Aluminium Hull

[Yinka]

Hi, I'm a cat sailor looking for wise words on the pro's and cons of aluminium hulls.

I plan to sail for about 20 decades exploring the islands of the Pacific and the Indian oceans.

Thats going to be a lot of shallow water, a lot of reefs and a lot of hot sun.

My main interest in Aluminium is mostly concerning it's strength and durability if scraped or

struck.

Hmmm... what say the wise heads ?

Happi days

Yinka

[Lighthouse]

Welcome aboard "Yinka" - good to have you here.

Be interested to see what others say but for me, it is best to avoid any scrapes and knocks with any hull matarial. I think that if you do have a scrape, it is probably easier to repair GRP/Glass than Aluminium in those out-of-way places.

For what it's worth, we sailed the Indian Ocean Islands with a steel hull for a few years and never touched anything.

Welcome aboard.

[MMNETSEA]

Check these links for info on marine grades of Aluminium

5083 Structural Marine Aluminium

Sealium

[Jan]

Hi Yinka,

We circumnavigated in an aluminium yacht (www.pinical.nl). It did matter to me before we left, but now it doesn't really matter anymore.

I don't think it does matter that much. There are a lot more critical issues. Seaworthness depends on details, ALL details, of ship (and crew). Regarding the hull that's i.e. the sea cocks, rudder mount, keelconstruction, etc. etc.

This site has a readable article about hull materials: http://www.windpilot.de/en/Ra/rayacen.html

(No shares; just a satisfied customer).

Fair winds,

Jan

[Trim50]

Having some metallurgical background, I find it mind boggling that anyone would place aluminum in contact with sea water and call it home. The only thing that protects aluminum from corrosion is corrosion.

I know there are some hardcore aluminum hull people out there, but nothing in my mind can beat the specific strength, durability, ease of repair and cost of Glass-Epoxy composite. Also, the disadvantages of GRP listed in the previous link are completely misleading because they are all design and process related and not material related. Each one can be addressed by design.

[roverhi]

Aluminum can be the best and worst building material for a boat. The stuff survives very well in seawater without any protective coating. It was about the lightest building material until the new exotic fiber/resin composites became available. The French have been building 'naked' aluminum boats for years that have lasted without exterior coating or maintenance. Aluminum does not have the tensile strength of Steel but it's not needed for hull sheathing, for the most part. If you did run into something solid, aluminum will deflect and stretch but is very hard to pierce. It would be better to hit a reef with a steel boat but aluminum is a close second. one of the biggest advantage to aluminum, like steel, is it's leak free. No matter what you do, the hull to deck joint on FRP boats will eventually leak if it doesn't from day one.

The nightmare with aluminum is electrolysis. If you get in a 'hot' marina, close to a hot boat, or have a disimilar metal, aluminum can disappear in a very very short time. Some boats are just 'hot' in themselves. Had a friend who worked for the Driscoll yard in San Diego when they were building a lot of aluminum boats. He said they had a couple of boats they called 'fizzies' because they had electrolysis problems the minute they hit the water. You have to be very careful with electricity on an aluminum boat. It's not insurmountable, it's just you have to be very careful as a dumb mistake can sink a boat in a matter of hours.

I'm amazed our metallurgist thinks that there is anything unusual about corrosion protecting aluminum. Oxidation is what protects all bare metal. The problem with steel is it exfoliates, the corrosion constantly exposing new metal to rust. Aluminum, bronze, silver, lead, etc. all develop an oxide coating (corrosion/rust) that protects the underlying metal. It makes the metal nearly impervious to additional corrosion and works excellent as long as there is not constant abrasion that wears the oxide off.

BTW, we're talking marine aluminum alloys as non marine alloys don't like saltwater at all.

Aloha

Peter O.

[Yinka]

Hmmm... thank you 'wise heads' that was very enlightening. Knowledge dispels fear... (eventually).

Happi days

Yinka

[MMNETSEA]

Yinka

Check this web site:-

Aluminium Yacht

Here is an answer given By Jimmy Cornell to a question regarding Aluminium hulls :-



Do you have some suggestions regarding the prevention of hull corrosion by electrolysis or do you have other instruments other than the zinc anodes to face this potentially "lethal" phenomenon?

"I have had an OVNI 435 for 5 years – and sailed some 45.000 miles – and – thank God – have not had any corrosion problems. I have renewed the zinc anodes regularly (one which goes quickly – after about 6-9 months – is the propeller anode - I have a MaxProp). Apart from that all you need to do is regularly check the current leakage meter (Controle d’isolement) which is installed as standard on all OVNIs. Also, take normal precautions, such as never staying too long docked next to a boat that is made of steel, or boats that run their generator too much... by not too long, I mean not weeks. Also, it is very important when you connect to shore power in marinas that you make sure that the polarity is correctly wired – a lot of marinas are not (which doesn’t really matter on 220 Volt but apparently is not a good idea on an alu boat)."

