Full or fin keel? - Page 13 - SailNet Community
Old 03-22-2012
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Re: Full or fin keel?

Quote:
Here is the problem with that statement, dampening (the ability of a boat to dynamically to resist rotational motion) is directly proportional to a moment of inertia the amount of which results from the resistive force of the rotation and the distance that resistive force is from the instanteous rotational axis. In calculating a dampening moment, the force is a linear factor, but distance from the center of that force to the instanteous rotational axis is to the third power.
Hello Jeff, I pretty much stopped reading after this paragraph. Damping has nothing to do with inertia. Inertia is related to the square of the distance and the force is a function of acceleration. Damping is related to area and the moment is proportional to distance. The force is a function of velocity.

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Old 03-22-2012
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Re: Full or fin keel?

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Originally Posted by Faster View Post
there are plenty of fin keel boats that are 'stiffer' than some full keelers.
I would say in general that most any newer flat bottom, wide beam fin keel is more stiff than most any full keel. That is exactly the characteristic of such boats. Heavy full keel boats are typically not stiff. However, heavy full keel invariably have considerably more mass righting moment. That's what makes them so much better in rough water.
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Old 03-22-2012
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Re: Full or fin keel?

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Originally Posted by Jeff_H View Post

... dampening (the ability of a boat to dynamically to resist rotational motion) is directly proportional to a moment of inertia the the amount of which results from the resistive force of the rotation and the distance that resistive force is from the instanteous rotational axis....
Hi Jeff,

There are other factors that contribute to dynamic stability and I don't think that one is the more important. Really important is everything that contributes to dissipate the wave energy in kinetic lateral movement and detrimental is everything that contributes to transform that energy in a rotational movement.

I would say that the main factors that contribute to allow the dissipation of energy through a lateral movement are a small area of submersed surface, a small freeboard and a big righting moment (not necessarily at big angles of heel because when boats slide sideways in a wave the heel is not much) and a low inertia.

Regarding inertia and roll moment of inertia, let’s consider two boats with the same positive area under the RM curve, and therefore needing the same energy to be capsized. One is a long keeler, small draft, narrow heavy weight boat the other one is a beamy light boat with a big draft and all the ballast on a bulb at the end of a fin keel.

As I had said before the dynamic behavior of these boats when hit by a breaking wave will be very different in what regards the capacity to dissipate the wave energy moving sideways, but let’s consider that the full keeler would not trip in its keel, that the extra surface would not have a damping effect on the rotational movement and that the low mass and small under water surface would not permit the lighter boat to move much more easily sideways.

Let’s consider that the same amount of energy of the wave that hits the boats results in a rotational movement for both boats.Both boats require the same energy to be capsized so in what regards the results (capsizing or not) the effect would be the same but the kind of movement due to inertia would be very different.

The heavier boat would start to roll much more slowly but because it has much more inertia once started the rolling movement it would be much harder to stop it. On the lighter boat the roll movement will be faster but will be also stopped faster because the inertia is much smaller. Both boats will roll to the same point but the duration of the roll movement (to capsize at 90º and back to its feet) will be much longer on the heavier boat.

We could say that the slower movement is a more comfortable one but the fact is that in what regards seaworthiness the heavier boat will be much more time deeply heeled, exposing its side much longer to another breaking wave that will catch him with little stability left and therefore will have potentiated effects, resulting probably in a capsize.

That’s why I think roll moment of inertia is not a determinant factor in Dynamic stability and that its effect in Dynamic stability effectiveness is many times hugely overrated.

Regards

Paulo

Last edited by PCP; 03-22-2012 at 12:07 PM.
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Old 03-22-2012
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Re: Full or fin keel?

To use water analogies: things are "getting deep" in this discussion, and I find more and more that it's "over my head"!

My copy of the book "Seaworthiness" has been shipped, perhaps if I can digest some of that, I'll at least have a handle on the 30 yr. old side of the discussion.
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Old 03-22-2012
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
To use water analogies: things are "getting deep" in this discussion, and I find more and more that it's "over my head"!

My copy of the book "Seaworthiness" has been shipped, perhaps if I can digest some of that, I'll at least have a handle on the 30 yr. old side of the discussion.
Why to pay or wait for it? As I have said it is an old book and has many old books it can be downloaded for free:

Regards

Paulo
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Old 03-22-2012
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Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
Hello Jeff, I pretty much stopped reading after this paragraph. Damping has nothing to do with inertia. Inertia is related to the square of the distance and the force is a function of acceleration. Damping is related to area and the moment is proportional to distance. The force is a function of velocity.

Bryce
Bryce,

You apparently are mistaking simple 'moments' and simple 'inertia' for 'moments of inertia' the term that I used in the quote from my explanation.

As I am using the term 'Moment of Interia' I am using the term defined as, "In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation. The moment of inertia plays much the same role in rotational dynamics as mass does in linear dynamics, describing the relationship between angular momentum and angular velocity, torque and angular acceleration, and several other quantities."

