A boat’s bottom and center of gravity play a big role in how well it will perform
A boat’s bottom and center of gravity play a big role in how well it will perform
Last month we considered a few of the things to look for on a prepurchase sea trial — how you can’t have both speed and interior space, the impact of hull shape on head-sea ride and down-sea course-keeping, and steering responsiveness. So what else should be on the sea trial agenda? Let’s have a look at what makes a boat wet or dry and walk down a few abutting alleys while we’re at it.
Hull shape and appendages
One of the things people want to know about boats they’re considering is how dry — or wet — it is under way. Of course, a sea trial is the best time to find out for sure, assuming wind and sea conditions are right to put this attribute to the test. In the meantime, it may help to consider that how dry a boat runs is, besides relative wind and sea state, mostly a function of three things: the shape of the hull and appendages such as spray rails and chine flats below the waterline, the boat’s center of gravity and how fast the boat is being run.
What makes one boat wetter than the next? All else being equal, a boat with convex sections tends to be smoother-riding but wetter, since even a very slight curvature in the surface of the hull spreads out wave impact over just a few more milliseconds. A boat with concave sections, which trap rather than dissipate wave impact, tends to be dryer but harder-riding.
The inverted bell-shaped bottom (viewed in section — imagine slicing the boat like it’s a loaf of bread) isn’t a bad way to go, with convexity in the bottom half of the hull near the keel to smooth out the ride and a little concavity up near the chines to deflect spray outward. In the real world most planing hulls are straight from keel to chine, which isn’t a bad compromise. But whatever the shape, it’s the hull below the waterline that largely determines how wet the boat will be, not the flare above.
With semidisplacement boats that cruise up to 18 knots or so bow flare makes a difference, since if there’s enough of it pronounced flare will catch some of the spray before the wind can blow it back onto the topsides. The reason flare helps with slower boats is that most of the bow is in the water, even at cruise speed, so the spray is generated farther forward, giving it a better chance of making it to your windshield. For this reason, some of the wettest hulls on the planet are semidisplacement boats driven hard, like an old Grand Banks 42 running at 14 knots with a stiff relative wind 40 degrees on the bow. Not good.
With planing hulls running at 25 to 30 knots or more, bow flare makes much less of a difference, since the forward third of the hull is out of the water most of the time, and the boat has a better chance of driving past the spray before the wind can catch it and blow it at you. When the spray does get blown up and toward the 30-knot boat, most of it lands well abaft the windshield area, or even abaft the stern. That brings us back to the point that for any boat moving at planing speeds, it’s the bottom of the boat, not flare, that has the biggest impact on dryness.
So when you see a North Carolina-built sportfisherman with its exaggerated bow flare, like an older Buddy Davis, don’t think dry; think pretty, at least if you like the look. If anything’s amazing about those boats it’s that they were originally built of wood, considering the tortuous twists and turns those sheer strakes were subjected to complying with that shape. And I, for one, wouldn’t want to stuff a North Carolina bow in a following sea, since the deck at the bow is so full in plan view (viewed from above) — talk about a show-stopper. Better to be on a boat with a finer bow at the sheer in those conditions.
In addition to the cross-section shape of the hull, those appendages mentioned earlier also influence a boat’s dryness. Semidisplacement hulls, like a Down East lobster boat with round bilges and full keel, often have spray rails that extend from a foot or two above the waterline at the stem, then aft and downward until they’re below the waterline a third of the way to the stern. These semidisplacement-hull spray rails act like planing-hull hard chines, separating water flow from the hull and adding lift. In the process, they help a lot in keeping a round bilge boat running dryer, since that’s the only way for the water and spray to be deflected down and away from the hull. Planing hulls, on the other hand, usually have hard chines with chine flats as well as running strakes below the chines that knock spray down and out to the sides. Keeping the spray low is key to keeping the wind from getting underneath it and blowing it up on deck before the boat has gone by.
