To achieve a bow wave shift, whilst at the same time bringing about a possible reduction in the height of the bow wave, lies in the use of the bulbous bow, but that is another story.īTW. if taken to any kind of extreme, however, additional drag would result owning to the size of the bow wave formed a problem usually encountered with full bows. In general, this is achieved by an increase in volume of displacement forward. Overhang at the bow causes the bow wave to creeps forward, and because the of the sectional form usually produced by the overhang, the displaced lenght is increased substantionally when heeling.Ī full bow generates an earlier bow wave, though the same effect would result from a deep forefoot which retains reasonable waterlines forward. It pays to maximize displaced LWL, thus enhancing the hull's capability of generating a longer wave system by a bow wave which forms earlier and a stern wave which extents as far aft as possible. The quest for good performance starts with the shape of the hull, particular below the waterline. However, take note that wavelength is determined by velocity, and not the hull's lenght. Maximum displacement speed is indicated by a Froude number of about 0.4 depending on the hull's displacement length to be more accurate than the rule of thumb method of approx 1.3 LWL. At maximum displacement speed, the hollow of the stern wave is reinforced, causing the boat to trim up the bow (squatting the stern), a state which carries a lot of resistance.Ī boat's velocity with reference to its wave system and lenght waterline is judged by a Froude number. The transverse wave system results from the interaction of the bow and stern wave systems, and varies with the boat's velocity. The nature of the wave system generated around the hull is the stranglehold which inhibits planing and is an important factor affecting the performance of displacement craft.Īs it moves through the water, a boat's hull produces both transverse and divergent wave system, although only the former appears to affect the resistance of the hull. I have read a bit about the Dashew boats recently and have been moored in Newport Beach California for a couple of months where I have seen Deerfoot, some sundeers and a number of Macgregor 65s. On the topic of high aspect ratios generating more lift, why then would a plane like a DC3 or a deHavilland Beaver, two real workhorses with great load carrying capacity at low speeds have such low aspect ratio wing designs relative to a glider? Is that perhaps because the advantages of high aspect ratios where poorly understood 50 years ago? (Albeit in a lesser forum! Who knows what value?) Any thoughts? I have also seen the size of a wake referenced as an indication of the "efficiency" of a hull. Unless maybe a finer entry with a long taper (like the Dashew boats) creates a bow wave differently? It seems to me that with adequate power (strong breeze, large sail area) all of the friction or wetted surface issues can be overcome but that the length of that wave is the true limiting factor. Thanks Wynand, I hadn't heard that 1.3 figure before.Īm I right to think that the function normally referred to as hull speed is related to the length of the bow wave - a wave's speed being a function of it's length? Is my perception that a dispacement hull has begun to exceed this speed when it begins to sit lower in the water and the bow wave begins to ride higher and higher on the hull? ![]() There are some good books available check out the book store on the top of this page High aspect ratio's generates more lift that low aspect ratio's - ever wondered why a glider's wing is so narrow and long? I believe they would make it narrower still if they can find a way of attaching them safely to the fusalage! Another indicater of hull speed is the ability of the hull to carry it's sail, and so we can go on and on. Another issue that will make a hull fast or slow is the foil shapes of keel and sailplan. Yotphix, there are a lot of issues that have an influence on hull speed, but in essence friction and drag are the main critters slowing a hull down and this is directly a result of wetted surface area. However, because of different hull shapes, this is not always true - therefor actual hull speed can varies from about 1 - 1.6 times approx theorotical hull speed. A simple thumb rule is that the boat would do about 1.3 times theorotical hull speed. In short, a displacement hull speed is rooted in the waterline.
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