New Mast Tangs

[Bill Roosa]

eye of newt and toe of frog

Roger calls it eye of newt and toe of frog, we in the hollowed hall of the great 5 sided castle quotiux call an "engineer estimate" or a "SWAG."

An engineer estimate == properly designed structure which will fail in predictable ways when overloaded.
SWAG==properly designed structure that has an unknown failure mode and unknown ultimate load(ing).
The bottom line is cost; we can do it faster, cheaper, or better. Pick two.
If you want it delivered faster and cheaper it will not be better.
If you want it better and cheaper you will not get it any time soon.
If you want it better and delivered soon it will not be cheap.

 

[Roger Long]

You can design structures so that they break where you want ....

Sure, outboard motor sheer pins are an excellent example. Most of the time when you hit something, the sheer pin breaks and the prop is undamaged enough to just put in a new pin and you're good to go.

Now, you are motoring along a dangerous shore or being carried out to sea by current and you hit a log and the sheer pin breaks. You die. If there hadn't been a sheer pin, the motor might have been vibrating so badly that you needed a whole new prop or even sustained enough damage that it is doomed to bearing failure in the next few hours. It might have kept running long enough to save your life though.

For a sailing rig of conventional design, which I'm stuck with, my preference is just to have it as strong as possible. That said, I'm quite interested in the idea (another conversation going on here http://forum.woodenboat.com/showthr...tarting-Proportions-in-Boat-Design&highlight= )in using a sliding gunter rig on an ocean cruising sailboat. The mast of such a rig is fairly short so can be of heavy section and rigging for maximum chance of surviving a roll over and still spread a lot of area in lighter winds.

I actually like your idea of having a mast that is likely to be standing after a rollover but it is a design objective that would have to be considered right from the get go. It can't be done by tinkering with the sizes of components in my existing rig. Doing it with any conventional mast would be a difficult design problem. The great thing about the sliding gunter rig is that the part that creates a lot of the stress in a roll over is already likely to be down and lashed on top of the boom in the conditions where a roll over is likely to occur. That makes it a very different design problem than trying to create a great enough disparity in the strength above and below the spreaders to insure the desired failure mode.

 

[Rick D]

Or, you could just have a free standing rig ;-)

 

[tommays]

I have managed to live through all my seat of the pants bicycle frame designs by following the bicycle tubing manufactures fairly simple guidelines and not doing anything to stupid with the frame geometry

And i have found if i take a good look at most successful machinery its not to hard to come up with something that wont self destruct

 

[Roger Long]

Or, you could just have a free standing rig ;-)

That won't necessarily withstand a roll over. Ask me how I know.

 

[RichH]

This would be my preference ... with end-stitch welded gussets to retard flex in that 'kink', .... no welding nor heat affected zone in the area of the vulnerable 'bend'. All welds ground smooth after assembly.

 

[Roger Long]

This would be my preference ...

Which?

 

[Rick D]

OK, I'll bite, how do you know?
I was only josh'n, but I do expect a free-standing rig would be somewhat more able to withstand a roll over simply by being able to absorb the shock loading more readily, but I'm a long way from being an engineer!

 

[tommays]

Rich

There are just so many ways to make that part wrong

I get regular grief from SS pressure vessels were they polished that weld a tad to much and that's on the board certified stuff

 

[RichH]

..... with the triangular gussets. If the sketch. isn't clear I can upload to photobucket, etc. and repost.

 

[Roger Long]

..... with the triangular gussets.

I got it. For some reason, the picture didn't appear in your post the first time I looked at it.

Nice design.

 

[RichH]

Rich

There are just so many ways to make that part wrong

I get regular grief from SS pressure vessels were they polished that weld a tad to much and that's on the board certified stuff

An ASME code cert'ed welder should have no problems if he/she uses full penetration welds and then the 'finisher' doesnt 'go overboard'. Same for code 3 as well as code 8.

