Can somebody pro/con a full vs. fin keel for a newbie (will learn to sail on said boat) and taking it thru the Caribbean? All I can seem to come up with so far is fin keel is better to the wind, and a full keel will protect your rudder.
Full or fin keel?
- Tags
- boat buying newbee advice
-
SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more!
utchuckd
21 - 40 of 847 Posts
Joined
·
9,377 Posts
- Reaction score
- 1,366
After skiing the black runs, the blue runs are easier too.
Joined
·
7,660 Posts
- Reaction score
- 6,085
Agreed!
Actually, this is what I was trying to get at with my comments. Knowing something about your sailing/cruising plans will help with your choice of boat (and keel). If my cruising was mostly about marina-hopping, then I would definitely NOT want a full-keel boat -- especially not in the Med
Joined
·
9,377 Posts
- Reaction score
- 1,366
MikeAgreed!
Actually, this is what I was trying to get at with my comments. Knowing something about your sailing/cruising plans will help with your choice of boat (and keel). If my cruising was mostly about marina-hopping, then I would definitely NOT want a full-keel boat -- especially not in the Med
While the standard Med moor to stern-to, I learned to do mine bow-in with a stern anchor. Much easier and more privacy. Having a U-shaped pulpit helps.
Joined
·
7,660 Posts
- Reaction score
- 6,085
That's good to know Jackdale. I love my Rafiki. It's a wonderful cruising boat for a short-handed crew, but I'd rather take on Force 9 gale than face a tight marina
.A keel is your underwater wing, just like your sail is the in-air wing. Remember, the sails generate lift around the leeward side of the airfoil, the angle of the boat as it moves through the water causes the keels to generate lift on the windward side of the waterfoil. The vector sum of these forces is what pushes you forward. As I understand it, the advantages and disadvantages are as follows.
A full keel is flat and has a lot of wetted surface to cause friction. They are generally slower and don't generate lift at all. Instead of producing lift they simply provide resistance to sliding directly downwind, which the wind would otherwise blow you. Full keels typically don't go as well to weather as other designs. On the other hand, the draft is shallow and damage is light to non-existent when the boat is run aground. Full keels are very traditional and tend to be very strong too. That's why they are used most often on blue water cruisers where speed is less important than standing up to a harsh environment. They can make maneuvering difficult, especially if the prop is in an aperture.
Fin keels are shaped much more like an airplane wing and do provide lift. The most efficient kind is the high aspect keel. They are narrow and deep and are efficient for the same reason a sailplane's wing is. Because they generate lift, they don't need to be so big -- and so they have less wetter surface and less friction (read fastest). They also put the lead ballast down the farthest, which moves the Vertical Center of Gravity (VCG) down the lowest. That gives you the best righting arm moment to keep the boat more vertical for a given weight. The high-aspect keel also goes to weather the best, but it is the most easily damaged upon grounding, and since it is the deepest draft it is the most likely type to touch ground. Needless to say, you can't get into shallow areas with a deep, fixed keel.
Low aspect fin keels are a compromise. This kind of keel is the shoal draft keel and is the best kind to have where I sail. They are very popular here in the Chesapeake Bay and along the Bahamas, simply because the water is so shallow. Low aspect fin keels don't generate as much lift since the wing chord is stretched longer. And because they don't generate as much lift, they don't go to weather as well and they also have to be bigger, so there is more wetted surface to cause friction. VCG is not as low so they require more ballast. therefore making the boat heavier.
There are a lot of variations on keels -- the full keel with cutaway forefoot. The shoal keel with bulb. The shoal keel with wings. The bulb at the end of the high aspect keel. The lifting high aspect keel. The keel stub with a swinging centerboard, and many more. All have advantages, disadvantages, and some have maintenance actions you need to accomplish every so often. Which kind is best for you depends on the kind of sailing you hope to do and the depth of water you want to get into.
Several before me have spoken about tracking, so I won't cover that.
Hope this helps!
Tom
A full keel is flat and has a lot of wetted surface to cause friction. They are generally slower and don't generate lift at all. Instead of producing lift they simply provide resistance to sliding directly downwind, which the wind would otherwise blow you. Full keels typically don't go as well to weather as other designs. On the other hand, the draft is shallow and damage is light to non-existent when the boat is run aground. Full keels are very traditional and tend to be very strong too. That's why they are used most often on blue water cruisers where speed is less important than standing up to a harsh environment. They can make maneuvering difficult, especially if the prop is in an aperture.
