Nautical Expert

Motorboat magazine

Planing, displacement and semi-displacement yachts. Explanation without formulas

Planing, displacement and semi-displacement yachts

Updated on September 9th, 2023

The pebble that you threw into the lake or into the sea as a child, making it jump on the water surface (stone skipping), is nothing more than a planing mode. Of course, you know that in order for the stone to do this, it must be fairly flat, have a high flight speed and an optimal angle of attack, about 20 degrees.

The flat surface of the stone, which is in contact with the water surface, is similar to the bottom of a motor boat with low deadrise, which is a prerequisite for planing. It is obvious that an absolutely flat boat bottom, with zero deadrise, has less drag, more lift and, therefore, the highest coefficient for the planing mode.

Planing mode of the motorboat

Planing yachts

However, moving on a completely flat-bottomed boat will shake your brain quite well even with weak waves, and besides, it is fraught with the destruction of the hull due to strong shock loads. The seaworthiness and handling of such a boat, of course, will be disgusting. But the engine power to switch to planing mode will require a minimum.

On large water bodies, where at least small waves are present most of the time, you will have to choose a compromise: a boat with variable deadrise. This is a bottom with a V-shaped bow, smoothly turning into a flatter surface at the transom. Such a hull can significantly increase seaworthiness and reduce shock loads when passing through a wave, while maintaining the possibility of planing. At the same time, the aft part of the bottom cannot be made completely flat, as this will greatly increase the yaw.

Planing yacht bottom

Planing is a mode when the hull of the boat stops floating due to only the Archimedes force of buoyancy. When the required speed is reached, the hull begins to support the oncoming flow of water and, to some extent, air. That is, it is supported by hydrodynamic forces, and the value of the Archimedes force in this case is significantly reduced.

Remember stone. When he jumps over the surface of the water, the buoyancy force of Archimedes is extremely small. In a static state, the pebble cannot float on the surface of the water. The planing effect helps pilots make an emergency landing on the water.

Planing hull with redans

Planing yacht hulls often have transverse redans – ledges on the bottom to reduce the wetted area and cut off excess water, as well as stern plates – to stabilize the boat and reduce excessive trim.

If you are looking to buy a new outboard motor for your tender or RIB but are unsure of the minimum power required for planing, there is a simple calculation. For every 25 kg of boat weight, 1 hp is required. Please note that this refers to the total weight of the boat, including the motor, skipper and beer. At the same time, if the tender has a pronounced deadrise, then it is necessary to reduce the estimated weight to 20-22 kg.

In other words, to achieve the planing effect, it is necessary that the boat has a sufficiently high power-to-weight ratio – at least 30 hp per ton of gross weight of the boat and a special hull shape.

Of course, do not forget that a lot depends on the parameters of the propellers, which is a separate large and interesting topic.

Displacement yachts

Deadrise has its limit, after which the hull of a motor yacht ceases to be planing. A yacht with displacement lines has a much greater deadrise throughout the entire bottom, and the chines in the bow are smoother. The absence of flat surfaces does not allow such a boat to enter the planing mode. Therefore, such a hull has to push the water mass in front of it, and not fly over it. And, although a displacement yacht has a low speed limited by the ratio of its length at the waterline to the width, seaworthiness, efficiency, safety and cruising range increase incredibly.

Displacement motor yacht hull

The speed limit is related to the wave formation process. Surprisingly, a megayacht and a small displacement fishing boat, having similar proportions and hull contours, when moving at the same speed, form the same wavelength. As the speed increases, the wavelength also increases. Given the length of the hull of a megayacht, one can imagine how many such waves can be along it. That practically does not affect the speed and power consumption. But the size of a fishing boat at some point may turn out to be less than the wavelength that it itself forms, and it turns out that the boat is between two waves, at their soles.

Increasing the speed will not help in this case. This will only lead to an increase in the height of the bow wave and a sharp increase in fuel consumption. A boat of the same length, but less width, could go faster, since its hull would form transverse waves of a lower height.

Displacement superyacht

In addition to increasing the length-to-width ratio, one way to reduce this wave drag on a displacement hull is with a bow bulb located below the waterline. When the yacht moves, the bulb creates its own additional wave, which partially dampens the wave created by the hull.

At the same time, if the boat had planing bottom contours and sufficient engine power, it could pass over the crest of the bow wave. So the boat would switch from displacement mode to planing mode.

To overcome the bow wave somewhat more power is required than to maintain the planing mode. Therefore, the transient mode consumes much more fuel. But after overcoming it, excess gas should be dumped and switched to cruising mode.

Catamarans

A catamaran, from the point of view of hydrodynamics, should be considered not as a single vessel, but as two separate ones. Each hull is displacement. But you will notice that these hulls have a very high length to width ratio, which allows them to move with minimal wave formation and drag. This is what determines the high speed and efficiency of multihulls.

Semi-displacement (semi-planing) yachts

Today this is a very popular solution for motor yachts, but it is not suitable for all hull sizes. Trying to get the best of both worlds, as always, is full of compromises, but meets the basic requirements of buyers. These boats feel good in rough seas, and in good weather they are able to reach high speed.

Semi-displacement (semi-planing) yachts

At its core, to get this kind of hull, you simply take the bow of a displacement boat and connect it to the stern of a planing boat. Smooth chines and deep deadrise, turning into an almost horizontal plane of the stern, provide a wide range of speeds. That being said, you are guaranteed a good autonomy if you stay in displacement mode.

Important parameters

Speed is fun, but let’s look at what interests us much more, namely range and economy.

The cruising range, of course, strongly depends not only on the technical features, but also on the size of the yacht. However, planing boats with a range of only 250-500 nautical miles clearly do not claim to be the most autonomous. Semi-displacement yachts can achieve much better cruising ranges of up to 1000 NM or more if you don’t push the throttle too hard. Displacement yachts will leave everyone far behind if there are no filling stations along the way, because they are able to remain autonomous at incredible distances from 2500 to 8000 NM and more.

Average power consumption for planing yachts is 60 hp/t or more. Semiplaning boats have an average of 10-40 hp/t. And here, displacement yachts win everyone not only with incredible seaworthiness, but also with an average power consumption of 5 hp/t. It becomes clear how they manage to achieve a range of several thousand nautical miles, and why we love trawlers so much.