Bolger Advanced Sharpie Proa, now with Videos.
Othmar is on the right track, but it will take two very stiff battens held at a constant angle to each other to make it work. The apex of the “V” created by those two battens will scribe a circular arc provided it is translated such that the two legs always pass through your two end points (I’m assuming we’re talking about something like the corners of the forefeet at the ends of your hull, yes?).
The math is a bit crazy if you want to figure it out mathematically; using Othmar’s diagram:
r = (b^2 + (c^2)/4)/2b, where r is the radius, b is the ultimate distance between the centrepoint of the arc and the centrepoint of chord c. Chord c is the straight line distance between your two known points bow and stern (err.. I mean bow and bow…)
It looks daunting, but it’s not really; r and c are known constants and it simplifies down to a quadratic variant that looks like this:
b^2 - 2rb + (c^2)/4 = 0
You can fill in the r and c values which will leave you with two possible solutions for b: one will define Othmar’s circle above chord c (which is the one you’re after) while the other would define the rest of the circle below it. If you run battens from your discovered point at the apex of b passing through (and extending past) points at either end of chord c and fasten them together so as not to allow the angle btween them change you can scribe the radius by moving the whole thing along. At least one of the legs will need to be as long as the distance c, then flip it over to draw the other end.
You could also use your favourite CAD program to draw it up and find the height of b and the included angle required between your battens.
Confused yet?!?! I can try to explain better if you’re stuck, or you can just give the details [I just need the end to end length of c] and I can solve it for you.
Confused yet?!?!
Yes!
As i said before. I suck at math. I´m totally worthless! Without my pocket calculator i´m lost!
I will try to understand it, but it will probably take a while. (Years…)
Thank you for trying to explain it!
Johannes.
Yeah, the ratio of “b” to “c” is going to define the included angle “a”. set up a straight batten perpendicular to “r” at each end once that angle is defined, then pull the center of a loooooooong springy one down to “b” with the ends guided.
a lot would depend on your batten. I think I’d calc offsets at 1ft intervals and fair it.
Tom.
It’s not as difficult as it might sound; this is our go-to trick for scribing deck camber onto bulkheads and deck beams that are already installed in a hull. It relies on using stiff battens, not bendy ones… (Bending battens is great for creating fair curves, but not so great at recreating a specific radius or curve)
Here’s a doodle to give you some idea of how to do it with a few sticks and a handful of tacks.
Thank you MTP!
That is a very simple and usefull way of drawing an arch.
I tried it today when drawing the lines on my monohull-shunter.
Johannes.
I have not updated this thread in some time.
I´m building two models at the same time to minimize the amount of time working with epoxy.
Today i cut the parts to a 8:1 L:B Advanced Sharpie Proa.
I use the same 5 cm rocker for both the bottom and the sides as my first one.
From the left to the right:
My new 8:1 AS-Proa, 120 cm long, 15 cm wide and with 5 cm rocker and 5 cm blunt ends.
My oldest AS-Proa, 10:1, 121 cm long, 12 cm wide, 5 cm rocker and 2 cm wide blunt ends.
My more slender test. 12:1, 143 cm long, 12 cm wide, 4,5 cm rocker and 3 cm wide blunt ends.
I hope and believe that the 8:1 L:B Proa will sail better then the 12:1 L:B Proa did. I´m actually quite disappointed with that one. It seems to be able to sail very fast, but other than that it had more leeway and was very unruly to steer. Much more sensitive than the first one. I think its because it had to little rocker compared to its length and weight. The rocker is needed to suck down the hull somewhat into the water and enable the hull to act as a lifting foil. A higher speeds the longer and more slender hull behaved much better, but at a knot or so it skidded sideways - almost to the point of not sailing. I had to load it down with rocks to get it to behave as it should.
These are load-carriers and mainly for extended cruising. I have to able to sail slowly to see reefs and other obstacles in the water. I need a couple tons of water, food and gear on board.
I want to be able to sail fast (sustaining more than 10 knots), but not at the price of an unruly and unsafe boat.
Johannes.
I just found this one. A scow with a really wide bow. there is not much waves and the are sailing downwind, but i think it sails really well. It tries to lift the bow and is close to planning sometimes according to th e article at Duckworks.com.
I think a more slender and lighter Scow with a lot more righting moment from an outrigger will get up and plane real easy.
