Great information Jeff---it certainly shows that tightening on the hard is not
necessary--and as soon as I can round-up the proper torque wrench I will give
it a go while she is in the water. If I can tighten those that I can reach, it
might stop the leak--or at least slow it significantly.
If the leak continues after I get the bolts I can tightened appropriately, I
will have the yard pull the mast to get to and tighten those that I cannot
reach.
The bolts I can reach are 7/8" diameter with nuts that are 1 1/2" diameter. Any
idea of what torque is appropriate for these?
I seem to remember values from some list postings that I probably need values
above 200 ft-lbs. This likely requires a torque multiplier IIRC--nothing is
quite as simple as it seems!!
Thanks,
Charlie NelsonWater Phantom1995 C&C 36 XL/kcb
-----Original Message-----
From: Jeffrey A. Laman via CnC-List <cnc-list@cnc-list.com>
To: cnc-list@cnc-list.com <cnc-list@cnc-list.com>
Cc: Jeffrey A. Laman <jlam...@outlook.com>
Sent: Sat, Feb 16, 2019 2:45 pm
Subject: Stus-List Prepping for 35-1 Keel Bolts/Smile Fix
#yiv9856812885 P {margin-top:0;margin-bottom:0;}Regarding bolt torque and
corresponding bolt pretension: there seems to be some misunderstanding about
bolt torque. Worries about stripping threads when boat is in the water,
thinking about lifting the keel up, must do it on the hard, etc.
Just as an example for the sake of discussion, let's consider a 1" diameter,
316 bolt
- Fy = 30 ksi = 30,000 psi (yield strength)
- Fu = 75 ksi = 75,000 psi (fracture strength)
- Area = 0.79 in2
- Area through threads approx 0.60 in2
The correlation between torque and bolt pretension is not high due to the many
factors involved, but generally, a commonly used approximaterelationship is as
follows:
T = (K x D x P)/12
K is a torque or friction coefficient, D is diameter, P is pretension force. K
= 0.20 would be common. Rearranging the above to solve for pretension:
P = 12T/(KD)
Lets also presume, again for discussion, that the 1" dia 316 bolt is
pretensioned to 60% of yield. That means the pretension force:
P = (0.60) x 30,000 psi x 0.60 in2 = 10,800 lbs
T = (0.2 x 1" x 10,800)/12 = 180 ft-lbs
Which is certainly in the torque range suggested in recent posts.
Let's take these numbers, again for discussion and consider a boat with (8) 1"
diameter keel bolts. The pretension total would be:
8 bolts x 10,800 lbs/bolt = 86,400 lbs.
The weight of the keel (4000 lbs? 6000 lbs?) is not particularly significant
-- it's within the margin of error of this entire calculation and then some.
Observations:
- Concerns about lifting the keel and causing damage are unwarranted
- Doesn't matter much at all whether the boat is in the water or in the
cradle
- Bolt torque is the torque is the torque -- it doesn't matter if the boat
is in the water or not relative to stripping the threads.
Sorry for the very long post, but wanted to help out with some of the
discussion. I intend to torque my keep bolts this spring and have read the
posts on this subject with interest.
Best,Jeff Laman1981 C&C34 "Harmony"Ludington, MI
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_______________________________________________
Thanks everyone for supporting this list with your contributions. Each and
every one is greatly appreciated. If you want to support the list - use PayPal
to send contribution -- https://www.paypal.me/stumurray
