DUDLEY DIX YACHT DESIGN

 

THE CASE FOR LOFTING

 

 

The aspect of boatbuilding which seems to be most terrifying to the amateur builder (and sometimes the professional) is lofting the lines.

This is an irrational fear, probably born from the mystique which the professional loftsman has been known to wrap around his craft to protect his position.

There are benefits to be gained from lofting. Some professionals do it even if computer generated lines and offsets are available.

The lines drawings on this page are optimised for 600x800 or greater resolution. If using lower resolution, please scroll horizontally to see the full drawing.

Shearwater 39 yacht design
This Shearwater 39 hull was the plug for the GRP production moulds.

The aspect of constructing a boat which seems to be most terrifying to the amateur builder (and sometimes the professional builder tooling up for GRP volume production) is the lofting of the lines. This is an irrational fear, probably born from the mystique which the professional loftsman has been known to wrap around his craft to protect his position.

Full Size Patterns

The intimidated prospective builder will do everything that he can to bypass the lofting operation and is at his happiest when he is able to start his construction from full size hull patterns. If he were to understand how lofting is done he would find that there is a much larger range of designs available to him than what can be bought through the marine equivalent of the mail order catalogue, including many which were drawn before the computer age reached the yacht design profession. Without the self-imposed need for full size patterns, he can make his design choice based on far more important factors and end up with a boat which better suits his needs.

There is nothing wrong with building a boat from full size patterns as long as the design was not chosen because of the full size patterns. They should be considered only as a small time saving benefit and not as a reason for the design choice. There are designs being sold specifically for amateur builders, with full size patterns to simplify construction, but with almost non-existent detailing. The patterns will only give you the shape of the hull or the panels which make up that hull. They will not give any information on how to build the boat. If the design is marketed around the feature of full size patterns, look carefully at the detailing which is supplied in the package because it may be minimal.

Lofting time for a design will vary from a day for a single chine hull to maybe two weeks for a large and complicated round bilge hull. Taken as part of the total duration of the boat building project, that is a very small proportion and certainly not worthy of compromising the final result.

If full size patterns are used, they should be printed on Mylar to be of full value. It is no good for the designer to draw the patterns on Mylar and then print onto paper for supply to the builder. Atmospheric conditions, primarily humidity, affect the overall dimensions of the paper sheets and with them the accuracy of the patterns. Changes in atmospheric conditions over, firstly, the period when the patterns are being printed by the designer and, secondly, the period when they are being traced off by the builder, can cause unfair spots in a hull. From one day to the next there can be a few millimetres dimensional change to a large sheet of paper and both printing and tracing may well be spaced over a few days.

Computer Aided Design (CAD)

Many design offices now use computer aided yacht design facilities which include mathematical fairing of hull shapes with a high degree of accuracy. The computer generated offsets (which can be provided at either traditional measurement stations or frame positions) eliminate the need to loft the hull in the traditional manner, requiring only that the sections and bow profile be accurately drawn out on the loft floor so that the frames can be built. Where the necessary plotting facilities are available, the computer can produce full size patterns but, again, they should be printed on Mylar for supply direct to the builder.

It has been stated many times in the boating media that one can tell which yacht design software package was used to design a hull by looking at it's shape and that one can see which designers are using the same software. The inference is that the resulting hull shapes are what the computer wants and not what the designer wants. This is a fallacy and I have produced a wide range of hull styles on my computer, from very traditional to very modern, with full control over the final hull shape. That argument against the use of computer generated offsets or patterns simply does not hold water.

The Lines Drawing

If the design which has been chosen does not include full size patterns or computer generated offsets, it will be necessary for the builder to know how to convert the information supplied to him in the form of the lines drawing and table of offsets into the final hull form. The principle is very simple and need not be of concern to anyone who is capable of building a boat. The lofting of the lines is one of the easiest operations of the entire project but it's accuracy is vital to the final result.

The reasons for lofting the designers lines are as follows:

- The lines drawing is to a small scale which needs to be enlarged to full size so that patterns can be taken from it for the fabrication of frames, bow profile, etc.
- The table of offsets is measured from the lines drawing where a thin pencil line can be as much as 5mm wide to scale so there is a fair chance of inaccuracy in the offsets. Any inaccuracy must be found and corrected on the table of offsets.
- Bevel angles can be taken off the loft floor to pre-bevel the frames, stem, keel and other structural members before they are set up on the building stocks. This applies mainly to timber construction.
- Templates for semi-bulkheads, gussets, knees and many other small parts can be made for pre-fabrication of the parts.

The lofting operation is essentially a process of drawing out the designer's lines drawing full size on the floor so it is important that the lines drawing be thoroughly understood. To this end I refer to Fig 1 which is a typical lines drawing for a modern round bilge yacht with fin keel, transom hung rudder and planing boards. It has been simplified for the sake of clarity in this reduced format and would normally include information on deck configuration as well as a curve of areas which shows the volume distribution of the submerged section of the hull. There are also many measurements which have been omitted to simplify the drawing.

