Almost-A-SCAMP Progress Report #5 . . .

[Charles Brennan]

Most of my progress to date (while waiting for temperatures more conducive to using epoxy) has been sanding coarse fillets, which is even more boring than it sounds, so I'll spare you pics of that.
One thing that speeded it up, was a set of sanding drums from Harbor Fright.  These different sizes match up reasonably well to the different inside fillet curves.
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The SCAMP has a 6 mm �(¼") deck. 
I tend to hang out on decks and clamber around on them when boarding from shore, etc. and that deck piece just felt a little thin and flimsy to me, so I wanted to increase the support.
Nor did I trust just a fillet to support the deck to bulkhead 2 junction, so I added, shaped and beveled, a cleat for the deck to rest on.
While I was at it, I added two more deck braces, to either side of the 2 main center beams.
MAN!    Those were some  miserable compound bevel angles! 
I got tired of making mistakes with the compound mitre saw and finally resorted to simply using a wood rasp and a Mark I eyeball, which worked perfectly.
(Pic prior to gluing.)
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My next issue was what to do about a rudder.  They sell a kit that includes two centerboard halves and two rudder blade halves, already formed into the proper foil shapes.
When I purchased this partially completed kit there were the centerboard parts, but no rudder parts.
(Durn.)   
BUT!! I had a set of plans!     So, I'll just loft a rudder from the prints and make my own from scratch, just like generations of boat builders before me. 
After all, Noah didn't have plans and a kit available to him like I do and HE managed to build a boat, so what could go wrong?   
Problem  1: No reference points.
But ya gotta start somewhere, so I marked out some reference lines on a scrap piece of 70's era paneling to use as a template.

Problem 2: Y co-ordinates, no X co-ordinates.
THAT was a stumper. 
How was I going to get enough points to fair the curves?  Or even figure out where they were?
Some co-ordinates could be inferred by working backwards from one measurement to determine another measurement, but that only worked in about three places.
Then, staring at the plans for about the 12th time, I noticed in the lower left corner: Scale 1:5
DOHHHHH!!!!   
Simply measure right on the plan with a metric rule (just like the one that I carried in my nerd-pack for 35+ years) and multiply by 5! 
That got me a whole buncha points!
Umm . . . .  mostly.
Seems if you make a mistake in measuring by oh, say, 1 mm on the plan, you're off by over 5 mm on the template.
Nothing quite matched up.
Anywhere.

Turns out, I SUCK at lofting!   
Goes back to my childhood, when the other second graders learned to draw a straight line with a ruler and I ended up with an obtuse angle, when my ruler slipped.
Once got accused of taking a drafting class final exam with a Hershey bar.   
Hey! I was sweating!!

I'm not good at drawing curves either, but I figured I could use the inside of the rudder cheeks to approximate the rudder blade pivot top.
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Well I got a curve, but almost NONE of the curve intersected any points where they were SUPPOSED to be.
(Notice in the pic below, the arc outside the cross-hairs point?)
Became very frustrated, that after over 2½ hours of  drafting and sweating (why do I always sweat, when there's a pencil in my hand?) and swearing, I didn't have anything remotely close to what was going to be a usable template.
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Ah! But I DO know how to use a computer.   
CAD came along in the 80's, barely in time to salvage a technical career and was a godsend for me, since while I might not be able to draw a straight line, I can tell a computer how to do it, all day long!
Although ironically, CAD wouldn't save me this time, because I couldn't trust my imperfect measurements off the prints to input valid co-ordinates that the CAD program could print out.
But, I still had a computer, didn't I?

So I folded up plans page 2 to the rudder section and scanned it on my all-in-one printer/scanner/copier and saved it as an image file.
Then I input the file into a drawing program, lined it up on the ruler line in the program and printed it out; then I enlarged it and printed it again. 
Now, knowing the difference between the original size and the modified size, versus the desired size, (e.g. 805 mm over-all length)  I used some of that algebra (that I swore to my 8th grade teacher I would never have any use for) to determine the correct multiplier  and scaled up the size, once again.
It was now of course, much larger than a sheet of paper but having a full size image, I could position portions of the image in the printable page boundary area and print out sections of the rudder, a piece at a time.

