Steel or Aluminum

[scoobert]

I will be building my cat, unless i find a better idea the project will be started in the spring.
I have seen many pluses and minus about building in each.

Aluminum:
pluses:
no need to paint everything
looks, cool factor

minuses:
need to weld in a shelter
need to buy a very expensive welder
material costs
must keep an active corrosion system in operation
unnoticed corosion

Steel:
pluses
can weld outside
i own the welder already
cheaper material costs vs aluminum nearly 3 times cheaper


minuses
must paint every inch inside and out
must keep up paint
rust


myths:
aluminum is lighter: per volume, yes it is, but when you need strength it takes the same weight of aluminum to be as strong as steel



i am assuming steel would be easier to strip and repaint, as i could use a steel wheel on my drill, if done to aluminum it would cut a hole in the hull.
i also assume if there is an underwater strike against a hard object the steal would be less likely to get scraped due to hardness.
i assume steel would be an easier repair overseas as materials would be easy to find.
steel is easier to weld.

did i miss anything?
whats your opinions on both?

[44'cruisingcat]

It's not a myth. Aluminium IS lighter. Seen any steel aircraft lately?

[paulf]

Should you build with steel....Even if you paint, I would sand blast and flame-spray galvanize the Hull inside and out 100%. The zink coating on a clean tooth should last the life of the boat.
Paulf

[roverhi]

Think you really need to learn about the reality of living with a boat in salt water and how the designs perform.

Multis have to be built light for performance and structural survival. Doubt that you could build a steel multi at anywhere near the light weight required without cutting the metal thickness to tinfoil dimensions. It would be a 'holey' mess with the slightest bit of corrosion sending it to the bottom.

If you use the proper marine aluminum alloys, they don't need to be painted. FWIU, 50' is the cut off between aluminum and cored fiberglass for suitsbility in building out of alloy.

[scoobert]

remember they use to be cloth. strength is not a factor for the skin.
also there is no rust on the often unpainted airframe.
i am a pilot. i do pre-flights on the plane. i see all kinds of cracks in the ultra thin aluminum. lets not compare planes to boats. two different beasts.

[scoobert]

Originally Posted by roverhi

Think you really need to learn about the reality of living with a boat in salt water and how the designs perform.

Multis have to be built light for performance and structural survival. Doubt that you could build a steel multi at anywhere near the light weight required without cutting the metal thickness to tinfoil dimensions. It would be a 'holey' mess with the slightest bit of corrosion sending it to the bottom.

If you use the proper marine aluminum alloys, they don't need to be painted. FWIU, 50' is the cut off between aluminum and cored fiberglass for suitsbility in building out of alloy.

cat will be 65' and weight will not matter in my application as much as others.
this is not a sail boat. it is being built as a slow trawler cat.

[scoobert]

i have already planned to use thicker steel then is called for. i would like a minimum of 1/4" under the waterline. but i need to do the math on 3/8 too. if the weight is not too massive i would do that. i understand that unpainted steel can rust in salt water at a rate of up to 1/16" each year. at 1/4 inch i would have to miss it for 4 years.

[noelex 77]

Aluminium is much lighter than steel.
Steel is not a very practical material for a sailing cat. The weight is just too much for adequate performance. The thin gage steel that is needed is difficult to weld without distortion and the resulting craft has very little thickness in reserve to allow for corrosion.

If you are going to home construct out of aluminium you need to be an excellent welder. When buying an aliminium boat weld quality is vital. To be able to do it well you need to be a professional, welding aluminium everyday. People that weld steel professionally are generally not good enough.

However the hull cost is only about 1/3 of the total cost of a boat and the labour for welding is a much smaller fraction again. So having a hull professionally constructed and fitting out the boat yourself. Or working with a professional aluminium welder are all viable options that are worth considering.

A steel boat can be safely constructed with normal welding ability.

[motion30]

I think steel is a loosing deal . Listen to those here that own steel boats, Cheaper in the short run much more expensive in the long run

[neptunesjester]

Aluminum does not require active cathodic protection. Properly zinced marine grade aluminum alloy lasts damn near forever or will at least out last me.

I had to remove my active cathodic system from my AL vessel when it malfunctioned and started eating the hull. I've just used regular zincs since then. I know the purists will say I should be using magnesium zincs, but 15 trouble free yrs and counting in saltwater with this setup demonstrates the hull is adequately protected..

Later,
NJ

[Matt sachs]

The ultimate strength of aluminum 6061-T6 is 40,000 PSI and it weighs .0975 pounds per cubic inch. A36 steel yeilds at 36000 PSI and weighs .283 pounds per cubic inch. I'm comparing apples to oranges here but it makes the point I think. Built with aluminum to the steel standards the boat is many times stronger than steel.

[noelex 77]

Originally Posted by neptunesjester

Aluminum does not require active cathodic protection. Properly zinced marine grade aluminum alloy lasts damn near forever or will at least out last me.

I had to remove my active cathodic system from my AL vessel when it malfunctioned and started eating the hull. I've just used regular zincs since then. I know the purists will say I should be using magnesium zincs, but 15 trouble free yrs and counting in saltwater with this setup demonstrates the hull is adequately protected..

Later,
NJ

Zinc is the correct anode material for aluminium in salt water. Zinc will not work on any boat in fresh or brackish water, however.

Alumium boats are often not fitted with any anodes other than the normal propsaft zincs.
If there are disimilar metals in contact with hull underwater, such as bow thrusters and hydraulics (such as Ovni with their retractable rudders) additional hull zincs close to these locations are necessary.
Some manufacturers do fit hull anodes like those seen on steel boats (although much fewer in number than would be seen on a steel boat), but just as many do not.

Active systems are not generally used on pleasure boats.

[scoobert]

Originally Posted by noelex 77

Aluminium is much lighter than steel.
Steel is not a very practical material for a sailing cat. The weight is just too much for adequate performance. The thin gage steel that is needed is difficult to weld without distortion and the resulting craft has very little thickness in reserve to allow for corrosion.

If you are going to home construct out of aluminium you need to be an excellent welder. When buying an aliminium boat weld quality is vital. To be able to do it well you need to be a professional, welding aluminium everyday. People that weld steel professionally are generally not good enough.

However the hull cost is only about 1/3 of the total cost of a boat and the labour for welding is a much smaller fraction again. So having a hull professionally constructed and fitting out the boat yourself. Or working with a professional aluminium welder are all viable options that are worth considering.

A steel boat can be safely constructed with normal welding ability.

yes, it will be home built, completely.
our goal is to finish the hull, power and drive systems, have the finish work done overseas. EG cabinets, hardwood floors, seating.
i have never welded aluminum. my welder will not accept a alumium kit, and the proper welder would cost me $5000. i would also have to learn to weld it, build a shelter, i would be out $20,000 before i could even start my first weld. with steel, many parts can be gotten down the road from me. i cant even find a 5383 alu supplier. so i would have to save up the $150,000 for the raw aluminum just to start the job. where as $2000 in steel will take me a long way.

[scoobert]

Originally Posted by Matt sachs

The ultimate strength of aluminum 6061-T6 is 40,000 PSI and it weighs .0975 pounds per cubic inch. A36 steel yeilds at 36000 PSI and weighs .283 pounds per cubic inch. I'm comparing apples to oranges here but it makes the point I think. Built with aluminum to the steel standards the boat is many times stronger than steel.

Steel vs. Aluminum

[Matt sachs]

yhea, what he said...