On Powerboats - Finding the right powerboat hull

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Taking it out in real-life conditions can tell you if this is a boat you’re going to enjoy owning

Taking it out in real-life conditions can tell you if this is a boat you’re going to enjoy owning

On Powerboats

In this, my first monthly column for Soundings, I’ll consider a few hull design and handling elements that contribute to a boat’s offshore capabilities.

It’s important to find out how well a boat rides, tracks and handles before you buy it, and taking it out on a real-world sea trial is the best way to determine if it measures up. By “real-world,” I mean fully loaded with fuel, water and the amount of gear and people you’d ordinarily carry, and preferably with a good breeze blowing so you can really put the boat through its paces.

First, a quick primer on hull shape and its impact (so to speak) on ride quality. Boat designers have competing interests, but mostly they want to sell you a boat. As boaters themselves (let’s hope) they should want their craft to run well in the sea conditions they’re expected to encounter. But here’s the rub: Boats often are sold when they’re high and dry at boat shows and at dealerships, and since internal volume is what sells many boats, the biggest 40-(or whatever)-footer often gets the nod. The problem is that a smooth ride depends on moderate beam and a sharp entry, both of which cut down on interior volume.

While the only place to find out for sure how a boat rides is in big waves at high speed, you can learn a lot initially from a look at the boat out of the water, or even from a brochure or Web site. For a decent ride, look for a length to beam (l/b) ratio of 3 to 1 or more — a 36-footer with a beam of no more than 12 feet, for example. Also, you’ll want a fairly sharp entry with chines that start to narrow up well abaft the bow, and plenty of deadrise forward.

You really can’t have both a smooth ride and tons of space, since a beamy boat will pound more and start to do so at lower speeds than a narrower model. The corollary is that some brands deliver a much better ride than others with similar interior volume. The sea trial is the time to ferret out where on the spectrum the opposing interests of ride quality (smoothness) and interior roominess fall.

Don’t judge a hull solely by its transom deadrise. Although salespeople often point to it as the most important ride-quality criterion, transom deadrise is only a starting point for determining ride quality. The fact is, some boats with 18 degrees of transom deadrise ride smoother than others with 20 degrees. Generally speaking, it’s the shape of the hull forward of amidships, where most of the wave impact takes place, that’s important — depending, of course, on the sea state and the speed and running angle of the boat.

Only with high-speed boats do you have to worry about pounding in the aft 30 percent of the hull. Designers divide boats into 10 stations from the bow at the waterline to the transom, and most planing hulls hit waves primarily at stations 2 to 6. That’s why ride quality depends primarily on hull form in this area. Finally, the only way to get deep deadrise forward is to have the chines narrow up more in the bow, producing a fine entry.

So on your sea trial, find out how fast you can run the boat comfortably when the wind is blowing. Ideally, you’ll arrange several test rides on the same day so meaningful comparisons can be made in similar sea states and while your memory of the last ride is still fresh. Here are a few things to watch for during your ride.

Head-sea ride

The ride going into the waves should be smooth enough at the speeds you intend to travel so as not to be uncomfortable. A deeper-vee bottom spreads out wave impact over a few more milliseconds than a flatter bottom, giving your appreciative buttocks a more comfortable, less jarring ride. If the boat is outboard- or sterndrive-powered, or has trim tabs, you can play around with trim to increase comfort. Generally, you’ll want to lower the bow to smooth out a head-sea ride, but the added wetted surface will slow you down and make for a wetter ride, so you’ll want to find a middle ground based on the circumstances.

Down-sea ride and course-keeping

Running down sea, with the seas directly astern or 20 to 40 degrees on the quarter, is the hardest test of a boat’s course-keeping ability — its inherent tendency to stay on a given course. On a sea trial, run down sea and determine whether the boat tends to stay on course (within 5 degrees or so) pretty much on its own or if it wanders (yaws) all over the place. If it’s the latter, this boat will wear you out offshore and will be less enjoyable. Also, make sure those trim tabs are raised when running down sea, otherwise the boat will tend to bow steer and may even broach.

Intrinsic course-keeping ability is a function of hull form and weight distribution, both vertical and longitudinal. A boat also can be too directionally stable and actually hard to turn, but that’s pretty rare and usually involves boats with big, full keels and small rudders. You want a boat that tends to hold its heading, and yet can be readily turned with its steering system.

Steering

Pay attention to the number of turns from lock-to-lock, ease of turning, speed of a 360-degree turn and rudder angle. From the skipper’s perspective the boat should respond readily to the helm, which means the number of turns of the wheel from hard left to hard right should be low enough to allow for a quick heading change in an emergency. It also means the effort required to turn the wheel should be minimal.

To save money, many boatbuilders install manual hydraulic steering in boats that really ought to have power steering, and they get away with it by increasing the number of lock-to-lock turns for more mechanical advantage, which in turn reduces the physical effort required to turn the wheel. With manual steering, you are the motor driving the steering pump.

Most sterndrives have power steering, and they are typically set up for about 2.5 turns of the wheel lock-to-lock. I find that 2.5 to 3.5 turns lock to lock is ideal for the average planing hull, producing good steering responsiveness with just a quarter- or half-turn of the wheel. Anything more than four turns lock-to-lock, and the response time slows dramatically. It becomes a safety issue if you find yourself unable to avoid objects spotted directly ahead, as well as making you work harder to keep the boat on course. Fewer than two turns, and the boat may well turn too fast, or heel excessively and throw your passengers around.

If you get on a new boat that has a suicide knob on the wheel installed by the builder, it’s likely to have a manual steering system with too many turns. Such a boat really needs power steering. Any outboard-powered boat with more than 200 hp arguably needs power steering. Any inboard boat that takes more than four turns lock-to-lock probably needs it, too, or it needs better-balanced rudders. The faster a boat is capable of cruising, the more responsive the steering should be.

Any inboard rudder ought to turn a full 35 degrees to both sides for a total 70-degree arc of travel. Any less, and the boat’s turning rate will suffer; much more than 35 degrees, depending on the rudder’s cross section, and the rudder will stall. A 30- to 50-foot boat should turn 360 degrees at cruising speed in no more than 45 seconds; the best-handling boats in this size range will do so in less than 30 seconds. If it takes longer, it’s likely the rudders don’t turn 35 degrees and/or they’re too small.

Eric Sorensen was 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 has spent 40 years operating charter and commercial fishing boats, Coast Guard vessels and Navy ships and patrol boats.