Developable Surface Boat Designs

First season report

2011-09-07T14:42:00.000-07:00

Boating season is about over for this year but it has been very enjoyable. The only changes I have made so far to the boat are to move the throttle control over about 1 1/2 inches away from the steering wheel, to allow more clearance, and to replace the plexiglass windshield with auto safety glass for a clearer view. After trying three propellors, we have boat speed up to 33 mph (at a 7500' altitude). The engine trim control allows us to bring the bow down in waves and trim it up in smooth water for more speed. My wife enjoys the boat which is important. She spent more time driving it than I did on this last trip. That is great; my great joy is to start with a vision, create a plan and then make it a reality. This winter I will start on a new boat.

2011-06-12T18:47:00.000-07:00

We took the boat out this weekend and had a great time. We were part of an Antique & Classic Boat Society club get-together. We were surrounded by beautiful traditional mahogany runabouts. I had changed the engine propellor to one with a more aggressive pitch and the result was more speed- 28 mph. The engine was still achieving 5950 rpm, at the top end of its recommended rpm range, and I am wondering if an even higher pitch will bring the rpms down a little and improve speed further. You can see in the attached image that the boat's bow is out of the water at speed. This is due to the slight rocker in the run. By calculation, the bow should rise about ten inches to achieve its planing attitude, and I don't think that is far off. From a visibility standpoint, the bow rise is not an impediment. At low speed the boat rests at its marked waterline. The ETEC engine is so smooth and quiet, and we are still using our first six-gallon tank of gas. My wife is not much of a boater, but seems to really like this one. With the intense sun in Colorado, our bimini top is an essential.

Finally in the water

2011-05-05T18:07:00.000-07:00

Springtime in Colorado. After ten days of cold and light snow, the temperatures finally got up in the 60's, and we were able to launch the new boat this week. Everything went perfectly. I had re-inforced the deck at the bimini support attachment points, put flotation foam in the stern, installed a new battery, and readied my list of supplies. Amazingly nothing was forgotten; everything performed as designed; and the setup-launch-retrieval was easier than expected.

Out of the Shop

2010-12-19T10:51:00.000-08:00

I had planned on using beer and thirsty friends to get the boat out of the shop and onto a trailer, but then a I reconsidered the logistics of getting six friends here at the same time with late fall weather cooperating, and devised a new plan on the first warm day. As when I previously turned the hull over, I suspended ratcheting tie-down straps from the ceiling beams and lifted the boat up off its building cradle. Then I moved a small cart under the center of the hull and lowered the boat onto the padded cart. I backed the 8' wide trailer up to the 6' wide shop door; pushed the hull out the door, rolling on its cart, until about 8' of the hull was out the door and on the trailer; then simply finished winching the hull onto the trailer.

Crowley Marine was ready to install the engine, steering gear, engine controls, and wiring, and they recommended a canvas shop for the travel cover and bimini. So, in a short period of time, the boat is now basically ready for the water. Of course, there are always more finishing projects. I need to (1) reinforce the deck underside where the bimini fittings are fastened; (2) place flotation foam in the bow and stern; (3) touch up the cetol finish in spots along the sheer where it was disturbed when the sheer strip was placed. After pricing simple custom upholstery for the plywood-framed cockpit seats, I found that I could purchase quality standard bucket seats for less than half the price. I should get the seats in the next two weeks.

But with winter's snow in our yard and the ski slopes beckoning, that will all wait until warmer weather.

Ready to leave the shop

2010-10-21T20:52:00.000-07:00

Painting is not my favorite thing, but I have been doing quite a bit of it to complete this build. I still need to do some touch up but I have been trying to get this boat out of the shop and onto a trailer so I can get the engine and controls installed. Of course the boat will still need cushions, and bimini top and a travel cover. The weather forecast mentions possible snow next week, so I am just a little late to launch this season.

2010-07-27T17:07:00.000-07:00

Finally was able to get the transom and splash pan covered in glass and resin. I wasn't looking forward to this step. It was like developing the pattern for a garment; I had to make patterns for the complex adjoining surfaces using six different sections. Had to decide which edges to put tabs on so that I had overlapped joints, and had to decide the exact order of placement to minimize interference. All concave junctions for adjoining surfaces were first filled with a fresh fillet of resin putty to prevent dry spots where the cloth couldn't adapt to the abrupt bend. Most of the surfaces were vertical, so the cloth needed to be taped in place while I spread resin and tried to avoid drips and runs. Because the glass cloth distorts easily, I had to note reference points for careful placement. This covering helps not only to protect the wood, but also to add considerable strength to this area which will receive maximum stress from the engine. To my relief, this session went well. Next up will be covering the entire deck with glass and resin.

