Stress (stiffness) testing a small tri hull

I've nearly finished building a small tri / outrigger hull. It's an overbuilt 24ft 6mm ply / 240gsm epoxy basalt Gary Dierking waapa design that i'll be using with H16 hulls as floats and a H16 rig.

I was thinking of sitting the hull ends on supports and loading the mast step area with sand bags to check for hull deflection.

Would a 5ookg test weight be a realistic max figure for this sort of hull and rig?
(as in to simulate max rig loads)
and what sort of deflection if any would be acceptable?

Cheers

 

You want to check hull or crossbeams deflection ?

 

The hull first, but I'll test the crossbeams as well.

 

You won't have such a load while sailing.
Even if you load 500kg on the water the hull will behave differently. It's a pity to torture the structures.
How will the H16 mast be connected?

 

I'm thinking of changing the mast step to a deep ball and socket, but it will be raised 500mm above deck level on a small cuddy. pretty much the same rig geometry, unless there are advantages to changing it? The cross beams will fold near the middle (1200mm from centre line) so this is the widest I can go with the shrouds.

I say the same geometry, but as the mast base will be raised it will enable a lot more rake / adjustability, whether its needed or not I don't know?

 

How did you even determine the mast mounting point? You can't adjust the position lengthwise? The original design had different sails.

 

The plans have a normal marconi rig option in the same position, (but at ~ 1/2 the sail area) this also pretty much lines up with the early Tremolino masts location. In my head the Tremolino is kind of the same beast.

I can play with lee board mounting, mast rake, jib size to hopefully compensate for balance / trim issues. I hope.

 

I thought that originally there was some crab claw, that's why I asked.

I want to make a cat with cc sail and I have to leave a lot of space for adjustments.

Raising the mast and more adjustment is ok. Distance of 1.2m for shrouds , should be ok if the crossbeam is stiff enough.

 

Did you build from plans and specifications?
You need to be talking to the designer.
The design specifications, if followed closely during construction, should be more than adequate to insure that your build is stout enough.

 

For sure, It's built 100% to the plans - the specifications are probably a bit of an unknown though. As in - I used a combination of marine ply, good quality unmarked random ply (backyard lab tested though) , different timber types for deck supports, gunnels, what nots.
everything from using 19 x 19mm garden stakes doubled up and glued to salvaged hardwood roadside marker posts for stems.


I was thinking of the stress test as a sanity check, I'll be testing the cross beams anyway. I'm kind of surprised this test is not a thing with small diy sail boats?

I'll also be testing the (new) standing rigging to 2/3 it's breaking strain

 

The load condition you are proposing assumes the compressive load on the mast comes from fore and aft tension in the stays and there isn't one aft. A small boat driven by sails is beyond statically indeterminate. That being said a conservative deflection might be L/480 which would be 0.6". Doubt you would get that much would be interesting to hear what happens.
Enough small boats have been built and tested in real world conditions that there is some confidence in standard practice.

I remember the case of one of the monohull Americas Cup boats that did break as you describe, virtually snapped in two from rig tension and mast compression; but load conditions were far different in that case. The most memorable thing about the event was the picture of the group of architects, engineers, builders, material suppliers et al standing around the boat stiffly erect as if no one was willing to bend over and expose their backsides.

 

He doesn't want to build a flying Pinto.

The sense of a 500kg load on the mast foot is not entirely unreasonable.

I'm not a sailor and shrouds are exotic to me.

 

It might be more like a flying gecko, which are pretty cool critters.

What cat design are you thinking of building?

 

I was interested in crab claw sails not because they are aerodynamically exceptional but because they are relatively simple and seem useful to me in another boat project.
7.5m hulls , 4.5m beam , planing hulls , cheap tarp sails , two 6-9.8hp engines . After @magwas called his project microcruiser I liked the idea .
My very first approach was pure experimental , but why not to make something more usefull .With very limited sails experience I decide to put two small outboards.

After @magwas started cat capsize thread , I decided to make it reversible. I'm intrested in heavy wind conditions , I know that this shape of hulls will act poor in light wind .
Crew 1-2 . If the hulls shape wil be not so hydro efficient I will switch to the foils . This should be relatively easy with that kind of build .

 

Go Sailing.
If it breaks it's too light
If it doesn't break it's too heavy