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Old 04-01-2006, 20:28   #1
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Keel discussion

I am amongst the wannabes on this board and I am closing in on the right boat for my wife and I to take long term cruising. After many months of research and as much sailing as we can cram in, there are still some issues that I think only experienced unbiased salts can help me gain perspective on. One is Keel design.

To kick things off I'll provide my superficial conception of matters.

Full Keel
Pros: Tend to have a shallower draft, handle grounding well, often enclose the prop providing resistance to prop fouling, provide good rudder protection, track well, offer good motion comfort
Cons: Unresponsive handling characteristics (hard to tack in a blow, tough to maneuver in a marina, etc.), slow (large wetted surface)
Examples: Island Packet, Cabo Rico

Semi-Full Keel
Pros: Better handling than the full keel with most of the benefits
Cons: Reduced grounding and rudder protection, more draft
Examples: Pacific Seacraft, Valiant

Fin Keel
Pros: Points higher than other designs, sails better to windward, less wetted surface (faster on all points of sail), best turn in response
Cons: Reduced directional stability (wont hove to, more prone to broach), deeper draft, vulnerable to substantial damage in grounding (especially with spade rudder)
Examples: Hylas, Saga

I'd like to know what folks think about keel design, including practical experiences.
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Old 04-01-2006, 22:12   #2
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Keels

If the hull is the right shape and weight, you can hang whatever you want on the bottom. If the hull is not balanced a full keel will not fix it, neither will a fin keel. Many full keel boats have a lot of bad habits as the keel is trying to cover up for an unbalanced hull. Some have awful weather helm. Comparing two well balanced boats, one with a fin and one with a full keel, some may prefer the motion of one over the other. The full keel will not be as quick to vary its course. The claim for the full keel is that it holds its course, but what if it is not pointed where you want it to go. Broaches usually occur at higher speeds. All the fast boats have fin keels. A full keel will broach just as easy as anything else, if the speed is pushed. Only the fin keels seem to be able to really push the speed up. But the hull is also the big issue as well as the keel, the full keels seem to be on heavy boats that tend to trip on their nose. I would be looking for a balanced hull as my first priority, and if it is balanced I would prefer a fin keel because I think going to windward is important, and drag kills speed.
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Old 04-01-2006, 22:18   #3
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Well let's get the boats in the right categories first....

Fin keels
These are longer planform fin keels, but still fin keels none the less:
Pacific Seacraft, Valiant. Alos in this category would be the C&C Landfall 38, Peterson 44 and lots of other boats.
Fin keels can also be short fore-and-aft metal structures bolted to the hull or molded fiberglass keel stub. If they are tall you end up with a large lever arm and the POTENTIAL for a weaker structure.
Examples everything from recent America's Cup to the S&S designed Catalina 38. Farr, Mumm, the list goes on an on. (Most are pretty good)
Other fin keels are encapsulated (fiberglass filled with lead or other ballast). Encapsulated fin keels are usually tough as nails and are often better foil shapes than castings because of the higher quality mold. Examples Outbound 44 and Newport 41. Not sure, but suspect Amel also has an encapsulated keel.

Semi-full keel
Actual examples would be some of the earlier Hallberg-Rasey, Bristol 35, and Hinkley Bermuda 40. All have forefoot that is largely cut away.

Motion comfort
This has very little to do with keel type and everything to do with hull design (ie rocker and shape at the stations) and the relationship between mass distribution and rig. Check out books by Marchaj or this website
http://www.image-ination.com/sailcalc.html

Hope that helps some. Personally, I like tough boats that go to windward very well.
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Old 04-01-2006, 22:19   #4
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It all comes down to the intended use of the vessel.

If you want to sail the Carib. then a shoal keel would be on the menu. They can be built like a full or a fin.

If you want to cross the ponds in the worst of conditions and in no hurry, a full or heavy displacement is in order. Or your semi-full keel. They seem to have more room per foot.

If you're just going to be a coastal cruiser looking to get back and forth quickly and in/out of the harbors with tight manuvering, then the fin keel is the most suitable. Theses are more flat bottomed and have less space. But can be a rough ride in a storm.

Then there is the close in with extreme high/low tides, that's where the twin keels come in. Then can be roomy but take a lot of windage on a reach or close haul (bad performance).

There are those who would argue that they can do the same with anyone of these keels. That's when it comes down to personal preference OR "This is what I'm stuck with, so I'll deal with it".

Some people just like the looks of a paticular design and are willing to go through the trouble and take it anywhere.

My $.02.................................._/)
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Old 05-01-2006, 05:16   #5
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First of all 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; they theoretically form 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.

They 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.

Fin keels:
By the classic definition of a fin keel 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 we assume that fin keels mean a separated rudder (skeg hung or spade) but in fact early fin keels had the rudder attached in a worst of all worlds situation. They offer all of the disadvantages of both full and fin keels, but with none of the virtues. 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 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 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 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 37 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.

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 are very efficient 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. In 2002, the Naval Academy did a study of keel types and grounding. They found that the popular perception that wing keels are harder to free is accurate. In their study, wing keels were extremely harder to free. Straight fins were much easier to free, especially when heeled, and the easiest keel to free was the bulb keel.

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.

