the boat's bottom, keel and rudder that are submerged in the water. (A racing hydroplane, to use an extreme example, usually runs with only it's prop, rudder and a few square inches of its hull touching the water. That minimizes drag, and enables it to use most of it's engine power to generate boat speed, rather than to push a large mass of water aside as it moves forward.) A full keel sailboat has more wetted surface than a fin keel, so a full keel boat will generally be slower than a boat of equal proportions (width of beam, waterline length, etc.) with a fin keel.

Generally, the faster a sailboat moves through the water, the faster it is capable of going, and the closer it is capable of sailing to windward. I like to say, "Speed begets more speed, and better pointing ability." As the boat's speed through the water increases, the surfaces of the keel and rudder become more efficient in generating lift, and that helps the boat point higher. The same is true of the ability of the sails to generate lift, as they move through the wind. An ultra-light racing sailboat can generate enough speed so that it can even rise up onto a plane, lifting part of the hull out of the water, and, when that happens, the amount of wetted surface decreases dramatically and suddenly, resulting in a dramatic and sudden increase in boatspeed.

I hope this helps.

Last edited by Sailormon6; 11-13-2007 at 12:51 PM.
Sailormon6 is online now  
Quote Share with Facebook
For the best viewing experience please update your browser to Google Chrome