Steel vs Aluminum
Steel or Aluminum? And I love both.
Originally we had planed on building in Aluminum but the price of
aluminum would have required waiting another year for us to have
enough cash. So the cost was the most important factor to us
because the other differences in the end were a wash.
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
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
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 choose 45 ft sheets because they fit on a standard truck and
will allow 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 is 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
banging your head against the flat of an angle is much nicer that
the edge of a flat bar.
||Hot Rolled 1/4" in 6 x 45ft plates
||3/8" 5086 H116 8ft x 20ft
|Hull plate cost,
||10.2 lbs. per sq. ft. of 1/4
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 wait will be lower and do the same job
|5.19 lbs. per sq. ft. of 3/8 inch.
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.
||Yield Strength: 36,000 psi
Ultimate strength: 60,000 psi
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
|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 mare 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
||Quick and easy provided you have a
plasma cutter. Slow and messy if you have to use a
||Cuts perfectly clean with normal carbide bit
blades in any wood working tool.
||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 suite case 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.
||One word: Rust. You will need find,
prep and paint chips and scratches soon after they
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 say in marinas where stray electrical currents
are often a problem, as well is the presence of
steel or iron in close proximity. Organic growth will hasten
corrosion and bottom paint must be designed for
Aluminum is very
susceptible to corrosion from copper. A penny or
piece of wire laying in a bilge and 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 shop
in the cargo hold for metal working, 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
||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 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.