Tortured Plywood - Design & Construction

I am considering using a tortured plywood approach to design and then build a 5 metre pedal powered catamaran. I use Rhino. I have 2 questions.

1. Is there anyway to use CAD tools to get even a first cut at the flat shapes that need to be cut?

2. When building are there techniques to allow some degree of control over the final shape? Perhaps using some sort of female mould? I understand that Tornado catamaran hulls were originally build using this technique. I can't imagine that any shape that came out for them was OK.

Look forward to your advice

Brian Rodwell

 

1. Make a 3 x 3 control point surface. Move points to a decent half hull. Split hull surface to 20 or more slices with vertical lines. Unroll slices and orient a puzzle with sheer and keel line corners. There is your flat sheet.

2. Glue wood strips to the sheer lines inside the hull. Cut the strip ends to the final half angle. Stitch (copper cable or banduit) the halves at the ends and keel line. Bend the hull ends up and the middle down so that the end have the final angle (strip ends are tight). Use a cross strip to keep the amidships open (near horisontal). Tape the halves with glass and epoxy.

Make a ladder to the shape of deck from wood strips. Use up to 10 or more tie lines to torture the hull to the final shape. Damp the halves to help torturing. Force the sheer lines inside the deck ladder. Glue bulkheads inside.

Or have the Gougeon Brothers on Boat Construction, it all there.

 

Thanks

That will all work for me.

Interestingly I have had a copy of Gudgeons Book for yonks. After your post, I just pulled it off the bookshelf and there it was. The joys of internet to get a reference to a book you already have!!

Brian

 

Brian,

What are you thinking for a propulsion system? Chain or shaft to prop? I think there are some commercial pedal power cats around here.

 

For propulsion I have settled on propellers. My inclination for drive units is to use ones that are made commercially and save a lot of work. So far I have found:

www.seacyle.biz
which appears to be a double bevel gear drive using a vertical drive unit
and
www.prophish.com
which appears to be a single bevel gear drive using a inclined drive unit

I will probably go with one of them.

I suspect that they both use metallic bevel gears and it looks like the gears are completely enclosed to protect from corrosion. What I would really like to do is use large non-metallic bevel gears and leave it exposed. I mention large because the gears would need to be large in non-metallic to handle the torque. If I could find suitable gears, I would consider making the drive myself - probably almost completely non-metallic. So far I haven't be able to find suitable gears- and I am certainly not going to make gears myself.

The HPV guys use a twisted chain drive (similar to the chain on a bicycle) to solve the same problem. I am not keen to go that route because I would either have to enclose the whole assembly (more complex) or suffer the effects of marine exposure.

If anyone knows of suitable gears, please let me know.

Brian Rodwell

 

Perhaps a toothed belt would do the job. Harley Davidson is using them on some of their motorcycles in lieu of chains. They're pretty common so the cost would be reasonable.

You might be able to locate nonmetalic or stainless pulleys - try the commercial food processing equipment industry.

Here's an example belt: http://www.daycoproducts.com/daycoweb.nsf/Product_Gold_Label_Top_Cog_Cog_Construction?OpenForm

 

The biggest problem is the right angle change to the axis of rotation from the pedal axis (across the boat) to the propulsion axis (for and aft). Hence the comments about bevel gears. OrtThis is gives information about what the HPV guys do with twisted chains:

http://www.recumbents.com/wisil/hpb/compact_drive.htm

I have never see examples of a toothed belt being twisted the way chain drives can be twisted. Has anyone heard of that?

Brian Rodwell

 

Haven't heard of it, but seems that it'd be more efficient than twisting a chain which causes a lot of friction within the chain links. With enough length the angle caused by the twist should not cause alignment problems at the pulleys. Might want to talk to a belt company.

I've had this idea of a drive system for a catamaran that uses a ribbed belt like a snowmobile drive belt running lengthwise down the tunnel between the two hulls, on the water surface. There'd be more mechanical friction in the system, but am thinking that the efficiency of the belt in the water might be better than a prop - no cavitation. Definitely less exposed to damage.

Here's a supplier of conveyor equipment that has polymeric chains and gears for conveyor systems: http://www.rexnord.com/portal/

 

"I am considering using a tortured plywood approach to design and then build a 5 metre pedal powered catamaran. I use Rhino. "

I have a question. Why would you bother to go to the difficulty of torturing plywood in building your hull when you have a software package that allows you to easily create developable surfaces?

 

First question: Rexnord seems to have lots of chains includinig non-metallic chains. However I didn't notice any chains that could be twisted and achieve the 90 degree change in the axis of rotation.

Second question: I am interested in tortured plywood because it could be faster, cheaper, lighter, prettier and stronger. The approach I have used to date in other designs is to use developable micro surfaces to produce a macro surface that is compound. For a minimum wetted area hull (as you particularly need for this type of application) this would translate to multichine if you were using plywood directly. I would normally assemble the developable plywood in a female mould. Tortured plywood certainly works (the Gudgeon book was first written in 1978 and thousands of hulls have been produced this way) and it may have the attributes that I have listed above for my application. That is why I am checking it out.

It will certainly involve experimentation by me to produce my catamaran tender. But I may be able to also produce a set of instructions and dimensions that enables some else to repeat the process very easily.

More explanation: Another value behind my efforts is to do it lighter than is currently available. An obvious question to me is, " If you are going to buy the drive, why not buy the whole unit". I think the Seacycle weighs 160 lbs, the other one weighs 155 lbs.

I am keen to use all the techniques (and tortured ply is one of them) that are available to reduce the total weight. If I could get it down to 80 lbs (and still be robust) , I would be thrilled. This would be a product that could be used very differently from the existing products(eg cartopped, more easily loaded onto a mother ship etc). This widens its area of application. I am not sure I can get that low, but that is my target. Once I have completed my research, I can calculate whether that is feasible.

So does anyone know of suitable large non-metallic bevel gears?


Brian Rodwell

 

Some reference material from the past: http://www.svensons.com/boat/?p=RowBoats/PedalCat

 

Twisted chain prop drive

Hey Guys,

I couldn't help but provide a link to the work of a friend in British Columbia. He has developed a propeller drive system for human powered boats that is simple, powerful, uses a twisted chain to turn the rotational forces 90 degrees and is easy to incorporate in the design of a new boat.

His name is Bob Stuart and his product is called SpinFin. I've test driven the drive system on a 17' kayak and it's quite fast and easy to use.

You can read about the SpinFin here: http://microship.com/bobstuart/spinfin.html

Chris Ostlind
Lunada Design

 

http://www.mitrpak.com/product_datasheet.php?product_id=65
http://www.zero-max.com/products/crown/2_way.asp
http://www.mitrpak.com/product_datasheet.php?product_id=27
http://www.wcbranham.com/products3.cfm?prodCatID=85&prodNum=4304-0403

The last company has many other options in all stainless, different reductions if you want a worm drive etc. I have done a lot of research on the subject for my own pedal boat project and have a few novel approaches if your interested.

Geno

 

tried 'unroll surface' yet?

Hi Brian,

You may have heard this already, but the current version of Rhino 3 has an 'unroll surface' command that lays out a flat pattern from a twisted panel shape...its pretty cool. I have not built anyhting from it yet, however. But proving out panel shapes in a paper model shd be pretty informative.


John Larkin

 

gearing

Here is the gear I'm using for my 20' aluminum pedalboat: http://www.andantex.com/anglgear.html