“Hull Speed” is a commonly used colloquial term that refers to an imaginary speed beyond which it implies a hull cannot move through the water. In fact, there is no such actual limit in physics; i.e., no physical limit on how fast a hull can move through the water. There are, however, very real practical limitations in the “real world.” So, be aware of the distinction the theoretical and the practical when you hear this term in conversation.
Hull speed calculations are based on hull shape, waterline length, propulsion horsepower and the displacement of the boat. The math is extremely complex, but the conditions are within all boater’s experience and observations, which makes discussing and visualizing them fairly easy.
When a hull starts moving through the water, it begins to make a wake. The physical distance between the waves of the wake is small at slow speeds. As the hull moves faster and faster through the water, the waves of the wake get farther and farther apart. Finally, at some speed of the hull through the water, the waves will be separated by the length of the hull at the waterline. That condition – speed at which hull length equals wake wavelength – is the practical definition of “hull speed.”
This is a very over-simplified description of a very complex physical phenomena, but for those of us who are not math majors, it’ll do.
Hull speed is not an actual, physical limit to how fast the hull can be driven through the water, but this factoid is cold comfort in the practical world where sailors and cruisers live. That’s because to make the hull go faster through the water, enough energy is required to make it climb its own bow wave. Achieving that is extremely energy inefficient, and unbelievably expensive. Hull type can be displacement, semi-displacement or planing, with lots of variability within those general categories. For displacement hulls, its virtually impossible to move through the water above the speed at which hull length equals wake wavelength. For semi-displacement hulls with deep vee bows, extremely unlikely. For semi-displacement hulls with shallow bows, possible, but expensive. For planing hulls with minimal bow deadrise, not to hard. The math for calculating hull speed is very complex.
For displacement and semi-displacement hulls – like those of the Monk – there is a formula that approximates the constellation of variables. The formula is <Hull Speed =1.34*LWL^(1/2)> . In English, that’s 1.34 times the square root of the water line length. For Monks, it’s 1.34 times the square root of 33 feet, or 7.7 knots.
What that means for a Monk is that once you get the hull to 7.7 knots, it’s extremely energy inefficient to try to drive the hull through the water any faster. The rate of energy consumption increases as the cube of the speed (or there abouts; a steep, non-linear curve), so it’s very expensive in fuel and energy dispersal to try to make the hull go faster than it’s wake wavelength speed.
Remember, this discussion is about a theoretical speed calculated by Naval Architects for driving a particular hull shape through calm, still water. It has little practical value in our real world. If you’re going down stream in your Monk on the Mississippi River, you may be making only 7.7 knots through the water, but with a 4.0 knot following current, you may well see 11.7 knots SPEED OVER GROUND. However, you’re not exceeding “hull speed” THROUGH THE WATER. If you’re going upstream against a 4.5 knot current into the Lachine Rapids of the St. Lawrence River at Montreal, you may be making 7.7 knots through the water, but may only see 3.2 knots SPEED OVER GROUND.
This phenomena is also what you see when surfing down the face of a large wave at an ocean inlet. The real problem comes in the trough of the wave, when the stern wants to keep going at 11.7 kts but the bow wants to slow to 3.2 kts. That confluence of forces produces a net rotational vector on the hull that causes the boat to turn quickly to one side or the other. If that vector is sufficiently large, the boat will broach. Broaching will be, at a minimum, very, very disconcerting! Broaching can be fatal to the boat (capsize). Bad! To be avoided!