Eight steps to a voyaging design

From Ocean Navigator #132
September/October 2003
Suppose we want to design the ultimate voyaging boat. Should it be beamy? Should it be narrow? What kind of overhangs should it have? Just what hull shape should the best cruising boat have? I’ve heard sailors say that the best hull shape must be beamy like single-handed around-the-world boats, that it must have a long and narrow hull, that it must have a blunt bow, and so forth. However, most of the hull shapes generated for various race boats are governed by the rating rules, and these rules affect the hull shape in dramatic ways.

Figure 1: The sections of a voyaging boat. To improve keel performance, the bottom of the hull is almost flat, and the waterline beam is carried as far outboard as possible to improve stability. To keep bilge water in the right place, the boat has a sump in the top of the keel.

Because there are no rules for voyaging boats, we can design the hull to move easily through the waves, to rest peacefully at anchor, and to be handled by a couple of sailors or the autopilot under almost any conditions. But what would such a hull look like?

1. Captain & crew

First, let’s look at the sailors who will be aboard the boat. They usually are an older couple who likes to take their kids or maybe their grandchildren sailing, who may take the boat to the Caribbean for the winter, and who prefer to be reasonably safe under most conditions.

They don’t want to experience the extreme thrill of surfing down a Southern Ocean graybeard at 22 knots, praying that they won’t wipe out and put the rig in the water. They also don’t want to take forever to get from one port to another because they are experienced enough to know that the longer you stay out in the ocean, the more likely you are to be pasted by a storm. They also like to have a hot shower, be warm in colder climates and enjoy reasonably fresh food in a civilized manner. In other words, they want space, comfort and amenities.

2. Displacement

To enjoy the amenities suggested above means the boat will probably carry a moderate to large main engine – or for more efficiency, a smaller engine and a generator – a watermaker to ensure that water is available at any time, some reasonably sophisticated electronics, a good anchoring system, a water heater, probably a propane (or LNG) stove and transom grill, a life raft, and a few smaller items.

These requirements suggest that the boat will be of moderate displacement. A lightweight boat may have a problem carrying the weight of gear required for voyaging, plus lightweight boats tend to have a quick motion that some people find extremely tiring. While a heavy boat can easily carry the amount of gear required, it will require larger sails, a bigger engine, a heavier rig, etc., and it will take a long time to get from one place to another, making it more likely that it will be at sea in heavy weather.

What is moderate displacement? In talking with many, many boat owners and having designed a few boats for moderately experienced owners, I find that most prefer a boat around 44 to 48 feet long. They feel that this is the largest hull that they can handle safely without resorting to having a professional onboard.

We can assume that a moderate-displacement boat 48 feet long has a light-ship displacement of 22,000 to 26,000 lbs. Light ship means that the boat has no stores, fuel, crew or sails onboard. By the time gear and people are loaded aboard, the half-load displacement is around 26,000 to 30,000 lbs, depending on gear. (The half-load condition is with half stores, half the fuel load, half the water capacity, a full crew and sails.) At one time, this displacement was thought to be moderately light, but times have changed, and now it is thought to be moderate.

A better criterion may be the displacement-length (D-L) ratio. Using this ratio, we can determine whether the relationship between the waterline length and displacement is about what it should be. A ratio between 50 and 100 is generally considered to be an ultralight-displacement boat, or ULDB. Between 100 and 150 is light displacement, and between 150 and 220 is moderate. A 220 to 300 ratio is now considered reasonably heavy, and more than 300 is very heavy.

If the light-ship displacement is 24,000 lbs and the waterline length is 43 feet, the D-L ratio is 135. But boats rarely sail with no sails, no crew and no gear, so the half-load ratio gives a better idea of the boat’s ability. If the half-load displacement is 28,000 lbs, convert to long tons (1 long ton = 2,240 lbs) and divide by the cube of the waterline length divided by 100. So the D-L ratio is (28,000/2,240)/(43/100)3 = 157, which puts it in the moderate category. When the boat is ready for sea and fully laden with stores and gear, the weight will probably be around 30,000 lbs, which gives a ratio of 168. This suggests that the boat is a moderately fast voyager.

So what hull shape suits that style of cruiser best? While we can design the ultimate cruising hull for the hull suggested above, it is better if we determine a few more dimensions before we come up with the lines plan.

3. Beam-to-length ratios


Figure 2: A three-dimensional view of the hull showing the sections clearly. Note the slight roundness of the forward sections and the bow overhang, intending to provide reserve buoyancy as the hull slams into a wave when heeled.

