It really comes down to the relationship between material density, tensile strength and bending/puncture strength.
Density:
-
Fiberglass: 100lb/cubic foot
-
Aluminum: 167 lb/cubic foot
- Steel: 488lb/cubic foot
Of course that comes with a
trade off in tensile stength (there is a lot more variability depending on specifics here both production of GRP and specific alloy of the metals):
-
Fiberglass: 43,000psi (basic fiberglass not carbon fiber or other high end)
-
Aluminum: 65,000psi
- Steel: 85,000psi
As you can see, the steel weighs 3-5 times as much bu is only 1.5-2 times as strong. But the real impact comes when considering bending strength. Relative bending strength can be compared by looking at thickness times tensile strength (this is a relative comparison not the actual bending strength). Assuming a 1sft panel has the same weight:
- Fiberglass 4.9" thick by 43,000psi = 206,000psi-in
- Aluminum 2.9" thick by 65,000psi = 190,000psi-in
- Steel 1" thick by 85,000psi = 85,000psi-in
As can be seen, Fiberglass & Aluminum are pretty close. Steel is far weaker for the same weight.
In smaller
boats, the required bending strength is small enough that even though heavier, the steel plate is so thin that puncture strength becomes a concern (also working with very thin
sheets is more difficult), so you need to up-size the thickness and make the
hull even heavier. This can affect aluminum but to a lesser extent.
As
boats become larger, the thickness required to handle bending strength becomes thick enough that aluminum thickness handles puncture issues and is thick enough to be easier to
work with and you start to see lots of larger cats done in aluminum.
Steel never really gets to be effective. In very large monohulls, steel can have cost and simplicity advantages even though it is heavier but since monohulls pay far less of a performance cost by being overweight, owners can overlook the weight issues.
So far the analysis has assumed solid GRP. If you do a cored
hull, the GRP easily has the best strength to weight ratio but it does come with requirements for quality construction methods. You can also get into carbon fiber and other high end composites that outperform just about everything.
PS: I believe the premise that there are lots of
marine steel welders desperate for
work is a false assumption. In most areas, steel pleasure boats are a rarity, so there are probably 10 times as many fiberglass guys as there are steel welding guys and the skilled welding guys are specialists in marine applications.
Unless you are building your
boat, it's really simpler to work on fiberglass boats. Any chemical issues are largely offset when you realize, you need to sand, prime and
paint regularly on a steel
boat if you don't want it rusting away.
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