Instant Naval Architecture of Sailboats

The purpose of this article is to explain in easily understood terms the basic laws and concepts that underlie the boat construction, the design and performance of sailboats. Several simple experiments you can perform at home will be suggested to help you understand these concepts. No great attempt will be made to get into all the finer details or ramifications of these concepts, since you are not trying to become a naval architect, but simply a more knowledgeable boat buyer, looker, dreamer, or whatever.

Basically, this article is going to use these fundamental concepts in order to indicate answers to questions which I’ve been asked many times: «This boat is only 4 feet longer than that boat; why does it cost twice as much?» and «How can a boat be tender if it has a 50 percent ballast ratio?»

As you will see, many interrelated factors are involved in the answers to these questions and many others like them; they involve:

• hull form;
• speed;
• seaworthiness;
• rig;
• directional stability;
• comfort;
• suitability;
• weatherliness, and so on.

Basic Laws

Displacement

Probably the most fundamental thing you can know about a boat is its total weight.

If you imagine a boat being slowly lowered into the water by a crane or marine railway, you can see that the hull of the boat shoves aside, or «displaces», water. It makes a hole for itself shaped like its underbody, and, when the weight of the water trying to fill up the hole equals the weight of the boat pushing the water out of the hole, a state of equilibrium is reached – and the boat is afloat.

You can get something of a grip on this concept by taking a child’s building block and slowly lowering it into a sink of water. Before the block touches the water, it has a perceptible weight. As you lower it, though, it seems to weigh less and less until, finally, it has no weight you can discover and it is buoyed up by the water it has displaced.

If you were to try this experiment with a small block and an absolutely full glass of water, you would find that the amount of water that overflowed the glass would be exactly equal in weight to the wooden block. Similarly, if you were to lower a boat into a swimming pool filled to the brim, the weight of the water that overflowed would equal the total weight of the boat.

Now, what is really important is not the weight or displacement of the boat per se, but its displacement in relation to its size. Thus, a 30-footer that weighs 15 000 pounds is said to be of heavy displacement (literally, a «heavyweight»), while one that displaces 5 000 pounds is said to be of light displacement, meaning it is light for its size.

To be technically and strictly correct about it, a boat’s displacement should be compared to its waterline length. But, when you’re looking for a boat, you are not going to think of a boat as being a boat of so many waterline feet, but of so many overall feet.

In other words, you are going to tend to categorize boats by their total length from bow to stern and compare one boat to another in the same overall-length category rather than same waterline-length bracket.

The relative lightness or heaviness of a boat has a lot to do with other characteristics to be discussed later on, but Figure 1 serves as a rough guide.

The figure gives the approximate weight in pounds for boats of moderate displacement at overall lengths of between 25 and 45 feet.

A boat of 30 feet overall, for instance, that was significantly (say 25 percent) heavier than 8 000 pounds would be considered heavy displacement. Likewise, a 30-footer that weighed, say, 5 000 pounds would be quite light for its size.

The reason that the relative heaviness or lightness of a boat is important is that this factor determines certain fundamental characteristics of the behavior of the boat in the water.

To illustrate this behavior, take an empty Band-Aid can and tape the top so that it will not leak. Get a block of wood about the same size and put the can and the block into a bathtub half full of water. Make waves with your hand and observe what happens.

The heavy-displacement boat – the block of wood – tends to be submerged by the waves, while the light-displacement can bounces around on top of the waves, riding them and giving way to them. If these objects were, indeed, boats, which one do you think would be the drier? Which one would have the more comfortable motion?

Obviously, the light-displacement boat (the Band-Aid tin) would be drier, but its motion would be more violent. Heavy displacement, on the other hand, means an easier motion, but a much wetter boat.

Right here, we are in the thick of the seaworthiness question which has been hotly contested for centuries. Many sailors maintain that heaviness, per se, means seaworthiness. But we have seen that, while heaviness may mean less violent motion in a seaway, it also means taking lots of solid water on board. This means that the boat needs to be very strongly constructed in order to stand up to the repeated blows of waves.

The light boat, too, needs to be well built, because she has to be able to stand blows of solid water against her sides, even though these may be eased somewhat by the boat’s ability to give way and, thus, absorb some of the blow.

All in all, it seems to me that the argument between heavy and light boils down to tendency to equate heaviness with strength and lightness with flimsiness. While these equations may often be true, they are – today, at any rate – not necessarily so.

A boat can be weak and heavy just as surely as it can be strong and light. The key is strength rather than light or heavy, in and of themselves.

As you can see from the bathtub experiment, relative lightness or heaviness merely determines the characteristic motion of a boat in rough water. Whether she can survive this sea route will ultimately depend on how well and correctly she is constructed. Read the article «Manufacturing of Fiberglass Boats and Design FeaturesFiberglass boats», which is devoted to ways that will help you evaluate the fundamental design of the boat.

