I had been in the Bahamas for about six weeks doing pretty much nothing other than some fishing and cruising the island of Abaco, by boat and motorcycle. I was on my second Five Star Barbancourt, a rich Haitian rum, using the free WiFi at Curlytails Bar in Marsh Harbour when I saw the “Hello from Boat Digest” in the subject line of an e-mail. Uh-oh, he found me. I always feel guilty when I get an e-mail from George, our Editor at Boat Digest. George started the e-mail with a few pleasantries and then moved on to the more serious “hey Mr. Technical Editor, do you think you could find it in your busy day, between naps maybe, to do a piece on hull design?”

After forty years of boatbuilding experience and thirty-six years in my own boat business it seemed a bit daunting to do a “piece” on hull design. Maybe a book, but a “piece” is a more difficult deal. But what the heck, lets get started and see where we go.

I would expect that somewhere back about the same time fire was discovered and swimming invented, man needed to cross a bigger body of water than he could swim so he found something he could float on when he was tired. Not exactly the first boat but pretty close. Things evolved from there and pretty soon they where hollowing out logs and using poles to push with and crude paddles to help move their “boat” along. Yachting was born!

It was a surprisingly short time from the first log to some remarkably high-tech boats. These boats were an amazing array of designs from animal skin covered wood framed kayaks, Polynesian proas, and some incredible boats the size of a football field, all designed and built many thousands of years ago. The basic design elements were established way back then for hull design and a key element elucidated about 250 B.C. by a fellow with the name of Archimedes. Archimedes came up with the rules of buoyancy also known as hydrostatic force. More cool to me is that he designed a vessel named Syracusia that was of nearly cruise ship proportions that supposedly carried over six hundred men. It leaked badly and we can credit him with also designing one of the earliest bilge pumps, the Archimedes screw. No kidding!

In an effort to keep a complex topic simple I am going to take some risk and boil hull designs down. Much like a herpetologist might identify a reptilian species first as being either warm or cold blooded we are going to look at hull designs as falling into one of two groups: either displacement or planing. Before a bunch of you start e-mailing me about what a nitwit I am, keep reading. If then you can find some errors, by all means let me know.

If we look at the world today, our pleasure boats are primarily planing hulls. Ninety percent-plus of the boats that travel at more than twelve miles per hour are planing hulls. All the ski boats, sport cruisers, fishing boats, personal water craft, you name it. As a rule if it moves fast it is a planing hull. There are exceptions and we will talk about them later.

Displacement hulls are the other group. Nearly all single- hull sailboats and many of the long-distance pleasure trawlers are displacement (reference the Nordhavn photo on page 50). As you can probably guess, a good rule of thumb is if it can ONLY go slow it is probably a displacement hull.

There are dozens of formulas that help designers classify hulls but the simple fact is that planing hulls are planing when they displace (push out of the way) less water than they actually weigh. If a vessel weighs 4,000 pounds on a scale, then it is planing when anything less then 4,000 pounds of water is pushed out of the way as it moves. Some of that water that is being pushed out of the way is often recognized as wake. A hull, when it is planing, actually lifts out of the water. This lifting is typically known as a hydrodynamic force. In the case of very fast boats they may displace (push out of the way) virtually no water at full speed and are riding on only the very top of the water surface. (See the Baja 30, bottom right.) This is the case with a racing hydroplane. You might have noticed that once a boat is on a plane the wake often gets smaller as the boat goes faster. That is because it has more hydrodynamic lift the faster it goes and pushes less water out of the way. On the other hand displacement hulls always push an amount of water out of the way equal to their own weight.

It gets very sticky when folks, often marketing guys, try to break down all the different types of either planing or displacement hulls. Many of you have probably heard that a displacement hull has a “hull speed” and that it is limited to this by the laws of physics. Well, I am here to tell you that in my world that is not true. The hull speed many people talk about does exist and is normally expressed as the formula of 1.35 X √ of the waterline. If the actual waterline of your boat is, say, 25 feet you multiply the square root of 25, which is five, by 1.35 to yield 6.75 which is the theoretical maximum speed of a full displacement vessel at maximum hull speed in knots, to convert that to miles per hour you would multiply by 1.15 for a speed of 7.76 MPH. Whew!!

