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Next step in Design Evolution

Just a snapshot of the frames and keel in approximate position. We are looking from stern to bow. The bow and three more frames will be added. The frames themselves will be trimmed further; the pieces have just been bonded and are still rough.

Every time I build a boat, I learn something and get ideas on how the process could be improved. Now I am starting on a new design. I am getting comfortable with using custom solid lumber planking with no plywood. By breaking the sheathing process into fairly narrow planking, we can create surfaces unrestricted to curvature in only one direction at a point. This should result in a more rounded surface shape.

Current plans are for a boat about 13 1/2 feet long and 46 1/2 inches wide with a somewhat wineglass shaped, squared off stern. The sheathing will be 1/4 inch by 2-inch cross section, solid wood planking. I have an entire table of dimensions and have drawn full-size patterns for the frames, stem, and keel. In fact, the wood keel is ready to accept frames. My wood source was low on inventory; thus, I had to scarf a short piece unto the keel stock to get the length needed. (Another foot added to its length would be more spacious and preferable for many uses.)

Back to making progress. I am currently installing the sheer; once that area is re-enforced, then I can turn the hull over and start figuring out the planking sequence. Notice that no strongback has been needed. Because everything is calculated, the frames, keel, sheer, and forefoot only go together in one position. Looking at my table of dimensions, about four hundred calculations were needed to come up with all these values. But I have been using this approach for many years, so it is quick and routine for me. Does this look like a developable surface hull design? With the plank keel, triple chine, and continuous 3D curve of the sheer, there is almost no flat surface to observe.

My first hull of this type was 14 1/2 feet long, then I did a 15-foot hull, then a 14-foot hull, and now a 13 1/2-foot hull. Each time it wasn't just the length that changed. Materials have varied, beam has changed, keel width has changed, and this hull has a squared off stern. I think that improvements have been carried about as far as I can imagine for this type of hull. Let's see if I still feel that way when the boat is completed.

Sheer has been completed; a chine strip has been installed. The hull has been turned over and the plank keel has been rabbeted to create a landing area for the first plank. Rabbeting the lengthwise center section of the keel was simple because the rabbet was cut at a constant angle on a straight edge. At the ends, the edge curves, and the angle becomes progressively steeper; an area to be careful. I used a router initially and then did touchup with a small plane. When starting to fit planks, I discovered that my 1/4" thick planks were thicker than needed, causing extra weight and decreasing their flexibility. I ran them through a planer to reduce the thickness to 6 mm.; a slight reduction but enough to make them follow the hull curvature more easily.

The hull was faired; board edges which were initially cut at right angles were tapered so that the hull sheathing would lay across adjacent frames with full contact. At the ends this meant using an angle grinder with a 60-grit flap disc to quickly cut away the 1 1/2' blunt edges to a tapered "V". The angle of this "V" (initially 23.2 degrees) was dictated by the designed hull projection. That projection in an X:Y:Z coordinate system is expressed by the ratio 7:3:1. A straight edge can lay across sequential frames, or ends, forming a straight line when faired. This was just the gross fairing; a more exact fairing will be done when fitting each plank.

We are looking at the stern. The underwater shape of the hull will be double ended like a canoe. Two straight edges clamped in place to illustrate the ruling lines defined by the projection of length (x), width (y) and height (z) in the ratio of 7:3:1. Having a constant ratio facilitates fairing the frames and ends. Also, you can see the rabbeted plank keel. Now I am scarfing planks to achieve the full 14' needed length for this part of the hull.

The first planks, port & starboard, have been bonded into place. Planks were placed in a shallow basin to be thoroughly wetted, then they are clamped in place and "ironed" with a steamer. After being allowed to dry (easy to do in our dry Colorado climate) in place, the clamps are removed, epoxy resin is applied, and the plank is then bonded permanently with little clamping pressure required. This wetting process will not be required above that first chine as the required curvature is less severe.

I always obsess over what I could have done better. Length: this 13' 6" length is easier to haul, store, and handle but spacing the frames further to produce a longer hull would make for better performance on the water. Plank keel: I have experimented with different widths and rocker (this is the 4th hull of this type and 1 of 11 boats I have built). This keel is about 10.5" wide because I found a beautiful board of that width at the lumber store and had to try it, but I actually think about 9" would be ideal for this design. Good lumber is getting harder to find here.

