WHAT THE HULL: Some musings on Hull Shape

Posted By: SkipperEd
Date: 9/6/99 11:23a.m.

Report: outside forum guidelines

What the Hull: Some Musings on Hull Shape by SkipperEd A Preface Books have been written about hull shape. Lonnnggg books with lots of mathematics, formulae, equations, diagrams, water tank test results, hydrodynamics, and other important physics. This tiny little article does not pretend to be anything even approaching a comprehensive discussion of all the elements of hull shape, and is an even less comprehensive look at boat design in general. There isn't the time or space, nor do I have the qualifications. I am not a naval architect, and don't even play one on TV. That's what this article ISN'T, so don't write a flaming post to the TSBB proclaiming that I didn't mention "this or that," and that "this or that other thing" should have been included. We all know how incredibly complex sailing can be, and how many variables are involved. I am not trying to address them all. The article hopes to be a launch pad. A launch pad to start the reader thinking about hull shape when looking at boats. A launch pad to send the reader off to look into the entire area of hull shape and boat design on his or her own (many of you at TSBB already have). This modest little article also wants to be a smidgin of an introduction to some of the basic factors involved in hull shape, and the effect of those factors on sailing. The entire subject is fascinating to me, and I hope this will help people to understand better the cause and effect of hull shape, and what to expect from different hull forms. On a beginning level. The article will not go into multihulls. Why? Sailing means a great deal to me. I love it. A lot! I encourage others to try it. But there's a problem. Sometimes a novice sailor (newbie) is bitten by the sailing bug, and then goes out to buy the first boat! Hooray. However, as a newbie, the person doesn't know what to look for. Some newbies ask around. Some sign on to TSBB and ask "What should I buy?" Some just buy what looks cool. But even when a newbie asks before buying, he or she is susceptible to a couple of other factors: the inherent biases that all sailors have about the sort of sailing they enjoy best, and the even more common problem of the newbie not providing enough information about their needs and lifestyle for the veteran sailor to provide really useful advice. (We too often assume that other people will like what we like, so we recommend our favorite boat. And often, our favorite is all wrong for the new sailor.) Why Number Two. Even some experienced sailors haven't paid much attention to the significance of hull shape during their sailing years. They may have learned that a light, beamy boat with a fin keel and a tall rig will sail faster than a heavy, narrow boat with a full keel and low aspect ratio rig, but they may not be aware of the other significant effects the differences may cause. And so, with these two "whys" in hand, let's cast off the dock lines and head out on a brief and simple introduction to hull shape. First, a little "glossary" so that we're all talking the same language. Start with dimensions: BALLAST: the amount of weight placed low in the boat to provide stability BEAM: the width of the boat at its widest point (Waterline Beam is the maximum width of the boat at its waterline) DISPLACEMENT: the weight of the boat (actually the weight of the water it displaces) DRAFT: the depth of water required to float the boat LOA: length overall, the maximum length of the boat (doesn't include bowsprits, davits, etc) LOD: length on deck LWL: load waterline length, the length of the boat at the waterline BULWARK: a raised ridge above the edge of the deck, similar to but more substantial than a toerail CENTER OF LATERAL RESISTENCE (or lateral plane): the center of the underwater portion of the hull important factor in weather helm CHINE: usually limited to flat bottom boats (or v-bottoms), it is the angled corner where the bottom of the hull meets the topsides ENTRY (or entrance): the bow just above the waterline, where the boat cuts the water FAIR: smooth, a hull that doesn't suffer from bumps, indents or other misshapes FREEBOARD: the height of the boat from the waterline to the deck KEEL: (technically, the backbone of a boat) for the purposes of this article, we'll refer to the keel as the portion of the boat designed to provide lateral resistance, including fin keels, full keels, keel-centerboards, centerboards, swing keels, etc. It isn't exact, but it'll work. OVERHANG: at the bow and stern, the amount of the boat that extends beyond the waterline QUARTER: the part of the boat that is aft of the halfway point (beam). A "quarter berth" is a berth located in the aft corner of the boat. SHEER: the line the deck makes when view from the side. Commonly, traditional