Fin keels are shaped much more like an airplane wing and do provide lift. The most efficient kind is the high aspect keel. They are narrow and deep and are efficient for the same reason a sailplane's wing is. Because they generate lift, they don't need to be so big -- and so they have less wetter surface and less friction (read fastest). They also put the lead ballast down the farthest, which moves the Vertical Center of Gravity (VCG) down the lowest. That gives you the best righting arm moment to keep the boat more vertical for a given weight. The high-aspect keel also goes to weather the best, but it is the most easily damaged upon grounding, and since it is the deepest draft it is the most likely type to touch ground. Needless to say, you can't get into shallow areas with a deep, fixed keel.
Low aspect fin keels are a compromise. This kind of keel is the shoal draft keel and is the best kind to have where I sail. They are very popular here in the Chesapeake Bay and along the Bahamas, simply because the water is so shallow. Low aspect fin keels don't generate as much lift since the wing chord is stretched longer. And because they don't generate as much lift, they don't go to weather as well and they also have to be bigger, so there is more wetted surface to cause friction. VCG is not as low so they require more ballast. therefore making the boat heavier.
There are a lot of variations on keels -- the full keel with cutaway forefoot. The shoal keel with bulb. The shoal keel with wings. The bulb at the end of the high aspect keel. The lifting high aspect keel. The keel stub with a swinging centerboard, and many more. All have advantages, disadvantages, and some have maintenance actions you need to accomplish every so often. Which kind is best for you depends on the kind of sailing you hope to do and the depth of water you want to get into.
Several before me have spoken about tracking, so I won't cover that.
Hope this helps!
Tom
Joined
·
295 Posts
- Reaction score
- 67
Thanks Tom, that was very informative.
I'm currently shopping, but I haven't sailed anything but fin keels and centerboards, so I'd like to know more about these other kinds of keels. I'm having a tough time deciding and am afraid I'm going to wind up buying one of each.
Joined
·
108 Posts
- Reaction score
- 25
While the prop in the aperture makes backing up an adventure on my full keel attached rudder Vanguard, it makes sailing through a field of lobster pots a lot less stressful. Have never snagged a pot in 20 years of sailing in Maine-I don't even try to dodge them. I'd not want to snag a pot line at 2AM with a spade rudder or foul one on my exposed prop. Most trap lines have many weighted pots attached, very easily could bend a rudder post or prop shaft. Hitting a ledge with a fin keeler even at slow speed could sink your vessel a lot easier than it would a full or 3/4 keel boat.
+2 backing is a b***h. But I keep telling myself, the boat was designed to go in the other direction!
Of course that is kinda what it was designed for. :thumbs up:That's good to know Jackdale. I love my Rafiki. It's a wonderful cruising boat for a short-handed crew, but I'd rather take on Force 9 gale than face a tight marina
Joined
·
7,660 Posts
- Reaction score
- 6,085
Joined
·
6,696 Posts
- Reaction score
- 1,865
I did a lot of research on keel designs for cruisers over the past couple of years, and the conclusion I came up with was verified in books on cruising by Lin and Larry Pardey, Capable Cruiser and Storm Tactics.
First and foremost, at age 71 I'm not what some would refer to as a seasoned sailing captain, however, I've owned 16 powerboats and been on the water since age 5. I've only been sailing for the past 7 years, with an average of 45 trips per season. I've only owned two sailboats, a fin-keeled Catalina 27 and a full-keeled 33 Morgan Out Island and most of the time I sail single-handed.
One of the primary benefits Lin and Larry pointed out in their books was in the likely event you strike an immovable or nearly immovable object with your keel, the fin keel would likely incur more damage to both the keel and other parts of the hull than a full-keeled boat, which would likely slide more gently onto the object. Additionally, the rudder would probably not incur damage on the full-keeled boat because the rudder would be somewhat protected by the keel, whereas the fin-keeled boat would likely be subject to rudder damage under these circumstances.
In my case, often when sailing at night in Chesapeake Bay, crab pot markers tended to slide down the length of the full keel without snagging the rudder or prop. When I sailed the Catalina at night I could almost count on snagging at least one crab-pot marker, and some nights up to a half-dozen in a 20-mile trip along the channel edge.
I've been caught in some nasty storms with both boats and unequivocally the Morgan with the full keel handled far better in a quartering, following sea than the Catalina. The Catalina was much more tender to wind gusts, while the Morgan just kept plodding along at the same speed.
Now, the Fin-keeled Catalina pointed a bit better than the full-keeled Morgan, but not significantly better--maybe a couple degrees at best. As for the speed, both are relatively fast, which was surprising to me because everyone told me the Morgan would really be slow--even in a 10-knot wind.
When I purchased the Morgan I was also looking at several other boats in the same price range and similar size. However, the only one with a full-keel was the Morgan and having sailed the boat for one season I believe the choice I made was sound, especially for blue-water cruising. I'll be heading south from Chesapeake Bay to the Florida Keys, Dry Tortugas and across to the Bahamas this coming fall and I'm confident the Morgan with its full keel and wide beam will handle the task with ease.
