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HOBBY MAGAZINE JULY 1966 Very little alteration has been made to the lines of the canoe body of Red Herring apart from a small modification to the bow section and the elimination of the canoe type stern. In fact the fin and bulb for Caller Herring could be fitted without any difficulty to an existing Red Herring hull if the owner wishes. I know that a number of glass fibre moulds exist and it would be no problem to fill in garboards of the mould and produce a suitable hull. It should be possible to get worthwhile comparisons of performance between Red Herring and Caller Herring and any differences can be attributed to the change in keel and sail plan. This should be a very interesting exercise. Caller Herring is 2 lbs. lighter displacement than her predecessor, but this is because there is little displacement in her fin and I did not wish to alter the canoe body by putting this 2 lbs. into the hull and thus destroy the value of comparative sailing. At 32.5 lbs. Caller Herring is not a light displacement type but may suit inland lakes better than Red Herring. I must say here that I do not believe that the low C of G of a bulb keel and the obvious increase in righting power when heeled is a substitute for reasonably heavy total displacement. One should not confuse the need for sheer weight to overcome natural forces in heavy weather with the need for power to carry sail, which is a function of righting moments. Both factors are important and I cannot say which should be given preference, although I am a bit inclined, at this stage, to say the former. This new design will help to make this point clearer. The 22 1b. lead bulb and its low position should give Caller Herring a big advantage over Red Herring in sail carrying power and as a result the new sail plan is of ultra high aspect ratio. One of the disadvantages of a lead bulb is that it possesses considerable wetted surface. In this case it is nearly 90 sq. inches and this surfaces contributes nothing to the lateral resistance of the keel and only produces drag. The fin itself is smaller in area than the conventional arrangement so that the total wetted surface is reduced, It is possible that the bulb increases the efficiency of the fin by virtue of acting as a fence. However, we do find ourselves with possibly less effective lateral area and a big slice of unproductive wetted surface. It remains to be seen whether or not the smaller fin needs to operate at a higher aig a of attack in order to develop the required side force and so in- crease the drag of the whole boat, or whether the improved righting moment of the low c.g. ballast and high sail plan are a sufficient advantage to end up with a total plus in performance. 291 | | fase / / / [ susene ce ab tome =~ — OaNeS30 ONINYIH YITIVD why I do not produce a 10 rater of fin and bulb keel design. Thus urged, and supported by the growing popularity of this type of appendage both in model classes and in full size application, it seems worthwhile having a look at the effect of such a keel on an existing 10 rater. Red Herring has proved to be one of the most successful 10 raters for some time, so I thought a fresh look at that design would be profitable. Hence the name of the new boat! a8) I HAVE had a number of letters recently asking me A® nee “GESr ‘CLMaN “ORLSEeN TRAGH “As ABULB-KEEL 10-RATER BY JOHN LEWIS smart wr CALLER HERRING ey 1966 3DIANIS SNV Id Y3xVW 1300N JULY MODEL BOATS LEAD WEIGHT ABOUT 4 L85S. FIG, 4. MIDSHIP SECTIONS NOT TO SCALE CARGO SHIP | | WATER LINE N STERN TRAWLER | TUMBLEHOME | | SMALL PIECE” Part two WATER LINE | OF FINE s . SANDPAPER | (continued Gluta TO from May BASE INCHES x toa et 2 iS She x x coe he issue) HARDWOOD BASE 7″ x 13″ x yd IN USE, WIDTH OF FLAT PLATE KEEL RISE OF FLOOR WEIGHT SHOULD REST ON SPLINE THUS SPLINE = CPAPER Thoughts on Lines by J. W. Holness Ale shapes of various types of hull appear very different but the same general principles apply to them all. Water does not like to go round sharp corners, least of all at speed, so that any hull intended to be driven economically should have smooth fore and aft curves with no abrupt discontinuities. The part of the buttocks rising to the waterline at the stern should be a flattish curve to just above the waterline and as shallow a slope as the hull depth will allow. A very round form of buttock here will Cp, waterline length and desired displacement, the area of the midship section can be calculated. The easiest way to measuring the area of the sections is to draw a grid of squares to a suitable scale on a piece of tracing paper, place this over the sections and count the number of whole squares and squares half or more within the boundary line. The accuracy of this relies on the accuracy of the grid and the size of the squares. Obviously, the smaller they are the more accurate the result, but if they are to small one goes cross-eyed counting them. To calculate the volume of displacement, the waterline should be divided into an even number of parts, usually ten, and the cross-sections drawn at these points. When all the sections have been drawn, the underwater area of each is measured by the grid method. It is worth plotting a Curve of Areas to check on the fairness of the hull and to see that the areas of the sections have been correctly measured. If the lines appear fair but the spots for the areas will not lie on a fair curve, measure the areas of the offending sections again. tend to encourage a large stern wave. At the bow, the shape of the waterline is the main consideration, in particular, the angle of entry of the load waterline, see Fig. 1 (May issue). This angle