Josh,
Not quite sure I got this right, are these horizontal bolts into the
side of the engine or vertical to the boat? In either case, though, the
bolts and plate are carrying the load - most of which is the thrust
loading of the propeller through the mounts and driving the boat - by a
combination of shear and friction. To be only shear, you'd have to have
fitted bolts in the plate which I doubt is the case, so here most of the
load is probably through friction. The 10.9 designation, while related
to hardness, is a strength measure for metric bolts, about equal to
Grade 8 in SAE bolts. So you have high strength bolts allowing greater
torque, thus greater friction, which increases the plate's ability to
transfer thrust.
Having said all that, for sure there's overkill in this. You've been
operating fine for 9 years with 7 instead of 8 bolts total, so putting
the last one back in even with somewhat compromised threads should be
fine. If you can run a tap into the hole to clean the threads it would
help, but my thoughts are to just go with it.
Neil Gallagher
Weatherly, 35-1
Glen Cove, NY
On 2/23/2021 10:41 AM, Josh Muckley via CnC-List wrote:
Engineering discussion for the group.
Here's the background. For about 9 years I have known about having 1
of the 2 bolts broken on the forward starboard engine mount. These
bolts hold a piece of 1/2 inch plate steel which was bent at a 90°
angle to the vertical side of the engine. This creates a horizontal
shelf. The shelf then rests directly on the vibration dampening mount
which is then bolted to the boat. Periodically, I have made attempts
to extract the bolt but only a few days ago was I finally successful.
The bolt was a M12x1.75. I don't know what hardness the OEM called
for. Years ago when I discovered the broken bolt I was quick to
replace the remaining good one with one having a hardness of 10.9.
For a fresh start, I now intend to replace both of them with new and
again a hardness of 10.9. In removing the old bolt, my drill hole was
off center and ended up cutting some of the threads. I approximate
not more than 1/4 of the thread integrity was compromised.
https://drive.google.com/folderview?id=15GCOH3qKIWXl1lmjsbZqsf-ECGCrl1fN
Here comes the question(s). Just for kicks, I decide to attempt to
figure out what the sheer strength of the engine mounting bolts are
individually. I came up with an answer but would like hear what the
group comes up with.
I also now have a bunch of other questions tumbling around like: How
much weight would it take to break one? How much margin to failure
did I have while operating for the last 9 years? What could have
possibly caused this bolt to break prior to my ownership? Theories
and ponderings are welcome.
With 1/4 of the threads degraded on the newly available bolt hole, I
plan to only torque the bolt to ~50% of max (90 ft-lbs) which would be
~45 ft-lbs. I'll also make sure that the lock washer is fully
collapsed but I'm confident that that will happen well before final
torque is reached. In sheer, I believe this bolt is providing as much
or more strength simply by being there than anything to do with its
torque. All though, I have heard the engineering theory that (when
built appropriately) the bolts apply the pressure which creates the
friction to prevent the pieces from moving in sheer. Under this
argument the 2 pieces of metal don't actually ride on the bolt shank
specifically, and the surface area where the 2 pieces touch is of
particular importance. Looking at the engine castings, it would appear
that maximizing surface area contact was not considered. As such, I
can only fall back on my previous thought that the bolt shank is in
fact carrying the load in sheer and torque is more or less irrelevant
except for the purpose of keeping the bolt tight. I'm also aware of
various thread repair options but in the awkward location I am fearful
that I would cause more damage than good by attempting any of them.
Particularly if the value added by the second bolt truly is its
presence not its torque.
All thoughts and insights are encouraged.
All the best,
Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD
Thanks to all of the subscribers that contributed to the list to help with the
costs involved. If you want to show your support to the list - use PayPal to
send contribution -- https://www.paypal.me/stumurray Thanks - Stu
Thanks to all of the subscribers that contributed to the list to help with the
costs involved. If you want to show your support to the list - use PayPal to
send contribution -- https://www.paypal.me/stumurray Thanks - Stu
