Today racing yachts strives for a wave piercing bow. To slice through a wave instead of having to fight your way through it sounds like what a strong man should do.

Racing yacht and cruising yachts are different kind of boats. A racing boat is often built light and driven hard to windward a cruising boat is heavier and the sailing is more relaxt, its course is mostly downwind.

A cruising boats max speed or hull speed happens when the bow wave and the stern wave join each other and become one, creating a trough so deep that the boat is captured in it and can only travel as fast as the wave. The speed of the wave is the square root of the wavelength times a constant. Thus the longer between the bow wave and the stern wave the faster the boat can sail. Accordingly the earlier a boat can create a bow wave the faster she can sail. That’s why yacht designers give fast boats, boats that travel at a high Froude number a high prismatic coefficient – the prismatic coefficient is how many % of the underwater hull is left after you shaped a prism with the boats midsection and its waterline length. A high prismatic means full end, the opposite of wave piercing. Common numbers for sailing yachts is 0.54 % powerful powerboats more towards 0.7 %

The ocean sailing yacht may not have such a high average speed but out on the ocean there are big waves and in the trough, sailing downwind, there is a counter current often 2 to 3 knots fast if you add that speed to the average speed you are in a high Froude number speed range and need a high prismatic not wave piercing bow.

One more thing sailing fast down a wave and meeting a counter current is to ask for a broach. A spoon bow reduces the risk of broaching.

Further a spoon bow increases a boats initial stability as it makes the boat wider in the bow than the wave piercing bow and it’s the beam integrated over the length of the boat that is the initial stability.

Below two illustrations

A boat at hull speed. The bow wave and stern wave have created trough in wich the displacement boat is trapped.
The spoon bow of Exlex Minor

To be continued…

Regards Yrvind