Full or fin keel? - Page 22 - SailNet Community
Old 03-30-2012
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Re: Full or fin keel?

In effect Paulo is saying "the wider boat with fin keel will snap faster than the full keel boat, but both will heel to the same angle"
So what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?

I'm confused as well Peter. I would also have thought that "rotate much easier" would mean "snap faster" as well.

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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
In a static situation without waves I don't know but recent tank testing showed that in a dynamic situation with waves the mast will help the boat to right itself up faster. Yes I know, it looks odd, but I read the paper and against facts there are no arguments...just the need to understand why

Regards

Paulo
Faster righting from inverted (turtle) position with rigging and mast, than without, that is not very intuitive.

In science I learned that one can usually devise an experiment to get the results desired.

Here is a possibility. A beamy shallow hull, compared to a deep V hull, is very stable upside down, and will resist righting. Thus the fascinating tilting keel in the video in this thread, to change the dynamics of the beamy hull.

A mast will hang down deep, well below the wave action, in deep still water. So if beam on, the waves could carry the hull along while the mast stays behind, thus in effect starting the roll towards righting.

But generally a mast and rigging stabilize the rolling of the hull, so generally it would greatly slow the rolling back from an inverted position. If the boat was not beam on to the waves, I would think it would have to wait until (and if) wave action slewed it around to beam on. A rogue wave may not have another behind it strong enough to push the hull beyond the mast. If wind capsized the boat, the waves might not be strong enough, and lacking the leverage of the mast, the wind will slide over the hull.

As an extreme example, a flat (beamy) board will resist rolling, but a round log will roll very easily. When I fell a tree it tips over very slowly, like the mast and rigging. Once on the ground, I can roll the log easily, like a dismasted boat.

This has remained an incredibly interesting thread, thank you to all who provide their insights, thoughts, and experiences.
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Re: Full or fin keel?

Quote:
Originally Posted by GBurton View Post
In effect Paulo is saying "the wider boat with fin keel will snap faster than the full keel boat, but both will heel to the same angle"
So what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?
....
Snap is a bad word to describe the movement but since you are using it:

So what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?

No, the boat will snap back almost as fast as it snap before on the opposite direction. When the light boat is already "snapped" back, the heavier boat is probably still rolling down slowly or perhaps it is starting slowly to come back to its feet.

Regards

Paulo
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
Snap is a bad word to describe the movement but since you are using it:

So what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?

No, the boat will snap back almost as fast as it snap before on the opposite direction. When the light boat is already "snapped" back, the heavier boat is probably still rolling down slowly or perhaps it is starting slowly to come back to its feet.

Regards

Paulo
Still waiting for the damping explanation......
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
In a static situation without waves I don't know but recent tank testing showed that in a dynamic situation with waves the mast will help the boat to right itself up faster.
That's really odd, could you give us a link to that paper?

(I hate to mention, but Marchaj's book also references tank tests showing boats are more likely to capsize without the mast, but he is saying they'll be slower to recover with the rig because of it's dampening effect.)

Peter
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Re: Full or fin keel?

Quote:
Originally Posted by pvajko View Post
Well, Paolo, I (the forum engine) quoted that from your own post, click on the little arrow in the quote box and you'll see.

But we seem to be talking about two different things. I'm talking about your tripping over the keel scenario where you seemed to say that the beamy, fin keel boat will slip sideways rather than roll over. That is, I think in contradiction with what Jeff said (today's fin keel boats have better dampening than full keels) because if a fin keel as at least as good dampening as a full keel, I can not see why would it slip more easily sideways, but again, I may be wrong here.
I am sorry.Yes, regards that quote it seems to be some confusion. When I read what you have posted :

"Now, if a fin keel allows the boat to "rotate much easily" as you say, then how can it have a "much higher dampening moment" at the same time?.."

I interpreted “rotate much easily” as a referring to a roll movement.

When I said that I was referring to rotating not on a horizontal axis (roll ) but on a vertical one. That has to do with the lesser resistance a much smaller underwater area will provide and also to the shape of that area: A long one along all the hull, in the case of the full keel and a central vertical one in the case of the Fin or foil keel.

