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Old 06-03-2004, 06:27   #1
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Question Fin Keel? Long Keel? Jeff?

hi everyone,

still looking for a blue water cruiser. after reading some feedback on this sight we're more confused than ever!

thanks, bob & deb
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Old 06-03-2004, 08:54   #2
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I think that there is no one universally right answer here but a whole lot of strong opinions when it comes to keel types. If you are traditional in your view point then you would lean towards a boat with a full length keel. If you care more about performance and ease of handling then you might lean towards a fin keel.

There are good and bad offshore boats with all kinds of keels and so buying a boat with a full length keel will not guarentee that you will end up with a good offshore boat any more than buying a boat with a fin keel will guarentee a bad offshore boat. These are at best subjective decisions.

The way that I personally look at this, If your goal is to spend almost all of your time offshore and in really remote areas of the world, then a full keel boat probably makes sense. If you are going to island hop and perhaps occasionally make longer passages, and you will be traveling in places like the US, Carribean and Europe, then a properly designed and engineered fin keel boat makes more sense. BUT again that is only my opinion.

The material below is exerpted from an article that I had written for another venue but which might help you as well.

Full keels:
These were the earliest keels and they 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 spirol 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 whise 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. On most boats under 40-50 feet, there is a tendancy for dynamic directional stability to be more critical to course holding than the directional stability that comes from the a long keels greater longitudinal moment of intertia. Since directional stability as a product of the dynamic balance between the sail plan and underbody is so important to directional stability, in practice many fin keel boats actually hold a course as well as a full keel. In general though you can expect to make more small course adjustments with a fin keel. It is sometimes argued that the lower helm loads on a fin keeler requires less energy to make each of 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 sail boats. 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.

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 them in with 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 occurance 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 aprpoaching that theory on large bilge keel boats. While bilge keels do allow shallow draft though, they extremely difficult to free once aground.

I hope this was helpful although I don't know if it really answers your question since this topic is so subjective.

Respectfully,
Jeff
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Old 06-03-2004, 10:13   #3
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Well Jeff, that sure gives them something more to think about! Nice job explaining the differences.
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Old 06-03-2004, 11:16   #4
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Thanks Jeff !

I'm copying your excellent treatise into my personal archives.
Best regards,
Gord
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Old 10-03-2004, 05:08   #5
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Bob & Deb, we're missing a huge 'inbetween' category...

As usual, I like a lot of Jeff's content but, because he chooses to define 'fin vs. full' keel as the naval architect he is/has been, the discussion to my way of thinking blurs together with narrow-chord fins the largest single group of offshore cruising-suitable boats on the marketplace these days: extended fins, sometimes with partial- or full-skeg rudders and sometimes with semi-balanced rudders.

Personally, I think the distinction is far more important than trying to determine whether a 'full keel' boat is, by conventional NA definition, a fin keel with an attached rudder...because this latter group of boats are all long in the tooth these days and generally - altho' I would argue with some exceptions - not as desireable a set of choices as boats with extended fin keels.

Why am I mentioning this? Because in many cruising venues (BC/AK, parts of Cen'l America, Oz/NZ, Western Europe to mention some but not all), a boat's ability to stand on her own keel is very helpful if not at times essential. Haul-out facillities are sparse in many places, sometimes may simply be a marine railway, and scrubbing grids are in common use. Standing your boat on her own feet is a risky thing to do with a short-chord fin but commonly done with extended fins. Moreover, 'modified' or 'extended fin' keels are almost always in conjunction with hull lines that are suitable for handling heavier load-outs, one inevitable byproduct of going cruising. The typical racer-cruiser of the 70's, performance cruiser of the 80's, etc. typically have the small wetted surface hull and narrow chord fin that, today, we find on a Hanse, Elan, Dehler, Bavaria and so forth - as compared e.g. with Najad's, H-R's, Pacific Seacraft's, Caliber's (and many more) extended fin. As just another way to illustrate this distinction, check the specs on a H-R 37, a PS 37 and a Hanse 37 and compare the tankage in each boat, one of many critical design features when picking a cruising-suitable boat. (And that's looking at a PS hull that is a small 37). This is before you consider boats that design tankage into their hull form, something that can be done in either a lousy or decent manner (e.g. Whitby 42, the current LRC Caliber line).

More important than the point I'm trying to make above, it sounds (from reading between the very few lines of your post) that you're concentrating on one tree in a very large forest, and thinking about a keel choice somewhat in a vacuum. If you start with what I guess I'd call the fundamental criteria - budget and intended cruising plan - tons of 'choices' will elminate themselves. After that, just sorting thru the myriad of remaining variables (draft, tankage, rig preferences, 'must have' systems like bullet-proof self-steering and excellent sails) will, I suspect, quickly shift keel type from the abstract or conceptual to the real world weighing of multiple variables and the compromise inherent in any boat choice.