Some boatbuilders get carried away with these appendages, however. One thing to look for when the boat is out of the water is the width and down-angle of the chine flats — too much of either isn’t good, since a wide chine flat can make an otherwise good boat pound unmercifully. You can have a great hull design, then add a set of overly wide chines or strakes to it and beat yourself senseless offshore. The shape of the chines (and lifting strakes) in cross section also is important. Whether the chine flats meet the hull at a hard inside corner or are gently radiused makes a difference, in my experience. A gentle curve tends to deflect spray down and out more effectively, and pound less, than a hard corner. The same goes for the lower inside corner of the running strakes on the bottom.
Other than hull form below the chines, appendages and speed, what else affects how dry a boat will be? The short answer is trim, or running angle. That’s because a bow-down hull tends to be wet, since the hull meets the waves and generates spray farther forward, putting you into a semidisplacement scenario. Running bow-up will keep you dryer, but you’ll also ride harder because of the more acute angle at which the hull hits the waves.
The trim at which your boat runs depends on two things: the lifting force of the water (both buoyancy and dynamic lift) and gravity. From a hull’s perspective, there are two kinds of lift: the buoyancy created by any immersed object at rest or up to displacement speeds, and dynamic lift created by water flow once the hull gets moving fast enough to get up on plane. The center of lift is determined by hull shape and trim, and the location of this center of lift in relation to the vessel’s center of gravity is what determines trim.
This point about weight distribution is hard to overemphasize when it comes to planing vessel performance. Every boat has a center of gravity (CG), the exact three-dimensional point about which all the weights (solid, liquid, cargo, passengers, etc.) in the boat can be considered to be concentrated. What matters to a boat’s trim is fore-and-aft, or longitudinal, CG. If CG shifts forward, the bow sinks and the stern rises. If CG is farther forward than it should be, the bow will be deeper in the water, which, as discussed earlier, makes the boat wetter. Being trimmed by the bow also makes a boat slower, since the increased wetted surface adds frictional drag.
On the other hand if CG is shifted aft, the bow rises and the stern sinks, which makes the boat run dryer, since wave impact — and spray generation — takes place farther aft along the bottom. This makes it more likely you’ll be able to drive past the spray before it gets a chance to make its way up to the windshield. Of course, as mentioned, the hull will ride harder, since wave impact (slamming loads) becomes more pronounced as the bow rises and trim increases.
It’s important for the designer and boatbuilder to get the center of gravity and center of lift situated correctly in relation to each other so they produce the desired running trim. For a planing hull with constant deadrise in the aft half of the hull, which includes most deep-vees, trim at cruise speed should be 4 to 6 degrees, a little less for a boat with a warped bottom design, which Hunt, for example, favors for a number of reasons.
(A warped bottom has constantly decreasing deadrise as you move aft; think of it as the chines running downhill slightly but continuously as you move toward the stern — more on that in another column.) This 4- to 6-degree trim works well in a constant-deadrise hull because the bottom is at just the right angle in relation to the water it’s passing through and over to generate the right amount of lift to plane properly and “fly” on the surface. Running through the water at the proper angle puts the center of lift in the right place in relation to the boat’s CG. Plus, the wetted surface is optimized for best performance, which involves a balancing act between ride quality (bow down) and resistance (bow up).
In short, to have a dry boat you’ll need an appropriate hull shape below the chines; well-placed and proportioned spray rails, reverse chines and strakes; and proper weight distribution so the boat runs at its proper trim. Flare helps with a 16-knot semidisplacement vessel, but it won’t make much difference at higher planing speeds in a hard-chine boat. Flare does, however, give a boat the curves it needs to look good.
Fore-and-aft (and vertical) weight distribution has a number of ramifications for matters near and dear to a boater’s heart, other than dryness, as we’ll see next month. Course-keeping, helm visibility and hump speed are a few things we’ll look at. Stay tuned.
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 has spent 40 years operating charter and commercial fishing boats, Coast Guard vessels, and Navy ships and patrol boats.