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  #461  
Old 07-31-2012
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Re: Full or fin keel?

Reserve buoyancy is a function of displacement...not shape.

It manifests itself...not only fore and aft, but athwart-ships as well. It makes a sailing ship "stiffer"...able to carry more sail.....it makes her ride up and over the sea-state...not plow into it! Furthermore, when a marine architect purposely adds reserve buoyancy to his loftings, he is compromising his choice of "shapes". And they will pretty much dictate the chine and tumblehome of the final drawings.

This thread is about full vs fin keel.
Given that modern (Non -Brit) designers tend to favor the beamier, plans....afterall, they can carry sail....are very roomy below...and they don't require keels that would plow up the Grand Banks! These more modern designs
do have more wetted surface....so the choice of underbody is obviousy fin and spade!




That's why we shot you out of the water in 1812....we took "America's Cup a couple of decades later....and thwarted all your efforts to win it back. Should I go on?
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  #462  
Old 08-01-2012
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Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
Wide sections either end of the boat do not necessarily mean reserve bouyancy. That depends on how they are shaped.

But wide bow sections mean a harder collision with waves and less moderated impact, so more pitching, and more rapid deaccelleration in each wave collision, and therefore a much less comfortable motion combined with less speed upwind in a chop.

Wide stern sections generally mean better dampening so a little less pitching at a slower speed up wind.
My boat is really wide on the foredeck (but not at the waterline) to keep the cockpit dry (an Atkin trait), this would make the boat a bit saucy unless you carried the amount of chain and anchor it was designed to, balanced out by the engine. Being a heavy displacement boat (15,000lbs) it does not decelerate when going through a wave. The big wineglass transom also provides a lot of reserve buoyancy and keeps the cockpit dry from that end.I guess that is just one of the advantages of a full keel over a fin.

Last edited by wolfenzee; 08-01-2012 at 01:37 PM.
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  #463  
Old 08-01-2012
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Re: Full or fin keel?

Quote:
Originally Posted by Navstarr View Post
Reserve buoyancy is a function of displacement...not shape.
Reserve Buoyancy is not really about displacement in the usual sense of the term. Reserve Buoyancy is about the change in volume as vessel places portions of the boat normally above the static waterline below the surface of the water. This occurs as trim changes or dynamics forces cause the boat to temporarily displace more or less than its actual weight or static displacement. Because of that, shape becomes important, especially if the discussion is considering the behavior of the boat in a seaway, which was my point above.

In and of itself, simply adding reserve buoyancy does nothing good for the boat, and improperly shaped or excessive reserve buoyancy can be detrimental to motion comfort and seaworthiness. But if the change of shape and increase of buoyancy occurs progressively as the boat changes trim, that increasing buoyancy can dampen rotation and slow the rate of change of motion.

Quote:
Originally Posted by wolfenzee
My boat is really wide on the foredeck (but not at the waterline) to keep the cockpit dry (an Atkin trait), this would make the boat a bit saucy unless you carried the amount of chain and anchor it was designed to, balanced out by the engine. Being a heavy displacement boat (15,000lbs) it does not decelerate when going through a wave. The big wineglass transom also provides a lot of reserve buoyancy and keeps the cockpit dry from that end.I guess that is just one of the advantages of a full keel over a fin.
This is full of less than perfectly accurate statements but I'll see whether I can straighten them out for the record.

Quote:
Originally Posted by wolfenzee
My boat is really wide on the foredeck (but not at the waterline) to keep the cockpit dry (an Atkin trait), this would make the boat a bit saucy unless you carried the amount of chain and anchor it was designed to, balanced out by the engine.
This begins to address my point about shape, which is that Atkins was a master of modeling hulls and many of his designs are shaped so that progressively increase displacement above the waterline dampening the dynamic tendencies and minimizing accelleration and deaccelleration. Both the nicely flared bow and wine glass stern on your boat are perfect examples of that. Adding an anchor and chain in the bow and an engine in the stern, would also slow the accelleration and deaccelleration rates, but would increase the pitch angles which is why people think of boats of this era as having excessive pitch angles. (hobbyhorsing)

Quote:
Originally Posted by wolfenzee
Being a heavy displacement boat (15,000lbs) it does not decelerate when going through a wave.
All boats, regardless of their weight decelerate when going through a wave. Its basic physics, but the rate of deacelleration is proportionate to the inertia of the boat, and the force it encounters. At this point, how much your boat 'feels' deacceleration relative to a lighter boat is only true relative to wave size.

