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Old 03-09-2008, 11:21   #1
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Overkill

Is there a reason for not doing overkill in dimensioning?

I understand weight being an issue, but is there another reason for not increasing the dimensions on critical parts.

Stays?
Fittings and linkages etc.

Anchors?
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Old 03-09-2008, 11:28   #2
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No problem with rigging. Too big an anchor might not help the trim of the vessel.
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Old 03-09-2008, 11:41   #3
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Not meaning to be picky here but in naval architecture your dimensions of structural members, hull plating etc are called your scantlings. wiki/Scantling I know, boats have a different name for everything!

The safety factor with things should varie with how critical it would be if something failed. Another consideration is does this device get shock loaded? Does this device NEED to bend?..like an aircrafts wings or a mast in order to change sail shape? Is this the correct material for cyclical bending?..will there be metal fatigue with this type of material? With those kinds of things and others things, increasing the safety factor may be necessary.

This of course needs to be balanced against unnecessary weight. Too much of a safety factor and something could get so heavy so as to not make it worth building in the first place. Or it becomes non-competitive. An airplane comes to mind concerning building in too much of a safety factor making it too heavy to get off the ground or to fly safely. Boats have similar considerations, the only difference is boats cannot fall out of the sky.

Designing a boat really is the art of compromise. Make one thing better and you almost always make something else worse. Larger or lighter then more expensive. Smaller then less durable. More sail area then more likely to capsize. More stuff onboard then slower. A taller mast then fewer bridges you can get under. The list is endless.
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Old 03-09-2008, 11:44   #4
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Shock loaded? Please elaborate on that one a little.
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Old 03-09-2008, 11:50   #5
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This is specific to the application. Case in point, when designing the pilot house on my trimaran, I overbuilt. I estimated the weight that I could allow, and anticipated the need to reduce that weight once the boat was launched. I built in areas that would be easy to decrease weight by removing material, while not detracting from the strength, or asthetics.
When it comes to anchors, oversize is good, but remember, you need to be able to get it off the bottom, and you do not want the bow to be down when the anchor is stowed. Hauling a 65# anchor from 30' with 150 feet of chain when the windless fails is no fun. For some, it is not an option. Doing this 2 or three times trying to find good holding will wear out anyone.
Building is a fine balance between performance, and strength. If you build in more strength than you need, you will lose performance, or, at the very least, capacity for other items that you may need.
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Old 03-09-2008, 12:56   #6
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"but is there another reason for not increasing the dimensions on critical parts. "
MONEY.

Check the prices on your favorite structural materials and metals. It adds up fast. Add machining costs, shipping costs, all oversized to match the oversized parts, and you're into significant money.

And as any racer will tell you (boat, car, or plane) if you didn't break anything--you're not racing, or else you're being slowed down by overbuilt parts.

The trick is not to "use more" but to know how much is necessary in the first place. If concepts like shock loading are foreign to you--you need to learn more about the engineering aspects of boats in motion.

Suspend a 200# man from a rope. The rope only needs to support 200#s. Now take the same man and have him fall twenty feet while using that rope as a safety line. The rope takes a SHOCK LOAD way in excess of 200#s, the result of the 200# man accelerating under the force of gravity.

Drop a boat off the crest of a 20' wave, everything on it gets shock loaded. Boats move in two fluids, three rotations, and three dimensions, the numbers can get complicated pretty quickly.
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Old 03-09-2008, 13:11   #7
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Drop a boat off the crest of a 20' wave, everything on it gets shock loaded.obv

That was kind of my point.. everything gets shock loaded to one point or another.

