Your boat’s bottom … and yours

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Taking pains to choose the right former can save you regularly inflicted pain in the latter

Taking pains to choose the right former can save you regularly inflicted pain in the latter

If you buy a boat that rides poorly, nothing else matters. Your dream boat can have the neatest wiring panels — fastidiously routed, color coded and numbered — the most advanced resin-infused, close-molded composite construction; the fanciest power Stidd helm seats; and Feadship-quality joinery. But if the boat beats you up cruising in a 6-inch chop, you’re not going to get much enjoyment out of all those wonderful features.

Surprisingly, there are plenty of boats on the market that will do just that — beat you up in a half-foot chop. I’ve run 50-plus-foot cruisers that started pounding in the Intracoastal in Florida before we even reached the inlet, let alone the ocean. I was out in an express sportfishing boat last fall — a new model from a well-known, long-time builder that should know better — and this $400,000 mid-30-

footer pounded hard and regularly in a 6-inch chop. I wasn’t expecting a good ride, given its hull shape, but this was worse than I had imagined. The reasons became clear when I saw the boat out of the water, as we’ll talk about in a minute.

You can measure, and therefore quantify, the comfort of a boat’s ride using an accelerometer. This little gadget tells you how hard the hull is impacting the waves by displaying and recording the speed with which it changes direction and velocity. When a hull comes up to the wave and rides over and through it, you might think of it like this: The hull rides up on the back of one wave, the bow comes out of the water a little way, then falls back down on the next wave, and so on. When it lands on the next wave, it stops falling and starts to rise again. The key to comfort is the speed — measured in milliseconds — with which the boat stops falling upon impact with this incompressible water and starts to come back up again.

From a hull’s perspective, it works like this: A boat with a sharp bow and lot of deadrise will meet the wave gradually. The sharp bow enters sequentially, slowly developing dynamic lift and buoyancy as the forefoot drives deeper into the wave. The entry (the forward section of the hull below the chines) is shaped so this lift is developed and maximizes over a comparatively long period — measured in milliseconds — and you, the passenger, feel this as a soft landing. The bow pitches down and immerses, equilibrium is reached as the water flow and buoyancy develop enough lift to support the boat’s weight and the inertia of the hull’s pitching, and it starts to lift back up out of the water, ready to meet the next wave. It’s precisely the deep-vee deadrise and fine entry that produce the gradual immersion, gentle deceleration and subsequent rise back up as you meet the next wave. You feel all of this as a soft ride.

Now consider a boat with little deadrise and a blunt, full entry. This boat launches off one wave and then starts to fall onto the next. Only this time, the hull picks up dynamic lift so quickly that the bow reverses direction almost instantly, which you feel as pounding. That’s because this big, wide, nearly flat surface presents itself to the wave almost all at once, rather than gradually. When it hits, it’s all over; the boat isn’t going to pitch down any further — and it’s like hitting concrete.

While the deep-vee dissipates energy gradually, stopping its fall and rising back up gently as it meets a wave, the boat with the full, broad entry does so all at once, and boy do you feel it. This is more than a comfort issue; it also affects safety. It’s easy to get hurt on a hard-riding boat, since the motions are so violent. The stresses on the hull and the rest of the vessel’s structure also are increased because the boat is made of materials that are unyielding — until, of course, they fail.

The engine beds, galley cabinetry and GPS/chart plotter are all subject to the same motions you are, only with no shock absorption built in. At least your knees flex. Not only are hard-riding boats uncomfortable and unsafe, they’re limited in their usefulness. They can’t take you where you want to go as fast as you want to get there, and sometimes they can’t go at all.

So why do some boats ride hard? I think there are two primary reasons. One is that people often buy boats at boat shows, and in this environment it’s the size of the accommodations that most impresses. If one 40-footer has a king-size berth and head with separate shower forward, and the next 40-footer has a queen and a combination head and shower, and smaller his-and-her lockers to boot, which usually gets the nod? The problem is that bigger berth, head and lockers take up more space, and the way boatbuilders create the extra volume is by pushing out and lowering the chines forward. Now you have the room, but you also have a hard-riding hull.

