This is a topic that keeps coming back like a bad penny. It is not that difficult if you take your time with it. On the other hand this has been a topic that was ignored for by designers until fairly recently. It has come in to the forefront in the 25 years since the Fastnet disaster. It has also become an important frontier in racing yacht design (which will ultimately provide a lot of useful information for the design or cruising yachts) because of the realization that large angles of rotation and quick motions are slow. With the advent of smaller more powerful computers and miniaturized accelerometers full sized boats have been instrumented and motions measured. As a result theories on the factors involved in the dynamics of motion are being more clearly understood.
At the same time there has been a clearer understanding of what makes for seakindliness. If you look at motion that wear people down and cause seasickness it is both the speed and the angle of rotation. Marchaj’s slightly dated tome ‘Seakindliness the Forgotten Factor’ includes a chart that shows a navy study on the causes of seasickness and both rate and angle are equal factors. A more recent study indicated that individuals have different thresholds for discomfort and that the triggers of discomfort are purely individual as well. In other words, some people are bothered by the speed of the motion more than amount of the motion and other people are affected more by the amount of motion than speed of motion. And still others are equally affected by both. On that basis a boat that rolls through wide angles slowly is no more universally seakindly than a boat that snap rolls through small angles. This means that an ideal boat from a motion comfort standpoint would tend to roll, yaw, or pitch slowly and through a small angle.
To agree on terms, I will take a moment to define the basic terminology (at least as I will be using these terms). There are six types of motion:
-Three directional: surge (for and aft), leeway (sidewards), and heave (vertically), and
-Three rotational: Pitch (fore and aft), yaw (rotation as seen in plan view) and roll (which of course is side to side)
The amount and speed of motion a boat experiences is related to the amount of energy that is imparted into the boat, the ability of the boat to store that energy, and the ability to dampen that stored energy.
If we look at the issue of absorbing energy, most of the energy imparted into a boat is through simple physics, mostly due to inclined planes and impact. While there is comparatively little to do to minimize the absorption of energy due to inclined planes, much can be done to reduce the imparted energy due to impact forces through hull and foil shaping. Vee’d bow sections and finer bow sections reduce the impact loads of frontal seas for example. Another example that comes out of tank and instrumentation of actual boats which shows that a heavier boat passes through the waves while a lighter boat tends to move over waves. This suggests that heavier boats therefore experience greater impacts. This is somewhat mitigated by the greater momentum of the boat but depending on hull shape those impart greater energy into the boat. On the other hand going over waves a lighter boat is more likely to absorb more energy due to heave (vertical motion).
There are three ways that boats develop stability; form (also called initial stability), ballast, and dynamic. Of the three, dynamic has the least relevance to displacement boats as they rarely have enough speed to create a useful amount of dynamic stability.
As the name implies form stability derives from the shape of the hull of the boat. A shallower and wider hull generates more stability than a deeper narrower hull. Visualize a piece of wood lying in the water. On the flat it has a lot of form stability. On edge it has next to none. This is because stability in any boat comes from the distance between the center of gravity (the balance point for all of the weight in the boat including those things that are part of the boat and those things that can be moved around) and the center of bouyancy (the single point that is the center of all of the volumes under water). In a wider, shallower boat the center of bouyancy moves more quickly toward the low side for small increases in heel angle. A boat with a low vertical center of gravity tends to positively increase the lever arm between the center of gravity and the center of buoyancy at smaller angles of heel and therefore develop stability more quickly.
Form stability has several problems. First of all at large angles of heel, usually approaching 90 degrees form stability drops off dramatically. This is the point where it can be needed most. Second form stability tends to give the boat a quicker motion because more energy can be imparted into the hull either from impact or inclined planes. Lastly, in the extreme conditions of a blue water passage, a boat that depends on large amounts of form stability also tends to be more stable in an inverted position.
Ballast stability has mostly positives associated with it. All other things being equal, the deeper and heavier the ballast the more stability a boat will have.
Ballast that is heavier and deeper also gives the boat a slower, more comfortable motion. Of course like most things in yacht design, there are some tradeoffs in this area as well. If the ballast occurs at the end of a deep keel, the boat cannot get into as shallow water. As you start to shorten the keel length you substantially give up performance for the same stability because you either end up with a low aspect ratio foil or you end up with a big bulb or wings also increasing drag. No mater what the ads say, nothing performs like a deep fin keel. Of course deep fins have their own compromises but that is not the point of this.
Another factor that comes into play is the issue of the loads imparted into the boat. The larger the loads relative the more motion there will be. But because the loads involved can be so large the amount of load felt by the boat tends to be proportionate to its weight and exposure. In other words all other things being equal a heavier boat will have higher impact loads, but also proportionate resistance to those loads.
One thing that has been determined that the single most important factor controlling comfort in seakindliness is length. All other things being equal a longer boat will have a more comfortable motion. While sources do not universally agree whether it is waterline length or overall length that is critical, the general agreement seems to be the longer the boat the more comfortable the boat will be. That said a short waterline boat compared to overall length boat will have a greater range of pitching motion and may have a quicker motion as well, because the reserve buoyancy of the ends can suddenly come into play jerking the pitch to a stop and back the other way.
Next comes form stability as a culprit. Form stability allows a light boat to have a lot of initial stability but at the cost of poor ultimate stability. Form Stability’s has two affects on seakindliness, it allows more energy to be imparted into the boat and form stability results in a lot quicker motion as the boat quickly builds stability to resist the rolling motion. Form stability comes from wide shallow hull forms be they light like the IOR boats of the Fastnet era or moderate like the Moorings 38 mentioned earlier or heavy like the Island Packets. Of course without some form stability the boat would sail at large angles of heel (look at the English lead mine cutters for example) and that is not very seakindly either.
When we look at the relationship of a boat’s weight and seakindliness we are really talking about the effects of moment of inertia. The inertia of a boat has several affects. A boat with a large moment of inertia takes more energy to get the boat in motion. Once in motion, it is harder to stop from moving so this stored energy of a boat with a lot of inertia causes a boat to roll further than a boat that has less inertia. Of course a low inertia boat will accelerate and de-accelerate quicker.
The amount of inertia that a boat has is a result of the weight of the boat and the distribution of the weights. A small weight located a long distance from the axis about which the weight is rotating can have an equal inertia to a much heavier object closer to the axis of rotation. Since weight is linear but distance from the roll axis is to third (or fourth power), distance from the roll center is far more critical to the overall moment of inertia. If we consider a heavy boat, that has a short heavy rig, a lot of weight in the hull and a lot of low density ballast in a long shallow keel, its roll moment of inertia may actually be same or less than a much lighter weight boat with a deep bulb keel and a tall light rig.
Another factor that had little study in any detail is dampening. Dampening is the ability of a boat to absorb the energy once it has been imparted to it. The two best examples are the affects of a deep keel and a tall rig. If you visualize these rotating in a circular motion the air pushing against the sails and keel far from the hull creates a resistance to rotation and supresses the effects of inertia. In other words, dampening results in less motion and slower motion; a definite win/win situation.
So where does this leave us, in terms of seakindliness, a light weight boat, with a fine bow, and a narrow beam, Vee shaped hull sections forward merging into elliptical hull sections aft, with a deep bulb keel and a tall light rig can have very admirable seakindliness characteristics and would be a much faster boat in all conditions than a heavier displacement full keel boat.
Obviously there are trade-offs being made here but seakindliness or weatherliness isn’t one of them. Properly designed, the ability to carry the weight of cruising gear is not lost either. The price comes with deeper draft and these boats do require greater care in their engineering and greater skill to build.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 07-23-2009 at 09:31 AM.