The idea for increasing dimentions on stays and other rigging would be to prevent faliure from shock loading.. Quite frankly. a couple of good pieces of string will hold the mast standing in calm conditions and in a dry dock.

anchors were more of a observation as to how dangerous and apparently common dragging is. I've never been in a situation where we did start to drag more than 10 feet or so before it caught. So i guess i've been lucky. Full time sailing, i'm sure will give me that experience sooner or later.
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Old 03-09-2008, 14:46   #8
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The idea for increasing dimensions on stays and other rigging would be to prevent failure from shock loading..
So how much over is over? What if the designer computed the loads properly? What if the computation included factors such as duty cycle and peak loads? Repeated lower level of stress also include stress fractures. Saying something is "overkill" presumes you know exactly the correct amount it should have been designed for. If you know the correct number then you don't require overkill. Overkill is just a fancy way of pretending you know and asserting the value is much higher than required. Overkill is what boat sales people call it.

When you consider the large loads of supplies and gear required there is no room for overkill.
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Old 03-09-2008, 15:03   #9
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One needs to think of weight and where it's going. Given the number of failures of rudder fittings and the tiny weight effect of going overkill there, It would be foolish not to do an overkill there. Doubling the size of gudgeons and pintles would have negligible effect on a boat's performance. An oversize anchor is such a great insurance factor that I will gladly accept an performance costs for a good nights sleep in a storm, without taking overkill to extremes.
Going from 1/4 inch to 5/16th inch rigging wire for my 31 foter is a total of 17 lbs added for the whole rig, a small priice to pay. I did see a steel Roberts 38 that had gone for 1/4 inch hull plate and greatly regretted it. 3/16th plate has the same toughness as 7 inch cold molded fir, so that was a bit ridiculus.
Put the overkill on small items that frequently break or fail ,like rudders , and leave alone those that rarely break.
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Old 03-09-2008, 15:20   #10
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The engine of "Shock Loading" is mass. Increasing mass for no reason creates problems but solves none.

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Old 03-09-2008, 15:22   #11
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Given the number of failures of rudder fittings and the tiny weight effect of going overkill there
They don't always fail from undersized fittings. I have no gudgeons and pintles. I don't need them.

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Going from 1/4 inch to 5/16th inch rigging wire for my 31 foter is a total of 17 lbs added for the whole rig, a small priice to pay.
I'm not so sure going bigger will make it last any longer. I have 8 shrouds instead of 6. Making the wire thicker won't always make the rig stronger. Wire is just one part. It's sort of an adventure in junk science saying to overkill this and that.

Overkill on things that frequently break may just be the wrong thing. What about the thngs poorly designed. Bigger won't make it stronger in almost all cases. Better designs ofter are stronger yet lighter. The weight of the extra bulk forces oversizied mebers just to deal with all the overkill.
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Old 03-09-2008, 15:49   #12
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See my thread on aluminum chainplates. Just as stong as 316 SS but lighter and cheaper. No one agrees with me that I should use them. It's what came on the boat, and the designer/builder was a racer. His son is the boatwright for the US olympic team. So this guy probably does not know what he is talking about, taught his son wrong way to do things, and his son doesn't what he is talking about or doing either.
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Old 03-09-2008, 15:54   #13
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No one agrees with me that I should use them.
I would not disagree that they can be made. They are just made differently. AL alloys can be quite strong and salt water resistant. Being in the Olympics does not convey the knowledge of metallurgy.
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Old 03-09-2008, 16:19   #14
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I would not disagree that they can be made. They are just made differently. AL alloys can be quite strong and salt water resistant. Being in the Olympics does not convey the knowledge of metallurgy.
He was not in the Olympics, he was the US teams boatwright for the last 3 or 4 olympics. I think he may know a thing or 3 about design. I am sure he does not design parts that will fail or that are overweight. My point is there are ways of making things stronger and lighter and they don't have to cost more, they might not be the standard acceptable way to do things. Synthetic rigging, composite chainplates, hybrid drive systems, unconventional sail materials. I bet that when mylar sails first came out there were naysayers. This forum needs to think outside the box once in a while and the people need to be more accepting of others ideas. Tolerance is the key.
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Old 03-09-2008, 16:46   #15
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Yup, gonna take my Soling accross the ocean. The only thing I said nay to on mylar when it first came out was the cost! (and longevity)

There are some very interesting solutions with regard to weight. Looking forward to nanotechnology in composite core material. Nice thread on composite rigging.
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