The boat’s beam is critical if it is going to play in the deep ocean. If we go back to the Fastnet disaster of 1979 and the subsequent safety-from-capsizing report from the Society of Naval Architects and Marine Engineers and the U.S. Yacht Racing Union (now called U.S. Sailing), we find that large beam played a significant role in leading to a capsize. In fact, the report suggests that narrow-beam yachts stand a significantly smaller chance of being capsized than do extremely beamy yachts. The range is given as length-to-beam ratio between 2.4 and 3.0. (Note, the report uses a calculated value for L, which corresponds to slightly more than the waterline length and a maximum beam. We will use the waterline length of 43 feet.) Thus, for this hull, the maximum beam comes out as being between 14.3 and 17.9. I chose to go with 14.75 feet, which should give plenty of beam for interior accommodations and a lower probability of capsizing.

A quick recap, length overall is 48 feet; beam is 14.75 feet; waterline length is 43 feet; and half-load displacement is 28,000 lbs, for a moderately fast, moderately beamy boat that can be handled by two experienced sailors. The next step is to determine the hull draft. As the boat is likely to spend winters in the Caribbean, we will say that the draft is not to exceed about 6 feet 3 inches when fully loaded.

To get the most efficient keel design, the hull will be relatively flat-bottomed. Once again, this represents a trade-off. The most efficient hull shape for a flat-bottom hull is one with an almost vertical blade and a bulb keel. However, such a keel shape is terrible at an anchorage. It simply does not have enough lateral profile to prevent the boat from veering around the anchor line when any kind of breeze is blowing. So, because the boat will spend a lot of time at anchor, the lateral profile of the keel will need to be increased.

Consequently, by defining the draft limit, we have defined the hull-bottom shape; and from the type of sailing the boat is likely to undertake, we have come up with an approximate keel profile.

4. Freeboard

The freeboard on this boat will be determined by the interior layout. Unlike boats that spend their lives in northern climes, the headroom in this boat will be about 6 feet 6 inches to allow good air circulation in the tropics. This will necessitate a small cabin trunk, but the higher freeboard and small cabin can be made to be aesthetically pleasing, and they make life easier belowdecks. However, relatively high freeboard poses another problem: How does a crew get from the dinghy onto the deck or retrieve a crewmember who falls overboard?


Figure 3: A view of the hull from the side showing the freeboard is not unattractively high. The hull is projected beyond the transom to give the builder an extra frame to carry out the hull and to ensure the hull stays fair when built. The cabintop is simple with space aft of the mast to stow a dinghy when at sea. The sides of the cabintop enclose the cockpit.

For a number of years now, I have advocated open transoms on boats to make it easier for a man overboard to get back aboard. The hull we are designing will have a transom with a pair of steps into the cockpit and a stern platform. With a permanently mounted drop-down ladder, an open transom makes the ideal platform to step aboard and keeps the topsides unmarred by dinghies coming alongside or ladders being lowered over the side.

5. Prismatic

coefficient

We also need to consider the prismatic coefficient – that is the fineness of the ends of the boat. The prismatic coefficient gives an indication of the boxiness of the hull. Prismatic coefficients vary widely, depending upon whether the boat is designed for light-, medium- or heavy-air sailing. In general, the higher the prismatic, the more power it takes to make the boat go fast.

On a sailboat designed for light-air sailing, the hull’s prismatic coefficient (without keel) is usually in the range of 0.48 to 0.51. For a boat intended for medium-air sailing. the range is about 0.5 to 0.55, while heavy-air boats range from about 0.54 to 0.59. For example, a boat designed with a prismatic of 0.59 might be fast in heavy winds, but only rarely do winds blow strongly enough to make the boat go that fast. For our boat, we will keep the prismatic in the mid-range of about 0.55, to ensure that the hull has moderate to good performance over the entire range of hull speeds.

The hull shape

Now we can define the hull shape. Rather than taking an existing hull, we can look at the three major sections of the hull and then make the remainder fit the three parts. The three major parts of the hull are the midship section, the bow and the stern. We want to define each part within the context of the other portions. We don’t want to design a snub-nosed cruiser with a long J-class style (such as Endeavour or Shamrock). Both ends of this boat will have moderate overhangs (about 15 to 20 percent of the total hull length), a sloped bow profile with some special features, as discussed in the bow-shape section.

6. Midship section


Figure 4: This design is very similar to the wireframe hull shapes in previous figures, but with greater overall length. Once the hull shape has been set, then keel and rudder shapes can be added and the topsides refined, here, for example, by adding a pilothouse.

