Offsets for Sloop MERRYWING
Our title would at first glance appear to suggest a confusion of bad grammar and bad taste. In truth it is an old, proper, and excellent definition of the first task in the art and craft of boatbuilding: the lofting process. In simple terms, the operation consists of drawing (“laying down”) an accurate, full-sized picture on the floor, from which patterns are “taken off.” The process is neither mysterious nor difficult, but there are some simple and essential truths. (In the learning process, there are one or two shocking truths, as well!)
When I was very young I held most naval architects in awe, and considered myself very smart indeed to have mastered the mechanics of lofting those sacred and untouchable drawings-waterlines, buttocks, magnificent diagonals, finally the body plan-and I crawled reverent miles on hands and knees, correcting tiny (and not-so-tiny) errors.committed by men who had mastered the greatest and most thrilling of all arts: yacht design. It was an honor and a privilege to be chosen as one who would, however inadequately, bring this vision to being in wood, metal, paint, and whatever else the specifications called for (most of these items bought by the Designer at one-third off, and furnished to me, or our common victim, the Owner, at list price, as I finally learned)-and it was wonderful.
Time passed, my work improved, my knees and my faith became worn and battered, and I discovered a shocking truth: Practically anyone who can read the figures on a yardstick can lay down a body plan and a profile; and that’s all you need to lay down, anyway. If the designer howls betrayal for some reason, there are two suggestions you can make about that set of lines and offsets. The polite one is that he take them back to his drawing board and correct them himself. If he can’t develop a curved transom, he shouldn’t expect you to do it for him. If he says you can’t get the angles for the stem rabbet from the scale drawing, he needs further education. And finally, if this lofting were half as difficult as you have been led to suspect, some of us old pros would still be chopping dugouts out of tree trunks, and managing to make that look like quite a mystery, too.
So let’s lay down what is strictly necessary, and no more; build a set of molds; make two dimensional patterns of stem, sternpost, tail feather (the late Sam Crocker’s term for the keel-of-the-counter, usually called the horn timber), and transom knee; and make the pattern for the ballast-shoe casting. This last item is the only difficult one of the four, and will be treated separately and at length later.
The basic grid
For this laying-down business you need a reasonably smooth floor, slightly longer than the boat and slightly wider than the total of maximum draft plus greatest freeboard-in the case of our example, 10 by 40 feet. You can manage on half this length or even less, but you’ll have some confusion of lines-over-lines to cope with. If the floor is good enough, and the owners don’t object, give it a coat of flat white paint, and work directly on that. If it’s the floor of your living room, or as rough as my shop floor, cover it with light-colored sheathing paper (40 inches wide, 500 square feet to the roll, available at any lumberyard), which you spread out and let lie for 24 hours before you stretch and tack (or tape) it in place. Don’t worry if the experts tell you this is all wrong, and that you’ll never be able to do accurate work on a surface that changes dimensions every time a cloud goes by. You’re going to have expert trouble from now on, anyway, and you might as well get used to it. (One of them used to haunt us with the threat that he’d get out his astrolabe and prolapse and show us the scientific way to figure out the shape we were seeking; and for a while we wondered that one small head could carry all he knew.)
Get yourself a 10-foot straightedge. A 4-inch strip off a ½-inch plywood panel makes a good one; or you can true up the edge of a dry board with your jointer. While you’re at it, make a 6-footer and a long one-maybe 16 feet long. Stretch a string (nylon is best) the length of your loft floor, far enough from the edge to allow for the full draft of the boat, plus 2 or 3
inches. Do not chalk and snap it; instead, crawl along and mark where it lies at 3- or 4-foot intervals. Take up the string, mark this line with a good black number 2 pencil, using your 10-foot straightedge, and you have the load waterline, from which everything else develops. Using the same technique, mark the other full-length lines (three above, four below the load waterline, and exactly 12 inches apart) parallel with the load waterline. Now mark on the load waterline the locations of all the perpendiculars shown in the lines drawing-face of stem, station numbers 1 through 8, intersection of the sheerline projected through the centerline of the transom, and all the buttock lines you’ll need on either side of station number 5. Draw these lines in, exactly at right angles to the load waterline. The safest way to do this is to erect station number 5 by the old high-school geometry method of swinging intersecting arcs above and below the line, marking station number 5 through these intersections, and then working forward and aft (with your 50-foot steel tape) from station number 5 along the 36-inch load line and the 48-inch waterline. If your straightedge, joining these new marks, passes precisely through the corresponding marks on the load waterline, you will know your measurements were correct. All this is dull business, and perfectly obvious, I’m sure.
