The elements of seaworthiness
Posted on 01 September 2009
Written by Eric Sorensen
A seaworthy boat handles well, is stable, sheds water quickly, and has plenty of reserve buoyancy
A seaworthy boat to my mind is one that can reliably and safely carry out its function or mission. In that context, you would expect a runabout to stay afloat, handle well and ride comfortably in a light chop on inshore waters.
Obviously, I’d expect a lot more from a long-range, oceangoing displacement passagemaker like the Nordhavn 86. This mini ship should be able to handle rough seas, have a large range of stability, be able to stay afloat when any one compartment is flooded, and have a reliable and redundant propulsion system.
Clearly, seaworthiness is a relative term that depends on the intended purpose or mission of the vessel. What’s important is that the boat meets reasonable expectations, whether it’s pulling a skier, gunkholing on Chesapeake Bay or crossing the Pacific. Let’s begin by looking at some of the elements of seaworthiness.
For me, the question to ask first is, What sort of damage would it take to sink this boat? A boat has to displace its own weight in water in order to float, and the obvious way to do that is to keep the ocean on the outside. But boatbuilders also play a role, filling the area between the hull and deck liner with enough foam or dividing it into enough small compartments to still displace its own weight if the hull is damaged.
If you own a typical 24- to 30-foot center console or express cruiser, the answer to the question about what sort of damage would be required to sink the boat is: not much. A single 2-inch hole practically anywhere in the bottom would do the trick — and quickly. If that 2-inch hole is just six inches below the waterline, it would flood the hull at 55 gallons per minute, or 3,300 gallons per hour. A 500-gph bilge pump — with a typical 40-percent efficiency loss — would actually be able to pump about 5 gallons a minute over the side; at that rate, it would take about 11 pumps to keep up.
Most boats 20 feet and longer have neither compartmentation nor sufficient foam to keep them from sinking — the Coast Guard only requires boats smaller than 20 feet to have level flotation when swamped — but some fishing boats like Grady-White, Key West, Sailfish and Boston Whaler do. (Making a boat “unsinkable” adds cost and weight, which affects performance, and takes up space.) The funny thing is, it is often boats smaller than 20 feet and ships more than 100 feet (or so) that are hard to sink, as they use lots of foam and compartmentation, respectively.
Most boats in between put all their reliance on keeping the water on the outside to start with. In other words, many would go straight to the bottom if a single 2-inch transducer came loose. Fortunately, the reality is that relatively few boats do sink due to flooding from a hole below the waterline. And if an unsinkable hull is breached and you lose power, under the worst-case scenario you’ll be waiting for a tow in a very large life raft.
Stability refers to the natural tendency of a hull to return to an even keel after an upsetting moment is applied — like a wave or gust of wind or an on-board weight shift. In normal (undamaged) conditions, boats develop a righting arm when heeled, created by two forces pushing in opposite directions, separated by distance. These two forces are gravity (pushing down) and buoyancy (pushing up). The higher the boat’s center of gravity, the less energy there is to right the boat.
At smaller angles of heel, beam and hull shape have the most influence on stability because it’s largely the distribution of buoyancy, not the center of gravity (CG), that determines initial stability. Planing hulls are very stable — stiff, even — through the first 15 degrees or so of heel because their hulls are so buoyant outboard at the chines. On the other hand, a narrow, deep-displacement trawler will be tender at initial angles of heel because of its slack bilges. However, its very low CG will create tremendous reserve stability once it starts heeling farther. The result is that just when the righting arm (the energy available to right a hull) is actually decreasing in the planing hull at around 20 degrees (or even less), it’s just starting to pick up on the displacement vessel, which is far more stable, ultimately. The planing hull might capsize at 40 to 45 degrees of heel, while the displacement vessel might easily go 70 degrees or more, as in the case of a sailboat with external ballast.
It helps to know, when understanding stability, that the center of gravity moves toward a weight addition and away from a weight subtraction. In other words, add weight above the CG — it’s a little above the level of the top of the engines in many inboard planing boats — and it becomes less stable. Or remove weight below the CG, and the same thing happens — stability diminishes.
That’s why adding a tuna tower or hardtop, or replacing old, heavy diesels with new lighter ones, or even just burning fuel will make a boat less stable. Keep this in mind when modifying your boat in any way. You can have a naval architect determine your boat’s stability if you’re in doubt — particularly if you’re adding a tower — or at the very least check with the builder. Read the tea leaves: If the boat hesitates before rolling back to an even keel, or just feels sluggish or has a longer roll period than feels right, get it checked out.
If your boat is floating at the dock with no one on board, you know it has enough buoyancy under those benign conditions. But what about when it’s loaded with fuel, water, passengers and gear? And what happens when you load the boat up with a cockpit full of fish, then take a wave over the side and have 6 inches of water on deck? Well, what happens depends on your boat’s stability and something called reserve buoyancy.
Reserve buoyancy is a lot like reserve stability, which is the energy stored up on account of the hull’s shape and weight distribution. Reserve buoyancy is simply a function of the watertight volume of the hull above the waterline. The “watertight” part is what matters; it means the deck has to prevent water from down-flooding to the bilges, and, in fact, it must quickly shed that water back over the side, since all that unencumbered water sloshing around up high above the waterline creates a free-surface effect that dramatically reduces stability. The higher the watertight cockpit deck is above the waterline, the more reserve buoyancy there is. So check out how high above the waterline the deck is when the boat is fully loaded.
