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What Should Be Regarded as
Essential Design Data
Copyright 2001 - 2010 Michael Kasten
When reviewing various custom or production boat designs I am often amazed at the lack of meaningful information presented with the design. In most cases the data may not have been presented for the sake of simplicity, however in a surprising number of cases the data may not even exist...! It is the latter case that's of interest here, since the implied notion is that the vessel's designer might not even know some of this basic information!
The ability to look at the lines of a boat and immediately see whether the design has merit requires a practiced eye. This is mostly a matter of understanding what the vessel's lines represent, and then of passing judgment based on what has been observed to be a good design in the past. To the practiced eye, the lines drawing showing a boat's shape will give considerably more information than the numbers alone can ever provide.
Still, it is highly useful to know some of a vessel's calculated parameters. There need not be an overabundance of data for a cursory look. A few basic numbers will suffice for a preliminary judgment. The following is what I consider to be the essential information when reviewing a design. This first data group is usually given within the Study Plans for any vessel, or this data can easily be derived from a boat's drawings by scaling and then doing some basic figuring...
- Length on Deck
- Beam on Deck
- Waterline Length
- Waterline Beam
- Displacement at the Designed Waterline
- Sail Area
To assess the relative merits of variations in length, beam, displacement, sail area, sail carrying ability and so forth, we can make use of several commonly used evaluation criteria. In so doing however, we must recognize the differences imposed by the "scale" of a vessel. For example, when making a vessel bigger in all directions the displacement will vary as the cube of those changes (l x w x b). You can have a look at these various relationships in our PDF describing the Similitude of Scale.
It might seem silly to define some of the above terms, such as "Length" or "Beam" but you would be surprised at how often they are incorrectly quoted, many times even intentionally... Here are a few of the basics, so you can avoid being misled by incorrect data.
- LENGTH: When referring to a boat's length, it is the "Length on Deck" that is of interest, since that defines the length of the enclosed and presumed to be water tight envelope. When asked, "How long is that boat?" the correct answer is to reply with the Length on Deck. It is unfortunately all to common, especially among charter vessel owners to claim that the boat is as big as everything that sticks out...! Over-quoting the vessel's size is intentionally misleading, for obvious reasons.
- Occasionally a boat's length will be quoted in terms of its "Length Over Rails" which would then include any substantial bulwarks or guards that extend the length of the hull structure itself. This will not include appendages, such as a separately attached swim platform or davits or a bowsprit that could be removed, say for shipping.
- Sometimes it is of interest to know the "Length Overall" inclusive of appendages - typically sought by the harbormaster so you can be assessed for everything that sticks out, or possibly for a canal transit to be sure you fit the locks.
- BEAM: Similarly a boat's beam is taken to be the maximum width of the watertight hull, usually but not always the "Beam on Deck." For example if there is significant tumble-home, the maximum beam might be below the deck level. This dimension is of interest because it defines the maximum extent of the water-tight envelope.
- On occasion it is of interest to know the "Overall Beam" more or less for the same reasons as one would need to know the Overall Length of a vessel - usually for clearances during haulout or dock-side, in which case the maximum beam is given, including permanently fixed guards or other non-removable appendages.
- WATERLINE LENGTH: This should be an obvious definition, being the length of the immersed body at the floatation water plane... but there are subtleties..! For example on most motor yachts there is a "light" condition with fuel and water tanks nearly empty, and a "loaded" condition with fuel and water tanks nearly full. If there is a long counter-stern, these two load cases can produce quite a different WL Length. As a result of the load-case ambiguity, I usually refer to the Design WL or Datum WL as the "Reference Waterline". This is the waterline plane ordinarily used for most of the calculations. In the case of stability, usually it is advantageous to consider the light load case, since that is ordinarily the "worst case" having the highest center of gravity. In the case of propulsion, it is advantageous to consider the fully loaded condition, since that load case will require the most power and fuel to push the vessel through the water. Due to the stability consideration, for most yachts I prefer that the "Reference Waterline" or "Datum Waterline" represent the "lightest" load case.
- WATERLINE BEAM: The width of the immersed body at the floatation water plane - with the same load-case caveats as are the case with the Waterline Length.
- DRAFT: The immersed depth of the hull at its deepest point, taken from the "Datum Waterline" as defined above. Obviously the Immersed Depth will vary with the load-case, so on occasion a fully loaded draft may be quoted. For a given load case, draft will also vary according to the salinity or density of the water. In fresh water, which weighs less than sea water, a boat will float more deeply. See the next definition...!