The midship section of the hull defines the overall hull shape. On this boat, it will be relatively flat-bottomed to emphasize the performance of the keel, but it will have a built-in sump to allow bilge water to accumulate in the hull bottom. Waterline beam, unlike racing boats, will be fairly large to ensure that the boat has good stability. (Waterline beam plays a large part in the boat’s stability.) The waterplane-area shape will also be wedge-shaped rather than diamond-shaped to help contribute to stability. The midship section itself will have an almost semicircular shape to minimize wetted surface, but the semicircle will be modified to increase waterline beam and to flatten the hull bottom slightly.

7. Bow shape

The bow has to cleave the water cleanly, ideally without allowing green water to flow across the deck. The design of the bow is complex in that it must operate successfully in the upright condition (downwind) and in the heeled condition (upwind). Under both conditions, it needs to support the hull as it slams into waves and confused seas. If you look at the bow shapes on today’s yachts, you may have noticed that they are becoming more vertically sided, with shorter overhangs. In fact, some bow shapes end just at the forward end of the waterline and have vertical sides. As this shape drives into a sea in the vertical condition, water is free to flow up either side of the bow and onto the deck. (Compare that with a powerboat designed to drive into large seas. Such a powerboat has extreme flare in the bow to drive the seas to one side and keep them from landing on deck.) In the heeled condition, the flat panels of the U-shaped bow tend to reverberate as the boat slams into a sea. On a V hull, by contrast, the bow panels have a much more acute angle to the water, making them more likely to pound and reverberate as the boat works its way upwind. However, when heeled, the V hull tends to throw the water outward, away from the hull, as the bow plows into a wave. This helps keep the deck drier and the foredeck safer.

In my opinion, curving the bow panels slightly tends to make them much less resonant, and putting some V in the bow sections gives the boat some reserve buoyancy to shoulder waves aside as the bow drives into a sea. It looks slightly old-fashioned, but remember, we’re designing to suit the waves, not a rating rule.

8. Stern shape

Like the bow, the stern should be designed to suit the waves. From midships to aft, the profile view should show a gradual tapering to allow water molecules to flow smoothly past the keel and rudder without undue turbulence.

With the transom steps a desired feature, the stern shape will be governed by the width of the transom and the shape we desire it to be. Personally, I like to see a transom shape that closely mirrors the shape of the mid-body, so that the after body of the hull remains a constant shape throughout the aft part of the hull.

At this stage, we can go to the computer and start developing the final hull shape. Using a computer to draw the lines of the hull is unlike the old process of drawing lines. Imagine, if you will, a net laid out on the ground. This is your lines plan in the computer. For the designer, pulling the net into the shape of the hull is the process by which the hull is designed. Consequently, the old standards by which the lines were developed has been thrown out the door. For example, in the sections shown here, the lines running fore and aft are part of the mesh frame, rather than conventional lines. We need the computer calculate the waterlines, buttocks and other features separately.

That is a look at how some of the basic considerations are made. There is a great deal more work to do before we have a finished plan. But a voyaging sailboat has to make sense at this early stage before we proceed.

Roger Marshall is a multi-talented boat designer and writer who has authored 12 books. Current projects include a 24 ft. powerboat and a 54 ft. cruising sailboat as well as a new book, Rough Weather Seamanship due from International Marine this fall.

A three-dimensional view of the hull showing the sections clearly. Note the slight roundness of the forward sections and the bow overhang, intending to provide reserve buoyancy as the hull slams into a wave when heeled.

These requirements suggest that the boat will be of moderate displacement. A lightweight boat may have a problem carrying the weight of gear required for voyaging, plus lightweight boats tend to have a quick motion that some people find extremely tiring. While a heavy boat can easily carry the amount of gear required, it will require larger sails, a bigger engine, a heavier rig, etc., and it will take a long time to get from one place to another, making it more likely that it will be at sea in heavy weather.

What is moderate displacement? In talking with many, many boat owners and having designed a few boats for moderately experienced owners, I find that most prefer a boat around 44 to 48 feet long. They feel that this is the largest hull that they can handle safely without resorting to having a professional onboard.

We can assume that a moderate-displacement boat 48 feet long has a light-ship displacement of 22,000 to 26,000 lbs. Light ship means that the boat has no stores, fuel, crew or sails onboard. By the time gear and people are loaded aboard, the half-load displacement is around 26,000 to 30,000 lbs, depending on gear. (The half-load condition is with half stores, half the fuel load, half the water capacity, a full crew and sails.) At one time, this displacement was thought to be moderately light, but times have changed, and now it is thought to be moderate.

