Guess how it broke

codprawn · 27 September 2008, 01:39

I was horrified last week to find that my RIB had somehow managed to come untied at the bow. It was still tied at the A frame but the bow was free to hit the pontoon and the boat next to it - no damage thanks to large rubber tubes!!!

Because of the lack of suitable cleats on a RIB and the fact that my harbour is extremly rough I had 4 ropes attached to the bow eye. This is one of the strongest points on a boat - how the hell did it fail?

I would like to point out that it was NOT corrosion or similar. I have worked out the answer but I was stunned and nobody I know has ever seen anything like it.

The eye is a VERY heavy duty unit - anyone know where I can get one exactly the same?

PeterR · 27 September 2008, 03:34

If it wasnt a close encounter with Ury Geller How about friction/fatique .It would suprise me but it has snapped at its narrowest point on the thread

bedajim · 27 September 2008, 05:49

Quote:

Originally Posted by codprawn

I was horrified last week to find that my RIB had somehow managed to come untied at the bow. It was still tied at the A frame but the bow was free to hit the pontoon and the boat next to it - no damage thanks to large rubber tubes!!!

Because of the lack of suitable cleats on a RIB and the fact that my harbour is extremly rough I had 4 ropes attached to the bow eye. This is one of the strongest points on a boat - how the hell did it fail?

I would like to point out that it was NOT corrosion or similar. I have worked out the answer but I was stunned and nobody I know has ever seen anything like it.

The eye is a VERY heavy duty unit - anyone know where I can get one exactly the same?

What is the thread size

Limey Linda · 27 September 2008, 06:21

Were any of your four ropes tied in apposing directions? If so then the stresses are trying to bend the fitting from side to side. Over time this will cause fatigue and fracture. I also want to bet the fitting has a rolled thread not a machined one. Easy to tell the difference. Rolled threads are bad as they impart compression and brittleness ( work hardening)

Channel Ribs · 27 September 2008, 06:25

Quote:

Originally Posted by codprawn

The eye is a VERY heavy duty unit - anyone know where I can get one exactly the same?

I am sure we either have one or can get one, what are the specs?

JSP · 27 September 2008, 06:27

If there were 2 ropes on this, was one of them set up as some kind of spring? (although with no cleat around the centre of the boat I'm not sure how you'd do it) So when the water has pushed her about you've had opposite forces which has made it go clink ?

If not I haven't got a clue and I'm calling in Scooby doo!

Hightower · 27 September 2008, 08:01

Yep, stress fatigue. The only way this would happen would be a take off in opposite directions and the boat moving in a sideways manner or a take off all to one side with a 90 degree angle to the Bow, the boat would be tugging from one side.

You should find a good selection of U bolts from hardware internet sites like here or Here at the Bottom of the page

This should get you started. Just check the dimentions from the online drawings in the websites to identify which one is suitable.

If this might happen again, it might be an idea to look for a different soloution than using a U bolt?

Good Luck.

Chris · 27 September 2008, 08:48

Polwart with an angle grinder

Poly · 27 September 2008, 08:52

Quote:

Originally Posted by Chris

Polwart with an angle grinder

I would have subcontracted it to garfie!

IBWET · 27 September 2008, 08:59

I'll be interested in the answer to that one Codders. I know someone with a brig rib approx 3mtr long, as far as I'm aware it's (the u bolt) only ever had minimal use ,never any more than one rope yet we tried to pull it up on the pontoon with this and got the same result...straight pull......so .... lets hear it

codprawn · 27 September 2008, 12:48

Well it's my own stupid fault but I never expected this to happen.

Although the bow eye is seriously strong the hole is quite small. I had 4 ropes through it and another 2 old ones that I hadn't bothered removing when they had snapped.

I can only assume that the ropes expanded due to water absorbtion and caused the fracture. The forces must have been immense - I would guess at about 5 - 10 tons.

When I looked at the ropes the remains of the bow eye was still in the knots - it was so tight I could not pull it out - I had to cut each rope through to remove it.

Thanks God for a strong hull - I couldn't belive that it hadn't just pulled through. When I measured the thickness of the hull there it is well over an inch thick - prob about 35mm as you can see from the position of the nuts.

