Full or fin keel? - Page 48 - SailNet Community
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post #471 of 847 Old 08-05-2012
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Re: Full or fin keel?

Sea Hunter- Could you please explain what you are talking about? It may be a linguistic issue but much of what you have written has little, if anything, to do with the way that these terms are normally used in yacht design or design theory, or with the behavior of a vessel.

For example: "Certainly while loading has all to do with reserve buoyancy" The way that the term "reserve buoyancy" is classicly used, loading has little, if anything t0 do with reserve buoyancy. As I read your sentence, I would suggest that you are mistaking (reserve) "carrying capacity" for "reserve buoyancy". These are two very different and not necesarily related concepts.

Or, "I believe that the CG has more to do with keel design." CG is only affected by weight distribution. While the type of keel and the profile and section of the keel affords the designer more or less ability to move the CG around, the type of keel and the profile and section of the keel does not in and of itself impact the CG of the boat. To provide an example to explain this, you could have an three boats the identical type of keel and with the same profile and section of the keel, but one uses cast lead, the second cast iron, and the third has a timber keel. Obviously each of these materials would move the CG around vertically, and in most cases, longitudinally as well.

Similarly, "A long broad keel offers a more forgiving ride yet becomes more stiff on the reach, regardless of reserve buoyancy." A broad, meaning a transversely wide keel in conventional yacht design useage, while more forgiving in terms of stalling has litte or nothing to do with stiffness (form stability), reaching ability or reserve buoyancy.

None of the rest of "Designers alleviate this by cutting back the keel. Reserve buoyancy may have more significance in boats with finely shaped fin keeps where the shape of the hull changes the shape of their hulls at speed, but displacement is displacement." makes any sense or has any accuracy, except that "displacement is displacement."

Respectfully,
Jeff


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post #472 of 847 Old 08-05-2012
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Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
Sea Hunter- Could you please explain what you are talking about? It may be a linguistic issue...

Respectfully,
Jeff
Thank you Jeff, I read and reread that post several times, and could not understand any of it. Glad it is not just me!
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post #473 of 847 Old 08-05-2012
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Reserve buoyancy is a hulls ability to carry a load over and above displacement. Anything above the waterline, (with exception to a non watertight structure ie wheelhouse is not included) is reserve buoyancy.
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post #474 of 847 Old 08-05-2012
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Re: Full or fin keel?

Sea Hunter,

I am glad that you clarified what you are thinking. At least as you are describing it, (i.e. "Reserve buoyancy is a hulls ability to carry a load over and above displacement".) that is not in keeping with the way yacht and ship designers use the term, "Reserve buoyancy". What you are describing rarely if ever is considered as a part of yacht design. To some extent you are describing something like the total displacement of the submerged vessel less its normal buoyancy, which is not terribly relevant even in an extreme seaway.

More relevant in understanding the behavior and characteristics of the vessel is carrying capacity which is the amout of weight that a vessel can carry before the load has a significant impact on performance and safety. Also more relevant is the impact of hull shape on reserve stability, the angle of vanishing stability and area under the stability curve.

But none of these are inherently associated with whether the boat has a full or fin keel.

Jeff


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Last edited by Jeff_H; 08-06-2012 at 08:37 AM.
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post #475 of 847 Old 08-05-2012
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That's my point, reserve buoyancy has nothing to do with keel type. Displacement and CG are more important.
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post #476 of 847 Old 08-06-2012
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Re: Full or fin keel?

And just to be clear for someone trying to follow this, as the terms are normally used, displacement and Center of Gravity have little to nothing to do with 'reserve buoyancy' either. But when evaluating motion, carrying capacity and stability of a vessel, weight distribution and buoyancy distribution are far more critical than the overall displacement of the vessel, or its keel type.


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Last edited by Jeff_H; 08-06-2012 at 09:00 AM. Reason: spelling-clarity
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post #477 of 847 Old 08-06-2012
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Quote:
Originally Posted by jeff_h:905984
and just to be clear for someone trying to follow this, as the terms are normally used, displacement and center of gravity have little to nothing to do with 'reserve buoyancy' either. But when evaluation motion, carrying capacity and stability of a vessel, weight and buoyancy distribution are far more critical than the overall displacement of the vessel, or its keel type.
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post #478 of 847 Old 08-07-2012
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Re: Full or fin keel?

Reading this thread makes me want to throw my hands up, sell my mono, and buy a cat...


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post #479 of 847 Old 08-07-2012
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Re: Full or fin keel?

Ah so, but cats are far more complex to discuss in terms of tracking and motion comfort, not to mention fur balls and puking, and catamarrans are even more so.
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Re: Full or fin keel?

I agree, my cats motion comfort is really terrible. All the scratching and biting really ruins an otherwise nice day on the water.


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