To explain, being a heavier displacement boat, your boat tends to be slower changing direction for a given wave impact and so would tend to have less deaccelleration than a lighter boat. But mitigating against that is that because it does not change direction as easily, it also buries itself further into the wave, experiencing a greater impact force. In practice this means that in small waves, you would feel the impact less than a lighter boat, but as waves become larger and steeper you would feel each collision more than a lighter weight boat. At some point as the waves start to get large enough, neither a light or heavy weight boat will inherently feel the waves any more or less than the other due to their weight, with shape being a more significant determinant of how boat behaves due to the impact with the wave.

Now then in really big waves, a light boat will heave at a similar rate to the rise and fall of the surface of the wave. A heavier boat is more likely to get out of phase with the wave and feel a greater impact in the trough and negative gravity at the crest than the lighter boat.

Quote:
Originally Posted by wolfenzee
I guess that is just one of the advantages of a full keel over a fin.
Actually it has nothing to do with whether you have a fin or full keel and not much to do with overall weight, and mostly to do with buoyancy and displacement distribution.

Jeff
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Last edited by Jeff_H; 08-02-2012 at 11:10 AM.
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  #464  
Old 08-01-2012
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Re: Full or fin keel?

Everyone knows the best keel is a bilge keel, nuff said.
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  #465  
Old 08-02-2012
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Re: Full or fin keel?

Doug Peterson?!?!?!?!?!

oh yeah, the guy the designed the peterson trapezoidal keels on many boats from the late 80s into the 90's. This included my mid 80s Jeanneau........

I would still take a current day fin/bulb over my current keel if I had a choice. Not sure if the boat I have would like the keel or not, ie do better per say. I would like a bit more ASSet if you would, for a bit more room in the cockpit.

At the end of the day, ALL boats are compromises! as to how the end user wants to fill certain uses etc of the boat.

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  #466  
Old 08-04-2012
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Re: Full or fin keel?

Some of these light weight fin keels almost come to a full stop when confronted with a large wave, yes mine does decelerate some, but not considerably.
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  #467  
Old 08-04-2012
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Re: Full or fin keel?

Quote:
Originally Posted by wolfenzee View Post
Some of these light weight fin keels almost come to a full stop when confronted with a large wave, yes mine does decelerate some, but not considerably.
And some do not, its not about the keel or the weight of the boat, but the shape of the hull and the distribution of the buoyancy and weight. But more significantly, a properly designed, longer boat with the same displacement as yours would feel the waves even less than you do since the impact would be less, but the momentum the same.
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  #468  
Old 08-05-2012
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Re: Full or fin keel?

Wolf,

I don't know what modern light weight fin keel boats you have sailed, but the ones I have been one do just fine in waves. There may be a lot of reasons to prefer a full keel over a fin, but performance is not one of them.
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  #469  
Old 08-05-2012
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Certainly while loading has all to do with reserve buoyancy, I believe that the CG has more to do with keel design. A long broad keel offers a more forgiving ride yet becomes more stiff on the reach, regardless of reserve buoyancy. Designers alleviate this by cutting back the keel. Reserve buoyancy may have more significance in boats with finely shaped fin keeps where the shape of the hull changes the shape of their hulls at speed, but displacement is displacement.
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Old 08-05-2012
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Re: Full or fin keel?

Some boats are just well designed and well balanced -

It is a careful balancing act between wetted surface area vs. trim vs. center of effort vs. center of lateral resistance. There is quite of bit of art in balancing the oft conflicting parameters.

For example Steve Schock designed the Harbor 14,20,25, and 30 to have low wetted surface and balanced helm. He spent a lot of time adjusting the keel shapes and location to achieve the neutral helm these boats are known for.

A Harbor 25 sailor on SF Bay single hands most of the 100 days a year he sails. He sails out of Sausilito so he spends a lot of time in and around the slot. No autopilot. He says the boat is so nicely tuned, that he can leave tiller unattended for minutes at a time.

The longest he left the tiller unattended was 20 minutes ! He says the boat just tracks along.




Last edited by WDS123; 08-05-2012 at 10:23 AM.
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