INTRODUCTION
This article was originally prepared as a response to questions posed on the Trawler World mailing list on the subject of Beam vs. Ballast as these two factors affect a vessel's Stability and Rolling Behavior. There are many opinions among boaters regarding the quantity of ballast as it relates to stability and on the amount of beam as it relates to comfort.
Most of what we read in that regard presents an incomplete picture at best. Applying actual science to the question rather than hearsay, we find many popular notions to be entirely incorrect.
Occasionally a few truthful bits do emerge...
ON ROLL MOTION...
BEAM: Increased beam definitely does not provide increased comfort. Increased beam also does not provide increased safety, or what we would call “seakeeping” ability... Consider the following...
A relatively light weight vessel with a wide waterplane will naturally have a very active roll behavior on the water. In other words, such a vessel will react to the shape of the water's surface very readily. This describes the majority of semi-displacement vessels and virtually all planing vessels. Adding ballast or making the water plane wider will only result
in a more "harsh" roll motion.
While fairly wide beam is generally beneficial to a true planing vessel, with displacement or semi-displacement types adding beam or ballast will only serve to degrade performance due to increased displacement and wavemaking.
Roll accelerations are well documented as being the primary culprit inducing seasickness. In general we observe that while greater beam will provide less roll angle, greater beam will also provide much more harsh, rapid, aggressive roll accelerations. Other factors being equal, this effect varies as the cube of the beam.
We necessarily conclude from this that widening the water plane (increasing beam) will reduce comfort and degrade seakindliness.
BALLAST: We are so often barraged with obsolete notions about ballast. One such notion is the commonly held belief that a certain 'ballast ratio' is desirable for sail boats. Similarly, among power boaters there is the popular but incorrect perception that power boats need a certain amount of ballast in order to have adequate stability.
Why are these two notions false...?
First, we can point to the disadvantages of increasing the amount of ballast. We generally observe that heavier displacement equals greater resistance, therefore increasing the amount of ballast will only slow the boat down. Conversely, in order to sustain a given speed it will cost more in terms of fuel or sail area.
Second, we have observed that adding ballast to any of these vessel types will not provide an improvement in comfort. Why?
As with added beam, if we add ballast the roll motion will become more aggressive. Even though the added ballast may reduce the typical roll angle, there will be a less gentle "return" at the end of the roll, i.e. roll accelerations will be greater, the motion will be less comfortable, and the incidence of seasickness will increase. More or less the
same thing applies whether aboard a displacement power vessel or a sailing vessel.
DISPLACEMENT: Other factors being equal, greater displacement ordinarily equates to greater comfort; the quality of 'seakindliness' we all seek. The reasons for this may not be so readily apparent.
Displacement vessels (sail or power) will usually have a less aggressive roll motion, a longer roll period, and a more gentle "return" at the end of the roll than semi-displacement or planing types. This is primarily due to the displacement types having less wide waterplane and greater displacement. Comfort or seakindliness is therefore enhanced by keeping
beam to the least amount necessary for initial stability and for sail carrying ability.
On the other hand we have observed that adding ballast will be counter productive in terms of comfort. How then can displacement benefit comfort...?
ROLL MOMENT: While the amount of displacement has an effect on a vessel's motion, it is the distribution of displacement that has the greatest effect on roll motions. We can improve comfort, and we can actually improve safety by increasing the "roll moment of inertia" of the vessel. This is accomplished by spreading out
the various weights aboard rather than having them highly concentrated. This is very much counter to the usually assumed requirement for a specific 'ballast ratio' on sail boats, or that there must be a certain amount of ballast present for the safety of a power boat.
For the most basic understanding of this, we can say without doubt that an object with its mass distributed toward the perimeter will have a higher resistance to changes in motion. Therefore, it will be more dynamically stable. This can be intuitively thought of as the “gyroscope” effect.
On boats, it has been well proven that the very distribution of weights athwartships and into the rig are in fact favorable to stability in a 'dynamic' environment. We have observed that boats that have been dismasted are much more likely to be rolled over. This is due to the lack of inertia (lesser roll moment of inertia) and the relative ease with which a heavy roll can be suddenly
induced. This cannot be demonstrated by static analysis as one would normally expect, since the dismasted boat obviously has 'more' static stability without a mast - in the actual ocean though, it is the opposite.
As compared to a fiberglass vessel, a steel vessel will inherently have its mass distributed farther from its roll center, therefore a steel vessel will inherently have a higher roll moment of inertia, and will be less active 'dynamically' both in terms of roll and in terms of pitch.
