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Buttocks, Waterlines and Diagonals

On June 7, 2017, in Designer Articles, by Glen L. Witt

This discussion primarily applies to vee bottom boats, although the terminology is similar to those with a round bilge.

The terms buttocks (butt), waterlines (wl), and diagonals (diag) are referred to on the “Lines Drawing” of a set of plans which is usually on sheet 2. Unless the boat is being lofted, they will have little purpose as far as building the boat is concerned. However, understanding why these lines are important, and their purpose will enable a builder to better understand how the lines of a boat are generated.

The “Lines Drawing” of a boat will basically consist of three views: Plan or top view, Profile or side view, and cross Sections taken through the hull at intervals commonly called stations, which usually coincide with the location of the frames or forms. Three primary lines dictate the shape of the typical vee bottom boat: chine, junction of the side and bottom planking; sheer, top edge of the side planking; and keel, the junction of the bottom planking along the centerline.

The keel definition is loosely defined to simplify the description. Often this line is called fairbody, the junction of the outside of the planking as it meets the keel.

If the section lines, chine to sheer and chine to keel, are straight, no other lines are required to illustrate the shape of the boat. However, most craft will have some curvature of the sides or bottom as shown in section. Most builders think of a sheet plywood boat as having straight sections; some may, but most will be of convex shape. Concavity is not practical unless it is at the extreme bow or stern, usually minimal in nature.

Plan View

Buttocks are used to show the amount of deviation from a straight line the bottom sections (chine to keel) have. Buttocks are shown in plan view as straight lines parallel to the keel, usually two or three at least on each side of the boat. In section view they are straight vertical lines parallel to the centerline and spaced from the centerline identical to those drawn on the plan view. In profile they are curved lines that illustrate the height the buttock crosses the section line. These curved lines must be smooth and fair. Altering section lines and re-fairing the profile buttock line is an ongoing process. When properly faired, the lines in section and profile must be smooth flowing and fair.


However, the section line may intersect a buttock at such an angle that determining the exact point is difficult. To improve accuracy, a diagonal is used, shown as a straight line in section, usually to cross the planking as close to 90 degrees as possible. One is usually at forty five degrees to the centerline, others as required to make the definition point of a crossing line accurate. The distance is plotted from the intersection of the diagonal at the centerline, along the diagonal to the planking, at each section of the plan view. This should result in the plan view diagonal being a fair curving line. If not, the entire process is repeated, adjusting the points as required to assure each buttock and diagonal are fair lines and the bottom sections smooth flowing lines.

Waterlines are shown in profile as straight lines parallel to the base or reference line that in turn is at right angles to the station perpendiculars. Usually a minimum of two and usually more are used to define the side contour at each section. They must be smooth curving lines on the plan view and develop smooth curves on the sides as shown on the section views. The points of curved lines are shifted to accomplish this; this is quite similar to what was done for the bottom.

Section lines are the frames or forms that shape the boat. If the lines are not accurate, the boat may have unsightly dips or bumps that will be an eyesore and detrimental to performance.

The finished lines are still inaccurate. On a small scale a pencil line width may represent 1/8″ or more. To finally true up the lines and get accurate sections, the boat must be lofted and this means going through the same process in full scale but, of course, the small scale drawing will make the task easier.

Many years ago, Glen-L decided that lofting was not that easy for most builders, nor something they really wanted to do. Imagine laying even the smallest boat in actual size, incorporating all views shown in the “Lines Drawing” for the boat.

So, when building with Glen-L plans and patterns, you can forget all of the above and not really care what a diagonal or buttock is; you go directly to building the boat with our full size patterns. This will save you a lot of time that’s better spent building your boat. To see all the Glen-L plans and patterns available for the beginning builder, choose a category from our online catalog.

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One Response to Buttocks, Waterlines and Diagonals

  1. Krish Salian says:

    Brilliant way of describing boat plans.

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