Thinking ahead, I started to finalize the windshield design. No experience, so I started looking at pictures of old launches to see how it was done traditionally and what might work on the narrow, cambered foredeck. I sat in the cockpit and tried to imagine how a bimini top will join to the top of the windshield. Because the entire boat surface is developable and mathematically created, I was able to select x,y,z coordinates for all points defining the windshield and then calculate lengths and angles for creation of a cheap plywood mock-up of the windshield. Looks pretty good, so now to select real wood and put together the actual windshield. This will be a nice side project while waiting for resin to dry on the deck.

Wood Decking

2010-07-13T17:35:00.000-07:00

The plywood under-decking was bonded in place with temporary staples to hold it until the epoxy hardened. Then the plywood was trimmed around the sheer and cockpit. The process was repeated with cypress planks, about 3/16" thick. Now the planks have been sanded. Finally starting to look like more than just the hope of a boat. I need to glass the stern, recessed transom, and splash pan before glassing the deck itself.

2010-06-20T14:14:00.000-07:00

Sometimes you can be working diligently on the boat, and progress seems slow. This is one of those times. I want to put the deck on ( a big visual change), but first I needed to consider everything under the deck while access was good. So I have been working on the instrument panel, floor boards, seats, and interior finish. Also placed reinforcements in the frame where deck cleats, chocks, and bimini supports will be positioned later. One photo shows the deck plywood panels resting in place. Those will be bonded and stapled (temporary), then a layer of cypress will be added, then 6 oz. fiberglass. And then it will get painted. I know that the cypress wood grain is beautiful, but this is Colorado, and the sun's intensity would vaporize that varnish. I think I have selected paint colors. I have made an engine selection; expect to use hydraulic steering because of its compact installation; and need to select gauges for the instrument panel. I am debating a couple alternatives for the windshield design; I'll be making some mockups to help visualize the effect.

2010-04-04T15:39:00.000-07:00

Progress has been slow due to the intervention of ski season and work, but today finally I was able to turn over the hull so that work on the deck and interior can begin. In the past few months I have been sanding, putting down epoxy and 6 ounce fiberglass cloth, and then painting. All that just takes time. On the vertical surfaces each coat must be thin to avoid runs but the epoxy must be built up enough to fully fill the weave and provide a sandable surface coat. Because the cloth and resin is clear, it is difficult to spot small surface irregularities. I chose Pettit Easypoxy for the bottom of this to-be-trailered hull. It went down fairly well with a roll-and-tip technique; the critical element seemed to be getting each coat just the right thickness. I used a laser to strike the waterline, connecting 3 points on each side of the hull which I estimate to be about where the hull should float. I may later have to experiment in placement of the battery and fuel tank to achieve proper flotation. On the topsides I elected to use Sikkens Cetol. It seemed easy to apply. The directions say to apply it "liberally" for three coats. I got a bit sloppy (lighting wasn't good enough) on the second coat and experienced some sags, but it should be easy to fix.

The hull was easy to turn over. I had anticipated using beer and multiple neighbors to get the job done. Instead I used C-clamps to suspend ratcheting tie-downs from the overhead. Tightening the tie-downs lifted the boat up in a suspended sling. Rotating the hull in the sling is fairly simple. I removed the strongback and substituted a simple plywood support cradle which I then lowered the hull onto. Although I didn't weigh it, I noted that the hull is light enough for two people to lift at this point; very good for an 18.5 foot hull.

I have been looking at outboard engines. All the dealers are accustomed to fiberglass hulls which weight much more than the wood-epoxy. Thus, they have been recommending at least 60 hp, but I think that 40 hp will probably reach my powering goals.

Planking the hull

2009-11-15T10:32:00.000-08:00

Planking the hull has been fairly straighforward. First I had to assemble my new band saw and learn to properly adjust and check it. That proved a bit more difficult than expected since it was shipped to me minus a screw (which took a while to discover) needed for proper tensioning. I also needed to choose which wood to use. Surprisingly, I found a local source in Colorado for cypress at moderate cost. I resawed the boards into a 3" by 3/16" cross-section. Most boards have some kind of curve to them; so I had to be careful to mark and maintain a straight line when laying them out on the curved hull surface. Then it was simply a case of coating each board and the underlying hull plywood with epoxy and temporarily stapling them in place with minimum mess. I used a bit of excess to make sure that they were thoroughly wetted and bonded which then required that I sand the entire hull to remove blebs and discontinuities.