It is clear that would be much more easier to make the fin keel to rotate around its keel than the full keel boat.

Everybody that tried to turn around on a marina or in a tight spot a full keel boat and a fin boat will know of what I am talking about.

Regards

Paulo
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
...
In science I learned that one can usually devise an experiment to get the results desired.

....
If it was that way we would still be in the dark ages. It all depends on who makes the testing, amateurs interested in proving something or scientists trying to broaden knowledge on hydrodynamics. In this case it was the last one. The Tank testing was conducted in a main university by researchers.

Regards

Paulo
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Re: Full or fin keel?

Quote:
Originally Posted by pvajko View Post
That's really odd, could you give us a link to that paper?

(I hate to mention, but Marchaj's book also references tank tests showing boats are more likely to capsize without the mast, but he is saying they'll be slower to recover with the rig because of it's dampening effect.)

Peter

Yes, I will give you the name of the paper and the University (I have to look) but you have to pay to access it or to receive the papers home.

But it is easy for you to mention what test tanking is Marchaj referring (just say the page and the chapter). To my knowledge all tank testing regarding sailboats stability was done after Marchaj book.

If I remember correctly he refers not tank testing under controlled conditions but what happened on the 1979 Fastnet . It was found that a boat after losing is rig was easier to capsize than with the rig intact.

Off course, without controlled conditions and without isolating causes saying that is due to a lesser roll moment of inertia is just a hypothesis that has to be confirmed with tank testing.

Personally I think that the reason is other and it has to be with a boat sailing or sitting on the water at the mercy of waves. With 40 or 50K winds even if you don’t carry any sail, the boat is sailing, moving on the water with directional stability and much less at the mercy of waves.

There are some great images of an Open 60 on bare poles sailing at over 10K.

Regarding Marchaj's saying that a capsized boat will be slower to recover with the rig because of it's dampening effect, it is just an hypothesis that has to be verified under controlled conditions. It was what those researchers have done: Marchaj hypothesis is wrong, it is not in accordance with reality.

Regards

Paulo
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Re: Full or fin keel?

I am not 100% certain that Paulo and I are in agreement on all of this, in fact I am certain we are not, but that is okay. Through discussion I would expect that we will end up perhaps a little closer in our views. This is my understanding of some of the issues being discussed above.

Starting with the issue of relative dampening force of a deep fin vs a shallower greater area keel. (Trying to avoid the semantics issue) The dampening force generated by a keel rolling through the water is proportional to the area of the keel, and the distance (from the roll axis to the center of the dampening) to the third power. So while a deep fin keel will have much less surface area than a traditional, because it has a significantly longer dimension from the roll axis to the center of the dampening than a more traditional keel it may actually develop greater dampening forces.

The potential of a deep fin keel to produce dampening is somewhat mitigated by the fact that high aspect ratio fin keels of stall more easily than lower aspect keels and so potentially produce less side force per unit of area. That said, these types of discussions are hard to broadly generalize and the specific reality will vary with the specific design of the keel, boat and wave shape. In most larger-wave induced rolls, the side speed of the keels through the water is so great that virtually all keels are effectively stalled, and so the side force per unit of area may actually be the same between a deep fin and a shallower full keel.

Further complicating this discussion is the shape of the keel, shape of the hull/keel intersection, and proximately of the keel to the hull. The more vertical geometry of a deep fin keel would result in the face of the keel operating more perpendicular to the flow of the water. In a stalled state that would produce a higher unit force. The area around the hull is highly turbulent during a roll, and might entrain more air, therefore producing less unit side force, and since a full keel has a larger portion of its keel in this region, it might be expected to produce a relatively lower unit side force. But because there is a gentler curve in this area of a traditional full keel, there is also likely to be less turbulence in the area of the keel to hull intersection, therefore somewhat mitigating the impact of the larger percentage of the full keel operating in this zone.

Another factor that complicates this discussion is that traditional full keel boats tend to (but not always)roll at a slower rate than more modern designs (which tend to be lighter for an equal length). That slower roll rate means less turbulence and so a greater dampening unit force per area, but the keel passes through the water at a slower speed suggesting in reduced unit dampening force per area.