- DISPLACEMENT: Literally taken, displacement refers to the cubic feet or cubic meters of water that are "displaced" when the vessel floats. For a given weight, the amount of water that is displaced in order to float the vessel will vary according to the salinity or density of the water. Even so, the weight of the displaced water is always equal to the actual weight of the boat, therefore "Displacement" is ordinarily expressed in pounds, or long tons of 2,240 lb. (35 cubic feet of sea water), or kilograms, or metric tons (1,000 kg. or one cubic meter of sea water) - but never in short tons, which are irrelevant to boats except when ordering materials...! Displacement will obviously vary according to the load case as described above.
- SAIL AREA: In my own usage of this term, Sail Area refers to be the total sail area of the actual sails. In "racing" terminology, the entire area of the fore-triangle is included regardless of the size of the jib that will be used, and the main sail area is taken as the area of a triangle bounded by the mast hoist and the boom outhaul, disregarding any roach or hollow to the leech or foot of the sail. Most yacht designers will quote sail area as being the area of the actual sails, not the race-rated sail area, but for racing sail boats or even cruiser-racers, unless this is specifically identified it remains ambiguous.
- POWER: Alas, even simple horsepower is subject to interpretation. This is so because engines have different ratings depending on whether you are quoting "continuous" horsepower, or "intermittent" horsepower - the maximum momentary output. When I refer to horsepower, I ordinarily mean "continuous" horsepower - that which the engine will be able to sustain day in and day out on a long voyage.
Hull Design Ratios
The next group of numbers can only be provided by the designer (or by your own detailed analysis of the lines). These are quite basic, and should always be provided within the Building Plans for any design!
- Prismatic Coefficient
- Wetted Surface Area
With the above information in hand, one can learn quite a lot about a design using a few simple calculations to derive the various Ratios that follow. These Ratios will provide a deeper look, but still they will not be the whole story!
Although it's rare for this data to be supplied with a simple article or advertisement, the following ratios should always be provided within the Building Plans. If they haven't, one can easily calculate them from the Prismatic and Wetted Surface information. The formulae for these four calcs are well worth learning. They are available from any good book on boat design, such as those by Skene or Larsson and Eliasson, or you can download our PDF that explains the various Coefficients of Form. The most commonly quoted ratios among them are:
- Displacement to Length
- Sail Area to Displacement
- Sail Area to Wetted Surface
- Horsepower per Long Ton of Displacement
Each of the above measurements and ratios provide information about the basic geometry of the hull itself and how the hull relates to the driving power provided. All of it is to be considered essential, and should always be readily available on the drawings or from the designer.
With all of the above data a word of caution is in order... It is the height of folly to presume that any one of these numbers must be any particular pre-determined value. Instead, there is generally a considerable "range" of suitable values that will be more or less appropriate for the design in question. Further, many of the various design "numbers" are interrelated in ways that may not be entirely obvious.
We will usually ask ourselves, "What does the displacement number actually represent? Is it the light load case? Is it the fully loaded displacement? Is it somewhere in between?"
As noted in the "Definitions" section above, it is customary to quote Displacement at the "Design Waterline" also called the "Datum Waterline" or the "Reference Waterline." Typically this is with the vessel in level trim. In calculating displacement, it is usual to assume a basic set of spares, tools, safety gear, ground tackle, "average" stores, and the tanks about half full. Among these variable weights one must also account for the inevitable accretion of stuff put aboard by the owner. This stuff can often be very much an unknown!
Depending on vessel type, the quoted "Design Waterline" displacement can just as correctly represent the "light ship" load case. This is most common for power vessels which may have quite a large variation in displacement due to the fuel load. In these cases, the lines will have been designed to allow the vessel to also be more deeply loaded, and the full load displacement will also be quoted. With a power vessel or a power-oriented motor sailor, since the weight can vary quite a lot, the trim and stability must be calculated for each of the light, average, and heavy load conditions. Using the IMO as an example, the 10% and 90% load cases are required to be considered in calculating compliance with IMO stability criteria for ocean going motor vessels.
What Other Information Should be Available?
There is yet another category of information that relates to the behavior of the boat. The data mentioned above is simply the static data based on the upright at-rest condition.
The following is data is calculated with the vessel at various heel angles and is also to be considered essential. Oddly, it is quite unusual to find much meaningful data published on stability and performance. This should be considered to be essential information, so should be provided within the Estimating Plans or Building Plans, or should be readily available on request.
- Stability Curve: This is the curve of righting arms. The curve is usually expressed in feet or meters along the vertical axis, and in degrees along the horizontal axis. With the vessel upright, the curve is at zero. As the boat heels, the center of buoyancy moves outboard, while the center of gravity remains stationary. This creates a Righting Arm or Righting Lever. It is the horizontal distance between the upward buoyant force and the downward force of gravity. The Righting Moment at any point on the curve is simply the product of the righting arm at that point (length) times the vessel's displacement (weight). In other words, Force times Distance equals Moment. The result is conventionally expressed in foot-lbs., or in newton-meters in the same way as would be the torque applied by a wrench.
