Re: Tartan 34C LWL question.
I apologize that I wrote this for another discussion so it is a bit more wordy and a bit off topic, but it deals with the question about hull speed on a boat like the Tartan 34 which has long overhangs, but moderately full stern sections.
The science on waterline length is much more advanced than it was back when boats were routinely designed with long overhangs. Much of the discussion above does not conform with the current science. While it is true that increasing the length of the waterline will increase the hullspeed of the boat some, it is not true that the waterline length of a boat with long overhangs will increase its boat speed with heel angle to the same extent as it would on a short-overhang boat with the same static waterline length as the long overhang boat when it’s heeled. It has always been known that it would not but now we understand more about why that is true. What long overhangs did was to make a number of obsolete racing rules believe that a boat with a proportionately short static waterline would be a little slower than it was in reality.
To explain, as has been understood since the late 19th century that hullspeed is not a fixed number, but an approximation of a point at which the energy required to propel the boat increases rapidly. As most of us know, the phenomena of hullspeed is created by the bow wave moving closer to the stern wave with speed. The speed of a wave (in knots) is equal to the square root of the wavelength (in feet) multiplied by 1.34 and that is where the well-known hullspeed number comes from.
As the boat moves faster the bow and stern waves move closer together and as they move closer they begin combining. Once they begin to combine, it takes greater energy to climb up the combined heights of the two waves. At some point in the speed range the amount of energy to climb the combined wave height increases sharply, and that increase is so sharp, that at that point it becomes very difficult for a displacement boat to generate enough forward force from its sails to overcome that rapidly increasing drag. That approximate point is hullspeed. In theory, by spreading out the points on the boat at which the bow wave and stern wave are produced, the boat is able to go faster before hitting the point at which the induced drag increases rapidly.
When a boat with long overhangs initially heels, the waterline length increases at both ends of the boat. The further that the beam of the boat is carried into the ends of the boat, the smaller the heel angle at which the waterline increases. Different designers handled this in different ways so if you look at a typical Alberg design, they generally had very full bows to bring the waterlines forward, and if you look at many of Bill Tripp’s CCA designs, they often had full lines aft.
Ignoring the issues associated with heeling that impact comfort, leeway, and weather helm for a moment, there are reasons that extending the waterline by heeling do not result in the magnitude of speed benefit that the heeled water would seemingly produce. To begin with the area aft of the stern wave tends to be low pressure and that works on the counter to cause the stern of the boat to squat (pull down into the water). That creates a dynamic drag and it also adds wetted surface adding drag in that manner as well. When Tripp made his runs straighter and wider, this provided the ‘bearing’ to reduce squatting, and so was more effective in producing speed under sail or power than the narrower stern models and models with more upswept counters in profile.
One way of extending the hullspeed of a boat is to reduce the size of the bow wave, and one way to reduce the size of the bow wave is to have a finer bow. A longer waterline forward allows the bow to be finer for the same displacement and center of buoyancy. That is in part the theory behind the near plumb, finer bows used on modern designs.
But a finer entry also helps with the way a boat behaves in a chop. A blunter bow tends to collide with more force into each wave. That larger collision force tends to promote more rapid pitching, a greater loss in speed from each wave, and a larger pitch angle. A finer bow as seen on Herreshoff designs tend to slice through the wave, increasing in buoyancy incrementally and with less violent force. In that regard, Alberg’s fuller bows got in wrong on all counts. Not only does Alberg’s fuller bows produce a bigger bow wave making hullspeeds occur earlier than a finer bow, but they are more prone to pitching and losing speed in a chop.
The trend toward short aft overhangs also results from another phenomena. When you truncate the stern of a boat sharply, as in the case of a vertical or reverse transom, the water leaves the stern cleanly and continues aft on a trajectory as if there was still more stern back there. In doing so, it moves the stern wave further aft, tricking the water flow into thinking that the boat is longer than it, and thereby increasing the speed at which the steep rise in drag occurs. But because the stern ends where it does, there is not as much of a tendency to squat with the associated drag, and because the water is not sliding up the counter, there is less wetted surface. This results in a boat which sails and motors faster than a boat with the same waterline length achieved only by having a long overhang heeled into the water.
Short overhangs also offer very large advantages in term of motion comfort due to pitching since there is a tendency to reduce pitching moment of inertia while better matching the dampening characteristics to the pitching moments of inertia.
(When a boat is surfing, two things happen, the height of the bow wave is diminished by the geometry of the back of the wave reducing the induced drag, and the force of gravity is added to the force from the sails which in concert allows an increase in the speed of the boat.)
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay