Adjustable float dihedral

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Kurtz

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I'm drawing up a cut file for cross beam hinges on a small tri

I can make the float dihedral adjustable (at setup) - question is, is this worth doing?
Just thinking out loud here, it's a zero cost option if I do it now.
(a copy of this hinge setup below)

51233146689_985b75c29d_c.jpg
HINGE.png
 
Highly loaded connection , every play on the pins will make difference. Crossbeam material ?
 
2 x 20mm x 90mm hardwood with 45 x90mm pine or a better softwood inner core. epoxied together with a covering of light glass for abrasion resistance.
The hinge brackets will be 3mm 316 with 10mm bolts and Aluminium cylinder distance pieces at the hinge bolts.
The forward inner cross beam will have structure on top that will stiffen it as well.

The outer beams (from the hinge out) will be ~ 1200mm long
 
Crossbeam crosscut drawing if its not top secret ?
 
montero said:
Crossbeam crosscut drawing if its not top secret ?
Click to expand...

Top secret? LOL - I'm hacking this together with hardware store timber

Does anyone know which orientation the beam should be for maximum stiffness in the up and down direction?
I've done it this way for through bolt strength - no other reason.
(it probably wont matter with this small tri, just curious)
image_2025-04-24_155932314.png
 
Theoretically you should have the planks rotated 90 degrees to act as top and bottom flanges of an "I" beam. Practically at this scale it's not likely to be critical and there is the connection issue to consider.
For adjustability if you are clever the outer plate could have the pin holes staggered such that you could have two different angles depending on which set of holes serves as pivot. Two sets of plates could equal four different angles, probably not worth much in a craft where amount of weight and location is far more important.
 
Hi Kurtz, 3mm plates will have their holes elongated quickly; the stresses and slight movements will do that to any material that thin, and may induce metal fatigue. Go for thicker plates, with bushes to spread the pin loads. Agree with Skip on orientation.
 
 
montero said:
Crossbeam crosscut drawing if its not top secret ?
Click to expand...

Here's some pics of how the whole thing will go together.
The beams are maybe looking a bit under done?

Screenshot 2025-04-25 182802.png

Screenshot 2025-04-25 182927.png
 
Put a cap piece of timber on top, taper down to almost nothing at the end, maximum moment is going to be at the lashing at hull. And run some unidirectional glass continuously along the bottom. If it's going to break my money is on the front beam where the mast compression falls. Impossible to tell likelihood of such at the moment, would need the total displacement of ama and some details of sail (area & shape) and dimensions. Practically I think you'll be fine; as has been said the weakest link is the bolt and pin arrangement at the hinge. Finish it, sail it and let us know how it goes.
Best wishes
 
seasquirt said:
Hi Kurtz, 3mm plates will have their holes elongated quickly; the stresses and slight movements will do that to any material that thin, and may induce metal fatigue. Go for thicker plates, with bushes to spread the pin loads. Agree with Skip on orientation.
Click to expand...

This has kept me awake for a few days now, thanks :)

I'm thinking if the hinge bolts are torqued down to a reasonable figure with quality bolts and structural washers, this should mitigate some of the issues?
2000kg clamping force is easily attainable on a 12mm bolt with a 30mm diam distance piece. Up until the (clamped) friction of the plates is overcome the pin holes should see no load.

It does mean every hinge mating face will have to be clean and the bolts torqued properly on every assembly, which is a PITA and a sure example of poor engineering. I think I'll run with it though.

image_2025-04-30_082437566.png
 
Increasing the hinge bolt spacing is always ok until it starts to get cumbersome.
Plate thickness ,plate material ,plate lenght .Maybe you dont need triple screws the middle ones seems to be unecessary.
To avoid shearing of bolts, rough surfaces can be used, such as in the connections of truck drive shafts, with slotted surfaces.
 
I mean , in this case increasing vertical distance between hinge bolts.
 
montero said:
Increasing the hinge bolt spacing is always ok until it starts to get cumbersome.
Plate thickness ,plate material ,plate lenght .Maybe you dont need triple screws the middle ones seems to be unecessary.
To avoid shearing of bolts, rough surfaces can be used, such as in the connections of truck drive shafts, with slotted surfaces.
Click to expand...

Yeah don't worry about the middle hole - I was thinking maybe add a third to stiffen up the hinge for - fore / aft bending forces. I'll include it in the cut file just in case.

here's a link to small tri site with some discussion on the hinge setup as used on a Onyx 16 and some of Gary Dierking’s boats

ONYX 16 Trimaran Launching & 1st Ride https://smalltrimaran.co.uk/onyx-16-trimaran-launching-1st-ride/

Crossbeam Hinge https://outriggersailingcanoes.blogspot.com/2011/11/crossbeam-hinge.html
 
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