PaulRainbow
Well-Known Member
If you draw a line through the turnbuckle and lines through the ring bolts (parallel to their threaded sections) the angle where those lines interest will be approximately 45 degrees.
"Chainplate" for an inner forestay...
- Thread starter Kukri
- Start date
If you draw a line through the turnbuckle and lines through the ring bolts (parallel to their threaded sections) the angle where those lines interest will be approximately 45 degrees.
BabaYaga
Well-Known Member
The load direction on each ring nut will be the bisection of the two sides of the 'dyneema triangle', will it not?
This load direction is, while not perfectly in line with the bolt, pretty close to it.
Laminar Flow
Well-Known Member
The force is, roughly the resultant of the direction of pull which is not perpendicular, at least not as it appears in the picture, to the alignment of the ring nut bolts; they are being pulled "sideways" by the dyneema strapping and that constitutes a sheer stress.
My other point was that the strapping leaving the ring nut in two different directions has a similar effect to a turning block where the forces of each end of the line add up.
BabaYaga
Well-Known Member
It seems you fail to recognize the fact that the pulling force on the ring nut is in equal parts transmitted by the horizontal section of the strapping and the strapping section connecting to the rigging screw. Therefore, as explained in the post above, the aggregate force must be in line with the bisection of these two pieces of strapping.
This direction of pull is, while not perfectly perpendicular, far from the 45 degrees that you stated.
In fact, the decision to use a dyneema strapping and two opposing ring nuts to distribute the load to the hull was in part made just to counter the potential hazard that you outline – excessive side loading of the anchoring bolts.
That said, it will be difficult on any boat to find a section of the hull in the bow area that is at right angles to the direction of pull from an inner fore stay. Thus, some side loading of anchoring bolts is hard to avoid. This must be compensated for in dimensioning etc.
Not sure what your concern is here. The ring nut will break? It has a breaking load of 4.5 tons. My boat is 9 metres and displaces 3.6 tons.
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JUSTINHALEWOOD
Active Member
This is great, thank you
Laminar Flow
Well-Known Member
Not sure what your concern is here. The ring nut will break? It has a breaking load of 4.5 tons. My boat is 9 metres and displaces 3.6 tons.
[/QUOTE]
Fair enough, the size of the boat is not evident from the picture.
I do not think the ring nuts in themselves are the problem, it would be the threaded part of the bolt holding them. Be that as it may, it is not exactly elegant engineering as the entire chain of of parts starting at the attachment of the stay to the mast should be of equivalent strength with all vectors of force correctly resolved.
Strength of a 8mm bolt in kg:
In tension: 2021
In sheer: unthreaded 3450, threaded 1737.
I this sense, the breaking strength of the ring bolts is irrelevant.
To engineer something properly you really need to know the true forces involved and how the vectors apply.
[/QUOTE]
Fair enough, the size of the boat is not evident from the picture.
I do not think the ring nuts in themselves are the problem, it would be the threaded part of the bolt holding them. Be that as it may, it is not exactly elegant engineering as the entire chain of of parts starting at the attachment of the stay to the mast should be of equivalent strength with all vectors of force correctly resolved.
Strength of a 8mm bolt in kg:
In tension: 2021
In sheer: unthreaded 3450, threaded 1737.
I this sense, the breaking strength of the ring bolts is irrelevant.
To engineer something properly you really need to know the true forces involved and how the vectors apply.
pvb
Well-Known Member
And, of course, to know the strength of the GRP laminate the ring bolts are fixed to...
BabaYaga
Well-Known Member
For the record: The bolt in question is M10, not M8.
It is not loaded at 45 degrees, but likely about 10 degrees off the perpendicular. Not much to worry about IMHO.
All in all, I find your quick dismissal in post #18 to be a bit...quick.
Laminar Flow
Well-Known Member
My deepest apologies if I offended you.
Truthfully, if I had found such an arrangement on my boat, I would have changed it. Not because of any ego, but because I know what it is like to be on the verge of losing a mast offshore, 1500 miles from the next land, when both the twin forestays come down.