Why Does a Boat Plane?
In technical terms, planing means that your boat is being supported by hydrodynamic lift instead of buoyancy. Boats typically, but not always, rely on one or the other when in motion. Buoyancy, or hydrostatic lift, is what keeps you afloat in a displacement hull vessel. But a planing hull relies on hydrodynamic lift instead. When a planing vessel achieves enough speed, this lift is the dominant force keeping it up. The effect is that the vessel is riding across the surface of the water or planing. Let’s take a look at the difference between displacement and planing hulls.
What is a Displacement Hull?
A displacement boat is designed to glide through the water smoothly with a minimum of power (like a canoe with oars or a sailboat under sail). Generally, these boats are very stable and ride smoothly. Larger displacement vessels were also designed to efficiently and safely carry lots of cargo or people. Displacement speed isn’t on par with planing speed, but doesn’t need to be.
The name refers to water displaced by the hull. Think of the hold of any ship, and even the bottom of your canoe below the waterline. That empty space in your boat displaced an equal amount of water. The vessel must be balanced and secure to remain afloat as a result.
Displacement hulls are the most common and the oldest types of boats. Planing hulls did not become feasible until we had the technology to make them. So all older style boats, the ones that existed before engines, were displacement hulls.
The displacement boat has smooth lines and curves that gently move water out of its way, both sideways and down. Because the water moves gradually, there is less resistance against the boat. The general shape of a displacement hull is like the letter V when you look at it from the front. It cuts through the water at the narrow point at the bottom, then widens out to sit in the water as it reaches the top.
When a boat begins to move faster, there will be more resistance. You can experience this yourself by simply putting your hand into the water. Slow and smooth meets with little to no resistance. But the faster you do it, and the more surface area, the more the water resists and pushes back. This is why a belly flop can hurt so much. A lot of surface area hitting the water fast and it feels like you just jumped flat on the ground.
What is a Planing Boat?
Planing boats, on the other hand, are designed to rise up on top of the water. They can go very fast, but need more power to get up on top of the water. The heavier the boat, the more power required to get it “on plane.”
Because a planing boat moves so fast, the water can’t move around it like it would with a displacement hull. This is similar to when you skip a stone across a still pond. The boat is being met with a lot of resistance and essentially rides on the surface. Unless a V-shaped displacement hull, a planing hull has a much flatter bottom. Again, this is that belly flop principle. If you dive into the water, your hands cut into it like the V in a displacement hull allowing you to smoothly glide right in. But if you belly flop, you’re like a planing hull, hitting flat with a wide surface.
A planing boat can’t go fast all the time, that’s obvious. You need to slow down and of course stop sometimes. In these cases the planing boat operates very much like a displacement boat. Without speed it doesn’t face resistance. Therefore, it no longer planes.
Slow speeds and increased weight can both prevent a planing boat from actually planing. Too much weight puts you at greater risk of tipping or capsizing in a planing boat. The hull isn’t designed to remain afloat as well as a similarly sized displacement hull. You’d need to push your engine much harder to achieve the speed necessary to keep the boat on top of the water, which of course could be dangerous.
This doesn’t mean a planing boat weighs less than a displacement boat. They can weigh exactly the same. In fact, a planing boat could just as easily weigh more. It’s only a matter of how the hull hits the water at speed that separates how they work. The shape and speed will always ensure a planing vessel stays on the surface. Unless something goes wrong, of course.
Planing boats can obviously get you where you’re going very fast. A potential downside is that the ride is often much less smooth. Planing vessels can bump and skip on the water’s surface. If you hit the wake of a larger vessel or waves in a planing boat at speed, it could spell disaster.
It’s also possible to imbalance the weight of a planing boat in a dangerous fashion. If all the weight in a cigarette boat is in the back and you hit speed, the nose could rise right out of the water and flip the boat. If you ever watch speed boat races you’ve probably seen this happen more than once. It’s one of many good reasons to wear your PFD on a boat.
Which Hull Design is Better?