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 boatís 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 boatís 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 boatís 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 1950ís and into 1960ís.They fell out of popularity with the advent of the wing keel in the early 1980ís. 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 Iíve 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 donít 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 Bolgerís 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 raised to be hinged feather so they do not need to be raised.

So thatís about it. The final is tomorrow- multiple choice and essay.

Jeff
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Old 05-01-2006, 14:02   #6
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Thanks to everyone for all of the feedback. In my case my wife and I will be cruising the Caribbean (full loop) and potentially running through the Panama canal out to the Galapagos and then down to the Marquesas. I am very interested in thoughts on keel design in general however, not necessarily specific to our application.

Sailor Paul: You mention the Outbound boats. I have been looking at them on the internet and recently received information in PDF. What do you know about them?

Jeff: Wow, thanks for all the data. ThatĀfs a great overview. Synergy is a beaut as well.

All:
What are your thought on encapsulated versus externally bolted on keels?
Also what are your thoughts on Lead versus other materials such as iron?
What are your thoughts on centerboards?
(Sheel Keels are covered fairly well under a separate thread.)
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Old 05-01-2006, 16:35   #7
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Randy - If you use the Search option button and plug in those key words you will find we recently had threads on each. Good reading. If you have questions, just post them at the end of that thread and it will come back around. Enjoy

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Old 05-01-2006, 17:31   #8
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Keels

In my earlier post I said I prefer fin keels because it is a hull balance issue not a keel issue that causes boats to wander or stay on course. Long keels can have some nasty bad habits, and they can have some nice habits, motion being one of them.
If I wanted a small boat for the Wednesday night races ( and I did ) I would prefer a modern deep fin. If I wanted a boat for crusing I would still want a fin but strength would be more critical for me. So the attachment point would need to be larger than a deep fin would provide. So that means a lower aspect ratio fin, but a much stronger fin. My T8.5 has a cast iron keel. The keel has to be larger when using a lighter material. But not a lot. My keel is also a lot fatter at the bottom, and has a mushroom shape at the top. I can hit rocks with this keel and not sustain any damage. The iron needs to be adequately coated, but so does steel or lead. Keel bolts in lead can come lose and cause problems. I would check how the keel is attached, any keel. On an older boat I would change the keel bolts if it had not recently been done. On a lead keel I would check the design and change the bolt system if required.
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Old 05-01-2006, 20:56   #9
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Cruising? I like the k/cb

For my money, a well-designed keel-centreboard boat is ideal for cruising, although my experience is limited to Great Lakes so far.

In our experience, with the board down we are essentially as efficient as fin-keeled cruising boats to windward (perhaps the key word is cruising). We have the added advantage of being able to "go shoal" when necessary, which gives us much more water in which to roam ... that extra foot or so over so-called shoal draft fixed fin boats can open up a lot of opportunities!

Another feaure of the design that I like is downwind - when we raise the board our wetted surface is dramatically reduced, and we generally seem to get more oomph out of the same wind as our fixed-fin buddies. (Downwind control issues with a small attached rudder is a different discussion, right? )

The added maintenance is there, no denying that. We also lose some significant bilge space for the centreboard trunk. I like the funky off-set companionway, though.

My two cents. (BTW, note the name of our boat is Leeway)

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Old 09-01-2006, 23:22   #10
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If you are looking at the Outbound 44, I have to ask. How much are you thinking about spending on this first boat?

How does the target boat compare in size to what you are sailing now? The following is my personal opinion, and it's just that.... If you are working with less than 400 hours under sail, you may have a more sucessful cruise by keeping the long distance boat within 20% of the size you most often use for practice. That could mean aiming for a smaller boat, OR increase the size of the practice boats you are renting.
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Old 10-01-2006, 05:44   #11
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Somebody get a hold of loc Doc,If he wants scary then this is where he ought to be!Every one above sounds like they have degrees in ,OPPS,sailing.**** you people know a lot and fastly I am apreciating the fact.I know Im out of the thred but I have been reading everything you people have to ask and say.Ive been seriously thinking about a twin keel since Im new to it all and I like the Idear of pulling in close to high tide and just sitting at low,I dont want a boat that will go fast and I dont mind motoring a bit neither but do I have to take in all of what you are saying or should I say,explaining,in order to just say,sail off somewhere?
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Old 10-01-2006, 11:51   #12
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Hi Mudnut, I guess you should look at it with these coloured glasses. There is no one perfect design. Every design has advantages and disadvantages. Otherwise you would only ever see one design being built and I imagine someone would have patented it and thus only one make.
Now take that to a personal level. What can you afford and what boat captures your heart when you see her. Now go sail.
It's great to have the info in the back of your head, but....unless you are full on racing and I mean seriuose racing, there is one majorly important factor in owning a boat and going to sea. It's romance, adventure and feeling the world under your feet so to speak. It's good to have the knowledge, but don't let it cloud and confuse what you are wanting to do in life.
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Old 10-01-2006, 17:17   #13
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Wheels,the romance I truly understand and the adventure"I traveled around OZ for 25yrs,decided to see it from the outside now",I was on the turps last night,althouh I didnt put my Question the right way ,you did answer it definitivly,thanks Mudnut
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Old 10-01-2006, 19:19   #14
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Mudnut

If you have any questions on the twin keeled boats, just ask.

One of my other boats is a twin keel. They have their advantages and faults.

What I don't like about them is one can not make good headway to the wind in a strong breeze........................._/)

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