image of sailboat Planning downwind off Brighton
Planning downwind off Brighton

 Planning downwind off Brighton

By William Mills

Hull Speed

Occasionally we hear nautical jargon like displacement and semi displacement hulls and their effect on the speed which the hull travels through the water, or boat speed. What is meant by these terms?

Upwind

When a yacht is sailing to windward its maximum speed is limited by its ability to force, or displace, the seawater in which it is moving through. Both the front and rear of the yacht create a bow and stern wave. The longer the waterline the faster the yacht can go.

The mathematical equation is the square root of the waterline multiplied by 1.4 to 1.7. So for example an X332 has a waterline length of 28.9 feet, the square root of which is 5.35. Multiplied by times 1.4 is 7.5 knots, and by times 1.7 is 9.1 knots. So depending on the condition and configuration of the under water area of the hull, this yacht cannot exceed a speed of around 8.5 knots when sailing in upwind displacement mode.

It is a useful calculation because in very rough airs once the boat reaches hull speed putting up more sails won’t increase this but may well strain the rigging and cause unnecessary heeling.

If we are out sailing and the wind gets stronger and stronger, when hull speed is reached it is time to reduce sail. Many a times a yacht has been on a fine reach in windy weather and reefed their main to discover there is no reduction in speed. More alarming is when having taken the headsail down as well the boat still isn’t going any slower.

If after removing all the sails, we are still going at full hull speed under bare poles it must be very windy indeed! It is also useful to know our hull speed because if we ever get towed by a large lifeboat we can tell them our max speed is not more than 8.5 knots.

Semi-displacement

Firefox at Sussex Yacht Club-the white underneath is the part submerged
Firefox at Sussex Yacht Club-the white underneath is the part submerged

Someone once said some yachts can never go semi-displacement. All yachts can depending on the force of propulsion. If a yacht had a towing rope attached to a much bigger vessel, say a Royal Navy destroyer capable of 30 knots and more, and the ship started to go quickly, and assuming the tow didn’t break, the yacht would follow along behind.

If would become rather like a child pulling a toy boat on a piece of string. it would bounce along on top of the water similar to a coin or flat stone being skimmed across a still pond.

If the force is sufficient, and an extreme example would be bolting on a solid rocket booster, the boat will seek the point of least resistance. It will go airborne.

When a yacht is sailing down wind, if the mast is high enough with sails big enough in strong winds the boat starts to rise up, and lifts up over the water in front of it rather like a seaplane does in WWII movies. This is called surfing or planning. We see fast RIBs doing this all the time.

So an X 332 although restricted in up wind sailing to a hull speed of 8.5 knots, downwind given sufficient sails and wind strength, it will lift up so less of its hull is underwater thereby enabling it to reach boat speeds upwards of 20 knots and more.

The modern America’s Cup boats are at the cutting edge of technology as they try to configure very long hull lengths with ultra lightweight hulls.

The first gives a long waterline for upwind displacement sailing, and the latter coupled with a huge sail area takes on the Jules Verne like properties of a hot air balloon resting on a water ski skimming over the surface and achieving speed in excess of fifty knots!

This in turn leads to arguments as to when does a craft stop being a boat and start becoming a flying machine!

662 words

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