boats have a sheer that curves downward from the bow and stern. Often, modern boats have a fairly straight sheer. SKEG: A solid, non-moving part of the hull just forward of the rudder which helps to support and protect it. SPADE RUDDER: a rudder that is separate from the keel, and has no skeg. It is supported only by the rudder stock (or shaft) TOPSIDES: not the deck. The topsides are the portion of the hull from the waterline to the edge of the deck. Pretty much the same area as the freeboard. TRANSOM: the stern panel of a boat (boats with canoe sterns don't have a transom) TURN OF THE BILGE: the curve where the topsides turn to become the bottom WETTED SURFACE: the amount of surface area under the water; the more wetted surface, the more friction and the more power needed to drive the hull through the water Before looking at the different hulls, keels, and rudders, it's important to know the difference between FORM STABILITY and WEIGHT STABILITY. Some boats derive their stability from the shape of their hull. Consider a sheet of plywood floating in the water: it's not going to flip over because of its flat, broad shape rather than its weight. Now, consider a round plastic barrel floating in the water: it will roll and roll, bottom to top, top to bottom unless something is added to stop it. If you add a heavily weighted keel to the barrel, it will stop rolling. The plywood derives its stability from its form, the barrel from the added weight. A broad, flattish bottomed boat (or the wide stance of a catamaran) derives much stability from its form. A boat with a round hull must add a weighted keel. Further, there are two other kinds of stability: INITIAL STABILITY, and ULTIMATE STABILITY. Put a mast and sail on the sheet of plywood, and put a mast and sail on the barrel. At first (initially), the plywood remains stable and flat, while the barrel heels over a little, and rocks back and forth. Now add wind: the plywood will remain flat, not heeling at all, until the force of the wind is enough to overcome the breadth of the shape, then it will flip over suddenly. On the other hand, when we add wind to the sail on the barrel, the barrel will begin to heel early, heel further and further until the weighted keel reaches the point where it is exerting maximum downward force, and the wind begins spilling from the sail. Here is the point of ultimate stability. The barrel heels earlier (it is more "tender"), but it has greater Ultimate stability. OK. Now for a quick rundown of the basic types: V-BOTTOM: Most common in power boats. Has a hard chine (angular), straight sides that meet the v-shaped bottom at the chine. Example: West Wight Potter. Advantages: inexpensive to construct, easiest to home-build, excellent initial stability Disadvantages: has a tendency to pound in waves, stability changes are abrupt rather than gradual, more difficult to sail when heeled FLAT-BOTTOM: Most common in punts, and dories, but distinctive in sharpies. Again, it has straight sides that meet a flat bottom in a hard chine. Example: Sea Pearl. Bay Hen Advantages: inexpensive to construct, excellent initial stability, extremely shallow draft Disadvantages: pounds in waves, needs to be sailed flat, best in protected waters Note: racing scows fit the description of flat bottom boats, are very fast, but are commonly limited to flat water, lake sailing ROUND-BOTTOM: there is a wide range of round-bottom hulls, from just slightly rounded (or u-shaped) as in a dinghy, to Y-bottomed as is common in most racer-cruisers, to the "wine-glass" bottom of a Cape Dory Typhoon. WINE-GLASS: Advantages: smooth motion in seaway, strong hull form, moves through waves well, good directional stability as it is commonly equipped with full keel, seaworthy Disadvantages: low initial stability ( heels easily, "tender"), significant wetted surface increases friction and slowing boat, expensive to build, deep draft U-SHAPED: Advantages: shallower draft, low wetted surface, often able to plane, more initial stability than wine-glass Disadvantages: less directional stability, more pounding in waves, shallow bilge Y-SHAPED Advantages: good compromise between smooth motion in waves and lower wetted surface than wine shaped, seaworthy, appropriate to cruisers Disadvantages: slower, more wetted surface than U-shaped, less initial stability than U-shaped Note: most trailer-sailors have U-shaped hulls to make them more easily launchable. Compare a Cape Dory Typhoon to a Catalina 22 for an example of the differences. THE ENTRY: A "FINE" entry indicates a pointed bow shape that remains fairly narrow for some distance back. (Examples: J-22, Capri 22) Advantages: cuts through waves, better speed, less pounding Disadvantages: limited interior volume A "ROUNDED" (or Full) entry indicates a broader, rounder bow shape whose beam increases