(See why I say this is hard to generalize about and why I said 'may result in greater dampening')

As a word about my personal opinion in these discussions, and in an effort to provide a filter which would help provide a fair minded sense of where my personal prejudices tend, when it comes to offshore cruising boats, I am not a fan of the Open Class style boats that Paulo likes. They are clearly faster than more moderate designs, but that speed comes at a price in terms of motion comfort and self-righting.

While these Open Class based designs have demonstrated an enviable record as race boats, and may even be a reasonable basis for coastal cruisers, I personally prefer more moderately narrow beam boats for distance cruising containing significant amounts of offshore use.

Which brings me to the issue of boats in waves and GBurton's question about what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?

I think that these questions have little to do with the keel type, and everything to do with form stability. Fin keel or not, a high form stability boat will want develop a greater force trying to make it heel parallel to the plane of the surface of wave. All other things being equal, a higher form stability boat will tend to move more in sync with the wave face. (Not a good thing in big waves)

To one degree or another, mitigating against that is the tendency of modern deep fin keels with bulb designs to have a proportionately high righting moment due to very low centers of gravity. And also mitigating against the impact of form stability is that deep fin keel boats tend to develop very high roll moments of inertia relative to their displacements often having roll moments of inertia similar to much heavier displacement boats. That would tend to slow the roll rate some, but of course not as much as would be the case of a more moderate beam design, in other words a boat with less form stability.

As I said earlier, in big waves, a large roll moment of inertia does two things, at the top of the wave, it delays the rotation of the boat relative to the rotational force. A good thing, but at the bottom of the wave, its greater stored kinetic energy, tends to cause it to get out of phase with angle of the wave face and continue to roll as the bottom of the wave flattens out so that there is a greater danger of dipping a spar in the water (never a good thing). At the bottom of the wave, the boat with greater form stability would generate more righting force, remaining in sync with the wave surface and so would be less likely to dip a deck or spar or keep rolling as far as a boat with a high roll moment of inertia and/or less stability.

I am running out of time but I also want to touch on the issue of inverted stability. Pretty much any reasonably normal sailboat of most eras of history, placed in perfectly calm water exactly upside down, will remain so. When you talk about a boat with a large angle of positive stability, it simply means there is a smaller angle at which it is stable inverted. But since it requires wave action to invert almost any reasonably safe design, there is usually some wave action trying to rotate the boat past the point that it again starts to develop positive stability. When you talk about traditional designs, or more moderate modern designs (either with an LPS approaching 150 degrees), this can happen at inverted heel angles as small as 25 to 30 degrees. But when you consider a design like the open class boats, with the keels centered, the limits of positive stability can be down in the 125 degree range.

What is impressive to me about the video is that the inverted Open Class boat with its keel canted, begins to develop positive stability at an angle which appears to be somewhere around 15-20 degrees. That is amazing! But to me, in my personal opinion, having to cant a keel of an inverted boat to right the boat is no way to go cruising, especially if you visualize the potential weight shift in an inverted distance cruiser.

Lastly there is the question of the impact of the mast on righting. Like most of this, this discussion point is complex and situational. As someone said above, the reason that a mast may be seen as being helpful to righting a boat is that in a large enough wave situation that a boat can be knocked into an inverted position, the mast being deeper into still water, acts a very deel keel so that the force of the wave is amplified and there is more force exerted to rotate the boat toward upright. But that is in part offset by the ballast affect great weight of the sails and rig below the surface, and damping resistance of trying to move the sail sidewards through the water once positive stability is achieved.

There is also a thought that few boats will survive a roll to inverted with their rig intact. The flat water test, therefore is supposed to demonstrate what happens in most likely condition, (inverted without rig) and the hardest to re-right (inverted without wave action to re-right). I am not sure that I buy that, but that is my understanding of the rationale.

Cheers….

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Last edited by Jeff_H; 03-30-2012 at 07:44 PM.
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Old 03-30-2012
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
But it is easy for you to mention what test tanking is Marchaj referring (just say the page and the chapter). To my knowledge all tank testing regarding sailboats stability was done after Marchaj book
See attached.

Now let us see your paper.
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Last edited by pvajko; 03-30-2012 at 07:56 PM.
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