Lines of round bilge sailboat hull

Fig 1 - Lines of round bilge sailboat hull

The drawing shows three views of the hull, in this case each occupying it's own section of the drawing. The three views are in the standard format of side, plan and end elevations as are used by any high school woodwork student. However, because the hull surfaces are curved and with few convenient corners to use as measurement reference points, a different method is used to define the hull shape. This is done by means of contours which define the hull shape in exactly the same way that the contours on a chart define the shape of the sea bottom or a contour map of a mountain defines the shape of that mountain. Whereas contour maps of geographical features only look at them from above, on a hull lines drawing we are looking at the hull from three directions and each view has it's own story to tell about the character of the hull.

Side View and Buttocks

The side elevation shows the boat as if it were being viewed on the dock, with the sheer line being the extreme upper edge of the hull and the profile line forming the lower perimeter of the hull from stemhead to transom, usually along the centreline for a monohull. Between the sheer and profile lines are a number of curved lines called buttocks (Butt 1 to Butt 3 on the drawing), those closest to the profile line being pretty much parallel to it and those further away being progressively less parallel. The buttocks are the side view contours of the hull and each connects up all points on the hull surface which are the required distance from the hull centreline. If the buttock under consideration is a 500mm buttock (Butt 1 in this case), all points along that line are 500mm from the hull centreline. If the hull was cut through longitudinally 500mm from centreline, the 500mm buttock shape would be the shape of the cut.

Most hulls will have buttocks which are gently curved in the bow areas with some flatness in the lower areas aft and fairly tight curvature in the topsides aft. A slab sided hull will have fairly vertical aft endings to the buttocks while the top ends aft will turn forward on a hull with tumblehome. A clipper bowed hull will have buttocks which curve forward high in the topsides near the bow and a hull with a firm turn to the bilge will show it with a fairly tight curve in the buttocks near the waterline in the bow areas.

The buttocks are the best indicators of the off-wind performance potential of a hull. At the stern they show at what angle the water will be flowing as it comes out from under the hull. A steep buttock indicates a large quarter wave and a flat angle shows a flat and clean wake. An upward curve to the buttocks closest to centreline as they approach the stern will cause the hull to squat as the stern is sucked down by the water following the curve.

Plan View and Waterlines
The plan view shows the boat as if it were upside down with the viewer looking down at it. At the extreme outer edge is the sheer line while all the lines inside of the sheer line are known as waterlines (WL -1 to WL 3 on the drawing) and are the plan view contours of the hull. The line at which the designer intends the boat to float is called the design waterline and is normally designated DWL on the drawing. The shape which it is drawn shows the shape of the water plane on which the boat rests when it is stationary and is equivalent to a horizontal cut through the hull at that level. The other waterlines are all related to the DWL by their heights above or below DWL and each shows the shape of a horizontal cut at it's own level.

A fine bowed boat will have waterlines at and close to DWL which are straight lines in the bow area and meet the centreline at a relatively small angle. Conversely, a full bowed boat will have considerable curvature to the forward waterlines and they will meet the centreline at a large angle. Below DWL, a hull with U-shaped bow sections will have waterlines which turn in suddenly and meet the centreline at 90 degrees while V-shaped sections will give waterlines which have little turn in and meet the centreline at an angle. A boat with a flat stern will have a lot of curvature to the waterlines aft, meeting the centreline at close to 90 degrees while those of a hull with Vee aft will have less curvature and meet the centreline at an angle. Looking at the topsides, a slab sided boat will have the waterlines above DWL converging on each other for much of the hull length around the maximum beam and a boat with tumblehome will have one or more waterlines projecting beyond the sheer line for the length of the tumblehome.

The waterlines are the best indicators of the up-wind potential of the hull, telling the eye whether the bow will cut cleanly through headseas with little effort or require a lot of brute force to push the seas aside. The relationship between the bow and stern waterlines indicates the extent to which the boat will change trim as she heels over. A very fine bow combined with a very full stern will cause the bow to be depressed considerably as the heel angle increases.

Body Plan and Stations

The end view of the hull is known as the body plan or the cross-sections and is sometimes drawn superimposed over the side view. Only half of each section is drawn with those from the bow to the point of maximum beam drawn on one side of centreline and the rest on the other side. This gives a view looking aft and one looking forward. Each section shows the shape of the hull at a specific position along it's length.

The body plan gives the clearest picture of the overall shape of the hull with regard to stability characteristics and seakindliness. It shows whether the hull has firm or slack bilges, Vee or U-shaped bow sections, Vee or flat stern sections, amount of flare, flam, tumblehome and a host of other subtle characteristics.

None of the three views tells the complete story of the hull by itself. They all need to be read and interpreted together.

On the lines drawing is a grid which forms a background to the lines of the hull and, on inspection, it will be seen that the grid for each view is different from that of both other views. Looking at the side view of the hull, the grid is composed of horizontal lines which are the waterlines on the plan view and vertical lines which are the sections of the body plan. The plan view has longitudinal lines which are the buttocks of the side view and transverse lines which are again the sections of the body plan. Similarly, the body plan has vertical lines which are the buttocks of the side view and horizontal lines which are the waterlines of the plan view. Each curved line in each view is, thus, a straight line and part of the grid in both of the other two views.

