bolts into my cabin roof

[blackrose]

I've bolted stuff to my roof with M5 brass bolts tapped straight in, and sealed with marineflex.

 

I've done the same thing and put bolts in sealed with Marineflex.

[Ron T]

As Mickey44 said brackets would seem the best bet.Less heat with the welding and no holes through the deckhead.

[David Schweizer]

No.

 

The silicone is the wrong material- it is a very poor sealant in thin layers and will leak sooner or later. A better solution would be to keep the neoprene gasket under the brackets and put a bonded seal (dowty washer) under the washer beneath the bolt head. Alternatively some non-setting butyl rubber (like Seamseal CV) would do both as a gasket and as a bolt sealant.

 

Tapping the bolts into the roof is a good concept- but will only work if the roof is more than usually thick. M8 has a thread pitch of 1.25mm ( M8 fine is 1mm) so into even a 5mm roof ( most are 4 mm or thinner) you will not get more than 4 full threads. A standard nut is 6.5 mm thick and designed to be weaker than the bolt thread, so I reckon you need at least a total threaded thickness of 8mm. I think you risk stripping your M8 threads out just tightening the panels up properly and certainly they are not going to stand much wind loading unless there are an awful lot of them. The best thing to do would be to put proper nuts and washers, and some butyl sealant if you like, on the inside. If you cannot get to the inside have some doubling patches at least 6 mm thick welded to the roof and then tap into these.

 

If the holes in the brackets are small enough, or you can get some top-hat washers to reduce the hole, then M6 is 1mm pitch and the nuts are 5mm thick so you may get away with that size tapped directly, but it's still risky on a 4mm roof.

 

If the brackets are stainless then A2 or (best) A4 stainless bolts would be the proper fastener, rather than BZP passivated.

 

 

Regards

 

N

Whilst I accept the theoretical basis of your arguement, personal experience suggests that your concerns may be unfounded. I fitted four side steps to the 5mm cabin sides on our boat, each with two M5 stainless steel countersunk machine screws. Fifteen year later they are still securely attached despite my 14 stones being regularly imposed upon them.

[dor]

My panel was fitted with brackets using M4 SS set screws into tapped holes in the 4mm roof. I had similar concerns about security.

 

 

Note brackets on the corners to stop ropes getting caught underneath.

 

The installation was written up in March Waterways World - I can let you have a copy of the article draft if you are interested.

 

Whilst I accept the theoretical basis of your arguement, personal experience suggests that your concerns may be unfounded. I fitted four side steps to the 5mm cabin sides on our boat, each with two M5 stainless steel countersunk machine screws. Fifteen year later they are still securely attached despite my 14 stones being regularly imposed upon them.

 

Agree absolutely! Same situation for me.

[BEngo]

Whilst I accept the theoretical basis of your arguement, personal experience suggests that your concerns may be unfounded. I fitted four side steps to the 5mm cabin sides on our boat, each with two M5 stainless steel countersunk machine screws. Fifteen year later they are still securely attached despite my 14 stones being regularly imposed upon them.

David,

 

FWIW my brass steps are secured with 3/16 WW brass csk machine screws (coarser threads and weaker screws than yours) but which have now done 23 years, so I can see whence you come. However...

 

There are two significant differences from the OP's proposed application. The steps are fitted with M5 screws into 5mm sides. An M5 nut is 4mm thick, so the sides actually contain more threads than a standard M5 nut. The other thing is the load direction. On the step bolts it is principally in shear (across the screws). The whole of the screw core is available to resist this. I assumed the panels might be canted, so the load on a set of solar panels in a gale is likely to present a significant pull along the bolt which is only taken by the threads in the roof. I did suggest to the OP that he might be better off with more M6 screws as these more nearly suit the thickness available. Dor's M4 solution takes this further as an M4 nut is only 3.2 thick

 

Finally, solar panels are a lot more expensive than a step. I think any person using the step were it to fail (it shouldn't because of the reasons above) would get fair warning that it was dodgy. Failure of the panel fasteners when the owner is away is more likely.

 

 

 

N