Folding arm materials - Any reason not to use Aluminum?

Corley said:-
"Most aluminium beam trimarans use a waterstay to put the tube into compression its a valid approach and means you can cut down on wall thickness of the tube and hence weight you also reduce point load and reduce metal fatigue problems. In a trimaran the size your proposing I'd look at the Buccaneer 24 trimaran design and utilise a similar system (lots of threads on the multihull forum about it). There is no need to reinvent the wheel its all been done before. If you want a curved beam a composite cored or strip plank beam is stronger, lighter and less subject to fatigue issues. I thought the scarab plans included aluminium beams anyway? He will already have addressed the issues discussed previously If I recall correctly his folding system is a bit like Farrier's and uses waterstays to put the beam into compression."

And I think that says it all.

 

First things first: Oldsailor, my name is Damian. I recently got some gwahir plans from you. Just so you know who I am, these silly forum names....

I'd like to throw another red herring into this thread. Assuming Mr Kendrick is still using his top hat beams on the 18 would you consider fabricating some that shape in aluminium ? Carefully engineered they could be similar weight and stiffness, and if you did clobber something at least they would bend before tearing. Also of course no modifications to the connecting structure needed.

Just a thought.

Personally I think the 18 is quite nice as is and if he reengineered the 9m with those "new" ideas it'd be a very nice design.

I'd still build a farrier though.

 

Yes. Still using the top hat style beams. The reason they are built that style. It's understandable why. The two link sets fit nicely inside the beam and are hidden. It looks really nice. I'm still not sure I'm not going to go that route. In aluminum because of its superior mechanical properties I don't think it's necessary to duplicate the shape to have stronger, lighter beams.

I like the farrier designs too. They are WAY more expensive though. $10k just for a set of beams. That's more than the total build cost of a Scarab 18.

And I want a day sailer that I can park in my garage. The smallest farrier is an f22. That' s a bit big for me.

 

Yes Damian, I'm sure that some very nice crossarms could be fabricated from suitable thickness alloy sheet.
But why bother when you can buy round or square section tubing more cheaply.
When we built the three Buccaneer 33s we had Alcan supply the oval tubes by simply putting round tube through the roller forming machine to flatten them to the specified dimensions, at no extra charge.

 

I get the size issue.

Personally if I were after a multi less than 20' I'd get a mini bridgedeck. I have a jarcat J5, and these have been streched 10% to 5.5m (18'). No folding, full sized double plus 2 quarter berths, cockpit stepped mast easily raised by hand. Mine is 600 kg (1320 lb) on the trailer.

The cost issue isn't necessarily correct. You don't have to buy anything from farrier for most of his designes. I've got F82 plans. They show how to make everything, and are the best plans you'll ever see. A farrier of a given length can be built for similar money, given like materials, as a Kendrick.

Ray Kendrick offers the option of ply, but he sometimes glasses both sides. The ring frames on the earlier designes added signifigantly to the cost if you got them done for you. I saw examples of his boats in build where the results were not what I would have wanted. Mr Kendrick himself has built some really nice boats, but I came to believe his designes were less idiotproof than some others. This was some time ago.

The F22 is a special case. Ian Farrier took the decision that he wanted to use the special beams and these had to be professionally made. He justifies that cost on the basis that if you factor your labour at any sort of cost the factory beams work out ok. I am not arguing that is correct but that's the logic behind it. One could argue, validly, that if you attribute any value to your labour you wouldn't build a boat at all. You could also argue, validly, that the "fun" part is building the hulls and some of the rest is tedious. You yourself have balked at the beams on your little boat.

I need to make myself clear, because I am sometimes misunderstood.

Mr Kendricks designes are very nice. The 18 is particularly nice. I'm not slagging off anyone. My interest is in the bigger sizes and it's quite clear that like for like a farrier will get a LOT more money resale.

Your probably not concerned by this, and that's fine. The 18 should be so cheap and quick to build resale differences will be trivial.

There are a number of reasons to stick to the top hat design. Apart from the aforementioned connections top hats are used in all sorts of engineering situations because it happens to provide good stiffness and strength in several load conditions.

Anyway, whatever you decide remember to have fun

 

The problem is with the connections. The Buccaneers don't fold. Tube beams are a great simple solution, but retain the end connections and you start digging yourself a hole. If you don't mind the bolts showing, with some careful layout, top hats could be knocked out in no time with a folder and guilotine. Put some rubber/plastic beading on the edges and drill holes and your done.

I can not remember how the beams attach to the floats on the 18.

Anyway...

 

I like Kurt Hughes approach the beams slide past each other inside the hull and have a waterstay that comes tight when the floats are in position simple, light and practical almost like a spiritual successor to the Buccaneer trimarans. He actually credits Lock Crowther with the original implementation of the idea on those early tri's on his cylinder mold video.

http://www.multihulldesigns.com/designs_stock/d32tri-rapido.html

 

Is just as important being able to enjoy the boat when sailing.

So it comes down to how long it takes from trailer to water and how often you have to reassemble the boat.

Alloy tubes could be the cheapest, and lightest, but takes longer to reassemble.

