Sunday, March 14, 2021

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.

Monday, February 08, 2021

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 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.  
First coat of paint

Second 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 end decks, and interior.  I built two oars recently which will need varnish as well.

More to come as I find time.

Friday, December 11, 2020

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. 

Monday, May 25, 2020

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.

Friday, May 01, 2020

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. 


Sunday, April 26, 2020

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.          

Wednesday, September 25, 2019

Planking the Guideboat

The hull bottom is now sheathed in 4.8mm Okoume plywood.  It provides a nice continuous covering which distributes stresses evenly.  It didn't bend exactly the in the manner I hoped for, but I'll go back with some fairing putty to improve that.  I learned that the best way to created a tight junction with the plank keel when bonding the ply in place was to use #4 x1/2" flat head wood screws as temporary fasteners.  I tried using staples but they didn't have enough holding power.  I also tried using clamps, but couldn't get the proper leverage/clamp placement at the keel.

Sheathing the bottom in three sections; nice to have plenty of clamps.  The keel joint is temporarily fastened with small flathead screws.  I had no problem removing them after the epoxy set.  In the center section, the screws were spaced every 12 inches; at the ends, the spacing was every 3 inches.

Significant curvature in the hull ends.  Need to do some fairing of the sheathing and put an external cap on the hull end ply junction.

With the hull bottom sheathed, I now need to finalize how to plank the topsides.  I am thinking to use 5mm thick solid planks, carvel style, narrow widths, with one or two staggered scarf joints per plank.  But that could change.  I have some planking material on hand, but not enough to finish anything; need to find enough stock lumber to finish it before I get committed.