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  #11  
Old 05-22-2013
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Re: Rigging Math

Ahh, the English language. It's so good at expressing precise things.

Yes, the chainplate can be made smaller but certainly not arbitrarily. I don't guess at chainplate questions. Too much is at stake. There are several important variables, i.e. how many bolts, what size bolts, spread out over what area, attached to what? and they all have to be evaluated carefully and a reasonable safety factor applied.
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  #12  
Old 05-27-2013
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Re: Rigging Math

Bob-

On the beefing up chainplates issue; did you weld a secondary plate on to make it thicker at the eye?
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Old 05-27-2013
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Re: Rigging Math

Quote:
Originally Posted by davidpm View Post
What resources would you recommend if someone wanted to run the numbers themselves for some rigging puzzles?

For example:
You replace your standing rigging with synthetic and figure you could shorten the keel a couple inches and have the same stability. How would you calculate that?
Why would you do this? You only get an increase in performance if you make the rig lighter relative to the righting moment. Having a boat that is too stiff is not ideal either but you are not going to change the ratio enough with lighter standing rigging to make that much difference. To answer your question though; you would subtract the aloft weight lost and come up with an estimate of how much percent you changed the TOTAL weight of the aloft rig (spars, running rigging, sails, etc included), then you could remove that much percent from the weight of the keel (evenly, without changing the airfoil shape of the keel). Say you made the aloft weight 2% less; you could trim 2% of the keel weight; but it would have to maintain the CG location or it would need to be less weight off of the keel.

If you just want to cut an amount off of the bottom you would have to re-calculate the CG of the keel relative to where it is presently and calculate righting moments (lots of math to do this). With the above scenario in mind you might end up taking 1% off the keel at the bottom.

These are only suggestions at how you would do this; I'm not a naval architect.

Quote:
You are comparing two boats and want to calculate the margin of safety that the two builders used based on observed rigging.
It depends entirely on the rig. You'd have to estimate maximum required strength for the rig and then figure out how much stronger the wire is than that estimate. Two boats that are different will have different maximum loads so you would have to do these calculations separately for each. Also realize that if a boat has been re-rigged there is the possibility that it was oversized on re-rigging which is probably not a good decision (in regards to righting moment).

Quote:
You are thinking of replacing your chain-plates with titanium, how much smaller can you make them considering that titanium is several times stronger than steel.
I'd just replace with a carbon copy and then you have a chainplate that is several times stronger than it needed to be; and weighs less than it would in stainless steel anyway.

Quote:
You use a load cell to measure the force required to banana a boat with the for-stay and back stay. How do you calculate is that is the right amount of stiffness or if the boat has lost structural integrity?
If you make a banana you are putting too much force on it. Static force should be ~12-15% breaking strength of the wire (or rod); measured with a Loos gauge. Dynamic loading is a matter of how much you are willing to bend the mast and stress the boat further than the loads imposed by the sails. If you have a load cell you would want the maximum load (including sail loads)on the forestay to be no more than 75% of the yield strength (not breaking strength). That's a safety factor of 1.3. Backstay force required to do this would be less; calculated by a ratio of the angles between the forestay and backstay (trig calculations with vector analysis).

Quote:
In every case the answer is ask a navel architect. But if such a person is not available or you want to check their work how would you make the calculations yourself?
Either you have enough background in engineering and math to figure it out; or you would have to hire a Mechanical Engineer or Naval Architect to do the math for you. I gave you some examples of how you might attack these problems but if you don't know how to work these sorts of calculations then you probably should not do them yourself and rely on the answers you get.

Please don't take my comments as a blueprint for trying to do these sorts of calcs; if I were doing this as an actual problem I'd be asking someone like Bob Perry to be checking my math and critiquing my method of getting a proposed solution.

Last edited by KeelHaulin; 05-27-2013 at 04:17 AM.
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Old 05-28-2013
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Re: Rigging Math

Keel:
You can do it several ways. You can just weld on a doubler plate on one side. But the really elegant way of doing it is to weld on large washers each side of the main strap and to taper the edges of the washers.

A good rule of thumb for s.s. chainplates is that the plate thickness at the pin hole has to be equal to the wire diameter.
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Old 05-28-2013
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Re: Rigging Math

I'd say in general, consulting a Naval Architect is the answer. I'm an engineer and have a degree in structural engineering, but I would not attempt to tackle some of the problems you are asking about. Most problems are not as are as straight forward as they might seem to someone without the background/education/experience associated with the problem you are looking at. For instance, the titanium chainplates. Not only do chainplates need to be strong enough to transfer the load, they need to cover an area large enough to accept the load, and they need to be thick enough for the load to spread out. This much I (or anyone with access to the formulas) could work out. Add the dynamic loads, the salt water environment, the effects of dissimilar metals, and now your are dealing with the need to consult someone who has experience with this particular application.

