JayDee
Yes, waterline length has a lot to do with boat speed. Yur example of half a knot represents a 20% speed difference for a boat that is sailing 2.5 knots, so that is a pretty extreme example. By cmparison, the IACC designers will spend $1 million for a design idea that is good for 0.2 knots for a boat with a theoretical hull speed of 11.5 knots, so there you are talking about less than 2% increase.
For a one meter boat, a 2% speed increase is .05 knots. To get that kind of speed increase from just waterline length, you need to increase the length of the boat by about 1.5 inches.
That is why the hull length is so tightly regulated.
But getting more to your real question does it matter how you displace the water. Yes, of course it does! Take a look at the picture of my fairwind again. You can see that the displacement wave is quite short. The bow wave crest is bck pretty far from the bow and the stern wave crest is forward pretty far from the stern. Thus, the effective length of the displacement wave for that boat is much shorter than the waterline length of the boat. Thus this boat is speed limited to well below it’s theoretical speed by that very short wave. THis is because the displacement distribution of the boat is concentrated near the keel. The keel itself is perhaps an inch thick t its thickest point. This causes a lot of displacement in that area of the boat. Thus, this is a very inefficient design.
A higher performance displacement hull would have the bow wave much closer to the bow and the stern wave riding right at the stern. In fact the overhangs on the classic boats (Js and old meter boats) were designed to do exactly this and move the waves further apart as the boat heeled.
Several factors in hull design will influence this effect. Rcker and prismatic coefficient are two obvious ones. But there are other tricks designers paly with the shape of the hull to smooth out the displacement and move the waves toward the ends of the boat.
In aircraft design in the 1960s there was a theory known as area ruling which creasted the characteristic “coke bottle” fusalage shapes. The idea here was that the cross sectional area of the fusalage as a function of length should follow a nice smooth parabolic curve. They had discovered that sharp increases in the cross sectional area at any point along the fusalage would cause shock waves to form at speeds near the speed of sound. By using the area rule, they could delay the onset of those shocks until closer to Mach 1.
Bow and stern waves on a sailboat are very similar to shock waves on an aircraft, so in a simplistic way, the area rule applies to hull design as well. There are some big differences between a free surface wave and a supersonic shock wave, but there are also some important parallels.
One important design element that we have not really addressed in this thread is total displacement. If you can reduce the total displacement of the boat, you can reduce the size of these waves and make it easier for the hull to plane. In full size boats (and model boats to a certain degree) designers talk about the sail area to displacement ratio. That ratio measures the available power (sail area) relative to the wave making resistance (displacement). Planing hulls like dinghies, scows and skiffs have very high SA/Disp ratios. Thus, they create only small waves and have plenty of power to push through them and get planing. With a displacement boat, the displacement is neded to create righting moment so you cannot go as light as a dinghy or skiff. But you can cheat that equation as well by simply moving the ballast lower (increasing the righting moment arm length).
This is why model boat do so well at exceeding their teoretical hull speed. THe keels on model boats are generally much deeper on a scale basis than their full sized cousins. This allows them to create more ighting moment frm less ballast and therefore reduce their displacement and increase their sail area to displacement ratios. Thus high performance Model boats (IOM, US1M, 36/600, M, etc.) create very small waves and have plenty of power to push through those waves. This is why most of those designs can exceed their theoretical hull speeds quite regularly.
- Will
Will Gorgen
wavespeed.JPG