My wife Beverly and I were looking for a lifestyle change. So we opted for early retirement and life as live-aboards. It has been our dream to leave California, go through the Panama Canal, cross the Atlantic Ocean, and cruise the Mediterranean Sea and the Greek isles in our own power yacht.
We had several criteria for the new boat. It had to be a true passage-maker with a real pilot house, it had to be livable, it had to be affordable, and, above all, it had to be seaworthy.
I read everything I could get my hands on about long-range power boats. I was most helped by Robert Beebe’s book Voyaging Under Power. This book has recently been revised by James Leishman and is a treasure-trove of information.
From this source and others, I learned about loaded waterline length (LWL), above-water-to-below-water ratio (A/B), displacement-to-length ratio (D/L), hull-speed ratio, and prismatic coefficient (PC). Not to mention the all-important “trawler-truth ratio.” These add together to form the mathematical description of a boat.
The D/L gives you a number to use in determining the relative heft of various boats. The larger the number, the greater the interior volume and the corresponding load-carrying ability. The D/L ratio is equal to the displacement in long tons (2,240 lbs), divided by the cube of the loaded waterline length divided by 100. You shouldn’t have to derive the number; it should be readily available from a vessel’s builder.
Most passage-making power yachts in the 50-foot range have a D/L near 270. The smaller the boat, the larger the D/L must be to give load carrying capacity. The Nordhavn 46, with a LWL of 38.4 feet, has a D/L of 383. The Krogen 48, with its LWL of 45.5 feet, has a ratio of 267.
The above-water-to-below-water ratio (A/B) describes the height of the boat above the water divided by its draft. Capt. Beebe felt that a ratio of 2.6 was the maximum allowable for boats that were going to venture into waters that were historically very stormy, such as the North Atlantic. If the A/B is too high the stability of the boat is adversely effected.
Boat speed in a displacement hull is directly related to the length of the waterline. Hull-speed ratios can be determined by dividing the speed in knots by the square root of the loaded waterline length (LWL). Conversely, the boat’s speed at a given ratio can be determined by multiplying the hull-speed ratio by the square root of the LWL.
For example, take a boat with a LWL of 36 feet. The square root of her LWL is six. Her speed at a hull-speed ratio of 1.0 would be 1 x 6 or six knots. At a ratio of 1.2 it would be 1.2 x 6 or 7.2 knots. Her maximum theoretical hull speed would be 1.34 x 6 or 8.04 knots. She would be most economical between six and 7.2 knots (1.0 to 1.2). The boat could probably be driven to a hull-speed ratio of 1.4, or about 8.5 knots, but with a significant increase in fuel consumption. When making long passages, for reasons of economy and range most boats, regardless of hull type, are run at hull-speed ratios between 1.0 and 1.2. This means relatively slow speeds unless you have a very large boat. In boats you can go fast or you can go far, but you can’t go fast very far. Containerships doing 20 knots or better are traveling at hull-speed ratios of about 1.0 to 1.1, their most economical speeds.
When I first heard the term prismatic coefficient my eyes crossed! It’s a mathematical description of the “pointiness” of the ends of the boat. Each prismatic coefficient has a corresponding hull-speed ratio at which the hull is most efficient. In the speed regions in which most passage-makers operate (hull-speed ratios of 1.1 to 1.2), the most efficient prismatic coefficient is 0.6 or a little under. Sometimes compromises in prismatic coefficient are made in order to increase the D/L for more internal volume. This results in a “blockier,” less efficient hull, requiring more power to push through the water along with a corresponding increase in fuel consumption.
The “trawler-truth ratio” estimates how closely the boat in question resembles a true trawler. Most long-distance yachts have a reasonable relation to true trawlers. They do not have, or need, the load-carrying capacity of a fishing trawler. That is to say, they have a smaller displacement-to-length ratio. They do, however, have the needed internal volume to carry sufficient crew, fuel, food, and supplies for self-sufficient long-distance voyaging.
According to Beebe, the weight of the ballast should equal at least 50% of the weight of the fuel. This prevents the boat from becoming too light and too high in the water as the fuel is burned.
We thought a cruising speed of 8.5 knots would be adequate and give us room to slow down if needed for more range. A LWL of 45 to 50 feet would give us the desired cruising speed. We wanted a displacement yacht with a LWL of 45 to 50 feet, a D/L near 270, a prismatic coefficient near 0.6, an A/B ratio of 2.6 or less, and a high “trawler-truth ratio.” We also wanted a real pilot house with room for electronics, charts, reference books, and a watch berth. I also wanted a Portuguese bridge. A good Portuguese bridge allows the watchstander to go outside of the pilot house at night without fear of being lost overboard.
I also thought that a single engine would be more economical to operate. In addition, the full keel and rudder skeg would protect the propeller. This is in contrast to twins, most of which have exposed rudders and props. In my opinion, single engines are more fuel efficient than twins. Plus, the shaft and prop are protected by the keel. Usually the rudder is large and is protected by a large skeg. Far safer in waters with its floating debris, logs, and ice. The large central rudder and full keel are also of great help in following seas.
I also feel that an alternative form of propulsion is necessary for safety if you intend to be in areas where self-rescue may be the only alternative. Ideally this should take the form of a wing engine with its own prop and shaft and a source of clean fuel.
In addition to seakeeping, housekeeping is also important, especially if you are going to live aboard. We wanted a large master statetroom with generous hanging lockers. We needed an “office” where we could use the computer and keep our files. And I had to have bookshelves! A place where you can keep all the manuals, reference books, cruising guides, and other publications you’ll need.