should be no larger than necessary and for fast hulls should be as small as possible, with the maximum beam at the waterline well towards the stern. Width at the waterline is one of the factors that help stability, so small sailing vessels usually have fairly large angles of entry as stability to carry sail is a primary consideration. A sailing vessel also needs beam, a stiff section with a hard bilge, and enough draught to enable her to sail to windward, and must carry some ballast low in the hull or on the bottom of the Me Shen volume can then be found using Simpson’s first ule: keel. The problem of how much beam, ballast and bilge to give a sailing yacht has taxed yacht designers for many years and will probably continue to do so. Powered cargo ships and the later commercial sailing vessels have a very small rise of floor amidships with a small bilge radius and vertical sides with slight tumblehome above the waterline, see Fig. 4. Most cargo ships have a greater or lesser length of parallel middle-body, i.e. a number of frames amidships have the same shape. Tankers and bulk carriers may have as much as 30% of their length as parallel middle-body whereas a fast cargo liner will have about 10% of her length. Smaller vessels with a greater beam length ratio do not usually have much parallel middle-body, otherwise the ends become excessively bluff. Small working craft such as fishing boats, pilot cutters, and ferries, which travel at high V//L values do not have any parallel middle body. Such vessels have much greater rise of floor and bilge radius than cargo ships. It is all a compromise between length, narrowness and easy sections for speed, bluff lines for cargo carrying, beam for stability, etc. Before embarking on the actual drafting of a set of Lines, it is advisable to have some idea of the required displacement. The achievement of this is a matter of trial and error but it is helpful to know the correct prismatic coefficient or Cp for the type. The Cp is the ratio of the underwater volume of the hull to the volume contained in a solid, having the waterline length of the hull and the same section as the midship section. Some guidance figures are given in the accompanying table. Knowing the for a waterline divided into ten parts. If h is the distance between sections in inches and y; y2 y3 etc. are the areas. of sections 1, 2, 3, etc., in square inches, the volume is in cubic inches. If the areas are for half the section, the answer should be doubled. The weight of an equal volume of water can be calculated from the fact that one cubic foot of fresh water weighs 62.4 lbs. If the answer comes. out less than the weight estimated for the vessel, the Lines. must be filled out and the displacement checked again. An alternative Rule is the Trapezoidal: Voldine=h (%: + Y2 + Y3 ons Yo. 7 ¥i0. -F 1) 2 2 This is only accurate for hulls whose curves are very easy, but not for a vessel with a lot of shape as it assumes that the Curve of Areas is a series of straights between sections. Unlike Simpson, it can be used whether the waterline is. divided into an even or odd number of parts. If desired, the position of the longitudinal centre of buoyancy may be found by cutting out the shape of the Curve of Areas in paper and balancing it on the edge of a ruler to find the centroid. This will be in the same fore and aft position as the centre of buoyancy. As regards the instruments required, to buy the proper equipment would be prohibitively expensive. One can, however, get by quite well with cheaper alternatives. Some kind of drawing board is desirable but this need only be hardboard or thin ply, if it can be rested on a flat surface. Thicker plywood or blockboard is better as it is then stiff enought to be supported on a few points. This is am 300 JULY advantage if one is using a polished table, as the board can then rest on two or three pieces of wood which are padded underneath. A piece about 4 in. or 3 in. thick should be stiff enough, other dimensions depending on size of model to be drawn. If a very thin material is used for the board, the paper will have to be fastened to it with Sellotape or masking tape. If any case it is difficult to push drawing pins into ply because of its hardness. Whatever the board is made of, the surface should be smooth and without a prominent grain. In use, it should be covered with two or three layers of smooth, hard, white paper, cartridge paper being the ideal, otherwise the pencil point may follow the grain of the wood. Unless one has a proper drawing board there is little point in having a T-square, though some form of straight edge is almost essential. This can be made from a thin piece of hardwood or plastic and should be the same length as the board. Much ship draughting consists of drawing curved lines. As many as possible are drawn using a flexible batten or spline of wood or plastic held in place with lead weights. If the line has too much curvature for a spline to follow, a plastic or wood shape, called a ‘curve’ or ‘sweep’, depending on type, is used. This may fit only part of the curve to be drawn, so it is necessary to have a selection of different curves. Stiff splines can be made using selected strips of ramin, carefully sanded, but flexible ones are more difficult and it may pay to buy one or two—they cost about 10/- to 15/-, depending on the type. Elbows, books and spare hands may be used as a substitute for lead weights but, for serious work, the real thing is required. Five is the minimum number that would be any use, ten would be enough for most things. The accompanying sketch shows the appearance. A foundry will cast them quite cheaply if one provides a pattern and