The diagonals for the body plan are only a bit more complicated (see Figure 1-1). Notice that in this design they all start at points where station number 5 (the centerline for the body plan) intersects the horizontal load line and waterlines, and they all pass through intersections of buttock lines with these same horizontal lines. Thus, diagonal Dl starts 12 inches below load waterline at centerline, and passes through the intersection of the 24-inch buttock and the 36-inch waterline; and diagonal D5 starts 36 inches above the load waterline and intersects the 12-inch load line 5 feet out from the centerline. Draw them all in, then, forward and aft of station number 5, and you are ready to start working from the table of offsets. The hard work is over, and the fun is about to begin.
The table of offsets
Figure 1-1
I have known bright people to whom a lines drawing resembled a cross section through a barrel of frozen angleworms, and meant but Iittle more; and these same people thought of a table of offsets as something you might expect to come from the maw of a mad computer that had been fed on Pictish runes, rock and all. Both these conceptions are faulty and exaggerated. If you have managed (as I did, rather late in my childhood) to master the technique of drawing a line from 1 to 2 and so on in proper sequence to 87, and got for your diligence the picture of a nice horsie, you should have no trouble with a table of offsets. Any figure in any one of the little boxes simply tells you to start from a known point, proceed along a carefully labeled line for an exact number of feet, inches, and eighths of an inch, and there put a pencil mark. For heights, you start at the load waterline and measure up or down, as common sense and a glance at-the scale drawing indicate. For breadths, you start at the centerline and measure out horizontally. For diagonals, you start where the diagonal starts (at the centerline of the plan) and measure along the line of the diagonal. When you’ve located and marked all these spots, you draw a fair curve (or sometimes a straight line) through them, and get, full-size, a line that I hope looks amazingly like the corresponding line on the scale drawing. Occasionally some sadistic N.A. will take all his vertical measurements from a base line, or something he prefers to call the designed waterline, but he usually gives you a hint that you’d better watch out.
Now that the above is all clear in your mind, you are probably itching to get at the body plan and make the molds. For these, however, you need to know the exact height of the top of the backbone (keel, stem, stern knee, tail feather) where each mold stands, the corrected height of the sheer at each station, and the half-widths of the backbone, from the centerline to the rabbet, where the molds straddle it. So curb your impatience, and lay down just enough of the construction profile to show the shape of each piece of the backbone assembly, the line of the rabbet, the exact location of the shaft alley and rudderport, and the line of the sheer in profile. Indicate (and label, lest you mistake these lines for something else later) the positions of the principal fastenings in the backbone. If these are not shown in the scale drawing of the construction plan (as they certainly should be), demand them from the designer. Finally, lay off from one of the full length horizontal lines (assuming it, for the purpose, to be the centerline of the keel in plan view) the half-breadths of the rabbet, for each station, as given in the table of offsets. The load line 36 inches above the load waterline is the best one to use for this particular half-breadth, because it’s in the least-cluttered section of the floor. If you want to lay down the sheerline in plan view, use the waterline 12 inches below the load waterline for your theoretical centerline, lest you measure yourself right off the side of the floor.
So much for the general plan of attack. Now let’s choose weapons and carry it out.
The construction profile
Start with the rabbet line on the keel ( see Figure l-2). You will observe that this is a straight line from station number 3 to and through station number 6, and that the table of offsets therefore omits the height of the rabbet on stations number 4 and 5. This straight section of rabbet is the most important line of reference in the entire laying-down and setting-up processes, so get it right-and extend it to station number 2 forward, and to station number 7 aft. Use your nylon string all the way, and be sure. Now note that the top of the wood keel is exactly parallel to, and 3½ inches above, this straight rabbet line. Mark this in, all the way from number 2 to number 7; repeat the performance for the bottom of the wood keel, exactly 1 ½ inches below the line of the rabbet. This last line is also, of course, the top of the ballast keel. It might be worth your while to use a red pencil for these last two lines and all other construction (as distinct from design lines) details. Note, finally, that all the heights in the boat are based on the line representing the top of the wood keel: the stem, the stern assembly, the four principal molds, and, eventually, the underside of the cabin sole. Check the offsets, check your measurements, check your youthful exuberance, and get it right.
Figure 1-2
While your straightedge is still warm, and before we get to the subject of battens, mark some more straight lines: the centerline of the transom; the profile of the tail feather, from its intersection with the transom, across the top of the stern post; the lines of the rabbet on the tail feather; the centerline of the rudder stock from the deck to the heel of the stem post; the centerline of the propeller shaft; the straight portion of the profile of the bottom of the ballast keel; the straight portion of the rabbet line on the sternpost. These straight lines, each joining two points exactly located (by measurements on the lines drawing or from the table of offsets), will precisely determine the starting points of the curves you are about to draw.