The swing arm folding cross beam, would be usefull if it is trailered most of the time.

I my self prefer the Ian Farrier style for the added cross beam clearance.

It also would depend on expected conditions you could expect.

 

The OP started with the assumption he was going to build the Kendrick 18 and modify the beams. He has indicated a willingness to add a bulkhead, but implementing sliding beams is another level of modification. I am not suggesting it's a bad way to go, but if you start with a plan for a horse and try to make a canary...

My Horstman has a lovely self build swing wing trimaran but it comes in lengths from (as I recall) 23' to 26'.

At 18' I suppose the float/beam assemblies become light enough to contemplate demounting or single strut folding.

 

What type of design loads would you calculate for the beams?

If we start with a 750lb boat, add 750lbs of people and gear, should I guess that each beam could be loaded individually with 1500lbs? Or should I figure flying both hulls, big wave, 2gs all on one beam and shoot for a load capacity of 3000lbs per beam? That would give each ama a combined 6,000lb design capacity - WAY more than the buoyancy. Is that overkill?

 

I think this is correct. IMO the ease of trailer to water is a top priority. I go to a lot of different sailing locations, and marina slips are expensive here. There are times I might leave it in an overnight slip, but otherwise it's a day sail type situation. The lake I go to most often does have dry storage so I could leave it rigged, but it's both expensive and unsecured and I don't feel comfortable leaving it in an unsecured parking lot where anyone can just drive in. I believe in the inverse relationship of boat set up time to boat use. I'd like to be in the water sailing in 15 minutes. I can do that pretty easily with my current boat.

I really would like the ability to fold the boat on the water. The farrier folding design works nicely for this. I'd like to stay with very similar geometry to the original design. If I were going with Aluminum I'd just mimic all the original geometry, simply substituting an aluminum beam for the fiberglass one because it's easier for me to manufacture.

 

I'd like to be in the water sailing in 15 minutes.

I couldn't rig and launch my beach cat in 15 minutes.

Splaying the floats, either sliding beams, swing wing or the farrier control arms isn't going to be your problem. Also with an 18' boat the mast should be small enough to step by hand so you save the time and trouble of rigging a jin pole, but surely you give the boat a good check over before sialing off ?

Dunno. If your aiming for those sorts of times to water I'd be looking at mini bridgedecks.

http://www.teamscarab.com.au/5.6cat/design.html

Personally I'd have a jarcat ahead of double shuffle or the waller 620 or any of the others, but YMMV.

 

From what I under stand the Ian farrier folding style takes about 15 to 20 minutes, from trailer to launching. I am not sure of the Ray Kendrick style for rigging. but the corsair uses or used a hyfield lever to allow for different length of shroud length when folded and open.

 

My hobie cat (especially if I left the rudders on) would go trailer to water in under 15 min. There's just not much to a hobie 16 rig.

I plan to stick with an non-spreader mast. Simple rigging, like a beach cat. On my current boat I step the mast by hand in about 2 minutes. Having a mast cradle on the rear of the boat really makes it easy for one person because I can just slide it back until the base is at the mast step and pin it to raise it. I'm planning to use a similar rig - possibly exact same rig to start which is a spare hobie cat mast that I've converted to use on my current 17' monohull. I'd like to find a taller hobie 18 mast, so I'm keeping my eye out. Until then the one I have will work.

Combined with slugs and some quick attach lazy jacks, and a roller furler for the head sail and the sails go on really quick. If I start adding a asym and other additional sails, then it'll get more time consuming to set up. I'm not opposed to doing so, but I'd like it to sail reasonably well under just main and jib if we're wanting to go out for an evening cruise.

I also leave all the "stuff" on the boat - and usually pack the cooler etc. on before leaving the house, so there's no loading from car to boat at the marina. It's ready to roll. Just jump on and go.


Anyway, I don't think 15 minutes is unreasonable. The only additional time would be folding out the amas, and that should only take a couple minutes. Ideally I'd like to be able to put the bare pole up, then put it in the water and fold out on the water. That way I can finish setting up while my wife parks the car. Plus sometimes the marina ramps are really busy, so there's not always two side by side lanes open. Being able to drop it in the water down a single ramp lane would be nice some days. And if for whatever reason I did want to rent a slip for a day or two, I could use a single slip.

If it takes 20 minutes - no big deal. If it takes an hour - I probably won't use it very often. I really can't see what could possibly take an hour to set up though.

 

The best way to determine safe design load is back analyze the original beams for strength (in all directions and torsion), to see what the original designer had in mind (with some adjustment from builders on their experience of the original beam design in terms of durability, if any feedback is available). Than you can compare it to estimated loads based on worst case loading, flying whole boat on one ama, etc.

If it all seems reasonable than you just design the new beams to those loads using properties of the proposed new material, in this case aluminum. That would also include all of the pivots, mounting fasteners, connections, etc.

I have designed alterations and repairs this way many times. It would be the safest approach. The only time you might do something different is if you were attempting to lighten the design, or for what ever reason you the strength of the original design is suspected of being inadequate.