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Developing the body plan and checking with diagonals

With all the offsets table measurements depleted in the profile view and half-breadth it’s time to move on to the body plan. The lofting grid for the body plan consists of datum and waterlines at the same spacing as in the profile view and a centerline, and buttock lines at the same spacing as in the half-breadth view. As the name suggests the centerline is in the center ( marking the vertical center of the keel/boat) and perpendicular to the datum. Buttocks are placed two to the left from the centerline (for the aft view) and two to the right (for the fore view) with B1 being closer to the centerline and B2 further out.

Now, on every station three locations need to be picked up:

We’ll get away with only these three only because there’s no vertical curvature in the hull’s shape so for all frames rabbet to chine is a straight line and chine to sheer is a straight line for all frames except for station #1 but we have a specific measurement for laying down that curve.

For every one of these locations two separate measurements are needed, one from the half-breadth (imagine this as the body plan x-axis value) and one from the profile view (body plan y-axis value). In order to transfer these measurements from profile and half-breadth so-called tick-sticks can be used.

So-called tick-sticks used for transferring measurements from one view ofthe boat plan to another one.

In order not to have to roll my plans open-closed all the time I decided to take as many measurements from other views in one go and mark them distinctly e.g R2, CH2, SH2 for rabbet, chine and sheer at station #2, though having different sticks for profile and half-breadth views.

Essentially the process was to:

Lines completed for the fore (stations #0 - the stem, to station #5) in blue.The lofting grid has been laid down in brown with (from left to right) thecenterline, B1 and B2 in vertical and (from down to up) datum,lowest-point-rabbet, waterline and transom height in horizontal. Note thatstation #1 it was mistakenly connected from chine to keel height at the stationnot rabbetline.

This should be done for all stations.

When this is completed the next step is to draw the waterlines based on the body-plan to the half-breadth view, and buttocks based on half-breadth and body-plan to profile.

For waterlines to the half-breadth we’ll need tick for every station in the body plan for the distance from the centerline at a right angle to the location of the intesection of that waterline and that station. As the centerline is perpendicular to the waterlines then it’s easy to achieve by simply measuring along the specific waterline. These measurements are then transferred to the half-breadth. Waterline ends in the aft and in the fore can be determined using the same set of techniques described in the previous post for determining chine and sheer ends in the aft/fore using measurements from the profile view at the waterline’s intersection with two neighboring station lines as a reference. Faired curves should then be drawn using the already described method of nails and a long batten.

Next up is transferring buttock lines to the profile view.

Fitting buttock lines from the body plan to the profileview.

As before - buttock and station line intersection points are picked up on the tick-stick from the body plan and then - relative to the datum - transferred to the profile view. Both buttock lines intersect the sheer in the fore, but in the aft (because of the way they were measured initially) B2 ends at the intersection of transom and sheer (as a side note: this also was not most probably the wisest of choices… but we’ll see), and B1 somewhere along the transom profile. Simply tick the location on station #9 and station #8 - then using a straight-edge batten where the line intersects the transom, that’s where the B1 ends.

And a again nails, batten and draw a fair curve.

All completed lines in the profile view at station #5… and reverselymislabeled buttock 1 and buttock 2 - these should be the other way around.So it is important to label everything ASAP because later on you will notsimply remember which line was what.

With the buttock lines transferred to the profile it’s time to check up on the work so far. Using a tick stick on every station line on the profile view:

Luckily - I guess because of the shape of the hull - I did not have to do any re-fairing of station lines in the body plan and could simply get on to drawing the diagonals to the body plan.

In the book “Lofting a Boat: a Step-by-step Manual”, Roger Kopanycia writes:

Once plotted and faired, Diagonals are used to correct any final discrepancies in the Body Plan. They’re generally considered to be the most accurate lines because ideally they are drawn square to the Body Plan curve, almost as if they’re following the line of the planking. In practice this is not possible, as all of the Stations have different curves, so a ‘best-fit’ line is drawn.

As the station lines for this boat are not curved, I decided to go with two pairs diagonals, placing them so that one would pass through the rabbet-to-chine (D1) and the other chine-to-sheer (D2) portions of the station lines. Both of the diagonal pairs intersect the body plan centerline in the same location, although this intersection location is arbitrary (and based on the shipwright’s best knowledge and experience). The most important thing is that a pair of diagonals (to the fore side and to the aft side of body plan) should start in the same location and have the same angle from the centerline.

Completed body plan with two pairs of diagonals from fore looking aft on the right, andaft looking fore on the left. Notice the fixes for the height of frameson keel for stations #9, #8, #7 and #6 on the left hand side aft view whichwere ticked incorrectly but luckily discovered during transferring waterlines tothe half-breadths view described earlier in this post. Also: station #5 has beenlaid down in both views as I got too carried away with ticking the fore stationmeasurements and forgot that it should be in the aft view instead.

The diagonal(s' curves) should be plotted just underneath the half-breadth view centerline but as already mentioned on numerous occasions before - free space is a problem so instead I plotted these on top of half-breadth lines using a different color pen.

Using yet another tick-stick, place it on one of the diagonals and mark the intersection of the body plan’s centerline and the location of all station lines that it passes, for both, fore and aft sides of the body plan. Then transfer these to (in my case) the half-breadth view using the centerline as a reference and mark the positions of diagonal-to-station-line intersection points on all stations. Similar to the sheer-to-half-breadth and chine-to-half-breadth (check previous post) the locations of diagonals intersecting the rabbetline in the fore and transom in the aft are picked up from other views.

For the rabbet:

for the transom:

Repeat the same procedure for all the remaining diagonals if any. When done then bang your nails and draw a curve with a batten to see if it passes through all the marked positions. If it does then we can assume that the rest of the lines are correct. If all the measured locations are not on the curve then corrections to the body-plan, half-breadth and profile are required.

All was good on the first go with the diagonals I had plotted and so I could move on to the next step which is developing the transom and transom fore face.