Steel or Aluminum? And I love both.

Originally we had planned on building in aluminum but the price of aluminum would have required waiting another year for us to have enough cash. Cost was the most important factor to us because the other differences in the end were a wash.

Metal Estimate

Originally thinking in aluminum it is ideal to get the plates wide enough to avoid as much welding as possible. In aluminum everything would be 3/8 inch except the pilot house which would be 1/4 inch.

Hull: 15 wide x 75 long x 2 sides; 16 plates – 8 x 20ft
Deck 16 x 70; 8 plates – 8 x 20ft
Bulkheads 8’x 16′ x 5; 5 plates – 8 x 20ft
Keels 20′ long x 2 deep x 2 sides x 2 keels; 2 plates – 4 x 20ft
Skeg & Rudder 15 long x 8 deep x 2 sides; 4 plates 4 x 20ft
Pilot House: 15 x 16 roof + 4 x 62′ perimeter; 2 – 8 x 20 and 4 – 4 x 20

When converting to steel, the wide plates are no longer important but steel comes off a roll so lengths up to 65 feet are available if you don’t mind paying extra for the oversize truck to deliver them. If I could get 75 foot long sheets it might be worth considering, but I chose 45 ft sheets because they fit on a standard truck and allowed the butt joints in the hull to be staggered.

The pilot house will still be aluminum on a steel boat; so to convert the aluminum sizes to steel I just took the square footage for all of the 3/8 inch plate which came to 5,120 and divided that by 6 ft, which is currently the widest steel plate available without having to pay extra and that comes to 854 feet. Divide that by 45 ft plates and it comes to 18.9, but 20 is a nice round number. In July of 2008 the price was 61 cents a pound delivered, plus tax. It would have been half that cost if we had purchased in January, but that is spilled milk.

In addition to plate we also need 1000 feet of 2 1/2″ to 3″ Sch40 for the bulwark and for rounding the corners. We chose to go with 3 inch. It’s considerably more but I have bent 2 1/2″ pipe and never bent a 3 inch pipe yet. Also 800 ft of 1/4 inch, 2 x 2 inch angle that will be used for ribs and stiffening the deck. We could go with 1/4 thick, 3 inch flat bar too, but I think banging your head against the flat of an angle is much nicer that the edge of a flat bar.

Comparison

Mild Steel

Aluminum

Material

Hot Rolled 1/4″ in 6 x 45ft plates

3/8″ 5086 H116 8ft x 20ft

Hull plate cost,
July 2008

$36,456

$76,800

Weight

10.2 lbs. per sq. ft. of 1/4 inch.
Steel is heavy. The only good thing about that is a heavy boat tends to roll less, but you can always add more ballast to an aluminum boat and that weight will be lower and do the same job better.

5.19 lbs. per sq. ft. of 3/8 inch.
Hands down, aluminum boats are going to save weight, which translates to more cargo, less fuel, and more speed. Some of this weight savings will need to be used to reinforce around welds and in areas of vibration.

Endurance

Yield Strength: 36,000 psi
Ultimate strength: 60,000 psi
The difference between the yield and ultimate strength represents the “plastic range”. Steel is stronger that aluminum by volume, but when you make the aluminum 50% thicker it is stronger than than the steel.

Steel has superior abrasion resistance. Drop a sharp object on steel and you will just get a scratch in the paint. Steel is much less susceptible to fatigue due to vibration.

Yield Strength: 30,000 psi
Ultimate strength: 45,000 psi.
To compensate for it’s lesser “plastic range”, aluminum hulls are made150% thicker than steel. The thicker plate provides additional stiffness and a hull that is about 30% less likely to dent and about 12% stronger before it fails.

It is easier to mar the surface of aluminum with a pointed object, like the end of a pipe. Additional reinforcement is required around engine beds and chain plates to reduce fatigue from vibration.

Cutting

Quick and easy provided you have a plasma cutter. Slow and messy if you have to use a torch.

Cuts perfectly clean with normal carbide bit blades in any wood working tool.

Welding

Stick weld to pull the hull together and then finish it with either stick or wire welding. A stick welder power source, a long welding lead, and a suitcase for wire welding with shielded wire will set you back about $4000. You will still need a spool gun for welding aluminum that will be used on the pilot house and hatches.

MIG wire weld is easy even for the novice provided you spend about $6000 for a welder like the Miller 350P with a 30ft push-pull wire gun. You will have to work in doors or wait for the right weather. It needs to be 90 degrees so that thermal expansion is not a problem, and you have to be protected from wind and rain. Welds on Aluminum are weaker than steel but backing bars can more than compensate for the loss of strength in critical areas.

Corrosion

One word: Rust. You will need to find, prep and paint chips and scratches soon after they occur.
Both steel and aluminum boats need to be protected by zinc anodes. But steel is much less susceptible to galvanic corrosion.

Corrosion can be a serious problem if you plan to stay in marinas where stray electrical currents are often a problem, as well as the presence of steel or iron in close proximity. Organic growth will hasten corrosion and bottom paint must be designed for aluminum.

Aluminum is very susceptible to corrosion from copper. A penny or piece of wire laying in a bilge can eat through a hull in days. Any non aluminum metal laying against the hull will be a problem. Fasteners must be plastic or stainless steel. For this reason bilges are often painted to help insulate them.
This is a big drawback for us, because our boat will have a shop in the cargo hold for metal working including a welder and a plasma cutter. The spray from a plasma torch will immediately burn into the surface of aluminum impregnating it with steel which immediately starts to corrode.

Paint

Two coats of epoxy paint are required for both inside and outside after you have sandblasted the metal. Sandblasting a 70 foot steel boat will be around $6,000 which is a sizable chunk of the original savings on the material.

No need to paint the inside of the hull. Painting the outside is strictly for controlling the heat. No paint will stick really well to aluminum so plan on repainting every few years. No paint is an option but it will be hotter in the tropics. Only routine coats of bottom paint is required, but that is needed for steel too.