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Old 11-02-2007
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Shoal Draft V. Deep Keel, Part II

This is a continuation of another thread first posted here:

Shoal Draft vs. Deep Keel

NOTE: IF YOU WANT TO TALK ABOUT GENERAL PROS AND CONS OF SHOAL VERSE DEEP DRAFT BOATS, such as the pros and cons of shoal draft vis a vis getting through the Intracoastal Waterway, or cruising the Bahamas, or visiting certain Pacific atolls with your family and the implications thereof, etc, GO TO THE LINK ABOVE.

THIS THREAD IS INTENDED TO BE A TECHNICAL DISCUSSION CONCERNING DESIGN ATTRIBUTES AND CHARACTERISTICS OF SHOAL AND DEEP DRAFT BOATS. THIS IS THE THREAD TO DISCUSS CONCEPTS SUCH AS MOMENTS OF INERTIA, RIGHTING MOMENTS, METACENTERS, ETC.

Posts #41 and #42 of the other thread raised the discussion to a higher technical level. I have taken the liberty of quoting them here so as to continue the technical discussion in the new Design and Construction forum.

Diva 27 said:

Quote:
Originally Posted by Diva27 View Post
There's a lot of fuzzy thinking about sailboat stability out there. As coauthor of Yacht Design Explained (shameless plug: buy it, read it, memorize it), I'll touch on the basics.
Keel draft matters with respect to how it affects how deep the boat's centre of gravity is located. It's important because as a boat heels, its stability depends on the horizontal separation between the centre of buoyancy and the centre of gravity. That separation is the lever arm that describes the righting moment. Generally speaking, the deeper a keel is, the lower the boat's centre of gravity becomes, and you can achieve the same righting moment in the same hull with a shallower but heavier keel. Keel draft is critical to windward performance. Deep and narrow is more efficient than short and long. So with a shoal draft, you're generally accepting reduced windward performance in pointing ability, as well as an overall heavier (and slower) boat. But draft is critical to determining where you can anchor, of course. I have a vintage C&C 27 that draws 4 feet 3 inches, and I'm glad, given water levels on Georgian Bay in recent years.
Bottom line is that a shoal draft design can be as stable as a deeper draft design, provided the designer has done his/her homework. Where it's a matter of putting a different keel on the same hull, the shallow draft keel is going to need enough volume to hold more ballast, or else the additional ballast has to be placed in a much less efficient location, up in the bilge. Shortening the rig also helps in this case, as this lowers the centre of sail force. The distance between the centre of sail force and centre of keel lift, measured down through the mast and keel, determines the heeling arm. The longer the heeling arm is, the greater the potential heeling moment (force times distance), and that requires a larger righting moment to oppose. I've been grossly simple here, but maybe you get the idea of what's involved.
To which Robert Gainer replied:

Quote:
Originally Posted by Tartan34C View Post
Nice short and concise description. But because the distance term in the polar moment of inertia is squared and the distance term in righting moment is liner the amount of energy to flip a shoal draft boat that has the same righting moment as a deep draft boat is less. Also as the draft increases the horizontal separation between the center of buoyancy and gravity increases more rapidly at a greater draft for a given heel ((depth of CG )X(tangent of heel)) so you really need a higher righting moment on the shoal draft boat to get a similar feel as the deep draft boat. Look at an ice skater in a spin to see the effect of changing the polar moment of inertia. Bringing in the arms increases the speed of rotation. Everything is a tradeoff and nothing is straight forward.
All the best,
Robert Gainer

Last edited by JohnRPollard; 11-03-2007 at 10:46 PM.
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Old 11-02-2007
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Nice discussion. It's getting well beyond my level of expertise, but I do follow it for the most part. I just wanted to mention two concepts that I understand somewhat differently, in the hopes that others will comment and help me to makes sense of this puzzle.

First, for purposes of this question let's compare a shoal draft long cruising fin keel to a deep foil with a bulb, both generating the same righting moment on the same hull. Robert said: "But because the distance term in the polar moment of inertia is squared and the distance term in righting moment is linear the amount of energy to flip a shoal draft boat that has the same righting moment as a deep draft boat is less."

