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Old 11-02-2005, 05:24 PM   #1
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Default FULL KEEL V/S DEEP FIN

hi folks

i would like to understand why most "cruising" boats seem to have a "full keel" that runs almost 75% of the boat's LWL; or around that much of the skeg length. being a dinghy and keelboat racer, i am usually used to seeing "deep fin" keels such as those on a J/24 or a X/372 or a Farr 25.

what are the relative advantages of full keels, deep fin keels and bulb keels?

thanks

chetan
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Old 11-02-2005, 09:44 PM   #2
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Ahhh, today seems to be my day for answers!

OK, there are a few big advantages with a long keel, and a couple of disadvantages.

Firstly, a long keel is integral to the hull. It is virtually impossible to break the keel without the type of forces that would rip the boat apart anyway. The advantages of this can be seen from the 1979 Fastnet, where the more modern racing yachts lost their keels and rudders in a force 10. Losing a keel or rudder is still far more likely in a fin keeled yacht in a storm that losing the mast, or capsizing.

Secondly a long keel acts less like a pendulum than a fin, so offering a much smoother ride than a lighter fin keeled boat, and is generally more 'seakindly'. This is a key reason why blue water yachtsmen like them... it doesn't do anything suddenly, so the G&T stays safe.

Thirdly, the keel holds her on her course much better, making leeway or twitchy steering less likely.

Forthly, the centre of mass of the boat (with the exceptions of the super-deep fins of racing monohulls) is lower, so providing a far greater righting moment. This combined with a narrower beam means that a capsize is much less likely, and re-righting more likely should teh worst happen.

Lastly they tend to be shallower, so making shoal draft work easier

On the down side, long keels have greater wetted area, meaning they create more drag, and so are slower. They are also heavier... which doesn't help!

They also have a different way of handling themselves in heavy weather. Modern boats are designed more to take a battering to the stern, and so modern heavy weather experts claim that running before a storm is the best option. With long keel yachts, they are more buoyant in the bow, and so should be faced into the wind, and hove to.

Personally, for a compromise of seaworthiness and speed, a combination between the two is best, as you would find on some of the 60s and 70s yachts, like the Rustler and the simply awesome Contessa 32 - the only boat of it's size to survive the 79 fastnet.

3 months 29 days and 3 hours to go
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Old 11-03-2005, 12:30 AM   #3
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Hi,

You asked - what are the relative advantages of full keels, deep fin keels and bulb keels?

The above answer gives praise to a full keel and all the positive points are valid especially for a slower cruiser.

One can't argue a full keel will hold its direction better than the others. It will also tend to go to windward with an easier motion (less slamming) usually due to hull shape fairing into the keel shape.

But it is always easy to forget the typical full keeeled narrower yacht will also tend to be wetter on all points of sail, will make more leeway (than a a fin) when beating, will tends to heel more when reaching (if both keels are same weight), and for me the worst - can often be unmanageable downwind easpecially in a following big wind / sea.

I've found long keelers with integral rudders are usually a difficult vessel to turn in a confined space (and indeed sometimes on a race course) and often impossible to steer in reverse.

But as we've both said - they will hold hold their course - and do feel kinda nice underway - so suit most cruising people.

A deep fin keel will on the other hand maximise your windward performance. I'd disagree with the other writer as equal weight for weight, a fin should keep the boat more upright on all points of sail. But this keel will not help hold the boat in any straight line - other than in lightish but steady breezes. It will take more helm work by you or your self steering.than light breezes without the rudder being used.

It will usually spin on a dime, and usually steer well in reverse - but it won't allow you into those secret anchorages and coves that shallower draft cruisers love!

A bulb is usually a compromise to the fin keeler.

Added to give more righting moment to a shallow draft keel - it will open up the chances of getting into those shallower coves. But with that shallower keel it will not be as good as a deep fin - and maybe not even as good as a full keeeler - when working to windward. It will make more leeway than the fin - and slam more than the full.

In truth - I believe all these shapes are just fine - each has advantages and disadvantages. For me? We went for a fast deep finned cruiser - which gets us over the water to the next anchorage, port or party - just as quickly as we can get there.

Hope this helps

Cheers

JOHN
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Old 11-03-2005, 02:24 PM   #4
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hi folks

most definitely does help and thank you all for the replies

chetan
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Old 11-03-2005, 07:09 PM   #5
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IMO some of the above comments confuse the characteristics of hull form with the type of keel that may be associated with it. E.g. some full keel boats track poorly (I could give you a list...) and some fin keelers track wonderfully. And it should be added that every vessel needs to have a course maintained, and so how well the boat steers is IMO at least as important as how it tracks. In this regard, full keel boats almost universally have 'barn door' rudders which are going to require more helm pressure than a semi-balanced blade found on a fin keeler.

Righting moment is hardly a reflection of keel type; fin keel boats can have a far greater righting moment than a full keel hull. CG (what I think Ben is referring to by 'center of mass') is a function of the combined weights of all the boat's parts and is independent of keel type; imagine sailing on a full-keeled steel boat built with a steel cabin structure and a steel mast (ugh...). Usually, fin keeled hulls suffer from lower GZ (righting moment) than they otherwise would because the builder wants to appeal to buyers sailing in shallower waters. Bulb and wing keel ends are favored by race boats with immensely tall rigs in part because they keep the boat upright, not because they have lower GZ's.

