Diesel SCR and DEF: Quick Facts for Yacht Owners

• Miles Yachts
• April 6, 2026

By: Andy, Miles, President Miles Yacht Group

What is Diesel SCR?
Selective Catalytic Reduction (SCR) is an exhaust aftertreatment system that injects Diesel Exhaust Fluid (DEF) into the hot exhaust stream to convert harmful nitrogen oxides (NOx) into harmless nitrogen and water.

Why do newer large yachts have it?
International and U.S. regulations now require much lower NOx emissions from many new yachts 79 feet and up that operate in U.S., Caribbean, and certain European waters. SCR is the practical way builders meet those standards without sacrificing performance.

What is DEF and how much will I use?
DEF is a ureaandwater solution carried in its own tank and consumed as you run. Most yachts use roughly 2–5% DEF by volume compared to diesel fuel, with many realworld boats landing around 3–4%. If your yacht burns 80 gallons of diesel per hour, expect something in the range of 2–3 gallons of DEF per hour.

How big are the DEF tanks?
On an 80–100 foot yacht, it’s common to see 150–200 gallons of DEF capacity, which typically covers several long passages with a comfortable reserve. Your exact range will depend on your fuel burn and cruising profile.

Is DEF easy to find?
In the U.S. and major Mediterranean yachting centers, yes. DEF is widely available through truck stops, commercial fuel docks, bunker suppliers, and yacht agents. Small, purely recreational marinas and out of the way islands may not stock it, so your captain should confirm DEF availability when planning remote itineraries.

Does SCR change how I run the boat?
Day to day, the boat runs the same. The main differences are:

• **Your crew will monitor DEF levels alongside fuel.
• **The exhaust system has more hardware (catalysts, pumps, sensors) that needs routine inspection and eventual replacement.
• **If DEF runs out or the system isn’t working properly, the engines may reduce available power until the issue is corrected.

What should I ask when I’m buying?
When you’re evaluating a yacht with SCR, ask:

• **What is the DEF consumption at my usual cruise (gallons per hour)?
• **How many operating hours do I get from full DEF to a safe reserve?
• **What is the maintenance schedule and expected life of the catalysts and dosing equipment?
• **Who supports DEF supply and SCR service in the areas where I plan to cruise?

At Miles Yacht Group, we help clients understand how systems like SCR fit into the bigger picture of ownership—so you can focus on where you want to go, not just how you’ll get there.