Lofting Requirement and Technique for a Ferrocement Hull

8/8/2019 Lofting Requirement and Technique for a Ferrocement Hull

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LOFTING REQUIREMENT AND TECHNIQUE FOR AFERROCEMENT HULL

2.1 Introduction

2.2 Boat plans 2.3 Mould loft floor 2.4 The grid 2.5 Laying off profile and plan view 2.6 Battens, splines and straight edges 2.7 Body plan 2.8 Additional information gathered 2.9 Transom projection 2.10 Patterns - general 2.11 Camber pattern

2.1 Introduction

The lofting requirements and techniques for a ferrocement hull do not differ in any greatway from those needed for other materials. Because it is easier to alter a pencil line thana section of steel once frames have been made, great attention should be paid toaccuracy of measurements and fairness of line to reduce any errors to a minimumwhich, in turn, reduces rectification cost and provides a fair shape to the frames ontowhich the hull reinforcement is applied.

There are many publications available written on the art of lofting to which the reader may refer. This chapter will provide a reminder to those already familiar with lofting but

not familiar with ferrocement construction.

2.2 Boat plans

A complete set of plans for the construction of a fishing boat will probably vary from onedesigner to another, but should include at least the following drawings for a ferrocementfishing boat:

1. Lines plan and offset table2. Hull construction profile and plan3. Hull section/frame detail4. General ferrocement reinforcement details

5. General arrangement profile and plan6. Sterngear and liner tube details7. Rudder gear and liner tube details8. Fuel tank(s) construction details9. Water tank(s) construction details10. Deck construction11. Superstructure profile plan and details12. Fish hold construction details13. Hydrostatics and Stability curves



and as many other details as requested by the builder from the designer, viz: deck gear fabrication details, fuel, water and electrics schematics, exhaust system, nozzle details,propeller details, and accommodation details. Usually, the drawings may be read inconjunction with a detailed specification drawn up by the designer which will normallycover all principal characteristics, scantlings, material specifications, machineryrequirements, electrics, deck gear, ventilation, cathodic protection, painting and safety

equipment.

2.3 Mould loft floor

Lofting cannot begin unless a good level platform has been prepared to ideallyaccommodate the full length and depth of both the profile and plan half breadth allowingan additional 600-1000 mm each way. It makes it easier, particularly if the lofting area isto be used frequently, to lay either hardboard or plywood sheets onto the lofting surfacefirst and paint the boards with white emulsion or similar before laying any lines.

Hardboard sheets should first be soaked on the underside with water and allowed to dry

before painting and fixing into position. This will help prevent the sheets from distortingonce laid.

The sheets are pinned at their joints allowing for easy removal and subsequent re-use if required.

At the same time sufficient boards should be prepared for the subsequent laying off of the body plan, which will ultimately be the final frame making shapes of the hull at thelocation dictated by the design. The number of boards required will depend on whether afull frame or half frame drawing is required for frame fabrication, or indeed whether theproduction schedule calls for two frames to be made at the same time.

2.4 The grid

The first stage after the lofting platform has been prepared is to lay the grid for the profileand plan of the hull. The designer will have indicated his stations spacing (which ideallyin the case of ferrocement construction should also be the frame locations) coupled withthe spacing of water lines and buttock lines (ref. Fig. No. 1).

Figure 1. Lines plan



2.5 Laying off profile and plan view

On the profile view the designer's offset table, coupled with any useful measurementsgiven on the lines plan, are used to lay off the underside of the keel, stern, sheer, rabbet,deck at side, knuckle (if indicated), buttocks and profile of the backbone and transom.

The offset table measurements will usually be given in millims to the outside of the hull,and either measured +/- from the DWL or LWL (datum water line or load water line) or avertical measurement from the baseline (ref. Fig. No. 2). Additional fore and aftmeasurements for the landing points of the lines should also be indicated on the linesplan by the designer, but invariably these points need to be picked up during lofting.

Figure 2. Table of offset related to lines plan



On the half breadth plan, similarly, measurements are taken from the designer's offsettable and laid off for the sheer, deck at side knuckle point (if indicated), water lines, anddiagonals. The designer should provide (sometimes he does not) measurements for finishing points for all the lines that fall between stations on the lines plan where they runinto the half breadth of the rabbet at the stern and the half breadth of the keel, andcentre line, at the aft end.

All the lines as they are laid are checked for fairness, and half the skill in lofting is in thechoice of either easing an irregular measurement, in or out, up or down, whilst keeping

the changes moving in a particular manner, at the same time remembering what hasbeen done on previous lines checked, without drastically altering the designer'sintentions as regards shape. The fairness of each line is 'in the eye of the beholder' andultimately is in direct proportion to the skill of the loftsman and designer.

2.6 Battens, splines and straight edges



At this point perhaps a note about splines, or battens, and straight edges would beadvisable. A fair line is better represented by choosing a spline of adequate length for the full extent of the line to be drawn, and thickness and width near to the maximum non-breaking dimension to suit the curve being drawn. The splines or battens should have along straight grain which should not split when bent. For the sheer line and other longeasy curves, a size of 40 x 40 and 50 x 15 mm will be enough. For the plan view lines, a

size of 25 x 10 mm and 20 x 20 mm will be useful. For the frame/station lines, battens or splines of 10 x 10 mm or a little heavier or lighter will give a fair line. Teak was always agood material to use for this purpose, but as long lengths of teak are now nearly a thingof the past, one needs to experiment with whatever is available prior to the final laying of the lines.