Another aspect of displacement is cost. Boats are made of materials, which fundamentally cost so much per pound. Therefore, one way to reduce the price of a boat is to cut down on the amount of material used in building her (i. e., reduce her displacement). This is why the test of dividing displacement into dollars is so useful; it lets you discover if one boat is truly less expensive than another (costs less per pound) or cheaper simply because you would be buying less boat (less stuff, to put it crudely).

Reducing the price by carving away material quickly reaches a limit, however, because in order to be strong, something light must be very carefully made. This means more man-hours must be invested and, so, you will find that very light boats (a 30-footer, say, weighing 3 000 pounds) will cost about $10 per pound by the time they are equipped.

Size

A boat is basically a box. This is to say that it has a length, a width, and a height. Now, you know that if you increase one dimension of a box and increase the other two in proportion, the box gets a lot larger, very fast.

To illustrate this, take four sugar cubes and arrange them into a larger cube, two units on each side. You have a box containing four cubic units and weighing four times whatever each of the basic cubes weighs.

Now, increase each dimension of the box 50 percent, or by one cube. What is the size of the «box» now? Twenty-seven cubic units! So, by increasing each dimension of the box 50 percent, you have increased the size (and weight) of the box by more than six hundred.

Read also: Comprehensive Collection of Common Sailboat Rig Types and Designs

This same effect operates in the case of a boat and is the answer to the question, «If it’s only 4 feet longer, why does it cost twice as much?»

A 30-foot boat is just about twice as much boat as a 26-footer having the same proportions: it weighs about twice as much and has twice as much interior, or cubic space. This means twice the room for:

• people,
• stoves,
• galleys,
• bunks,
• storage,
• or whatever,

in other words, a lot more liveability with just a little more length.

Sheer size is important for many other reasons. One of the main ones is what is called «stiffness», or the ability of the boat to stand up to its sails and go rather than simply lie on its side.

As you have seen, the hull of a boat can increase in three dimensions. The sail plan, on the other hand, can only increase in two dimensions.

Say you had a Boat A and scaled it up to Boat B by doubling all dimensions. Boat B would be eight times as heavy as Boat A, but its sail area would be only four times as large.

Since it is the sails which heel the boat and the weight of the hull is one of the factors that tend to counteract this heeling, the anti-heel forces gain faster than the heeling forces as the size of the boat increases.

Stability

In the discussion on size, we touched on one aspect of stability – the fact that the sail area (force that makes the boat heel) does not increase as fast as the hull weight or force that counteracts the sail plan.

There are several other factors in stability. Perhaps the most important of these is Basic Hull, Keel, and Rudder Shapeshull form.

Try this experiment: get an empty soft-drink can and seal up the opening with tape so water can’t get in. Float it on its side in the sink or tub. Now get an empty cigar box or other flat-bottomed container, seal it, and put it in the sink or tub, too.

Using a small bit of modeling clay, stand up a drinking straw on top of the cigar box. Try to do the same with the soda can.

Without going deeply into naval architecture theory, I think you can see that the cigar box has natural stability by virtue of its form (shape) and that the soft-drink can does not.

Now tape a lead weight under the can and try to set up the straw «mast» again. This will succeed, provided the lead weight is large enough – and you will obviously have a small model of a typical, ballasted sailboat.

The question logically arises:

• Why is it necessary to have the lead?
• Why not just go with the flat bottom, which has natural stability by virtue of its shape?

Of the several answers to this question, most important is probably that the cylinder, or the can, has less surface area in the water for a given weight – displacement – than the cigar box. Surface area in the water, the so-called «wetted surface», is a major deterrent to speed, and naval architects spend lots of time trying to reduce it as much as possible.

As a result, sailboat underbodies are a compromise between the cylinder and the cigar box. One of the ways you can judge the degree to which any boat relies on ballast or shape for stability is to walk around behind her and look at her underwater shape in cross section.

Figure 2 shows a boat which derives much of her stability from her shape – she is more like a cigar box than a soft-drink can.

Figure 3 is the converse.

While you are studying the boat from this angle, keep in mind that the sail plan is the factor that works against stability. Thus, a shape like the cylinder, which derives all of its stability from the ballast, can be stiff (or sail at a small angle of heel) if the sail area is small. Yet, a shape like the cigar box with ballast added – a very stable combination – can be tender (or sail at a large angle of heel) if the sail area is large and lofty.

Given the same sail plans, a boat that relies largely on ballast to keep her upright will need a relatively larger proportion of ballast than a boat whose hull has stability by virtue of its shape.