Tom, look again at my (edited) sketch .... not welded nor heat affect zone near the vulnerable 'kink'. The gussets prevent/retard flexure at the 'bend'; I dont care HOW THICK the part is if its 'kinked' is going to 'flex' and flex propagates and accelerates fatigue failure. 3/16 - 1/4" would probably be 'fine' for the gussets.

 

[RichH]

I got it. For some reason, the picture didn't appear in your post the first time I looked at it.

Nice design.

Thanx.
The combined I^3 including the gussets would tend to prevent/lessen undue flexure due to any static or dynamic misalignment (due to rig stretch under varying wire load).

A rough/est./SWAG calc. of your rig. indicates that cap shroud wire will elastically stretch out by about 2.7" when the wire changes from 15% to 50% tension, ~5" at 85% tension ... I assumed a ~48ft. long cap shroud.

 

[Ross]

The lower bolt holt should be counter bored so that the bolt bears on a flat surface.

 

[Roger Long]

OK, I'll bite, how do you know?

I was in a friends catboat with an aluminum mast that was flipped by a 20 foot breaker (estimate of the USCG crew chief who rescued us and had been keeper of the light a couple hundred yards away for 20 years). I saw the mast crumpling over even before the wave had completely inverted the boat and washed over us as I fell through the air.

An unstayed mast will be designed according to the same principles as a stayed one except with the mast section functioning as a pure cantilever. It will therefore break at about the same loading being swept through the water by a wave roll. Less rigging will reduce water resistance and slightly increase the chance of the mast standing but just being un-stayed is no guarantee that the mast will be there when the boat rolls upright.

 

[Roger Long]

A rough/est./SWAG calc. of your rig. indicates that cap shroud wire will elastically stretch out by about 2.7" when the wire changes from 15% to 50% tension, ~5" at 85% tension ... I assumed a ~48ft. long cap shroud.

That works out to a pretty small change in rig geometry though as the spreader will keep the wire a fixed distance from the mast. The whole deck stepped mast will tip to one side.

Here's a scale drawing of the tang bolt area to the actual angle of the upper shroud as picked off a photograph I took to check spreader angles:

Bend radius is 1 t (3/8") inside and 2 t outside.
Note the extension of the outside line of the tang showing the material not subject to bending forces. This is the effective thickness of the 1 1/2" wide tang. Deducting 1/2" for the bolt hole means there is .281 square inches of material.

My upper shroud tension of about 3200 pounds at peak righting moment, plus 20% of wire B.S. for prestress x 1.5 for dynamics / .281 = 27,864 still under the fatigue limit even if all of those cycles put the cockpit coamings violently under water.

I think I'll be O.K. without the welded gussets but I'll file your idea away for something where I'm trying to save weight.

 

[RichH]

Totally agree with ... "27,864 still under the fatigue limit"

'Fun' discussion. ;-)

 

[CaptJake]

Got into "crap" section of this a little late so bear with me.
While teaching an EOD class at a little known station somewhere in the world; A student (while attempting to disassemble a Claymore APM) suddenly stopped working, put down his tools, and asked a very intelligent question. "Chief?" he asked, "what's the factory failure rate on these things? I'd hate to find out I was killed by an engineering error. "0%" I replied. He went back to work happy with that answer until I added; "But they do test them before they put the detonators in." He dropped out the next morning.

 

[LloydB]

Wouldn't the ultimate breaking strength of the shroud normally be less (by some factor) than the ultimate breaking strength of either tang or chain plate? Just wondering about weakest link in original installation.

 

[Roger Long]

While teaching an EOD class ...

Warning, thread drift:

EOD eh? Cool. I got a chance to meet and watch the EOD guys at Naval Weapons Station Earle work when I appeared in this NOVA show.

http://video.pbs.org/video/1377494854/

You can see me right at the end as they blow up models I designed to study the effects of the self destruct charges the subs carried. Not many people of my station in life get to hold C4 in their hands. It was great fun.

To maintain thread relevance, my tangs and chainplate modifications will be done by the same person who welded the sub models.