Designers, owners and naval architects have long debated the benefits and compromises of staying under 500 gross tonnes. Marilyn Mower investigates how some 50m-plus superyachts achieve the limit, and what trade-offs owners face in space, performance and cost.
At any yacht show, misinformation on the 500-gross-tonnage topic flows like the libations at the VIP hospitality lounge. Does it matter if a yacht is above or below 500GT? That turns out to be a good question.
The 2025 Global Order Book included 987 yachts under construction below 500GT, or 86.7 per cent of the total. Meanwhile, the 2026 Global Order Book lists 932 yachts under construction below 500GT, with 156 of them targeting the 499GT limit *.
Obviously, staying under that figure is hugely popular, but why? Opinions vary on what the implications are on build and operating costs. Brokers generally quote a ballpark price per GT, but are they quoting a simple-to-build open-plan boat or one that will require extreme engineering (as staying below 500GT on a larger yacht implies?).
Most sub-500GT yachts hover around 45 metres, so how can yachts of 50 metres LOA, or even more, all measure less than 500GT?
First, what is gross tonnage? It measures cargo capacity, which means enclosed volume — accommodations, machinery and amenity spaces and bits you don’t even think about, such as chain lockers, AC ducts and the overhead spaces crammed with piping and wiring.
In 1969, the United Nations, through its International Maritime Organization (IMO), sought to codify the patchwork of centuries-old rules for measuring cargo ships to unify manning requirements and safety rules. Volume — how much a cargo ship could hold — was more important than length and served to calculate registration fees and most port fees.
The bigger the volume, the more safety rules to protect goods and ensure an adequate number of crew, with both their number and proficiency requirements rising at each step. The benchmarks were set at 200, 300, 400, 500 and 3,000GT based on considerations such as the Convention on Limitation of Liability.
Although yachts were not mentioned in the 1969 document, the rules were applied to mandate the number of yacht crew, their skill sets and ship safety equipment.
At 300GT, yachts over 24 metres shift from home country regulations (for instance those of the US Coast Guard) to IMO rules. Chief among the additional requirements at this level are stability studies and adherence to load line requirements, STCW certification for the captain, first mate and engineer, compliance with Marpol pollution regulations, MLC rules for crew accommodations and working conditions.
Life jackets and life rafts, watertight bulkheads, more advanced navigation and communications equipment, and a fire suppression system are all mandatory. Go over 500GT and you need SOLAS safety equipment, specialised training for officers and engineers and generally an additional engineer, plus enhanced navigation and communication electronics, compliance with ISPS code for security, and ISM for a dedicated manager ashore.
“We are trying to apply the logic of a 56-year-old rule about cargo ships to measuring luxury yachts,” naval architect Perry van Oossanen says. “For yachts, what we have are interpretations of a rule. The reason this is getting more discussion now is that there is interest in bringing the interpretations and definitions of the various flag states in line with each other.”
When discussing the pros and cons of a yacht measuring over or under 500GT, two things became clear: 500GT is an arbitrary number, and selection of a yacht over or under 500GT should be based on what the owner intends to do with the vessel.
The dock talk myths are that sub-500GT yachts are cheaper to build and operate and that a bunch of tricks can be used to build a spectacular superyacht that somehow measures under 500GT.
It used to be that yachts began barging over the 500GT threshold at about 43 to 45 metres LOA. But, by stretching a definition here and pushing a limit there, designers, builders and their clients realised that it was possible to expand the envelope, literally.
In design offices, the search for the holy grail is about creating 50-metre-plus yachts that measure below 500GT. Are there yacht designers who have exploited loopholes or vagueness in the definitions? Of course. Are these legitimate tonnage exclusions? Yes, at least at the time of their construction, but they imply compromises.
The custom yachts 58.5-metre Phi and 52-metre Special One, launched by Royal Huisman, and the 50-metre Sanlorenzo Almax, all below 500GT, are among recent examples that seem to defy mathematics. How did they get there? Almax, for one, among other steps, shrunk the size of its engine room using fuel cells in place of big diesels — a bold but not inexpensive step for sustainability.
