Stability explanation progress report

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Roger Long

Roger Long

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Nov 22, 2008
3,562
Endeavour 32 Portland, Maine
I've got the stability part of my web site done as far as the explanation of the basic mechanism.

http://www.rogerlongboats.com/Stability.htm

It may have to sit there for a while but there's lots more to come. It looks like I'll be doing some more TV and I've got a stack of books and DVD's to go through for research. Got to go down now and check the boat shed before the big storm.
 
Tim R.

Tim R.

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May 27, 2004
3,626
Caliber 40 Long Range Cruiser Portland, Maine
Nice write up roger. Your link from the bottom of waterlines to righting arms is broken though. You are missing the "s" at the end. A moot point considering you have not completed this but thought you should know.

I wonder if it is possible to build a working model to demonstrate CG. Maybe a clear box that has a dish shaped surface in it with a red ball that is free to roll around the dish. It would obviously only work to a certain level of heel but may be usefull in a classroom environment.
 
Roger Long

Roger Long

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Nov 22, 2008
3,562
Endeavour 32 Portland, Maine
Tim R. said:
I wonder if it is possible to build a working model to demonstrate CG.
Click to expand
Thanks, the link is fixed although there is nothing on the page yet.

I plan to start incorporating animations as I get to the more complex parts. They should be almost as good as a model and a lot easier to build.

Wish I could continue but I've got to go do a mound of "homework".
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
This thread is especially interesting to me as it is an expansion on C.J.Marchaj's "Seaworthiness-The forgotten factor". I look forward to the next chapter.
 
Dan Johnson

Dan Johnson

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Oct 14, 2005
2,191
1983 Hunter H34 North East, MD
Latest issue...

of Practical Sailor has an article on the comparitive stabilities and heeling arms of older versus current designs.
 
tweitz

tweitz

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Oct 30, 2005
290
Beneteau 323 East Hampton, New York
Roger -
Very interesting site. You should take a look at the "Waterlines" page, there are a couple of instances of CG and CB being confused (I suspect the sentence "...the CG must be below the CG..."). Very usefulinformation.
 
J

Jim Kolstoe

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Jun 5, 2004
209
- - Eugene, OR
Nice and informative material, although I was a bit startled by your statement that most modern boats are designed with the CG higher than the CB. That seems inheirently unstable and therefore overly dependent on form stability. Given that most ballast keel boats have 30-40% of their mass located in the keel at the lowest part of the boat, I had thought CG would be below, or at worst no higher than CB. What factors drive that design decision?
Jim Kolstoe, h23 Kara's Boo
 
Roger Long

Roger Long

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Nov 22, 2008
3,562
Endeavour 32 Portland, Maine
Jim Kolstoe said:
ugh I was a bit startled by your statement that most modern boats are designed with the CG higher than the CB. That seems inheirently unstable and therefore overly dependent on form stability. Given that most ballast keel boats have 30-40% of their mass located in the keel at the lowest part of the boat, I had thought CG would be below, or at worst no higher than CB. What factors drive that design decision?
Click to expand
Good. Being startled is the most fun part of learning. CG location is a very different design decision than locating something like a tank. It's the sum of everything and very hard to move around. Normally proportioned cruising and cruiser/racers will have have a CG located between 50% and 70% of the distance from the bottom of the hull (keel hull junction) and the deck amidships, AKA Hull Depth. CB will be below this in 99% of the boats you have ever seen. Powerboats will have CG between 60% and 110% of the hull depth. The rig and ballast just about offset each other on many sailboats.

There will be new stuff soon on the site.
 
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