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Old 11-07-2011, 07:56   #1
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Breaking Strain of Timber Beam Davits

hello I was just planning to 'add a patio out the back' - not on land but on the yacht. I think I would need two 3m x 100mm x 100mm jarrah beams, going out astern with one-third of that length secured by bolting to the deck. then I would lay some timber 'decking'. at sea I would put the tender on it, using the platform in place of davits, and at anchor it would be a nice bbq area.
oh, how to support it?
- I currently have twin mizzen masthead running backstays which attach to the aft deck. I would replace them with twin standing backstays which would transit the far ends of the beams, finalising at the bottom of the transom.
question: what is the load bearing capacity of a beam of those dimensions (weight of dinghy and the structure itself). and what is the compressional/torsional strenght, exerted by the new standing backstays? and how to calculate?
i have searched everywhere for some basic data sheets - does anyone know of any? in desperation I have 're - soughted' to steel beam data, with equally little success. surely the math can't be that arcane? i have yet to find any simple data sheet or 'physics for dummies book'. any website or similar source would be welcome!.
thanks, Cap'n John
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Old 11-07-2011, 08:07   #2
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Re: breaking strain of timber beam davits

got to the library, get a statics engineering text book and a mechanics of materials text book... everything you need will be contained within. finding accurate material property data for that exotic wood might be difficult, but surely some numbers could be found for a similar, more common wood... factor in a good margin of safety... good luck
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Old 11-07-2011, 08:10   #3
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Re: breaking strain of timber beam davits

Your residential building code will have this info, or your national org. that sets the specs on wood products.
The info is usually easily available.
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Old 11-07-2011, 08:31   #4
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Re: breaking strain of timber beam davits

Typically structures like that, especially floors, are not designed with regard to a breaking strain but rather a stiffness. Usually specified as some load (kg, pounds) per unit area (m2, square foot). This is because the structure should be stiff, 'feel' sturdy. This is how dwellings are designed. The breaking strength of a floor in a dwelling is far beyond what it will ever experience, but undersize the joists and nobody will like living there.

So the suggestion to look at home building references is a good one. Just guessing, I'd think two 100mm square beams cantilevered 2m would feel weak. 50mm x 150mm would be much stiffer.
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Old 11-07-2011, 08:31   #5
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Re: breaking strain of timber beam davits

You are going to need a cross piece at the end of the structure to keep the backstays from comming together and the stresses on your mizzen mast will be different. You should check to see that you have enough strength in your other standing rigging.
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Old 11-07-2011, 08:50   #6
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Re: breaking strain of timber beam davits

You would be setting up a "boomkin" situation, with considerable compression loads on the beams, and shear loads on the deck fastenings.
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Old 11-07-2011, 08:53   #7
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Re: breaking strain of timber beam davits

Quote:
Originally Posted by Shanaly View Post
. . . - I currently have twin mizzen masthead running backstays which attach to the aft deck. I would replace them with twin standing backstays which would transit the far ends of the beams, finalising at the bottom of the transom. . . .
thanks, Cap'n John
Using or converting your mizzen mast running backstays to permanent backstays and using them to "hold up" the new "porch" is - IMHO - very dangerous and is a disaster waiting to happen.
- - The "porch" should be independently supported by poles (stainless steel) that attached from the underside of the ends of the beams to the boat's hull down near the waterline (depending upon the overall length of the extension back from the transom.)
- - If you have a "jackstay" from the top of the Mizzen to the top of the Main Mast and convert the Mizzen running backstays to permanent backstays attached to the "porch" there is the possibility that a large wave breaking over the stern will poop the porch and exert strong downward pressures that will pull the Mizzen mast out of column and break it. If the Mizzen is attached to the Main mast then it too will be pulled aft ripping the forestay from its deck attachment (or breaking it or a connecting pin or tang) and take down your Main Mast.
- - So, if you want to build a "Porch" - fine - just do not screw with the standing rigging of the boat. It is a finely tuned and designed system by the designer to withstand the stresses and loads generated by the sails and it not designed to be used to "hold up" anything else.
- - The "porch" should be supported from beneath by poles or braces.
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Old 11-07-2011, 09:34   #8
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Re: Breaking Strain of Timber Beam Davits

I am not sure that jarrah is the way to go, that will be a lot of weight hanging out the back.; maybe hollow box section built up out of plywood would be stiffer and lighter- perhaps 50 x 150 as daddle suggested.
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