(Jimmy Cornell of Noonsite fame - his excellent work "A Passion for the Sea" reviewed by Cruiser Log - pages 248 and 447 - also covers the subject further :-

He gives great emphasis to the use of Zinc anodes to be used on an aluminium boat and these to be checked on a schedule frequently.

Richard

[Trim50]

Originally Posted by roverhi

I'm amazed our metallurgist thinks that there is anything unusual about corrosion protecting aluminum. Oxidation is what protects all bare metal. The problem with steel is it exfoliates, the corrosion constantly exposing new metal to rust. Aluminum, bronze, silver, lead, etc. all develop an oxide coating (corrosion/rust) that protects the underlying metal. It makes the metal nearly impervious to additional corrosion and works excellent as long as there is not constant abrasion that wears the oxide off.

BTW, we're talking marine aluminum alloys as non marine alloys don't like saltwater at all.

Aloha

Peter O.

Yes, Peter...I've worked with aluminum (numerous alloys) for many years and have several patents for various alloys and processes. I know enough to never consider it for the hull of my vessel. US 7,036,550, US6,848,494 and US 6,997,232,

I guess it is really a matter of choice. In the final analysis, it is all a matter of electrochemical potential. GRP will alway win an FMEA comparison on this basis.

I'll gladly go toe to toe with anyone on the discussion of GRP vs Aluminum if they are willing to take the time.

[Lighthouse]

Stupid question - what happens if a couple of copper coins roll down into the bilge and are left there?

[Nausikaa]

As far as aluminium's strength goes I can vouch for it. I was absolutely amazed once, years ago, when I was serving on a small, aluminium coast guard cutter. We had received a distress call. A girl had gone through the ice in the archipelago where we were based. Her brother, mother and father had also gone out to try and save her and had also gone through the ice. Our little cutter was aluminium framed and hulled but with stainless steel propellers and axles.

We knew there was little chance of us finding any of the family alive but, as long as there was a chance, we pushed that little cutter to the edge of the envelope and beyond. She was never built to go in ice but she performed magnificently. Even the cooling water intakes remained un-blocked as we forced our way through the ice.

When it was all over we went directly to the shipyard and took up the cutter on the hoist. Anodes had been ripped off, the rudders damaged but the hull had held. Not a leak anywhere although forward of midships the shell plating had been severely stretched and pressed in between the frames. From the outside, every rib was visible. The engineers and dockyard technicians scratched their heads a bit and concluded that the best thing to do would be to leave the plating as it was and fair the hull with Plastic Padding - buckets of the stuff. That was over 20 years ago and the cutter, with its plastic padding, still looks good today, although it is no longer in government service but a private motor yacht.

Amazing stuff aluminium but because of the risk for electrolytic corrosion and the lack of repair facilities in many remote cruising areas it would not be my choice of material.

To end the tale I should tell what happened to the family. Unfortunately, despite our best efforts and that of others rescue did not come in time. An entire family was lost. A very sad thing indeed, especially as with a little training in ice rescue the daughter could have been rescued by the parents.

Aye // Stephen

[Swagman]

All materials have benefits - and I've yet to see a GRP destroyer.

We had a race yacht constructed of aluminium for 4 years and one soon got used to the protective actions required to minimise corrossion.

But do have to say it is probably the one material I'd prefer to have around me if we ever hit the bricks. Like it or not aluminium can take some pounding other materials - including wood and steeel and most certainly GRP or ferro-cement - would hate.

Check out the materials used by most river racing powered craft who expect the bounce off things - and you'll see what I mean.

Cheers

JOHN

[seaescape]

Sorry but I don't know to much about electroisis and I have got an Alii boat so I should learn something about it.

My yacht is 20 years old and has no corrosion below the water line at all. She under went a hull ultrasonic test in 2004 (I think it was) and is reported in a1 condition. I hauled her 12 months ago to do the bottom paint and while talking to others around the yard I found that most boats up on the hard were doing the same.

My ketch looks like it is GRP due to what I think is an excellent fairing job (check the members gallery) so I got alot of questions on weather I had any OSMOSIS is her. I had heard some scary stories on this prior to buying my yacht. So it formed my decision and reasoning on buying Ali and not grp. The age of a boat that I could afford was also a big factor. I found that age is also a determining factor in osmosis and to that, how has the boat been looked after under the water.

Back to bottom painting (and yeah I have to use special paint) all those other guys doing their bottoms were confronted with some degree of OSMOSIS, I don't know where this comes from, I guess the same for everything in salt water (its those dam saltwater fairies) but I do know it can destroy a boat without being able to see it or it can be very costly to repair if you can't do it yourself.

As for copper coins in the bilge, I don't know, Keep the salt water fairies out and check for electricity leakage.

So here I sit hoping that my keel doesn't fall off.

Always Learning.

Kevin

[Nausikaa]

Originally Posted by seaescape

As for copper coins in the bilge, I don't know, Keep the salt water fairies out and check for electricity leakage.

There are so many stories about copper coins or brass keys dropped in the bilges of aluminium boats forming a perfect coin/key shaped hole in the hull. Unfortunately, there is a lot of truth in them.