Good Luck,
Gary
First and foremost, at age 71 I'm not what some would refer to as a seasoned sailing captain, however, I've owned 16 powerboats and been on the water since age 5. I've only been sailing for the past 7 years, with an average of 45 trips per season. I've only owned two sailboats, a fin-keeled Catalina 27 and a full-keeled 33 Morgan Out Island and most of the time I sail single-handed.
One of the primary benefits Lin and Larry pointed out in their books was in the likely event you strike an immovable or nearly immovable object with your keel, the fin keel would likely incur more damage to both the keel and other parts of the hull than a full-keeled boat, which would likely slide more gently onto the object. Additionally, the rudder would probably not incur damage on the full-keeled boat because the rudder would be somewhat protected by the keel, whereas the fin-keeled boat would likely be subject to rudder damage under these circumstances.
In my case, often when sailing at night in Chesapeake Bay, crab pot markers tended to slide down the length of the full keel without snagging the rudder or prop. When I sailed the Catalina at night I could almost count on snagging at least one crab-pot marker, and some nights up to a half-dozen in a 20-mile trip along the channel edge.
I've been caught in some nasty storms with both boats and unequivocally the Morgan with the full keel handled far better in a quartering, following sea than the Catalina. The Catalina was much more tender to wind gusts, while the Morgan just kept plodding along at the same speed.
Now, the Fin-keeled Catalina pointed a bit better than the full-keeled Morgan, but not significantly better--maybe a couple degrees at best. As for the speed, both are relatively fast, which was surprising to me because everyone told me the Morgan would really be slow--even in a 10-knot wind.
When I purchased the Morgan I was also looking at several other boats in the same price range and similar size. However, the only one with a full-keel was the Morgan and having sailed the boat for one season I believe the choice I made was sound, especially for blue-water cruising. I'll be heading south from Chesapeake Bay to the Florida Keys, Dry Tortugas and across to the Bahamas this coming fall and I'm confident the Morgan with its full keel and wide beam will handle the task with ease.
Good Luck,
Gary
Travelineasy :
On my own ship, a Union Polaris 36, for me, there is no substiture for the full (or near-full) keel.
In summer, in lighter airs, most of the other ships pass me easily.
But you wait for when things get rough, in the bigger seas, particularly long hours in the bigger seas.
It is different then.
.
On my own ship, a Union Polaris 36, for me, there is no substiture for the full (or near-full) keel.
In summer, in lighter airs, most of the other ships pass me easily.
But you wait for when things get rough, in the bigger seas, particularly long hours in the bigger seas.
It is different then.
.
Joined
·
19,468 Posts
- Reaction score
- 3,889
Different horses for different courses... There's no right or wrong answer here.
As has been mentioned already, perhaps numerous times this decision is going to come down to the type of sailing you do, the area you sail in, the performance level that 'turns you on', etc etc....
If a full keel was really the absolute optimum for offshore sailing we'd still see the Suhali's on the Volvo Ocean Race. (and I imagine a few of those crew, crawling soaking wet into an equally wet berth might well wish they'd chosen that ride from time to time)
For our type of sailing fin/spade suits us fine. Were we going offshore we'd probably choose a middle of the road compromise, with Perry's Passport 40 being at the top of the list..
But there's also no denying the "Aaarrgh" appeal of the heavily teaked up, massive tillered, rock solid, bowspritted, cutter rigged rugged cruisers like the Baba's and Westsails and so many others.. clearly there's a demand for those too.
So let us fin keelers have our day on that light air beat... you'll get your chuckle when it dusts up to 30 knots and you're finally leaving us in the dust...or at least looking a bit more comfortable as we sail alongside...
It's ALL good...
As has been mentioned already, perhaps numerous times this decision is going to come down to the type of sailing you do, the area you sail in, the performance level that 'turns you on', etc etc....
If a full keel was really the absolute optimum for offshore sailing we'd still see the Suhali's on the Volvo Ocean Race. (and I imagine a few of those crew, crawling soaking wet into an equally wet berth might well wish they'd chosen that ride from time to time)
For our type of sailing fin/spade suits us fine. Were we going offshore we'd probably choose a middle of the road compromise, with Perry's Passport 40 being at the top of the list..
But there's also no denying the "Aaarrgh" appeal of the heavily teaked up, massive tillered, rock solid, bowspritted, cutter rigged rugged cruisers like the Baba's and Westsails and so many others.. clearly there's a demand for those too.
So let us fin keelers have our day on that light air beat... you'll get your chuckle when it dusts up to 30 knots and you're finally leaving us in the dust...or at least looking a bit more comfortable as we sail alongside...
It's ALL good...