In the pic below, the lower right sheet is from the original printout of the scan and the lower left sheet is a test of the scaled-up image.
The paper template at the top of the pic is actually 8 different sheets, taped together.
Those reference numbers on the drawing were useful, since I managed to get them on most every sheet and was able to align the different sheets by overlaying the numbers on top of each other.
(That one curve on the left that looks crooked, is because a corner of the page lifted up when the pic was taken.)
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Yeah, over 2½ hours by hand, versus 30 minutes with a computer, a laser printer, some scissors, and scotch tape.
Checked all the template measurements against the reference measurements on the plans and they were perfect!
Color me HAPPY!   
I used some spray adhesive on the paper to attach it to the paneling and cut out the panel with a jig saw.

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Had some Douglas Fir marine plywood left over from replacing the main cabin bulkhead on Urchin, which is now going to become a rudder.
An upcoming issue is how to form a foil, but the plans include a full size foil cross-section, so I'm going to make a half-foil out of some aluminum I have laying around, to use as a gauge.
I did something similar when I re-furbished the cast-iron keel on Urchin and needed to check the NACA-15 foil shape as I faired it.
Every day, it's a little closer to becoming a BOAT!!   

Charles Brennan

[westcoaster]

Charles, you've always been a Scamp.

[Norm L.]

Good thought went into that Charles. I would have gone through the cardboard boxes of 8 refrigerators and the Mark I eyeball to get a template of a rudder that would work. Maybe not well, but work.

As my 11th grade physics teacher said, everything is easy if you know the answer. I hung around Eaton's boat yard in Castine. They were several generations building wooden lobster boats and a few lobster yachts. No CAD there. Just thousands of hours of experience mostly working by eye.
In the hurricane that hit the Maine coast last fall, storm surge and high tide came at the same time and the waterfront in Castine was under several feet of water and docks torn up by wave action.
The Eaton boat yard was flooded. Some friends set up a GoFundMe account with a goal of $15,000 to help replace lost equipment and material and to help rebuild the ancient building. Right now, the collection is just under $100,000.
The Eatons were/are ornery classic Maine characters. But most people love them, and all respect them. It's nice to know there is still respect for historic boat building.  Eaton's is one of the very few waterfront boat builders left in Maine.

[Noemi - Ensenada 20]

Re:  compound bevel angles....my friend once built a stage set full of compound bevel angles on a bandsaw.  It was the only tool that could make what she needed.

[Wolverine]

Charles,  I see you're signed up for the FL120.  I'll be there with my Compac 19, and friend Chris with his Suncat.  Any chance the Scamp will be ready by then?

[Spot]

The nice thing about plywood for a foil is that the alternating glue and wood layers give you a topographical 'read' as you grind them away. So if you are methodical and go slow you and have a mental picture of how it should fair, you can get pretty close...here is a pic from a software for this purpose, keep in mind I did not inport the actual cross sections so the rendering is a bit more robust than the drawings would suggest.

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[Charles Brennan]

Westcoaster, glad to see you make an appearance!  Was just thinking the other day it's been a long time since I saw a post by you.  That's a serious concern for all of us "Sailors Of A Certain Age", ya know.   

As far as being a Scamp my whole life; can't help it.
Got a bad rep as a troublemaker I have to keep up!   

Charles Brennan

[Charles Brennan]

Norm, That should give you some idea of just how "graphics challenged" I really am.
I eschewed refrigerator cartons in favor of paneling, because I frequently accidentally drive the pencil point into the cardboard and it jams, then the pencil point breaks.   
Still getting over the childhood graphics trauma of the art class folding up paper lace doilies into thirds and cutting decorative notches into them.  When unfolded, they had beautiful 6 sided lace snowflakes for winter decorations at home, to adorn Mommie's refrigerator.  I always ended up with three triangles; never could figure it out.

You'd think somebody with enough dexterity and eye-hand co-ordination to play musical instruments could draw or paint, but NOOOOOOOOOO!!   
Why I really admire Jeff Fuch's ability to draw those terrific pencil sketches.

