BOATBUILDING WITH A DIFFERENCE VI
(For Aspiring Amateurs)
by Barend Migchelsen 
Migchelsen@aol.com
http://ca.geocities.com/bmboats2002/
http://members.aol.com/_ht_a/migchelsen/myhomepage/

Barend Migchelsen, (pronounced Mikkelsen) learned to sail in The Netherlands in 1943. In 1975 he started to build boats and boat models as a hobby.  Today, he organizes and teaches classroom courses in boat building, and has published several books on the subject.  The following is an excerpt from one of these books.  Click here to check out Barend's books at our store


Nearly all amateur boat builders of small boats start off by buying a readymade set of plans before, if ever, they get the urge to design their own.  It is the best way to get the experience without that it has to cost you an arm and a leg.

But even if they stick to buying plans, at one time or another, they just will have to make at least a full-sized Body-view drawing.

Accuracy in drawing of Side Panel Planks without Extensive Software

In art supply stores, they sell at a reasonable price 24"x36" easel pads with a very accurate one-inch grid of thin, light-blue lines.  Drawing on this kind of paper is easy and enhances the accuracy.  If you stay an amateur, a pad will last you a lifetime.

In the Body view of a constant-flared hull, the parallel running station lines of the side panels are always skewed.  Unless the chine line runs parallel to a straight sheer line in the Body view, this line becomes a concave curve on the expanded side panels.  That curve is plotted onto the material by transferring the measurements from the station lines from the Body view and connecting the plotted points by a line that is drawn along a flexible batten.  Next to transferring these measurements with a divider compass, working with a tick strip is far more accurate than measuring the lines.

After having drawn the Body view to scale in the conventional way as shown below in figure 6-1A, I make a full-sized, second drawing, see figure 6-1B.  In this drawing, the side panel is tilted by the amount of the flare angle of 22.62º.  For a constant-flared hull the station lines in figure B run parallel to the vertical grid lines of the easel pad.  It gives an easy double-check of the correctness of the dimensions transfer.  Note that the 26" sheer line has become the horizontal base line for this second drawing.


Fig. 6 - 1   For accurate plotting of the width of the expanded side panels, 
Figure B is made full size.

When the tilted drawing 6-1B is constructed full size on the special one-inch-grid-easel-pad paper with a sharp pencil, the measurements of the plotted width of the side panels on the station lines are accurate to two decimal figures.

Expanded Side Panels

In a hard-chined hull design, making the expanded side panels is not difficult.  The sheer line is a straight line.  It is possible, and very easy to design a hull with the chine line parallel to the sheer line.  The (half)-Body view looks then as is shown in figure 6-2A.  The chine line and the sheer line are straight and run parallel in the drawing of the expanded panel.


Fig. 6 - 2   Chine lines

In the (half)-Body view of figure 6-2B the chine line diverges from the sheer line.  In figure 6-2C the line converges into the direction of the sheer line.  In nearly all designs figure 6-2B is the hull shape fore of Beam for a higher bow, while figure 6-2C represents a sweeping stern end aft of Beam.  In the Profile view the chine lines of figures 6-2B and 2C are concave curved lines as is shown in figure 6-3.


Fig. 6 - 3   Expanded side panel

When the cross frames, the stem, and the transom are properly aligned on a building jig, transferring the dimensions of the side panels becomes easy.  The distances between the station lines on an expanded panel are not constant but increase from Beam toward fore and aft.  For accurate measurement, place an inwale in the frame notches.  Mark the exact position of the station lines on the inwale.  Along the inwale, measure the distances of the station lines with a tape measure, or a tick strip made from Bristol board.  Transfer the distances of the station lines to the straight sheer line edge of the material for the side panels.  Construct the vertical station lines on the side panel material.  Plot the width of the panel at each station location.  Clamp a flexible batten along the plotted points.  Draw the chine line and cut the panel at the chine line at flare angle.

This is the way to do it with the stitch-and-glue, or the tack and tape methods.

When the set of plans shows an inside chine batten, it becomes even easier.  Clamp the panels on the protruding shoulders of the cross frames.  Draw the chine line along the chine slat on the inside of the side panel.

