Keel Bolt Removal on a 1984 Beneteau First 38

Oct 9, 2013
72
Beneteau First 38 Belmont Harbor
The instructions from Beneteau specified the use of a small amount of Never-Seez for each bolt and also provided table with min & max torque values for each size of bolt. The instructions are saved and available on the OneDrive cloud server.
 
Jan 27, 2008
3,086
ODay 35 Beaufort, NC
I'm pretty sure I read on SBO that bolts to be torqued should have clean and dry threads for proper readings. Just say'n.
Not sure where you read this but it sounds fishy. As bolt torque increases the friction on the threads and under the head of the bolt to the mating part increases dramatically. Having worked in aerospace manufacturing for decades I can assure you that thread lubricants were extremely common. Mo-lith No. 2 thread lubricant was a very popular engineering specification for fasteners and I recommend it for this application of keel bolts or nuts. Here's an outfit that sells single tubes at a reasonable cost. Might be a good addition to the SBO store! https://www.skygeek.com/lubriplate-...h-no-2-moly-lithium-lubricant-14-oz-tube.html
Dry film lubricated bolts is another option if you can find them or have the coating applied by a competent shop.
You can't do this with a bolt but where you have a stud and a nut you can calculate how much the fastener should stretch for the torque specification and then use a set of dial calipers to measure the length of the stud before and after tightening the fastener to make sure the clamped joint has sufficient clamping force. You can also figure out how much stress is in the fastener versus it's yield strength to figure out your safety factor.
Here is a good reference on use of thread lubricants, friction, and so forth for those so inclined to research the subject. https://books.google.com/books?id=N...L#v=onepage&q=molith thread lubricant&f=false
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Minimum engagement depth for full structural integrity, will vary greatly depending on the materials in question (grade of the fastener) & the thread form.
I’m quoting a Beneteau engineering doc with that statement.
 
Dec 31, 2016
319
Beneteau Oceanis 351 Charlottetown
I’m quoting a Beneteau engineering doc with that statement.
I personally wouldn't be all that trusting of Beneteau' s specs. This is the info I received on keel boats torque for my boat, note the M20 have a min. of 94 ft. lbs. and a max. of 195 ft. lbs., 101 ft lbs difference, kind of a rinky dink scale,haha
TABLE 6.5.1
DACROMATISED STEEL BOLTS TYPE 8-8.
DIAMETER
M14
M20
M24
M30
TORQUE in M.Kg - minimum
5
13
23
45
TORQUE in M.Kg - maximum
9
27
46
90
TORQUE in ft.lb - minimum
36
94
166
325
TORQUE in ft.lb - maximum
65
195
333
651
 
Jan 27, 2008
3,086
ODay 35 Beaufort, NC
For standard UNC threads three full threads engaged fully will give the full strength of the fastener. This is based on the cross section area of the root diameter compared to the cross section area of the thread at the pitch diameter for three threads. Another way of expressing what Jack offered
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
I'm pretty sure I read on SBO that bolts to be torqued should have clean and dry threads for proper readings. Just say'n.
There are separate torque spec's for dry fasteners & lubricated fasteners. The numbers for the lubricated fasteners are lower. Torque spec's vary with fastener material & thread form. General recommendations for various fasteners can be looked up in charts. https://www.google.com/search?q=tor...WBwFMKHQ0TAWcQsAQIVw&biw=1280&bih=887#imgrc=_

Engineers often specify torques that are outside the limits of these charts for specific applications that have nonstandard directional load stresses or other pertinent factors to consider.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
I’m quoting a Beneteau engineering doc with that statement.
It might be helpful for you to define that sort of thing when you make the kind of statement that you originally did. Within a specific set of parameters, using a specific bolt in a specific material, your original statement MIGHT be true. But if someone were to take what you originally wrote as a blanket statement & apply it in a different situation, they could cause a mechanical failure.

I'd be interested to read the document that you are quoting. Please post it if you can.

Thanks,
Jim
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
For standard UNC threads three full threads engaged fully will give the full strength of the fastener. This is based on the cross section area of the root diameter compared to the cross section area of the thread at the pitch diameter for three threads. Another way of expressing what Jack offered
This still varies with the material in question. Not all fastener materials have a consistent ratio of tensile strength to shear strength. In the case of a machine screw going into a tapped hole in a material that is different from the material that the bolt is made of, this standard goes right out the window.

If you want to get more in depth, you also need to consider the class of the thread fit & the percentage of thread, based on pilot diameter of the female. UNC threads can be & frequently are made to several different standards with varying pitch diameter ranges & varying engagement widths.

In the case of any sharp V thread, like a UNC, there is also the concentration factor to consider. That sort of thing is not so much in play with other thread forms like a Whitworth, which has a rounded root.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
I personally wouldn't be all that trusting of Beneteau' s specs. This is the info I received on keel boats torque for my boat, note the M20 have a min. of 94 ft. lbs. and a max. of 195 ft. lbs., 101 ft lbs difference, kind of a rinky dink scale,haha
...
In the case of something like an aircraft, where everything has been engineered down to the last detail to save weight & there is a high expectation that only AFP's or other skilled individuals will be torquing fasteners, it is common to see torque ranges that are very narrow & even detailed instructions on torque intervals & patterns.

