Lower Shroud Under deck mounting broken

[dLj]

I would be interested if there is a dummies explanation for "One reason is that allows for reducing a stress riser at the transition of the central bar to threaded end" I had assumed it was all just a simple matter of increasing the cross section in the low par of thread.

I wish I had two bars, one with a threaded end as you describe and one as was made originally. That would make it much easier to explain. But let give it a try on using only words. Where threads end, you have the "notch" where the thread root exists. That notch is a stress riser. Let me see, think of taking a pencil, do not sharpen the point leave it blunt. Take that pencil, place it on your hand and apply 100 grams of force. You will hardly feel it because the end is blunt and it is very easy to hold 100 grams in your hand. Now, take that same pencil, sharpen the point to very sharp. Now place that on your hand and apply the same 100 gram force. This now hurts because the same force is pushing the tiny sharp point into your skin. Same force, but now you have a stress riser, the sharp point. Notches cause the force being carried along a smooth surface to greatly magnify at a notch. I don't know if this helps I can try again if you'd like.

So I assume this "So what does this tell you? To avoid this you need to allow good access to air allowing maintenance of the passive oxide layer on the surface of the part" was the bases for hunter not sealing this, other than the bolts. So a) if it got water there it was not trapped and b) it could get air as well.

@dlochner has answered this question very well. I don't need to repeat it.

The link I provided above shows very little difference between 304 and 309 and I was hoping the increase in cross section would compensate for difference in strength and potential for stress related fatigue or cracks. However ultimately I have only a couple of choices, go now with 304 and a 30% increase in cross section or order the full thing from Selden which will take weeks.

I would disagree with this. If you look at the numbers found on that page, there are really three of the listed properties I would consider most important to your specific application: Yield strength, Fatigue strength and PREN number. All three of these are higher for the 309 than the 304. The fatigue resistance is notably higher. Interesting that this page gave ranges for the 309 while only providing a single number for the 304.

The PREN number is a measure of a materials resistance to pitting corrosion. This test is done through a specific set of parameters and can be useful when comparing alloys, but in any application where corrosion is a concern, you really want to know the materials resistance to corrosion in the medium it is being exposed to. In your case, you want this to be resistant to sea water. You are in an ocean environment.

It is known that both 309, and the 316 alloy I mentioned before, are superior to the 304 alloy in your specific case.

Can you also tell me how this oxidizing layer that protects stainless might apper to the human eye, or are you saying it has no visible singns.

The passive oxide layer on stainless steels runs in the 1 to 5 nanometer range of thicknesses. It cannot be seen by the human eye.

If this were my boat, and I would get the original part sent to me, even if it took several weeks. But it's your boat, you are free to do what you feel best.

dj

 

[DArcy]

Here is an interesting example of a stress riser. A friend was sailing from Bermuda to Antigua last fall when the autopilot quit. Investigation showed the bolt fastening the ram to the quadrant failed. Looking at this picture you can see there is a big change in diameter between the threaded portion and the shoulder, right where the failure occurred.

In this case it was a fatigue failure. The lighting isn't great in this photo but you can see the smoother part towards the outside and the jagged bit in the middle, this is the telltale of fatigue failure.

This was a fairly new part, probably less than 100 hours on it and it was "professionally" made by the autopilot manufacturer. The BIG problem with this part is the change in diameter is abrupt, with virtually no root radius and the result is predictable.
@Screen Saver the design of your original chain plate is good because it avoids the diameter change. Making a bigger rod stepping down to the same size thread would actually result in a much weaker part. This can be mitigated by using a large root radius but you still don't improve the overall strength since you have that week point at the change in diameter.

 

[Screen Saver]

Himmm… Been there done that as they. Also name brand professional installed (Production boat) I was solo on 7 day passage In my case it had done 40,000nm. Still no excuse as far as Im concerned. I personally believe in my case the stud which was pressed through the actuator arm ( of Electric stepper motor) had a grove cut in to it on the underside to give the welding something to bite into. It broke at the grove on the underside and not at the shear point. Again it looked to me simply based on shinny bits and older looking bits that it had been broken for a while.