However, there are a number of hull designs that always push an amount of water out of the way equal to their weight (displacement hulls) BUT go much faster than “maximum hull speed”. These hulls almost always have a very “fine” shape to them. Their beam, or width, is small compared to the length of the hull. Think of a canoe, racing kayak or the single hull of a catamaran. Whereas some boats may have a beam that is as much as nearly half of their length. I am talking about hulls that have a beam that is only one tenth of their length. Most of these displacement hulls are often called “piercing” hulls. These hulls can be very efficient and actually harken back to power boats built before we had unlimited horsepower. Many of the “Commuters” of the 1920s and ‘30s used remarkably low horsepower engines to achieve speeds way in excess of hull speed but many, not all, were still displacement hulls. They would be great boats today except for some realities. One of these is the fact that nearly every marina on the planet charges by the foot of length. If we were charged by the square foot we would probably see these boats making more of a comeback than we are. Another very visible category of boat that meets the criteria of displacement hulls that exceed hull speed are catamarans. While there are planing catamarans, the bulk of large cruising catamarans, sail and power, today are displacement and exceed max hull speed. The fact that they have two hulls instead of one does not change the argument.

I am going to swim up stream here a bit more and declare “semi-displacement” and “semi-planing” are not only one and the same but also, simply, planing hulls. Even though there are some distinctive characteristics of the breed the bottom line is that they are planing when they are above hull speed and achieving a degree of hydrodynamic lift. A pleasure trawler with a 36-foot waterline that makes something over ten or 11 miles per hour is a planing hull no matter what anybody else calls it. The argument over the distinction between displacement and semi-displacement is very wide spread. All the pros seem to recognize the breed but few can agree on the pedigree. Is it marketing hype or real distinction in design? My feeling is that they are a legitimate attempt to bring the qualities of a displacement boat to a planing hull. Please do not construe this to mean that I disapprove of hull designs that display the characteristics of the so called semi-displacement hull, but I do feel it is a marketing term.

Different hulls do different jobs. The more closely you can identify your own mission on the water the better decision you can make in choosing the right hull shape. What kind of seas will I be in? How much weight do I need to carry? How much speed do I need? How much range do I need?

The reason most of our pleasure boats are planing hulls is because they fill the mission we want from them. Usually it is that all too short weekend on the water and we want speed to get to places that would not be practical in a conventional displacement boat. Many of the vessels in this category are also trailerable, perfect for lightweight planing hulls. The weekend boater has figured out his mission and it fits a fairly narrow profile.

The trouble begins when we start asking more of our boats. We want it to do everything. This is where the saying comes from that boats are a compromise. What you gain in performance in one area you lose in another. Given the same hull, when you increase speed you decrease range. When you increase the weight you decrease speed. There are compromises for every decision that a designer makes as he designs a hull and the buyer is faced with trying to understand those trade-offs.

Displacement hull design is not nearly as riddled with goofy compromises as planing hulls are. Displacement hulls tend to only face one hurdle and that is the quest for more speed, but otherwise displacement hull design tends to be a more honest approach to doing at least what the designer thinks is the right thing to do to get the performance he desires.

Planing hull design is too often a different story. This is where the marketing guys have a big say in the critical elements of design. Speaking to one of the salesmen of a very old and well known builder some years back about why they had made the bow so full, instead of sharper on their new boat I was told it was to make the forward V berth bigger. Seeing my surprise he then told me that most people don’t use their boats much away from the dock anyway! The company was willing to take the gamble that a bigger berth was more important to their customers than a smoother ride.

The design did not have a very long lifespan and was pulled from production after a couple of years. I have heard owners say that they really love the inside but the ride stinks in a bad chop. So, in a bit of self-fulfilling prophecy, they don’t leave the dock much anymore. What is important to you?

I remember as a builder in my twenties hearing a fellow really bad mouth a boat that I knew something about. As a kid in my teens I had worked on the construction of this particular boat. It had been contracted by the Miami Seaquarium. It was designed and executed for the very specific purpose (GULP!) to catch porpoise alive for display. It was an exceptionally fast 26-foot hull with a ton of gas guzzling horsepower and had very low freeboard aft and virtually no transom so the poor porpoises could be hauled over the stern after they were netted. The boat did the job incredibly well but when it was sold to new owners they had another mission in mind and their thinking was poor. It was terrible at its new task and was nearly lost a couple of times. Bad hull design? Nope, great hull design for the original purpose but very poor for the way it was being used by the new owner. Again the importance of identifying your mission in the narrowest terms possible will lead to the greatest happiness afloat.

Well I have gone back and read all that I have written and realized there was some stuff that I had to read twice to figure out. If George will let me, and you are willing, next month we can go into the real reasons our planing hulls have many of the features that they do.

Why some fast boats can be dangerous going slow in bad weather and what you can do about it. How to drive certain hull designs differently to enhance their comfort and performance. You might be aggravated at the salesman who sold you your boat after reading next month’s article. I am not building much of a fan base in the boat industry these days but maybe together we can make some changes out there.

Education is the key to making the right choices in just about every part of our lives, including the choices we make about boats. And stop by and say hello. Slip W28, Conch Inn Marina, Abaco, Bahamas. Or if you have any specific questions I can address please e-mail me at Ken Fickett@BoatDigest.com.