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Discussion Starter #1
While doing research on various types keels, I've seen discussion that motion and stability result more from the design of the hull than from the keel design, so that a "balanced hull" will have better motion, steadiness of helm, rolling and heeling, and maneuverability regardless of the type of keel. But it was not explained what a balanced hull is. Hulls all look the same to me, more or less. Does anybody understand the design issues with hulls? What are some examples of well balanced hulls?

Thanks
JIm
 

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Old soul
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Never heard this term before. Balanced rudder, certainly. But balanced hull? Hopefully some of our designers will chime in, but this is new to me.

There are a wide range of hull designs. Some are dramatically different. And for displacement boats, the hull includes the keel.
 

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Discussion Starter #3
Never heard this term before. Balanced rudder, certainly. But balanced hull? Hopefully some of our designers will chime in, but this is new to me.

There are a wide range of hull designs. Some are dramatically different. And for displacement boats, the hull includes the keel.

Thanks Mike. What is a displacement hull, one with an integrated keel? Like a full keel?

Thanks.
 

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What is a displacement hull
Its' a hull design that pushes the water aside, rather than trying to ride up on top of water. They are fuel efficient, but limited in speed. These are typically found on sailboats, trawlers, cargo ship, tugboats and the like. Planing hulls are found on go fast powerboats.
 

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Cape Dory 30
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Its' a hull design that pushes the water aside, rather than trying to ride up on top of water. They are fuel efficient, but limited in speed. These are typically found on sailboats, trawlers, cargo ship, tugboats and the like. Planing hulls are found on go fast powerboats.
Not sure if true, but possible an US Aircraft Carrier, but you will never see that in print. Dennis
 

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The concept of a balanced hull from is a very old concept that was first voiced in the 19th century. Conceptually, a balanced hull form is a hull shape than does not change trim (bow up or down) and doesn't change center of lateral resistance with heel angle.

In its original conception, and without the tools to actually analyze the dynamic forces at play, initially the theory would have been implemented by boats that were true double ended and with cylindrical hull sections.
That hull form turned out to produce a hull that statically appeared to be balanced, but in practice did not behave as a balanced hull form.
Over the next century there were a wide variety of theories about how to produce a balanced hull form. One by one, these have largely been discredited.
It's only with the ability of computer modeling of the impact of the dynamic and static forces and the changes in buoyancy distribution with heel angle, that a truly balanced hull form can be achieved.
That hull form could not be more different than the original concept. It turns out that the 'pizza slice' hull forms with comparatively flat sections, ironically remain balanced across the heel angles that most boats sail within.

And yes, the keel type has little to nothing to do with balance. As a broad generality, full keels tend to produce the highest weather helms because the true center of lateral resistance is closer to the leading edge of the keel than static calculation would suggest.

Jeff
 

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Discussion Starter #8
The concept of a balanced hull from is a very old concept that was first voiced in the 19th century. Conceptually, a balanced hull form is a hull shape than does not change trim (bow up or down) and doesn't change center of lateral resistance with heel angle.

In its original conception, and without the tools to actually analyze the dynamic forces at play, initially the theory would have been implemented by boats that were true double ended and with cylindrical hull sections.
That hull form turned out to produce a hull that statically appeared to be balanced, but in practice did not behave as a balanced hull form.
Over the next century there were a wide variety of theories about how to produce a balanced hull form. One by one, these have largely been discredited.
It's only with the ability of computer modeling of the impact of the dynamic and static forces and the changes in buoyancy distribution with heel angle, that a truly balanced hull form can be achieved.
That hull form could not be more different than the original concept. It turns out that the 'pizza slice' hull forms with comparatively flat sections, ironically remain balanced across the heel angles that most boats sail within.

And yes, the keel type has little to nothing to do with balance. As a broad generality, full keels tend to produce the highest weather helms because the true center of lateral resistance is closer to the leading edge of the keel than static calculation would suggest.

Jeff
Thanks very much Jeff. Can I assume that designs from the 60s to the 80s are a serendipitous mix of well balanced and not well balanced? I understand they are not pizza slices so not as balanced as a newer design, but wondering how this should affect choice of an older (less expensive) monohull, like a Pearson, Irwin, Island Packet, Passport, Hallberg-Rassey, Swan, Valiant etc. etc.

Thanks again.
Jim
 

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Discussion Starter #10
See, I knew Jeff would be able to answer. And I see you got your answer to the displacement hull question.
Thanks Mike, yes, this is a great resource. Much better than watching endless Youtube videos, that kind of info doesn't occur in sailing videos.

Thanks.
 

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Barquito
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I would suggest getting a book or two on the subject. One book I found useful in describing almost every choice you will need to make in a cruising boat is Nigel Calder's Cruising Handbook. Keep in mind, he tends to lean toward suggesting the more complicated, more expensive systems.
 

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Discussion Starter #12
I would suggest getting a book or two on the subject. One book I found useful in describing almost every choice you will need to make in a cruising boat is Nigel Calder's Cruising Handbook. Keep in mind, he tends to lean toward suggesting the more complicated, more expensive systems.
Thank you Barquito. I have ordered it, along with Sailing Theory and Practice by Marchaj, C. A. which was also recommended.

Thanks.
 

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Thank you Barquito. I have ordered it, along with Sailing Theory and Practice by Marchaj, C. A. which was also recommended.

Thanks.
Marchaj's book is a great resource for basic theory. I will note that it is somewhat out of date in that a lot has been learned in the nearly 60 years since it was written.

Jeff
 

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Discussion Starter #14
Marchaj's book is a great resource for basic theory. I will note that it is somewhat out of date in that a lot has been learned in the nearly 60 years since it was written.

Jeff
Thanks Jeff. It's the 1985 edition, not the 1964, so it's only 35 years out of date :). His 1989 book Aero-Hydrodynamics of Sailing must be out of print, Amazon wants $262.

Surely it would be nice to have something that includes more modern hulls. There are several books on sailboat design on Amazon but it's hard to know which ones are worth the price. If you have suggestions please let me know.

Happy sailing...
 

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Marchaj's book is still a very useful book to read. The 1985 edition has quite a bit of updated information that came out of the early research following the Fastnet Disaster. But my recollection is that it predates the next traunch of research results that was associated with the development of the EU Regulations.

One of the biggest issues with the post Fastnet research is that the boat designs were IOR hull and rigs. Since then, boat design has moved in a very different direction that actually addressed many of the issues that Marchaj's book rightly identifies.

Jeff
 

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Discussion Starter #16
Marchaj's book is still a very useful book to read. The 1985 edition has quite a bit of updated information that came out of the early research following the Fastnet Disaster. But my recollection is that it predates the next traunch of research results that was associated with the development of the EU Regulations.

One of the biggest issues with the post Fastnet research is that the boat designs were IOR hull and rigs. Since then, boat design has moved in a very different direction that actually addressed many of the issues that Marchaj's book rightly identifies.

Jeff
Thanks. What do you recommend I read to understand the different designs from the 60-90s hulls and newer hulls?

Thanks again, much appreciated.
 
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