Inquiry for information from experienced boatbuilders and boat designers

Hi Rumars, thanks for your post, highly appreciated since it clarifies a lot

can you just specify what do you mean between the panels, is it the side panels of the ama you are reffering to (so the block of ply or solid wood would be installed horizontally and below the top panel/ama deck) ?

It is indeed a cantilever beam without waterstays

awesome info. In that regard, If I understood correctly, the middle plate should be omitted and bulkheads every 20-30cm should be installed, as well as the side plywood plates should be cut so the fibres are 45deg from the horizontal line of the beam, as well as to use solid wood for the top and bottom flanges as every other layer in the plywood would not align the fibres with the longitudinal plane of the beam as their angle would be offset by 90deg. XPS foam is clearly then not adding anything structurally.
From comments here and this post: Design of wood epoxy trimaran beams https://www.boatdesign.net/threads/design-of-wood-epoxy-trimaran-beams.68629/ by Adrian Baker, as well as Kurt Hughes' website and his insight into the matter, it seems to me that due to the complexity of building the crossbeams out of ply and GRP, the cost, and the larger dimensions required, all this points to aluminium round beams being perhaps a better choice. Also it seems to me a round groove is better due to glassing the right angles for the square beam.

I would appreciate any thoughts about the following:

I found 6082-grade aluminium round tube beams with a 150mm diameter and a 5mm thickness, they are 6.15kg /metre. From what I understand (from other posts, especially the abovementioned by AdrianBaker) this is perhaps more than sufficient since the tri in question is to be 22.6', with the total beam from ama to ama being 13'. 400-450kg hull dry weight, each ama 70-80kg, 950-1100kg displ.

Since each aka would be 2 metres, that would be about 4x2x the difference in mass between perhaps a sufficiently dimensioned alu beam (i.e. 140mm dia. / 4mm thickness), which would be about 13kg difference. I would prefer to have the akas stronger than neccessary

Understood; so basically from what I can see, a good slot for the beam, then two or more connectors as well as polyester webbing with ratchets (as can be seen from SolGato's picture above?) would be a good combination. The attachment to the ama's side below the beam to which the webbing would be lashed, would it need to be metal or is solid wood element sufficient?


Thanks

 

The blocking needs to be installed inside the aka where the bolt passes trough. Otherwise when you torque the bolt you run the risk of collapsing the aka walls. On the outside you just need a big washer or backing plate under the bolt head or nut.

You understood correctly, make a hollow rectangular beam, plywood sides, solid wood top and bottom, with bulkeads and blocking. In the thread you mention in post #17 there is a drawing of a similar beam where you can see the blocking and bulkheads. It's not complicated at all to make one, it can be done from hardware store materials. You need a few planks of clear spruce/pine/fir and some plywood. You space the top and bottom flange apart with the bulkheads and compression blocking and glue them to it with thickened epoxy. Then you glue the plywood sides on. Few people actually bother cutting the sides in such a way that the plys are at 45°. Round over the outside corners and cover with 200gr of fiberglass, then paint.

I don't do structural engineering for others, so I won't give my opinion. Please do the math for a cantilever beam with point load at tip. The asumption is that the ama is bolted to the wall and the entire weight of the boat hangs on the beams. This is equivalent to the boat flying the main hull.

There are many right ways to attach the aka to the ama. As I said, I would not use lashings. For example you can extend the bulkhead above deck and bolt the aka directly to it. For a round tube you can do glassed sleeves, above or below deck. You simply insert the tube into the sleeve and use a bolt to secure it from sliding out. You need to look at some pictures of demountable cats and tris to get some ideas, including the lashed ones if you want to go that way.

 

Thanks for the guidance on what calculation to do. And especially for this illustration what this calculation actually describes it solved some of my further questions because I have very little experience in structural calculations.

There were, for each alloy, very varying maximum tensile and shear strength limits, depending on material shape and thickness, so for example maximum 6082-T6 alloy yield tensile strength information varies from 250 to over 300 mPa so I noted the lowest figures.

So I arrived at the following result, for mentioned dimensions and point load on tip:

-----------------------------------------------------------------------------------------------
(2 metre beam, round tube aluminium, 15cm diameter, 5mm thickness)

Load: 1000kg

the stress amounted to 8928 psi, with a maximum deflection of 61.5mm

- compared to the two alloys' maximum stress figures:

(6082)
tensile strength 12342 psi
shear strength 18870 psi
compression ?*

(6082-T6)
tensile 36300 psi
compression 35200 psi
shear 24650 psi
----------------------------------------------------------------------------------

Since there are 2 beams on each side, do they share this load, or should it be considered that during movement (in this particular situation when flying the hull on one ama) one of the beams could at a certain instant take on the complete load?