- Reaction score
- 7,948
I apologize that this is very long primarily because it was written years ago for another purpose but it does broadly address issues associated with selecting a keel type. In reality, as quite a few posts above have suggested, boats behave as systems, and there is no one universally right answer on any of these broad questions.
"Your question seems to be about appendages. In principle Appendages keep a boat from making leeway. They come in many shapes and sizes. Keels are supposed to be a fixed appendage and centerboards generically are moveable appendages that occur on the centerline but centerboards are just one kind of moveable appendage. In more detail:
Keels:
The earliest form of a keel was simply the backbone of the boat extending through the bottom planking. (Like a Viking ship) That works OK with running and reaching sails but when you try to point toward the wind you slip side wards at great speed. As sails and rigs were invented that allowed boats to point toward the wind the keel was extended below the boat either by planking the hull down to a deeper backbone or by adding dead wood (solid timber below the backbone. A planked down keel permitted the space between the planking to be filled with heavy material (originally stone), which served as ballast keeping the boat from heeling. After a while it was discovered that there were advantages to bolting a high-density cast metal ballast to the outside of the deadwood and interior ballast dropped out of fashion.
Full keels:
These earliest keels pretty much ran from the point of entry at the bow, to the aft most point of exit at the stern. Those are full keels in the fullest sense of the word.
They have some advantages; the keel theoretically forms a long straight plane, which keeps a boat on course better (greater directional or longitudinal stability). If you run aground they spread out the load over a larger area reducing the likelihood of damage. Once really planted they keep the boat from tipping over fore and aft. They are easier to haul and work on. You can spread out the ballast over a longer distance and so they can be shallower for the same stability. You have a greater length to bolt on ballast so it is a theoretically sturdier and simpler connection. You often hear that the rudder is better protected on a full keel, but that is not usually the case. On most fin keel boats, the rudder is designed with a significantly shallower draft so that even if the keels hit bottom, the rudder doesn't. But generally, the rudder on a full keeled boat is only inches from the bottom of the keel placing it at greater risk. Repair yards here on the Chesapeake talked about the frequency with which Island Packet came in with damaged rudders. In their case, this was a post hung rudder as deep as the keel, which is an example of a worst of all worlds solution.
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. As a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This is drawn behind the boat creating drag in a number of ways. The longer the keel, the bigger the vortex, the greater the drag. So they need more sail area again to overcome this drag. To stand up to this greater sail area the boat needs more ballast and a stronger structure, which is why long keelboats are often heavier, as well. (Of course, then the spiral starts again as more sail area is needed to overcome that additional weight as well. It is the classic weight breeding more weight design cycle) Full keels tend to be much less maneuverable, especially under power.
Fin keels:
By the classic definition of a fin keel, now out of vogue, any keel whose bottom is less than 50% of the length of the boat is a fin keel. Fin keels came into being in an effort to reduce drag. Cut away the forefoot or rake the stem, as well as, move the rudderpost forward and rake it sharply and pretty soon you have a fin keel. Today, there seems to be a widely accepted belief that a fin keel implies a separated rudder (skeg hung or spade) but in fact early fin keels usually had their rudder attached. No matter what we call them, these comparatively short keels with attached rudders were in all ways 'a worst of all worlds situation'. They offer all of the disadvantages of both full and fin keels, but with nearly none of the virtues of either. Unknowing or unscrupulous brokers will often refer to boats with fin (or near fin) keels as full keel if they have an attached rudder.
Fin keels with separate rudders seem to be the most commonly produced keel form in the US these days. (I could be wrong, there is a slight resurgence of full keels these days)
Fin keels have some advantages as well. They have less drag as explained above so they typically make less leeway and go faster. You can get the ballast down lower so in theory they are more stable for their weight. They are more maneuverable. They take better advantage of the high efficiency of modern sail plans and materials. They work especially well in terms of roll dampening and so offer advantages in terms of motion comfort.
They have some disadvantages as well, many of these have been offset or worked around by modern technology but at some level they are still accurate critiques. They often have less directional stability than long keel boats so the tend to wander more under sail. Since directional stability is also a product of the dynamic balance between the sail plan and underbody, in practice they may actually hold a course as well as a full keel. In general though you can expect to make more course adjustments with a fin keel. It is sometimes argued that the lower helm loads requires less energy to make these corrections so a fin keel may also require less energy to maintain course. This I think is a product of the individual boat and could lead to a debate harder to prove than the number of angels that can dance on the head of a pin.
Fin keels are harder to engineer to withstand a hard grounding and when aground they are more likely to flop over on their bow or stern. (Although in 40 plus years of sailing, I have never heard of anyone actually experiencing this.) Fins typically have deeper draft. They are easier to pivot around and get off in a simple grounding.