ROLL ATTENUATION: On power vessels, the most effective strategy for roll attenuation, and very certainly the most effective strategy per dollar spent, will be the use of properly designed paravanes. These will degrade performance somewhat, particularly at higher speeds, but for those times when performance is imperative, the paravanes can be retrieved...
On power vessels, if cost is less of a consideration and convenience of use is a paramount concern then active stabies will possibly be the best choice, even though they are only effective under way.
On sailing vessels, the weight of the rig and the wind in the sails do an excellent job of reducing roll motions.
These are among the various factors that affect a vessel's perceived "comfort" which is the usual consideration with regard to roll motions. Of course the comfort of the crew is a safety factor, and it should not be overlooked...! For further discussion of these phenomena, please see my web page that describes other possible Roll Attenuation Strategies.
ON STABILITY...
The above factors primarily affect a vessel’s initial, or "perceived" stability. In a completely separate category is a vessel's ultimate stability.
BALLAST RATIO: Ultimate stability, i.e. the ability to resist or to recover from a large angle roll, ordinarily is enhanced by the addition of ballast. Whether adding ballast will provide an improvement in ultimate safety for any given vessel is a question to which only a detailed analysis can provide an answer.
On the one hand, in a 'static' sense, more ballast lowers the center of gravity, and should therefore be beneficial. It is obvious that for sail carrying, yes more ballast is beneficial. For comfort though, it is not. For resistance to being rolled in actual dynamic conditions, it is not.
A light weight vessel having a large concentration of ballast will have a much lesser 'roll moment of inertia' so will be much more easily put in motion and therefore will be more likely to experience large roll angles due to wave action.
While the 'ballast ratio' may have some utility as a measure of seakindliness (i.e. more equals less), it is in fact quite meaningless as a measure of either stability or seakeeping ability. Why?
We know nothing about stability without considering the distribution of weights ( the vessel's actual center of gravity), and the shape of the boat.
A/B RATIO: Popularized by Beebe in Voyaging Under Power, the A/B ratio was originally promoted as a quick way to judge a power boat's seakeeping ability. It simply compares the Above water area to the Below water area. Small numbers are viewed favorably, large numbers not.
We are so frequently taunted with questions about the A/B ratio of power boats that we should be clear about one thing: The A/B ratio is very misleading, therefore obsolete and nearly useless. As a criterion of stability or sea keeping ability it is a gross oversimplification of the factors that should be considered. We have much better tools at our
disposal for the analysis of stability and we should make use of them.
The range of positive stability of a sail or power vessel depends entirely on three factors. The third though equally important is often ignored:
- Shape
- Center of Gravity
- Prevention of Flooding
To the above we would ordinarily add a fourth factor: Movable Weights. For the moment we'll simplify the question by assuming all weights to be fixed and therefore that the CG remains in one place.
After numerous stability analyses on a variety of craft, we quickly observe that increasing the volume of enclosed space above the waterline increases large angle stability. We achieve this by increasing freeboard and the volume enclosed by the vessel's superstructure. More enclosed space equals more buoyancy and a greater righting force.
This must not ignore the center of gravity. Therefore the weight of superstructures must be kept within limits. Those limits however are not determined by some arbitrary A/B ratio, but instead by a thorough study of a vessel's weights, combined with a rigorous analysis of a vessel's large angle stability.
As an interesting and somewhat contrary example, we observe that the large angle stability of power boats will nearly always be greater than that of a typical sail boat in the same size range. This may come as a surprise, but it is proven time and again by rigorous analyses of different vessel types.
Why is this so...? After all, don't they have a higher ballast ratio and a smaller A/B ratio...? It is due to the relatively much larger enclosed volume of the superstructures. In fact we can say that the relatively greater large angle stability of power vessels is because of their relatively higher A/B ratio...!
Of course in any such analysis we must make rational assumptions with regard to flooding...
FLOODING: The primary key to prevention of flooding is to have a strong superstructure and adequately strong openings that are actually capable of keeping the water out in the event of a capsize. In terms of ultimate stability, a vessel's potential 'downflooding' points are also a primary consideration.
On power vessels for example, the location of engine room ventilation openings are possibly the most common violations of common sense and of good practice in terms of downflooding, and therefore of ultimate stability. Sliding glass doors and picture windows come to mind as being a close second...
Sailing vessels are ordinarily well fortified against downflooding. Still, dorades, hatches and companionways are possible sites for downflooding.