Looking up the formulas and working out the math at the exclusion of considering the other factors involved is know as "handbook engineering." If you are unfamiliar with where the formulas come from or theories behind them, you could inadvertently apply these formulas, resulting in structural failures.
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  #16  
Old 05-28-2013
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Re: Rigging Math

Quote:
Originally Posted by davidpm View Post
What resources would you recommend if someone wanted to run the numbers themselves for some rigging puzzles?

For example:
You replace your standing rigging with synthetic and figure you could shorten the keel a couple inches and have the same stability. How would you calculate that?

You are comparing two boats and want to calculate the margin of safety that the two builders used based on observed rigging.

You are thinking of replacing your chain-plates with titanium, how much smaller can you make them considering that titanium is several times stronger than steel.

You use a load cell to measure the force required to banana a boat with the for-stay and back stay. How do you calculate is that is the right amount of stiffness or if the boat has lost structural integrity?

In every case the answer is ask a navel architect.
But if such a person is not available or you want to check their work how would you make the calculations yourself?
Such 'numbers' arent directly ascertainable as each 'architect' uses and applies additional 'safety factors' to the structure. The applied safety factors (increasing the strength or geometrical shape/amount of material, etc.) are based on a 'historical average' of what worked, what didnt work, how LONG in time the component has been in service, etc. .... and usually are arrived at or are reported by underwriters or insurance carriers --- called 'historical scantlings'. Especially in a 'boat' where there are repetitive applied or cyclical stress is applied, how 'strong' (tensile or compressive stress) a material is comparatively insignificant when fatigue (embrittlement, etc.) is probably the predominant failure mode; and where good design for fatigue and the special properties that lessen fatigue failure is more important than 'strength'.

I wouldnt go so far as to say 'ask a naval architect' either, as marine structure does fail, and fail more often than 'static' design. I'd rather delve into the material sciences that govern all the 'weirdness' (of shape/geometry, of materials morphology, etc.) that controls 'dynamic' applications. The 'ultimate' is of course to dynamically and destructively test in exacting/same conditions in order to understand and to develop 'time or cyclical' under dynamic loading constraints so to better apply the 'correct' safety factor.

As regards 'titanium', unobtainium, and impossiblephorus ... the jury is still out as titanium, etc. has not been in service all that long especially in marine design so that engineers and designers are SURE that the long term effects in service are 'all that well understood'. 300 series stainless, when most of our current boats were designed (and that includes their rigging) had not come to the 'evolution of knowledge' that it has today .... about 40 years after the fact; and, builders still use this vulnerable material - vulnerable to fatigue, crevice corrosion, etc. etc. and improvement is by simply by 'bumping up' the amount of safety factor and the 'geometry' of the component 'may' work, or 'may not' work. The applied safety factors are to cover 'unforseen and unpredictable untowards effects' --- dont mess with it, let it 'evolve', naturally, and by the reports by the insurance carriers who advise of such scantling failures in marine service, including your insurance carriers recommendations to 'its time to replace'.

Youre definitely not going to improve by building or changing to 'stronger', its much much more complicated than that.
It takes a wee bit more than a cookbook/handbook and list of 'material strengths', it takes design and materials 'evolution'. Imagine the cost of a boat if all the metal structure was 'titanium', even if we dont know all the long term effects of titanium - called the cost of diminishing returns.
If you really want to be 'sure', build a new one and dynamically test it until total failure under the exact expected conditions, understand correctly & completely the failure, and then make small changes so that you 'slowly evolve' into 'better'; rather than, changing the whole set of underlying circumstances that puts you back firmly to 'square one' in the constantly evolving design process.

Lastly, if your 'redesign fails', and wasnt 'approved' by your insurance carrier or you cant prove that the redesign completely and under all conditions was 'better', guess who is going to pay for and be ultimately responsible for all that includes the catastrophic failure caused by your 'redesign'?
;-)

Last edited by RichH; 05-28-2013 at 08:59 PM.
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Old 05-28-2013
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Re: Rigging Math

Not sure I agree with the comments about Titanium alloy not being well understood for it's properties. It's been used plenty long enough in military applications to know what it's strength properties are and resistance to corrosion.

Is it your opinion that any changes to the original design of a boat's systems puts the owner in the position of voiding their insurance? I think that's a bit of an extreme position to take, as a huge part of the boating industry relies on modifications and upgrades of existing boats.

The last I heard, wrought Titanium alloy was lower in price (and easier to machine) than stainless. :-)
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Old 05-28-2013
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Re: Rigging Math

Quote:
Originally Posted by bobperry View Post
Keel:
You can do it several ways. You can just weld on a doubler plate on one side. But the really elegant way of doing it is to weld on large washers each side of the main strap and to taper the edges of the washers.

