Too much steel makes the ship heavy and expensive. Too little makes it weak. You need the right plate in the right place.

You optimize hull design by matching steel grades to stress zones. Use high-strength AH36 or EH36 in high-stress areas (bottom, deck) and Grade A in low-stress areas (superstructure, internal bulkheads). Replace Grade A with AH36 to reduce thickness by 15-20% and save weight. And involve your steel supplier early to avoid designing non-standard plates. I have helped shipyards in Vietnam and Mexico save 12% on steel weight and 8% on cost using these methods.

Good hull design is not just about strength. It is about putting the right steel where it works hardest. Let me show you how.

How to Match Steel Plate Grades to Hull Stress Zones for Weight Savings?

The bottom of the ship takes huge wave loads. The superstructure takes almost none. Why would you use the same steel for both?

Match higher-strength grades (AH36, DH36, EH36)¹ to high-stress zones like bottom, deck, and side shell. Use mild steel (Grade A) in low-stress zones like superstructure, internal bulkheads, and non-structural parts. This cuts weight because AH36 gives 50% more strength than Grade A for the same thickness. I worked with a buyer in Qatar who switched from Grade A to AH36 for his bottom plates. He saved 18 tons of weight on a 100-meter vessel.

Let me map the hull zones to the right grades.

I am Zora Guo. A buyer in Malaysia once designed an entire hull with Grade A steel². The hull was heavy and thick. I asked him: “Why not use AH36 for the bottom and deck?” He said: “I did not know the strength difference.” We redesigned his steel selection. The new design used AH36 for 60% of the hull and Grade A for 40%. The weight dropped by 15%. The cost dropped by 8%.

Hull stress zones – where the loads are highest

Zone	Stress level	Recommended grade	Why
Bottom shell (keel to bilge)	Very high	AH36 or DH36	Wave bending, slamming, cargo pressure
Main deck (especially hatch corners)	High	AH36 or DH36	Cargo loads, wave impact
Side shell (waterline area)	Medium to high	AH36	Wave pressure, berthing loads
Longitudinal bulkheads (cargo holds)	Medium	AH36 or Grade A	Cargo pressure, but less than bottom
Transverse bulkheads	Medium	Grade A or AH36	Watertight divisions, moderate loads
Superstructure (accommodation, bridge)	Low	Grade A	Only wind and small waves
Internal non-structural parts	Very low	Grade A	No significant loads

The strength-to-weight benefit

AH36 has a yield strength of 355 MPa³. Grade A has 235 MPa. That means AH36 is 50% stronger. For the same load, you can use a thinner plate⁴.

Example: A bottom plate needs to carry a certain bending moment. With Grade A, you need 18mm thickness. With AH36, you need only 12mm. That is a 33% weight reduction.

Grade	Yield strength (MPa)	Relative strength	Thickness for same load	Weight for same load
Grade A	235	1.0x	18mm	100%
AH36	355	1.5x	12mm	67%

How to apply this in your design

1. Identify the stress level for each zone using your design software (or class society rules).
2. Set a target grade for each zone.
3. Calculate the required thickness using the grade’s yield strength.
4. Check that the thickness meets minimum class requirements (some zones have minimum thickness regardless of strength).

I always tell designers: “Do not use Grade A where you can use AH36. The weight savings pay for the higher grade many times over.”

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What Thickness and Grade Combinations Reduce Overall Hull Weight Without Sacrificing Strength?

You can reduce weight by using a higher grade with a thinner plate. But you need to know the right combinations.

The best thickness and grade combinations use AH36¹ for plates 10mm to 25mm thick. For plates under 10mm, the weight saving from switching to AH36 is small because the minimum thickness rules⁵ apply. For plates over 25mm, consider DH36 or EH36 to avoid excessive thickness. I helped a buyer in Romania reduce his hull weight by 22 tons by changing from 15mm Grade A⁶ to 10mm AH36 in the superstructure – a zone where the load was low enough to allow the thinner plate.

Let me show you a practical weight-saving table.

I am Zora Guo. A buyer in Thailand had a design with all plates at least 12mm thick. Many zones could have used 8mm or 10mm Grade A. But he used AH36 everywhere. That was over-engineering. I showed him a table of optimal combinations.

Rule 1 – Use the minimum thickness allowed by class

Class societies have minimum thickness rules for hull plates. For a vessel over 100m, the bottom cannot be less than 10mm regardless of grade. So switching from Grade A to AH36 below 10mm gives no weight saving (you cannot go thinner than the minimum).