Jack
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Old 10-03-2004, 18:24   #6
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All boats must stand on their keel, even when in the cradle. Any support along the hull merely ballances the boat, and caries no load.
This applies to Fin, Cruising Fin, and Long Keel boats (or whatever terminology works).
Properly designed & built, a Fin Keel will bear the full load of the boat.
Respectfully,
Gord
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Old 10-03-2004, 21:10   #7
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Keel

Gord I agree with you, but some boats are a bit weak around the keel and the keel will try and push its way up through the bottom, so more weight must be put on the pads and then the same boat deflects where the pads rest. I think all boats should be able to support their weight on the keel and they also should be able to be supported by pads on the hull but that is not always the case. The yard will know which boats are more durable and which ones are not. The ones that flex will wobble around when they are being trailered. On some boats when hanging from slings, you can grab the keel on the bottom and pull it side to side, while placing the other hand on the hull you can feel it flex. Michael Casling
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Old 10-03-2004, 22:42   #8
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No offence, But.............

I'm not sure I would even want a boat that couldn't rest on it's keel. In the good ole' days they use to build them from the keelwood up.
Any fixedkeel boat that has a lose keel that wobbles, I would think needed some new keel bolts and packing (sealer)! And any hull that oil cans around the keel would have some structural damage!

Regards........._/)
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Old 11-03-2004, 02:29   #9
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Sorry about this showing up as a new thread...

Perhaps I didn't make my point clearly. In cruising venues, being able to stand on one's own keel isn't about 'strength' but rather about the length (chord) and width of the keel relative to the overall length of the hull. When taking the hard, a boat is initially tied off to a bulkhead or pilings but, as the water recedes, it's left to balance itself on its keel's footprint. No supplemental jackstands are in sight. As the tide is running out, one typically will attach a masthead halyard to the bulkhead midships and take a strain, so as to lean the boat slightly into the bulkhead, adding a 2nd balancing point...but that's not going to help any tendency the boat has to pitch by the bow or stern, and the halyard will tend to rotate (yaw) the boat about its keel if the contact point is small and balance of the boat invites it. (Keep in mind that the wind hasn't stopped blowing, nor the wx stopped changing, while this 12-hr process unfolds).

As with the examples I gave in the above post, some boats would leave me quite uneasy about doing this on their short-chord keels. This is just one example where I think we do a disservice when talking about 'fin' keels as tho' they all inherently share the same characteristics re: cruising.

Jack
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Old 11-03-2004, 10:52   #10
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Keels

Jack, agreed, fin keels come in many flavours and the bigger footprint on the bottom is going to help. Michael Casling
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Old 11-03-2004, 11:20   #11
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Exclamation Careening

Good point Jack!
Careening seems to have lost interest in the modern day sailors agenda. Cruisers would seem to have more of the need being away from home port and facilities for long periods.
The new racing boats with the canting keels would be out of the question. A run on the muck could even be a problem.

............_/)
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Old 29-04-2004, 08:35   #12
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Talking

I Chartered a 62' ketch for the New Year in the BVIs in 1999. We went to Anagota island for a couple of days. At one point, the boat started to lean to one side just a little. I asked the Captain if we were on the bottom. He said," Ya, but its a full keel and we are not leaving till tomorrow." He didn't have a care about being in shallow water. When I bought my boat, I looked for a full keel. I'm not saying it is the only way to go, but for me, I enjoy the thought of it being down there when I need it.
We don't sail as fast or do as good in light air, be we don't worry about floating containers or other debree as much either.
Just my opinion.
JT
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Old 29-04-2004, 15:26   #13
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fin keel

anything a full keel can do a fin can do far better including: incomparable hull speed with smaller sails, far less torque and stress on the rig, much cheaper bottom jobs & better handling with far more fun. full keel boats push water around creating unnatural stresses and huge bow/stern waves basically submerging the boat.

a friend who is a hull desinger told a customer " why have a boat that cant get out of its own way resulting in being battered when you could have one that that is attuned to the dynamics of the seas"
my personal experience is with my schock 34 which i consistently sail at 9 knots in the trades with a 100% jib and main.
my friend with a morgan 41 sails at 6-6.5 and in 15 knots on a reach and cannot tack in less than 5 knots when i am sailing at the windspeed.
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Old 02-06-2004, 07:21   #14
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I've owned a shoal draft Jeanneau Sun Odyssey 40 for about four months now and am getting the hang of its performance characteristics. It has a relatively shallow (4'11") draft with a 5000 lb bulb on the end. The actual keel and bulb is only about half of the draft.

Most of my prior experience was on higher performance oriented boats that had deep fins. Different objectives and obligations have me cruising now....

The thing I've learned about the bulb keel is that it stalls very easily at low speeds. In light air, going to windward, I'm much better footing off until I get boat speed up in the 2-3 knot range before I try to point much higher than 60 degrees to apparent wind. Once boatspeed is above 2+ knots, the boat will point right up to 40-45 degrees of apparent.

I haven't really noted any other idiosyncrasies as far as sailing performance. The boat handles, tracks, and backs well.

The bulb is flat on the bottom so grounding is at least comfortable. I am VERY cautious of the rudder, though, as it is only a few inches shallower than the keel.

Curtis
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Old 02-06-2004, 08:44   #15
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Seakindliness

Does the type of keel (for this purpose, fin or full) make a difference in the motion of the boat (sea kindliness)? It seems that way to me, but I don't know for sure. My full keel (w/cutaway forefoot) seems to have much better (less) motion than did my fin keel.
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