Shoal keel
A shoal keel is just a keel that is not as deep as a deep keel. Today the term seems to be applied mostly to shallow fin keels. Shallow full keels seem to be referred to as shoal draft boats. A shallow fin is a tough animal to classify. Like a fin keel with an attached rudder, I really think it has few of the advantages of either a deep fin or a full keel and has many of the worst traits of both full and fin. This can be partially offset by combining a shallow fin with a centerboard, which is a neat set up for shoal draft cruising.
Bulb Keel:
A lot can be done to improve a shallow fin. One way is to add a bulb. A bulb is a cast metal ballast attachment added to the bottom of the keel. They concentrate the ballast lower providing greater stability and sail carrying ability than a simple shallow keel. Traditionally bulbs were torpedo or teardrop shaped. They have been re-contoured to provide some hydrodynamic properties. Recalling the discussion on tip vortex from above. Shallow keels need to be longer horizontally than a deeper fin in order to get enough area to prevent leeway. This means that a shallow longer fin would generate more tip vortex and more drag than a deeper keel. The bulb creates a surface to turn the water aft and prevent it from slipping over the tip of the keel thereby reducing tip vortex. This does not come free since a bulb increases frontal area and surface area.
Scheel Keel:
Scheel Keels are a specialized form of shoal bulb keel which is shaped to reduce induced drag (tip vortex) and surface area, and lower the center of gravity. Like any shoal bulb keel ity offers a very big improvement over a simple shoal draft fin but do not sail as well as a properly designed deeper fin.
Wing keels
Wing keels are a specialized type of bulb keel. Instead of a torpedo shaped bulb there are small lead wings more or less perpendicular to the keel. These concentrate weight lower like a bulb and properly designed they also can useful in reducing tip vortex. There has been some discussion that wings increase the effective span of the keel when heeled over but this does not seem to be born out in tank testing of the short wings currently being used in production sailboats. Not all wings are created equal. They potentially offer a lot of advantages, but they are heavily dependent on the quality of the design and I really think that many wing designs are not really working to their potential.
Then there is the whole grounding issue. The popular perception is that wing keels are harder to free is accurate. This seems to be born out by discussions that I have had with towboat skippers on the Chesapeake. According to them wing keels were extremely harder to free. Straight fins were much easier to free, especially when heeled. There also is some evidence that bulbs may also be easier to free than fin keels.
Keels that are not really keels:
Swing keels are ballasted centerboards and drop keels are ballasted daggerboards that are ballasted beyond what it takes to submerge themselves. They are really forms of centerboards. More on these in the discussion on centerboards.
Keels that are keels that move.
I said in the introduction that keels do not move. That used to be true. We now have canting keels, which can be pivoted from side to side. They are best designed to be light fins with heavy bulbs that can be canted to windward increasing the effectiveness of the righting aspects of the keel. Just one problem, a keel canted to windward losses efficiency to prevent leeway so they really need other foils to keep leeway in check. I frankly do not like the idea of a canting keel. I think canting keels are too complex and potentially problematic to find general useage in daysailers and cruisers.
Centerboards:
Centerboards are appendages that can be raised and lowered on or near the centerline of the boat. They can rotate up into a trunk or rotate below the boat. Daggerboards are a type of centerboard that raises vertically or near vertically in a trunk. Swing keels are a type of rotating centerboard that actually contains a substantial portion of the boat’s ballast. They may be housed in a trunk like a Tartan 27 or 34 or hung below the boat like a Catalina 22. In the case of the Tartan 27 or 34 they are more frequently referred to as a Keel/ Centerboard (abbreviated k/cb). A swing keel is intended to act as a fin keel when lowered and allow some sailing in the partially raised position. My biggest problem with swing keels is that most do not have a positive lock down. In an extreme knockdown they can slam up into the hull greatly reducing the boat’s stability. This is a pretty rare occurrence and usually requires big wave action combined with a lot of wind, but I have experienced it out in the Atlantic.
A drop keel is a daggerboard that actually contains a substantial portion of the boat’s ballast. These are easier to lock down but can be more easily damaged in a grounding. They generally have better shape than a swing keel and can be more robust, but not always are.
Other appendages: (besides the rudders)
Bilge keels (or twin keels for our English friends) are a pair of keels (usually fins these days) that emerge on either side of the boat and angle out. They offer some advantages. If you let the boat dry out the boat can stand on the two keels and wait the next tide. There are dubious theories about increased efficiency since one is vertical like a good leeway resisting foil and one is canted like a good stability inducing foil. With computer modeling there has been greater success in approaching that theory on large bilge keel boats. While bilge keels do allow shallow draft though, they extremely difficult to free once aground since having the two keels on the ground prevents heeling the boat to get free. In practice bilge keels have enormous wetted surface creating a lot of drag at lower speeds, and produce two very large tip vortexes creating a lot of drag at speed.