If your bilges are bone dry, cobwebby and dusty as a Pharaohs tomb then nothing will happen but if they are wet (and how many bilges are not wet?) then putting two dissimilar metals together in salt water will generate small electrical currents which will cause galvanic corrosion in a metal hulled vessel.

Galvanic corrosion is caused by an electric current generated by two different metals in an electrolyte (any substance containing free ions that behaves as an electrically conductive medium) such as seawater.

Differences in metal potentials produce galvanic differences, as can be seen in the following galvanic table. If an electrical circuit is made between any two of these metals in an electrolyte, current will flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate will be. Metals or alloys at the upper end are noble whilst those at the lower end are active or anodic. The more active metal is the anode or the one that will corrode.

Exactly the same thing happens between the metal parts of a boat which are underwater. That is why we place zinc anodes on our boats. Look at the following table and you will see that zinc is way down, almost at the bottom, of the galvanic series and is, thus, a very good anode (in this case a sacrificial anode) which will protect metals higher up on the scale from galvanic corrosion.

NOBLE - LEAST ACTIVE

Platinum

Gold

Graphite

Silver

18-8-3 Stainless steel, type 316 (passive)

18-8 Stainless steel, type 304 (passive)

Titanium

13 percent chromium stainless steel, type 410 (passive)

7NI-33Cu alloy

75NI-16Cr-7Fe alloy (passive)

Nickel (passive)

Silver solder

M-Bronze

G-Bronze

70-30 cupro-nickel

Silicon bronze

Copper

Red brass

Aluminum bronze

Admiralty brass

Yellow brass

76NI-16Cr-7Fe alloy (active)

Nickel (active)

Naval brass

Manganese bronze

Muntz metal

Tin

Lead

18-8-3 Stainless steel, type 316 (active)

18-8 Stainless steel, type 304 (active)

13 percent chromium stainless steel, type 410 (active)

Cast iron

Mild steel

Aluminum 2024

Cadmium

Alclad

Aluminum 6053

Galvanized steel

Zinc

Magnesium alloys

Magnesium

ANODIC - MOST ACTIVE

So, anyone the wiser? Electrickery is just another name for black magic!

Aye // Stephen

[Trim50]

Originally Posted by Swagman

All materials have benefits - and I've yet to see a GRP destroyer.

John,

I have only found one substance that can destroy GRP, and that is Methanol. It takes a couple of days, but pure MeOH will turn most cross linked polymers into mush.

Ken

[Swagman]

Originally Posted by Trim50

John,

I have only found one substance that can destroy GRP, and that is Methanol. It takes a couple of days, but pure MeOH will turn most cross linked polymers into mush.

Ken

Ok Ken I'll try again.

Never seen a GRP Frigate.

JOHN

[Trim50]

Originally Posted by Swagman

Ok Ken I'll try again.

Never seen a GRP Frigate.

JOHN

Ahh frigate about it

[JeanneP]

Now, John, you know as well as I do that the choice of steel for building large ships has less to do with the material with the greatest strength to weight ratio than with the ease of construction. Can you imagine a hull mold for a frigate?

When we stopped at Beveridge Reef in the S. Pacific, a completely submerged atoll, there was a modern GRP fishing boat up on the reef, had been there for about 6 months or so. About 60' long, the hull was two pieces with a bolts joining the forward and after parts of the hull. It had gone up on the reef during a storm when it had (reportedly) lost its electronics (though they called for help and were rescued, so they hadn't lost everything - there wasn't any civilization within VHF hailing distance). The force of the seas pushing the boat over the reef ripped its huge propellor off (the windward/seaward edge of the reef was as wide as a two-lane highway, barely awash at low tide, at most 3 feet below sea level at high tide), and the reef did put a hole in the side of the boat, but not a large one, and that might have been the result of six months of pounding and storms rather than damage incurred when it first went up. Salvage was near impossible since the boat was sitting on the reef with at least 60 feet of reef separating the boat from clear water on all sides. Nothing could get to it except a dinghy at high tide.

Back when Peter and I were restoring old buildings, we had to replace a support beam. The 100+ y.o. wooden beam was about 8 inches square. In order to get the same load-bearing capacity we had to go with a 12" steel beam and still had to add an additional support halfway along its length. (dimensions might not be exact here - it's been 30 years since we did the work).

Steel is not necessarily the strongest or best construction material, but often the easiest to work with, especially on the scale of military vessels or freight carriers.

[Nausikaa]

Originally Posted by Swagman

Never seen a GRP Frigate.

No, nor have I but I have sailed on a minehunter which was built of GRP and she was 48 metres long with a displacement of 360 tonnes.

It is horses for courses. The minhunter, even though she was the size of a corvette, was built in GRP to be non-magnetic. Of course, aluminium would have worked as well but at what cost?

Jeanne is of course right. It is the cost of building which, oncce basic parameters of strength etc have been met, that dictate the material to be used. I suppose, if cost was no object we would all be sailing boats made of titanium.

Aye // Stephen