The proportion of ballast in a boat is found by dividing the weight of ballast by the weight, or displacement, of the boat. A boat displacing 15 000 pounds and having 7 500 pounds of ballast has a ballast-to-displacement ratio of 50 percent.

Since we have seen that the underwater cross-sectional shape of the boat, as well as her rig, contribute importantly to her stability, it’s pretty obvious that ballast ratio alone will not tell the story about the relative stiffness or tenderness of a boat.

Sail Area

The sail area of a boat is its power plant. And, since most boats must sail in a wide range of wind strengths, judgment must determine just how much or how little sail a boat should have.

Here we are talking about her basic, working sails, since it is obviously possible to increase the area somewhat with special sails such as genoa jibs and spinnakers or to reduce the sail area by reefing.

What size basic Self-Survey Criteria for the Rigrig a boat carries, therefore, depends upon the wind strength in which she is usually expected to operate.

Generally, a boat expected to sail in winds primarily in the range of 6 to 15 knots – summer conditions in the middle latitudes – will need basic working sail area (main and jib for a sloop) of about 100 square feet per ton (2 000 pounds) of displacement.

As mentioned before, however, small boats (below 30 feet) tend to have more than this and large boats (above 30 feet) tend to have less. This, again, is due to the fact that the hull can grow in three dimensions, while the rig can grow only in two. An attempt to make the rig grow in proportion to the weight of the boat is quickly defeated by the problem of providing enough guy wires to hold a very tall and spindly mast upright in the boat.

Speed

If the bathtub still has water in it from the earlier experiment, stick in a finger and wiggle it around to make some small waves. Notice the length of time it takes these waves to reach the far end of the tub.

Now put in your whole hand and make larger waves. You will notice that these waves move much faster through the water.

It is a basic fact of nature that a wave of a certain length moves through water at a given speed, and the longer the wave, the higher the speed.

Scientists have clocked waves and found the formula which predicts the speed of any length wave. The formula is speed of wave in knots is equal to 1,34 times the square root of the length of the wave in feet:

Thus:

• a 16-foot-long wave travels at four times 1,34 knots, or 5,36 knots;
• a 25-foot wave travels at five times 1,34, or 6,70 knots;
• a 36-foot wave at six times 1,34, or 8,04 knots, and so on.

Now, a ballasted sailboat moving through the water makes waves, and the faster she travels, the longer the waves she makes, and there comes a time when she is making one wave: one crest of the wave is at the bow and the other is at the stern (see Fig. 4).

This is the longest wave she can make, and she can only travel as fast as this wave, which we know is limited by nature to

$1,34\sqrt{L}$

. To travel faster on the water, you have to get out of the wave-making business and either skip upon the surface (called «planing»), which requires much more power than the sails of a cruising boat can generate, or slip beneath the surface (called «becoming a sub»), which requires more money than many governments can muster.

 
Anyway, as you can see from Figure 4, the length of the wave made by the boat is going to fall somewhere between the overall length of the boat and its length at the waterline; so, for practical purposes, you can estimate the speed potential of a ballasted sailboat by taking the square root of its waterline length and multiplying by 1,34. This speed is referred to as the «hull speed» of the boat.

It will be interesting: Self-Survey Criteria for the Basic Boat

To assess fully the speed potential of a boat, however, you need to know not only how long a wave she can be expected to make, but how deep a wave she can be expected to generate. Two waves can be the same length – travel at the same speed – but be of different depths.

Shallow waves are formed with less energy than deep ones and, since light boats make shallow waves, it can be seen that they need less power (sail area) to reach hull speed than do heavy ones.

This is to say that, for a given sail area and waterline length, a light boat will reach hull speed in lower wind velocities than will a heavy-displacement hull, all of which suggests that if you plan to sail in a region noted for light breezes, you should probably lean toward light boats with generous rigs and vice versa if your sailing grounds have a history of strong winds.

Hulls and Rigs

Basically, there are only two hull types in ballasted sailboats: the so-called full-run keel with attached rudder and the fin keel with separated, or spade, rudder. There are many variations between these types – fuller full keels and «finnier» fin keels.

What I have developed over the years is a mental spectrum into which I classify boats based on their hull type. I call it the «mooseto-butterfly» system. On the left end (moose), I put the really full-bodied, heavy-displacement type represented by Figure 5.

I place the really cutaway, light-displacement types represented by Figure 6.

As you can see, many boats fall between these two extremes; what I have done is to indicate under each picture the general behavior of each type.

Moose to Butterfly

As you can see, if sailing comfort is your object or if you are in an area where winds are usually strong, you would tend to pick a hull on the «moose» side of the spectrum (Fig. 5 below). If the excitement of a livelier, more responsive How to Choose Your First Boat?boat is for you, or if you are in an area where winds are normally light to moderate, you might tend toward a «butterfly» (see Fig. 6 below).