Does this look like a developable hull shape? All I see is beautiful fair curvature due to the increased sheer curvature, triple chine, and use of 2" wide planks. Yet, due to its mathematical design, all dimensions are accurate to 0.01" or better, more accurate than you can cut or assemble. I made two minor mistakes when measuring boards (inattentive), but there are no errors in the calculated dimensions.

Planking completed to the first chine. This surface is a single developable projection, thus, straight planks were used and only trimmed at the ends.

Planking the bottom, between the keel and first chine, was relatively straight forward. Each 2" wide plank was clamped into place and trimmed to size; then the planks were immersed in a shallow water trough for 3-4 hours until they were thoroughly soaked to increase flexibility. Next, they were clamped back in place and allowed to dry. Once dry, they were bonded into place with epoxy; minimal clamping force was needed because they were pre-warped to fit the curvature. In a few spots where clamping was not feasible, #6-3/4" screws were placed until the epoxy cured, then they were removed.

When ripping 2" thick lumber into thin planks, the resulting planks are seldom straight. What starts out as a straight plank may end up with a curve due to released stresses in the wood. Thus, I then "map out" each plank with pencil marks noting any convex or concave areas. Even if these variations are minor, they can make a significant difference when fitting a plank to the hull surface. Boat building is a game of details.

Above this chine (when the hull is upright), the planks need to curve. In order to do this, the planks must be created and fit in three sections and then scarfed in place on the hull.

Above that first chine (two more minor chines were included in the design), straight planks could no longer be used. The needed surfaces are banana shaped. Instead, each 2" wide plank was subdivided into three sections which were scarfed together to accommodate the curvature. Spiling was also required of each section prior to scarfing in place, a slow exacting task, but speed improves with practice. The amount of required curvature decreases near the sheer.

In this photo you can see that a next plank is being fitted in three sections. The forward and aft sections have been fitted. Next the midships section will be fitted and then the three parts will be scarfed and bonded together.

Compressing the hull from 14 1/2 feet to 13 1/2 feet accentuates the curvature required in the planking and increases the amount of fitting required for each plank. Just 1-2 more planks to go, but the planking must meet the sheer edge in a very finished manner.

Bonding the sheer strake in place. When using thin planking (6mm) the clamping pressure must be closely spaced. You are looking at 92 clamps of various types and sizes. What I have left over is either larger or much smaller. There are two scarf joins to each side. The sheer strake is full width, leaving narrow gaps at some places below it which I will fill in at the next step.

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A Real Sharpie

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.

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Starting again: Another guide boat

What if I shorten my guide boat design to 14' to make it easier to transport? What if I widen the plank keel slightly to make it more stable? Should I increase the freeboard? Can we reduce the weight? Wouldn't it be neat to use all dimensional lumber with no plywood? Perhaps I can finish it clear. I want to use a new and stiffer material, "Ramboard", for making patterns; it should be more accurate. So many reasons to build another boat; none of which have to do with actually using the boat.

For me, designing and building a boat is the equivalent of an artist starting a new painting. What artist only paints a few paintings? Each painting expresses a separate vision. Creativity proves we have control over at least part of our lives. And with the demanding dimensions required in a boat, there is the mental challenge. And so it begins.

The bow profile built from two scraps of 2x4 Douglas fir. I built two of these, and they will require further modification for the changing bevel along the lower edge. I have also roughed out the plank keel, the edge of which will also need to be modified to accept the garboard plank.

This shows a double stack, 12 frames. Two more frames need to be built; both will be bonded to the bow profiles. Then I can start putting things together. Of course, these frames will require further modification; notches for the stringers and sheer and further finishing. They are half-lapped and bonded with epoxy. Built from lumber left over from other projects; you can notice the varying grain.

The bow profiles have been bonded to the first frames. Next, these assemblies will be bonded to the keel as the sheer is installed. The chine and sheer curve lengths and stations have been calculated and, when installed, will force other frames into alignment; thus, eliminating the need for a strongback.

