Rules for Aluminum Hull Penetrations

Hi, this is my first post to this forum. I'm designing a bow thruster tunnel for an aluminum-hull houseboat. I have tried to find references in the ABS rules for aluminum hull penetrations, but the only things I've found so far are for steel. I am specifically looking for rules regarding doubler plate thickness and width, and appropriate welds for doubler plates, in aluminum. Can anyone here help me locate the relevant rules? Thanks.

 

Welcome to the forum, Ryan.
Could you show us a drawing or diagram of what you are projecting because it is very strange to have to put double plates, etc. for a transversal bow propeller.
This hole is not studied as a penetration but simply as a hole in the hull to be reinforced. The tube of the propeller itself can be sufficient as reinforcement, without the need for double plates.

 

Welcom to the forum Ryan,

As a general rule most Classification societies require 1.5t plate thickness iwo of penetration.s
We typically arrange them like this:



The doubler, which is 1.5t hull thickness, is the doubler you can see. It is arranged to allow a full pen fillet weld either side to the penetration. Also the aperture in the hull is just enough for the penetration allowing enough room for the exterior fillet weld to the penetration and also the doubler to the hull plate and no more.

In recent years LR, for some reason, no longer allow this method, despite approving it many times in the past.
In which case you merely extend the penetration beyond the hull plate to allow the same, fillet weld either side of interior/exterior fillet weld.



Also for penetrations, you need to support the item that is penetrating in 2-axis for stiffness.

 

I wondered when I saw the plates in close contact if crevice corrosion potential could be the reason that they went away from the standard that they used before.
Salt water, lack of enough oxygen to form a corrosion resistant surface?

 

I did query them on this, as this is a new development from LR.
They "kind of" eluded to such, but it was easy enough to counter any 'technical' argument they cited.

The joints are done with easy access and importantly fully welded. Crevice corrosion can occur when there is a "path", ie in-between a stitch/intermittent weld - a route for the water ingress.
Failing that the only other route the water may take is through the weldment via porosity.

Again, this only occurs if the two allows are dissimilar, such as 5000 series to 6000 series.
Welding 5000 to 6000 series there is always porosity, it is next to impossible to eliminate it, owing to the chemical composition of the 6000 series alloy.
What occurs is that with 5000 to 5000 welds there is no porosity - when you follow proper QA procedures. Which is the case of a doubler which is the same alloy as the hull plating.

But where the penetration - usually a pipe - has often been the more easily source 6000 series alloy, which is not allowed below the waterline, for its inferior seawater resistant characteristics - corrosion occurs. So firstly never use 6000 series for a pipe/tube that is below the waterline... that prevents 99% of the problem.

We did a lot of testing with different alloys and filler wires, and the results are very obvious:



You can see that the 5000 to 6000 series weld is riddled with porosity - not the the 5000 to 5000.

Thus given the history of endless boats we have designed and built (to many different Class societies), all using 5000 series tubes/pipes and the doubler being of same alloy, we have never experienced any issues of corrosion. Only when the yards have ignored our design drawing and selected 6000 series, has there been corrosion - but that is their problem/responsibility for ignoring our design.

Hence I think LR is basing their new position on yards that do not use 5000 series alloys for the penetration, i.e. taking a very conservative approach and assuming it will be a poorly selected alloy grade.
I have not given up and shall keep trying to persuade them to alter the rules to reflect the difference in alloy grade and their approval of such penetrations.

 

Thanks everyone for the responses. Thank you very much Ad Hoc for the examples, that is exactly the kind of reference I was looking for.