Stainless screw heads shearing off.....

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mrgrumpy2

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I have just antifouled my sports cruiser and noticed that two of the eight Stainless screws that fix the starboard trim tab to the underside of hull have had their heads broken off.....one is also broken off on the Port trim tab. I've also noticed that the small anodes on the trim tabs are particularly worn and I'm worried that this could be the cause. Do you think I should try and replace all off the remaining screws? My final question is whether it will be ok for me to drill additional holes in the trim tab hinges and then put three new screws into the hull....I dont think I will be able to get the threaded section of the 3 screws out .....Thanks in advance.
 
underside? I thought they were supposed to be fitted on the vertical of the transom, having a 90 degree hinge, that way the screws are protected and there's no step at the rear of the hull. Of course I could be dead wrong but that is the way ours are fitted as are most others I have seen. Also the anodes should be changed when half of the material has gone.
 
Thes fastenings may have failed due to crevice corrosion. When you replace them, in new holes where necessary, be sure to use sufficient sealant to block any crevices that could lead to this. It would aslo be wise to remove renew and seal all the remaining fastenings as well. Expect a few more heads to go before you have finished.
 
The hull has a recessed area on both sides in which the trim tabs sit....The hull therefore offers some protection to the fittings.
 
If you lift the trim tab flange from the hull re bed that on sealant as well as bedding the screws on sealt and make sure you use A4 or 316L screws.
 
The anodes on the trim tabs have just been replaced because the previous pair had almost fully eroded. This I suspect is why the problem has arisen....I am now worried that over time that the remaining screws will continue to break off and therefore they all need to be replaced now, but I'm fairly sure that when I try to unscrew them the heads will shear as soon as I apply pressure with a screwdriver. There must also be a limit as to how many fresh holes I can drill in the plate and into the hull as it will have a weakening effect on both. Bolts would be a better solution but there is no access from inside due to the shelves that sit the generator and waste tanks......This is proving to be a real conundrum.
 
Stainless steel fastenings should not be used below the water line for starters. One of three things may be going wrong: 1 as suggested crevice corrosion - the result of using SS in this location, and is caused by exclusion of oxygen preventing the surface from forming a protective oxidised film. The screw ends will appear pitted in the break area. The bttom of the pits may appear blackened.

2. The trim tabs may be moving slightly against the screw heads, causing the heads to flex. The heads could have broken off because of metal fatigue. SS fatigue failure is very sudden, and is often impossible to detetct until failure occurs. Examine the broken screw end to see if there are a series of rough lines across it. You may need a magnifying glass to see them.

3 The screws may have been overtightened in the first place. In this case the shank of the screw just below the head twists and is weakend so that when it comes on load the head pops off. In this scenario the metal will appear torn but not discoloured.
 
You are correct in having ss below waterline here. 316 Sounds like corrosion, obviously. Remove the tabs and maybe some more will break off. Try removing the bust screws using an extractor device see for example Axminster on line tools catalog. If not successful smooth them to the hull and se if you can refix the tabs 5mm to one side. If that isn't possible it is perfectly fine to drill new hole in the tabs for re-fixing them. Use fatter screws so they take longer to corrode. and bed down on sika 296

Best of luck
 
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