initial hull sheathing

2009-08-29T10:58:00.000-07:00

Once all the frames and the anterior keel are mounted and aligned on the strongback, the entire framework was then faired so that panels would lay flat instead of merely touching one edge of each frame. This provides more panel support and more bonding/nailing surface. Once faired, rosin paper patterns were made for each proposed panel of the hull sheathing. I was planning to make thin ply patterns but found that stiff paper was accurate enough in this case. I had to decide the order of panel placement and the position of junctions between panels. I wanted to place the panels which required the most bending first, and I wanted to join separate panels in places where hull curvature was at a minimum. On my previous boat I scarfed all the panels together before placement, but found that handling an 18+' long, narrow panel wet with epoxy resin, precisely placing it, and getting it securely fastened in place quickly was a difficult task. Fortunately, this hull has convenient panel juncture points where either a butt plate could be placed or a nearby frame could back an in-place scarf joint.

Those plywood panels which required significant bending, the bottom forefoot and stern side tumblehome, were submerged in a shallow basin of water for several hours and then quickly clamped on all edges into place of the frame and left to dry for a couple days. The panels were then removed, trimmed for a more exact fit, and bonded into place. As more panels were placed with adjoining edges, clamps could no longer be used on all edges. For these locations I used a nail gun with a 3/4" or 1" 18 gauge nail and scrap 1/4" ply placed under the head to facilitate later removal.

At this point, the entire hull is sheathed, and we can begin to see the full shape of the hull. I now want to cover the entire hull with a second layer; 6 mm. just isn't enough thickness for me. Sure, I could used multiple layers of resin and cloth, but that stuff is heavy and expensive. The strength-to-weight ratio for wood is excellent, so I will add about 4 mm. of thin wood planks; sand it as needed, and finish with a layer of resin and cloth. I've had to research band saws and blades to get the right setup for "resawing", cutting a normal plank into 3-5 mm. thicknesses. The band saw I bought was missing an important set screw, and I was unable to properly adjust it until I figured out that the mechanism was not functioning properly. Looks like it is now properly set, and producing thin planks is my next task. I have picked cypress wood for the planks- available, relatively cheap, fairly light, and rot resistant.

The Design Becomes a Reality

2009-05-13T20:01:00.000-07:00

I have already posted on the design of this boat, so I will skip that subject other than to mention that my initial "plan' consists of tables of computed offsets in an X,Y,Z three-dimensional framework. I am having computer issues with importing photos, so they will get posted a little later.

1. First job was to convert those dimensions into full-size patterns for all the frames and the curved keel forefoot. I used stiff rosin paper for the patterns, the largest pattern is about 36" by 68". The offsets only define the outline of each part; thus, each frame was then drawn as a series of boards with joining angles and gussets. A horizontal reference line was drawn at the same level for each frame; this was used to align them all vertically when mounted on the strongback. The centerline can be used to align them horizontally.

2. With full-size patterns it is relatively easy to build the actual frames, all 14 of them. An outboard engine is to be contained in a semi-well which intersects with three frames, so those internal details had to be included in the frame designs.

3. A strongback was built with vertical support posts attached at each frame position. The frames were sequentially clamped in place using a laser beam to align them. Then longitudinal components, starting with the keel, were bonded in place. I really was anxious to get the engine well fit and bonded in place. Being integrated with the deck, hull bottom, transom, and three frames and structurally important to transmit the power of the engine its fit was critical.

2008-12-24T12:07:00.000-08:00

The question of how to create more than just simple angular shapes from sheet materials caught my interest quite a few years ago when I built my first plywood-sheathed boat. To visualize the problem I was facing, I first used a flashlight to project the shadow of a wire of various curvatures and orientations onto a flat screen, a crude form of conic projection. Then I realized that if I could model the curved wire mathematically, the same thing could be done virtually with highly accurate results. From this, I developed an approach that has worked well for me and, to my knowledge, hasn’t been described elsewhere. I hope to describe it here to learn whether anyone else is using a similar approach or if anyone might benefit from it.

