Types of Hulls - 1 : The Planing Hull

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Imagine making a sumo wrestler run a 100 meter-sprint...even the thought is hilarious and probably scary.

But why talk about sumo wrestlers running on blog a about boating!? - Simple...since boats have a body too - the HULL. And hulls are not only meant to keep the boat afloat, but their shapes and types have a reason too.


In our Types of Hull series, we are going to discuss only a drop-worth of what different kinds of hulls are about...the first being the Planing Hull.

With most of the boats in the harbour at the moment having these kind of hulls, it comes first on our list.

WHY 'PLANING' ?

The term 'planing' is given to any craft which is predominantly supported by hydrodynamic lift, rather than hydrostatic lift (buoyancy).

Simply put, it means that the boats weight is supported when in motion by the pressure of the water surface.


Planing allows the boat to go faster by using its speed and hull shape to lift the front part of the hull out of the water. The boat travels on top of the water, greatly reducing the hydrodynamic drag on the vessel.

The increase in aerodynamic drag is small by comparison, and can be compensated for by the increased power from the sails due to the faster speed of the craft, and by the crew trimming the sails.

HOW DOES IT WORK?

When a planing hull is either not moving or going very slowly it works like a displacement hull. However, by adding lots of horsepower, a planing hull will rise up over its bow-wave, causing the boat to displace much less water. As a result, there is much less wetted surface on the hull bottom, meaning less friction as well. The speed of the boat will now increase at a great rate. With this hull, the more horsepower you add, the faster the boat will go.


However, this great speed comes with some big downsides. The boat is now skimming over the waves which is great in flat water, but even in a mild chop the pounding and vibration become very severe. (ASK US, WE WENT FROM MUMBAI TO GOA...in 'slightly' choppy waters!)


At best, this is just uncomfortable for crew and passengers, but worse is that it greatly adds to wear and tear on the yacht. If the yacht slows down and reverts to displacement mode, the flat bottom of a planing hull and short waterline length make for a rolling and pitching ride.

The same can be said for the ride at anchor. The flat bottom of a planing hull also means that the yacht suffers greatly from windage, and so can be hard work to berth, even if fitted with bow and stern thrusters.

HOW TO GO FAST WITH PLANING HULLS ?

Again, its simple...MORE HORSEPOWER !

High speed planing yachts are often combined with Arnesen surface drives for yet more speed, which makes the captain's job even harder when manouevring at low speed.

And it is nigh on impossible to combine a planing hull with stabilisers because of the drag they cause.

Planing hulls are generally used by smaller dayboats, but some fairly large open yachts are designed to be fully planing hulls.