rapidly. (ComPac, Seaward) Advantages: much more interior volume, greater buoyancy Disadvantages: more resistance in moving through water, slower, more pounding An "EASILY DRIVEN" HULL We've probably all heard this term, and it's an important one. Simply put, the more easily driven a hull is, the more easily it can be propelled through the water whether by sail or motor. A knife moves through the water effortlessly it needs to move very little water aside to makes its way through the water and thus, takes little power to do it. For the sake of this discussion we will exclude PLANING HULLS (i.e. hulls that can skim across the top of the water), and limit ourselves to DISPLACEMENT HULLS. (i.e. hulls that must move water aside in order to travel). It makes sense that a very narrow hull with a sharp entry will be easily driven if it is not too deep to create lots of wetted surface and drag. But, a very narrow hull doesn't have much buoyancy, and certainly doesn't have much living space. So a compromise is made: the beam is made wider to provide more buoyancy and living space below. But now that the boat is wider, it is more difficult to drive through the water. In turn, the boat's bottom is made flatter instead of deeper, so that it has less wetted surface, but now, it pounds more in the waves, so ..... Designers are in a continual search to find the right combination of compromises for the right kind of boat. A Shields is a 30 foot boat with a 6 ft 5 inch beam and almost no freeboard. Long, low, and narrow. Sleek with no room below, a daysailer. A Cape Cod Cat is 17 feet long, with a beam of almost 8 feet (a foot and a half wider, but only about half as long!). Two boats clearly designed for different purposes. But let's compare two well-known trailerables of the same length: both boats have exactly the same LWL of 18 feet, one has a beam of 8'0, the other 8'2", one has a sail area of 250 sq feet, and the other has a little more at 308 sq feet. But that's where the similarities come to a screeching halt. The first one, the Flicka 20 weighs in at 5,500 pounds, while the other, the Ultimate 20 weighs only 1,100 pounds. Be sure to consider ALL the numbers. HULL SPEED A non-sailing buddy of mine teases me about hull speed. "Here's a 25 foot boat, and here's a different 25 foot boat. If both of these boats have the same hull speed, how can you say one is faster than the other?" Or looking at the Flicka and the Ultimate 20, if they BOTH have the same LWL, and hull speed is determined by the equation: 1.34 times the square root of the LWL, why aren't both boats equally fast? It's a good question. But the answer is easy, and it has to do with a hull being "easily driven." Excluding the concept of "planing" here. The Ultimate 20 is a faster boat than the Flicka even though both have the same LWL because the Ultimate 20 is: 1. able to achieve greater speeds in less wind than the Flicka 2. able to reach hull speed more easily than the Flicka. Using some totally hypothetical numbers: imagine the true wind is 10 knots. Both boats are sailing on the same tack. The Ultimate 20 would be able to reach 5.5 knots of boatspeed while the Flicka would reach only 3.5 knots of boat speed in the 10 knots of wind. NEITHER one reaches hull speed, BUT the Ultimate is considerably faster. FREEBOARD: This is one of the least understood and appreciated elements of hull shape. Partly because it is difficult to measure, and because measurements are never given in boat statistics. HIGH freeboard is an indication that the boat's topsides are comparatively high from the waterline to the deck. LOW freeboard is the opposite. HIGH freeboard (Example: Seawolf 25 cruiser, a 25 foot boat with standing headroom) Advantages: more interior volume, more headroom, increased reserve buoyancy Disadvantages: greater windage (which can result in greater leeway, difficulty in sailing to weather, difficulty docking), often aesthetically "chunky" looking LOW freeboard (example: J-105, a 35 foot boat without standing headroom) Advantages: low windage, sleek appearance, better sailing performance Disadvantages: often no standing headroom, little storage, uncomfortable KEELS FULL KEEL Advantages: greater directional stability (easier to steer in a straight line), strong,increased protection for hull, rudder and propellor, seaworthy, generally good motion in seaway, good ultimate stability Disadvantages: difficult turning, slow due to increased wetted surface and weight, decreased windward sailing ability FIN KEEL There are many kinds of fin keels from extremely deep, narrow keels with huge bulbs at the end such as in high tech racers, to moderate draft fins, to shoal draft fins. In each case the advantages and disadvantages are a function of the depth and length. For example, a high tech racer's very deep, but narrow keel has the advantage of great righting moment