Instead of attempting a comprehensive description (which would put everyone to sleep), I’ll go through one sample design which illustrates the approach and try to leave out tedious details. The technique does not require a drafting board or a computer. An inexpensive pocket calculator is the minimum equipment needed. The approach relies on two simple concepts: First, a chine description is created from which you can pick off many closely-spaced exact offsets. I use mathematical relationships to do this, but any method which results in multiple well-distributed exact offsets is adequate. Second, for any line in a three-dimensional x-y-z coordinate system, as we move along such a line the changes in x, y, and z are proportional to each other according to the slope. Thus, if we know the change in one coordinate, we can calculate the change in the other two coordinates. By calculating the new coordinates of many exact points when projected into a different plane, we can derive the offsets for as many transverse sections as desired, create another chine, describe the entire length of the keel, and establish the sheer. A table of accurate offsets is the first result; then the hull sections can be drawn or sketched on graph paper to visually confirm what has been created. I use X to designate length starting at the forward end of the chine; Y designates width from the center line; and Z is the height above base, usually selected as the lowest point on the keel.

Bottom: With some idea of the desired length, beam, displacement and other characteristics of the proposed hull; the forward chine is created. For our example hull {see related drawing}, a few of the offsets starting at the bow are (0, 0, 20.25), (14, 6, 17.25), (28, 11.6, 14.45), (42, 16.4, 12.05), until we reach (119, 28.5, 6) at the point of maximum beam. A parabolic relationship is convenient to use, but other mathematical relations may be preferred depending on the purpose. For this example it was convenient to calculate offsets every 7 inches; that could be increased to every 3.5” if necessary but commonly provides more data than needed. The point of maximum chine beam when projected to establish the keel coordinates becomes the lowest point on the keel. Several different types of projections could be used. I chose a parallel projection (constant slope) instead of a conic projection because, for this situation, the differences were small. The parallel projection is also easier to work with and minimizes severe localized curvature. I set the lowest point on the keel to be located at (63, 0, 0). The bow half angle, desired deadrise, and type of hull play a part in choosing this point. Realizing that a unique line can be defined by a point and a slope, each offset on the chine gives us a point, and the slope to be used is the difference between the maximum beam at the chine (119, 28.5, 6) and lowest keel point (63, 0, 0); that difference is 56/28.5/6.0. We know Y=0 at the keel; thus we can calculate the values of X and Z for each chine offset when projected to the keel or center line. A plot of these coordinates provides the keel shape {see area B1}. Similarly, we can find the offsets for all planned frames by using the established values of X for each frame (i.e. 14, 28, 42, etc.) and solving for Y and Z. Dividing the X:Y:Z slope by 4 reduces it to 14/7.125/1.5 which simplifies computations with changes coinciding with most frame intervals.

Having completed offsets for the forward portion of the bottom, the midsection is defined next {see area B2}. For a variable deadrise hull, a conic projection for the midsection helps reduce deadrise aft. This can be done by creating a short curve with the desired slopes at the keel; then creating a proportionally larger/longer parallel version of the same curve (in this case, twice as big) to extend the chine aft. Finding the apex of this cone is not necessary; it is enough to know that proportional parallel curves are part of a conic development. Continuing aft, the keel Z coordinate will increase more rapidly than for the chine because it achieves its end slope sooner; thus, the deadrise will diminish producing a flatter run. At the aft end of these curves the keel and chine have parallel slopes which are then extended by straight lines to the stern to produce a straight run of constant deadrise {see area B3}. For this hull, the deadrise is about 60 degrees at the bow, 20 degrees six feet aft, and 5.7 degrees at the stern.

Side: Locating an apex for conic development of the hull side laterally in line with the bow half-angle results in a plumb bow profile. Moving the apex further laterally creates more flare to the bow section. By moving the apex moderately laterally the bow profile angle can be matched to the rise of the forefoot of the keel, avoiding a knuckle in the profile {see area S1}. Vertically the apex should be located high enough above the proposed sheer level to avoid any localized severe curvature to the sheer. After trying out a few alternatives, I settled on locating the apex at (63, 40.5, 66) Because any two points also can be used to define a unique line, using this apex and the series of chine offsets, many lines can be projected and their intersections with proposed frame locations in the anterior hull calculated. Because a tumblehome stern was desired, a new apex for conic development was established closer to the sheer line once development from the previous apex reached the midsection. The new apex was connected by a common ruling line to the previous apex in a segment of slight curvature preventing transitional distortion of the hull surface in this area. Using the new apex, conic development was continued to the aft end of the chine curvature {see area S2}. This apex also provides one point and the series of chine offsets provide second points so that many unique lines are defined. For each line, its intersections with every applicable frame location can be calculated to any desired exactness.