It seems to me, for 6082, that a 1.5 tensile load limit is not sufficient?

What x load factor should be taken into consideration as minimum? x=3 ?

How does the 61.5mm deflection over 2 metres seem?

* I couldn't find a compression yield limit for 6082.

 

Thank you Rumars also for this information.
There was a post on this forum where a member answered a question about scantling, I couldn't find it at the moment, it was a post from 2007 and said that for scantling purposes, for smaller dimensioned crafts that it would be a good practice to compare other boatbuilding projects since over time designs and techniques accumulated or rather to say experienced designers architects and boatbuilders realized pretty much what to do and what not to do for anything; if I remember the words. So it was a cool post and after that I went to this forum and googled around, and studied cca 900 or 1200 photos as well as schematics etc. but couldn't find much for this aka-to-ama connection for demountable/detachable beams. Folding ama trimarans use mechanisms for rotation while I was interested in attach-detach fixing elements like bolts and panels, but now also lashing with the webbing. And the few photos show the exterior. So thank you for all your insight into this as it is quite clear now.

I have one remaining question: when you say "use a bolt to secure it from sliding out" of the glassed sleeve - do you mean drilling vertically through the round beam, so two holes, and then bolting it

 

You should look at beach catamarans for beam attachment methods. I mean Hobie, Tornado and consorts. Even the TS3 with its massive box carbon beams uses essentially the same system, big straps bolted down to a saddle. (This is the system I would employ for round Al beams.)
TS3 Сatamaran - Galerie https://ts3catamaran.com/galerie/#
Tiki 21 with ratchet straps: https://photos.google.com/share/AF1...?key=cHlITWJjWk5UQkZ3VGN4M1RQTHBNNFFfQ0d5UlNn
Round Al beams, socket and lashings (Mbuli), wood beams bolted to bulkheads (Madness): Pacific Proa Beach Cruiser https://www.clcboats.com/shop/wooden-sailboat-kits/proa/
Making hollow wood crossbeams: Hollow Crossbeams https://outriggersailingcanoes.blogspot.com/2020/09/hollow-crossbeams.html
Diverse ama lashings: Outrigger Sailing Canoes https://outriggersailingcanoes.blogspot.com/search/label/ama?updated-max=2009-02-21T17:04:00-08:00&max-results=20&start=20&by-date=false
Many photos of small trimarans: Small Trimarans https://smalltrimaran.co.uk/

 

On my Catamaran and others I know of, the threaded studs are glassed/epoxied into the hulls around the “pocket” that the beams sit in.

This area is usually reinforced to handle the “connection”, so the studs are well anchored.

The clamping straps then fit over the beam, slipping over the studs, with washers and nuts added to clamp the connection together.

As Rumars mentioned, another popular design is to have a pass through hole molded into the hull that the beam slips through with bolts locking it together passing through vertically.

One thing about both options above is that you lose freeboard. If your floats (amas) are tall with lots of volume, it may not be a problem.

A way outrigger canoes and other small multihulls get around this is by using a block or stand off so the beam can mount higher up off the water and the float can be pressed hard without the beam dragging in the water.

The first generation of Hobie Catamarans is a good example of low volume floats that use posts to raise and keep the trampoline and cross beams high out of the water.

Newer beach cat designs have bigger hulls with more volume, so many have beams that are mounted closer to the hulls using the pass through or clamp down method,

On beach cats where the beams pass through a hole in the floats as Rumars described, the trampoline in what holds the system together. There are stop plates inboard of the beams that keep the floats from sliding any further along the beams to maintain the proper beam width. The tension of the trampoline and the lashing then draws the floats and beams together to create the structure, and to keep the floats from rotating around the beams, oval mast extrusions are often used instead of round.

Experienced beach cat sailors that like to sail fast know the importance trampoline tension plays in performance.

 

Rumars thank you for this detailed post with lots of examples. Some of those I have seen googling, but nonetheless after studying all the examples, all of these reiterate what you and others posted above in this thread. So basically I am nearing a clear perspective on the attachment of akas to amas, thanks

 

SolGato, thank you for your answer.
A long time ago, I had the opportunity to see a Hobie catamaran on a beach, it was pulled out. Going around it I remember looking at the exact thing you mention, this must have been a newer generation, as the crossbeams went right through the amas (so, without a vertically exended raise). Back then I was thinking what was the method of connection, but after all these posts I finally have a clearer picture.

The tension of the trampoline understandably provides an additional force keeping all the structure together. But in my perspective, without the trampoline net, the utmost importance was in understanding what you and Rumars already exemplified.


I wonder now, what seems as the only question I can think of, is if the crossbeam on the hull itself can be attached on the perimeter of the hull (the gunwale) by the same means, or should it be attached as closely to the centre of the hull, to reduce the moment of the forces.