Keel- Centerboards are a wonderful choice for coastal and offshore cruising. Properly designed they offer nearly the performance of a fin keel, and yet permit access to shallower venues. They can be partially raised to precisely control the center of lateral resistance and therefore offers the ability to have a very neutral helm and great tracking in a wide range of conditions. Properly constructed they have proven to have a long service life. Keel-centerboard boats really proved themselves offshore during the late 1950’s and into 1960’s.They fell out of popularity with the advent of the wing keel in the early 1980’s. The downside is that they are a little harder to maintain, and because the ballast is closer to the center of buoyancy they require more ballast and so end up requiring a higher overall displacement, a higher ballast to displacement ratio, or are more tender, or some combination of the three.
Bilge boards (for the scow guys), are a pair of centerboards that angle out of each side of the boat. They work well on scows but I’ve never been able to really figure out scows anyway. Seriously, You raise the windward board and lower the Leeward one on each tack and because they are close to vertical they can be small and efficient. I still don’t get the scow thing.
Last but not least- Leeboards. Leeboards are foils that are bolted to the side of the hull like on Dutch Jachts and Herreshoff Meadowlarks. Phil Bolger’s sharpies use them a lot as well. They have some advantages but they drive me nuts. They are vulnerable in docking and ideally are raised and lowered on each tack also. Some are designed to hinge outward from the hull so as to feather and so those do not need to be raised.
So that’s about it. The final is tomorrow- multiple choice and essay.
Jeff
"Your question seems to be about appendages. In principle Appendages keep a boat from making leeway. They come in many shapes and sizes. Keels are supposed to be a fixed appendage and centerboards generically are moveable appendages that occur on the centerline but centerboards are just one kind of moveable appendage. In more detail:
Keels:
The earliest form of a keel was simply the backbone of the boat extending through the bottom planking. (Like a Viking ship) That works OK with running and reaching sails but when you try to point toward the wind you slip side wards at great speed. As sails and rigs were invented that allowed boats to point toward the wind the keel was extended below the boat either by planking the hull down to a deeper backbone or by adding dead wood (solid timber below the backbone. A planked down keel permitted the space between the planking to be filled with heavy material (originally stone), which served as ballast keeping the boat from heeling. After a while it was discovered that there were advantages to bolting a high-density cast metal ballast to the outside of the deadwood and interior ballast dropped out of fashion.
Full keels:
These earliest keels pretty much ran from the point of entry at the bow, to the aft most point of exit at the stern. Those are full keels in the fullest sense of the word.
They have some advantages; the keel theoretically forms a long straight plane, which keeps a boat on course better (greater directional or longitudinal stability). If you run aground they spread out the load over a larger area reducing the likelihood of damage. Once really planted they keep the boat from tipping over fore and aft. They are easier to haul and work on. You can spread out the ballast over a longer distance and so they can be shallower for the same stability. You have a greater length to bolt on ballast so it is a theoretically sturdier and simpler connection. You often hear that the rudder is better protected on a full keel, but that is not usually the case. On most fin keel boats, the rudder is designed with a significantly shallower draft so that even if the keels hit bottom, the rudder doesn't. But generally, the rudder on a full keeled boat is only inches from the bottom of the keel placing it at greater risk. Repair yards here on the Chesapeake talked about the frequency with which Island Packet came in with damaged rudders. In their case, this was a post hung rudder as deep as the keel, which is an example of a worst of all worlds solution.
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. As a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This is drawn behind the boat creating drag in a number of ways. The longer the keel, the bigger the vortex, the greater the drag. So they need more sail area again to overcome this drag. To stand up to this greater sail area the boat needs more ballast and a stronger structure, which is why long keelboats are often heavier, as well. (Of course, then the spiral starts again as more sail area is needed to overcome that additional weight as well. It is the classic weight breeding more weight design cycle) Full keels tend to be much less maneuverable, especially under power.
Fin keels:
By the classic definition of a fin keel, now out of vogue, any keel whose bottom is less than 50% of the length of the boat is a fin keel. Fin keels came into being in an effort to reduce drag. Cut away the forefoot or rake the stem, as well as, move the rudderpost forward and rake it sharply and pretty soon you have a fin keel. Today, there seems to be a widely accepted belief that a fin keel implies a separated rudder (skeg hung or spade) but in fact early fin keels usually had their rudder attached. No matter what we call them, these comparatively short keels with attached rudders were in all ways 'a worst of all worlds situation'. They offer all of the disadvantages of both full and fin keels, but with nearly none of the virtues of either. Unknowing or unscrupulous brokers will often refer to boats with fin (or near fin) keels as full keel if they have an attached rudder.