In the case of less critical equipment that is likely to be worked on by less qualified individuals, it is common practice to oversize the fasteners by a much larger safety factor. In this case, a much wider range of acceptable torques may be appropriate. Perhaps that may be the case here.
 
Dec 31, 2016
319
Beneteau Oceanis 351 Charlottetown
In the case of something like an aircraft, where everything has been engineered down to the last detail to save weight & there is a high expectation that only AFP's or other skilled individuals will be torquing fasteners, it is common to see torque ranges that are very narrow & even detailed instructions on torque intervals & patterns.

In the case of less critical equipment that is likely to be worked on by less qualified individuals, it is common practice to oversize the fasteners by a much larger safety factor. In this case, a much wider range of acceptable torques may be appropriate. Perhaps that may be the case here.
In my many years of mechanic/welding work it is very uncommon to see a range like this. Otherwise the instructions would be tighten until tighten. It's not uncommon to see a range of 2-4 ft lbs,variance but definitely not a 101 ft lbs .!
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
In my many years of mechanic/welding work it is very uncommon to see a range like this. Otherwise the instructions would be tighten until tighten. It's not uncommon to see a range of 2-4 ft lbs,variance but definitely not a 101 ft lbs .!
A mechanic is most often a skilled individual. An engine is a highly engineered piece of equipment. Your example is one step below an AFP working on a Cessna or a Dart, but far from the bottom of the staircase.
 
Jan 27, 2008
3,086
ODay 35 Beaufort, NC
This still varies with the material in question. In the case of a machine screw going into a tapped hole in a material that is different from the material that the bolt is made of, this standard goes right out the window
.
In the case of a bolt going into a nut how many "good" threads are in the nut? A quick check of a 1/4-20 nut shows it is 0.219" thick. So 20 X 0,219 = 4.38 threads. The first and last thread will be imperfect threads so if we assume about half of each thread is the full thickness of the thread form we get a little over three threads. For a 1/2-13 we get .427 X 13 = 5.5 again throwing out a full thread for imperfect form we end up with 4.5 threads. I can't argue that a bolt going into modelling clay will need a lot more threads engaged to develop the full strength of the joint. But the strength of the fastener is not effected by the modelling clay it is a property of the fastener.
 
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Ross S

.
Oct 20, 2011
120
Precision 21 Great Sacandaga Lake
I noted some questions about bolting theory here and thought I'd lend a little expertise.

1. Bolts generate clamping force by stretching (like a spring). When you tighten a bolt you actually are stretching it slightly. You can relate applied torque to bolt stretch, but it's a loose correlation because of the effects of friction and the friction factors are estimates with large associated uncertainties. Friction in the threads, friction of the bolt head, etc... This is why lubricated torque values are lower than dry torque values. Because with lower friction you need less torque to generate the same stretch. Torque is also one of the least accurate ways to generate a known clamping force (about 25% uncertainty). Most accurate is to hydraulically stretch the bolt to a known preload. A slight step down from there is to measure bolt stretch directly.

2. Torque ranges: In general, for a bolted joint to avoid loosening over time, you want the clamping force generated by the bolts to be larger than the anticipated loading of the bolt. If you can figure out the joint load you can then figure out the minimum clamp force required and the resultant torque. Also, you don't want to over stress the bolts either. For bolts that are intended to be reused, you generally don't want to preload them past 75% of yield stress (assuming the rest of the joint can withstand that load). So you can figure out a torque that equates to 75% of yield stress. If the bolts are oversized for the application then the difference between these two values might be large.

3. Thread form and materials do affect thread engagement requirements. The 1.5X rule of thumb does generally work for steel in steel. However, if you were threading an SAE grade 8 fastener into something soft like brass you'd actually want more along the lines of 2.5X. Why is that? Because in general you want the male threads to fail before the female threads fail. (i.e. you'd rather break the stud than strip the nut).

Hope this helps!
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
In the case of a bolt going into a nut how many "good" threads are in the nut? A quick check of a 1/4-20 nut shows it is 0.219" thick. So 20 X 0,219 = 4.38 threads. The first and last thread will be imperfect threads so if we assume about half of each thread is the full thickness of the thread form we get a little over three threads. For a 1/2-13 we get .427 X 13 = 5.5 again throwing out a full thread for imperfect form we end up with 4.5 threads. I can't argue that a bolt going into modelling clay will need a lot more threads engaged to develop the full strength of the joint. But the strength of the fastener is not effected by the modelling clay it is a property of the fastener.
A common 1/4 -20 (UNC) nut & a common 1/4-28 (UNF) nut generally have about the same outer dimensions, but different numbers of threads in engagement. Same with 1/2-13 & 1/2-20.