This is a common question for newcomers to boating. But it doesn’t really work like that. These are two different kinds of boats. They do different things and they can do them very well.
If you want to choose between a planing boat and a displacement boat, you just need to know the boats intended for. Do you want a lazy afternoon on a lake fishing? That’s going to be a displacement hull. Do you want to zip around the surface while you and your friends waterski? That’s done much better with a planing hull boat.
How to Get an Outboard Boat on Plane
So far it seems like all you need to do to plane is go fast. Yes and no, however. There’s a little more to it than all that. Every boat, as we said, works as a displacement hull at rest. Buoyancy holds it up and allows it to glide through the water at low speeds. But to get your boat on plane there are certain factors required.
You can’t plane a boat that doesn’t have a planing hull. Try it with a sailboat sometime, it’s not happening. At best, you’ll force the bow down and actually decrease efficiency and speed overall.
There are hybrid hull types that have a bit of a keel but can plane at speed. These require a lot of energy, however. You’ll be pushing the engine and spending money on fuel to make it happen. A true planing hull is what you want to use if your goal is to get your vessel on plane.
This kind of hull may have a small amount of shape at the bow. However, towards the back of the vessel it should be totally flat. They call this flat aft run. The design allows air to get under it when you achieve the proper hull speed, which is necessary for that lift. When all works according to plan, once you achieve speed, the front third of the boat should not even be in the water. The outboard at the back on the transom provides thrust and the front of the boat planes.
A boat needs to have even weight distribution for this to work. If you have six guys hanging off the port side, you’ll be imbalanced and won’t have enough power. Likewise, if everyone sits at the stern you’ll end up being bottom heavy and will likely be unable to work up the power to plane. So make sure everyone is spread evenly.
If you’re in a new boat or trying to plane for the first time, take it easy. Calm waters are best for a first time. No wind, no heavy traffic. Trim the boat motor towards the transom and start off slow and speed up slowly, don’t just gun it.
As speed increases your bow will start to rise. This requires some caution, especially if you’re new to this. As the bow rises, your view becomes obscured. Make sure no one else is around as you practice this.
To master this you need to adjust your trim position. Trim up away from the transom without increasing throttle. This will increase speed but not rpms. You’ll need to fine tune this a bit because at some point you’ll start slowing down. So you want to trim down again at that point. Engines can be finicky so you may need to take some time fine tuning this to your specific vessel. You’re looking for the sweet spot where the bow lifts, but isn’t bouncing.
Adjusting the trim can cause a lot of issues until you get it right. You may raise the bow too high or force the stern down into the water. You may also ventilate the prop by pulling it out of the water. But this is a trial and error process so keep at it until you get it.
Why Do You Want a Boat to Plane?
Once a boat starts planing there’s less resistance. There has to be because less of the boat is in the water. That translates to less drag and you can reach high speeds while expending less fuel.
So the big draw here is that you can travel fast in a planing boat. And that leads directly into the second reason people enjoy planing hulls. It’s fun. Water skiing, racing, all kinds of fun boating sports work best with a planing vessel.
Are There Other Boat Types?
There are also hydrofoil and hovercraft type boats as well. These are less common but do not fit into the category of either displacement or planing.
A hydrofoil boat will often have two or more wings that extend beneath the boat. These are referred to as foils. Large boats such as ferries are often hydrofoil vessels. As the boat picks up speed, the action of these foils through the water produces lift that raises the vessel. This allows for reduced drag and increased speed and a smooth ride.
A hydrofoil is sometimes confused with a hovercraft but these are extremely different vessels. You’ll often see hovercraft in swampy areas where the water would either be impossible to travel in a displacement hull vessel, or at least very difficult. The reason a hovercraft works is because, is the name suggests, it hovers.
A massive fan on the hovercraft produces air that blows downward below the hull. A rubber skirt around the vessel traps the air and creates an effect like blowing up a balloon. The difference in this case is that there’s a boat on top of that balloon so the air creates lift. The hovercraft can then drift over the surface of water and even land, though this can be dangerous, especially if the skirt gets damaged.