A fin keel boat has a separate fin keel and a detached rudder. There are short fins and long fins but the rudder ain't attached in a fin keel boat.
On full keel boats the rudder is attached to the keel. You can have boats with cutaway fore foots and/or stern posts but they are still full keel boats. You can also have boats like the old plumb bow boats where the keel typically runs from the bow all the way to the stern or lesser models like the Atkins design all the way to the extreme racing
boats of the '20s-'30s that had so much of the keel cut away that it was essentially a fin keel boat with the rudder attached.
The keels were cut away to cut down on wetted surface. Even though fin keel boats were designed and built in the 19th Century, Old Nat himself designed a couple, they weren't considered structurally sound. Given the typical wood construction methods of the time, they probably weren't able to build a truly seaworthy
structure with a fin keel. So the racing boats from the J-boats on down cut down on the keel as much as possible while still having an attached rudder.
Cutting away the forefoot affects directional stability somewhat. Not necessarily all that much, however if the hull is relatively balanced. cruiser/raicing full keel boats like the Alberg's, S&S, Hood
, Alden, etc. up to the early '60s have cutaway forefoots without giving up much in directional stability.
These designers all knew that they could get better boat speed in light air if they cut down on wetted surface, however. They also knew that they couldn't cut away the forefoots much more and maintain directional stability.
To maintain structural integrity, that left shortening the afterpart of the keel. The problem with that is shorter the aft part of the keel in relation to the center of effort, the closer the rudder was to the CE. They knew from outright racing boats designs that the rudders became less effective as the rudders got closer to the CE. With full keel, the shorter the keel aft of the CE, the less stable, and worse, the less controllable they became.
About this time, lightweight metal and fiberglass
came into the boating
world. These construction methods made the solution to the wetted surface conundrum possible. Voila, a short keel at the CE and separate rudder placed well aft so it had a long moment arm. Ah, the best of all worlds. Actually not quite. Many boats with fin keels have little if any directional stability and it can be relatively easy to stall out the rudder resulting in total loss of control. The Columbia
26 Mark II is an example of this interesting control problem.
Fortunately, most of the fin keel boats have excellent rudder control. Many also have good directional stability so they don't have to be driven, constantly.
So for a cruising boat, long keel vice fin keel is not a clear cut preference. Some fin keel boats are easy to single
hand and steer with an autopilot
or self steering
vane while some long keel boats don't have such great rudder control or directional stability so can be a chore to steer.
The big advantage of a full keel boat is the strength of the rudder. Being attached to the keel, it won't come adrift as long as the gudgeons and pintles don't give up the ghost. You will virtually have to tear off the aft part of the keel to lose the rudder. Not necessarily the case with a spade rudder or even a skeg hung rudder.
Also the full keel boat will withstand a gounding much better than a fin keel boat. There only two types of cruisers, those that have run aground again and those that are going to run b6aground.
First, with the rudder attached to the keel, it will take a tremendous beating and still function. Unlike a separate rudder that can be bent or broken off entirely.
It is also possible to twist a fin keel in a grounding. Sailed on a Chance 3030 that had a twisted keel. Mother would outpoint every body on one tack but slipped hopelessly away to leeward on the other.
The keel on a long keel boat is tremendously strongly attached. Even with an external keel, it's virtually impossible to tear off the keel without taking the rest of the boat with it. Not necessarily the case with a fin keel. Look at the structural stress on a hull with a fin keel. All the force of the grounding is taken over a relatively small area of the hull where the keel attaches. In racing boats, there has been a recent rash of keels just dropping off becase the hull or keel fails just from the dynamics of sailing. Take a boat sailing at hull speed
and stop the keel instantly and think what the strain will be A whole bunch of grounding where the hull continued on without the keel. Makes it real hard to control heeling angle with the stick, let alone sails, up.
Another advantage of full keel boats is they don't snag everything they run over. A big deal if you sail in 'Pot' country or accidentally run over a ghost net at sea. Untangling a snag turns into a life threatening situation, real quick, when the water temperature is under 70 degrees.
Last but not least, the modern flat bottomed fin keel boats have no bilge
sump. Any water that finds it way below will not be contained in a sump but slosh around everywhere from the gunnels on down. Nothing in the boat is safe from even a small amount of water. I'd never think of even coastal cruising in a 'no bilge' boat.
As far as performance on a full keel boat, you give up ultimate light air sailing ability. You can still sail in light air, not just as fast as as a fin keeler. You may gain that back when winds are stronger however. The usually easier motion of the full keel boats allows you to carry on when it starts to dust up. There is a difference between enduring a bit of discomfort thrashing to windward for an hour in a buoy race
to beating into 6-10 foot seas for days on end. Usually the boat can handle it but the crew can't. Incidentally, that's why windward ability is not such a big deal for a cruiser. Only DF's go hard on the wind
when the seas are kicking up. Anyway, the easier motion quite often allows the 'cruiser' to sail at optimal velocity while the racer
type is merely trying to survive.
What does it mean out in the real world. Full keel boats tend to be moderately heavy to very heavy displacment. You can add a few thousand pounds, and you will, without affecting sailing speed. The lighter boats suffer performance degradation disproportionate to the added weight. You'll be able to maintain optimal speed under more conditions than a lighter boat. When you add up the speed through the water over thousands of miles, the fat, full cruiser often gets there just as fast and, certainly, with a crew in better shape.
Our own experience with our Westsail 32 was 118 nm per day for over 15,000 miles. Not bad for a boat with a design displacement
of 20,000 pounds but 6" down on her lines as sailed and only 32' long. We did not motor
except in and out of harbor and to charge the batteries. That total mileage average included one day we did a whopping 15nm and 6 days in which we covered 900 miles. With the right wind and point of sail, we smoked a world champion 1/2 ton 37 footer and most every other boat under 50' we sailed against. The boat would sail in very light airs, however, and the Aries
would sail the boat if the boat would sail.
We gave up the ability to go to weather
in light air and a chop without the engine
. We could crack off a few degrees and comfortably sail along for days, however. The boat wouldn't set any records with winds under 10 knots tough it would still sail.
Would I have another Westsail if I was thinking of primarily coastal sailing, no way. They are ponderous creatures built for comfort and survival on the open ocean. Except with a following sea and 20 knots of wind, they just aren't exhilerating to sail. In coastal cruising, you often don't have the luxury of sailing on the best point of sail for the boat. Since I hate engines, would want a boat that would sail on any point of sail. I would also want a livelier boat. In coastal cruising the passages are short so I'd want to pack in the sensory pleasure of sailing. I'd give up the comfort, room and carrying capacity of the ultra heavy boat for a more exciting moderate boat.
Still want a full keel, though. Just had too many bad sailing days on fin keel boats that were simply too much work to sail efficiently or grabbed a pot. I'm driven when it comes to sailing at optimum speed and the fin keel boats I've sailed on just weren't 'fun' for more than a day sail.
BTW, a long narrow foil is more efficent at creating lift
than a wide short one. Fin keel boats are more efficient to windward because of this. Of course, the deeper keel makes some cruising areas not accessable. Also, having a deeper keel increases the righting moment. That can make for very abrupt and tiresome motion at sea as the boat constantly tries to jerk itself upright. It's especially abrupt in the flat bottomed designs so prevalent nowadays.