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The Pressures Working Against Your Hull

By: Captain Bill Jennings

Boating is fun, but every time you go boating, you are under a lot of 'pressure.' Yep - the pressure point of gravity, the buoyancy pressure point, the pressure point of balance, the hydrodynamic pressure point and the aerodynamic pressure point. It's hard to imagine that you can still have fun while dealing with all of these pressures. In reality, most boaters do a good job of handling all this by just using their 'seat of their pants' observations. Today, we are going to look at what these pressures really are and how to handle them using the controls in your boat.

The boat's center of gravity (CG) is the point where the boat and everything in it is concentrated (focused), in the downward force of gravity. While the position of the CG pressure point will vary with boat design, naval architects agree that for most planing boats it is between 28 to 35 percent of the hull length, measured forward from the transom, or just aft of midships. This is where you should find the best performance, handling and fuel economy.

The boat's center of buoyancy (CB) is where the upward (floating) forces are focused. In other words, the balance point where if you set the boat and all its contents on a pointed pylon, the boat would balance perfectly side to side as well as fore and aft. Technically speaking, the Center of Buoyancy is the center of the gravity of the volume of water which the boat displaces and it should line up vertically under the CG when the boat is at rest.

When an upward CB force moves away from the downward CG force, the two forces create a rotational force that is working in the same direction. This means that if extra weight is added to one side of your boat, the center of buoyancy will move to a position where the buoyancy and gravitation force create a 'moment' in the same rotational direction and the hull can capsize.

We all know that if a very heavy passenger chooses the wrong seat, it will change the balance and the boat will lean. If the weight is too far forward, the vessel will sit slightly bow down. Running a bow heavy boat will make it harder to

steer and it will not achieve its potential top speed. With too much weight aft, the boat will be difficult to lift on plane and exhibit excessive bowrise as it accelerates. The most annoying affect of a heavy rear end is porpoising, where the bow continually rises and falls while cruising.

Where is the optimum center of gravity located on your boat? How can we operate our boat in a properly balanced manner? The first step is to figure out if you need to change anything, and if you do, which way to go. To check the fore-aft balance point on your boat, look at your boat from the side when it is at rest and carrying a normal load. This simple visual observation can often tell you if the boat, from a fore and aft perspective, is in balance. By observing the boat from the stern, you can examine any listing at the transom. If either fore-aft or lateral balance appears to be wrong you can make adjustments. There are two ways that we can adjust weight to achieve the best CG and CB. The first is to move the location of items within the boat and the second is through the use of your boat's controls.

Achieving an optimum CG and CB on your boat can be as easy as moving things around. Within your boat there are many options to adjust weight, from removing unnecessary items and varying fuel loads, to moving batteries and trim pumps. The bigger the boat, the more heavy items you bring on board, so if you own a large boat, you will be surprised at how much weight you can shed by cleaning house. Ballast tanks are also an option in some cases, but of course this would add unwanted weight. Extension boxes represent an excellent method of adjusting your CG. These boxes will move your outboard or drives back around one foot and significantly change your CG. But even just considering the effects of placement and quantity when loading passengers and fuel will help prevent imbalance.

Once you have gone through the list of possible physical changes to your boat, your next line of defense is to operate the available controls to your advantage. For stern drives and outboards, the most important control is "trim." Every action where you are coming onto plane begins with "lowering" the trim to it's bottom position. With trim down, and you add power, your prop thrust adds lift to the aft portion of your boat, which effectively moves the center of balance aft. This levers the front of the boat over the boat's center of gravity and the bow stays down. With the hull's lifting strakes on the force of the boat biting the water, the hull transitions more easily onto plane. Once on plane, you trim the prop thrust out and the

reverse takes place. The stern of the boat drops, causing the bow to lever over your balance point in an upwards direction, resulting in less boat in the water. This means you have less drag and better performance. All controlled with a tiny

toggle switch. To keep the boat in the most advantageous position over the center of balance, a keen boater will be constantly making small adjustments with the trim button as speeds and water conditions vary. If your boat does not have

power steering, you can determine the optimum trim setting as the point where there is very little torque felt in the steering. If you trim out too far, reducing the length of boat in contact with the water, the bow will want to continuously rise and fall -- called porpoising. This could also cause the boat to dangerously 'rock' from side to side -- called chine walking.

Then there are "tabs". These are the two plate like objects that project off the base of a transom. They can be raised and lowered, either by a mechanical linkage or by hydraulic rams controlled by two switches at the helm. By lowering tabs independently, you move the center of buoyancy and can raise either side of the boat to level the ride. Using the oversized passenger on one side example, we can see how lowering a tab on the low side will lift the boat on that side in order to restore the boat to an even keel. When coming onto plane, lowering tabs will reduce the time and distance required. In rough water both tabs can be lowered slightly, tipping more of the hull into the water to add control.

Boats with inboard engines may have what is called a "cavitation plate." These plates usually run full width across the base of the transom and may be fixed or adjustable. Again, we are working with CG and CB. Moving this plate from a level

position down into the water creates upward pressure to the stern of the boat, and just as with hydraulic drive trim, moves the center of balance aft, changing the boat's attitude to run flatter in the water. They are not as popular as adjustable tabs because they do not provide lateral adjustment.

Your ever popular throttle can also be used to help your boat achieve ideal balance. Hard acceleration will increase prop thrust and the bow will rise. Throttle back and the bow will drop. Use this to your advantage when encountering wakes or large waves.

While CG and CB are two critical pressure points, there are more pressure points that we should identify. Both aerodynamic and hydrodynamic forces will have a center of pressure. Hydrodynamic forces have always been considered by designers. Simple proof is the fact that boats have almost always had a pointed bow or cutwater to slice rather than pound the water. The point where a boat pushes itself into water is a most obvious hydrodynamic pressure point. Interestingly, both aerodynamic and hydrodynamic pressures increase with speed. When at rest, relative to the water, a boat will not experience any pressure or drag. Both of these forces create a resistance force as the boat gets moving. For a boat running through water, it seems that this resistance or "parasitic drag" is proportional to the square of the speed. That means that increasing the speed of a boat requires disproportionally large increases in horsepower.

There are many components of parasitic drag, including surface roughness, shape, boundary layer separation and more. These are things that a boat owner cannot easily change himself, so boaters must look for these features when buying their boat. Recently, boat designers have been finding new and exciting ways to improve performance by modifying and reducing the negative effects of aerodynamic and hydrodynamic pressure points. New shapes and high tech materials can reduce the negative impact of both aerodynamic and hydrodynamic forces. We see stiffer composite materials and changes to the way strakes add lift. We now know that a simple thing like spaces between 'add-ons' to the hull, such as tabs, will increase drag. Under the hull, we find a good number of boats molding "steps" into the hull to introduce air bubbles into the water supporting the boat, thereby reducing the friction that creates hydrodynamic drag. Topside, designers are adding rake to a boat's windshield or shaping a foredeck to create added vertical lift to reduce aerodynamic pressure drag.

Understanding pressure points can help you operate your boat efficiently. You recognize that as equipment is brought on board and fuel and passenger loads vary, you should keep your pressure points in mind and position objects and passengers to obtain the best overall balance.

You can look at YouTube videos on this subject until you go blind, but the basics pressures that I have explained describe what happens every time you take your boat out for a Sunday drive. But, is understanding all of this theory really necessary when it comes to driving your boat? Maybe not, but it is nice to know what you are actually doing when working your controls. It takes some of the pressure off.

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