Lofting

[LBG]

On my plans there is somthing called " Buttock 1,2 and 3", they go out from the centreline at 300mm intervals. What are they for and are they important?, at the moment I carn't see a use for these dimensions. I'm building a 33' steel yacht if that is any help.

[Thunderhead19]

When lofting, the various lines forn a grid known as "The grid"...heh heh heh..uuuummm. In the body plan, you'll notice that the buttocks and the waterlines form a grid. Over this grid you'll then see the body sections scribed. The intersecting points of the body section and the bottocks and the waterlines allow you to accurately loft your frames.

[gonzo]

The buttocks allow you to fair the lines. It is one more check.

[LBG]

Thanks for your help guys, I'll start that grid pattern, nothing worse than a blue.

[LBG]

Well I've drawn the buttock lines in and they don't seem to intersect anything that I've drawn, are they used later on as a sort of datum?.

[Wynand N]

Originally Posted by LBG

On my plans there is somthing called " Buttock 1,2 and 3", they go out from the centreline at 300mm intervals. What are they for and are they important?, at the moment I carn't see a use for these dimensions. I'm building a 33' steel yacht if that is any help.

Buttock lines are vertical fore-aft slices parallel to centreline spaced evenly to describe the hull shape. Used to smooth the hull (with diagonals) especially at the stern. It is also an indicator of the flatness/shape (runoff) in the stern area.
I usually add the buttock lines last on the lines drawing and only very minor adjustments are used to smooth the hull.

BTW, before yacht design became a paper exercise, old world craftsmen/designers used to carve a half models of the hull shape out of a piece of solid wood. This was then split in slices (basically buttocks) longitudinally of which offsetts were taken to built the boat

[dr.j]

As has been stated, buttocks are, in addition to waterlines and diagonals, used as a means to creating a fair surface. In the body plan, buttocks are seen as straight perpendicular lines that are evenly spaced and parallel to the center line. In the half breadth view they are seen as straight lines going fore to aft and parallel to the center line. In the profile view they are seen as curved lines. Generally only the fore and aft sections are projected because it is here that there exists the greatest amount of curve.The curved line must be fair and pleasing to the eye. If there are bumps or holidays (small depressions...)in the line, then there exists inefficiencies in one's lofting. You must think of loft plan as merely three different views of the same object. Each view is simply seen by rotating the boat around. That is why the buttock appears as a curve in only one view. As has been stated , it is a "slice" from fore to aft, parallel to the centerline of the boat. When you put your buttocks into the body plan view (straight and parallel to the centerline...) you take all of the points that they intersect with the station lines, diagonals (if they are present...) and load waterline. Once they are projected to the profile plan, they will appear as curved, and hopefully fair,lines. You do the same for waterlines using the halfbreadth view. Just remember that lines that are straight in the body plan (water and buttock lines), are curved in either the halfbreadth(waterlines) or profile (buttock lines) view, and lines that are curved in the body plan (station lines) are straight in the plan or profile view. If you require more in-depth, precise explanations, a good source for beginners is in Howard Chapelle's book, "BOATBUILDING A complete handbook of wooden boat construction". My old copy was put out by Norton and co. I think Alan Vaitses also has one out on the subject. Most people think that it is a hassle ensuring that the lofting is accurate and complete. It is, however, way easier to erase and redraw a line than it is to rebuild a section of boat that did not turn out because of a lack of dilligence on the loftsman's part. I find that it is a good way to see if the finished product is exactly what you want in a boat.

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