Tortured Plywood - Design & Construction

[mattotoole]

Originally Posted by Deering

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.

There's quite a bit of friction (hysterisis losses) within belts too, which is why they're not used for bicycles (where power is relatively low and every erg counts). Chains are still more efficient, twisted or not. Modern bicycle chains are somewhat flexible to accomodate 9 and 10 speed gear clusters, so with enough length they'll twist OK too. Sure there's more friction but I bet it's still less than with a belt.

Also, bicycle chains, sprockets, and other hardware are widely available and inexpensive, and a working unit can be assembled with minimal machining or other custom work.

I do wonder how a twisted chain compares efficiency wise to a 90 deg gearbox though.

I also wonder how close that model airplane propeller comes to a purpose-built one (which would be many times more expensive).

[BG_Geno]

Originally Posted by SolomonGrundy

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

If you dont mind my asking, how much was that? They don't list prices on their site. Also, weight?

Geno

[SolomonGrundy]

Originally Posted by BG_Geno

If you dont mind my asking, how much was that? They don't list prices on their site. Also, weight?

Geno

I picked mine up surplus for $50, it weighs about 3.5 lbs.

[b_rodwell]

On the drive unit:

It was helpful getting feedback on the suppliers of angle drives. My latest thoughts are not to use an angle drive for the following reasons:

For a suitable three way drive, one supplier quoted US$350. This seems
a bit expensive.
I would expect a bevel drive to be less efficient than a twisted chain drive.
I would like to have a 4:1 amplification. The most I have seen with bevel
drives is 2:1

My latest preference is the twisted chain drive unit you can buy from China for around $250:

http://www.msu.edu/~pengchun/

I would be interested in critical comments about my thoughts and any experience in using this unit.

On CAD:

I have been able to use Rhino to derive the shape to cut the plywood. It uses some of the techniques previously described but with 2 additional steps. It would not unroll for me without the additional steps.
1. Slice the hull into segments using an athwartship plane.
2. Define the edges of each segment. (additional step 1)
3. Contruct a surface from the edges. (additional step 2)
4. Unroll the new surface.
5. Layout all the surfaces aligning the corners.

For most of the hull I was able to take slices 500mm apart. However near the bow and stern, it would not unroll untill I made it finer slices. The smallest slice was 20mm.


Brian

[BG_Geno]

the angle drives can be had much cheaper, and you don't use the angle drive for your gearing. Get a 1:1 angle drive, and use the sprockets (chain ring in the crank and freewheel on the angle drive axel) for any ratio you like, including 4:1 you mention. Twisted chain is a some what problematic solution from everything I have read. They often have derailing issues etc...some of the best boats I have seen, including long distance endurance type boats use bevel gear drives.

as for paying $250 plus shipping for that twisted chain unit...those are very easy to make for $50...plans can be found with materials list and instructions many places on the net.

Geno