Frames trial-mounted on keel for an initial view. The frames need to have notches cut for the sheer and two stringers before final assembly. It is snowing outside with below zero temperatures; a great time for a project like this.

Making progress. All the frames are in place and the sheer is in place, as well as two strakes. Note that no strongback was required. The measurement along the sheer for each frame has been calculated; thus, each frame can only be positioned at one exact place.

This hull will be one foot shorter that the completed hull nearby but with a wider plank keel. Why am I building two boats so similar? I have some new planking stock to try out; I have new pattern-making material (Ramboard) to try; and I think the shorter length may be easier to transport. I have also planed the plank keel down to a thickness of 0.4 inches to reduce weight.

I used an angle grinder (40 grit) to bevel the stems when fairing the overall hull frame. All ruling lines are calculated and parallel, which made it easier to visualize the required angles.

The edge of the plank keel has been routed to create a solid landing for the first plank. At the ends, the plank edge is curved with a constantly changing angle. This needed to be done carefully.

Carefully fitting 2 inch wide by 0.21-0.24 inch thick planks (not quite quarter inch). I picked two inch wide planking because that is the thickest wood my table saw will willingly cut, and that is the widest that many of my clamps will span. No fasteners used; only epoxy adhesive. The planks are falling into alignment quite nicely; much better than when I used plywood. To get the length I need, I am scarfing together shorter lengths using a 7 degree bevel on my chop saw. Perfect results using a simple jig, and it only takes seconds.

Using real planks have given me insight into the design of the original Guideboats. Upswept sheer in the ends of the boat facilitates easier planking. The greater length of curve from keel to sheer spreads out the bending stresses and provides a better landing for plank ends. That end curve had to be a conic projection in order to fulfill all requirements, although the first builders I am sure were not thinking of mathematics but instead just ease of planking.

I am enjoying the nice fair curves that the planks follow. Relying solely on epoxy adhesive means that I have to let the bonds mature on each plank before I start the next row. Slow, but I am not in a hurry.

With this triple chine design, the hull shape appears to be that of a nicely rounded hull form. Very pleasing. If I had used narrower planks, as in strip planking, the curvature would be even smoother.

First coat of paint

I had a quart of red on hand, so that is the color. It will need three coats. Then I get back to finishing the trim, the stem decks, and interior. I built two oars recently which will need varnish as well.

Close to completed; all it requires is a few more coats of varnish. I really like the carrying handles placed on the stem decks. They are bonded securely and allow convenient carrying or the securing of lines. All I started with were two scraps of 2x4; initially shaped with saw cuts, then a belt sander, and finally fine sandpaper.

Next to make are the removable seats.

You should know that I have already shaped the keel for a planned next boat. It will be slightly wider with a wineglass-shaped stern. Trying to decide if I should include features for sailing.

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Finishing Up the Guide Boat

New oars on the left; old oars on the right.

My latest guide boat is essentially finished; just adding varnish. I also built a new pair of oars for the new boat. Using published formulas for proper oar length, standard oars would be about 80 inches long according to the boat beam, but I also read a note, "Guide boat oars are usually 7 1/2 or 8 feet long." I have a fairly large stack of scrap wood which I need to use. From the scrap pile, I found some nice straight pieces to bond together creating 92-inch long oars with the inboard ends left with a chunky square profile to help counter balance the somewhat longish outboard shafts and blades. I am not going to go into oar construction because there are already plenty of Google instructions posted.

Taken on the US Air Force Academy. We are still discovering more trails in the Colorado Springs area; although we also took the bikes to Arkansas and Montana this summer. They are a great source of exercise and fresh air away from crowds of people (and COVID). Building boats is more enjoyable when the weather turns cold.

I want to apologize for the slow progress in recent months. Progress has been slow to non-existent during the better weather of Summer and Fall because my wife and my own infatuation with the new e-bikes we purchased. With the mountain e-bikes we feel confident to go further and steeper on our outings than we would with regular bikes. It has been a godsend letting us travel forest trails with fresh air, ample exercise, and no crowds.