Ah, well.  Ya work with what ya got.
Charles Brennan

[Charles Brennan]

Wolverine, I should only be so lucky.   
I am indeed signed up for the FL 120, but expect to be there in my Windrose 18, Urchin.  It's going up for sale towards the end of this month. I need to use the sale proceeds, in order to finish the SCAMP.  The worst of all worlds, would be for the boat to sell the week before the FL120 (or Cedar Key Small Boat Meet, for that matter).
Same with the BEER Cruise. At my present rate of progress, it would not be unreasonable for me to attend the 2025 FL 120 in the SCAMP, but those are lottery odds, this year.

Hope this clarifies,
Charles Brennan

[Charles Brennan]

Spot, What software did you use for that?
Sounds like it would be useful.
I probably won't be putting too much stock in the plywood striations, since the grains are angled differently on each side, owing to me trying to get the most usable area out of my plywood.

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Keep all the ideas coming!
I need all the help I can get!!   

Thanks,
Charles Brennan

[Spot]

Spot, What software did you use for that?
Sounds like it would be useful.
I probably won't be putting too much stock in the plywood striations, since the grains are angled differently on each side, owing to me trying to get the most usable area out of my plywood.
Keep all the ideas coming!
I need all the help I can get!! 

Thanks,
Charles Brennan

The software is called finFoil and is now available online as https://finfoil.io/. Even if the grains are biased, the glue lines should provide some guidance. I can almost smell the sanding dust...

[Brian S]

Sorry, I don't like using plywood for foils. Yeah, I know most designs nowadays specify fiberglassing it. And I also know I used it for my sailing pram's foils, but I also figured there wouldn't be that much load on them from the pram. For my bigger 10' skiff, I laminated mahogany staves from a dining room table leaf, end-for-ending them.

The problem with plywood for foils is that due to the 90º grain offset for each ply, the wood is 1/2 weaker in shear loading, because 1/2 the plies go the wrong way. I've stood on capsized boat's centerboards, and I don't want them letting go - or worse, hiking hard and having the foil go causing a capsize.

[Charles Brennan]

Brian, That was a thoughtful point you made, which sent me down quite the rabbit hole, researching it.
The Wood Handbook: Wood as an Engineering Material from the U.S. Forestry Service was quite useful.
Learned far more than I ever want to know about kPa, MPa, psi, shear, and tensile, of assorted plywoods.
I started by assuming that your premise that half the plies going in the wrong direction degrades the total strength by half.  There is some disagreement in the industry as to how much that is affected, but overall, while not as much as 50%, the variations are not really (IMHO) statistically significant.  So I'll concede that we can start with a working assumption of 50% shear load for plywood. 
The SCAMP's rudder and centerboard is nominally an inch and a half thick, so I looked up the shear loads for ¾" plywood and was greatly heartened to see that the loads exceed not only the hull weight, but also the anticipated total weight.
That was for the Okoume plywood used in my centerboard.  My rudder is being made of Douglas fir, since I had some on hand. Shear on the Okoume is listed as 5.90 MPa, and 8.20 MPa for the Douglas fir, respectively.
Got a severe headache doing all the math to turn that into psi's and whatnot, but the take-away was that I can easily cantilever either my rudder or my centerboard and still safely hold up the entire boat and everything ever liable to be aboard.

Mahogany measures out at 10.3 MPa shear, so you are definitely the winner in the shear contest!
Stout Skiff!

So while you are correct in your assessment of half the plys being in the wrong direction and not helping, it seems that the plys that ARE helping, will be more than sufficient for all the anticipated structural loads placed on them.
And if run into any UN-Anticipated structural loads, I imagine that shear loads are going to be the least of my worries, right about then. 

Thanks for the input, your advice made me smarter (from having to go look stuff up!); I find I never learn more, than when my assumptions are questioned and I have to understand them well enough to be able to explain them to someone else. 

Charles Brennan

[Monroe]

Charles,
That last sentence in your last post reminded me of an old story:

Two profs met each other in the hallway after a class was over and one said that the other day a student came up to him after class and said he was confused about something the prof just taught.
So, the prof said he explained it again with a bit more detail. The student replied that he now knew it much better, but still not exactly. So, the prof said he went over the concept for a third time and the student was satisfied, and what was even better was that he really understood the concept now.

[Wayne Howard]

At this stage in my life, I want to learn more from other people as it usually hurts too much to learn it myself.