can scrounge enough lead from somewhere. French curves may be bought quite cheaply and would do for a start although the shapes are somehow not right for ship work. Unfortunately, proper ship curves are expensive, hence most ship draughtsmen make their own in plastic, copying the shapes from their colleagues’ curves. The cheapest ship curves to buy are probably a set of Dixon Kemp ‘Pears’. These are primarily intended for yacht work but are very useful for drawing sections. Other items required are two 6 in. setsquares, a good ruler or scale, several pencils, H, 2H & 3H, hard and soft rubbers and some drawing paper. Cartridge paper is best but expensive. White lining paper will do as an inexpensive substitute but any erasing must be done very gently as it is easy to make holes in this paper. If itis proposed to do much work in ink, a ruling penis necessary, Otherwise a mapping pen will do. The actual drawing is commenced by very carefully setting out a grid, i.e. waterlines and sections are drawn where the profile will be, waterlines, buttocks and a centre line for the body plan and buttocks, sections and a centre line for the halfbreadth plan. All these will be straight lines either parallel or at rightangles to each other. This must be done as accurately as possible, great care being taken to get these lines square to each other, otherwise it will be impossible to fair the lines. It is advisable to draw centre lines and the datum waterline in ink, otherwise they may get lost in the inevitable rubbing out. It is practically impossible to draw a set of lines straight off, without erasing and altering as the drawing progresses. If tracing paper is used, the whole grid can be 1966 APPROXIMATE VALUES OF PRISMATIC COEFFICIENTS Type of Vessel LINER CARGO LINER CARGO SHIP TANKER Cp 0.60—0.68 0.65—0.70 0.65—0.85 0.75—0.85 TUG 0.55—0.65 TRAWLER 0.60—0.68 MOTOR YACHT 0.50—0.60 SAILING YACHT 0.50—0.53 CROSS CHANNEL 0.57—0.62 largely determine the appearance of the hull so some care should be taken to get them right, particularly the sheer. The midship section is then drawn, the underwater area being calculated as described above. It will be found easier to sketch this in freehand at first until the correct area has been obtained. Next follows the datum waterline and the quarter beam buttock. This is the buttock positioned at about a quarter of the beam out from the centreline. The midship beamand depth for theselines are taken from the midship section and the position of the ends are picked off the profile and half breadth plan, respectively. It is probably best to draw the waterline first, then the buttock, adjusting the waterline to suit if necessary. It should be borne in mind that the forward end of the waterline should be no bluffer than necessary and that the part of the buttock rising to the waterline no steeper than necessary and a fairly flat curve. In many types of vessels a short length of buttock near the waterline may be a straight line, faired smoothly into the rest of the buttock. The shape of the hull is now more or less fixed and it should be considered for a while before proceeding. Any major alterations made now involve much less rubbing out than if made later. It is possible to finish the design off in a number of ways. A good way is to draw a level line at about half the freeboard height, then a waterline one or two below the datum waterline, then sections, one division in from each end of the datum waterline, adjusting the waterlines if necessary. Two diagonals may then be drawn—one through the tuck and one through the turn of the bilge. They should cross the sections nearly at right angles. The remaining sections, buttocks and waterlines are then drawn in any convenient order, followed by the Curve of Areas, rabbet line, etc. If designing from ‘scratch’, a Lines Plan will take quite a few evenings work. It is a good plan, when one has got the main lines down, to prop the board up where it can be seen from one’s chair. A better idea of how the design is shaping can be obtained by studying it from a distance. For the same reason, it is a mistake to make the drawing too big, so if the intended model is a large one, it is advisable to draw it at a reduced scale such that the hull is not more than about 18 in. long. If there is a rapid change of shape between adjacent waterlines, sections or buttocks, either they are too far apart or the hull is unfair. Sections in particular should change shape smoothly from one to the next. A beginner may find splines awkward things to handle, especially if they are stiff. It must always be remembered that the draughtsman, not the spline, decides the shape of the curve, though on the other hand, the spline should not be forced into an unfair curve. The fairness, or otherwise, of a curve may better be seen by looking along it from low down at one end. For the first attempt it would be better not to design from ‘scratch’ but to start with one of those model plans that show profile, deck outline and two or three sections and to work this up into a complete plan. An alternative would be simply to copy a drawing. This would give practice in using the instruments. The first design from scratch could well be for a single or double chine hull with straight line sections, thus reducing the number of curved lines to be drawn to five or seven. By the time the beginner has drawn half a dozen or so Lines Plans, he drawn on the back of the paper so that none of it is lost in rubbing out. The grid finished, the sheer, keel line, stem and stern outlines are drawn on the profile, followed by the deck outline or sheer line on the halfbreadth plan. These lines (Concluded on page 303) 301