The other reason, in my experience, is a simple lack of understanding hull form on the part of these builders. Some think they know how to design a good running surface, but the proof to the contrary is in the pudding. Just look around at any marina or boat show to see bad examples, mixed in with lots of good ones, of course. It doesn’t cost any more to build a boat with a good-running bottom, so why build it with a senseless, aimless hull form?

Other than dry-land marketing and a lack of expertise, there’s a third reason for a bad ride: The boatbuilder wants to have the fastest boat for its size and horsepower. If you want speed, just flatten the bottom, since deadrise slows a boat down, though not all that much if done properly. The other way to increase speed is to take weight out of the boat, but weight also helps a boat ride better because you have inertia working with you, but that also can be carried to extremes.

Now back to our discussion. To ride well, the part of the hull that impacts the waves regularly at cruise speed has to be shaped to gradually deflect them, and it does this with deadrise. In a very fast boat that comes out of the water regularly — one running at more than 60 knots — deadrise at the transom means a lot. But in most boats that cruise at 20 to 40 knots, most of the hull remains solidly in the water in moderate sea conditions most of the time. So in these boats, it’s not transom deadrise that matters but deadrise farther forward, in the middle of the boat, which is where most of the wave impact is taking place. Remember this important detail.

A couple of other things matter, too. One is the fineness of the bow, which is referred to and measured as the half angle of entry. Think of it as the footprint of the hull in the water at the bow, the angle at which the waterline extends back from the stem. The finer this angle, the softer the ride. Like anything else, however, the angle of entry can be taken to extremes. Too fine, and the bow will immerse too deeply in a following or quartering sea and bow steer, especially if there’s a lot of superstructure (and therefore weight and inertia) up high forward. That’s because the hull has to drive down deeply into the wave before it picks up enough lift and buoyancy to start its trip back up again.

When the bow is digging down deep into the water, it creates a lot of drag and becomes a rudder once the boat starts to yaw. Then the stern tries its best to overtake the bow, and you have to fight this tendency continuously with the rudder. The boat with the fat bow won’t bow dive like this. And, in fact, it can make for a decent down-sea boat, which would be fine if you only ran your boat down sea. But that’s sort of like having a car you can drive to work but not back home.

The key here is moderation: a hull with enough fineness to deliver a soft ride but not so much that you bow steer down sea. The same moderation caveat applies to deadrise in that too much of it will slow the boat unnecessarily because a vee-bottom isn’t as good a lifting surface as a flat bottom. Also, a narrower hull of a given length rides better than a wide one, which is why you see 45-foot, 90-mph Cigarette and Formula monohull raceboats with beams of only 8 feet.

If you’re thinking about buying the beamiest 30-footer on the market, I can guarantee you it’s also going to be a harder ride, all else being equal. And since it’s wider, while transom deadrise might be the same as on the narrower boat (which is what the salesperson will throw at you), it will have a blunter entry and more surface area to impact the waves with, all of which makes it harder riding.

So what’s a boater to do? It helps to learn to read a bottom, both when the boat is in the water and out of it. You can tell a lot if you know what to look for, so examine the photos here to see some of the points I’ve covered. With a few reliable exceptions, don’t base ride-quality assumptions on brand name. Look the bottom over carefully. You can tell a lot about how it will ride just by looking at it in the water. Use this visual inspection as the first step in whittling down your list of possibilities.

No matter what anyone tells you, don’t expect to get a roomy cabin and lots of beam as well as a great ride. The laws of physics tell us you can’t have the ride of a Blackfin and the accommodations of a — well, you fill in the blank. Then make sure to validate your conclusions on a rough-water sea trial. Do your homework and have some fun.

Eric Sorensen was the founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at eric@sorensensguide.com .