Cutout for line exit plate

[memelet]

As part of adding my third reef I need to add a line entrance cutout in my boom (just forward of the cheek block).

I had on hand a schaefer exit plate that I was going to use. But this place has anti-chafe bar for inside the spar.


http://picasaweb.google.com/memelet/...38966149406242

Looking at where the chafe bar is relative two the mounting holes it seems that the cutout would need be square on that side. Indeed I found a picture on the web of a an actual cutout that did this.


http://picasaweb.google.com/memelet/...39779047714130

Is this simply a bad design on Scaefer's part?

(Ok, whats the deal? My images are not rendering using the IMG tag.)

[Ian McColgin]

I don't understand the question. The hole looks fine for the unit.

[memelet]

My understanding is that there should be no sharp corners in cuts in the spar. Only rounded. Is it enough that on the flat side of cutout there is a very small radius of rounding rather than like on the opposite side?

[Ian McColgin]

Yep. I would certainly get a small file out and clean up and round the edges of that cut out, but that roll in the plate is what will bear the line passing out. Without it you could have the line enlarging the groove in the mast, to the detriment of both mast and line.

[memelet]

Actually Ian I'm talking about maintain structural strength in the boom -- not anything to do with line chafe.

But since you didn't immediately say No No to that picture, I guess that cutout will not cause any issues.

[Ian McColgin]

In the normal run of things, a few cutouts with plates won't do the boom any harm. I guess if you had them tightly packed side to side at a point of bending stress on a boom that's already pushing the lightness envelope you could cause a problem but up towards the gooseneck the boom really experiences only mild compression strain. If you put a lot of holes in the attachment area of a mid-boom sheeting, then you'd have some trouble.

[allene]

Any sharp corner will be what is called a stress concentrator and if the boom cracks, the crack will start there. Better to remove the sharp corner with a round file so that it will take more stress for the boom to crack and have it crack somewhere else. Removing even a fair amount of metal to make the corner round will be stronger than having it have a sharp edge. A round hold in the boom will remove almost no strength. A square hole would weaken it a fair amount.