To confuse the reader, the section lines on the side and plan views are not called sections but are stations. The sections which are seen on the body plan are sections on the relevant stations.

The usual method of referencing the stations is for station 0 to be at the point where the profile line in the side view crosses DWL at the bow and station 10 at the point where it crosses at the stern. The stations between are equally spaced, giving a station spacing of 10% of waterline length. Other methods of spacing and/or referencing stations are sometimes seen, such as numbering from stern to bow. Often additional stations will be used to help define the bow and stern, such as stations 1/2 and 9 1/2. Large boats will sometimes have more than ten stations between the extremes of DWL and small boats may have less.

The section of hull ahead of station 0 may include a negative station or two but is more likely to have some dimensions on the drawing to define the curve. The tip of the bow will probably be referenced FP which stands for Forward Position and the stern would be AP for Aft Position. There will be dimensions on the drawing to position both FP and AP, both of which form part of the grid.

Waterlines are numbered or are referenced by their distances above or below DWL or, sometimes, their distances above a base line. Buttocks are numbered or are referenced by their distances from hull centreline.

Diagonals

On the body plan there will normally be one or more diagonal lines (Diag A to Diag C on the drawing) which are usually referenced alphabetically. These are used by the designer to define the hull mainly in the area of the turn of the bilge. The diagonals do not form part of either of the side or plan views but are drawn as curved lines on the opposite side of the hull centreline from the plan view.

Chine Hulls

The principles of a lines drawing for a single chine, multi-chine or radiused chine hull are similar to those described above for round bilge yachts. The difference is that such hulls have convenient corners in their surfaces which can be used as reference points for setting them out, eliminating the need for contour lines. As a result, waterlines, buttocks and diagonals are not required and the drawing, offset table and lofting are considerably simplified. The DWL will still appear on the drawing but, in the lofting process, will only be used as a datum. The sheer and chine lines will appear in all three views and the profile line will only be on the side view, coinciding with the centreline on the other two views. On the body plan, the chines of each section will generally be linked by straight lines.

Fig 2 shows a line drawing for a multi-chine hull with fin keel, transom hung rudder and planing boards. In this case, it has a flat central panel spanning from chine 1 on one side to chine 1 on the other side. As a result, chine 1 and the profile are the same line from station 0 to AP. A hull with Vee on the centreline will show a seperate profile line.

Lines of multi-chine sailboat hull

Fig 2 - Lines of multi-chine sailboat hull

On the multi-chine lines drawing are four broken lines running from top tp bottom and referenced A to D. These are frames and/or bulkheads which do not fall on station positions and their offsets will be included in the table of offsets or may replace the station offsets. Producing frame and bulkhead offsets from the lines drawing is a relatively simple matter for a chine design so the designer will normally supply them. In contrast, they will not normally be supplied for a round bilge design unless it has been computer faired so they would have to be produced from the loft floor.

A single chine design which has a large amount of twist to the panels (change of angle from station to station) will probably have conically developed surfaces. This will allow the sheet material from which the hull skin is formed to take it's natural curve without undue stress. The lines drawing for such a hull will have waterlines and/or buttocks in the areas of conical development so elements from both types of lines drawing will be present. A multi-chine hull will not normally need conical development because the hull form demands less twist and the narrower panels can accept more twist without problems appearing.

Table of Offsets

The table of offsets which accompanies the lines drawing is a tabular representation of the body plan, ie a list of measurements. There are various methods used in drawing up the table and it should be very clear by the wording on the table which method is used. The method which I always use is very simple and references everything vertically to the DWL and horizontally to the hull centreline, all measurements being to the outside of the hull skin. A minus sign in front of a vertical offset indicates a downward measurement from DWL while all others are measured upward.

Fig 3 shows a table of frame offsets for a motor yacht hull with two chines, the lower one being radiused so it has been called a ghost chine. The table for a round bilge design will have a similar layout with more rows. In the section for measurements above/below DWL the offsets for buttocks would replace those for chines and in the section for measurements from centreline the offsets for waterlines and diagonals would replace those for chines. The offsets for diagonals are always measured from centreline along the diagonals to the hull skin.

Table of Offsets for radius chine powerboat hull

Fig 3 - Table of Offsets for a radius chine powerboat hull

Further Reading

Those are the basics of the lines drawing. This article is the first in a series of three. The other two deal with the procedures of lofting a set of lines and form part of a set of articles which you can order here. Other reading for those wanting a deeper understanding is Boatbuilding by Howard Chapelle and The Gougeon Brothers on Boat Construction, both of which have comprehensive sections on the subject and can be ordered from our bookstore.


OUR CONTACT DETAILS

Click here to contact us by whatever method suits you.

This page was updated 3 September 2002

Web design by Dix Design