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I am looking at buying a 40' yatch with flybridge. My question is, does a thin sharp hull or a wide curved hull ride better in the sea and thru the waves? Which will swing more from side to side?
Stiff boats have a lot of ballast down low - probably spread for and aft too. This gives the hull an inertia that stops it reacting violently to every wavelet. A hull with a flybridge will be more tender than the same draft and ballast without the flybridge - so any hull with weight up high will roll more and stay over longer. A hard chine (like a square edge or right angle) will tend to resist rolling more than a round bilge hull, a pronounced full keel will also resist rolling quickly, but will make turning slower. A nice curve will be more efficient through the water than a boxy hard chined hull. A skinny boat will be very good into a sharp chop, although a little volume forward will reduce the chance of burying the nose downwind. Stern volume will also help with downwind stability and help resist broaching due to drag - yet too much aft volume will tend to float the stern too quickly with a following sea - but also you don't want to get pooped too easily. The thing is it is ALL compromise.
You are better to get out on a few different types in the conditions that concern you, and gain a practical appreciation for some of the options available. I am sure the owners will be willing to point out their vessel's various strong points! 
Given this is an expensive type of boat to buy and maintain, you are best to take your time with this.
Good luck.
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