Why did this metal fail?

[rotrax]

I cannot imagine leaving my outboard on the dink when sailing, and we have heavy duty American made Kato davits.
IMHO it is asking for trouble.
Keeping any dink from swinging in all directions while on the davits is a bit if a job. Having the motor on one end just makes it worse.

 

[john_morris_uk]

Just to add to the loading of davits comments. When I fitted our davits, the instructions were that the entire weight of the dinghy plus outboard should not exceed the maximum capacity of ONE davit. (In our case that's 85kg)

We do leave the outboard on the dinghy as the combined weight is well under the limit. We've had the davits for years and wouldn't be without them.

 

[auditdata]

Thanks for all the comments and advice. Just to conclude a local shop drilled out the holes, straightened the piece for 15 Euro same day service. It was remounted with the top bolts reversed. OK not as aesthetic but from looking at the posts it is likely to be stronger than the original.
Thanks

 

[nigelmercier]

As said above, the rating for the brackets is the static load, this will be exceeded many times over in a bit of chop.

I hope there is a big plate behind those screws, not just washers.

 

[nimbusgb]

As said above, the rating for the brackets is the static load, this will be exceeded many times over in a bit of chop.

I hope there is a big plate behind those screws, not just washers.

+1 without a plate the problem will just move to another area ........ tearing the bolts through the hull!

 

[auditdata]

Yes there are plates.

 

[PCUK]

Thanks for all the comments and advice. Just to conclude a local shop drilled out the holes, straightened the piece for 15 Euro same day service. It was remounted with the top bolts reversed. OK not as aesthetic but from looking at the posts it is likely to be stronger than the original.
Thanks

But presumably still overloaded?

 

[penfold]

Overloaded but a horrible design in the first place. I'd bet a couple of quid that the rust on the welded on tube is emanating from a crack at it's junction with the weld. The screws (welded because it would have been difficult to get a screw in once the v was welded?) just happened to beat it to failure.

+1
Terrible design and terrible realisation; the fasteners have failed through what looks quite like stress corrosion cracking right where the weld bead meets the shank of the bolt. Welding a drawn component like a bolt will compromise its strength and resistance to cracking, add on some salt water and stainless steel's propensity for crevise corrosion and it's bound to fail eventually.

 

[vyv_cox]

I see no evidence of any type of corrosion. High stress, low cycle fatigue initiated from several locations looks most likely to me but very difficult to tell on the photographs provided.

 

[30boat]

I see no evidence of any type of corrosion. High stress, low cycle fatigue initiated from several locations looks most likely to me but very difficult to tell on the photographs provided.

I agree.There is severe undercutting at the weld.This created a lot of stress points that would have initiated the cracking.

 

[auditdata]

But presumably still overloaded?

I can see from here the outboard is now on the transom!