Doug's BoatBuilding Primer
Copyright (C) 1996, 1998 by Doug Garmon
Doug's Boatbuilding Primer emphasizes "traditional" boatbuilding methods rather than stitch-and-tape, and employes them to construct an Inuit kayak. First-time S+T builders would probably benefit by reading it, also.
"Traditional", in this case means modern traditional western boatbuilding (which means building on a male mold with plywood and epoxy).
This article is intended to give a little information and also a little perspective about building a wooden boat. As with most instructional documents, it's a good idea to read the whole thing thoroughly before beginning a project.
There are multiple types of boat constructions you might choose for a small craft. These are the methods I prefer, all of which do not require internal "bulkheads" or fasteners (screw or nails) in most cases. Each method is "modern", as is requires a saturation resin (epoxy). Each method creates a very strong hull.
Tools I couldn't do without:
This isn't nearly as complicated as it may seem. Many of the 'rules' of wood for boatbuilding are suspended for small, light craft. Kayaks in particular, with limited lifespans, are open to all sorts of creative wood sourcing.
You have great flexibilty if your boat is to be used in the following ways:
Traditionally, two of the most used woods in small boats have been spruce and ash. Both are light, bendable and strong, but neither has much rot-resistence. I would use them without hesitation.
My favorite is douglas fir, somewhat heavier and more brittle but with decent strength and good durablility (rot-resistence). It's also easier to obtain and cheaper than the above two.
White oak, which has great history, is a little heavy for a kayak.
Cedar, Red or White, is an acceptable wood, and very durable. It is light, but does not have the strength of hardwoods such as ash. It takes glue well (it is very porous, so don't be stingy) however, and it would be preferable, certainly, to white pine.
White pine has little durability, only fair strength and is usually sapwood. Despite these downsides, it is light weight and I would use it if none of the other woods are available.
Red oak, easy to find, it as only one positive--it's fairly strong. Otherwise it is heavy and has little durability and is not appropriate for boatbuilding.
The preferred skin (hull planking) for a plywood boat is of course 4mm marine plywood.
I have used standard 5.2mm lauan underlayment--common these days for CHEAP homebuilt boats. The wood itself is fairly durable, only the glue used is suspect. If you're gonna build a boat and beat it to death, it's a way to go. Just be sure the lauan is sealed (varnished or epoxied) well on the INSIDE as well as the exterior of the boat.
A real monster could be build with 1/4'' exterior fir plywood. But you'd have to have galley slaves to row and carry the thing.
Lofting, or laying down, is the process of expanding the drafted plans (offsets) into a full-size drawing of the boat. Once enlarged, it's possible to lift many of the important measurements and angles from the lofting, usually with great accuracy. In traditional boatbuilding, this was one of the most vital steps in preparing for construction, as it eliminates much guesswork and allows the builder to check for inaccuracies in the offsets.
Many boats were built in large open buildings and the only way to subdivide such a space was to do so vertically. Lofts above the construction area were the norm, and coincidentally the same dimensions, so they were the logical place to enlarge the plans to 1:1 size. Thus the name.
Lines are layed down with long, flexible wooden battens, kept in place with nails or 'ducks' while the 'fairness' of the curves is judged.
I have skipped this step for some designs, as they were designed on computer (using a structured draw program, incidently, not a cad prog) and I felt the resolution was adequete for pulling accurate offsets. Although it worked, I'm not sure I'd recommend skipping this step. On the other hand, If a quick-and-dirty boat is your goal, lofting isn't really needed on a small double-ended kayak. Other small boats, such as those with angled transoms and boats which don't employ epoxy in their construction require lofting. Epoxy covers a mutitude of sins.
If you choose not to loft the boat, pay special attention to fairing in the stringers.
Certain sections WILL need to be drawn 1:1, most notibly the stems. There are alternatives, tho--placing the drawings in an overhead projector and tracing the outline directly to wood. Or using a 35mm slide and projector to do the same. Just be sure to get the dimensions scaled correctly in both x+y axis (slide must be on a parallel plane to work piece).
Or you could enlarge the drawings and tile them together.
A rock-solid frame is needed as a base for constructing the boat. Various alternative are available--the easiest way is to bolt two 1"x6"(or 8")x16' pine planks together at the ends and then force a spreader (2x4 or 2x6) between them in the middle. This makes a very simple, stable 'bow' strongback ('bow' as in 'bow and arrow'). Tension assures the frame will remain rigid. Be sure to true the planks with a plane BEFORE bolting them so they are perfectly flat on the side you intend to use.
A 'bow' strongback should be a little shorter (8-12 inches) and a little narrower (3-4 inches) than the finished boat.
Include a 'spine' down the center of the strongback. I simply mount 1x2 furing strips flush to the top of the strongback. Establish a centerline with a chaukline (drawn over with pen for a permanent line) or by installing a tensioned wire.
You can then mount the strongback on sawhorses or whatever. It's a good idea if your mounting surface is rigid also--string the sawhorses together with 2x4's or scrap wood.
Strongbacks are usually not required for stitch+tape boats.
Traditional wooden boats are build using stations, or the longitudinal bulkhead-like things that define the shape of the hull. In some instances, these patterns BECOME bulkheads, if they are left in the finished boat. Although this was the case with the original 'Qivitoq', I wouldn't recommend it for these reasons:
Standard material for station molds is 3/4" (or 5/8") plywood. Since they will NOT be integrated into the design, they don't need to be exterior or marine grade. If you wish to integrate the stations into the boat, use 1/4" exterior or marine plywood (or the same thickness you are using for the hull), and reenforce the perimeter (edges) with 1/2"x3/4" battens.