A good rule of thumb for s.s. chainplates is that the plate thickness at the pin hole has to be equal to the wire diameter.
Yes; my boat has doubler plates on the secondary shrouds; and the bow strap IIRC. There has been some discussion about crevice corrosion getting started between the plates because the weld is only on the perimeter, not at the eye hole.
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Old 05-28-2013
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Re: Rigging Math

Quote:
Originally Posted by KeelHaulin View Post
Not sure I agree with the comments about Titanium alloy not being well understood for it's properties. It's been used plenty long enough in military applications to know what it's strength properties are and resistance to corrosion.

Is it your opinion that any changes to the original design of a boat's systems puts the owner in the position of voiding their insurance? I think that's a bit of an extreme position to take, as a huge part of the boating industry relies on modifications and upgrades of existing boats.

The last I heard, wrought Titanium alloy was lower in price (and easier to machine) than stainless. :-)
Consider the SR51 'black bird' which was almost entirely built of Titanium ... and the entire fleet was retired en mass due to structural fatigue and other 'aging'. We certainly now know more about the 'foibles' of 300 series stainless than, say, 30-40 years ago.

In the age of "lawyers eventually win everything', for me, I would think it would be foolhardy for someone to radically depart from an established design, have a failure and the consequences of failure .... and expect an insurance carrier to bear such costs .... unless that person could prove without the shadow of a doubt whatsoever that his/her redesign was superior to the OEM designer. Most certainly if the OEM designer or builder has made changes that enhance the structural integrity and/or alleviate failure issues .... go for it, dont wait.

To be as simple as I can, metals and most other materials only last a for a finite amount for cyclical stress before they embrittle and then simply 'fall apart'; thats why most autos, equipment, airplanes and ships are in junkyards or scrapyards - the metal became 'tired'. Knowing this, it behooves one to do the proper prudent routine maintenance/inspection and simply replace such components when 'their time is up' (number cycles under a certain load value) --- for 300 series stainless, typically 1 million load cycles above 1/3 of ultimate tensile strength ... or about 1 circumnavigation.
Interestingly most offshore designers use a minimum factor of safety of about 3 for offshore design, probably to keep the peak stress (in stainless) at 30% or well below the 'fatigue endurance' limit; one I know (and have often 'back calculated')) uses up to 4X, and yet after a time even those 4X components break from fatigue. TIME in service and knowing when to change is the best method to prevent failure .... Im sure thats why most insurance carriers are increasingly 'recommending' complete rigging wire and terminal replacement after 10 years, as their 'stats' are probably saying so. .... and thats the reason why you seldom see DC-3 airplanes still flying or many 'Dusenbergs or Bearcats' driving around - their metal got 'tired' and the whole shebang got scrapped.

:-)
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Old 05-29-2013
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Re: Rigging Math

So a spy aircraft designed in the early 60's, built in 1964 and was not retired until the 1998 is your basis for Titanium not being a robust material? 34 years of use as a surveillance aircraft that flies at +3x the speed of sound is not strong enough? Retirement was for budget reasons and ability to maintain them because the parts were not available. Almost every American fighter aircraft built is made of Titanium now. Why? Because Ti alloys developed in the 60's to support Cold-War military applications like the SR-71 worked.

Quoting ASM International (Formerly the American Society for Metals)
Quote:
As a result of their high strength-to-density, good corrosion resistance, resistance to fatigue and crack growth, and their ability to withstand moderately high temperatures without creep, titanium alloys are used extensively in aerospace for both airframe and engine components. In aircraft, titanium alloys are used for highly loaded structural components such as bulkheads and landing gears. In commercial passenger aircraft engines, the fan, the low-pressure compressor, and approximately ⅔ of the high-pressure compressor are made from titanium alloys. Other important applications include firewalls, exhaust ducts, hydraulic tubing, and armor plating. Due to its high cost, titanium alloys are more widely used in military aircraft than commercial aircraft. For example, titanium alloys comprise approximately 42% of the structural weight of the new F-22 fighter aircraft, while the Boeing 757 contains only 5% Ti.

The excellent corrosion resistance of titanium makes it a valuable metal in the chemical processing and petroleum industries. Typical applications include pipe, reaction vessels, heat exchangers (Fig. 4), filters, and valves. Titanium is used in the pulp and paper industries, where it is exposed to corrosive sodium hypochlorite or wet chlorine gases. Due to excellent resistance to saltwater, titanium is used for ship propeller shafts and service water systems. The former Soviet Union actually developed large, welded titanium-hulled submarines.
316L Stainless has a fatigue limit of appx 280 MPa and Yield Strength of 290 MPa; Ti6Al4V (aircraft grade Titanium Alloy) has a fatigue limit of appx 490Mpa and a Yield Strength of 825MPa. Which is a stronger and less fatigue prone material?

Last edited by KeelHaulin; 05-29-2013 at 03:53 AM.
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