As to the original thread here. I thought my Deck Chain plate was a selden component. Turns out its not they don't make such things, Anyway its all back together now, just finished tuning the rig, which is not skill I am competent in, and no riggers here. it looks better than did before hand. I have reduced the excessive prebend and managed to take some excess rake out. So nearly within spec now, still a little too much prebend and rake. I'm still not 100% confident I have the tensions right, but it is tighter than it was, and the selden guide sort of implies if you don't put and extension on your wrench one person can not over tighten the 12mm Caps. The lose gauge I borrowed does not have my 12mm as an option so i had to do a bit of extrapolation, then went around the other 50ft's here and gave there rig a good shake... with there permission of course.

Supposedly there is a good rigger in Tiawan so when I get there I will do another check, and see if I can order myself a decent loose guage.

 

[howardfrost]

There seems to be some great responses to the above issue and I believe mine is related so would welcome any advice, please. I am considering the purchase of a 2002 H356 which shows a crack on ther s/board side shroud plate that also secures a 'whisker' pole of about 4' going from said plate to a point on the mast about 3' above the coach roof. The broker kindly sent me some pictures earlier today which I have shared and it does rather look like there has been some previous 'crazing' at this point - GRP/gelcoat filler apparent. I'd welcome opinions ion whether this is likely to cause a fail in the future (who knows?) or whether I do need to effect a stronger fix? I am unsure as to whether the plate is connected to a rod inside the cabin. Thank you for your thoughts. [admin - please if this is in the wrong thread, I'd welcome a steer - ]

 

[dlochner]

There seems to be some great responses to the above issue and I believe mine is related so would welcome any advice, please. I am considering the purchase of a 2002 H356 which shows a crack on ther s/board side shroud plate that also secures a 'whisker' pole of about 4' going from said plate to a point on the mast about 3' above the coach roof. The broker kindly sent me some pictures earlier today which I have shared and it does rather look like there has been some previous 'crazing' at this point - GRP/gelcoat filler apparent. I'd welcome opinions ion whether this is likely to cause a fail in the future (who knows?) or whether I do need to effect a stronger fix? I am unsure as to whether the plate is connected to a rod inside the cabin. Thank you for your thoughts. [admin - please if this is in the wrong thread, I'd welcome a steer - ]

It is probably not a structural issue, however, that can not be conclusively determined from a photo alone.

Gelcoat is a brittle substance, when subjected to stress it will crack. Typical places for it to crack are corners and inside radiuses. A couple of things happen, when the underlying fiberglass is laid up in the mold, it does not bend and make tight corners easily. That sometimes leaves small voids between the gelcoat and the glass. Another issue that comes up is where the glass cloth is laid up. There can also be small weak spots where the glass is thinner, fewer layers, and meets with a thicker layup. At this point there may be some slight bending and movement in the glass which causes the brittle gelcoat to crack.

This crack would not make me walk away from the boat. If the rest of the boat is in good shape, the price is right, etc. etc., then you might consider a prepurchase survey and identify your concerns to the surveyor. On the other hand, there are about 79 35-36 foot Hunters listed on Yachtworld.com.

 

[jssailem]

Jamestown Distributers has a video showing the repair of surface cracks. If they are more than surface then the cost of solving the issue may be 4 times the simple repair.

 

[DArcy]

@howardfrost The "whisker" poles you mention are struts which are a big part of the support of the B&R rig which Hunters are famous for.
Seeing cracks like that on fiberglass boats are fairly common but being around the chain plate may be cause for concern. An intimate inspection of the chain plate structure under the deck would be a good idea. A good surveyor would be a worthwhile expense and either put you at ease about it or caution you away.

 

[howardfrost]

Thank you to all who have responded and so quickly <3 I'll certainly have a closer look and try to upload some more photos and also seek the advice of a surveyor if the sea trial goes well - Fair winds, my friends