Boat Performance Factors Explained: Key Metrics and Analysis Guide

Discover crucial factors influencing boat performance, from waterline length to sail area ratios. Explore how these metrics affect speed, stability, and handling on the water.

If you are cruise-oriented, you should first narrow your choices by selecting a construction material (for example, fiberglass), a sail plan (for example, masthead sloop), and a hull-keel-rudder combination (for example, moderate aspect fin keel with skeg attached rudder). Then take boats with these characteristics and start reviewing specific performance criteria.

“Performance criteria” are the bench data you can acquire from the designer, the builder, and your own calculations that will give you an indication of how the boat will sail and handle. This enables you to review the sailing characteristics of a large number of boats without test sailing each in a variety of conditions. In many ways this is a better approach than test sailing because it provides more objective data.

Some of the criteria are based on an eyeball judgment, but most are specific figures that you can compare with the “normal ranges” and other boats you are considering. By making selection decisions on Basic Hull, Keel, and Rudder Shapesconstruction materials, sail plans, and hull, keel, and rudder shapes before you compare the performance criteria, you ensure that you are at least comparing apples with oranges (for example, sloops to light sloops) rather than apples with pastrami (for example, fiberglass boats for weekend cruising with steel boats designed to go to Antarctica).

If you are a racer or more racer than cruiser, you may want to do the reverse and examine the performance criteria first, later narrowing the field by considering other selection criteria.

While there are many performance criteria that could be on a list, these represent a good starter set to which you can add or subtract depending upon your level of expertise. Don’t be frightened by the formulas that have been included. The calculations have only been shown to indicate how the criteria have been derived. Most of this data should be available directly from the builder or designer, so you don’t have to be a math major or an engineer. You only need to understand what the product of each formula stands for and how it fits into a range of data for other boats.

To simplify your review of performance criteria, I have included only the basic criteria in this chapter. Those interested in pursuing a more detailed performance analysis will find several more complex criteria below.

Additional Performance Criteria

Prismatic Coefficient

This measures the fullness of the boat’s ends. Since it is difficult to calculate, it is best obtained from the designer or builder. It is the ratio of the boat’s displacement, in cubic feet, to the volume of a prism that has a cross section of the same shape as the hull’s midship section and a length equal to the hull’s waterline length. (For these types of measurements, the hull normally excludes the keel).

It is calculated by dividing the displacement in cubic feet by the maximum immersed section area of the hull in square feet multiplied by the waterline length in feet. The higher the prismatic coefficient, the fuller the ends. The maximum would be 1,0, which would be a rectangular box or a barge. Prismatic coefficients for sailboats vary with their speed-length ratios, but a range of 0,48 to 0,54 appears desirable when the boat is at rest.

A high prismatic coefficient translates into powerful ends, referred to as shoulders forward and quarters aft. These will better support the boat on its bow and stern waves, with earlier and higher planing speeds. However, if carried to extremes, particularly in the bow, this can result in pounding, reduced ability to windward, and slower speeds in light air.

Dellenbaugh Angle

This is a measure of apparent stability, or the ability of the boat to carry sail. It describes heel angle and the relative stiffness of the boat. Since it is rather complex to calculate, you should normally obtain it from the designer or builder.

The Dellenbaugh Angle is calculated by multiplying the sail area in square feet by the heeling moment (distance between the geometric center of the sail area and a point 40 percent of the draft below the waterline) and dividing the result by the righting moment at one degree of heel (see below). “Sail area” is defined as the mainsail area plus 100 percent of the fore triangle area, with each of these assumed to be triangles. Mizzen sails are generally calculated at 50 percent of their areas.

The Dellenbaugh Angle represents an angle of heel. The smaller the angle, the stiffer the boat will be under sail. A stiff boat can carry sail longer than a tender boat without being overpowered or having to reef or change to smaller sails. A small angle may also be the result of a minuscule sail plan that is inadequate to move the boat in light conditions.

Righting Moment

This is a true measure of absolute stability. It represents the resistance of the hull to being heeled one degree. It is calculated by shifting a known weight athwartship a specific distance, then dividing the moment (weight times distance shifted) by the change of the heel angle. It is measured in pounds and increases rapidly with boat size. A larger figure means greater stability.

For example, if you boarded a small sailing dink by stepping on the gunwale, you would tip it on its side so far it might swamp.

That boat has a low righting moment. If you boarded the Coast Guard training bark Eagle, it is doubtful that you would cause any measurable change in heel. The Eagle has a high righting moment or high absolute stability. You can make your own observations regarding this by standing on the rail at the beamiest or widest part of several boats and measuring the change of the angle of heel with a carpenter’s level. To obtain the specific figure for righting moment, I suggest you contact the designer or builder.