Bear in mind, too, as you look at these pictures, that I am assuming a normally proportioned rig on each of these hulls, with around 100 square feet of sail area per ton (less for the heavydisplacement boats, more for the light) Fig. 7.

I once owned a 31-footer much like the boat in Figure 8.

She weighed 5 500 pounds and had 500 square feet of sail (200 square feet per ton), which is overrigging with a vengeance.

In 8 knots of breeze, she was a total delight. In 12, she was on her ear and had to be reefed, even though 51 percent of her was lead ballast.

I sold her a few years ago, and on balmy summer afternoons I sometimes get wistful thinking about how she would tear up the water.

So far as general handling characteristics go, it can be said that the boats on the heavy side of the range are less sensitive to the helm and so tend to be easier to steer, requiring less concentration on the act of steering. They do, however, tend to build up large amounts of weather helm while reaching due to the fact that the entire flow of water to windward (caused by the leeway of the boat) has to flow past the trailing edge of the rudder. With a separated rudder, the bulk of this water can flow between the keel and rudder (Figure 9).

Both types of hulls can be made to sail themselves on the wind, since this is a function of the balance of the rig and the underbody, not of the basic shape. There are cranky boats of both shapes, of course. The fault is with the designer for not lining rig and hull up correctly; the fault is not in the basic shape of the hull.

The spade rudder boats do enjoy distinctly better handling characteristics under power; they are able to be turned in a tighter circle and to be steered in reverse. They also are much more sensitive to propeller torque and have distinct helm when under power; that is, if the helm is released, the boat will immediately round up.

This effect is considerably reduced by having a skeg in front of the rudder. A skeg also considerably improves downwind tracking under sail. All in all, I feel that an underbody like that shown in Figure 10, if combined with a good, carefully balanced rig, represents the best of both worlds.

The following will give you a quick look at rig.

Sloop

The most common rig on How to Choose the Perfect Sailboat: Tips on Selection, Ownership, and Alternativesauxiliary sailboats is the sloop. The fundamental reason for this may be that it is less expensive to build a boat with one mast. But the fact remains that it is an excellent rig for general sailing and cruising (Figure 11).

I like the rig because it is simple to handle, having, under normal circumstances, only two sails. With the powerful winches available today, handling large jibs is no particular problem. Sailing a boat with two masts is often a bit like trying to sail two separate boats. It really is more work. Still, as with everything else, split rigs have their advantages, so let’s discuss the two most commonly seen today – the ketch and the yawl.

Ketch

The technical definition of a ketch is that it is two-masted, fore and aft rigged, with the after mast shorter than the forward, or main, mast. The aft mast is stepped forward of the rudder post.

The aft mast is called the mizzen, and it is important to note – if you want to be technically correct – that the mizzenmast is forward of the rudder post, not necessarily the helm or the position from which the boat is steered (Figure 12).

The ketch rig has many adherents. Its chief advantage is that many combinations of sail are possible. For instance, in strong winds, a well-designed ketch can sail under her mizzen and working jib with main completely furled. Since many people find it easier to lower and furl a main than to reef it, the ketch obviously has great appeal for them.

The chief disadvantage of the ketch is that, in the usual sailboat with aft cockpit, the mizzen is often in the way: it is right in front of the helm, as a rule, and also the companionway. Careful design can pretty much eliminate this, but such design is the exception and not the rule.

Yawl

From a distance, a yawl looks like a ketch, with the exception that the mizzen on a yawl is relatively smaller than on a ketch and the mast sits farther aft. The technical definition is this: in a yawl, the mizzen is aft of the rudder post.

Yawls became popular during the days of the CCA (Cruising Club of America) racing rule. Having a mizzen reduced the rating of a boat by quite a bit and it was felt that the advantage of the lower rating more than made up for the additional windage and weight added by the mizzen. Basically, yawls are sloops with an afterthought. Perhaps they should be called hermaphrodite sloops.

In any case, there is an advantage to the yawl – and this goes for ketches, too. In heavy seas where you have searoom, a yawl or ketch can often be hove-to simply by lowering all sail except the mizzen alone and sheeting it in hard. The sail area aft tends to hold the boat close to the wind and often the boat can be left to herself while the crew goes below and gets some rest. Also, at anchor, the mizzen can be used to keep the boat from swinging.

It is my feeling that, for general pleasure sailing and summer cruising in boats to about 35 feet, a sloop is the best choice of rig. Larger than this, a split rig starts to be appealing because, although it increases the number of sails that have to be handled, it decreases the size of those sails, and that means easier sailhandling chores.

Author

Olga Nesvetailova

Freelancer

Literature

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