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From: Tom MacNaughton
Date: 22 Feb 2008
Time: 14:00:27 -0500
Remote Name: 66.252.35.152
We keep getting into concepts that really need extensive explanation to properly train someone in them. Planar sheers are such a concept. I’ll try to describe why they would be useful, what they are and how they solve the problems I mention, what their limitations are, and roughly how they are developed. The practicalities of actually drawing them will have to be reserved for those with enough passion for yacht design to actually take our YDS main curriculum or our “CAD Course”, which are, I’m afraid, the only places you can really learn this. For a very long time designers have understood that when you view the vessel in 3D in profile the ends of a conventional vessel, being further from the eye, will look lower than they do on a flat drawing. This can result in a sheer that looked beautiful on a drawing looking flat or lumpy in the ends in real life. Also as a boat heels away from the viewer in profile the ends will tend to turn into “reverse” curves quicker than the middle, thus at some angles producing sheers that look very wavy. That is one reason almost all photographers prefer to take pictures with the boat heeled toward the lens so that the sheer is strengthened rather than weakened. Further if you are looking at a boat from either the forward or aft quarter similar illusions may make the sheer looked reversed or lumpy in places. This is true not only of the main sheer line but of other sheered lines as well, including the boottop, coveline, cabin side sheer, etc. Many designers have developed rules of thumb for deciding how to compensate for these optical illusions. Those who either don’t use these rules or aren’t very good at it tend to get bland or ugly boats quite frequently. Designers with a good feel for adjusting sheers so that they will look good in 3D tend to be hailed as great geniuses and make a lot of money. Wouldn’t it be great if there proved to be some underlying principle which makes it easier to produce essentially perfect sheers, like the geniuses do, every time even though one may be a person of more humble abilities? Fortunately there is. Provided you have a boat of reasonably moderate proportions, once you have decided the proper height of the stem, sternpost or transom, and the midsection you can construct a plane which will cut through these points and the sections in such a way that it will form a sheer line at the intersection of this plane with the topsides of the vessel. Because every point of the sheer lies on a plane there will be no angle you can view the sheer from which will appear lumpy. While it takes practice to use this method you are pretty much guaranteed that the vessel will be beautiful if you get the three starting points correct. There are limitations however. If you have a sportsfisherman with an extreme flare such that the plan view of the forward deck edge is nearly round, or are designing a tug boat with a round stern, any planar sheer would be likely to turn up at the ends like Alladin’s shoes. Now one could argue that this may be evidence that the sportfisherman will never be as beautiful as she could be unless you draw out the bow overhang enough to allow a really beautiful planar sheer forward and we have done this with some of our power boat designs. They do look exceptionally beautiful. However there are several types or rounded sterns that really can’t be treated that way. The key here we think is deciding whether the curve in plan view will be continuous right across the centerline so that both sides match in tangency and curvature. If that is not the case the stern should be reshaped until it can have a planar sheer. If it is the case that the curve must be continuous than it will also automatically be both horizontal at the centerline and in plan view will meet at a precise right angle to the centerplane. In this case we design a planar sheer and just tweak the last part of it enough to achieve this meeting that is absolutely normal to the centerplane at all angles. This one deviation from the planar form has a logic and grace of its own and will look good from most angles. There is one more minor limitation. This is when there is a “broken” sheer as in a cabin trunk which at its aft end suddenly becomes a cockpit coaming. Or an old high quarterdeck type, or raised deck type, with a transition to a lower sheer part way along the sheer. In this case you will normally find that the “raised” part of the sheer and very often much of the lower sheer can conform to the planar form but the swooping curve that connects the two will probably not be able to be co-planar with the lower sheer. Of course it also won’t be co-planar with the upper. However this curve will normally be strong enough in itself so that it will never be a party to the type of illusions we have described. Now it only remains to give an idea of how the plane of the planar sheer is developed. It may help you visualize this process to imagine turning a half model upside down and resting it with its sheer on a flat table top. If you sight along the table top and the sheer line touches the table top at every point the sheer is planar. If it touches in some places but not in others you will have to shave away some of the parts between the forward most point and the aft most which touch the table. Gradually the gap would narrow until the sheer touched the table top all the way along at which point it would be planar. In our lessons we simply describe a graphic way to develop a plane and a sheerline that are co-planar. Re-reading this, I’m not sure how helpful it will be. Maybe this is just mumbo jumbo to the average reader. Nevertheless it makes developing truly beautiful vessels so much easier.