No matter how it is done, this tip makes things easier:  It is a good rule to measure twice and to cut once.  My method is to measure three times and to cut twice.

When there is a curved line to be cut, I make an extra cut about ¼" to ½" parallel to the to-be-drawn curve and remove the excess.  It becomes a lot easier to clamp the flexible batten along the plotted points of the chine line, which can now be done with the handy binder clamps.  The second advantage is that I can start cutting at the middle of a convex curve and prevent the saw from tearing into the line by the grain of the wood by cutting toward the ends.

For a concave line to prevent being drawn into the curved line by the grain of the wood, always start at the ends and work your way slowly toward the middle.

Multi Chine

In 1941, W. W. Norton & Company, New York, London published the book BOATBUILDING, a Complete Handbook of Wooden Boat Construction.  Written by Howard I. Chapelle, it still is considered the BIBLE for boat-building amateurs.

On page 44, he wrote, and I quote ad verbatim:  “MULTI CHINE TYPES.  Hulls having more than one chine on each side are sometimes designed for amateur construction.  Those boats most commonly have two chine lines on each side and are planked fore and aft.  It is doubtful if there is any particular advantage to such a hull shape, but some designers and builders prefer it to the orthodox single chine.  The planking has to be spaced and fitted as in a round bottom boat, though it is easier to get the plank in place.  The chines are generally backed with a batten, beveled to fit the angles of the frames and plank.  A better construction, though harder to do, is to lap the edges of the planks at these chines and omit the chine battens.  This is the sailing dory construction; by multiplying the number of chines, a round-bottom hull results.”

From this quotation, I come to the conclusion that Howard I. Chapelle did not think much of multi-chine construction.

Actually, multi-chine construction comes down to reducing the cockpit space, the width of the flat bottom, and hence the flotation capacity of the hull if you start off at the same initial sheer line and flare angle of both hulls as is seen in figures 6-4A and B.  The multi chine hull shown in figure B becomes (sometimes unpleasantly) nimble.  At the same time, it requires extra work.  The extra seam increases the chance of leakage.  Actually, it goes directly against the KISS formula:  Keep It Simple, Sailor.

On the other hand, when the initial side panel flare ratio and consequently the sheer lines are changed, roomy hulls with attractive line result as shown in figure 6-5B.


Fig. 6 - 4   Single chine compared with multi chine

What is a lumpy, ugly design of a slow hull in single chine form in figure 6-5A, imho, becomes a roomy, fast boat with beautiful lines when it is changed to a double chined design of figure 6-5B.


Fig. 6 - 5   Double chine versus single chine

The easiest way to construct a double-chined hull is by the tack and tape, or the stitch and glue methods.

In that case, difficult shaped stems, or gussets with two different, varying bevels are replaced by epoxy fill-ins.

For scows, dinghies, and prams an easy to cut bow board replaces a difficult-to-construct, comprehensive stem without diminishing the beauty of the lines as was shown in the runner-up design of the last contest in this magazine.

The measurement of the width of the bilge panels at the station lines are easily and accurately checked by tilting by 45º the Body view drawing of the bilge panel as is shown in figure 6-1B.  The top edge of the bilge panels in figure 6-5B runs parallel to the sheer line and becomes the straight horizontal base line in the drawing for the expanded bilge panel.  The chine (bottom) line is a concave curve as is shown in figure 6-3.

TUMBLE HOME

Instead of dividing the side panels into two, or more planks, in tumble home construction, an extra plank is placed on top of the sheer.  The plank on top of the sheer is placed at an inward, obtuse angle with the side panel.  The inward tilt could be called a negative flare angle.  The width of the plank often diminishes fore and aft at the ends to reduce windage.

An angled steel beam is much stronger than just an ordinary beam.  The same effect is obtained by placing the tumbled-home plank under an obtuse angle inward on top of the side panel.

Many Dutch boats are built that way.  Besides that it makes a very strong hull, it provides an ideal spot to attach the leeboards.

Angle

There are no fixed rules for the (negative) flare angle of the tumbled-home plank.  The lesser the obtuse angle between the two boards that form the side panel, the stronger the hull becomes.  However, the angle is never bigger than the flare angle of the side plank onto which this additional plank is placed.  This leaves a lot of choices to the designer.  Figure 6-6 shows a view of these choices.  In the choice of this angle, the practicality of attaching the guardrails must be considered.