What I had in mind is the left and right side crossbeams being connected near the longitudinal centreline of the hull

 

You need two connectors spaced as widely apart as possible, so on both gunwales. If the left and right beams are two separate pieces you either offset them, or splice with a tube inside.

 

In

So two connectors spaced as widely, laterally, as possible. Understood now, as this would reduce moment arms.
The middle, however, I'm not sure I follow you - if the tubes are 2 metres long, and the whole beam of the trimaran is 4 metres, then the beams would basically touch at the centreline of the mainhull. With the two connectors on the gunwales, an additional locking structure at the centreline would from what it seems to me be also good, ofcourse - yet the terminology exceedes my knowledge -

"splice with a tube inside" would it mean to set a shorter tube inside the two crossbeams, to connect the crossbeam tubes, then bolt this to hold them, in addition to the connectors on the gunwales?

Thank you

 

Yes, splicing means you put a shorter tube inside the two crossbeams. The ends of the shorter tube must not be cut straight to prevent stress risers, so you cut out a few triangles so that it looks like long teeth. This shorter tube is bolted or riveted to the crossbeam tubes, permanently joining them. You can avoid this by buying a 4m section of tube to use as the aka.

Now if you want to keep the crosstubes shorter for easier handling, the simplest method is to not have them inline, but next to each other. Each crossbeam is fixed separately to the main hull with its own hardware (or joined hardware).
This arrangement is commonly done on trimarans with sliding beams, for example the Astus series. Look at the pictures here Astus 22 https://www.astusboats.com/sailboat-astus-22.html you can clearly see the black tubes lying next to each other and the white brackets holding them on.

 

Take a look at today’s post by OldMulti in the Multihull Structure Thoughts thread to see yet another beam connection design that allows for simple folding and trailer ability.

It’s a bit more complicated than sliding beams past one another, but still simple enough for home building.

You mentioned wanting your boat to be demountable. If it’s the kind of situation where you will be assembling and disassembling seasonally, demountable makes sense. If however you are trailer sailing and need to assemble the boat for short periods of use, you might consider a folding or telescoping/sliding system.

 

Thank you Rumars for your reply.

Excellent information, highly appreciated, I had no idea of this method. It makes sense, in a geometrical way, with regards to the total "line" of the end of the shorter "splicing" tube being longer when "zig-zagged" around these triangles, rather than a standard even end 'cut' - thus the forces of pressure between the tubes are diminished due to this larger line of contact with the outer tubes, the crossbeams.

Can you please elaborate, since the dismountability of the crossbeams is a target - wouldn't bolting (not riveting) the shorter tube to the crossbeam tubes still allow the tubes not to be permanently joined?

Thanks for this suggestion since it and this example have further illustrated to me the importance of the crossbeam connection at the mainhull. If I understand correctly now, the goal is to have each "hull-to-ama" side connected in a way that the half-beam attachment is fixed to both sides of the mainhull - either through splicing or through non-inline crossbeam design?

In that case, does splicing in that regard require that the shorter, 'inside' tube be as long as to reach the gunwales of the mainhull (or even extending further from the sides, so that bolting can be done as close to the gunwales)?

I include the very interesting pictures of the Astus design for further reference to readers of this post who may be interested in it, cropped for a more specific perspective. One thing to note is these interesting white brackets, the two at each mainhull side but also the middle one.
Furthermore what is interesting to me, and I'm not sure if I understand why this is, is that the length of the central hull is indicated as being 6.95 metres, while the overall length is stated to be 8.30 metres. But from the pictures, I'm not sure where this 1.35m difference is, since the floats and the hull are almost of the exact length. I guess it is the extended longitudinal beam holding the headsail.

 

Hi SolGato, thanks for your reply and the link to oldmulti's post about this Stinger trimaran

A very sleek and solid design from what I can ascern. The horizontal circular pad structure provides a rigid surface for the beams to rotate, and it seems like a heavy-duty pin holding the beam to the ama

It is exactly so, seasonally disassembly is the target and so it is more preferable to go with the demountable design, but from what I have seen so far I must admit this folding system seems very convincingly rigid yet at the same time does not seem to complicated to fabricate

 

You can geek out a bit more on that style folding system by studying the Seaclipper trimaran which also has a simple and straightforward hinged kick-up rudder design that may be of interest.

Beams sliding past one another like the Atsus is a simple and straightforward method that makes trailer sailing a breeze.

A more “seasonal” method that uses tubes for beams would be something like this Condor 30 which has beam pass through tubes glassed into the main hull, so the one piece beam is passed through the main hull, fixed, and then the floats are clamped to the tubes in the same way as my Catamaran.