There's more to getting your outboard boat up on a plane than simply pushing the throttle forward. Here's what you need to know.
Even those new to boating have likely heard of a boat with a "planing" hull, or the phrase "getting a boat up on plane." But what does that mean? When any boat is at rest, the weight of the boat is supported entirely by the water in which it's sitting, known as "static buoyancy." As the boat starts to move forward through the water, speed creates hydrodynamic lift. As more power (and speed) is applied, lift increases, and the boat, in effect, rides over its bow wave, reducing wetted area and thus reducing drag. At this point, the boat is said to be "on a plane" or simply "planing."
How easily an outboard boat will plane is governed by several factors, most importantly, the size and shape of the hull and the power available. Hull design falls into one of three basic categories: displacement, semidisplacement, and planing. A displacement hull, such as a sailboat or trawler-yacht, won't plane regardless of the amount of power supplied. These types may dig in the stern with a lot of power, but this is not planing and is very counterproductive to efficiency.
The hybrid hull in the mix is semidisplacement, which spends most or all of its time in motion at displacement speed. Give it enough power and it will plane, but at a great cost in the amount of fuel consumed.
A true planing hull has little to no keel, and the underside of the boat is almost flat toward the transom. Said to have a "flat run aft," this type of hull is designed for trapping air and creating the necessary lift. If you watch a planing boat in motion, you'll notice that about the front third of the boat will be out of the water. With less boat in the water, there's less drag, and with that reduced drag comes better fuel economy, to a point.
Getting On Plane
The smaller the boat, the more effect crew positioning will have on the boat's ability to plane. If everyone sits on one side, the boat will not ride level; if all the crew rides in the aft cockpit, the stern will be depressed, and getting the boat up onto plane may be impossible. Passengers should be evenly distributed.
Here's how to get your boat on plane:
1. First, practice when the water is calm and there's little wind, so you'll more easily detect how throttle adjustments affect your boat. Start with the engine or sterndrive trimmed all the way in toward the transom, then increase the speed slowly. Don't just jam the throttle forward.
2. The bow will start to rise as the speed increases. Increase the speed until the bow starts to drop as the boat comes up onto a plane. A word of caution: Your view forward may be obscured in this initial stage as the bow rises, so make sure the way ahead is clear before increasing speed.
3. Without advancing the throttle further, trim the drive up, away from the transom. The trim control is normally incorporated into the throttle lever. As you trim up, you will find that the speed will increase even though the rpms stay about the same. Stop trimming or back down just a little when the boat slows. Trimming the engine(s) like this brings the bow up so that the boat is riding over the water and encountering less resistance rather than trying to push the water out of the way.
4. Depending on your rig, you may need to judiciously add trim to assist in the initial stages of getting up. This act calls for a deft touch, so don't add too much trim or the bow will rise too high, the stern may squat in the water, and if the prop is trimmed up too high, the prop may actually start to come out of the water, called "ventilating." The boat may "porpoise" as well, with the bow rising and falling continually, like a porpoise swimming on the surface. You'll also notice the boat will be harder to control, and the speed may even start to decrease if there's too much trim.
5. Listen to the engine as you trim. The sound will probably change. If the exhaust sounds much louder, you may have added too much trim. With careful practice you'll soon discover the sweet spot for your particular boat.
Some Plane Truths
Different boats have different characteristics but, generally, a planing hull is at its most economical cruise when it has just come up on plane. You'll find the need for a fair bit of power to "get over the hump," but once the boat's on plane and you're trimmed properly, you should be able to decrease throttle a little without losing speed.
Waves also play a significant role in planing. Sometimes they may make planing difficult or even dangerous; sometimes planing can help in handling waves. A discussion is beyond the scope of this article, but you should carefully note the effect of waves of different sizes, types, and direction on your boat.