- Dellenbaugh Angle: This is the angle that the vessel is presumed to heel given a force of 1 pound per square foot on the sails, assuming they were all sheeted flat amidships. It is an approximation only, and is based on the upright stability characteristics of the vessel. Being a very common and easily done calculation, it is highly useful as a preliminary tool for comparing one vessel to another in terms of a boat's relative power to carry sail. Good descriptions of this calculation along with graphs of what is to be typically expected are given in texts by Skene, Larsson & Eliasson, and Henry & Miller. Alternately you can download our PDF that explains the Dellenbaugh Angle and Other Sail Area Calculations.
Of course, these "predictions" only consider the boat at rest in calm water -- they do not predict how the boat will behave when it is in motion in a seaway. Nevertheless, these are a useful standard means of comparing one boat to another, and for such purpose they are highly valuable. So if your boat heels differently when in motion than the equations predict, it is to be expected...!
Given a hydrostatics program that is capable of performing a stability analysis using a static wave form, a much closer approximation of the amount of heel with the vessel in motion can be achieved. In spite of being considerably more accurate, a wave-form analysis will not necessarily provide a good comparison with other vessels for which the stability curve will have been prepared using flat water.
The Stability Curve
The stability curve provides the most information about the sea-keeping safety of the design, and also provides an indication of the boat's behavior. With the stability curve, one can predict the actual amount of heel using any given wind force on the sails or on the exposed profile of the boat.
To calculate the stability of a vessel of course requires that an accurate Center of Gravity be known. The weight and CG are usually calculated via a series of spreadsheets which document the weights of all the items aboard, all the structure, all the variable loads, rigging, fittings, machinery, electronics, systems, etc., as well as the longitudinal and vertical locations of each item. These various centers must be known, recorded, calculated, and finally the Center of Gravity of the entire vessel and her contents can be derived.
This is a tedious and ungrateful task, to say the least. Nevertheless, it is an absolute must for any new design. Without an accurate weight analysis, there is no stability curve. Without the stability curve, there is no real information about the boat's ultimate safety, or her ability to carry sail (except by an informal comparison to other vessels).
Knowing the vessel's center of gravity while upright, though, is not enough. After that, the boat's displacement and trim must be calculated at several angles of heel to obtain the heeled center of buoyancy at each angle. Once the CG has been calculated, if the vessel has been computer modeled the large angle stability analysis is relatively easily accomplished, requiring only a matter of hours to set up and run in a good hydrostatics program.
Done by hand however, a full stability analysis is extremely time consuming. Therefore in the past, a complete stability analysis would have ordinarily been cost prohibitive except on very well funded design projects. For yacht designs earlier than around 1985 the full stability curve will almost never be available. It will not have been calculated in the first place, since very few owners would have been able to afford to pay for it!
If the design is more recent, say after 1990 or so, and has been modeled by computer, the hydrostatics and stability analysis will usually have been performed by the computer. Any more, it is assumed that this information will be provided and that it will be thorough.
Sailing Vessel Stability Criteria
To judge adequacy of stability is a complex matter.
For sailing vessels, many have proposed a simple criteria based on a prescribed "range of positive stability." While relatively easy to assess and therefore tempting as a simple method, this is not a complete or necessarily adequate picture. Stability is a dynamic event and is affected by quite a number of different vessel parameters.
The most recent methods for assessing a sailing vessel's "dynamic" stability have been provided in two forms:
First, a proposed method is presented in Principles of Yacht Design, by Larsson & Eliasson. It is called simply the Dynamic Stability Factor (DSF). The DSF is the result of work done by Moon and Oossanen to propose a rational criteria based an a number of factors that contribute to a vessel's sea keeping ability. The factors are calculated which analyze Beam vs. Displacement; Sail Area along with Displacement, Beam and Length; Displacement to Length; Self Righting Energy; and finally the Relative Areas of the Positive vs. the Negative Stability Curves. A DSF "score" is accumulated and the vessel is rated for Ocean; Offshore; Inshore; or Sheltered waters.
Second, the DSF method was then expanded to include input from designers world wide and a refined method derived. The results of this research have been presented by Oossanen in a publication of the Chesapeake Sailing Yacht Symposium. This new method is provisionally being used as the standard within the European Union (Specifically, ISO-12217). It is referred to simply as the Stability Index (STIX). Although the STIX formulae differ completely from those used by the DSF method, the STIX method takes a similar approach. STIX analyses Righting Energy; Inversion Recovery; Knockdown Recovery; Displacement to Length; Beam to Displacement; Wind Moment; Downflooding Angle; and the vessel's Base Size. The resulting factors are accumulated into a "score" which again rates a vessel for Ocean; Offshore; Inshore; or Sheltered waters.