The American Boat and Yacht Council, the marine industry’s standards- and recommended practices-setting organization, tells us the minimum height of a cockpit deck, in inches, should be 0.22 times the boat’s length in feet. That means a 20-footer’s cockpit should be 4.4 inches and a 30-footer’s 6.6 inches above the full-load (the maximum amount of weight the boat would ever carry) waterline, at a minimum. If the boat ventures offshore regularly, I’d be looking for a couple more inches of deck height than that. If it stays inshore on protected waters, then the ABYC standard seems reasonable.
If you take water over the side in the form of a wave, or you ship a few hundred gallons when backing hard on a fish, you want that water back overboard as quickly as possible. Keep in mind that we’re talking about self-bailing cockpits here; with some runabouts, forget it, since the water goes straight to the bilge where that 500-gph bilge pump sends it back into the ocean at a piddling 5 gallons a minute. If the cockpit self-bails, as on a saltwater fishing boat or most express cruisers, it’s the size of the scuppers or cockpit drain lines leading overboard from the gutters that make the difference.
There are two issues: clogging, and scupper or drain line size. If it’s less than 1.5 inches in diameter, it’s going to be susceptible to clogging with pine needles, fish scales, seaweed and other debris. In a 20- to 25-foot coastal fishing boat, my first choice is always big scuppers cut directly through the hull side or transom (with no plumbing), recessed slightly below deck level, at least 2-by-6-inches (12 square inches) in size. After that, I’d be looking for a minimum of two 2-inch drain lines per side, in the aft corners and recessed slightly below deck level. You want them in the cockpit’s outboard corners, since that’s where the water is going to collect when the boat is listing to one side, which is what it’s liable to be doing with green water on deck.
The ABYC only calls for 1-inch (0.785-square-inch) cockpit drain lines. However, a line that small will clog easily and will drain at a quarter the rate of the 2-inch (3.14-square-inch) hose. The ABYC also calls for scuppers large enough so that a cockpit filled to the gunwales will drain 75 percent of the water within 90 seconds, which A) will take a lot longer than 90 seconds with 1-inch drains, and B) is about 75 seconds too slow, in my view, for an offshore-capable boat. Also, the cockpit should be pitched aft, and the deck, ideally, will have a camber to help herd the water toward the outboard scuppers.
Hull design, weight distribution
To be seaworthy, a boat needs to be manageable in rough water. This includes being able to steer straight and not bow steer or yaw excessively running down-sea; not roll too badly in the trough; and run into waves at a decent clip (comfortably up on plane) without pounding. This calls for a well-designed bottom, an appropriate center of gravity — both vertically and longitudinally — and responsive propulsion and steering systems. These are all important elements. A boat with too fine an entry and a CG that’s too far forward or too high will yaw and be unmanageable, even dangerously so, in a following sea.
A boat’s ride should not be too wet. Water coming on board impedes visibility and situational awareness, and makes the driver less able to see what’s going on in the immediate area, which could mean anything from breaking waves to other boats. And the hull should have plenty of freeboard, which affects both buoyancy and the ability to resist boarding seas. Before you buy any boat, sea trial it in rough water and in conditions of loading you’ll actually be operating in.
Speed and agility
This is a big one, and perhaps not given enough consideration. Speed can save your life, or at least keep you from getting beaten up coming home. If you get a weather report of wind coming your way, you can potentially get ahead of it in a 30-knot boat, but probably not in one that does 10 knots. In 10- to 15-foot waves, the combination of a deep-vee bottom and a lot of horsepower can help you dodge breaking waves, and it can give you the power needed to stay on the back of a wave crossing a bar. Speed gives you a lot more control over your location and, therefore, your local conditions. A 6-knot displacement trawler, with its great range of stability, is going to need it when the wind picks up.
Hardware & scantlings
It should be evident that in order to be seaworthy, a boat must be able to stay intact despite the tremendous abuse heavy seas can dish out. This means the hull, deck and superstructure have to be solidly constructed and able to withstand considerable dynamic loads, as do the weather doors, hatches and windows.
A boat isn’t seaworthy unless it can be competently operated, and this involves both a capable skipper and a well-designed helm station. This first design element is all-around visibility to the horizon. My design philosophy is that when the builder takes away any of my horizon as viewed from the helm, there’d better be a good structural justification for every degree I can’t see. If you can’t see a 10- or 20-degree sector of the horizon because of a radar arch or Euro window styling, shop for another boat. When running down-sea in rough water, it doesn’t matter if you have great visibility ahead when you’re about to be pooped by a wave on your starboard quarter that’s out of view behind the home-size refrigerator.
If the boat runs with the anchor pulpit blocking the horizon, or the window mullions are more than 2 to 3 inches wide — they don’t have to be any wider than that if competently engineered — visibility is going to suffer, as will your boat’s safety and seaworthiness. Also, make sure you can reach and see all the controls and instruments without straining and stretching. And there should be plenty of room between the wheel and the seat, since you might need to stand to see outside the boat a little better. There should be red lighting so you can see at night without losing your night vision.
There are many other seaworthiness considerations, including fire resistance and suppression capability, deicing capability in cold climes, the vessel’s susceptibility to wind, propulsion redundancy, dewatering capability, seakindliness (to keep the skipper healthy and alert) and so on. But this is a good start, and serves as a list of things to look for when considering your next boat.
This article originally appeared in the September 2009 issue.