Rough-cut notches to accept the keelson, chinelogs, etc, (if required) before mounting the stations. Think before you cut, as where the station will be mounted affect the cut.You can fine-tune the cut angles once the stations are in place. Stations which are farthest fore and aft should be also notched to accept the stems.
Remember that the boat must be lifted from the mold after completion, so enought material must be removed from the station NOW to enable this.
On most plans the offset measurements are usually taken from the keel upward, so the keel is baseline, or zero height. This orientation must be flipped for creating the stations. A convenient reference point (for kayaks) is the height of the bow stem (usually the highest measument), which becomes the baseline, or the top of the strongback. Points on the stations can be found by subtracting each offset height value from the height of the stem. If the stem is 14" high, and point at the base of the keel is 0", that point on the station (or point above the strongback) will be at:
Likewise if the chine is 1.5" above the baseline at that station, it's point on the station is:
Now mark the location of each station along the centerline, down the spine of the strongback.
Stations must be mounted square to the centerline, and perpendicular (90 degrees) to the top of the strongback. Install 1x4 cross-spalls across the strongback at each station. Use a carpenters square (from the centerline) to assure squareness and fasten the cross-spall to the strongback. Tack or screw each station into the edge of the corresponding cross-spall.
Use the carpenters square to verify that the stations are perpendicular to the strongback (i.e. the centerline). Brace the stations with 1x2 or other scrap lumber. The bracing can be tacked or screwed to the nearest cross-spall, or if the stations are too far apart, other cross-spalls may be added.
Before any structural component can be added to the station molds, they must be faired-in.
A batten can be used to check the accuracy where the stations have been notched to accept any logitudinal components (stringers, battens, keelsons, etc.). The notches must have the correct angle--remember, the measurements at any station represent a plane of zero thickness, so you must compensate for the 3/4" thickness of the station. Depending on where the station is mounted will define how the notch is cut.
The station nearest the center of curvature of the boat will require little, if any, fairing.
Structural timbers essentially define the 'frame' of a boat. They are the longitudinal members, the dimensional and the internal bracing which help form and reenforce the shape. In a small plywood boat, their main purpose is to provide a glueing surface for the epoxy adhesive.
Important structural components include:
Other structural members certainly exist. Thwarts, frames and a multitude of design-specific details may be used.
With the stations installed, the general shape of the boat is becoming apparent. The following steps will define that shape significantly.
WOOD FOR THE STEM
Many choices here, all will serve.
Because the stations lift the hull off the strongback by height of the highest stem (the bow), the top of that stem falls on the same plane as the top of the strongback. Since the strongback is a little shorter than the boat, tack a 1x4 extention on one side of the strongback. Make it long enought to fit under the stem end (and a little more).
Do the same for the stern stem. It will not lie on the strongback plane, however, but higher. Use a piece of scrap wood with the 1x4 to set the correct height.
Find the endpoints of the boat by measuring out from the last stations on each end. Mark them on your extentions. Check the total length-- if it's wrong, you've made a mistake mounting the stations.
Fit the stems in place. They should fit in a notch in each of the outermost stations. The stem ends should be placed where you measured the endpoints of the boat. Be certain the stems are precisely on center. Tack them in place.
INSTALLING THE KEELSON
Generally the keelson extends across the bottoms of the stems. This is a design decision, but usually there is more glueing surface by following this model.
Prepare the bottoms of the stems by laying a batten in the stem notchs on the stations, and letting it overlap the stems. Assure the batten's fairness, then scribe a line on the stem where the stem and keelson will join. Mark both sides of the stem. Remove the batten and carefully cut off the excess stem, using the finest-tooth handsaw (japanese, x-acto, etc) you own.
Clean up the cut with the block plane for a good glueing surface. Clamp the keelson in place on the stations, then epoxy it to both stems. Clamp the ends, or alternately, screw the keelson in place with a S.S. or bronze woodscrew (make sure the screw will not be in the way when the stem/keelson unit is shaped to accept planking). If the stations are not integrated into the hull, do not glue the keelson to them!
The stringers define the plank edges. If the design is batten seam, the stringers will be lifted from the mold and integrated into the hull. Fasten them from the inside.
If glued lapstrake, the stringers will be used only as a guide and remain permanently affixed to the mold. In this case the battens must be covered with a release compound (such as parafin wax) so that the hull may be removed after glueing.
Look carefully at your building mold, especially from the ends. Do the curves look smooth and true? Is each side symetrical?
Your eye is more important than the designer's offsets. If something looks off, correct it now. Trust your eyes.
Small corrections can be made by planing the battens in place. Otherwise, the batten must be removed and the station mold itself faired in correctly.
Since plywood is manufactured (generally) in 4x8 foot sections, longer planks are achieved by scarfing shorter sections of wood together. Various methods exist, the two most useful are:
Both methods are easy to understand, but the glass-butt scarf is by far the easiest and fastest. Diagional scarfs tend to deflect the woods less, and render smoother curves.
When scarfing dimentional timbers, use the diagonal scarfing method. The scarfs can be cut on a table saw, or a number of timbers may be clamped together and the scarf cut with a plane.