While many other criteria have been proposed, the DSF and STIX methods are by far the most comprehensive, and provide the greatest amount of information about a sailing vessel's survivability.
Power Vessel Stability Criteria
For power vessels, criteria for stability are well established. The most comprehensive stability criteria are those published by the IMO (International Maritime Organization). Originally researched by contributors to the FAO symposia regarding fishing vessels, the IMO criteria assign minimum acceptable values for the area below the righting curve up to varying degrees of heel, measured in foot-degrees, or in meter radians. Further, the IMO assesses dynamic conditions by superimposing a heeling moment curve due to weather conditions, with minimum acceptable values assigned.
The IMO criteria are both rigorous and relatively easily calculated directly from the stability curve. The IMO criteria are accepted world wide, and by the European Union. The IMO criteria are generally presumed to apply to vessels over 80 feet LOA, however they are also excellent criteria for smaller power yachts. The IMO criteria should be considered the minimum in all cases.
Before a designer can know any real information about stability, the structure must be specified so that its weight and center of gravity may be calculated. During the design of a boat, the CG can only be derived by a rigorous weight analysis, including knowing the boat's structure, the structural weights, and the weights and centers of gravity of every item on the ship.
For the prospective boat owner, the most reliable indication of structural adequacy is whether or not the boat has been designed and built to one of the usual "class" standards, such as the ABS Rule, Lloyds, Det Norske Veritas, German Lloyds, etc.
Carrying the computations one step further, having done the stability analysis, the design should be supplied with some indication of performance. For this, the hull's resistance must be calculated. With the assumed resistance in hand, the following will become possible:
- Sail Boats: If the design budget allows, a Polar Diagram of anticipated performance on different points of sail in varying wind strengths can be prepared.
- Power Boats: Power and Range Calcs. At the very least, this takes the displacement, the waterline length, quantity of fuel, horsepower available, and the various vessel speeds, and gives a table or graph of expected range and power usage at different vessel speeds.
What to Expect...
By now, it should go without saying that all these calculations require a substantial investment of time on the part of the designer of the vessel, and a consequent investment in the designer's time by the prospective vessel owner... The time spent by the designer will either be paid for by the hour or will be paid on a percentage basis when a vessel design is commissioned.
Even with a stock design, one cannot expect to obtain a complete design package at a bargain basement cost. The question comes down to what you expect to end up with in the boat... When shopping for a cheap set of plans, a realistic view of what you are getting must be kept in mind. If you purchase a cheap set of "stock" plans, you must be willing to accept a fairly large compromise in terms of the known safety of the design, and generally also in terms of the quality of construction details provided. For designs being marketed without full stability analyses, you should expect a very steep discount, and you should be willing to accept the inevitable consequence of what amounts to an incomplete design package.
On the other hand, if you have spent a considerable sum on up to date design work, regardless whether it is for a custom or a stock design, you should expect the kind of information described above to be readily available.
The question naturally arises, "What does it all cost...?"
Prior to the availability of computer modeling and analysis it was not even remotely practical to provide a complete stability analysis for the common small yacht. Prudent designers would instead take a pre-existing vessel and make modest changes. By taking conservative steps, a designer would know the result in advance based on their experience with the prior vessel. This method is not to be dismissed; it has served for centuries...
The advantages provided by a rigorous analysis should be self evident. Using computer modeling and analysis we can now compare an entirely new vessel design to a known data set, rather than being restricted to comparison with a specific pre-existing similar vessel. By this means we can be assured of completely favorable results.
Of course the amount of time required for the analysis will vary depending on the specifics of the vessel under consideration. The amount of time required will escalate more or less directly in proportion to the displacement, size, and complexity of the boat. The time required will also vary depending on how rigorous the analysis is to be, for example, if a report will be prepared to illustrate compliance with some specific criteria, such as the IMO (International Maritime Organization), ISO (International Standards Organization), or the CFR (US Code of Federal Regulations).
One can begin to appreciate the amount of time and effort required on the part of a designer, and why the all-important weight study and stability analysis may have been neglected in favor of being able to offer a cheap set of drawings. While there are certainly benefits to an inexpensive set of boat building plans, one should be aware of what may have been left out, and of the potential consequences of that omission.
Being concerned about a vessel's survivability in ultimate conditions makes ultimate sense! In view of the cost to build a new vessel, and the cost of the entire expedition that any ocean voyage represents, the added cost of competent design work is not a significant percentage.
It does however amount to a high percentage of one's peace of mind when sailing on the briny deep.
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