In general, it’s a lot more expensive than one might think to reach the holy grail of yachting.
Read More/Future perfect: on board Sanlorenzo's groundbreaking fuel-cell superyacht AlmaxVan Oossanen is Phi’s naval architect, the longest yacht to stay under 500GT. “We really didn’t use any tricks,” he says. “Well, maybe the fashion plates… but the owner wanted a certain speed and range out of the yacht. And we did that with an efficient, narrow hull that reduced interior volume.”
Usually, he says, while clients ask for a specific length, shipyards focus on volume. “They ask for the biggest small boat we can design. Many things come as a result of that request. Some of them, such as large open areas, impact usability and lifestyle in bad weather; others are compromises the owner may not see on a drawing, such as limited deck storage, crew areas, storage for spare parts and maintenance issues. An open tender garage forward may solve a tonnage equation but becomes a maintenance issue later,” van Oossanen says.
While a later maintenance issue may not raise a red flag, the design and engineering cost to fit 10 pounds of goodies in a five-pound sack can be enormous and may reduce the field of yards able to build such a complex product.t.”
“We billed 64,000 man-hours of design and engineering time last year and 20,000 hours were spent on squeezing yachts to measure under 500GT,” Gregory C Marshall, a naval architect in Canada, says.
“Sub-500GT yachts have been a focal point of our work for 20 years. The dollars go up the harder you push the rule. It appears that about 55 metres will be as large as you can go and still stay under 500GT. We’ve done it twice and it’s a mistake, in my opinion.”
Marshall’s advice? “Build a 42-metre at 499GT. You’ll get the headroom and storage you want and you won’t need a pilot to cruise in Canada. If you want more accommodations, go over 500 and be done with it. You might add 100GT to the calculation, $5 million to the build cost, but the boat will be worth so much more.” As far as increased annual operating costs for crew, paperwork and the designated person ashore, it’s roughly $150,000 to $160,000, Marshall says. “That’s not nothing, but in the scope of a superyacht’s annual operating costs, it’s almost insignifican
Triggering angst now is the effort by classification societies to unify measurement. For example, storage within bulkheads used to be exempted from measured volume in some jurisdictions. Not anymore. Any enclosed space, including the ‘tween deck space above overheads, counts as volume. An open swimming pool doesn’t count, but if the bottom rises to make a solid deck for other uses, it does. And the popular plumb bow will hit 500GT faster than a raked bow.
“Let’s say there had been classification groups and surveyors who looked at things differently,” Vripack’s Bart Bouwhuis says. Vripack’s latest sub-500GT success is 52-metre Special One, a six-deck sportsfisher with an elevator measuring 499GT. Since enclosed spaces count toward tonnage, naval architects and builders have resorted to such things as boxes with grillwork on the sides and lattice tops covered with cushions. “There’s a percentage formula for how much of an enclosed volume has to be open to be excluded,” Marshall says. “It’s perfectly legal, but on a crossing, your storage becomes a washing machine.”
Sanlorenzo used the grillwork approach on its 50-metre 50Steel model to exempt the volume of its forward tender garage, as did Special One and a 50-metre Benetti. Mangusta stores the tender on its 50-metre Oceano 50 forward under a grated surface that can turn into a pool when the tender is launched. This technique is a trade-off, designers agree, because it means more crew time keeping the tenders clean and a bit more maintenance on the hydraulic systems that open the hatches and launch the craft.
“One of the issues we face trying to keep a yacht under 500GT for our clients is the limit it puts on headroom. A typical five-stateroom boat under 500GT will be limited to a maximum headroom of 2.15 metres,” naval architect Sergio Cutolo, founder of Hydro Tec, says.
