Thursday, September 11, 2008

Sailing innovation: the planing hull

Tillerman via his blog: Proper course, has given a group writting challange on what we believe is the greatest innovation for sailing.

The greatest innovation in the sport of sailing is in my opinion the planing hull. Although not the inventor (that goes to Cmdr R Munroe in 1898) Uffa Fox, in 1928 designed his International 14 with a planing hull and in one season won 52 races, 2 2nds and 3 3rds out of 57 races started. Thus started the revolution in sailing dinghy hull design. Today from dinghies to open day boats to ocean racers, any boat designed for speed has a hull designed to plane.

Why is a planing hull so important? There are two main reasons: speed! and thrills!

Prior to the development of the planing hull boat speed was limited by the waterline length of the boat with a formular thus Speed (in Knots) = Sqrt of hull length (in feet) times 1.34.

This 'displacement' hull produced a bow wave and a stern wave which, a bit like the sound barrier for subsonic Aeroplanes limits its maximum speed. And just as an Aeroplane needs to break the sound barrier to reach supersonic speeds so a hull needs to overtake its bow wave to travel faster.

Thus by designing a hull with a V entry bow section and a flat beam and stern section the hull, with enough drive power and the correct technique, can lift over its bow wave and accelerate to greater speeds than its displacement speed.

The hull is no longer supported by bouyance but rather by the water pressure created by its foward speed and so there is less of the boat in the water and thus less drag . Thus the boat goes faster, creates its own apparent wind which increases the power from the sails and in turn produces even more speed.

With skill (and thats the tricky bit) the speed increases until the the remaining hull drag balances out the driving force of the sails.

The danger is that a mistake can result in this precarious balance suddely going wrong and the result is quite likely to be a spectacular capsize:

All this adds up to real hair rasing, trail blazing, wet arse fun! Just like last weekend!

The key ingredient to getting a boat to plane is the correct hull trim by moving crew weight aft to lift the bow and keeping the hull upright so that the drive from the sail isn't pushing the boat into the water but lifting it and driving it foward. Maximise the sail trim to get optimum power, a common fault is over sheeting the sails.

The rules say that you are allowed to pump the sail once for every gust or wave so a well timed pump can give the boat the extra kick to get it over its bow wave and planning. Waves can be your friends too giving you the extra speed to get planing. The trick is to get the boat surfing on the wave front, this increases your speed and by triming the sails in time you can get the hull to plane and sustain the 'surf' and by catching sucsessive waves you can keep the boat planing a long time.

Once the boat is planing you may need to trim in the sails to the new apparent wind and maybe even bare away to keep the boat upright and the sails pulling. Avoid sharp helm movements as this will slow the boat and then you risk a broach and even breaking the rudder blade, try to hull trim to steer. If the wind dies, head up to keep the apparent wind and keep the boat moving as long as possible in the plane. Avoid hitting waves but try to steer through the troughs and gaps in the waves.

1 comment:

Tillerman said...

Good one Tim. I hadn't thought of this but you are so right. How different dinghy sailing must have been before this invention!