The initially established chine offsets, used to create the bottom, do not have to be used as the final chine position. In this case, aft chine is tapered through using a new curve to define the sides. In creating a tumblehome stern, the aft chine, instead of being a straight line, is modified by a calculated curve bending toward the midline {see area S3}. As with the original chine, exact offsets are calculated every 7 inches, but this new curve extends in virtual space about six feet past the proposed stern. For projection from this new curve, the slope established by the last ruling line connecting the amidships apex with the aft end of the chine curve was used. Thus, this is a parallel projection. For the tumblehome stern, we are using one known point and a defined constant slope. Choice of projections depends on the desired result.

Deck: Because the deck curvatures were to be determined independently from the hull, several possible deck configurations could be constructed depending on the desired boat character and planned usage. After considering the alternatives, I elected to use a fairly simple cambered deck. A transverse deck curve of the desired camber and defined offsets (2.5“camber in a 40” long curve) was created. This curve was projected fore and aft mainly by parallel projection except for those areas where a planned open cockpit would allow slight longitudinal camber to be incorporated without creating compound curvature. Y offsets were held constant while Z offsets changed slightly to reflect the fore and aft curvature. The final determination of the sheer location is thus defined as the intersection of the deck surface with the hull sides. Since both are fair surfaces, the sheer will also be fair.

With the shape finalized, I made one final modification. I elected to change the X spacing from 7” increments to 6.75” increments. This shortened the overall hull length by 8.125” to about 18 ½ feet overall. No frame offsets were affected by this change; however, the anterior keel will need to be redrawn due to changed X dimensions.

Summary: At this point all that has been created is a boat-like shape. Many steps remain in creating a complete boat design. However, the only goal here was to demonstrate a method of mathematical projection to create desired developable shapes with more complexity than just a few simple curves. Developable hull surface design can be a logical sequential process focusing on one portion of the hull at a time. Using an initial set of exact offsets for a chine and some algebraic relations, a highly accurate set of offsets for an entire hull can be created. Conic, parallel and planar projections can be combined to create multiple contours in a single, fair developable surface. Modifications can be made, or alternative shapes considered, by changing mathematical coefficients and recalculating resulting values only for those parts affected. Accurate drafting skills and/or complex computer software are not essential. Creating a form in abstract space removes physical restrictions on scale or degree of curvature. Routinely, dimensions are calculated to at least 0.01” accuracy, and many are exact. Curvatures are fair; no lofting to detect inaccuracies is required. Accurate dimensions are used to create drawings rather than the reverse sequence. A plan can be communicated using only sketches and a table of offsets/dimensions.

Design evolution

2007-07-28T19:09:00.000-07:00

Early runabouts were built on the warped-plane shaped, semi-planing hull bottom. This is the form that interests me most. I am not interested in the high horsepower, blasting through the water experience, but would rather glide smoothly, quietly, at moderate, breeze-creating speed and with good fuel efficiency. Achieving a smoothly transitioning warped plane hull bottom is also more interesting for me to design than a constant deadrise planing hull would be.

I built a series of three models in developing the current design. This design method does not use a drafting table or a computer program/screen to create the hull. The hull is shaped by the use of mathematical equations and geometric projections of calculated (x,y,z) coordinates until an entire table of dimensions to define all surfaces and/or intersections is created. The hull only becomes visible as the coordinates are plotted in a connect-the-dots fashion. Similarly, the entire boat is created by plotting the dimensions of each piece (coordinates of many points) to produce a pattern for each part, model or full size. Many dimensions are mathematically exact; the remainder are accurate within about 1/32". The "connect the dots" dimensions are closely spaced so that a short 2'-3' batten can be used when connecting them for a full size pattern.

Mathematically almost any shape can be defined. In the real world each piece needs to be cut, bent or laminated to create the actual shape. Thus, I always try to design the final form using long, gentle curves rather than abrupt bends which are difficult to achieve during actual construction.

New direction

2007-07-16T21:04:00.000-07:00

After I finished my model of a lobsterboat-style hull, I decided that , although fully functional, the boat looked drab and uninteresting. Rather slab-sided, not curvaceous enough, so I started over on the design. By changing co-efficients in some calculations, I gave the bow more flare, gave the stern more tumblehome, and flattened the run of the hull bottom. I also added a double cockpit and a fully cambered deck of classic runabout style. Then I started on building a new model. Drawings can't full visualize the three-dimensional form and proportions of such an object.

