This is the second part of my previous response. It took some effort to paint this word picture. If I have confused anyone, then speak up and I will try to make it more clear. As a boat moves though the water, it makes a bow wave. This is true of the smallest canoe, and of the largest ship. The wavelength of the bow wave, the distance measured from peak to peak, will become longer as the boat moves faster. When the boat is moving fast enough such that the wavelength of the bow wave equals the waterline length, then the boat is essentially riding in the trough of its own bow wave. When this happens, it is not possible for the boat to go any faster without passing its own bow wave. This is the hull speed of the boat. It corresponds to a speed-length ratio of 1.34 for the boat, which is also the S/L of any wave in deep water. So then naturally, the longer the waterline length, the faster this "hull speed" limit will be. The most common way to pass your boat's own bow wave is to get up onto a plane. A planning hull boat can do this, but a true displacement boat cannot, regardless of the amount of power applied. Generally, a boat is said to be on a plane when it attains a S/L of about 3.0. A displacement boat operates at an S/L of 1.34 or less. Boats that are able to operate between these two limits, in sort of a "gray area," are said to be "semi-displacement" boats. Another way for a boat to pass its own bow wave is to simply cut through it. For most normal displacement hull boats, the power required to do this is simply beyond reason; and would tear the boat to pieces if it were possible to apply this amount of power. (Or otherwise sink it, as noted below.) One common example of a boat which can cut though its own bow wave, however; is the Hobie Cat. It can do this because of its extremely small displacement, and its very high length to hull beam ratio. Many people think that the Hobie Cat actually plans. Technically, it does not; but the effect is the same: It passes its own bow wave and greatly exceeds its "hull speed" with little power. Unlike a true planning hull boat, it does not ride up and over its own bow wave, but rather simply cuts though it. The reason that this is possible with a reasonable amount of power is that a hull form of this type makes a very small bow wave, so that it requires little power to push through it and to pass it up. A true displacement mono-hull is cambered on the bottom, sort of like an up-side-down airplane wing. It actually produces lift like an airplane wing, but in the downward direction, tending to suck the boat deeper into the water. The faster the speed, the more the downward sucking force. This is why a large racing sailboat often has more than a quarter of its hull length out of the water when the boat is at rest. At low speeds, it has a very short waterline length, because of large sections of the bow and stern are cantilevered out above the water. This keeps the wetted surface area small, so as to provide better low wind performance. Then as the boat moves faster, it is sucked lower into the water. This lengthens the waterline length, thereby increasing the "hull speed" of the boat. So then, if you were to apply enough power to your full displacement sailboat or trawler, and if you did not actually tear it up in the process, you would only cause the boat to sink lower into the water, thereby causing the boat to require more power to maintain hull speed: but you would not cause the boat to go any faster than its hull speed limit. An interesting thing happened in Tampa Bay about twenty years ago. A large tug boat was towing a displacement mono-hull sailboat of about 25 feet in length. Very bad weather was approaching, so the tug boat increased the speed. The sailboat must have had a very strong hull not to have come apart. What happened is that the sailboat was sucked underwater and it sank. This is the result of applying perhaps a thousand or more horsepower to a full displacement hull boat. We used to have a 21 foot full displacement sloop. A four horse outboard would push it to hull speed. When we changed to a ten hp outboard, it would go no faster. We once tried to pull it faster with a large power boat. We estimate that the tow line transmitted about 200 hp to the sailboat. It did not go any faster, but did sit very low in the water, about the same as with a twenty knot wind with full sail on a broad reach. The fact of the matter is, that most sails are capable of transmitting a whole lot more power to your boat than any reasonable engine that you could mount in or on it. So far, I have talked about full displacement boats, which are limited to speeds below an S/L of 1.34, and planing hull boats with speeds in excess of an S/L of 3.0; but I have said very little about semi- displacement boats that operate between these speeds. It is the shape of the bottom of the boat that determines if the boat can operate at these semi- displacement speeds or not. Remember how I said that a full displacement boat will have a hull bottom that is cambered fore-to-aft like an upside down airplane wing? Well, a semi-displacement boat does not; or if it does, the amount of camber is very small. The actual nautical term that is normally used is "quarter beam buttock angle." The way to measure this angle on your boat would be to take it out of the water and chock it such that the water line is level and horizontal. Then put an inclinometer (angle gauge) on the hull bottom near the transom. A planing boat will normally have an angle of two degrees or less. A full displacement boat will have an angle of perhaps eight degrees as a minimum, and often more. A semi- displacement boat will have an angle in between these two numbers, and generally the smaller the angle the faster the boat in the semi-displacement mode. A boat that is operating at semi-displacement speeds will make a bow wave that is longer than its own water line length. It takes considerable power to do this, though a semi-displacement boat is designed so as to minimize the amount of power required. Think of a boat that is pushing its bow wave a little faster than the bow wave really wants to go. Long range power boats, some times called "passage makers," will slow down to well below the hull speed of the boat to save fuel. They often operate at a speed- length ratio between 1.1 and 1.2 or so. Doulos, the boat that we are building, is a 60 foot powered catamaran. I often tell people that it is a full displacement boat, simply because that is how I plan to operate it. The fact of the matter is that she will have a quarter beam buttock angle of less than two degrees, so that she could plane if I were to put enough power onto her--but I am not going to do that. It is actually much more like the Hobie Cat example, however; since it would not actually plane with sufficient power, but rather would cut through its own bow wave. Doulos will have enough power to operate at semi- displacement speeds, but only when loaded lightly. She will be able to do this because her length-to-beam ratio on the water line at minimum displacement will be almost twenty. This is very similar to that of the Hobie Cat. It will result in a very small bow wave, which I will be able to push or cut through with a reasonable amount of power, without actually getting up on a plane. This would be a good time to make a comment about "canoe" type transoms. Some monohull "double-enders" are built this way. I'm saving the transoms to build last for Doulos, since I have not yet decided how to build them. If I give her a canoe type transom, then that will have the same effect as giving her a larger quarter beam buttock angle. I would have to make it very long and pointy to keep the angles small, if I wanted to still be able to operate at the higher semi- displacement speeds. I can do this, in which case it will look very much like a large Hobie Cat; but that will greatly reduce buoyancy aft. On the other hand, I can carry the lines pretty much straight aft to a large and flat transom. This also would enable her to operate at the semi-displacement speeds; but it would make her much less efficient at slower full displacement speeds. Nothing wastes power and fuel like dragging a big flat transom through the water at displacement speeds. Boatless, but building M/V Doulos http://www.anyboat.com/kruse1.htm Paul Kruse plkruse@iu.net Port Canaveral, FL, USA