Is rigidity a religion? Amas on the move....

@Skeezix: I think you are forgetting the righting moment of the windward ama as it lifts off the water and the form stability of the main hull. They can be the greatest righting force.

 

Those forces are substantial indeed, though the windward hull is essentially moot in this thread.

Unless you have crew out to windward, its righting force is limited by its weight, while the leeward ama's force is the greater weight of the water it displaces. Otherwise amas would be prone to sink rather than float.

Of course, the smaller the trimaran, the greater the influence of crew to windward, even in the cockpit.

Form stability is a bit beyond my ability to discuss. Aren't those that are substantially beamer above the waterline unstable? Because tris depend so much upon the amas for righting moment, is not speed a stronger design factor for the main hull?

 

It depends on the design. Amas that are partially submerged at rest are different than the ones that don't.

 

Wise words.

 

I rather thought this was a good question. A shame it went off the rails.
Initially I thought this was about "roll" stiffness but actually it was about whether "trimaran ama arms" (lakos according to Gary Dierking) should have some flexibility or not?
This in my mind is a very good question.

Traditional outrigger sailing canoes have low bending and torsional rigidity.
Have a look at Gary Dierking's "Building Sailing Outrigger Canoes".
I was horrified by his "T2":

Wow, using sticks and string to attach the ama!

Compare this to my indestructible (designed to take adults using at my outrigger canoe as a see-saw):

(Yes those lakos are bent)

Now, both designs work but I now have a more open mind about "sticks and string".
In fact I would say I have over designed my lakos.

AlanX

 

Argh! Didn't realise Gary Dierking actually posted above!
Love your book, one of my favourites. It was the inspiration for my outrigger canoe.

AlanX

 

Well, it started off about stiffness, the EI, of the structure, but despite the best efforts of others to wade in and misdirect, it eventually emerged to be about stability, roll stiffness.
Thus, it is really down, as always, whether the posters understand their own question and/or using terms that think they know what they mean, but in engineering there is no room for ambiguity - and this leads down to polemics because posters don't tend to like being exposed as misunderstanding their life long beliefs!
It seems many posters don't like facts to get in the way of their argument, sadly.

 

Hi Ad Hoc,

I am not a really a forum person.
Hackaday is my usual place of residence with some 75 projects (include some of my boat design adventures).
So I am learning it is better to pick my posts here.
I have decide to post only if it adds to the thread.
I also think defending yourself is a waste of time.
Why am I here? There will be some "gems" to be found and some people will appreciate your input.

I really liked Gary's contribution, so the thread was worth the read.
With Gary's US2 design, he went full boat length with the amas.
Is the reason he wanted/needed more transverse rotational flexibility?
I assume that "riding verses piercing " waves is less of an issue for shorter amas.
Or was it just L/B ratio?

Regards AlanX

 

Good question!?

Without knowing the objectives, one can only speculate.

Since flexing is it good or is it bad?....the objective needs to known, there is no carte blanche statement. Since look outside the aircraft next time you fly...and ask the questions.
Is the wing flexing ...good or bad...or...as "designed"..... and that's the whole point!

 

I like your boat! I had a couple pirogues, and made one into a proa with a canted sailing rig. It was great fun.
I described it in this thread:
Mr. Smith's Amazing Boats https://www.boatdesign.net/threads/mr-smiths-amazing-boats.66741/#post-926331

 

In typical fast modern tris, going fast often entails flying two hulls, so the amas are high prismatic, and substantially greater than "full buoyancy". At the other extreme, (and extremely so) is my own Windrocket which had extremely low buoyancy floats, never intended to be in the water other than at rest. They served as seats and as aerodynamic aids, directing airflow up on the windward side and down on the leeward, creating righting moment in both cases. The center planing hull did all the work of floating the boat.

 

Hi TWG,
Does the wing pivot or do you use the flap to change the effective angle of attack?
AlanX

 

The wing , if the sheet is not attached, weathervanes, being pivoted on ball bearings (around a CF stub mast) at a point forward of its center of lift. Sheeting in engages the flap, and then alters angle of attack. When the angle of attack goes to zero as the boat accelerates, (because the skipper will have sheeted out to avoiding dipping a float) then the top half of the wing is creating no lift and negligible drag, and the lower half of the wing is creating all the lift. The center of effort was then 5 feet off the deck, and the lever arm that the skipper was perched on was a little over 10 feet.
The main sheet could be rigged for either one part or two part purchase.

 

I find the claim that Wharram's boats were a failure .. *bizarre*, as they cruised everywhere...

I also find it bizarre that flex is presumed to not coincide with boats, given that glass-epoxy construction ( glass-polyester should be outlawed: the polyester hasn't got the elasticity to do anything other than crack, given enough flexing/stretching .. see a youtube channel called "Tech Ingredients" for an excellent guy's video on epoxy composites ) *is normal* in trimarans.

Please consider that different domains ( of trimaran, e.g. ) have different modes of operation:

Say a dinghy tri: meant to surf ON the water, not displacement, right?

So, its amas slap hard onto the water, when one tacks.

Putting flex into the amas to mitigate that makes much sense, to me.

However, if one is making a cruiser, the amas need to *not* slap hard onto the water, they need to slip quietly into the water, increasing their resistance as the heel increases, right?

So, those ones are more V shaped, right?

I find the wisdom in Japanese earthquake-resistant traditional wood architecture to be pertinent:
flex, dissipating the forces, instead of breaking, the way Western masonry does.

I think the for-to-aft flex of an ama may be very important to sea-worthyness, but one must then make certain that
a) the bow of the hulls *isn't* stopping when it digs into a wall of water ( for safety's sake: a hard learned lesson, from what I've read: make the bows *not* blunt ), &
b) the bow of the hulls has sufficient buoyancy to prevent the bows from digging-in ( axe bows can go to hell, in my view, and those longer-lower bows are dangerous idiocy: curved, slight overhang, go deep enough into the water, make the front of the boat behave properly, ie make the bows *deep*, or, as a boat designer said "chesty" )

As in aviation, flex is inescapable. Metal-fatigue is BAD ( *especially* for non-steel metals: steel has a self-limiting thing, in fatigue, that no other metals have, which is why aluminum aircraft eventually need to be decommissioned or have structure replaced ), but non-brittle-fiber/epoxy composites *aren't* metals, so that's not a competent criticism of such structure.

( stay the hell away from carbon & graphite, if you don't want brittleness, btw )

Just opinions, of course, of a miscellaneous autist, but getting this right is important to me & my designs.

*shrug*

 

Composites also fail from fatigue. It is not a problem specific to metals. In fact, as it appears you refer only to fiber/epoxy composites, their failure would be in brittle mode.