Fin keels with separate rudders seem to be the most commonly produced keel form in the US these days. (I could be wrong, there is a slight resurgence of full keels these days)
Fin keels have some advantages as well. They have less drag as explained above so they typically make less leeway and go faster. You can get the ballast down lower so in theory they are more stable for their weight. They are more maneuverable. They take better advantage of the high efficiency of modern sail plans and materials. They work especially well in terms of roll dampening and so offer advantages in terms of motion comfort.
They have some disadvantages as well, many of these have been offset or worked around by modern technology but at some level they are still accurate critiques. They often have less directional stability than long keel boats so the tend to wander more under sail. Since directional stability is also a product of the dynamic balance between the sail plan and underbody, in practice they may actually hold a course as well as a full keel. In general though you can expect to make more course adjustments with a fin keel. It is sometimes argued that the lower helm loads requires less energy to make these corrections so a fin keel may also require less energy to maintain course. This I think is a product of the individual boat and could lead to a debate harder to prove than the number of angels that can dance on the head of a pin.
Fin keels are harder to engineer to withstand a hard grounding and when aground they are more likely to flop over on their bow or stern. (Although in 40 plus years of sailing, I have never heard of anyone actually experiencing this.) Fins typically have deeper draft. They are easier to pivot around and get off in a simple grounding.
Shoal keel
A shoal keel is just a keel that is not as deep as a deep keel. Today the term seems to be applied mostly to shallow fin keels. Shallow full keels seem to be referred to as shoal draft boats. A shallow fin is a tough animal to classify. Like a fin keel with an attached rudder, I really think it has few of the advantages of either a deep fin or a full keel and has many of the worst traits of both full and fin. This can be partially offset by combining a shallow fin with a centerboard, which is a neat set up for shoal draft cruising.
Bulb Keel:
A lot can be done to improve a shallow fin. One way is to add a bulb. A bulb is a cast metal ballast attachment added to the bottom of the keel. They concentrate the ballast lower providing greater stability and sail carrying ability than a simple shallow keel. Traditionally bulbs were torpedo or teardrop shaped. They have been re-contoured to provide some hydrodynamic properties. Recalling the discussion on tip vortex from above. Shallow keels need to be longer horizontally than a deeper fin in order to get enough area to prevent leeway. This means that a shallow longer fin would generate more tip vortex and more drag than a deeper keel. The bulb creates a surface to turn the water aft and prevent it from slipping over the tip of the keel thereby reducing tip vortex. This does not come free since a bulb increases frontal area and surface area.
Scheel Keel:
Scheel Keels are a specialized form of shoal bulb keel which is shaped to reduce induced drag (tip vortex) and surface area, and lower the center of gravity. Like any shoal bulb keel ity offers a very big improvement over a simple shoal draft fin but do not sail as well as a properly designed deeper fin.
Wing keels
Wing keels are a specialized type of bulb keel. Instead of a torpedo shaped bulb there are small lead wings more or less perpendicular to the keel. These concentrate weight lower like a bulb and properly designed they also can useful in reducing tip vortex. There has been some discussion that wings increase the effective span of the keel when heeled over but this does not seem to be born out in tank testing of the short wings currently being used in production sailboats. Not all wings are created equal. They potentially offer a lot of advantages, but they are heavily dependent on the quality of the design and I really think that many wing designs are not really working to their potential.
Then there is the whole grounding issue. The popular perception is that wing keels are harder to free is accurate. This seems to be born out by discussions that I have had with towboat skippers on the Chesapeake. According to them wing keels were extremely harder to free. Straight fins were much easier to free, especially when heeled. There also is some evidence that bulbs may also be easier to free than fin keels.
Keels that are not really keels:
Swing keels are ballasted centerboards and drop keels are ballasted daggerboards that are ballasted beyond what it takes to submerge themselves. They are really forms of centerboards. More on these in the discussion on centerboards.
Keels that are keels that move.
I said in the introduction that keels do not move. That used to be true. We now have canting keels, which can be pivoted from side to side. They are best designed to be light fins with heavy bulbs that can be canted to windward increasing the effectiveness of the righting aspects of the keel. Just one problem, a keel canted to windward losses efficiency to prevent leeway so they really need other foils to keep leeway in check. I frankly do not like the idea of a canting keel. I think canting keels are too complex and potentially problematic to find general useage in daysailers and cruisers.
