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post #118 of Old 04-24-2013 Thread Starter
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Re: Bob Perry's take on Wolfenzee's dream boat

Originally Posted by hellosailor View Post
I can understrand how a sea anchor contributes stability by adding drag, but overall the "drag" on a hull? Slows it down but I've never heard to that as adding stability of any kind. Directional stability comes from many things, drag is just a parasite that may be along for the ride AFAIK.
I am not 100% sure what context was said or that is being asked within. Drag does not add to stability per se, but reducing drag can help with heel angle and weather helm in a gust. When a boat gets hit with a gust the force from that gust has to get disbursed somehow. Some of it goes into producing drive which in turn accelerates the boat, some of it produces drag trying to slow the boat down. some of it goes into heeling, some of it goes into leeway, and so on. In theory, the more force of the gust that you can use for acceleration, the less that should be available to do the less desirable things. So if there is less drag, and the boat can accelerate, it will heel less and develop less leeway, when a gust hits. This is very apparent on my boat when she has a dirty bottom.

Directional stability is generated by a collection of forces generated by the hull, the keel, and rudder passing through the water. It has traditionally been thought that a long keel in and of itself generates a lot of directional stability, but much of a long keel operates in turbulent water, and so does not have the kind of steady flows that would automatically mean that any boat with a long keel will be directionally stable.

But beyond that on a boat the size of the one in question, good balance between the rig, hull and foils will be a larger determinant of how likely the boat is to hold a course relative to the wind.

And frankly for distance sailing, a boat with a light helm which can be easily steered by a windvane is probably a better choice than a boat which will track solely on the length of its keel.

To answer Ron's (Faster) question, we kept the skeg on part because Wolf had been very concerned with tracking. I had hoped that the skeg would help with directional stability at higher boat speeds. I am not sure whether we will retain the skeg as we move into the next phase of this.

I have to admit that I drafted the bulb, and have not done that with a whole lot of scientific care. At this point its a place marker and more of a symbollic illustration.

But to answer your question to the best of my knowledge the fillet at the top of the bulb is there to reduce wetted surface and mimimize the turbulance that would occur as the water passes downward and aft over the bulb.

A bulb acts in a very different condition than a keel root especially on slower boats. This area is exposed to water sliding down the foil and colliding with water being turned aft from the free stream over the top of the bulb. Its my understanding that a square edge at that point does little to help with that condition but does add drag.


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