Cross beams/aka's for a 6m (20ft) trimaran

G'day all,

I'm building a self designed 6m /20ft lightweight demountable trimaran that separates into 3 sections; 2.4m bow section, 2.4m midships section + 1.2m aft section.

Why you ask?

I have a steep driveway that rules out a trailer but car-topping the boat is just doable. I have a ute (that's a pickup for you yanks!) which can hold the main hull sections while the rest of the boat, Mk3 Boyer A class cat hulls for ama's, mast and cross beams, is on the roof racks.

My 3 questions for the collective wisdom of the forum are...

1. If I were to use unstayed aluminium tube section as the aka's what outside diametre and wall thickness would be appropriate?

2. Same question as above but for carbon fibre tube...? (Ha!Like I can afford it!)

3. What are my alternatives to aluminium/carbon fibre to keep them light but strong?

Specifications of the boat for reference...

- A class cat 4mm plywood hulls for ama's. Estimated weight is 15-20kg / 33lb-44lbs (I have not weight them as yet, anyone know the weight of these?)
- 6.8m 22ft stayed aluminium mast.
- would like to have at least 2 people on the tramps at any given time.
- estimated mainsail area will be 12sqm / 130sq foot
- jib 3.7 sqm / 40sq foot
- screecher ? not sure but tack will be mounted on a 600mm prodder
- main hull is 6mm ply, EPS foam and fibreglass/epoxy.
- estimated main hull weight 100kg / 220lbs.
- main hull draft at midships 600mm / 2ft
- main hull beam at widest point 603mm
- sailing beam 5.5m / 18ft
- distance between aka mounts on ama is 2250mm / 7.4ft

Thanks.

p.s. It's my first post so be nice!

 

Hi Aaron, I have similar size boat but 4.8m wide when open and a lot more sail area. I used 100mm ally x 3mm wall thickness, it seems very sturdy but I am no expert. The folding system I have is similar to Farriers so it may offer some support to prevent tubes bending under load.

 

I have just launched my new built Strike 20 Tri. It is 5.10 beam . 5.50 m is very wide beam but you have very low buoyancy outriggers. Mine' are Nacra 5.80 hulls. I use 100 mm OD , 3 mm thick aluminium tubes and water stays. With such a wide beam you need waterstays. On my boat I have estimated waterstays max load was around 3000 daN on main beam.
Best wishes

Patrick

https://www.dropbox.com/sh/wi2pj01emza07lv/AADocN3LlA25DjzdTHe0uM4Ya?dl=0
https://www.dropbox.com/sh/ug32rkrarsn83cy/AACE0uUOSLksidTXjNGY8VURa?dl=0

 

Thanks Tamas, appreciate the info! I think you're right that the farrier style folding system will provide strength under load. What design is your tri? Is yours in the water or still under construction?

 

Thanks Patrick, great looking boat and sailing vids mate. Like the video of you blokes sailing past those sand dunes.

Building a Strike 20 would have been a whole lot simpler than what I'm trying to pull off, I'm a tad jealous.

What did you use for waterstays and how are they attached to the vaka and aka?

Thanks.

 

Further to my initial question here's a rendering of my design which I'm calling the RUDI 6.0.

In the graphic I'm comparing scale size of Hobie14 and Hobie16 masts.

[/IMG]

 

For the waterstays I use last generation of Heated pre stretched dyneema. I use twin 5 mm for each waterstay which gives 8000 daN breaking strength. I have also used 6 mm Vectran on the aft beams. Vectran is stiffer but has a lower breaking strength. The attachments are 10 mm bent U bolts on the main hull and 12 mm carbon rod through the aluminium beam at the outrigger side. I need to use very high tension on these stays to keep the platform tight, as my boat is on a mooring exosed to wind, waves and tidal current. Building Strike 20 was a lot of work too ! If you hit your 100 kg target for the mainhull, congratulation. My Strike 20 is about 350 kg total weight. Nacra 5.8 hulls are very heavy, 61 kg each !
Cheers

Patrick

 

Woah! 68kg is definitely much heavier that my A class hulls. Keeping your Strike on a mooring means you get to take her out all the time.

Dyneema was my first choice for stays if I needed them, probably use single 10mm diametre stays which will go forward and aft of the aka's to provide fore & aft tension as well as vertical.

Do you get a lot of spray in the cockpit from your waterstays? And at what speed?

Unlike your Strike 20 my RUDI 6.0 doesn't have a flared cockpit (part of how I can keep the weight down) so the trampolines will extend from the ama to the vaka. I expect anything above 6-8 knots wind I'll be getting wet!

 

Aaron, you may want to look up the TC601 build thread on this forum and build site http://lwr600.co.uk/TC601/index.html where you may get lots of ideas on build and such.

You may also want to read through the thread on small 20ft tri's where the use of an A Cat was widely discussed and some calculations submitted to justify the use of such low volume hulls.

I'll be updating the website to include how we have dealt with the beams, they have been really really difficult to get an easy and widely accepted method of building them without having weird looking attachment points to the donor hulls, that is widely available through the world. Sorry no the Farrier method is just too cumbersome and time consuming for the home builder, look more at the SeaCart 26 for inspiration.

 

Thanks Wayne, do you know the name / have a link for the thread discussing A Class cat hulls?

 

I think the three threads about under 20ft Tris' have a lot of info ( and also a lot dross in amongst it ) but the main one with mention and figures for the A class was

http://www.boatdesign.net/forums/multihulls/t20-new-development-class-39874.html

 

Some more practical data on 20 ft Tris. Two months after launching and solving some problems on our Strike 20, we sailed yesterday in a decent wind. You will find video here broad reaching around 15 kt :
https://www.dropbox.com/s/6uk10p6q6jx85qn/VIRB0082.MP4?dl=0
Top speed was 16 kt, with 14.8 mean speed over 500 m and 14.3 over a nautical mile.
Patrick

 

Hi Aaron,
Here is a blog I did on the build. http://husky6.blogspot.com.au/
I have been sailing for a couple of years now. I have since added a bit bigger jib and fat head main taking sail area up to about 27M2 which seems to have turbo charged it. I think I am just under 300kg, boat weight.
Cheers

 

Hi Tamas,
Your boat looks very nice, congratulation. What is the maximum beam ? I think my Strike 20 is around 350 kg, as the Nacra hulls are very heavy. You should be as fast as me (see my last post). Have you any recent GPS record ?
Patrick

 

Aaron-here is a thread with many tri's 20' and under:
http://www.boatdesign.net/forums/multihulls/small-trimarans-under-20-a-43650.html