I have understood that the larger surface area of the long fin keel would be more resistant to capsize due to the volume and mass of water that must be forced aside as the keel pivots around the boat's longitudinal axis. In other words, the increased wetted surface area whose friction is a liability in light air sailing becomes an asset in heavy going by offering increased resistance to roll as compared to the low wetted surface area foil. Thoughts?

Second, Robert also said: "...you really need a higher righting moment on the shoal draft boat to get a similar feel as the deep draft boat. Look at an ice skater in a spin to see the effect of changing the polar moment of inertia. Bringing in the arms increases the speed of rotation." Intuitively, the need for a higher righting moment in the shoal draft boat seems to make sense -- all else equal. But I could use a bit more help in understanding the "polar moment of inertia".

I guess I am having difficulty with the ice skater analogy. My understanding of the phenomena exhibited by the ice skater is that it demonstrates a physical law known as "conservation of angular momentum" which essentially states (forgive me, I am pulling this out of the cobwebs of high school physics) that an object moving around an axis will continue to move in the same direction at the same speed unless and until acted upon by other forces (such as friction). It's the same physical law that is responsible for the Coriolis Effect, from which we get our trade winds (with the sun as the energy source).

In the case of the skater, bringing in the arms from a distance that is further from the axis of rotation to a distance that is closer to the axis of rotation requires that the RPM of the skater increase in order to "conserve" the amount of momentum the skater already had. The skater still has the same amount of angular momentum, and the arms are moving at the same speed as they were, only now the rpms are more rapid because a shorter distance is being travelled by the arms. The opposite would happen if the skater extended her arms.

But that only works when the same object increases or decreases its distance from the axis of rotation. I'm not sure you can make an analogy when dealing with two different objects. In other words, an increase or decrease in the speed of the roll between the shoal draft and deep draft boat cannot be attributed to conservation of angular momentum (the ice skater phenomena) unless one or the other changes its own draft. Or can it?
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Old 11-03-2007
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What about the friction of the keel moving through the water? That has to have some effect on its rotation. Maybe too small to even consider? If keel A has less friction than keel B, it will help make it easier to rotate keel A around the axis.
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All I know is shoal draft= sailing in the bay, while deep keel means motoring 2-3 hours, out the channel past the jetty. 38' Irwin 3' keel with 6'CB.Art.
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John,
My thought with the ice skater was to show that with the same amount of energy having a different distribution of weight changed the speed of rotation. As the skater brings in their arms the polar moment of inertia is reduced so either less energy is needed to spin at the same speed or the same energy will spin you at a higher speed. I think you have it right and this is how "conservation of angular momentum" works. The polar moment of inertia is a description of how mass is arranged around a point of rotation. To put a number to it involves, as one part of the equation, breaking the body into infinitely small pieces and multiplying the distance squared of each piece to the axis of rotation. A real limitation from my lack of education is that I know how to use this in a cookbook way of design but I donít understand the math behind it so my explanation may be lacking clarity. I think the bottom line is that two otherwise identical boats with only the draft and ballast being different the shoal draft boat needs less energy to capsize. Now having said that there are several papers published that say shoal draft boats are less likely to capsize in a storm. The reasoning behind that has to do with the beam of a shoal draft boat and the way a wave moves. In this discussion we are only considering identical boats with different keels and not the design from scratch of a shoal draft boat with all the changes made to accommodate what is really an entirely different design so I am ignoring those studies for the moment.