Lobsterboats are traditionally built in the range of 28 to 38 feet. Since I didn't want to exceed 20', the proper proportions weren't quite achievable on the reduced scale. Classic runabouts seem to be attractive to everybody, and I am no exception. I went to a meeting of ACBS members and was disappointed in the selection of runabouts shown. Most were rather slab-sided, simple shapes; not a Riva among them. With the experience I have in creating developable shapes, I thought I could create something interesting.

I wanted fuel efficiency which generally means long and narrow. Twenty feet is the maximum length that will fit in most garages. The width had to be adequate for two people to sit abreast comfortably in the cockpit; that requires about a five foot width at the waterline. The result is a L/B ratio of 4:1 which is a reasonable compromise.

I am now finishing the 1/5 scale model of this new design and am extremely pleased with the result. This is the curvaceous and well proportioned shape I was hoping for. I hope to post pictures and more details in the near future. I don't plan to finish this model completely; I want to leave the deck open so that the structural details can be visualized- a guide for the future full sized hull. I do want to paint the outside of the hull and make it watertight so that I can check the hull buoyancy and loading.

Model Lobster Boat

2006-11-25T10:24:00.000-08:00

I am going to post these photos of the 4' model of my "lobster boat" even though I am not through painting it, because a cold front is moving in, and I want to get the boat in the water before the pond freezes over. Since this is a 1/5 scale model, I weighed the model; set it afloat; and added weight to bring it to its designed waterline. Indications are that the full size boat will be about 700 pounds and carry about a 1000 pound load at designed displacement.

Updated drawings

2006-09-09T20:32:00.000-07:00

I am working on a 4' model of my next boat using the calculated dimensions. In the mean time, I've used the frame patterns to create some approximate scale drawings of the full size boat. With a station spacing of 14", the bow half angle is 23.2 degrees. Bow height is 44". The deadrise at the stern is 6.2 degrees. Design draft is 8.4". The side view shows an enclosed bow from station 0 to 4, a small cabin from stations 4 to 7, an open cockpit with small coaming from station 7 to 14, and a planned stern well from station 14 to 16 for an outboard motor of about 15 hp.

Maiden Voyage

2006-05-19T20:27:00.000-07:00

In Colorado the boating that interests me is in the scenic mountain lakes. These are fairly large lakes with wooded shores and surrounding mountains. The wind is unpredictable and the water is cold. I wanted something big enough that we could bring friends and picnic supplies for a day's outing; something quiet and leisurely to enjoy the scenery. I had been thinking about a semi-planing powerboat, but was not ready for the cost, extended time schedule, or building space needed to produce what I wanted. The whitehall-type pulling boat seemed to be a decent option. The design steps weren't much different than a guideboat except for larger size and a transom. Picking the correct projections to get a proper wineglass shape was a new design exercise. A look at John Gardner's writings provided many hull details.

The final boat was 18' 4" by 4' 9" with two sets of oarlocks and a small transom where an electric trolling motor could be mounted. The deadrise midships is 16 degrees. Today we were finally able to take it out for the first time, and everything went well. There are some adjustments to be made, mainly on the trailer and custom cradle used to transport it. I haven't made custom oars for it yet. The ones I have are too short, but okay if the trolling motor does most of the work.

A 20' semiplaning powerboat

2006-05-10T07:54:00.000-07:00

I have completed the lines drawing (okay, not really the drawing, but all the mathematical measurements/offsets) for a 5 1/2' by 20' semiplaning powerboat hull. It will have lines similar in many ways to a scaled-down lobsterboat. At 20 feet and trailerable, I plan to use a bimini top instead of a fixed cover over the steering station. It will probably not get built until we move into a new house with a larger shop (current home is on the market). I do expect to build a 4' model in the near future.

Guideboat pictures

2006-05-09T12:03:00.001-07:00

Apparently my last post was too long to include the planned photos; so here they are.

Adirondack Guideboat Concept

2006-05-08T18:36:00.000-07:00

Up to now I had used a mathematical approach to boat design to make boats which were relatively quick and easy to build. Now I decided to use the same technique to produce designs with more complex and interesting curvatures. We were living in the Thousand Island area of northern New York, and I had read some articles about the Adirondack guideboat. I visited the museums at Blue Mountain Lake and Clayton, also the Canadian National Canoe Museum. I studied the curvatures of the hull forms, trying to picture the underlying curves and implied ruling lines. Most planked hulls can be thought of as many developable surfaces, each the width of a plank, whose intersections form many chines. By incorporating multiple chines and multiple projections, many hull forms can be closely approximated. The guideboat appeared to be one of those designs.