to enable to boat to carry more sail, but very poor directional stability making steering difficult. Whereas a boat with a long, but shoal draft keel will have good directional stability, but less righting moment. Obviously, an extremely deep keel has an increased risk of grounding, is more fragile and apt to damage. On the other hand, such a keel has exceptional windward performance due to the lift provided by the keel. Many non-trailerable boats have elected the moderate draft fin keel: that is, a draft of 4.5 to 6 feet as a compromise. Good lift, good resistence to leeway, and good windward performance, but without prohibitively deep draft The more shallow draft a keel, the better will be its steering, it's ability to "gunkhole," the ease of its launching, but the less efficient will be its ability to sail to windward. KEEL-CENTERBOARD This is why shallow draft keels are often equipped with centerboards to improve lateral resistence, and windward performance. The advantages of the k-cb keel are: fixed ballast, shoal draft, good windward performance with an easily raised and lowered unweighted board. Sometimes, the draft or towing weight of such boats are challenging. Keel-centerboarders remain ballasted keelboats. SWING KEELS and LIFTING KEELS (daggerboards) In addition to the keel-centerboard solution to providing a workable means of shallow draft for trailering, and improved windward performance, designers have devised weighted swing keels (as in a Catalina 22), or lifting keels as in an S-2 7.9 and others. Often, a portion of the ballast of such boats is bonded into the hull so that all of the ballast does not need to be located in the movable keel. Please note: in all of the foregoing, we have been looking at ballasted keel boats that use a substantial amount of weight as part of the boat SOMEWHERE to provide counterbalance to the force of the wind in the sails. Such boats are extremely difficult to capsize or even to knockdown in anything but severe weather. The next category does not meet these same criteria. It is essential that new sailors realize the difference between ballasted keelboats of any description, and unballasted centerboarders of any description. Unballasted CENTERBOARDERS. These boats range from car-toppable board boats such as Snarks and Sunfish, to trailerable dinghy sailors such as Capri 14.2s, Lasers, Snipes, Coronados, etc. Such boats depend form stability and on the CREW to provide the counterbalance to the force of the wind on the sails, and depend on the crew to be in the right place on the boat as well. Such boats can and do capsize very easily. SELF-RIGHTING: centerboarders will not right themselves. They must be righted by the crew swimming in the water. Many keelboats (though not all) will right themselves many will even right themselves from a completely capsized (or turtled) position. Some will self-right only from a knockdown. It depends on the design of the boat, including the hull, the deck, and the placement of the ballast. In the case of Around the World Alone Racers such as Isabelle Autissier, and the late Mike Plante, their boats are designed with such radically broad beam, and shallow freeboard, that they are not self-righting when turtled. However, many ocean cruisers such as Pacific Seacraft, Island Packet, Cape Dory, and others, will return upright even if fully capsized (provided watertight integrity is not violated). SOMETHING TO THINK ABOUT: It isn't an absolute truth, but there is enough truth in it to use as a guideline. Whenever you elect to emphasize one quality in your choice of boat, you have to give up an equal share of another quality. Want more speed? You might have to sacrifice a little comfort. Want more stability? You might have to sacrifice a little speed. Want more room below? You might have to sacrifice ease of towing. Want easier launching? You might have to sacrifice windward performance. Want a little, lightweight boat? Sacrifice sailing range and sea capability. Etc. Yesterday, while sailing on a broad reach in over 20 knots of wind with a following sea of 3-5 feet, Linda and I shrieked with delight as we surfed down the waves with a tremendous roar as our boat exceeded hull speed again and again because it was "surf assisted." (Put an asterisk in the record books.) It was an absolute ball! Our hull has a fine entry, only moderate freeboard, a 5'6" fin keel, a spade rudder with a partial skeg, but a fairly flatish (U-shaped) bottom that flattens out even more aft of the beam. As we were surfing, spray sparkling my smiling teeth, I said to Linda, shouting to be heard above the roar, "THIS is the benefit we get for having a lousy, too shallow bilge!" Our HULL SHAPE enables us to surf, but the price we pay is we don't get the wonderful deep bilge of a Cape Dory. Make your choices, but know the likely results of each one.