Downhilldai · 27 September 2008, 23:22

How is it moored now?

codprawn · 27 September 2008, 23:58

I bought one of the rubbish ones with 2 crimps - the washers/plate just pulled through but the load is outwards. I am looking for a decent quality one - most seem to have these crimped "ears" and they are useless - as least supply an extra 2 nuts but I would rather a proper shoulder/collar.

Limey Linda · 28 September 2008, 01:57

Quote:

Originally Posted by codprawn

Well it's my own stupid fault but I never expected this to happen.

Although the bow eye is seriously strong the hole is quite small. I had 4 ropes through it and another 2 old ones that I hadn't bothered removing when they had snapped.

I can only assume that the ropes expanded due to water absorbtion and caused the fracture. The forces must have been immense - I would guess at about 5 - 10 tons.

When I looked at the ropes the remains of the bow eye was still in the knots - it was so tight I could not pull it out - I had to cut each rope through to remove it.

Thanks God for a strong hull - I couldn't belive that it hadn't just pulled through. When I measured the thickness of the hull there it is well over an inch thick - prob about 35mm as you can see from the position of the nuts.

I believe your diagnosis to be incorrect as usual. I think you are saying that the failure was due to tensile stress caused by expanding ropes. Assuming that your U bolt was at least 3/8in. dia. it has a tensile strength of at least 20+ tons.
So when are you going to start making Bio diesel as you committed to last Jan.

or was this just more Codswallop.
We aere waiting for the results
Think again

Channel Ribs · 28 September 2008, 06:11

Quote:

Originally Posted by codprawn

I bought one of the rubbish ones with 2 crimps - the washers/plate just pulled through but the load is outwards. I am looking for a decent quality one - most seem to have these crimped "ears" and they are useless - as least supply an extra 2 nuts but I would rather a proper shoulder/collar.

I tried working out the dimensions from the photo, but the scale seems to be just out of shot.

hobbit555 · 28 September 2008, 06:12

crevice corrosion ( stainless steel's form of rusting) http://www.corrosionist.com/Pitting_..._Corrosion.htm

codprawn · 28 September 2008, 12:51

Quote:

Originally Posted by Limey Linda

I believe your diagnosis to be incorrect as usual. I think you are saying that the failure was due to tensile stress caused by expanding ropes. Assuming that your U bolt was at least 3/8in. dia. it has a tensile strength of at least 20+ tons.
So when are you going to start making Bio diesel as you committed to last Jan.

or was this just more Codswallop.
We aere waiting for the results
Think again

Yes I am saying the failiure was due to the expanding ropes.

I have seen enough snapped bolts in my time to know the difference. They look exactly like a bolt that has snapped from over tightening or stretching.

Sideways forces would not have acted there as the bolt was in a hole over 1" deep and was glassed in - I had to unscrew them all the way out.

As to the biodiesel I gave up on it when the price of cooking oil hit £1.20 a litre - diesel is £1.19 at the moment. Used oil is pretty hard to get as that sells for over £0.45 a litre now.

I have a great solution now - I run on 30% neat used cooking oil - and I get contaminated diesel cheap - stuff that has been drained from tanks where people have stuck in a bit of petrol as well. The 30% oil takes care of any lube problems the petrol may cause. It saves me about £50 a week on fuel costs.

codprawn · 28 September 2008, 12:52

Quote:

Originally Posted by hobbit555

crevice corrosion ( stainless steel's form of rusting) http://www.corrosionist.com/Pitting_..._Corrosion.htm

NOT!!!

codprawn · 28 September 2008, 12:53

Quote:

Originally Posted by malthouse

I tried working out the dimensions from the photo, but the scale seems to be just out of shot.

I will get the calipers on it later!!!

iandl · 28 September 2008, 13:24

from the pictures it looked around the 12mm x 150mm .....It seemed to fail at its weakest point ...just where the crimping hammer forces the lugs into it .....

if you can find 12mm stainless U bolts that arent crimped let me know cause i'm looking for another .......they seem to call them 'sealed' and offer them in 8 and 10mm sizes .....they do make them cause i already have 1 which i got from our local chandler ...but he only had one and it was old stock he got rid of and didnt know the maker

I