Over plating an aluminum boat

[ezyl74]

Hi I am the proud owner of a bit of a classic. 1962 Aluminum Cruisers Inc. Power catamaran. 38' x 13'6" welded aluminum project. The hull plate thickness is compromised thanks to years of neglect and corrosion throughout the entire exterior of the hull. My ?? I want to overplate the old hull plate with new plate. I would like to know the exceptable methods and welding techniques. I also need to know what adding aprox 6000lb of new hull plate and enclosed flybridge are going to do to my cat, and to top it off I am thinking about outboard motors instead of the inboards.

[Perm Stress]

You are in for quite a project...
Seek full design for modernization from a qualified NA.

[ezyl74]

I went down the NA road on another project. Not a good experience.

[CaptBill]

Use an aluminized coating. Use as many coats as needed to add some mil thickness. You will have a strong, continual bond. Should be better than new without adding tons of weight. You are basically painting on a layer of aluminum.

http://www.epoxyproducts.com/aluthane.html

[ezyl74]

I will look into it thanks

[Frosty]

It cant be all rotten,--can it?

It is not too difficult to make a plate that covers the part you want to repair and scribe around it and cut it out, then weld in.

Dont do it the other way round like cut out a hole in hull to match the plate you made,--- Duh--hard work.

[Perm Stress]

Originally Posted by ezyl74

I went down the NA road on another project. Not a good experience.

NA' not always say nice words about client's wishes...

"... I also need to know what adding aprox 6000lb of new hull plate and enclosed flybridge are going to do to my cat, and to top it off I am thinking about outboard motors instead of the inboards."

Do I understand correctly, that you want to weld new plates OVER existing and corroded old ones? If so, it is not a good idea. For more than one reason.

Now, you also want to add flybridge= lots of weight and windage high up. That's a good way to explore the limits of reserve stability; it is much safer to do this research on paper first.

You also want to install outboards instead of inboard motors. If existing motors have conventional shafts, not stern drive legs, this mean quite a lot of structural work to massively reinforce the transoms. Outboards also will change the trim towards more stern down. This may be important and may be not.

A little too much NA/Structural stuff to handle with a thread in a forum.

[ezyl74]

No it is not all bad. Some spots are pitted and some spots are through. It is kinda all over, I think it would be more work to cut patches than to cover the whole thing.

[ezyl74]

My thoughts were to go over original plate and some people I have talked to say ok and others are strongly opposed. I am trying to sort it out. The boat came in a couple arrangements. 1 of which is a full fly bridge. The big difference is my fly bridge is hard sides with glass. More weight but simmilar windage.

[Frosty]

Then go over the whole hull,--without welding to the hull, just tacking. When finished cut the old hull out. Or buy a new boat!!

Weight kills a cat.

[ezyl74]

I think after hearing some opinions that the thing to do is cut off old plate and weld new plate to old framing. No extra weight or crevice corrosion issues. Thanks but I will keep the boat its really cool!

[Perm Stress]

Welding over, also known as "doubling" is highly advised against (ant virtually excluded by some of them) by Classification Societies even in steel boat/ship building, where crevice corrosion is much less of an issue.
New plates on old framing is tried and tested repair method for metal hulls.
It ask for dismantling of interior, however...

[ezyl74]

I will follow the new plate on old framing method. The interior is already out. Thanks for your help

[Perm Stress]

You welcome.

[MikeJohns]

Originally Posted by ezyl74

I will follow the new plate on old framing method. The interior is already out. Thanks for your help

Would it be possible to post some pictures of the interior condition for interest.

When you re-plate alloy boats you need to be mindful of good alloy design practice and follow it when re-building. Use the strain tempered grades odf marine alloy which are a lot more forgiving of welding property changes than the heat tempered grades.

There were a few batches of dud alloy produced in the US in the 70's I think. The USCG had info on which batches were compromised and a list of bots that were built with it, seems they got the alloy wrong.