Fig. 6 - 6   Choices for tumbled-home angle

Guardrails

There are several ways to cut, and to attach the rails to the planks.  The best way is shown in figure 6-7.  Rip the four rail slats from two 1"x2" battens at a 37º angle as shown in the drawing on the right.  Attach the rails as is shown in the drawing on the left side of this illustration.

Flare Angle of the Tumbled-Home Panel

The obtuse angle between the two side panel planks is 180º - 37º = 143º.  The flare angle of the bottom plank is 22.62º.  The tumbled-home panel angle is 37º - 22.62º = 14.38º

(tan 14.38º = 0.2564).  See figures 6-7 and 6-8.

At first sight, this angle looks the same as a difficult setting like the flare angle of a dory (33º 41' 24").  But the angle between the long leg and the hypotenuse of a right-angled triangle template with a long leg of 11.7" and a short leg of 3" is exactly 14.38º

(tan 3/11.7 = 0.2564).

Bevel Angle of the two Side Panel Planks

The obtuse angle between the two planks is 180º - 37º = 143º = 2 x 71.5º.  Set the angle of the table saw blade at 90º - 71.5º = 18.5º when the cuts for this joint are made.  In that case, the cuts have exactly the same thickness.  See figure 6-8

Actual Tumbled-Home Plank in Body View

Figure 6-9 shows the complete Body view of the hull, albeit without the guardrails. 

Fig. 6 - 9   The completed Body view

Tumbled-Home Planks

The side of the tumbled-home plank above the sheer is a convex curve.  The other side that forms the topside of the hull is concave.  Calculating the width of the plank at each station location is not difficult and is shown in figure 6-10.

For clarity only the even-numbered stations and station BAC are shown.

At station #8, in the large right triangle, the hypotenuse is 26".  The total angle at station #0 is 22.62º +14.38º = 37º.  The length of the bottom leg is determined by the formula Dn = 26" cos 37º = 26 x 0.7986 = 20.76(45)".  The length of the station line leg in the large right triangle found with the formula Ln = 26" sin 37º = 26 x 06018 = 15.65" (15.6472").

In the bottom right triangle, drawn with the thick lines, the length of the partial station line is determined with the formula:

Ln  = Dn x tan (14.38º + 8.24º) = 20.7645" x tan 22.62º = 20.7645" x 0.416669 = 8.65". 

The width of the tumbled home plank at station #6 is: 15.65"– 8.65" = 7".

The measurements of the widths of the tumbled-home plank at the stations #6, #4, #2, and BAC are determined in the same way.  At station #6 this is 6.6", at station #4, it is 5.3", at station # 2, it is 3.18", and at station location BAC, it is 2.2".

It is easy to check the calculations with the method shown in figure 6-1B by turning the drawing 14.38º to the right as is done in figure 6-11 in the full-sized drawing on the one-inch-grid-easel-pad paper.

Template for the Expanded Tumbled-Home Planks

Note:  In he illustration 6-11 only the location of station BAC is drawn in for clarity reasons.  Although the location of station SAC aft is different from the location of station BAC fore, this does not make a difference.  The same drawing is used for fore and aft.

If the location of station SAC shown in the drawing falls exactly on the right place on the project, you have done a good job.

The least expensive way is to make the template for the expanded plank on a panel of 4 sheets of 22"x28" Bristol board that is available at any drugstore.  In Quebec, Canada, Bristol board costs $0.79 (plus 15.56 percent tax) a sheet.

Glue the sheets together over the 22" edges with masking tape.  Use the bottom edge of the panels as base line.  Draw the station lines at the same distance as on the expanded panel.  Plot the concave and the convex curves on the station lines.  Connect the plotted points with a flexible batten.  Cut off the excess under and above the plotted lines.  Make sure that the plotted lines are always clearly visible by cutting against the lines on the outside! 

CONCLUSION

For a novice amateur, it is still possible to solve lofting and construction problems with a high degree of accuracy with simple means, and a bit of ingenuity without the need of comprehensive, expensive CAD software.

Sheers and Chines,

Barend

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