Life Expectancy of Alloy Hull?

[mattplowman]

Originally Posted by Andrew Troup

OP: Which yard?

Wanganui Boat Builders

[David M]

Originally Posted by downunder

yes please.

The entire hull has at minimum 6 layers of an epoxy barrier coat, Interlux 2000.

I do not use copper based bottom paint. You absolutely can't do this with an aluminum hull. The bottom paint I do use, Trilux 33, is not very effective, especially in the summer, but it is better than nothing. I have tried a few different aluminum compatible paints over the years and the Trilux 33 is the least worse of the choices. The TBT based paint I used to be able to use was far more effective.

There are 6 fish shaped anodes welded to the keel. Three on each side from bow to stern. I also use regular elliptical shaped anodes on the propeller shafts.

The electrical system is a floating system, meaning none of the boats electrical system is attached to the hull. The engine is floating as well. The battery pos and neg goes directly to the engine.

I have a galvanic isolator on my shorepower ground but I do not have an isolation transformer, never needed one. Only AC current can go through a transformer and not DC. The field windings on each side of the transformer are electrically isolated from each other, so DC can't pass. It's a magnetic field expanding an collapsing that makes a transformer work...not a direct wire passing from one side to the other.

All my thruhulls are stainless steel. I know this sounds like heresy, but I tried the plastic Marelon thru-hulls without much success. The handles either broke off or the valve seized or both. What I do is thread the stainless steel valves onto aluminum pipe nipple using Tef-Gel or the medium strength 3M 4200. I then coat the inside of the pipe nipple with Interlux 2000 so the water does not "see" the aluminum to stainless steel interface. The valves are not bonded to each other or to anything. There is no corrosion whatsoever, and this boat has 12 below surface thru-hull valves. Do not consider using bronze valves, which is much more reactive with aluminum.

[Blue Stocking]

David M,

What are your thoughts on Mercury-type float switches in an aluminum hull.
Myth or fact?--a mercury spill from a broken vial will rapidly corrode/erode the hull plating.
Know of a 65ft CC roamer that was almost lost because of this--or so the captain says.

[surfmachine]

Hi, I have a 21 year old Feltz skorpion, alloy, built in feltz shipyard, Hamburg.

Slipped her in May and found two weep holes where the keel joins the floor, has lead inside, so opened the holes a bit and cleaned out the void and dried it and welded over the holes. Was around where the compression post sits on the floor and lift lugs for the lead ingots are located. rest of the hull is great, above deck, no corrosion, a wonderful yacht! In the shallow bilges, lots of whitish powder where the paint has bubbled off, but only surface corrosion?

All other places, looks like new, much better than a steel yacht, who builds them now?? And much stronger than FRP, fibreglass, I really like the alloy hull, no leaks from through hull holes, all one sealed unit, like a submarine!!?

All modern work and small naval boats are alloy now.

Good luck from Keith.........

[noelex 77]

Originally Posted by Blue Stocking

David M,

What are your thoughts on Mercury-type float switches in an aluminum hull.
Myth or fact?--a mercury spill from a broken vial will rapidly corrode/erode the hull plating.
Know of a 65ft CC roamer that was almost lost because of this--or so the captain says.

The corrosion would only be slow, and confined to the small area of the mercury spill. It also requires moisture. It is helpful to keep the bilge of a metal boat completely dry, which is what we do.

[SVPP]

I am thinking looking at buying this

SLOOP ALU 17M EXPEDITION for sale

any comments are appreciated

thanks Paul

[anontrolus]

A couple of thoughts on aluminum hulls:

- a some point some one will tell you how much money you could save by glassing the hull (but nobody will tell you how much glass you'll need)

- galvanic and stray current corrosion can be a bitch (I will never buy another aluminum hull without a complete interior below waterline survey)

- metal hulls are hot (curios to know if any aluminum boat owners have mitigated this issue with cork decks)

- all below waterline thru hulls will be plastic

- engine mounts need to be well engineered

- poorly engineered designs with poor welding results in fatigue failure and cracked welds

[noelex 77]

Originally Posted by anontrolus

- a some point some one will tell you how much money you could save by glassing the hull (but nobody will tell you how much glass you'll need)

This would be a terrible idea.

Originally Posted by anontrolus

- galvanic and stray current corrosion can be a bitch (I will never buy another aluminum hull without a complete interior below waterline survey)

The best electrical practices and a survey are essential.

Originally Posted by anontrolus

- metal hulls are hot (curios to know if any aluminum boat owners have mitigated this issue with cork decks)

Nearly all aliminium cruising boats are insulated. Mine has two inches half inches of foam insulation. It is much cooler than a fiberglass hull with much less condensation.
Generally only the ex racing boats are not insulated.

Originally Posted by anontrolus

- all below waterline thru hulls will be plastic

Marelon should be used not plastic, but all expedition boats ( like mine ) place the seacocks above the waterline, with solid aluminium tubes below the waterline.

Originally Posted by anontrolus

- engine mounts need to be well engineered

The engine bearers on an aluminium boat are very easy, just weld them to hull where wanted. The engine is bolted to these. Metal boats have the easiest most rigid engine mounting system.

Originally Posted by anontrolus

- poorly engineered designs with poor welding results in fatigue failure and cracked welds

The aluminium used and the quality of welding is critical. Avoid home built boats unless your absolutely sure of the quality of the welder.

[noelex 77]

Originally Posted by SVPP

I am thinking looking at buying this

SLOOP ALU 17M EXPEDITION for sale

any comments are appreciated

thanks Paul

It looks like a great, very tough, boat Paul.
There are not many details on the website about the construction details.
Have you got any more information?

[Lagoon4us]

Aluminium is a fantastic medium to build a boat from, what will lead an ally hull to fail.
INSIDE:-
1) Corrosion inside due mainly to contact of dissimilar metals = insulate with plastic sheet. i.e. if a bolt is used paste with 'DURALAC' liberally.
2) Corrosion inside due to battery acid or fume, be very careful of the yellow electrolytic paste used to raise conductivity on terminals = deadly and quick corrosion.
3) Concentrated salts from a leaking hose especially if the leak is carrying oxides from dissimilar metals, ie a dodgy hose clamp of inferior grade.
4) You must electrically connect all the ally tubes or pipes used for drains etc by running a wire under the hose end to the other hose end.
5) Dust/dirt/hair/fluff when mixed with moisture will set up a cell of corrosion.
6) Dont inbuild water or sullage tanks into hull use fibreglass only.
7)Run seperate earths.
8) Use nylon composite skinfittings and seacocks.

OUTSIDE:-
1) Once again insulate with plastic sheet/DURALAC every dissimilar metal connection.
2) If painted repair chips, a metal hull must be maintained as corrosion cells get in under the sharp chipped paint edge and oxide will bubble forth. (STEEL SAME PROBLEMS)

Never fibreglass an ally hull, it's NOT a core material.

Ally requires oxygen to form an oxide to defend from further corrosion, if painted it must seal 100% this i guess is the best tip to keep aware of.

Get an anode specialist, in Australia i use 'Wilson and Walton' to calculate anode requirement and measure latent/stray voltages. They used to give you anodes to drop into marinas attached electrically to the boat to prevent stray currents from underwater cables and marina wiring from actively stuffing your hull up.

Good design will alleviate cracking ie panel strengths etc.dodgy welding is probably the greatest let down, i have 40 years experience in ally and get quite shocked at some welders methods ie Cold welds, welds not running around the end of a stringer, frame or bracket and say 1 inch up the opposite side, if not done cracking will start there.

I cannot put a life on a good well built and maintained hull, indefinite really.

Cheers