Figure 3: A view of the hull from the side showing the freeboard is not unattractively high. The hull is projected beyond the transom to give the builder an extra frame to carry out the hull and to ensure the hull stays fair when built. The cabintop is simple with space aft of the mast to stow a dinghy when at sea. The sides of the cabintop enclose the cockpit.

A better criterion may be the displacement-length (D-L) ratio. Using this ratio, we can determine whether the relationship between the waterline length and displacement is about what it should be. A ratio between 50 and 100 is generally considered to be an ultralight-displacement boat, or ULDB. Between 100 and 150 is light displacement, and between 150 and 220 is moderate. A 220 to 300 ratio is now considered reasonably heavy, and more than 300 is very heavy.

If the light-ship displacement is 24,000 lbs and the waterline length is 43 feet, the D-L ratio is 135. But boats rarely sail with no sails, no crew and no gear, so the half-load ratio gives a better idea of the boat’s ability. If the half-load displacement is 28,000 lbs, convert to long tons (1 long ton = 2,240 lbs) and divide by the cube of the waterline length divided by 100. So the D-L ratio is (28,000/2,240)/(43/100)3 = 157, which puts it in the moderate category. When the boat is ready for sea and fully laden with stores and gear, the weight will probably be around 30,000 lbs, which gives a ratio of 168. This suggests that the boat is a moderately fast voyager.

So what hull shape suits that style of cruiser best? While we can design the ultimate cruising hull for the hull suggested above, it is better if we determine a few more dimensions before we come up with the lines plan.

3. Beam-to-length ratios

This design is very similar to the wireframe hull shapes in previous figures, but with greater overall length. Once the hull shape has been set, then keel and rudder shapes can be added and the topsides refined, here, for example, by adding a pilothouse.

The boat’s beam is critical if it is going to play in the deep ocean. If we go back to the Fastnet disaster of 1979 and the subsequent safety-from-capsizing report from the Society of Naval Architects and Marine Engineers and the U.S. Yacht Racing Union (now called U.S. Sailing), we find that large beam played a significant role in leading to a capsize. In fact, the report suggests that narrow-beam yachts stand a significantly smaller chance of being capsized than do extremely beamy yachts. The range is given as length-to-beam ratio between 2.4 and 3.0. (Note, the report uses a calculated value for L, which corresponds to slightly more than the waterline length and a maximum beam. We will use the waterline length of 43 feet.) Thus, for this hull, the maximum beam comes out as being between 14.3 and 17.9. I chose to go with 14.75 feet, which should give plenty of beam for interior accommodations and a lower probability of capsizing.

A quick recap, length overall is 48 feet; beam is 14.75 feet; waterline length is 43 feet; and half-load displacement is 28,000 lbs, for a moderately fast, moderately beamy boat that can be handled by two experienced sailors. The next step is to determine the hull draft. As the boat is likely to spend winters in the Caribbean, we will say that the draft is not to exceed about 6 feet 3 inches when fully loaded.

To get the most efficient keel design, the hull will be relatively flat-bottomed. Once again, this represents a trade-off. The most efficient hull shape for a flat-bottom hull is one with an almost vertical blade and a bulb keel. However, such a keel shape is terrible at an anchorage. It simply does not have enough lateral profile to prevent the boat from veering around the anchor line when any kind of breeze is blowing. So, because the boat will spend a lot of time at anchor, the lateral profile of the keel will need to be increased.

Consequently, by defining the draft limit, we have defined the hull-bottom shape; and from the type of sailing the boat is likely to undertake, we have come up with an approximate keel profile.

4. Freeboard

The freeboard on this boat will be determined by the interior layout. Unlike boats that spend their lives in northern climes, the headroom in this boat will be about 6 feet 6 inches to allow good air circulation in the tropics. This will necessitate a small cabin trunk, but the higher freeboard and small cabin can be made to be aesthetically pleasing, and they make life easier belowdecks. However, relatively high freeboard poses another problem: How does a crew get from the dinghy onto the deck or retrieve a crewmember who falls overboard?

For a number of years now, I have advocated open transoms on boats to make it easier for a man overboard to get back aboard. The hull we are designing will have a transom with a pair of steps into the cockpit and a stern platform. With a permanently mounted drop-down ladder, an open transom makes the ideal platform to step aboard and keeps the topsides unmarred by dinghies coming alongside or ladders being lowered over the side.

5. Prismatic

coefficient

We also need to consider the prismatic coefficient &mdash that is the fineness of the ends of the boat. The prismatic coefficient gives an indication of the boxiness of the hull. Prismatic coefficients vary widely, depending upon whether the boat is designed for light-, medium- or heavy-air sailing. In general, the higher the prismatic, the more power it takes to make the boat go fast.

