Bobstay attachment advice

[raz88 96]

Looking for some advice about what to do with my bobstay attachment. It appears to be gradually pulling forwards at the bottom of where it's mounted and I'm concerned if its over tensioned it will keep going.

 

The forestay goes to the prod meaning as the rig is tensioned the bobstay attachment fitting gets pulled up. There's a photo below - it's currently through bolted side to side, and when you look inside there's a decent chunk of timber in there that it's going through which has been nicely glassed into place.

 

My initial idea is to take the fitting off and have a new fitting built that either extends back further to allow for more/bigger through bolts, or somehow allows for a fore/aft bolt as well to counteract it being pulled upwards.

 

Does anyone with some engineering knowledge have a better idea?

 

http://s231.photobucket.com/user/Razzza_2007/media/Mobile%20Uploads/20160624_143902-1.jpg.html

[John B 106]

 

 My feeling would be to have a plate that extends upwards so it shifts the fulcrum away from that top bolt. If that didn't work well enough  then a plate also going down and a bolt through the stem ( fore and aft)

or, remake the plates so the top bolt is removed altogether. extended on the same line as the  bobstay and a second side bolt right through. That removes leverage and puts the  tension all on the same line as the bobstay , like a shroud tang on a mast.

[Steve Pope 243]

I would consider extending the forward face downwards with the existing webs also extended downwards and welded to to the existing fitting with a fore and aft bolt back through the centre of the stem with a backing plate on the inside. This would stop the plate twisting up as it is doing at present. Use 316ss.

[Island Time 1,239]

That's the right idea John B - spread the load. This a a substantial load!

 

Think about how your chainplates attach and how they always continue the line of the shroud and how they are attached to the hull.

 

The issue you have in this situation is that the bottom bolt, being the lowest, and in direct line with the bobstay, is taking almost all the load. This is why it is pulling out at the bottom.

 

I see a problem in that it is so close to the waterline - to extend the plate down, where is structurally needs to be, then you have connections and bolts under the water - less than ideal. Can you give us a pic of the inside? Was she designed like this, or has the bowsprit been added? 

 

I think I would move the entire lower bob-stay fitting up. If required, go to a heavier wire size to compensate for the increased angle. You need to have at least two of the bolts BELOW the bob-stay attachment points, and a decent SS backing plate on the inside. Remember that the bolts below the intersection of hull and bob-stay take most of the load and are in tension, the part above the intersection is more in shear. 

 

What movement can you see on the inside?

 

To be certain, get a registered engineer to design a new fitting for you - as failure of this would almost certainly cause the loss of the rig...

 

However, if you like, I can do a rough design for a replacement fitting, ( with the proviso that I am not a registered mechanical engineer, and the risk of using any fitting made with data I supply is entirely your own! )

 

If you want some more data, please let me know;

What is the diameter of the fore-stay?  The bob-stay?(give us an idea of the loads) Once I have that ,I can get an approximation of the angles from the pic, and (if I can remember how - it's been a long time!), I should be able to give you some basic numbers on the loading.  Once the loading is known, it's not too hard to design a fitting to withstand it....

 

Best to get a real engineer to do it, but it would be interesting to see how closely his loadings and design match mine!

 

Cheers

Matt

[Steve Pope 243]

Replace the term engineer with common sense and I endorse everything Island Time says. Engineer = dollars, and he / she will tell you nothing that common sense can't. Though If you want someone to blame if it all turns pear shaped, then  engineers are the way to go!  The way the existing fitting was designed there is no way that the present outcome could have been avoided.

[Island Time 1,239]

Probably KM, that would be fine, provided that there is a glassed in internal block and plate. However, personally I'd like to see a proper shear/bear or tear calc on the structure on a connection this important. Guessing leads usually to issues, or to excessive weight.

 

The risk with a single bolt like km suggests is that it tears a slot in the hull to relieve the load - which it won't do if the internal backing is sufficient AND glassed to the hull to transfer the load over a larger area.

 

I'd engineer it myself....

[raz88 96]

Thanks everyone, good ideas all.

 

Unfortunately the boat's in oz and I'm 3 weeks away from next visiting so will have to provide more details about the internal support and stay sizes etc then.

 

But this has certainly helped the thought process along. Thanks

[hb1849 12]

Replace the term engineer with common sense

 

And this is why people end up with stuff that breaks and/or is ridiculously overbuilt. Unless you class working out actual stresses in components using techniques learned during many years at univeristy and many more pracitising before becoming registered as common sense.... It's very rarely as simple as F/A!

[ScottiE 174]

Amen!

[Battleship 100]

Why re invent the wheel? have a look at the setup of similar boats, better yet the same design and see what they have.

[El Toro 97]

They're all different as were retrofitted and for different reasons....not that many 10.5s around.

 

Pork Chops prod was made to 1.2m to keep it in line with the 1050s and also fit into a 12m berth, The Butcher went for the montster retractable to do offshores, as well as moved both the rig (and keel?)as well as made the keel uber deep..... and Pighunters has only gone on very recently and is different again but still retains the orginal jandle configuration.

 

The reason the forestay is out so far is to reduce the 'oh sh*t we're going down the mine' moments the boats suffered...and is a lot cheaper than relaying bulkheads and moving the keel, which is what we did on Pork Chop, moving is 740mm back...as well as raising the forestay from 3/4 to 15/16s. Both actions stopped the down the mine issues, but increased the brown runny stuff down the inside of the leg when boosting into the near 20s.

 

The next plan was to square the back of the boat to get all the weight back beind the helm safely..but that is now for someone else to consider.

The boats love aft weight.

 

Still there is plenty of room up on the bow to play with, look forward to seeing what you do and the boat back in Auckland at some stage?

 

Hard to find many people who dont love sailing on them.

[raz88 96]

Thanks thats good info.

 

The other reason the forestay is out there is the spreaders have been lengthened and the shrouds come all the way out to the gunnels, so no overlapping headsails. Forestay forwards recovered some of that lost area.

 

Definitely bringing it back to Auckland, just working through the many bits that need to be sorted for that to happen.

[RushMan 31]

When are you planning the delivery across the Tasman?

[raz88 96]

Be a few months yet. Fair bit of organisation in getting sorted to do it.

[Fish 0]

Replace the term engineer with common sense and I endorse everything Island Time says. Engineer = dollars, and he / she will tell you nothing that common sense can't. Though If you want someone to blame if it all turns pear shaped, then  engineers are the way to go!  The way the existing fitting was designed there is no way that the present outcome could have been avoided.

So on the one hand you could just guess and hope, on the other hand, you could work it out properly (and ask an engineer if you can't work it out yourself). As its already failing, it's fairly clear guessing and hoping didn't work...

 

Engineers are not lawyers. Their qualifications enable the ability to give a straight answer.

[Island Time 1,239]

That's a bit unfair KM, indeed engineers, like everyone, have made mistakes in the past. When the owners press for lighter, higher aspect keels, deeper, with heavier bulbs, and now canting as well, this pushes the limits of current understanding. There is/was simply not enough data to fully understand the loads that can be encountered. That data is growing every year.

Then there is the make it strong enough for most users, make it cheap types, which is also faulty...

[Island Time 1,239]

Agreed on that.

[raz88 96]

"As its already failing, it's fairly clear guessing and hoping didn't work..."

 

You could argue it's done alright. From what I understand it's done 10+ years like that including most of a circumnavigation of Australia and one of two bolts has shifted 8-10mm - hasn't really actually failed as such. I'd just like to make sure it doesn't.