Beam design for a demountable catamaran

[guzzis3]

Alright here goes...

This idea has been rattling around in my head for maybe 10 years. I have always assumed because no one else (that I know of) has done it that it must be flawed. I'm sure one or more of you will tell me why it's a bad idea..

I would also like to acknowledge where the inspiration for the idea came from: The mast beam on the seawind 24 started me off, the trailer for Mr Woods Sango catamaran, and his cuddy cabins, and a trailer I saw years ago built as I recall for a tiki 26. Unfortunately I do no recall the builder.

So the idea is a demountable catamaran, nothing new in that, but this one might potentially go from trailer to water in short order and assemble with only one person needed. It is also only limited by the maximum trailerable width in your area being half the potential assembled beam.

The idea is:

A complex trailer with swing wings rather than sliding beams to spread the hulls, and a central section which raises on a parallelogram using the tailer winch. Swing wings mean no binding, the arms work in your favor about the pivots rather than the size of the hulls working against you to bind up.

A central section in one piece so beams and nets stay in place when detached from the hulls.

Joints in the beams so each hulls is 1.2 wide and the central bridgedeck is 2.4. On an 8 meter cat this gives a neat 60% length to beam ratio. If you can tow at 2.9 or 3 meters than you could go up to a 10 meter cat, notwithstanding the usual problems of towing weight and mast raising.

The joints in the beams are the big problem, with alignment and connection the lesser problem. It's an open deck cat albeit with a cuddy and beam strength and stiffness is critical. Adding joints is a problem. Adding joints which accommodate a bridgedeck approaching from above and facilitating secure quick easy fastening...well...

You may ask why not Sango ? and that's a great question. I've long admired Sango/Wizard they are a great design, and with the clip off cabins you get real accommodation flexibility. In fact by my eye if you make cabins that run just under the line of the central cuddy you could have 6' headroom in wizard, a 22' cat! It would not increase the silhouette but it would interfere with wind blowing at any angle across the boat. The real problem with moving accommodation to the hulls is their rotation. Any fluid, not just plumbing but fuel tanks or lead acid batteries n the hulls has to consider the rotation as the boat comes out of the water. Not an insurmountable problem but something that needs consideration.

Why not a folding trimaran ? Well you need to go to an F9 or similar to get 6' headroom and/or an enclosed heads. There are a few catamarans around at 8 meters with 6' headroom. Quite a few, Mr Woods' Elf for example, Mr Simpson's F1 is another. (I've been trying to contact Stallion Marine/Cure Composites but with no luck). As I've said before what I've always wanted was the minimum boat to get the accommodations I want and I do have a slight preference for cats over tris. There is a 30' mono on ebay atm for $7k and it doesn't look too appalling, but then your stuck with a 30'mono...

So that's the idea. I can design the trailer and build it, but the beams and specifically the joints have me worried a bit...

 

[catsketcher]

Drawings please

 

[guzzis3]

Oh ok. Is the description not clear ? I'll knock something up tomorrow.

 

[redreuben]

Yep

Sorry call me simple but I need pictures too !
Very interested in the size and concept too. Ideally 8m or less, the windage of the Woods cuddy bothers me.
My must have is a permanent double berth. And decent payload, WA coastline is sparse for stopovers.

 

[redreuben]

Just on the beams my later thinking has been around not trying to make the mast beam fold but to be more substantial and fitted later and using lesser beams as light mechanisms to facilitate folding and bolting the heavier mast beam in later. Just thinking aloud.

 

[Richard Woods]

Very interested in the size and concept too. Ideally 8m or less, the windage of the Woods cuddy bothers me.
.

The Wizard can be built as an open deck version - the Sango could as well but no one has done it

Having said that, the windage doesn't seem to affect the speed, even to windward. When we raced ours we were faster than a good Strider and a 26ft Shuttleworth open deck Arrow. (so 4ft longer and with a rotating wing mast)

Much more pleasing and surprising was that in one race we rounded the windward mark just behind Reg White who was sailing a 26ft carbon Firebird. He told us "slow down" Reg won the first Olympic Tornado gold, the Little Americas cup etc while the Firebird is one of the fastest open deck catamarans around - see here of a sistership

https://www.youtube.com/watch?v=ML7evMbVNjA

Of course we had a double berth, an oven, standing headroom in the cuddy etc

I will refrain from other comments until I've seen sketches, but is it similar to the Tomcat 6.2?

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[guzzis3]

Ok so PLEASE BEFORE LOOKING AT THE PICTURES BELOW READ THIS.

This is really quick and dirty. It seems my explanation isn't clear I just hope this helps. I hadn't intended to raise this here at least not now but
redreuben asked so It's a busy time of year...

In theory there are some jpegs below. They should show a picture and drawing of 3 transverse beams with a rear hard deck, pod and forward open area for the nets. Conventional open deck cat layout. The 2 things to note here are:

The longitudinal members connecting the beams. I've shown them as orange cylinders but they could be anything. Their job is to make the bridgedeck one piece, no assembly.