Let's not forget that, in replying to Chetan's question, we're discussing two ends of a continuum and omitting the middle: a hull form that utilizes an extended fin with encapsulated ballast and a separate, fully skeg-supported rudder. This reduces some of the disadvantages mentioned by John of a full-keeled boat while retaining some of its merits...at least to a degree. And just like the two extremes, it also is only one piece of the pie and, just like the other keel types, can't eliminate the effects of hull form and rig design.

Jack
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Old 11-03-2005, 08:02 PM   #6
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hi Jack

thanks for the reply. it is indeed a rather intricate discussion.

when you speak of a lower GZ do you mean lower in terms of a numerical value; or a righting moment that comes into effect at a greater angle of heel? or do you mean lower in terms of position; ie closer to the waterline; with reference to the "axis" around which the boat heels or pivots (if you imagine the sail and keel as forces acting on either side of a lever which is the mast above water and the fin below)

Apologies for the lack of terminology and perhaps primitive analogies but this is an area in which i would greatly appreciate any insight.
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Old 11-04-2005, 02:27 PM   #7
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Hi all,

Maybe I can shed some light (or confussion) in this issue?

The stability of a vessel depends on the position of its centre of gravity. Not surprisingly, a lower C of G gives increased stability. But stability in itself does not show how great the GZ (righting lever) is.

If you take pencil and paper and make a little sketch of a transverse section through the centre of a boat, with the boat lying upright and floating free and then mark the CofG relatively low down in the vessel. This point is known as G. Also mark the centre line with the force of gravity working vertically downwards from G.

On the same sketch, mark the centre of buoyancy B. This is the centre of the underwater cross section. Mark from B the force of buoyancy working vertically upwards from this point. If the vessel is still then force B should be equal and opposite to force G - thereby maintaining a state of balance.

Then redraw the sketch with the vessel heeled say at 20 degrees (where the heeling force in an external force, i.e. a wave) and again mark G and from this the force of gravity working vertically downwards from G.

Mark also the new underwater centre of floatation, B1. That is the centre of effort for buoyancy which will be relative to the new underwater shape.

You will now find that B has moved to B1 outboard of G and, if you draw the forces working in the opposite directions and then draw a line at 90 degrees from G to the vertical vector passing through B1 they intersect at point Z.

GZ is then the righting force or moment striving to bring the vessel into the upright position and is dependent upon its length and the displacement of the vessel. You will observe that a wider hull initially gives a greater GZ. Catamarans have a huge GZ to bein with.

In the second diagram, the point where force of buoyancy acting through B1 crosses the vessel's centreline is known as the metacentric height M

The above assumes that the vessel has positive stability. In this case G is below M. If a vessel has negative stability G is above M and a capsizing moment existis. In a state of neautral stability G and M are at the same height.

Stability is therefor increased by altering the hulls shape (not practical) or increasing GM by lowering G. This is achieved by having weights low down in the vessel.

What this in fact means is that the keel shape has no direct influence on the stability of the vessel but the mass of the keel and its distance below G have. A bulb will then move the keel's centre of gravity further downwards, thus increasing the metacentric height (GM) which give the vessel greater stability.

The righting lever or moment GZ is not a measure of the vessel's stability but the force which will bring the vessel upright after having been heeled. nObviously, as GZ is a function of GM then increased stability results in increased righting moments.

On a sailing vessel, the situation is further complicated by the force the wind exerts upon the sails, rig and superstructure. This effect is almost neglagible in a power driven vessel. We, i our sailing vessels, need to have a vessel which is stable even when the wind is trying to tip the vessel. Thus sailing yachts are much more stable than power boats. In fact, you would not want to be in a power boat if it had the same stability as a yacht as the righting moment GZ would be so great the boat would be flung from side to side. Even very large vessles, e.g. ore carriers, suffer from this. You can imaging the stability of an ore carrier loaded with something really heavy such as iron ore which only lies at the bottom of the hold compared with coal which would completely fill the hold. The iron ore cargo results in a very low centre of gravity which results in turn in a huge GZ. This makes the ship extremely "stiff" and has often caused vessels to suffer from huge stresses.

Getting back to the keel shape though; I favour the long keel for its course stability. I also favour the older 'wine glass' hull form as the curve of positive stabilty is much greater than a morw modern wide hulled vessel, although the wide hull ius initially much "stiffer" due to the position of B1 moving very quickly outboard for small angles of heel.

Another advantage of the long keel is that the rudder and prop are better protected and, if a small plate is placed over the gap between the bottom of the keel and the base of the rudder, lines can not enter that space and foul the rudder or the prop. Incidentally, my boat steers quite well when going astern too - one just needs top apply sufficient power.

Lastly, I find the deep and long keel makes for a much softer ride. The keel parts the seas rather than having them slap against a flat 'underbelly' which more modern fin keeled boats seem to have.

Having said all that, there are many fin keelers happily sailing around the world and crossing great oceans. The bottom line is, "you pay - its your call".
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