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Old 03-03-2007, 23:28   #1
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Keel Length

Actually the better term maybe Keel depth.
My wife was reading the discription of a lovely boat to me last night. She asked an interesting question. The discription said that the boat could ahve a keel of owners choice. She asked, so can boats have different Keels. Yes I replied, our design has two keel lengths. 5'6" and 6'9" which is what we have.
That got me to thinking and I must say I was a little confused. So if our boat could have a shorter Keel, does that mean it would have more ballast weight?? And apart from the obviouse plus of getting into shallower waters, how does affect handling?? Or should I say in another way, what advantages and disadvantages does Keel length have on the behaviour of the boat.
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Old 04-03-2007, 01:51   #2
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Usually on production boats that have 2 keel lengths available, the shorter or shallower keel is heavier - the righting moment being similar with a smaller moment.

Of course a proper boat doesn't need ballast at all..... ;-)
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Old 04-03-2007, 02:43   #3
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For the Hartley Tahitian the shorter keel is for a bilge keeler
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Old 04-03-2007, 08:57   #4
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Well, you start with your deep, fin keel. This is usually the initial design of a boat. Stability, righting moment, balance, etc. of a particular boat are usually engineered based on this keel. A deep, narrow (i.e.: not too long fore and aft) keel gives the best upwind performance. The lower down weight is carried on a keel, the less weight there needs to be in order to achieve a necessary degree of stability.

Think of a teeter-totter. When you were a kid and you wanted to play on the teeter totter, if you did it with someone who was heavier, they had to sit closer to the middle to balance with you. In the case of a boat, the "middle" supporting post of the teeter-totter equates with the waterline of the boat. The parts of the boat that are above the water are balanced to a degree by the parts of the boat under the water.

For an extreme illustration of this, look at the Mumm or Mount Gay sailboats with very long, skinny keels with a bulb of weight right at the bottom. This is not a practical design for cruising in a lot of areas, as you need pretty deep water to be able to sail, and because the long keels with bulbs are usually not particularly durable when the boat experiences grounding. So unless a boat is primarily deigned as a racer, you will usually find fin keels with a more traditional trapezoidal shape.

The deeper a keel is, the better the boat will perform upwind, due to a combination of the directional stability, and lift generated by the fin.

Historically, boats that were designed for use in close proximity to the shoreline, whether for coastal cruising or fishing, had shallow keels that ran almost the full length of the boat. These keels gave the boat good tracking ability, so they would tend to sail in a fairly straight line when the helm was balanced. In contrast to the deeper keeled boats however, the long, shallow, keels were slow to tack.

If you imagine holding a wooden slat in the water vertically, and then twisting it from side to side, you won't expect to feel much resistance to the motion. If you try to hold it under water horizontally and twist it from side to side, you will encounter significant resistance.

In these boats, stability was primarily provided through the use of ballast, often large blocks of iron called 'pigs' which could be moved fore and aft in order to achieve optimal balance and sail handling.

To avoid this lateral resistance, modern boat designers will usually attempt to keep the chord, or length, of a shoal keel fairly close to the chord of a fin keel, in order to not affect a boat's ability to turn too adversely. Because the keel is not as deep, and therefore does not provide as much righting moment to a design as the deeper fin keel, the keel itself needs to be heavier (usually thicker) than a fin keel, or the boat must carry some ballast. These boats will usually track (hold one direction) as well, or better than the fin keel version, but will not sail upwind as well.

In the late seventies, wing keels started to appear on boats. In a move that upset the racing community to no end, the upstart Aussies put a wing keel on their America's Cup boat. Immediately, protests were launched and much reading of rule books and scratching of wizened heads occured. Finally, because there was no rule explicity outlawing such a thing, the keel was allowed and the Aussies won the race.

Shortly thereafter, wings were everywhere. The Aussies originally put the wings on in an attempt to enhace the downwind ability of their boat. The theory was that the wings would provide additional lift to the hull, in the same manner as an airplane wing lifts the fuselage into the air.

Production boat designers immediately recognised the wing keel as the ideal solution to the shoal draft issue. By adding the wings to the keel, additional weight could be carried at the end of a shorter keel, allowing a shoal draft boat to carry a narrow fin with no extra ballast. Basically, they could lop off the end of the fin keel, reshape it and stick it on either side of the remainder of the fin.

Performance wise, the average wing keel does not achieve the same degree of lift that the Amrica's Cup boat did, but they tend to be slightly better downwind than the average fin, with a corresponding loss in upwind ability.

So in brief, the answer to your question is yes, many boats were offered with shoal keel options, and this continues to be the case. A lot of boats were offered with three keel options. If you were to place the three keels on a continuum of all around performance, with the shoal draft keel being 1 and the deep fin keel being 10, the wing keel wouold place somewhere around 7 or 8. That said, the average sailor is not likely to notice a difference between the wing and the fin, and only a slight difference between a fin and a shoal keel.
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Old 04-03-2007, 11:38   #5
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Well thanks Sailorman. That was awesome.
Alchemist, my blue prints show the same hull, just the two different keel lengths. unfortunatly I don't have anymore info on the hulls other than a blue print showing the outlines of the two configurations of keel length and Ketch/cutter
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Old 04-03-2007, 12:00   #6
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I have The book Hartley's ferro cement boat building. It gives two drafts for all his motor sailer's. In every one except Tahitian 45 the shorter keel is specified for bilge keels. so I'm not sure
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