The rolling resistance of different keels is not a factor in practical design so I never really thought about it. But I see some papers were published by SNAME and RINA on the subject of active and passive anti-rolling methods for ships. It looks like the conclusion of the studies of passive systems are that the greatest contribution of drag or resistance when rotating a fin is from the eddies at the end of the fin and not from the surface area of the fin. So less surface area and longer ends is more effective at reducing roll. I take that to mean if you take two keels that are identical and mount one on end and one on edge the one on edge is better at reducing roll because it has a longer edge making eddies. A practical application of this is the long shallow bilge keels on ships. Of course keeping the bilge keels within the box formed by a ships draft and beam keeps them from getting damaged by grounding on laying alongside a wharf so thatís another advantage of long shallow keels on ships instead of using keels that have a short root and extend a long way from the body of the ship.
All the best,
Robert Gainer
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Robert,

That is fascinating. Thank you for taking the time to explain further.
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Old 11-03-2007
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You guys really bring up some interesting ideas, one in which is the design of the boat to the keel....
So if the design of the boat is a factor, which came first, the shoal or the fin.. was the boat designed for the fin and the shoal was an afterthought to that boat or the otherway around..
And is there any information that would show that the same boat might be designed differently (internal ballest on a shoal) depending on the keel used.....
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Quote:
Originally Posted by RandyonR3 View Post
So if the design of the boat is a factor, which came first, the shoal or the fin.. was the boat designed for the fin and the shoal was an afterthought to that boat or the otherway around..
And is there any information that would show that the same boat might be designed differently (internal ballest on a shoal) depending on the keel used.....
RandyonR3,
The question is rather broad and may not have an easy answer. When you design a boat you usually have a list of requirement from the owner or company that commissioned the design and you lay out an outline or sketch of the boat and refine it until the owner is happy with the look and feel of the boat. Then you start producing the finished drawings needed to build the boat. So the hull keel combination is developed as a package early in the process to suite the needs of the owner. If you want to talk in broad terms about the difference between a shoal draft and deep draft boat I think the tendency is to have more beam as the draft decreases and also lower the sailplan as the draft decreases. Almost everything else is open to discussion. As far as a boat designed to be both shoal and full draft using the same hull I think you get a compromise boat that may be fine in one version but deficient in some way in the other version. But of course nothing is carved in granite and a well designed boat can be a reasonable, but only a reasonable boat in both versions. If you want the best boat possible you need to design the hull keel combination together.
All the best,
Robert Gainer
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Quote:
Originally Posted by Tartan34C View Post
If you want the best boat possible you need to design the hull keel combination together.
All the best,
Robert Gainer
In my case the keel was originally intended to be like it is now, and both hull and keel were "matched" from start.

But on the first trials we realized that we had added too much lead, and the boat was "too stable" for what we wanted. Also weight was a factor.

We found out in early tests that the postion of the bulb in relation to the boats's CofG was not right. Since we couldn't make holes to move the mast (which affected rating), we could only work from the water line down.

Being a prototype, we had only two options:

a) Spend a fortune in water tanks modulating the keel (which we did only on the first keel)

b) Install a "1st step" keel, and based on sea trials, test the keel and use comon sense and boat feel to "trim and "shave the keel.

As a consequence, we built an easy to remove keel, that inserts in an upside down box, so we could remove ti for alterations.

We modified it 3 times, and ended up lowering it almost 1 foot, and removed 400 Kg of lead from the bulb. I now have 9,5 feet draft.

Pointing is absolutely unbelievable and the last mod, made a 50% improvement in boat handling and responsivness.

One note: We had to adapt the rudder to the new keel.

My keel is designed for what it is intended, and where I sail draft is not a problem. A good thing.

We sail without reefing in up to 30 knots, and I bet, that no matter how hard we could try, a shoal keel would be un-sailable (SP)?

Each design has its own requirments
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Quote:
Originally Posted by Giulietta View Post
In my case the keel was originally intended to be like it is now, and both hull and keel were "matched" from start.

snip

We found out in early tests that the postion of the bulb in relation to the boats's CofG was not right. Since we couldn't make holes to move the mast (which affected rating), we could only work from the water line down.

snip

Pointing is absolutely unbelievable and the last mod, made a 50% improvement in boat handling and responsivness.

snip

Each design has its own requirments

Itís harder to design a boat like yours because everything is on the cutting edge so you have no room to fudge it. In a full keel boat the range of acceptable combinations is much larger so the boat is less sensitive to change then in a high tech boat like yours where everything is critical. I would never even think about trying to design a boat like yours. She is way out of my league.
All the best,
Robert Gainer
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