The hull ends are symmetrical and the oarlocks are set about a foot ahead of the hull midpoint. When rowing with two people onboard, the rower sits ahead of the oars. When rowing solo, the rower sits at the hull midpoint facing the other direction.

Seat positioned amidships for a solo rower. Yes, it is snowing outside.

Now that winter is here, I'll be looking for a project in my wood shop to keep me busy. Still working at the clinic, but only 2-3 days per week. My present thought is to build another guide boat using what I have learned building this one. Always trying something new; I anticipate using a 14' length (more easily cartop-able) and a wider plank keel. I was able to buy a length of flat lumber 12'2" long and 10.3" wide. I'll see how the wider keel changes the hull stability.

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Finishing the planking

I put off making choices for planking. Previously I have always used a plywood initial sheathing to define the surface shape and provide a substructure for bonding exterior thin planks; it works well with developable surfaces. I had some marine plywood on hand. Also had some mahogany planks left over from my last build, but there was a reason they were left over. The planks had flaws; they were the rejects. This new boat is somewhat of an experiment; didn't want to invest too much in materials; so I used those rejects. And I have been reminded how important it is to keep focused and not compromise at any step. Later on it catches up with you.

Now I am doing extra sanding and using fairing putty to make up for those inferior materials used. Next time I will spend more time and money to produce high quality planking at the very beginning. Even the garboard would be better with solid planking than the altered four-ply plywood I used. I would also omit two of the intermediate stringers in the frame work; they have not added much to define the build. A stringer at the initial chine and at the sheer would be enough framework.

Getting to the final touches. Need to finish all the trim, build some seating, and create oars.

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Creating a Developable Surface

This has nothing to do with any computer program and its underlying algorithms; instead, let's consider basic geometry, some algebra, a touch of trigonometry, and perhaps some optional calculus. Developable surfaces cannot include curvature in multiple directions at a single point. A straight line must pass through any point on such a surface; these are called "ruling lines". A unique line can be defined by either two distinct points in space, or one point and a defined slope. Surfaces can be created by projection from points: parallel in two dimensions (flat), parallel in three dimensions (think of a cylinder), or conic- radiating from a single point which I will call the focus or focal point (FP). Multiple projections can be connected to create a fair surface as long as those projections are linked by common ruling lines.

In this sketch of the topsides of a previous boat, I used four projections. I used a parallel entry for only a short distance. Then I switched to a conic projection (using a common ruling line) to create a flaring bow. Next, I moved to a different conic projection for a transition to stern tumblehome. Finally, a parallel projection was used to finish the tumblehome. You can see how projections are combined, using common ruling lines, to create varied surfaces and the desired shape. All projections were done from defined coordinates along the chine or extensions of the chine.

First, let's concentrate on ruling lines. On a segment of any line between two points in an X-Y-Z-defined space, the change in X is proportional to the change in Y and the change in Z according to the slope of the line. The slope can be calculated as (X1-X2)/(Y1-Y2)/(Z1-Z2) if we know the coordinates of two points on the line. We can calculate the coordinates of another point (or multiple points) on that line (X3,Y3,Z3) using (X1,Y1,Z1), the calculated slope, and any one of the X,Y,or Z coordinates for point 3. Let's assume we know X3, then the formula for Y3 would be the following:

Y3= Y1-(X1-X3)(Y1-Y2)/X1-X2)

Z3= Z1+ (X1-X3)(Z1-Z3)/(X1-X3)

In words, the change in Y is proportional to the change in X, and that change is then subtracted (or added, depending on direction) from the initial Y value.

Here comes the big concept which makes this whole system work: In order to make this work, we need some defined points in space to start describing our desired surface. We can't just draw a line on paper and say that this defines one edge of our surface. We need numerically exact discrete points. A mathematically-defined curve provides the solution. I generally use a form of trajectory curve, also called a parabolic curve, for this purpose. An example below:

Y=14.4-(72-X3)squared (14.4) /5184 (5184=72 squared)

Z= 4.8+(72-X3)squared (5.76)/5184

The X-Y slope at any point =2(14.4-Y3)/(72-X3) where X is measured from the bow and Y from the midline

If you integrate the curve (which I will not do here), you can calculate the length of the curve or a section of it. I actually did the integration calculations for the hull I am building. It eliminated the need for a strongback frame as the parts were self-aligning.

