Reverse engineering a Twiggy

[catsketcher]

In response to the thread on crossbeams I had a go at reverse engineering a Twiggy beam. Feel free to have a read and a comment.

Cheers

Phil

[keysdisease]

Nicely done. Steve

[rayaldridge]

Very good stuff, Phil. Now if you could just do that for a good little cat...

Reading your piece did make me wonder why waterstays haven't generally been used to stabilize cat beams.

[bruceb]

Phil, thanks, that makes me feel better about my re-engineering my Buc 24, I have been coming up with similar numbers. I didn't realize the Twiggys had double water stays, are the attach points on the hull separated fore and aft? B

[jamez]

Great paper Phil. Plenty of Crowthers over here (NZ) but I've never seen a Twiggy, although Cathy Hawkins book gives some insight. Didn't know the beams had an option to fold.

Originally Posted by rayaldridge

Very good stuff, Phil. Now if you could just do that for a good little cat...

Reading your piece did make me wonder why waterstays haven't generally been used to stabilize cat beams.

Where the stays join the main hull near the waterline drag in waves may be an issue. Most designs for tube crossbeams seem to employ them (eg. Kurt Hughes, B24). The 25 foot tri I'm building (which has wooden box beams) has them running from the bottom of the hull step, normally well clear of the water. On my boat these are 6x50mm S/S flat bar, not a wire stay, so I suppose its technically a strut. I've seen a similar arrangement on a Bucc 33 with tube beams which I may have a photo of somewhere.

[rayaldridge]

jamez I suppose a seagull striker is sort of an upside-down water stay. I guess I wouldn't be surprised to learn that some all-out weight-paring racing cats have similar strut and wire combos under the main beams.

[catsketcher]

Bruce - the Twiggy waterstays are about 200mm apart from each other.

Ray - I will dig out my reverse engineering of the Seawind 24 and write it up properly. It is a good little cat that has performed well and is solid too. It can use a dolphin striker to counter mast compression.

Jamez - the fold option was news to me until a few days ago. That's pretty sad considering I had my own plans that I sold with my Twiggy. I could do the calcs to prove the beams can't be cantilever but looking at them you can see that they aren't - they don't have increased depth at the root or any increased scantlings like a proper cantilever beam. I am sure that they are considered as compression struts only but need extra bulk to be walked on and take fore and aft loading - they are bigger fore and aft than deep - another giveaway for the compression strut.

cheers

Phil

[rayaldridge]

Phil, I'm really looking forward to it!

[langdon2]

Thanks for putting that up, Phil. Very interesting piece.

Cheers,

Steve

[upchurchmr]

Ray,

All of the day sailing cats I have used have a dolphin striker. Same exact concept as a water stay just no main hull to get in the way of the loads. I thought you had talked about those in an earlier blog. Hobie 18's have a cable if I remember right and the Tornado in my back yard has an aluminum strap.

Marc

[Samnz]

interesting

any chance you could share the scantilings for the foam and glass beam?

the 12 degree angle is the angle used for the diamonds on an alloy rig, no surprises he uses the same figure.

[catsketcher]

Gday Samnz

I can do a quick sketch up of the foam beam in a few days. At the moment I am doing my reverse engineering on the wooden beam.

cheers

Phil