Centerboards:
Centerboards are appendages that can be raised and lowered on or near the centerline of the boat. They can rotate up into a trunk or rotate below the boat. Daggerboards are a type of centerboard that raises vertically or near vertically in a trunk. Swing keels are a type of rotating centerboard that actually contains a substantial portion of the boat’s ballast. They may be housed in a trunk like a Tartan 27 or 34 or hung below the boat like a Catalina 22. In the case of the Tartan 27 or 34 they are more frequently referred to as a Keel/ Centerboard (abbreviated k/cb). A swing keel is intended to act as a fin keel when lowered and allow some sailing in the partially raised position. My biggest problem with swing keels is that most do not have a positive lock down. In an extreme knockdown they can slam up into the hull greatly reducing the boat’s stability. This is a pretty rare occurrence and usually requires big wave action combined with a lot of wind, but I have experienced it out in the Atlantic.
A drop keel is a daggerboard that actually contains a substantial portion of the boat’s ballast. These are easier to lock down but can be more easily damaged in a grounding. They generally have better shape than a swing keel and can be more robust, but not always are.
Other appendages: (besides the rudders)
Bilge keels (or twin keels for our English friends) are a pair of keels (usually fins these days) that emerge on either side of the boat and angle out. They offer some advantages. If you let the boat dry out the boat can stand on the two keels and wait the next tide. There are dubious theories about increased efficiency since one is vertical like a good leeway resisting foil and one is canted like a good stability inducing foil. With computer modeling there has been greater success in approaching that theory on large bilge keel boats. While bilge keels do allow shallow draft though, they extremely difficult to free once aground since having the two keels on the ground prevents heeling the boat to get free. In practice bilge keels have enormous wetted surface creating a lot of drag at lower speeds, and produce two very large tip vortexes creating a lot of drag at speed.
Keel- Centerboards are a wonderful choice for coastal and offshore cruising. Properly designed they offer nearly the performance of a fin keel, and yet permit access to shallower venues. They can be partially raised to precisely control the center of lateral resistance and therefore offers the ability to have a very neutral helm and great tracking in a wide range of conditions. Properly constructed they have proven to have a long service life. Keel-centerboard boats really proved themselves offshore during the late 1950’s and into 1960’s.They fell out of popularity with the advent of the wing keel in the early 1980’s. The downside is that they are a little harder to maintain, and because the ballast is closer to the center of buoyancy they require more ballast and so end up requiring a higher overall displacement, a higher ballast to displacement ratio, or are more tender, or some combination of the three.
Bilge boards (for the scow guys), are a pair of centerboards that angle out of each side of the boat. They work well on scows but I’ve never been able to really figure out scows anyway. Seriously, You raise the windward board and lower the Leeward one on each tack and because they are close to vertical they can be small and efficient. I still don’t get the scow thing.
Last but not least- Leeboards. Leeboards are foils that are bolted to the side of the hull like on Dutch Jachts and Herreshoff Meadowlarks. Phil Bolger’s sharpies use them a lot as well. They have some advantages but they drive me nuts. They are vulnerable in docking and ideally are raised and lowered on each tack also. Some are designed to hinge outward from the hull so as to feather and so those do not need to be raised.
So that’s about it. The final is tomorrow- multiple choice and essay.
Jeff
Joined
·
4,860 Posts
- Reaction score
- 2,512
One of my dockmates has a Cape Dory 30. This thread has got me curious, so I'll see if I can con him into taking me out when the wind is honking.
I've had my Pearson 30 out in the mid 20's and was quite comfortable. I've done two mid-length night races, and a couple of night cruises and found that the best way to avoid the crab pots was just to find deeper water. The Martec folding prop doesn't hurt either.
I fully believe that full keels have certain advantages, but it just doesn't blow hard enough, for long enough here on the Chesapeake to make me want one.
I've had my Pearson 30 out in the mid 20's and was quite comfortable. I've done two mid-length night races, and a couple of night cruises and found that the best way to avoid the crab pots was just to find deeper water. The Martec folding prop doesn't hurt either.
I fully believe that full keels have certain advantages, but it just doesn't blow hard enough, for long enough here on the Chesapeake to make me want one.
- Reaction score
- 7,948
Jack,
Thank you for the kind words...
Jeff
One thing that my article does not address is the perception that somehow a full keel adds stability or improves motion comfort. In and of itself, a full keel does neither and in many cases the full keel actually works against either.
Much of that perception comes from the fact that Full keels are often associated with vessels whose design lends itself to high stability or a gentle motion, but in some ways the greater sail area required to over come the higher drag means that a full keeled boat may actually have less useful stability, and so sail at greater heel angles. Similarly the heavier rigs and round bottoms associated with full keel type forms, often mean a roller motion.
Joined
·
6,696 Posts
- Reaction score
- 1,865
I'm not sure about that, Jeff. My Morgan doesn't heel much at all, even when the winds are really nasty. I'm guessing that it may be a combination of things, including the extra-wide beam, plus a real heavy, full-keel that keeps the boat upright.
Gary
Gary
21 - 40 of 847 Posts
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