The flanges in the beams. Each beam consists of a 2.4 meter yellow mid section, 2 1.2 meter long ends which fit inside the hulls (these would be trimmed to suit the width of the hull obviously) and in the join of each I've put flanges. There is a separate drawing showing what might be possible. Assuming aluminium round section beams I made them 300 by 6mm wall. The flanges are aluminium sized to slip inside the beams and glue to avoid welding. The outer end of the flange is 12 mm tapering down to maybe 3 to avoid too much of a stress rise in the beam. On the outer face of the flange is a plate welded probably and 4 holes drilled. I've drawn it as 10mm plate with 25 mm holes.

The other pictures show the bones of the trailer.

The hull cradles are not shown only the "swing wings". These rotate like a dragonfly trimaran's floats. Rearward and out.

Atop the trailer is another swing wing only this time vertical. There would be a platform structure on this, again not shown, to carry the bridgedeck forward and up.

Triangulation and details not shown.

Mr Woods: Yes that boat is somewhat similar in that it elevates the bridgedeck, but it looks like they tuck the hulls under on their side. I want to bring the hulls in in a vertical condition. If I were rotating the hulls I might as well stick with your very well thought out Sango.

I don't know about windage on Sango. There are other cats, like the F1 that have significant silhouettes thanks to 6' standing headroom in the hulls. Sango's cuddy is no higher and fairly short. If it didn't present problems on Wizard then surely it wouldn't be a problem on the longer boat ? 2c..

I might try to produce another picture of the beams in 3 pieces to make that bit clearer. The flanges as drawn could be made to work. I was pondering if glass beams could be made sensibly and what options that might present for the joints.

 

[UpOnStands]

just for fun

the cuddy is 1.56 m high x 2.4 m wide x 4.2 m long
each hull is 2 m high x 1.2 m wide x 7 m long
when assembled the cuddy floor is 98cm above keel line
the mailbox access to the hulls is likely to leak

the hulls are joined by fore/aft scissors arms and the trailer has slide out arms holding the hulls upright. The scissors when completely opened hold the hulls slightly wider than the cuddy. After dropping the cuddy in, the centers of the scissors are lashed up to the cuddy which tightens the hulls on to the cuddy.
packed the height is 3.6m and width is 2.5 m
assumes trailer envelop 4.11 m high x 2.59 m wide.

 

[guzzis3]

That has a lot of windage for a 7 meter cat.

 

[UpOnStands]

yes indeed.
I was just interested to see what could be packed into the trailer envelop.
The cuddy could be smoothed off more and the hulls of course are just temps to delineate the spaces occupied.
My main concern would be the weight.
The very small cuddy with beams and tramps cuts the weight down but then you have the problem of attaching the beams. Richard Woods has his Wizard arrangement but fitting it in the envelop really compresses things.
So, either concentrated point loading - pivots as in Wizard - or dispersed loads with extra weight?

 

[guzzis3]

The thing with wizard is you can make the clip on cabins for the hulls, so it's still 8' wide but you've got much more amenable accommodation in the hulls. The penalty is a bit more setup and open hulls when the cabins are off, unless you make covers.

The pivots aren't a problem. You've got to concentrate some strength at any attachment, including scissors, which have to pivot at the hulls anyway. The beams on wizard/sango from what I've seen are both adequate and not excessive in weight or bulk. They are a beautiful design, one of my favourites. My only issue is the liquid problem rotating the hulls. Otherwise they pack a remarkable lot into compact affordable boats that apparently sail like lightning to boot.

If I were reviewing your design I'd suggest either less headroom or lengthening to 8 meters and pairing down the front and back of the hulls and smoothing the cuddy. And lose the V hulls Soft v with some rocker, LAR, centerboard or daggers. As always my opinions are worth what you paid for them...It's an interesting idea though.

 

[guzzis3]

5 meters is a lot of beam for a 7 meter cat also. It's more like normal for an 8 meter though...

 

[UpOnStands]

the scissors are really under tension only -- could be Dyneema strops. Its the trailer's job to hold the weight of the hulls etc in position until she is assembled and floats.
The hulls need refinement - yes indeed.
I played around with non-symmetrical pivots -- working from Wizard but the 8 m width cramps either hull height or width or both.

just saw your reply; the cuddy is let into the hulls slightly so overall beam is 4.2 m = 60% of 7 m

 

[UpOnStands]

this shows the cut out for the cuddy

 

[waikikin]

Bridgedeck is the beam?

So Guzzis3,

If one abandoned the concept of deck interfaced "cross beams" and adopted the concept of something more like either a pipe flange interface or post stressed box beam where the hulls & bridge deck structure formed he boxes... you could for simplicity imagine the bridgedeck as a scow/punt shape(even with tapered indexing nodes) like with vertical sides and the inboard face of the hulls at the interface also being cut to the same plane then just squeeze those three components together with some wires & turnbuckles/bolts/whatever, the greater the interface depth the less the stress with the greater spread on the "nuts & bolts".... the bridgedeck component could possibly also be a stand alone scow/sea sled style vessel(with the "bolt holes" plugged)
The art would be in incorperating cockpit, cabin & tramp/forestay neatly into the center peice with similar art as you've mentioned into the trailer.

Maybe someone already did this too? & there would be some doubling of structure.

Jeff