“The flag states are getting stricter and stricter, even counting the space between fashion plates and the superstructure as volume,” Cutolo notes. Pushing the limit on maximum interior volume will result in a yacht with less hull depth. “This puts the main deck closer to the water, which people may like. But it also puts the make-up air intakes too close to the water and you have to do extra things to keep mist and water out of the engine room and ventilation system.”
Cutolo agrees that many brokers push the under 500GT idea to limit costs and imply naval architects have tricks to subtract tonnage. He offers the opinion that “tricks are a bit of a compromise.” One of those compromises is safety.
“Under 500GT, only the engine room is required to have fireproof walls, doors and a built-in firefighting system. Over 500 means fire insulation around the galley too, the entire yacht must be fitted with sprinklers or high fog and all escape routes must meet fire-proofing standards. A yacht over 500GT will also have to have an emergency generator above the waterline, a raised steaming light and higher levels of certification for the captain and officer of the watch and two deckhands. Plus, the yacht must adhere to ISM rules with a dedicated person ashore for emergencies.
“I understand the cost perspective, but you have to be willing to accept [those] compromises. Under 500 you will have less crew and less storage. We had a client who wanted a long-range explorer with certain accommodations and number of decks. We drew it and showed it to him and he said, ‘but where are my refrigerators?’ We had to explain that if he wanted his walk-in refrigerators, something else would have to go. A client has to be clear on the goal for the yacht and on the priorities. Yes, you can do an explorer under 500GT, we have done them, but then range and autonomy need to be the priorities,” Cutolo says.
He agrees with other designers that it’s more expensive to design and build a 50 metre under 500GT. In his view, 40 to 43 metres — and what you can comfortably fit in that — is the right length for an owner insisting on staying below 500 GT.
Thom Conboy, the US-based senior director of Ocean Independence and the North American sales director for Heesen Yachts, is not a fan of the 500GT limit. “I’d like to see it blown up. We should all just stop following it. It makes the interiors of a 50-metre feel like a 40-metre.
“Here’s the problem: MLC 2006 mandated an increase in the size of crew cabins and area by about 15 per cent. Next, the catalytic converters required for Tier III pollution control mean the engine room has to grow by 20 per cent. Where does that volume come from? It comes from the owner and guest accommodation. There is no exemption for these changes and the room they require.” To put it another way, these items gobble up 25 to 50GT, or five to 10 per cent of the total yacht’s volume.
“If clients are coming out of a 50-metre Trinity or Christensen from the 1990s or 2000s, they have a different mindset. Those boats measured under 500GT then. Built today, those boats would measure around 700 to 750GT. The only way to stay under 500GT with a new build today is to lower your expectations,” Conboy says.
“Why are we doing this? It’s a made-up rule that has nothing to do with yachts. Unlike LY3 and PYC that adapted SOLAS and commercial rules for yachts, we are still struggling with interpretations of an old cargo measurement. It makes no sense. You know what makes sense? A 52-metre yacht with a gross tonnage of 600 would probably be the perfect boat for the largest share of the market.”
Peter Bouma, senior naval architect at Vripack, and van Oossanen both say the Cayman Islands registry has been doing a great job at “reducing the grey area.” This is happening on two levels Bouma says: one is actually how to measure the dimensions and the other is in the actual rules on structure. “They are looking at it from the safety of the paying passenger and reducing risk.”
“The only way to deliver everything today’s owner seems to want in a below-500GT boat is to use a holistic approach to the dimensions, performance and amenities,” Bouwhuis says. “For example, if fast speed isn’t important, then you can use smaller engines meaning a smaller engine room. If range isn’t important, you can have smaller tanks. In this way you make smart changes that add up and you use the volume where it has the most value to the client. The only risk is if the client wants to push it to the extreme it can become too expensive to design and build. It’s always cheaper to build a large-volume boat,” Bouwhuis says.
The lesson is nothing new: you can’t have your cake and eat it too, no matter how adept the baker, designer, or builder.
* Updated after the article was originally published to include new data.
First published in the July issue of BOAT International US Edition. Get this magazine sent straight to your door, or subscribe and never miss an issue.