A strategy aimed at "keeping the water out" and assuring that all the various possible openings can be quickly covered will provide the most benefit in terms of ultimate or large angle stability for any vessel.
Whether they are displacement types, semi-displacement types, or planing types, if given sufficiently strong superstructure with robust windows, hatches and doors having adequate WT seals, most power boats will have an enormous range of positive stability. A large portion of those power boats will actually be fully self righting, which is much more than can be
said for the majority of sail boats. The key is to keep the water out...
WINDAGE: A thorough stability analysis must also account for the windage of a vessel's freeboard, superstructure, sails and rigging. Windage is a 'shape' related factor, and an important one.
Most of the various stability criteria that are in use around the world consider windage. In fact on close scrutiny we observe that windage is quite an important limiting factor. If there is any validity to the use of the A/B ratio as a measure of anything, it is possibly useful as a relative comparison of windage. It is important to note however that there is not any
given value here that means anything at all. It can only be a relative comparison, which even then is not of much use in an absolute sense, except to note that one boat has more and another less.
The real story lies in actually calculating a vessel's wind resistance and its heeling due to wind pressure, then comparing that to the vessel's righting curve. This is of course not simple, because in order to even have a righting curve we must first have calculated the vessel's CG and its shape, and we must then have done a large angle stability analysis.
Nevertheless this is the only way to derive a true comparison from one vessel to another. Actual calculation of windage and its effects on the stability of the boat in question is the only method internationally recognized as having any credibility among the various criteria of survivability.
MOVABLE WEIGHTS: The shape of tanks... In terms of movable weights, the shape of tanks is a primary consideration. Why...? Free surface effect. When you step into a partially filled skiff, you'll immediately discover the dramatic effect of free surface.... Numerically speaking, the motion of liquids in tanks degrades stability in proportion to the cube of the width of the tank. Suffice it to say that the liquids should be kept from sloshing, and that tall narrow tanks are generally better than wide shallow tanks.
WHAT CRITERIA TO APPLY...
PREVIOUSLY: We have looked at a few of the commonly used, but over-simplified and therefore outmoded and obsolete ways of judging stability, seakindliness and seaworthiness such as the 'A/B ratio' for power boats promoted by Beebe and the commonly quoted 'ballast ratio' for sail boats.
There is yet another commonly used but highly misleading criterion we may encounter. It is the metacentric height originally introduced by Frederik Heinrik Af Chapman in the 1600's. While quite useful as a gauge of initial stability and therefore as a preliminary gauge of sail carrying ability and of seakindliness, a vessel's metacentric height is not of much use as a
gauge of ultimate, or large angle stability. In other words, it is a useful piece of data re: initial stability, but it is not in any way definitive re: seaworthiness or survivability.
These various criteria have all had their place in the history of boat design, however we must look beyond the overly simplistic view they offer. More accurate comparisons are more subtle, and will therefore require more than a cursory inquiry.
PRESENTLY: We now have much better stability analysis tools available, and we should insist on using them to their best advantage. Computer analysis has made possible a much more detailed picture of the various elements of stability and seakeeping ability than had been practical even as recently as two decades ago.
In the case of seakeeping, stability and roll motion, we have observed that waterplane area, beam, VCG, displacement, and roll moment of inertia are all important factors. Also important are the shape of the underbody, the configuration of appendages, and the volume and watertight integrity of deck structures.
In terms of ultimate stability, there are many different standards being used throughout the world. Among the most familiar are the ISO (European Union), IMO (International) and US Coast Guard (US only), and others that are applied locally in various regions. Each of them have rigorous criteria that apply to commercial power or sailing vessels.
Within the EU, quite rigorous criteria have also been formalized for recreational craft.
For any new design, after a thorough weight analysis is done in order to determine the VCG, a large angle stability analysis can then be done. With the results of these analyses in hand, we can then determine the vessel's compliance with the ISO, IMO, USCG or other applicable stability criteria.
In terms of comfort, internationally recognized criteria for seakindliness are still under development. Seakeeping Software for personal computers that can easily perform the analysis of dynamic vessel motions has only recently been created.
SUMMARY
If there is one absolute truth in all of this, it is that with regard to seakindliness and seaworthiness there are no absolutes, only tendencies. As a result, we may attempt to apply generalities to the problem. However generalities are necessarily prone to oversimplification, therefore they will nearly always be misleading if applied too broadly.... or too
blindly...
Michael Kasten
Port Townsend, 2002 - 2004