All the criteria have been summarized in the Performance Criteria Checklist below.

Waterline Length (LWL)

Sailboats, particularly the heavier boats with deeper hull forms, usually function in a displacement mode. This means that an amount of water equal to that displaced by the hull is replaced at the stern as the boat moves through the water. As speed is increased, resistance to the boat’s forward movement increases geometrically, and the hull creates a deeper and deeper hole between the bow and stern waves.

As a result, proportionally larger and larger increases in power (engine or sail) are required to produce an increase in boat speed. A very heavy boat with a deep hull form and inadequate reserve buoyancy could theoretically become a submarine if enough power were available. This happens when the bow wave displaces more water than can be replaced by the stern wave, which could occur in a Force 10 storm.

Under the right conditions, most boats will move from the displacement to the planing mode and rise out of the trough to surf on the bow wave. While almost all sailboats will surf in the right wind and wave conditions, boats with lighter and shallower hulls will do so much more readily.

Most sailing is done at displacement speeds. Since these speeds are directly related to a sailboat’s waterline length (LWL), more length equals more speed. To describe this relationship, designers use the speed-length ratio, which is the speed in knots divided by the square root of the waterline length in feet. For trip and passage planning purposes a speed-length ratio of 1,0 is usually used since power requirements increase rapidly beyond this point.

A nominal speed-length ratio of 1,34 is used as the dividing line between maximum normal hull speed and speeds that require the hull to surf or plane. To calculate maximum hull speed, multiply 1,34 times the square root of the waterline length in feet. It quickly becomes evident that displacement speed increases, but only slowly, with longer waterline length. For example, a thirty-two-foot LWL on a forty-foot LOA would only be approximately one knot faster than a twenty-four-foot LWL on a thirty-foot LOA.

Beam

This is the maximum width of the boat. For best all-around performance, the beam should be moderate. Although increased beam provides heeling stability, excessive beam results in a loss of directional stability (particularly upwind) and more form resistance (drag caused by the shape of the hull). The ratio of beam to length overall (beam in feet divided by LOA in feet) provides a rough measure of the beaminess of various boats.

A nominal ratio to use for this comparison would be 0,33. Any design in which the ratio is larger should receive extra scrutiny. An even better measure of beam is the ratio of the beam to LWL, with 0,4 serving as a nominal ratio for comparison purposes. Keep in mind that many beamy boats were designed to accommodate larger interiors in order to enhance marketability, not for good sailing performance.

Draft

Draft is the amount of water that the boat draws, or needs to float without touching the bottom. Centerboard and daggerboard boats have two drafts – board up and board down. The draft of a boat is a critical factor in calculations whenever you anchor or are otherwise trying to avoid going aground. If you normally sail in deep water, the draft of your boat may not be a major consideration, although you always think otherwise after going aground. If you plan to sail in shallow waters or among coral heads, you will want to have the least draft possible.

While shoal draft may be an advantage in getting into a beautiful little bay, it will be detrimental when going to windward in open water. Other things being equal, the boat with a deeper, foil-shaped keel will point higher and exhibit less leeway.

Windage

The freeboard (area of the hull above the waterline), cabin trunk, rigging, and deck mounted and stored equipment all represent surfaces upon which the wind exerts pressure. All of these surfaces (except raised sails) are lumped under the term windage. All sailboats have windage to some degree. Boats with extensive above-water surface area in relation to their size are said to have excessive windage.

Boats with too much windage tend to sail around their anchors. This creates nervous skippers and crew in neighboring boats.

These boats are also more difficult to dock or maneuver in close-quarters situations since the wind often overwhelms the boat’s normal steering characteristics, particularly at low speeds. A boat with excessive windage will also be slower upwind and more difficult to tack.

An easy method for determining windage without resorting to complex calculations is to measure the freeboard at the bow and stern. The bow measurement is particularly important, since windage in this area has the worst impact on the boat’s handling. A more accurate measure would be a rough calculation of the superstructure’s surface area.

Measure the freeboard at its lowest point (usually near the shrouds) and multiply by LOA. Then add the height of the cabin trunk, multiplied by its length. This provides an approximation of the surface area of one side of the boat. Then Comprehensive Collection of Common Sailboat Rig Types and Designseyeball the rigging, deck mounted equipment and the like, and add a “clutter factor” for difficult to measure surfaces.

This might be from 5 to 10 percent of your base figure for the hull and cabin trunk. Using either of these two methods for calculating windage will point out some significant differences from boat to boat. Keep in mind that the lower the windage, the better the expected handling and sailing performance.