Plate thickness range	Best grade	Why
Under 6mm	Grade A	AH36 not needed; minimum thickness rules apply
6mm to 10mm	Grade A or AH36	Weight saving small; choose based on stress
10mm to 20mm	AH36	Significant weight saving over Grade A
20mm to 30mm	AH36 or DH36	Consider DH36 for cold routes
Over 30mm	DH36 or EH36	Higher grades allow thinner plates

Rule 2 – Replace Grade A with AH36 at the same thickness only if you need more strength

Do not use AH36 at the same thickness as Grade A if the load does not require it. That adds cost without benefit. Only use AH36 when you also reduce thickness.

Rule 3 – Use a weight-saving calculator²

Here is a simple table for a 10m x 10m plate area:

Grade	Thickness (mm)	Weight (tons)	Relative cost
Grade A	12	9.42	1.00x
AH36	10	7.85	0.95x (thinner plate, slightly higher grade cost per ton)
AH36	8	6.28	0.85x

For the same strength, AH36 at 10mm replaces Grade A at 15mm. Weight saving: 33%. Cost saving: often 5-10% because less steel is used.

Real example from a bulk carrier

A buyer in Vietnam had a deck design calling for 14mm Grade A. I recommended 10mm AH36. The strength was equivalent (AH36 10mm has similar bending strength to Grade A 14mm). The weight saving was 28%. The cost saving³ was 12% because the mill had AH36 in stock at a good price.

Here is a summary table for common replacements:

Original (Grade A)	Replacement (AH36)	Weight saving	Strength equivalent
20mm	14mm	30%	Yes
18mm	12mm	33%	Yes
15mm	10mm	33%	Yes
12mm	8mm	33%	Yes (but check minimum thickness)
10mm	7mm	30%	Often not allowed (min thickness)

My advice: For any plate thicker than 10mm in a high-stress zone, ask “Can I use AH36 at a thinner gauge?” The answer is usually yes.

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How to Use Higher-Strength Steel in Non-Critical Areas to Lower Material Costs?

This sounds backwards. Higher-strength steel costs more per ton. So why would you use it in non-critical areas? Because mills sometimes have stock of higher grades at lower prices.

Sometimes AH36¹ is cheaper than Grade A⁵. Mills roll AH36 in large campaigns. They have overstock. Grade A is less common for thick plates. So I have seen AH36 sold at a discount to Grade A. Also, using a standard size from mill stock (even if higher grade) avoids expensive custom rolling. I saved a buyer in Pakistan $5,000 by suggesting AH36 instead of Grade A for his non-critical deck plating because the mill had AH36 in stock and Grade A had a 6-week lead time.

Let me explain the market logic.

I am Zora Guo. A buyer in Saudi Arabia needed 8mm plates for his superstructure. He asked for Grade A. I checked with the mill. They had no Grade A in 8mm for 4 weeks. But they had AH36 in 8mm in stock. The price was actually $20 per ton lower because they wanted to clear inventory. He bought AH36. He got better steel for less money.

When higher grade is cheaper – three scenarios

Scenario 1 – Mill overstock

Mills roll AH36 in big batches. Sometimes they produce too much. They need to sell it quickly to free up warehouse space. Price drops below Grade A.

Scenario 2 – Thicker plates

For plates over 20mm, Grade A is rarely rolled. Most mills produce AH36 or higher for thick plates. So Grade A becomes a special order with a premium price. AH36 is standard and cheaper.

Scenario 3 – Combined orders

If you already buy AH36 for your hull, adding a few tons of AH36 for non-critical areas keeps your order volume high. You get a better volume discount² than splitting the order into two grades.

When NOT to use higher grade

• If the higher grade has stricter impact toughness requirements that you do not need (e.g., EH36 for tropical waters). You pay for testing you do not use.
• If the higher grade requires special welding procedures or preheating. That adds labor cost.
• If the buyer’s contract specifically requires Grade A (some owners specify grade for each zone).

How to check for cost-saving opportunities³

Before placing your order, ask your supplier:

1. “What grades do you have in stock in my required thicknesses?”
2. “What is the price difference between Grade A and AH36 for this thickness?”
3. “Is there a volume discount if I use the same grade for the whole order?”