My final design has a 7" wide plank keel and a 1/3 deadrise slope amidships. This facilitated a low seating position and a low center of gravity. Using a double chine, the hull flared from about 28-30" at the waterline to 42" at the sheer to provide a decent width between oarlocks. Length was limited to 14 1/2', since that was the amount of space between the water heater and furnace in the basement building location. I reduced the upward sweep of the sheer in the ends. I know such a sheer looks pretty, but it is also extra windage.

Each hull panel was formed from three projections. Each panel end had a conic projection with another amidships to link the other two. I use just a pocket calculator to do the math and freehand drawings (on engineering graph paper) to visualize the results. Because of the complexity of the hull panels with multiple projections, the hull shape is defined by the boat's skeleton, and the panel shape determined using patterns. Dimensional accuracy is within 1/32". Conic projection tends to produce changing bevels at each frame; thus, I used a plane to set the bevels after the frames were placed on a strongback. Knowing the exact direction of projection and using a straight edge, that is not difficult. Actual construction was done almost entirely with hand tools in our cramped little basement.

When designing, I visualize projections in three types. The parallel projection is the simplest; the direction of projection is contained within a plane formed by two axes in an x,y,z coordinate system and can be defined by a simple slope. It is characterized by identical panel slopes at each station from bow to stern. What I think of as a cylindrical projection cuts across all three axes in a coordinate system and can be defined as a ratio, x:y:z. The "cylinder" is not round but instead the shape of the underlying trajectory or other curve. This projection has many of the advantages of the conic projection but is easier to use. With the conic projection all lines emanate from an apex defined by a set of coordinates (x,y,z). If the apex is positioned too close to the surface being defined, an adjacent area of extreme surface curvature may result. If the apex is positioned too far from the surface being defined, the result has little or no advantage over a cylindrical projection. For me, the conic projection is best used where accentuated local curvature is desired or to link together other projections through converging ruling lines.

This little green faux guideboat performs great. It is light, dry, buoyant, maneuverable, and fast. I do think it would be faster if it were 1-2' longer. The two seats are removable and when positioned fit snugly around the frames. With one person, the rower's seat and attached foot rest is placed in the center of the boat. For two people, the rower's seat is placed on the other side of the oars and the second seat is placed at the other end to maintain balance.

A real disappointment

2006-05-07T19:46:00.000-07:00

We were living in Virginia and had a 60 acre lake just 1 1/2 blocks from our home. I was very busy at my job, but the temptation was there. It had been almost ten years since I built my last boat. Why do we build boats anyway? For me it is the creativity; making reality from just a vision in my mind. In this case my vision was cloudy. I was thinking, row, paddle, motor, maybe sail. A boat designed to do many things will do nothing well, and that's what happened. The boat was about 3 1/2' by 15', had a double chine and was designed with a parallel projection. This was a very quick boat to build (weeks). No frames. Instead I used a strongback and set up molds. All the panels were precut; tacked into place; seams epoxied; and a layer of glass laid down. Then popped it off the molds, and the inside was glassed. The weight of all that glass and resin made it heavy to cartop. The big error was that I added a transom, in case I wanted to motor, then added enough rocker to keep the transom out of the water. On a small, round-bottomed boat that takes quite a bit of rocker. Consequently the boat tracked poorly. The final insult was that the movers smashed the boat, punching a hole in the bottom, splintering the thwarts and delaminating the plies in the plywood with a smash to the bow. It now serves as a sandbox. And there are no photos I can find.

A Real Sharpie

2006-05-07T17:50:00.000-07:00

Our Panama skiff was very handy, and I had been reading Chapelle; thus, when we arrived in Alabama, with its big reservoirs on the Tennessee River, I decided to build a sharpie. Real sharpies have two masts and are over 20' long. I designed a sharpie 20 1/2' long by 6' wide (4' beam at the chine). In order to achieve considerable flare to the sides while keeping a fairly upright bow, I used conic projection. One apex below the bow and another amidships and lateral to make the transition front to aft. Again, I was able to calculate all lengths, offsets, and angles beforehand, thus not needing a strongback building form. However, handling sheathing panels over twenty feet long was challenging just from their size. The same mathematical techniques were used in shaping the sail panels as had been for the hull. To achieve balance between two sails and a pivoting centerboard, I used temporary mast steps, which could be adjusted, until after some trial runs; then I bonded them in place permanently. If you look at the shadow of the straight boom on each sail, you can see an almost perfect airfoil curvature, reflecting the shape of the sail. However, I made one big error. I was unable to find good information on the proper draft-to-chord ratio for sails, so I guessed. I used a draft-to-chord ratio of about 1/14 and later discovered that a 1/9 ratio would have been more appropriate. Thus, my sails were never able to develop the power that they should have, and the boat was not as fast as its potential.