Johannes.
Today i finally had some energy to cover all the seams with thickened epoxy.
Now i have to wait another weak before i will sand down the outside of the hulls.
So in another weak or two i hope to test these two ideas.
Johannes.
Hi Johannes
You wrote:
How does one draw an arch with a 25 meter radius on a piece of plywood? I have been thinking about buing 30 meter dyneema-rope, but its not cheap. I dont have any wire or chain…
Buy a laser meter in a diy-shop. There are quite cheap models. You can use it later for shure for any measuring between 1m and 100m. Very practical.
By the way. I was building boat models for more than 10 years, always making them better, having a new idea, making that better and so on. Just not in the sharpie style but in stressform. I can not remember how many models I built, but could have gone on another 10 or 20 years that way. Luckily, I must say now, I had a motorcycle accident that kept me at home for nearly a year and I lost my job. That was the point to think what I am really doing. And now I am doing it for real and build my proa. Drive savely, keep your job but start building.
Even the shape of a boat can be changed if necessary.
http://de.wikipedia.org/wiki/Panta_Rhei_(Schiff)
The text is in german saying that the newest cruising ship on the lake of Zurich was 15% (60 tons) too heavy because of a calculating error. They had to glue foam and fiberglass on the underwater ship to lift the stern and reduce waves the ship made (it was shocking, I saw it). It got approval then as a cruising ship again.
Even that is possible.
Cheers
Alexander
By the way. I was building boat models for more than 10 years, always making them better, having a new idea, making that better and so on. Just not in the sharpie style but in stressform. I can not remember how many models I built, but could have gone on another 10 or 20 years that way. Luckily, I must say now, I had a motorcycle accident that kept me at home for nearly a year and I lost my job. That was the point to think what I am really doing. And now I am doing it for real and build my proa. Drive savely, keep your job but start building.
There is some truth in what you say, but i believe its important to test the ideas before one commits lots of money and work to build something that maybe does not sail that good. Since there is very little information about what im doing i have to test it and get some experience this way. I am building a 24 feet proa, but it takes some time. Today i tested my assymetric monoproa and i think there is some real potential there. This is not something i could have learned about without my models.
But i agree with what you say. One never knows what life have in store.
Johannes.
A bit of confirmation for the scow bow comes in the form of 100’ mega yachts from Chevalier Taglang design.
Buy a laser meter in a diy-shop. There are quite cheap models. You can use it later for sure for any measuring between 1m and 100m. Very practical.
Thanks for a very good suggestion. I will buy one soon. They seem to be very useful tools.
A bit of confirmation for the scow bow comes in the form of 100’ mega yachts from Chevalier Taglang design.
Thanks! Its very interesting to read about all the advances in sailing. Sometimes you just have to go against “common knowledge” to make advances. Usually it is the “common knowledge” that stands in the way. Since i saw this clearly i have been having a blast developing stuff. Reading about what people experienced with Phil Bolgers Advanced Sharpies and Matt Laydens Paradox, seemed to go head to head with what most people think they should be like. Sven Yrvind crossed the Atlantic with a 14 foot flat bottom-chinerunner sailboat. He was sailing in the same hard weather as 2011 Fastnet Race, where there was race-boats that broke their mast and keel.
A couple of years ago most people considered flat bottom boats and keel-less boats to be totally unsuitable for open ocean, and some people still do, but its changing…
Johannes.
Today i have tested my 8:1 advanced sharpie vaka with the same outrigger and sail as usual. No videos this time. It was very gusty and it was hard to control the boat. Sometimes it started to sail well and than a gust hit and it it turned over and sank. I used some sand as ballast to slow it down a bit. When sailing down wind it started to plow water with its 5 cm wide (two inch) blunt end, but it never pitch-pooled. Even without the sand as ballast it did not pitch-pool, even though it was plowing water with at least one 2 inch of the blunt forward end under water.
The few seconds of good sailing i got i think it did what i expect it to. It needs some weight to push enough of the hull into the water, otherwise it had excessive leeway. It was fast and did something like 50 degrees towards the wind. I would not go with that wide blunt ends though. I think it would throw excessive spray in any kind of waves. It was much worse than my first yellow advanced sharpie.
Johannes.
I got a picture of the bow plowing water.
This is the worst i have seen. My older AS-proa would probably have pitchpooled in the same conditions.
Johannes.