Straight edges are initially required for laying the grid lines but as they are invariablyused later in setting up the frames and helping to check level lines generally throughoutconstruction, a mixture of sizes are required. Two straight edges the length of whichexceeds the maximum breadth of the hull by 150 mm are needed, 150 mm wide and 18-25 mm thick. Other smaller general purpose straight edges will also be used.

2.7 Body plan

Once all the profile and half breadth lines have been laid, the revised measurements arenoted on a new offset table No. 2. Attention can now be turned to the body plan for thefirst time. The body plan boards are laid and a grid as depicted on the designer's linesplan drawn with all the water lines, buttock lines, diagonals and centre line of the boatmarked off as shown.

Initially, fixed points of both half breadth and height (+/- DWL or from baseline), for thesheer, deck line, knuckle line, underside of keel and intersection of frames on centre lineor rabbet line are marked in. Using the revised offset table No. 2 each frame can be

plotted by marking the half breadths along the water lines, the heights +/- DWL on thebuttock locations and distance from the centre line down each diagonal.

Using suitably sized battens/splines, the frame lines are drawn. Visually one can checkto see if the curves have met all the measurement locations, as well as whether theframe lines are in 'sympathy' with each other. By easing the frame lines and retaking themeasurements one can fill in a further revised offset table No. 3 and recheck that thechanges on the plan and profile views fair in.

By now the errors should be minimal and the number of measurements requiring minor modification are nominal. The body plan frame lines are rechecked and if the body plan,profile and plan views coincide on all the measurements taken, one can now say that the

measurements are faired and a final offset table may be filled in and kept as a record for future use, especially if the frame making boards are damaged or not stored properly.

At this stage, the designer should be advised of the changes to the original offset tablemeasurements.

2.8 Additional information gathered



Whilst laying off the lines, many additional measurements that may not only be useful tothe builder but also to the designer, may be included in the final offset table. The majorityof errors are in the 0-12 mm category and this sort of difference is expected when theoriginal lines plan is to a scale of 1:20. Many designers will have greater errors than this,hence the reason for full-scale lofting out.

Additional information has been picked up, the AP (aft point) being an example. This canhelp by providing fairing points for the splines aft of where a line may actually finish, aswell as providing a pattern to check out the transom shape and fairing in of the frames atthe setting up stage. Additional information has been included for the deck at sidemeasurement also the crown of the deck, which are useful check measurements whenproceeding with the construction, particularly if a ferrocement beamshelf is to beincluded in the construction.

2.9 Transom projection

The transom of the boat may or may not be vertical. If it is vertical, the measurements

will be the same as the AP. 4 However, if the transom is raked, it will need to have itsexpanded shape drawn. The centre line of the transom shown on the profile plan,becomes the centre line of the half breadth view of the expanded transom. Each water line is drawn at right angles to the centre line which increases the distance between thewater lines in accordance to the rake of the transom (ref. Fig. No. 3).

Figure 3. Transom expansion



The buttock lines are drawn in parallel to the transom centre line and spaced as requiredby the design. The expanded transom intersection points on the buttock lines is made bysquaring from the transom centre line and buttock intersection on the profile view, towhere they cross the vertical buttock lines in the expanded view.

The half breadth measurements for each water line are picked up by squaring the water line intersection to the rake of the transom on the profile view down to where theintersection is made with the water lines in the plan view. The resultant measurementbetween the transom centre line and water line intersection for each water line will givethe half breadth measurement required for transfer to the expanded view.

Once all the points have been marked onto the expanded view a batten can be used tofair in the marks and the resultant shape is a view of half the expanded transom. The fulltransom shape can then be drawn out on a patterning material and can be used for making any number of transoms for that design. It should be remembered that the shapedrawn will be the aft side of the transom.

2.10 Patterns - general



Generally speaking a small number of patterns are needed for the construction of aferrocement fishing boat. It is useful whilst the profile shape of the boat is drawn out fullsized (and the designer is forward enough with his sterngear and rudder gear requirements), to make up a profile pattern of the backbone shape including marking onit frame locations, the shaft centre line and rudder centre line. If the reinforcement iscurved above and below the sterntube, not only will the pattern give the shape

accurately to bend the main reinforcing rods to, but will also give a check at the framesetting up stage to ensure that the space between the transom and keel is as it shouldbe, to avoid any later problems in rudder gear fitting or indeed if a nozzle is fitted.

Because it is normal to paint over the lofting area for future use, a pattern of the stem,with water lines, stations and location of main reinforcing bars will be useful. As for allpatterns, they should be clearly identified and kept with the frame making boards in asuitable place for any subsequent construction of the same design.

2.11 Camber pattern

Finally, the pattern that not only helps the hull constructor, but is also used at fitting out,is the beam camber board. The camber board is made from the designer's requiredcamber usually in the region of 15 mm in 300 mm, the actual figure varying with thebeam of the boat and the specified amount of crown/crop in the deck. It is useful at timesto have a male and female pattern depending on whether the boatbuilder can use thepattern either over or under the reinforcement which may come in the way.

The camber pattern will need to be made from material 18-25 mm thick, a few cm longer than the full breadth of the boat and a little deeper than the maximum crown of the deck.

Lofting Requirement and Technique for a Ferrocement Hull

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