Discussion in 'Multihulls' started by tamas, Nov 11, 2013.

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### tamasJunior Member

I was discussing the loads on the various components of the farrier folding system. He said there was a huge in-ward load on the main hull end of the beam. A load yes but maybe not massive and it is mostly down. I have tried to attache an image of the area I am talking about.

It would be interesting to get opinions.

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### brennoJunior Member

Folding system

Who is the he you refer to and what is the purpose of the question.

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### hump101Senior Member

Just take moments about the lower pivot point, and then the inward acting component of the force at the upper pivot is approximately equal to the vertical force at the float times the horizontal offset between float and pivot, divided by the vertical offset between upper and lower pivots, so typically the inward force component will be 5-10 times the vertical force, depending on your geometry.

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### tamasJunior Member

Thanks Hump101 I am surprised there is 5 to 10 times that of the vertical load. Thanks

Brenno, the "he" does not matter (I did not ask an opinion about he) and I thought the request was straight forward.

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### catsketcherSenior Member

Here are some quick calculations

cheers

Phil

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### hump101Senior Member

The 5-10 depends on the geometry, for a slender beam on an ORMA 60 it could be double that on the compression flange of the beam.

As Catsketcher has noted, the vertical load will be much more than the static mass of the boat in the real world. I would start with a factor of 5 to cover dynamic loading and fatigue, as per AISC for offshore moving structures (cranes) that carry people, then work up the calcs from there.

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### brucebSenior Member

The struts and pins as designed on several production folding systems seem to be close in strength to those values, but I do wonder about the bushings. Should they be designed to the maximum load or is something less ok?
B

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### hump101Senior Member

All components need to be able to sustain the applied loads, including the bushes. A failed bush would lead to further failure if the joint is moving around in a seaway. Calculated stresses in visco-elastic solid bushes can take account of rate dependent properties, so for certain arrangements very short duration peak loads can be accommodated, but care needs to be exercised in identifying such loads.

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### brucebSenior Member

bushings

Hump, That is the answer I would expect, but I hope you are quoting from actual practice. The bushings that are sometimes used in folding systems do not always seem to meet the max load requirements I would have expected. Real world practices compared to theoretical.
I am quite interested- I am ready to convert a 33' tri to folding, and I want to get it right the first time. One common folding tri design does not use bushings at all, with slight play in the folding joints taken out by clamping/loading the arms. It seems to work.
B

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### oldsailor7Senior Member

Bruce.
How are you getting on with the reno of the 33. Are you planing to use square section tubing for the new amas or have you found suitable round/oval tubes.?

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### brucebSenior Member

Beams

OS, I just had a hip replaced- on me, not the buc, so I have been away from the computer. My current plan is to use the beam design and folding system from a Scarab 32 and install it on the buc 33. I purchased the 32 plans, and it looks to be a very good fit, and the beams should be pretty easy to build. I will get pics posted soon, but you can see my interest in folding hinge stresses.

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### hump101Senior Member

I can't comment on a specific design without detailed information, but on what basis do they not always seem to meet the max load requirements you would have expected?

My point in the previous post is that the behaviour of encapsulated visco-elastic solids requires detailed modelling to capture analytically, and thus a bush that may appear to be inadequate based on a simple "force divided by assumed contact area" methodology may in fact have a higher load capacity when its actual response is calculated. Bear in mind that the bush is not required to operate under such loads, just sustain them in a fixed position.

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### brucebSenior Member

bush size

As an example, the main "water stay" strut bushings on an "F" design 33' folder are 3/4" inside 7/8" outside, 1/8" flange, 1 1/8" long used as a pair on each end of the strut. Nylon is the quoted material, although I know other plastics are often used. The struts are on a 7500 lbs tri design, mounted mid point as in catsketcher's drawing. I am interested in the acceptable bushing loading, but I have no idea how to calculate it, or access to any guidelines.
Sorry if I am asking hard questions, but I want to start building beams and brackets in about a month, and I need to get it right. I will have a drawing of my folding system in about a week. I still have to get final measurements off the boat as soon as I am able.
B

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### brucebSenior Member

tamas, I have the drawings for several tris, and a couple of actual boats to analyze (reverse engineer) for guidance. The Bucs and the early "F" designs used a VERY well attached carry-through upper beam. They never seem to have any failures, even in collisions. Later tris of most all designers today now have an engineered bulkhead/s to carry the compression loads, and they sometimes does not appear up to the job as built, and certainly not as abuse resistant. Build it strong
My only comment on the load vector diagrams are that the materials and sizes of pivots and bolts in the lower arms used by the current successful tri designs would indicate that the peak loads are at the low end of my calculations.
B

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### hump101Senior Member

Such a bush in a suitable grade of Nylon would reach yield with approx 30kN load applied, so 60kN for 2, provided that the structures inside and outside the bush are rigid relative to the bush, and with small tolerances. Obviously load would depend on geometry, but this bush capacity is not high based on vessel mass. I would look at using something more suitable than nylon (bronze, or some of the filled polymers give a significant increase in yield) or increase the diameter of the bush.

However, for the dimensions you give, this would mean the bush has a wall thickness of only 1.6mm (1/16")? Even if it deforms, the geometry of the joint won't significantly change, and the bush is almost just a lubricant/spacer. Am I interpreting your dimensions correctly?

Is someone checking the design calcs and scantlings for you? Really important that they are performed and checked by competent people. It's cheap compared to suffering failure of the structure in use.

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