Panama Skiff

2006-05-07T15:25:00.000-07:00

We arrived in Panama just in time that I could buy a few sheets of marine 1/4' plywood before the Panama Canal Company went out of business. No trailers or launch ramps available, shallow coral reefs off all the beaches, but beautiful deserted coves to visit with (by now) an entire family. So I designed a little yellow skiff, 4' (40" waterline beam) by 15 1/2 ', with a daggerboard, rudder and sprit sail. I was able to get Sailrite sail cloth and WEST epoxy through the mail. Again, I calculated the length of the chine, sheer, and sheathing panels and all offsets so that the parts went together smoothly without having to use a strongback. The only drawings I used were just sketches for visualization. I had estimated the hull rocker correctly in design so the boat performed nicely. We went picnicing, snorkeling, fishing and just exploring. It was light enough that we could cartop the boat on our Toyota to bays along the Atlantic coast.

Kayoe or Canak?

2006-05-07T15:13:00.000-07:00

I designed and built my first boat in the mid 70's, when we were living in Nebraska. I was a busy student with limited budget and time. The area has small lakes and shallow rivers and creeks but plenty of wind sweeping in off the plains. So I designed a sort of cross between a canoe and kayak. Double chine, plank keel, low freeboard, partially decked, approx. 16' by 3'. Also, it had to fit out the kitchen window after hauling it up the basement stairs. Using trajectory curve mathematics, I was able to compute the length of both chines, the sheer, and sheathing panel lengths as well as calculating all offsets. No strongback was needed; all parts fit together as cut. A friend had recently built a Windmill sailboat. He was impressed as I simply put the pieces together. I had used a parallel projection so all bevels were constant angles which were easy to precut. Consequently, I was able to get tight joints and use resorcinol glue successfully. As an experiment, I built a pair of leeboards, attached a push-pole rudder, and used a closet pole mast and nylon tarp for a sprit sail rig. With this I was able to maneuver up and down the lake, but sailing rigs don't get much more crude than this.

Where does such a wierd interest come from?

2006-05-04T19:32:00.000-07:00

Far back in college (1970), I took a sailing course which interested me; all sorts of subtle influencing factors in sailboat behavior. I was studying engineering and needed an idea for a senior research project. So I decided to study sailing hull design. For research I devised a simple hull towing routine: tow hulls with a range of measured forces and record resulting achieved speeds; vary the hull design and evaluate the change in performance.

The next question became, how to exactly describe a hull's shape and how to alter only a single or limited number of variables in the hull so as to independently test the influence of each variable in resulting performance? You can't just say that this hull looks this way and the next hull looks like that. You must be able to describe the hull shape mathematically in exact terms. I found some equations in the Proceedings of the US Naval Institute, but they were far too complicated for my simple project. I found the trajectory curve in physics textbooks, a parabolic equation form, had the right characteristics. Such a curve has the appearance of a boat waterline; it can be integrated to find its length; it can be differentiated to find the slope at any point. It is easy to work with. It describes the motion of an object displaced at some angle from its rest position by an external force which then is brought back to its rest position by gravitational forces. Not exactly the Navier-Stokes equation of general fluid mechanics, but at least it is analogous to the motion of a water molecule displaced by a passing hull form. The research project was successful but very limited in extent due to the complexity of the many variables and the need to build a new model for each design change.

Fast forward a few years: I changed careers forever, leaving engineering for a new profession. I didn't dislike engineering intellectually; I just didn't like the environment, being viewed as just a technical nerd, a contract-following migrant laborer. But I didn't want all my engineering knowledge to atrophy. So I picked up my senior research subject and, as a spare-time evening hobby, started playing with the mathematical hull design concepts to create hull forms of interest. Mathematical techniques match very well with the concept of conic projections to produce entire surfaces. Conic projections match very well with the use of plywood hull sheathing which speeds actual construction time. You have to actually build a boat occasionally to prove that your ideas really work (and that you are not completely crazy).

Over the next thirty years I mentally designed a number of hulls for various purposes and I ended up building six of them. My family and I have lived in many places. Each hull was designed for a particular set of circumstances and an envisioned purpose. I've learned something from each boat. If I were to revisit any design, I could find improvements to make, but that just makes it interesting. In subsequent entries I hope to post photos of some of the boats I have built and the lessons I learned from them.