Does anyone know how diagonals are applied to lofting a boat?
I understand what I'm looking at with the station, buttock and waterline line but I can't see to get my head around what I'm supposed to be looking at with the diagonals. What are they doing?
Thanks,
Frank
As I understand them, they are the third of the three measurements one needs to locate points in 3-dimensional space. I think of them as a check on range from the origin when viewing a "slice" athwartships that shows the 2-d parts viewed from fore or aft.. In practice, I just look at the points described by the lines matrix, and draw them on the mold, pattern or frame then fair with whatever tool works best...then I assemble based on the stations described by the side view. Usually I nest all the molds or patterns before attaching to a spine or keel(depending on the design). Except in the abstract, I've never formally used diagonals.
ReplyDeleteI guess that's where my problem is coming from. I can build a three dimentional shape using the station, buttock and waterline lines, but I was told that there would be less fairing to be done on the ribs if I used diagonals. When I asked the person to explain, he himed and hawed and finally admitted that it was something he read in "Boatbuilding" by Chapelle. I read the passage he talked about but I came away more confused than when I started. I guess that will have to remain one of those "Mysteries of Boatbuilding".
ReplyDeleteG'Morning. I've done a very small amount of lofting, and here is what I get from Diags. There are times where more information is needed to ensure that enough points are generated to make a fair curve. Diagnonals come into play at that point. If a loftsman is drawing up the lines and there is a section where more points are needed, he can insert a diagnonal, which is on a different plane then the other lines, this is just another double check, if the lines go through all the points we have an accurate representation. The other lines are usually spaced at a fixed distance, what you sometimes get is a line that crosses another at a lazy angle. When the lines cross like a Plus sign, they are very accurate, but when they cross like an X or a Lazy X they are not as accurate, Diagonals can help make up for this by providing additional points of verification. I usually don't bother with them. I did on the Catspaw Dinghy because they were easy to put it, (The plans were great), but I wouldn't go to a lot of trouble unless there was a question about a line. Then it might come in handy to verify that you got it right. Try Greg Rosell's book- Building Small Boats. Great book, the lofting section is very well explained and step by step. Best book around for "Building Small Boats". -John
ReplyDeleteI asked Greg Rossel, here is the response: John, "The whole idea for accuracy is to cut the curve of the station at as close to a right angle as possible. So..... near the sheer, the Waterlines are the most accurate, near the keel, its the Buttocks and at the turn of the bilge, its the Diags. They are used exactly like the waterlines -- distances measured out from the CL at each station. Plot 'em on the stations on the loft and fair with the batten. " Greg Rossel, author "Building Small Boats".
ReplyDeleteExcellent response from Greg Rossel--and if anybody knows, he does. Abbettjc--grateful for your followup. JR Sloan
ReplyDeleteGreat Thread fellas, I agree Spokanoe... Thanks to all who contributed to this thread and all who contribute to this forum and site. It wouldn't be the same with you all. Very informative. Thanks, -Kruez- PS: Hey John, Did tell Greg to stop by and sip some java with us? Grin...
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ReplyDeleteSorry I deleted the last message. It was a thank-you for everybody's help and thanks for the description. I sent it in and then re-read abbettjc's messages again and bam,... the light clicked on. When I re-read the description of the Pluses and the lazy X's and then Greg's statement that the waterlines were the most accurate at the shear and the buttock lines at the keel, diagonals suddenly made sense. They are drawn at the station lines out onto the buttock lines at 90 degrees to the shape line and back to the centre line. They are for describing a curve on the station lines. I always had some idea that they tried to describe the cut angle of the ribs from fore to aft so when you were cutting out the ribs (double-sawn), you could change the bandsaw table angle to get a fair curve from fore to aft in the planking without having to put the plank on, mark, shave, and repeat until you get a fair curve. Thanks to everybody for their help in this thread and especially John. I'm looking for Greg's book right now. Frank
ReplyDeleteI defen. recommend his book. He uses the Catspaw as his example boat (I just happen to be building that one). It is like a large instruction manual on how to build the Catspaw. I was able to loft that boat by following his instructions in his book paragraph by paragraph. It took a while, but I got through it. Lots of wisdom there. Picture of the Catspaw in progress enclosed.
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