When talking about dampening, the resistance to changes in rotation is not just the inertial mass of the object, but the resistive forces of the side force of water against the keel, and the side force of the wind against the sails, times the distance to the axis squared. But since the unit force is proportionate to the radius of the rotation, the formula for calculating roll moment of interia due to the keel, the distance between the center of the force and roll axis is to the third power.

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Last edited by Jeff_H; 03-22-2012 at 11:07 AM.
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Old 03-22-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
Why to pay or wait for it? As I have said it is an old book and has many old books it can be downloaded for free:

Regards

Paulo
Thanks Paulo, but too late for me, it's already paid for and on the way. At least I'll be able to highlight and mark it up, which always helps me to key in on and summarize important ideas.
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Re: Full or fin keel?

I certainly understand the theoretical advantages of the modern design but I question whether those advantages apply in at least one very common scenario. If this boat is 32 feet and used for very long distance cruising, ie a dinghy or two, 3 or 4 anchors and rodes, 40 days of food for 2, etc.etc, is it still going to be just as stable and fast as when it was test sailed by the owner? Personally, in my opinion, this modern design is going to prove to be slower and MUCH less stable than had it been designed a little heavier in the first place and with a little more boat in the water.
I once counted the rolls of 2 very different designs as they were sailing DW off Baja. One, a full keel 32, the other a modern 36. The full keel 32 rolled exactly 30% less than the 36. That is, the 36 rolled 100 times during the 32’s 70 times. Both boats rolled at what seemed the same angle. The 32 was sailing about 1/4k faster. Are future cruisers going to have to give up comfort for the modern design or go much longer in WL to get reasonable comfort?

Last edited by Oregonian; 03-22-2012 at 03:49 PM.
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Re: Full or fin keel?

Quote:
Originally Posted by Oregonian View Post
I certainly understand the theoretical advantages of the modern design but I question whether those advantages apply in at least one very common scenario. If this boat is 32 feet and used for very long distance cruising, ie a dinghy or two, 3 or 4 anchors and rodes, 40 days of food for 2, etc.etc, is it still going to be just as stable and fast as when it was test sailed by the owner? Personally, in my opinion, this modern design is going to prove to be slower and MUCH less stable than had it been designed a little heavier in the first place and with a little more boat in the water.
I completely agree with what you are saying if we compare boats based solely on length on deck. If the comparison is two equal length boats that are equally loaded, one a traditional long waterline relative to LOA, heavy L/D boat, whether full keel or some partially cut away keel form, and the other a more modern equal length boat, the newer boat will not be able carry as much weight in gear and consumables without seriously compromising performance and seaworthiness. But also, since design loads on the hull and rig are proportionate to the displacement of the boat, when equally loading these two boats a lighter modern design will be loaded proportionately more as compared to its design displacement and so would end up being a comparatively less robust boat when loaded to the max loading of the equal length heavy d/l cruiser.

It is for this reason that I have always suggested that a better way of comparing offshore boats is to compare boats of equal empty displacement rather than boats of equal length on deck or waterline. In that comparison, within reason, the longer boat of equal displacement will generally offer better performance across a wide windspeed range, offer better motion comfort and seaworthiness, be easier to handle, have better carrying capacity without impacting handling or seaworthiness and have roughly similar operating costs.

Quote:
Originally Posted by Oregonian View Post
I once counted the rolls of 2 very different designs as they were sailing DW off Baja. One, a full keel 32, the other a modern 36. The full keel 32 rolled exactly 30% less than the 36. That is, the 36 rolled 100 times during the 32’s 70 times. Both boats rolled at what seemed the same angle. The 32 was sailing about 1/4k faster. Are future cruisers going to have to give up comfort for the modern design or go much longer in WL to get reasonable comfort?
Anecdotally I can imagine how this would be true more often than not. Similar to the discussion above, a heavier (D/L) displacement boat will typically offer a slower roll rate, which can be a very good thing in a comparative short chop or in long wave length ocean conditions.

Depending on the wave frequency, and the weight and bouyancy distribution of the two boats, the heavier boat may also roll through a narrower roll angle as well.

But if we compare boats of equal displacement, one longer than the other, the longer boat will generally offer a similar roll rate, but if the boat is a modern design which has been properly designed for motion, the roll angle should be smaller and the edges of the roll more gentle. But also the collisions with waves when beating and close reaching should also produce less hobby-horsing and a softer collision with each wave.

On the other hand, depending on the wave frequency, and relatve speeds of the boats, the longer boat for the same displacement may not fare so well with regards to heave. Typically a longer boat with a smaller D/L, when encountering a single large wave or a widely spaced wave train, will heave at closer to the speed of the wave than a boat with a heavier D/L giving the heavier boat a much more comfortable heave motion in those conditions.

On the other hand, the heavier D/L advantage can get lost in a repetitive, closer spaced wave train, where the inertia of the heavier boat can take it our of sync with the wave train and so cause it to hit harder at the trough and peak of the wave resulting in a less comfortable motion in a narrow range of conditions.

Respectfully,
Jeff

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Old 03-22-2012
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Re: Full or fin keel?

Seems, too, that boats with hard bilges and high form stability are going to be more likely to 'follow the sea surface' in a beam sea and therefore 'roll' more dramatically as a wave passes underneath.

Ron

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