Using these particular curves, I insert values of X3=66,60,54,48,.... etc. and solve for Y and Z values at each of these X intercepts. This provides a table of exact coordinates, spaced every six inches along a chine curve. I could calculate values every three inches if needed, but the extra accuracy is unneeded.

Let's say I am designing a guide boat with a frame every 12 inches (which I did). We have the chine; what should the bottom look like? Displacement, stability, and construction considerations help decide this. I decided on a midships deadrise slope of 1:3 and a plank keel 7.2 inches wide (can use standard 1x8 lumber). Now for the projection.....

By setting Y=0 (width=0), we can solve for X and Z (length and height) and plot the bow profile Note that as the focal point (FP) is located nearer to the surface being described, the resulting projected curvature increases. This is generally true. Note also that the ratio of Y to Z at the FP is the same as the midships deadrise, z/y=1:3. This has to be true to create a common ruling line. Shifting the X coordinate fore or aft also affects the slope. If the X coordinate matches the stem half angle, it will create a plumb bow.

By setting Z=1.2, we can solve for X and Y (length and width) and create the plank keel profile with a width of 7.2 inches. Using a conic projection results in a longer keel with slightly more abrupt entry profile.

We have a choice of projections for creating the frames at X=,12,24,36, etc. This drawing was created using a projected parallel profile, X:Y:Z= 6:2.4:0.8.

These frame profiles were created using a conic projection. the FP is located at (-20,-9,-3) Recent experience suggests that the conic projection may be superior in this application; it depends on what characteristics you value most. A conic projection appears to shift some of the surface curvature near the stem from the Y-Z plane to the X-Y plane. At least that is my current best guess. The parallel projection creates a more aesthetic bow profile curvature, but a shorter waterline length.

Posted to answer a question asked:

Demonstrating the alignment of frames using the plank keel, and the calculated length chine and sheer as references. These pieces will only fit together in one relationship when aligned. No strongback needed.

Because this hull is designed as a developable surface, compound curvatures are avoided when planking.

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Solving the hull sheathing conformation

Spring is here but Pikes Peak still is snow covered.

Wow! Has it been so long since posting previously? Over six months? That has been time for three vacation trips and a major bathroom renovation. It is nice to stay busy. Now with the pandemic, things have slowed down some, and it's back to the guideboat project.

In the back of my mind has been that failure of the garboard sheathing to conform to my design dimensions. The plywood when bending aligned itself with a different curvature, lower stress pattern. Today, I did a simple investigation. By laying a straight edge (I used a four-foot level) on the hull sheathing and rotating around a tangent point it until the entire edge contacted the surface, I was able to discern some of the actual (approximate) ruling lines inherent in any developable surface. I had designed the hull bottom to be a parallel projection; the X:Y:Z ratio was 6/2.4/-0.8. It appears that, instead, the plywood adopted a conic projection toward the hull end.

Ruling lines marked on the sheathing plywood.

I marked some of the ruling lines on the hull surface. As you sequentially view the ruling lines from midships toward the ends, The first line is consistent with the design slope. The second line converges with the first line toward the hull midline. Converging lines indicates a conic focal point in that area. Subsequent ruling lines at the hull ends only slightly converge but with steepening deadrise (Y/Z slope). Looking at these lines and viewing the hull surface from various angles suggests to me a conic projection, or several conic projections, with focal points located forward and across the midline (keel). On my previous guide boat design, I had used a conic projection in that area. I used a parallel projection on this hull, expecting that it would be superior, and it is for general purposes.

Initial convergence toward the keel of the ruling lines from midships proceeding toward the ends.

Slight convergence with increasing deadrise for marked ruling lines.

I think I have solved this puzzle. The plywood was thin enough, and with only a four-ply unbalanced stress resistance, to relieve stress by distorting slightly at each of the frames creating an unpredictable conformation with a lower induced-bending-strain pattern; at least in this situation of a long, comparatively slender hull panel. I will make sure that I never face this situation again. I want predictability; not repetitive cut and fit construction.