Bolt strength comparison

Roly,

Don't. Never use the UTS value, ever! You the yield (or proof). The book is wrong. It is very misleading to use a high FoS with the UTS and incorrect. (Report attached why you MUST use the yield NOT the UTS, see lessons #1 at end)

The stress is simple = Force/area.

Then you have your factor of safety to add, so stress = (FoS x Force)/area

If in doubt what FoS to use, start with the best all rounder 5.

Also don't forget, when selecting the properties you need to account for the maximum toque the bolt can take. You can't just keep on tightening it up, as this also effects the clamping force.

To work out the maximum torque is simply this:

T = (Sig(y)) [the yield or proof stress] x 0.7 x Area x diameter/ 5000

Unit, yield stress MPa
Area, mm
Diameter mm

T = Nm

Report here:
View attachment MAIB yacht keel.pdf

 

The connection between the keel and hull is one of the most critical of all connections: failure here at sea under sail often results in death of crew. This is not a change to consider lightly.

Usually failure of this connections is failure of hull material through which the bolt is installed, or corrosion on the bolts themselves. Larger bolts means more hull material is cut out, fewer bolts means loads are not spread out over as large an area.

Without a detailed loads analysis of the installation, its geometry and configuration, the only acceptable substitution would be if you installed the same size bolts, that were stronger, and more corrosion resistant, and there was no incompatible materials contact.

Best to try and contact the original designer if he is still available, and see if he would accept the substitution. Otherwise is it not likely worth the risk.

 

From above, if I know the xsection area, I know the bolt size.

I am trying to get a little understanding of the basics here, not trying to engineer my keel, just gain a little extra confidence. I am replacing the bolts with stonger material bolt of bigger size, so considering the previous bolts did not fail over at > 20yr period I consider I am on the right side of the line.
Its just how big because I am limited to 16mm.

Using Gerr's formula & trying SF5.....is this correct?
Load/bolt=SF*keel moment*keel wt./2*bolt bearing width*Number bolts per side.
= 5 *1200 * 1500/(2 * 100 * 6)
= 7500kg
= 73575N / bolt area (16mm bolt) 201.14mm2
.'. = 366mPa Yield strength reqd, 16mm bolt, SF 5

Minimum yield (0.2% offset) of 2205 is 400mPa, so looks OK, at SF5.

With a little more accuracy, after a measure and no guessimating......
Bolts=6 per side + two center (half bearing width), call it 1 @ bearing width.
Bearing width = 125mm (Actual to center of bolt)
Bolt root diam.= 13.546mm
Bolt effective area=144mm2
Trying SF5:
=5*1200*1500/(2*125*7)
=5143kg / 16mm bolt (144mm2 root) metric coarse.
.'. =350mPa yield strength reqd, 16mm bolt, SF5
Considering the stock I am offered is UTS @875mPa/2 = >400mPa yield i have
a better than 5 safety factor.
Where have I erred?

 

All I can say is that your maths is correct. Beyond that not much.

Since without a diagram of the arrangement and knowing 100% what it is you're doing, I cannot confirm whether the numbers and formulae you are using are correct

 

It's just a keel.

What's the worst that could happen, it could fall off and be lost to the depths?

Of course that would be the least of your worries at that moment!

-Tom

 

A quick question, are you replacing 19mm 316 bolts that were originally fitted? If so as I mentioned before these may in fact have been stronger (material strength) than the 2205 that you are looking at. Then there is also the difference in strength between a rolled thread (if the bolts were a mass produced item) and a cut thread

 

Old bolts were 12mm, mixed hi-tensile & ms galvanised. I assume hi-tensile- I will have to put them in a vice to check.
Sandvik has given me a 536mPa 0.2% offset proof strength for the heat number for the 16mm stock current.
Saf2205 my studs will be cut machine threaded, custom.

 

When under sail and the keel falls off, the boat will almost certainly capsize. it will lose its ablity to make headway and with the sail still set, it will get dragged uncontrollably across the surface, until the rigging fails. This almost always results in some of the crew on deck getting caught in the failed rigging, and anyone below gets gets trapped and drowns. There have been many well documented accidents of sailing boats losing their keel, and crew getting killed as a result.

This is very serious business, the keel is an integral part of the sailing and control system, take it out and you are in big trouble.

The only time it would not matter much if you are lucky enough to lose the keel while at the dock side.

 

You have a dry humour, Tom.

 

I am replacing the bolts with stonger material bolt
Worth looking into
of bigger size
Why?
so considering the previous bolts did not fail over at > 20yr period
Again, why?

 

Well, I'm glad somebody appreciates it...

-Tom

 

Looked into- monel-too weak hence too bigger bolt
- ductile iron- hens teeth- too weak
- Hi-tensile steel, more prone to fatigue, sourcing problem
- mild steel too weak- hence too big diam.
- 2205 seems to fit the bill & readily avail. i am aware of CC. spendy.
Increased size- less bolts, 16---> 14

Latest available:
Hi-tensile steel 4140 0.25 proof 680mPa
4340 0.25 proof 755mPa with 8.8 nuts

SS 2205 0.25 proof 536mPa with 316 nuts
The relative fatigue resistance of these I have no idea.
What would you choose?

 

What shape is the hull in where the keel bolts to it? Will you reinforce it?
Hogging out the holes to fit bigger bolts reduces local hull strength strength while fewer bolts forces greater loads onto the attachment points.
My concern is that you are putting stronger bolts into fewer, weaker holes.

 

Completely rebuild floors to accommodate bigger fasteners. Bolts completely encapsulated.

I am having trouble sourcing 2205 nuts, (here) so I may have to run with
hi-tensile option.No problem in the USA.
Cheers