Here is a decision table:

Thickness (mm)	Grade A availability	AH36 availability	Typical price comparison	Recommendation
6-10	Good	Good	Grade A cheaper	Use Grade A
10-20	Good	Good	Similar or AH36 slightly higher	Use Grade A unless stock issue
20-30	Poor (special order)	Good	AH36 often cheaper	Use AH36
Over 30	Very poor	Good	AH36 much cheaper	Use AH36

I had a buyer in the Philippines who needed 25mm plates for a non-critical internal bulkhead. Grade A had a 8-week lead time and cost $850/ton. AH36 was in stock at $820/ton. He bought AH36 and saved $30/ton plus 6 weeks of waiting.

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Why Should Your Design Engineer Collaborate Early with the Steel Supplier?

Designers draw beautiful plans. Then they send them to purchasing. The purchasing team asks the supplier for a quote. The supplier says: “That size is not standard. It will take 12 weeks and cost 30% more.” Disappointment follows.

Early collaboration with the steel supplier¹ means the designer knows which plate sizes, grades, and tolerances are standard. The supplier can suggest small changes – like making a plate 2mm thicker but using a standard stock size – that save money and time. I have seen designs with non-standard thicknesses⁵ like 11.5mm or 17mm. Changing to 12mm or 16mm standard stock cut lead time from 10 weeks to 2 weeks and saved 15% on cost.

Let me give you a checklist of what to discuss before finalizing the design.

I am Zora Guo. A buyer in Mexico sent me a steel take-off list with 23 different thicknesses. Some were odd numbers: 9mm, 11mm, 13mm, 17mm. I asked the designer: “Why 11mm?” He said: “That is what the calculation gave.” I showed him that 12mm is a standard stock size. He changed his design to use 12mm instead of 11mm. The extra 1mm of thickness added 9% weight but cost 20% less because he avoided a special mill run. He saved $50,000 on the project.

What to discuss with your steel supplier early

1. Standard thicknesses

Most mills stock plates in even millimeter increments: 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 30, 35, 40, etc. Odd numbers like 11, 13, 15, 17, 19 are non-standard. They require a special rolling campaign.

Rule: Design with standard thicknesses unless you have a very good reason not to.

2. Standard widths and lengths

Common plate widths: 1,500mm, 2,000mm, 2,500mm, 3,000mm. Common lengths: 6,000mm, 9,000mm, 12,000mm. Design your plate layouts to use these dimensions to avoid cutting waste.

3. Tolerances

Standard tolerances (EN 10029 Class A) are enough for most designs. Do not specify tighter tolerances unless necessary. Tighter tolerances cost more.

4. Grade availability⁶

Not all mills roll all grades. Ask your supplier: “Which grades are readily available in my thickness range?” For example, EH36 is common for thick plates but rare for thin plates.

5. Mill lead times²

Ask for the current rolling schedule. If you need steel in 6 weeks, you may need to use whatever grades and sizes are in stock. If you can wait 12 weeks, you can order custom sizes.

A practical early-collaboration checklist

Item to discuss	Questions to ask supplier
Thickness	What thicknesses do you have in stock? What are standard mill sizes?
Width & length	What are your standard plate dimensions? Can you cut to custom lengths?
Grade	What grades are available in my thickness range? What is the lead time for each?
Tolerances	What is the standard tolerance? What is the cost for tighter tolerance?
Certification	How long for EN 10204 Type 3.2 certificates?
Quantity	What is the minimum order quantity for custom sizes?

The cost of non-standard design³

Design choice	Standard	Non-standard	Cost impact
Thickness	12mm	11mm	+20% cost, +8 weeks lead time
Width	2,000mm	1,850mm	+15% cost (cut from 2,000mm anyway)
Grade mix	One grade for similar zones	Many grades	Higher logistics cost, risk of mix-up
Tolerance	EN 10029 Class A	Class B (tighter)	+10-15% cost

I always tell designers: “Call me before you finalize your thickness list. I will tell you what is standard and what is not. One phone call can save you weeks and thousands of dollars.”

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Conclusion

Match grades to stress zones. Replace Grade A with AH36 at thinner gauges. Watch for cost-saving grade swaps. And involve your supplier early.

My Personal Insights (from 10+ years in marine steel export)
I am Zora Guo. My team in Liaocheng supplies marine steel plates for hull design optimization. We can advise on standard thicknesses, grades in stock, and lead times. We also help designers convert odd sizes to standard stock. Send me an email at sales@chinaexhaustfan.com or visit cnmarinesteel.com. Tell me your design thicknesses and grades. I will send you a recommendation for weight and cost savings.