Many modern designs have high freeboard and trunk cabins to obtain better headroom for the tall American sailor. Often the result is excessive windage and a boat that does not maneuver and sail as well as it should.

Weight Aloft

Any unnecessary weight above the waterline amplifies the pitching, rolling, and heeling of the boat. This slows the boat and results in a wetter and more uncomfortable ride.

The higher the weight is placed above the waterline, the more pronounced the problem. The rule of thumb for designing, building, and equipping the boat should be to keep the weight in the boat as low as possible. This means the owner should always be considering whether weight on the mast can be reduced, whether equipment carried on deck can be carried below, and so forth.

Of course it isn’t always possible to store every heavy weight in the lowest possible place, since there may be other trade-offs. A few typical problem areas include:

• nontapered masts and mast-mounted radar antennae;
• large, high fixed pilothouses;
• spare fuel or water carried on deck;
• anchors carried above deck;
• above-waterline chain lockers;
• and water and fuel tanks above the cabin sole.

Weight aloft could be given a numeric value (based on the center of gravity), but because it is heavily influenced by outfitting and equipping, I prefer to keep it more subjective. Look at the boat and give it a rating:

• poor weight distribution – a zero;
• average weight distribution – a one;
• and good weight distribution – a two.

Weight in the Ends

This is an important criterion. Extra weight in the ends of the boat leads to increased pitching in a seaway. This makes for uncomfortable sailing and slows the forward motion of the boat. The basic rule is to carry weight toward the center of the boat whenever possible.

The worst offenders include anchors and chain carried in the bow (3/8-inch chain weighs 1 1/2 pounds per foot); overloaded cockpit lockers; inboard engines pushed into the stern; outboards hung on the transom; and extreme bow and stern overhangs loaded with equipment.

Read also: How to Choose the Perfect Sailboat: Tips on Selection, Ownership, and Alternatives

As with weight aloft, it is easier to give weight in the ends a subjective rating: for example, a great deal of weight in the ends – a zero, and so on. Overhangs can be given a numerical value by comparing the LOA (length overall) to the LWL (length at the waterline). Divide the LWL by the LOA, subtract the resulting value from one, and you will have the percentage of the LOA that is in overhangs.

Displacement-Length Ratio

This describes the relative heaviness of the boat. It is calculated by dividing the displacement in long tons (2 240 pounds per ton) by one percent of the waterline length in feet cubed. Boats with ratios above 300 are considered heavy, while boats with ratios under 200 are considered light. Under approximately 100, the boat would be classified as a ULDB (Ultra-Light Displacement Boat).

Sailboats have been getting lighter for many years, with average displacement-length ratios getting smaller. Boats that were considered light a few years ago are now considered heavy. The reasons are dual: weight is inversely related to speed and directly related to cost. Racers and racer-cruisers are usually interested in low displacement-length ratios to increase their competitive advantage.

Cruisers, on the other hand, must also consider such factors as ease of motion and capacity to carry stores, which means they are usually interested in boats with somewhat higher ratios.

Boats with high displacement-length ratios, for a given length, will have greater carrying capacity for water, fuel, food, spare parts, and other items. For a given length and quality, they will also generally cost more, except for the ultralight exotic racing boats in which weight seems to be inversely related to cost.

Boats with higher ratios will have an easier motion in a seaway and will drive more easily through sloppy seas in light wind conditions. However, they will also require more sail area to perform as well as a lighter boat, and without adequate overhangs and freeboard for reserve buoyancy, they will be wetter.

Boats with low displacement-length ratios will surf and plane earlier with less wind and will have higher off-wind speeds. If bow overhangs are excessive, they may not be able to punch into a head sea with any success. Lastly, boats with low ratios will be drier to sail but will have a corkier, bobbing motion.

Ballast-Displacement Ratio

This is only a rough measure of stability, since it does not account for hull-form stability. It is calculated by dividing the weight of the ballast by the total displacement of the boat. The range for this ratio is very broad, but most boats cluster between .33 and .45. Usually, the higher the ratio, the more stable the boat.

In addition to providing an indication of stability, this measure identifies that portion of the total displacement that is available for food, water, fuel, equipment, and so forth. This is simply calculated by subtracting the weight of the ballast from the total displacement. For more precise indicators of stability, refer to Dellenbaugh Angle and Righting Moment in “Additional Performance Criteria”.

Sail Area-Wetted Surface Ratio

Light wind performance is primarily dependent upon surface friction. Since the hull moving through the water generates almost all the friction (windage is ignored since it becomes a factor only when sailing in higher wind conditions), boat speed in light wind conditions is proportional to the wetted surface area of the hull. Since “wetted surface area” is usually not known by the seller, this ratio should be obtained from the designer or builder.