On a sailboat designed for light-air sailing, the hull’s prismatic coefficient (without keel) is usually in the range of 0.48 to 0.51. For a boat intended for medium-air sailing. the range is about 0.5 to 0.55, while heavy-air boats range from about 0.54 to 0.59. For example, a boat designed with a prismatic of 0.59 might be fast in heavy winds, but only rarely do winds blow strongly enough to make the boat go that fast. For our boat, we will keep the prismatic in the mid-range of about 0.55, to ensure that the hull has moderate to good performance over the entire range of hull speeds.

The hull shape

Now we can define the hull shape. Rather than taking an existing hull, we can look at the three major sections of the hull and then make the remainder fit the three parts. The three major parts of the hull are the midship section, the bow and the stern. We want to define each part within the context of the other portions. We don’t want to design a snub-nosed cruiser with a long J-class style (such as Endeavour or Shamrock). Both ends of this boat will have moderate overhangs (about 15 to 20 percent of the total hull length), a sloped bow profile with some special features, as discussed in the bow-shape section.

6. Midship section

The midship section of the hull defines the overall hull shape. On this boat, it will be relatively flat-bottomed to emphasize the performance of the keel, but it will have a built-in sump to allow bilge water to accumulate in the hull bottom. Waterline beam, unlike racing boats, will be fairly large to ensure that the boat has good stability. (Waterline beam plays a large part in the boat’s stability.) The waterplane-area shape will also be wedge-shaped rather than diamond-shaped to help contribute to stability. The midship section itself will have an almost semicircular shape to minimize wetted surface, but the semicircle will be modified to increase waterline beam and to flatten the hull bottom slightly.

7. Bow shape

The bow has to cleave the water cleanly, ideally without allowing green water to flow across the deck. The design of the bow is complex in that it must operate successfully in the upright condition (downwind) and in the heeled condition (upwind). Under both conditions, it needs to support the hull as it slams into waves and confused seas. If you look at the bow shapes on today’s yachts, you may have noticed that they are becoming more vertically sided, with shorter overhangs. In fact, some bow shapes end just at the forward end of the waterline and have vertical sides. As this shape drives into a sea in the vertical condition, water is free to flow up either side of the bow and onto the deck. (Compare that with a powerboat designed to drive into large seas. Such a powerboat has extreme flare in the bow to drive the seas to one side and keep them from landing on deck.) In the heeled condition, the flat panels of the U-shaped bow tend to reverberate as the boat slams into a sea. On a V hull, by contrast, the bow panels have a much more acute angle to the water, making them more likely to pound and reverberate as the boat works its way upwind. However, when heeled, the V hull tends to throw the water outward, away from the hull, as the bow plows into a wave. This helps keep the deck drier and the foredeck safer.

In my opinion, curving the bow panels slightly tends to make them much less resonant, and putting some V in the bow sections gives the boat some reserve buoyancy to shoulder waves aside as the bow drives into a sea. It looks slightly old-fashioned, but remember, we’re designing to suit the waves, not a rating rule.

8. Stern shape

Like the bow, the stern should be designed to suit the waves. From midships to aft, the profile view should show a gradual tapering to allow water molecules to flow smoothly past the keel and rudder without undue turbulence.

With the transom steps a desired feature, the stern shape will be governed by the width of the transom and the shape we desire it to be. Personally, I like to see a transom shape that closely mirrors the shape of the mid-body, so that the after body of the hull remains a constant shape throughout the aft part of the hull.

At this stage, we can go to the computer and start developing the final hull shape. Using a computer to draw the lines of the hull is unlike the old process of drawing lines. Imagine, if you will, a net laid out on the ground. This is your lines plan in the computer. For the designer, pulling the net into the shape of the hull is the process by which the hull is designed. Consequently, the old standards by which the lines were developed has been thrown out the door. For example, in the sections shown here, the lines running fore and aft are part of the mesh frame, rather than conventional lines. We need the computer calculate the waterlines, buttocks and other features separately.

That is a look at how some of the basic considerations are made. There is a great deal more work to do before we have a finished plan. But a voyaging sailboat has to make sense at this early stage before we proceed.

Roger Marshall is a multi-talented boat designer and writer who has authored 12 books. Current projects include a 24 ft. powerboat and a 54 ft. cruising sailboat as well as a new book, Rough Weather Seamanship due from International Marine this fall.

By Ocean Navigator