tortured ply theory

A friend of mine may be has the offsets of the Lindahl and pics, I'll ask him. generally he answers fast. It's pretty old stuff and things have advanced but not so much in small cats: Look at the Tornado drawn in 1968 and with a modern rig it's pretty good.

The Lindahl and similar 18m2 were 18 feet long (5.50m) from 8 to 11 feet wide. The usual weight of a 3mm okoume, with 8-9 feet beams and 25-27 feet mast was around 240-250 pounds. Hull about 1 foot wide (30.5 cm), 1.64 foot high (50 cm), so very thin with a ratio of 18 to 1. Top speed around 22 knots. With good modern sails, rig, daggerboards and rudders even nowadays a Lindahl is pretty fast in light breeze. The hydrodynamic basis of the hull is sound. The hull underwater is elliptic in front, half circle at 50%, and flat at the stern. Rather little rocker. Big prismatic coefficient, with the center around 52-54% from the bow. Very good upwind if the rigging and sails are top. 18m2 are hard to sail with a wind over 15 knots and can be very athletic...It's a a doped Class A with 5m2 more sail...

I never built a Lindahl for me (but I helped a friend of mine to make one) and it inspired me for my own catas.
I can say that with a lot of care (and some money), specially made 3mm plywood, a bit of carbon fiber and foam in the hull, carbon beams, mast, boom, titanium hardware you can get the 18m2 cat, 10 feet wide configuration at 200-220 pounds (90-100kg). It's less than 1/2 the weight of an Hobie 18... At such a weight you do not beach, and the places where you can put your feet are painted in yellow...Very fast indeed: the advantage in speed was 5 to 10 %, largely enough to annoy the Federation and the catamaran importers around 1988-1991, so the rules and handicap were edited in such a way that a proto had no chance to win.

Although built with no safety margin, these ultralight did last a surprisingly long time. But a normally built 240-260 pounds 18m2 will last easily 15 years, with a careful maintenance. Sorry I have not more personal pics to show as all has been destroyed in 1995.

 

Ilan Voyager,

Thanks for the information. I had a ply tornado, but did not build it.

Marc

 

You're welcome. The tornado was designed in 1966 for compounded plywood, and with the new rig it remains a very serious contender. I helped to make 2: one for the gauge in 1990 at , and one for raid and coastal cruising in 1992 with a rig close to the actual one allowed by the gauge since 1995. The raid Tornado with special laminar boards and rudders, and 50 m2 (about 540 sq. feet!!) of mylar kevlar sails including the spi on a carbon mast was a very fast beast (around 26 knots on a half nautical mile). The jib was "automatic" with wishbone and a dedicated track on the main beam, the mainsail with 2 reefs and lot of original hardware (the owner had a shop making hardware for racing tris, and carbon pieces as booms etc...so you can imagine)
Width 3.60 meters (12 feet) so a helping system is needed to right up the cat in case of flip-flop. Weight around 150-160 kg, built with a good safety margin as the cat was often used in big waves and strong winds.

 

Is there freely available info on how boats like Attitude ( 8.5m) and Red Shift (10m) are built using this method?

Surely they cannot be turturing plywood thicker than 6mm.
Yet even this seems too thin for this size of boat unless they are only sailed inside harbours.

 

They're less than 6mm, probably 4mm - the important point with bending ply is that it stiffens to an amazing degree and once glassed is also strong. Groucho is now over 30 years old, built in 4mm.

 

Hi Gabor ... please send me a PN for further informations. This function is not activated in your user profile.

Best Regards, Michel

 

Something else to consider is that you can vac bag core onto the tortured ply structure if you're looking for extra hull thickness it can be strip planking, endgrain balsa or closed cell foam.

I'll be bagging core onto the inside of my cylinder mold Formula 40 project for panel stiffness and to remove the need for stringers.

 

If you could see the rotting end grain balsa in the old tornado in my back yard you would never use it. I am seriously thinking about taking it to the dump instead of trying to see how much needs to be replaced. FYI

 

You can use other cores apart from balsa but the advantage with balsa is its good performance at a relatively low cost. Kurt Hughes original Formula 40 was built with a cylinder mold outer shell and a vacuum bagged epoxy balsa core it was light and dry after being in the water nearly continuously for 15 years, careful building and good maintenance are important to a boats longevity regardless of core choice.

 

MY own 7.5m cat has 2 layers 3mm ply double diagonal and fibre glassed on the outside.

Going upwind in short chop in F4+ winds I do worry when it takes off from a wave peak and drops 1.5m in the following trough with a loud bang. I think I would worry more, on a 10m boat which probably has to weigh 50% more, if the bottom was only 4mm stressed ply even if sheeted.

Then there is the risk of getting punctured. This will eventually happen no matter how careful the owner himself may be. Repairing stressed ply does not seem easy.

Note that I am not deriding the method per se. I am in fact building a 5m cat in this method and I like the result.

I just wish one could get more meat in the skin in bigger boats.

regards

 

There is nothing wrong with thin ply skin and properly spaced stringers.
The sea wave loads are "Distributed" loads. Hitting rocks or reefs is an altogether different ball game. In that instance the sea and rocks will turn your hull into cornflakes no matter what it is made of.

 

I hear what you're saying, Farjoe, but fine bows with tight curves at base can handle the big wave drops. Maybe your cat is quite broad forward. By the way, you can laminate with carbon in areas that bang; I've never had any trouble with Groucho, which is very fine up front - although there have been times when I've also had second thoughts. OS7 is right; keep well away from denser objects than H2O.

 

The problem with boatdesign jargon is that statements are rarely exact so one's definition of a "fine bow" is not necessarily the same as that of somebody else.

I do not consider my boat to have fine bows but I don't think they are broad either.

In fact they could be broader to handle sailing downwind with a following sea.

Rather than having a finer bow I actually think I could fair better with more volume at the stern to be able to resist an oncoming wave from lifting the bows too much.

Coming back to puncturing the skin, I am actually more worried from third parties damaging my boat than me hitting H20_free stuff.

To date, the 2 punctures I had was when my boat was sitting at its mooring by persons/boats unknown.

 

When I was experimenting with models about 15 years ago, I used sheets of veneer from a crafts store, ~5" X ~36". I noticed as I twisted a sheet in my hands that there was a natural sweet spot that felt very strong, curved across the 5" dimension and then twisted by rotating the ends in opposite directions. It actually "popped" into a shape that looked like the basis for a great chined hull design, assuming some of the panel was trimmed at the center line, which would provide rocker shape to the keel.

I was never confident, though, that any CAD drawings I did would conform to the natural feel of the material. The closest I came to drawing it was like a candy cane stripe on a cylinder or torus.

Would probably work better to fit the bulkheads to the tortured shape instead of the other way around, but then displacement and prismatic are not so easily predictable - they would have to be calculated from the resulting hull, which is a rather dicey way to design a boat.

 

Hello Luckystrike,
Thank you for your reply regarding the contact for the German racing proa projet posted by you (http://www.boatdesign.net/forums/attachments/multihulls/56871d1304634428-tortured-ply-theory-img.jpg).
I tried to send you a message a few days ago. Maybe the messenger of this forum is not working as I was thinking, so I try again here:
My email is: kardosg (((at))) free (((dot))) fr
Please give me any contact for the German proa team (from Karlsruhe I guess), if you can.
Thank you
Gabor