It is calculated by dividing the sail area in square feet by the hull wetted surface area in square feet. “Sail area”, for this and other performance criteria calculations, is usually determined by adding the mainsail area to 100 percent of the fore triangle area, with each of these assumed to be triangles. Mizzen sails are generally calculated at 50 percent of their areas.

Better light air performance can be expected with higher sail area-wetted surface ratios. This is because as the ratio increases, sail area is increasing, wetted surface area is decreasing, or both. With practice you can eyeball a hull and keel and make a rough judgment about the relative amount of wetted surface. With a full keel, you can usually expect more wetted surface area; with a fin keel and spade rudder, you can expect less.

Sail Area-Displacement Ratio

Moderate wind performance is primarily dependent upon the relative displacement of the boat. Simply stated: light boats are fast, and heavy boats are slow. At moderate wind speeds hull friction becomes less and less a factor since enough power is being generated to overcome surface drag. As you approach hull speed, the power requirement to move the boat begins to increase dramatically.

Further increases in boat speed in moderate wind conditions are dependent primarily on the mass of the boat or the sail area. This measure is often listed in the boat’s specifications but can be calculated by dividing the sail area in square feet by the displacement in cubic feet (64,2 pounds equals one cubic foot of water) to the 2A power.

Values of this ratio usually range between 12 and 20, with some modern racing boats now appearing with ratios in the mid and high 20’s. Boats with ratios under 15 will generally be slow, and those under 12 will need a hurricane to get moving. This measure is usually calculated as a static ratio for new boats with little gear and equipment and with empty or half-full tanks. As a result, heavily loading a boat with a low displacement-length ratio will quickly lower the sail area-displacement ratio.

Sail Aspect Ratio

Performance, particularly to windward, is enhanced by high-aspect ratio sail plans. The high-aspect sail is higher off the water than its low-aspect counterpart. This puts more of the sail into faster and less turbulent air. The longer luff of the high-aspect sail generates more drive and lift to windward.

It raises the center of effort of the sail plan, which increases the Dellenbaugh Angle and decreases apparent stability.

The aspect ratio is calculated by dividing the height of the sail (usually denoted in sailmakers’ sketches as P for the main and I for the genoa) by the length of the sail’s foot (denoted E for the main and J for the genoa). Ratios range from 2,5:1 to 3,0:1 for most modern boats, though the ratio may be lower on older boats and higher on racing boats.

Analyzing the Performance Criteria

When reviewing the performance criteria, keep in mind that a boat’s relative ranking will vary for each individual criterion. This means that in order to have useful results, you must compare several criteria for each boat. While the comparison of one isolated performance criterion for all the boats is helpful in simplifying your analysis, one criterion can never adequately describe a boat’s potential performance characteristics and should never be the sole basis for making your selection.

For instance, Boat A might have the highest displacement-length ratio, which by itself would indicate a potentially slow boat. This same boat, however, might have the best weight distribution, both up and down and fore and aft. Depending upon your selection prerequisites, this may partially or totally compensate for the boat’s heavy displacement. As another example, assume you are interested in a “performance cruiser” with a displacement-length ratio from 220 to 250.

Several of the boats you are considering have ratios within this range. Boat X with a ratio of 270, however, might still have overall superior performance characteristics because of a better ranking on weight distribution, sail area-displacement ratio, sail aspect ratio, and so on. After considering all the criteria, Boat X may remain a strong contender for your selection.

You should also note that several of the criteria are directly related to your personal preferences and how the boat will be used. For instance, if you are racing, you will generally be interested in a deeper keel. If you are cruising in the Bahamas, you may be willing to trade off some upwind performance to lessen the time you spend aground.

When reviewing the performance criteria, always be wary of boats with extreme characteristics unless you are very knowledgeable and have specialized requirements for the boat’s design. Some extremes I would be careful of include:

• small boats with seven feet of headroom that look like house trailers (excessive windage);
• boats with a beam approaching 40 percent of their length (LOA);
• boats with displacement-length ratios over 400;
• and boats with sail area-displacement ratios under 12.

If you are having trouble deciding what to make of the performance criteria for a group of boats, keep in mind that this is only one aspect of your decision. The data are most useful in narrowing your choices and providing indicators on how you might expect the boat to perform.

On your final sailboat choices, you should always confirm what the performance criteria seem to indicate by considering the reports of other owners, the test sail if you take one, and the reputation of the designer’s boats.

Author

Olga Nesvetailova

Freelancer

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