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Old 17-11-2005, 18:26   #16
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Delmarey said, " "I've always wondered how a boat would handle without any chines. Basically a water wing. The draft would probably be more then normal and it would probably take more sail area to get it to move along. Much like a heavy full keeled boat."

You are describing the old English" Plank on edge cutters" also called "lead mines". They were popular in the late 1800's. They carried vast quantities of sail area, were submarines with a mast, and sailed on their ear. In their day they were pretty fast but of course really had very little speed compared to more modern designs.

Good design is about moderation, even in moderation.

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Old 19-11-2005, 12:30   #17
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Quote:
delmarrey once whispered in the wind:
Well, the nice thing about the round bilge with a keel, it's like a pendulum and it's EZ to lower the COG (center of gravity). Unlike a hard chine where most of the weight is spread over a wider area, in effect, raising the COG when it heels over. No pivot point.
Sorry to burst your bubble. A few basics in boat design.
For starters, COG (mass) is a fixed geometric point calculated by the designer. Its location is static and does not change with heel of hull, even in a 360 roll.
What do changes is the transverse COB (centre of boyancy) when a hull is heeled. When your hull is sitting upright, the COB and COG is in equilibrium - sitting above each other. Simply put, as the hull heels, the COB moves away from the COG and create a lever that is called the righting moment when multiplied by the boat weight. This is used by the designer to generate a stability graph. This shows the righting moment from zero degree throught to vanishing stability and beyond.
Never have I seen a stability graph that points to, suddenly, "Wham, and over she goes"!!. A designer take great pain to just prevent that with a boat. Loss of stability is gradual from max to vanishing point.
However, that said, "Wham, and over she goes" is really applicable to all boats when they reach their point of vanishing stability (inverse stability)
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Old 19-11-2005, 12:47   #18
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WHAM

A boat that has drums in the form of a catamaran, and a flat bottom boat with hard chines. A housboat with all the partiers on the top deck. No ballast on any of these. WHAM over she goes would be a good description. In fact that is close to the description used by witnesses.
Agreed they are not ballasted as I mentioned in my post, but it does help descibe the feel of a hard chine boat. By their very nature they have more initial stability based on the hull shape, and then the hull shape loses the stability in a more sudden ( some might say subtle ) manner.
A Thistle and a Lightning probably have very similar ultimate stability, but on the Lightning it does sneak up on you and WHAM it is gone.
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Old 19-11-2005, 14:08   #19
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Wynand N

This is as far as I'm going with this argument. But there are some things for sure!!!

If you take a weighted keel sailboat and put it in a calm harbor, WITH DECK HATCHES SEALED, then list it up on it's side, then let go, it'll right it's self. The COG remains constant above the waterline. Round bilge!

On the other hand, if you take a powerboat or catamaran, even with weighted keels, and pull it over on it's side, chances are that they'll continue over. The COG raise up out of the water!

As well, and upside down power boat or cat will stay upside down in most cases due to their beam and/or hard chine. Even a sail boat, as discribed above, will right itself if inverted. There is NO PENDULUM EFFECT in hard chines and cats. This is why mono sailboats are the choice of world cruisers.

I like Cats, and power boats too, but when the going gets rough it's time to throw out the parachute, and hang on. Whereas, a monohull, one can heave-to and go for it.........................._/)

I'll bet this guy below had a round bilge or at least a displacement hull. "PENDULUM EFFECT" Otherwise, he'd be at the bottom of the ocean.

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Old 19-11-2005, 15:31   #20
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Sorry to burst your bubble. A few basics in boat design.
For starters, the CG (weight not mass) is a fixed geometric point relative to the boat for any given loading but which will vary in position with how the boat is loaded no matter where the designer calculated it to be fpr some idealized loading.

And while the center of gravity will be fixed for any loading, and even assuming nothing shifts (like fuel and water in the tanks or supplies and gear in lockers) the CG will move vertically and horizonatally relative to the center of bouyancy or to the water even if it does not move relative the boat itself. Typically hard chine boats 'roll up' meaning that the center of gravity rises relative to the surface of the water as the boat heels, while round bottom and vee bottom boats tend to 'roll down' as they heel.

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Old 19-11-2005, 19:15   #21
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Old 20-11-2005, 05:41   #22
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Weight, not mass I believe it is the same thing my mate.

Without going into an argument let me put this straight. The sketch delmarrey so kindly supplied shows exactly what I said. Notice that the COB moved, not COG. That is fixed, regardless of how the boat was loaded, providing nothing moves. Something else that changes in heel is the meta centre (GM).

Quoted by Jeff "the CG will move vertically and horizonatally relative to the center of bouyancy or to the water even if it does not move relative the boat itself. Typically hard chine boats 'roll up' meaning that the center of gravity rises relative to the surface of the water as the boat heels, while round bottom and vee bottom boats tend to 'roll down' as they heel."

In all respect Jeff, you have it the otherway round. COB always move relative to the COG. Example: if a boat is badly loaded, the boat will trim itself when the COB lined up with COG.
OTOH, a fairly common way some designers trim a hull (me included) is when we have done weight/mass calculations and the basic hydrostatics, and we found that the longitudinal COG and COB are out of alignment, we trim the hull. I usually move the keel with ballast as this is the most effective way to trim until the CB and CG is aligned.

While we are on this issue of COG and without highjacking this interesting thread the following.
There is also a misconception that with twin keelers, when heeled, the weather side keel will pull down and increases the Righting moment. Nothing is further from the truth.....When the boat was trimmed statically by the designer, the CG of every thing on the boat, hull and deck included, was proccessed by the designer - on a vertical and horisontal plane. The keel are included.
Thus, as the twin keeler (and any other boat) heels, she pivots on the COG and only the COB changes due to the change of shape of the hull in the water as it heels. (Refer to Delmarrey's sketch.)

Yes, a round bilge hull is more tender than a chined hull due to the shape of hull.
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