Now about battens. You’ll be using these in practically infinite variety every time you turn around on this job, and for a long time to come, so you’d best start your collection now. You’ll need two immediately, at least 22 feet long, about ½ inch by 1 inch in section. These will overlap to mark the sheer line, here on the floor, and, later, on the planked-up hull. One of them will do for marking long planks. The curve of the stem requires a limber one, 1/z inch square and at least 16 feet long. The rabbet on the sternpost, and the forward end of the ballast keel, must be marked with very limber battens indeed-straight-grained oak or ash, less than 3/s inch square. These will do also for the body plan and molds, and at least two of them must be over 8 feet long. And when you come to the outline of the curved transom, you’ll be an old hand at this business and be able to judge for yourself what’s needed.
If you don’t already have your ribband stock, pick it out now (2-inch by 4-inch, 6-inch, 8-inch, 10-inch clear fir, if you can get it, at least half of it 22 feet long or better) and steal your battens out of it. Clear white pine is the best and most pleasant to use, but you’ll not be likely to find a board over 16 feet long. Saw out half a dozen of them anyway, from IO feet up, and about 3/4 inch square. Build a batten rack on the wall, out of reach of young fishermen and your own big feet.
So now you are equipped to finish laying down the construction profile. Do the face of the stem first: height at the sheer from the offset table. All other points (measured horizontally on the waterlines from the forward perpendicular) are taken from the scale drawing. Start a fivepenny box nail at each point, pull your 16-foot limber batten in against this curved line of fence posts, ease it in or out where necessary to correct for slight errors (holding it in place with nails driven against, not through it), and mark. Move in and mark the rabbet line in the same way, with your batten flowing into and following the straight line previously marked. Now draw, on the floor, the inside face of the entire stem, the scarf joint, and the jog at the forward end of the wood keel. You will have to scale some of these dimensions from the plans. Go aft now and do the same job on the entire stern assembly-the main and outer stern post, the tail feather, the knee to the transom, the completed rabbet line, the aperture for the propeller, and the bolt pattern.
If you are still able to bend over, mark the height of the sheer at each station ( dimensions, from the table of offsets, up from the load waterline), correct with long battens until fair, and mark. Do the same for the half-breadth of the sheer (working from the assumed centerline 12 inches below the load waterline-remember?) and for ·the half-breadth to the rabbet. Note that this width must be exactly 2 inches at the point where the rabbet leaves the keel and goes on to the stem, and exactly 2½ inches where the rabbet intersects the stern post-these figures being the halfsidings, of course, of the stem and the stern post, respectively. While you have them fresh in mind, draw them in as they must appear in the body plan: stem siding 2 inches forward of the station number 5 ordinate, and sternpost siding 2½ inches aft of it. Be very careful henceforth, when laying out half-breadths on the body plan, that you do not mistake one of these for your centerline. (Actually, when you get into the swing of it, you’ll find that you match the 3-foot mark on your rule with the 36-inch buttock, or whatever, and ignore the centerline altogether except when laying off distances on diagonals. Thus you avoid errors and save yourself much crawling.)
Now for the molds
Now is the time to lay down the body plan, which gives the exact outlines of eight cross sections through the hull, and from which (after subtracting the thickness of the planking) you will derive the shapes of the eight molds. (See Figure l-3.)
Figure 1-3
Start with station number 5. Find the point where the straight line of the rabbet in profile intersects the station number 5 ordinate. Get the half-breadth-to-rabbet distance from the
line you laid off, above, from the 36-inch load line. Square out this distance, forward, from your point of intersection, and mark the spot. This one is sacred and final. Locate the uppermost (sheer) point in the same way-out, forward, from the intersection of the sheer profile with the station number 5 ordinate, to the distance shown on your plan view of the sheerline or the breadth called for in the table of offsets, which should be the same thing. Be sure that this point is at the correct height above the load waterline, as taken at the station number 5 ordinate, and not as it appears so attractively before you where you squat 5 feet forward of station number 5. (I hate to belabor the obvious, but I have fallen into this error myself.) Now, working from the table of offsets, mark distances out on all horizontals; heights above or below the load waterline on all buttocks; and distances from the centerline on all diagonals.
Set up your row of five penny nail fence posts, and with trembling hand, bend your best oak batten into the curve …. Take a deep breath, calm yourself, make sure that you haven’t made any mistakes in reading or marking those offsets, and proceed to move this or that nail to get a fair curve on the batten, bearing in mind that of all the offsets, the diagonals are most likely to be correct. When you are satisfied that the curve is fair and yet as near as possible to the original offset points, draw it in and go on to the next one, and the next, until you have all eight done, with no more help from me. I can do this whole body-plan job in less than three hours, which indicates not that I am a fast worker, nor even a very careless one, but simply that it’s a quick and easy business after all.